PHYSIOLOGICAL REVIEWS Vol. 58, No. 1, January 1978

Printed

m U.S.A.

Cardiac Energetics COLIN Department

of Physiology,

Monash

L. GIBBS University,

Melbourne,

I. Introduction .......................................................... II. Cardiac Muscle Preparations ........................................... A. Oxygenation ....................................................... B. Mechanicalproperties .............................................. III. Energy Compartmentalization .......................................... A. 1nitial:recoveryheat ................................................ B. Resting heat production. ............................................ C. Activation heat .................................................... D. Tension and heat production ........................................ E. Shorteningheat .................................................... F. Workandefflciency ................................................. G. Fenneffect ........................................................ H. Hypertrophy and energy production ................................. IV. Whole-Heart Oxygen Consumption ..................................... A. Resting oxygen consumption ........................................ B. Determinants of active oxygen consumption .......................... C. Wallstress ......................................................... D. Coronary blood flow:oxygen tension .................................. E. Whole-heart heat production ........................................ V. Pharmacological Agents and Energy Expenditure ....................... A. Cardiacglycosides .................................................. .................................................... B. Catecholamines VI. Cardiac Metabolism ................................................... A. Energy sources and metabolic regulation ............................. B. Aerobicmetabolism ................................................ C. Anaerobic metabolism .............................................. D. Hypoxia and ischemia .............................................. VII. Energy Balance ....................................................... VIII. Muscle Models ........................................................ A. Muscle models and energetics ....................................... B. Muscle models and twitch contractions ............................... C. Cardiacmusclemodels ..............................................

I.

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185 185 188 190

193 196 200 203

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212 213 215 217 217 218 221 221 222 223 225 228

230 231 236 239

INTRODUCTION

It seems widely accepted that the basic mechanism of contraction is similar in skeletal and cardiac muscle. As might be expected, skeletal muscle studies, particularly those of A. V. Hill and colleagues, form the framework within which cardiac data have been interpreted. In general, cardiac physiology has benefited from such an approach but it has also led to some areas of confusion both in energetics and mechanics. A. V. Hill’s book Trails and Trials in PhysioZogy (201) summarizes the history of skeletal 174

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muscle energetics up to the mid-1960’s and subsequently there have been several reviews reflecting renewed interest in this area of research. The articles by Mommaerts (303), Woledge (443), and Abbott and Howarth (2) are recommended. For several reasons the thermodynamic background to muscle energetics is not considered here, but all the above-mentioned reviews cover certain aspects of this area and there is a definitive article by Wilkie (431). More recently the thermodynamics of the situation, particularly as applied to cardiac muscle, have been considered by Gibbs and Chapman (146, 148). This review has several objectives. First, it considers the energetics of myocardial tissue with data obtained with experimentally more tractable preparations, such as papillary muscles and trabeculae carneae. Second, the relevant data from studies on the oxygen consumption of whole hearts are examined for correlations with results from isolated, in vitro preparations. The physiological determinants of oxygen usage are also discussed, including the effects of pharmacological agents on cardiac energy expenditure. The metabolic sources that underwrite cardiac energy production and the current problems of energy balance are reviewed; finally, the use of models to predict the mechanical and energetic properties of muscle, particularly during twitch contractions, is discussed. II.

CARDIAC

MUSCLE

PREPARATIONS

Since most myothermic and many oxygen consumption studies are carried out with papillary muscles or cardiac trabeculae, some justification should be provided for their use. It is convenient to consider these preparations with respect to both oxygenation and mechanical performance. A. Oxygenation Authors working with papillary muscles must always expect to be asked whether their preparations are oxygenated adequately for the prevailing physiological conditions. The theoretical justification for the adequate supply of oxygen via diffusion through muscle goes back to Krogh (261) and the best treatment for muscle physiologists is probably that of A. V. Hill (201, chapt. 6, particularly sect. 6.0-6.4.4). It is usual, although in some cases arguable, to approximate the geometry of a papillary muscle by a cylinder, in which case the diffusion equation yields: Y = Yo - a0 G-o2 - fY/~

(1)

where y = oxygen concentration at r, y. = oxygen concentration at muscle surface, a, = basal oxygen consumption, r. = outer radius of the muscle, r = distance from the axis of the cylinder, and K = the diffusion coefficient of Krogh.

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Originally K, as defined by Krogh for standard temperature and pressure, was the volume of oxygen that diffises across an area of 1 cm2 in 1 min, under a partial-pressure gradient of 1 atm/cm. To be consistent in the use of units, y and y0 should be expressed in atmospheres, a, in cubic centimeters of 0, per gram per minute-i.e., milliliters 0, per gram per minute - and r and r0 in centimeters. At 20°C Krogh reported a value of 1.4 x low5 cm2/min atm for frog skeletal muscle; for each degree rise in temperature, K increases by about 1%. To express the diffusion constant in more conventional units, K is divided by the amount of oxygen dissolved in 1 ml of tissue at atmospheric pressure. At 20°C there will be 0.0254 ml 0, dissolved/ml of muscle; therefore

D=

K ml O,/ml tissue =

1.4 x lo-5

0.0254

= 5.5 X 10e4 cm2/min where D is the diffusion constant and represents the moles of oxygen transported across unit area when the concentration gradient is unity. The diffusion constant is temperature dependent because the solubility of oxygen decreases as the temperatures rises; from 20 to 40°C the average increase in k is 2.4%/“C. These days it is more common to express the diffusion constant in units of moles per centimeter per minute per torr. For 37°C this gives a value of 0.96 x low9 mmol cm-l. min- l torr-l for frog muscle. There is some disagreement within the literature about the correct value to take for muscle. Most authors use the figure provided above but Grote and Thews (174) report a value of 1.10 for rat heart slices and Kawashiro et al. (253) report a value of 1.31 for intact excised rat abdominal muscle. Using an indirect method to estimate the diffusion constant, Lentini (279) has argued that in rat heart the value should be about 2.40 x 10egmmol . cm-l. min-l torr-l. In order to account for this high value Lentini suggested that 0, diffusion might be facilitated in cardiac muscle. Although the facilitation of 0, diffusion by myoglobin is a definite possibility [see Wittenberg (440)], at the moment it seemssafer to accept a value of about 1.3-1.5. In order to calculate the critical diameter, y and r are set equal to zero so that equation 1 reduces to l

l

l

ro = d4Ky,lao

(2)

In the most quoted study that h.as been made of the critical diameter of papillary muscles Cranefield and Greenspan (97) measured the rate of oxygen uptake of quiescent cat heart and deduced a critical diameter 0.64 mm or less at 35°C. Papillary muscles of various diameters were used and oxygen consumption was highest in the smaller diameter muscles. They also

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reported that if muscles with diameters less than the critical value were stretched by 50% (in which case muscle diameter decreased 20%) there was no change in oxygen consumption, but in muscles whose unstretched diameter was greater than about 0.6 mm there were increases although there was a good deal of variability in the percentage increment. These results were used to reinforce the argument that the critical diameter calculated on the basis of the oxygen consumption data was about 0.6 mm. The difficulty arises that stretch per se increases resting oxygen uptake in many skeletal muscles (80, 123) and the calculation of critical diameter rests on the assumption that Krogh’s original value is not in error and is applicable to cardiac muscle. Furthermore Cranefield and Greenspan (97) reported a very high basal Qo, of 2.84 pl/mg wet wt per h. These authors compared their data with those reported in whole-heart studies. In a normally beating heart the oxygen uptake is generally taken to be about 9 ml/100 g per min, which they calculated to be equivalent to 5.4 pl/mg wet wt per h. Thus their measured basal oxygen-consumption value accounts for more than half the oxygen consumption of a beating heart. The usual assumption is that the

basal oxygen consumption

accounts for only l/4 to l/3 of the total metabolism

(see sect. IV). If the latter proposition was correct an isolated cat papillary muscle would have to be less than 0.16 mm in diameter in order for diffusion to supply enough oxygen at a stimulus rate approaching the normal heart rate for cats (150-180 per min). In 1963, Greenspan and Cranefield (164) reported a basal Qo, of 0.74 pl/mg wet wt per h for dog Purkinje fibers at

35°C: they attributed

the much lower value (about %I that reported earlier)

to the fact that this material is specialized conducting tissue; however, no direct comparison was made with dog trabeculae. Koch-Weser (259) examined the effect of rate on the force of contraction of isolated cat papillary muscles at 38OC. He measured developed force over the range 0.2-300 beats/min and concluded that provided the muscles were less than 0.88 mm in diameter their mechanical performance was not impaired even at rates as high as 188/min. If the stimulus frequency was kept at or below 18.8 beatslmin then muscles as large as 1.1 mm in diameter developed the same tension per cross-sectional area as the smaller muscles. Sonnenblick et al. (395), using cat papillary muscles at 38°C and paired stimulation to produce postextrasystolic potentiation, showed that these muscles would develop tension in the range 8.6-14.5 g/mm2. They also showed that actively developed tension of the potentiated muscles was directly proportional to the cross-sectional area over the range 0.41-1.44 mm2, suggesting that oxygen diffusion was not limiting force development. Frezza and Bing (134) recently reported data that seem to conflict with the results of the two studies quoted above. Using rat papillary muscles and columnar carneae muscles, stimulated 12 times/min, at 28OC they found an inverse relationship between tension development and cross-sectional area (range 0.32-1.68 mm’). They suggested that the poorer performance of the thick muscles was due to core hypoxia. This may be the case, particularly as rat basal metabolism is about double that found in other species (Loiselle

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and Gibbs, unpublished observations) but there are other possibilities, which are considered at the end of this section. Coleman (85), experimenting at 29°C with cat papillary muscles, has reported a basal oxygen consumption of 2.58 pl/mg dry tissue per h. Correcting this to a wet-weight basis and assuming a Q10 of 2.5, the basal Qo, at 35°C would be about 0.78 pl/mg wet wt per h. This value is therefore only about V4 of that reported by Cranefield and Greenspan (97) at 35°C using the same preparation. Over a small preload range of 0.1-1.0 g Coleman reported no change in basal QO, with altered resting tension. There have been some biochemical studies into the effect of size and type of preparation on selected biochemical properties. Pool et al. (345) examined the levels of high-energy phosphates in isolated cat papillary muscles at 26 and 37°C. The values were compared with those obtained from right ventricular samples in vivo. At a stimulus rate of 12/min there was no significant change in adenosine triphosphate (ATP) and creatine phosphate [phosphocreatine (PC)] levels even with muscles as large as 1.3-1.4 mm in diameter. At 26°C after 1 h of stimulation at 6O/min there were noticeable falls in ATP and PC levels. Dobson et al. (108) have compared the working guinea pig heart in vivo with isolated guinea pig papillary muscles. At 25OC papillary muscles as large as 1.2 mm in diameter, stimulated 12-36 times/ min, had PC, ATP, cyclic adenosine monophosphate (CAMP), and phosphorylase concentrations that were not significantly different from the working heart in situ. As a consensus view it would seem reasonable to suggest that for quiescent muscles the critical diameter is about 1.0 mm at 37°C when the solution is oxygenated with 95-98% 0,. A different value could then be calculated by making allowance for the extra metabolism needed to fuel activity. As a crude procedure it may be assumed that there is a linear relationship between energy production and developed tension and that a papillary muscle developing a force of 80 mN/mm2 will expend about 16 mJ/ g per beat (see sect. III). Thus if a preparation is developing a force of 60 mN/mm2 and is beating 30 times/min the predicted energy expenditures would be: active (30 x 12 = 360 mJ/g per min) + basal (400 mJ/g per min): i.e., the total energy rate would be about double the basal rate and accordingly the critical diameter should be halved. It should be stressed that the force per cross-sectional area should not be used as the sole criterion of whether a papillary muscle is adequately oxygenated. The mechanical response of papillary muscle is linked to several variables such as animal species, frequency of stimulation, extracellular calcium concentration, substrate, and temperature. There are wide differences between species in the force-frequency relationship and the change in this relationship with temperature or calcium concentration is not the same in all animals. A recent paper by Bodem and Sonnenblick (29) illustrates this point; at 30°C cat papillary muscles beating at 12/min produced greater force per cross-sectional areas than papillary muscles from rats or rabbits beating at frequencies of 6 and 36 beatslmin where these stimulus rates

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were approximately optimal for force development under the prevailing conditions. These authors suggest that differences in excitation-contraction (EC) coupling may account for the variable mechanical performance. The hazards of using force per cross-sectional area as the sole index of adequate oxygenation are apparent from the results of Fisher and Kavaler (127). These authors used in situ papillary muscles from dog right ventricles and showed that even under these conditions, where the muscles were being normally perfused, thick muscles developed less force per cross-sectional area than thin ones. It would be interesting to know whether this result holds for isolated preparations whose size is below the calculated critical diameter. It may well be that the mechanical network arrangement by which cardiac cells are held together becomes less optimal in larger muscles. Alternatively, larger preparations may have proportionally higher mitochondrial concentrations and proportionally smaller contractile protein concentrations. Attention might also be focused on the possible accumulation of higher than normal concentrations of extracellular potassium and lactate in the larger preparations. Such agents might modify the inward calcium current associated with an action potential and hence decrease contractility (see sect. VD). B. Mechanical Properties Heart muscle differs from skeletal muscle in several important ways. 1) Cardiac muscle cannot be tetanized without pharmacological intervention. 2) The tension contributed by the parallel-elastic element cannot be ignored in comparison with active force generation at lengths near Lmax (where Lax is the muscle length where maximum active force development occurs). 3) Most cardiac muscle fibers are not inserted into tendons; hence damage caused by clamps or ties and the physical properties of the hooks and knots used in securing the preparations produce a large series elasticity (262, 336). These problems are more apparent with the usual isolated cardiac preparations because of the small aspect ratio (length to diameter) of papillary muscles and trabeculae carneae. 1. Length-tension relationships Until recently most cardiac physiologists would have been happy to extrapolate from the work of Cordon, Huxley, and Julian (162) on skeletal muscles and to assume that the length-tension relationship of cardiac muscle had a similar basis at the sarcomere level. An increasingly large number of observations have been made that are difficult to interpret solely in terms of simple overlap of thick and thin filaments both for skeletal and cardiac muscle. This problem is discussed in some detail in a recent book [Ciba

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Basis of Starling’s

Volume

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(347)].

As Jewel1 (231, 231a) points out there are now some discrepancies between the predictions of the sliding-filament hypothesis and the length-tension relationship even in skeletal muscle. It has been shown several times (81, 162a, 216a, 359, 416) that activation is length dependent and that this dependency particularly influences twitch-tension development. Many authors have investigated the length-tension relations in cardiac preparations. Most authors obtain similar results with sarcomere lengths in the 2.0- to 2.3-pm range at Lax and in the 1.5 to 1.7.pm range at 0.75 Lax (168, 171, 242, 262, 343, 398, 400, 401, 403). It is a consistent feature of all results that at muscle lengths in the 0.75 to 0.8 Lmax range, tension development is usually 10% or less of tha .t recorded at Lax. Some authors, however, have reported larger sarcomere values, about 1.9 pm, ati muscle lengths where active tension development is either zero or less than 25% of maximum (314, 343, 438). It is of interest that these higher values are reported for atria1 tissue (314, 438) and it would not be surprising if there were species differences in the length-tension-sarcomere relationships. Gay and Johnson (140) first reported that there was no predictable relationship between muscle length and sarcomere length. Subsequently other authors confirmed this observation for muscle lengths below 0.85 Lax (168, 343, 398) but most investigators have reported a linear relationship between the two parameters over the 0.85, to 1.0. Lmax range (168, 242, 262, 314, 343). The relationship is not, however, 1:l. Thus Pollack and Huntsman (343) have reported that a 24% decrease in muscle length produces only a 17% decrease in sarcomere length and a 75% fall in active tension development. Krueger and Pollack (262) have shown that a 17% decrease in muscle length causes a 14% decrease in sarcomere length and a 75% fall in tension development in rat papillary muscles and they have pointed out that a similar decrement in skeletal muscle sarcomere length would only have caused a 7% fall in tetanic tension. Because of the discrepancy between predictions from myofilament overlap and force generation on the ascending limb of the active length-tension relationship several authors have suggested ways in which this relationship might arise. The most favored suggestion is that there is inactivation at the shorter sarcomere lengths by analogy with skeletal muscle results (81, 359, 416). Evidence for this effect in cardiac muscles comes from several authors (7, 28, 120, 216a, 231a, 242, 323). It is also possible that the large amount of internal shortening during force development could deactivate the contractile element; abrupt length changes certainly have this effect. Brady (36) and Edman and Nilsson (115, 116) have shown that shortening per se during a contraction will decrease the duration of the mechanical response. There is a consensus that it is difficult or impossible to detect a plateau region of constant tension development; in skeletal muscle this occurs over the 2.0- to 2.2-pm range. The considerable internal shortening that takes place during contraction (262, 314, 343) may account for the absence of a plateau (244). Krueger and Pollack (262) report that at Lax and 0.83 Lax

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sarcomere lengths were 2.35 and 2.01 pm but during activity these lengths decreased to 2.18 and 1.83 pm, respectively. Probably the most important result in their investigation is the clear demonstration that the high series elasticity of papillary muscles is caused by tissue damage at the clamped end (262). In cardiac preparations, resting tension starts to be detectable at lengths between 0.8 and 0.85 Lax, so that at Lax the muscle is very stiff compared with skeletal muscle. The explanation for this effect is by no means clear but there is a good summary of the position by Sonnenblick and Skelton (399) where various possibilities are mentioned: high collagen content, small cell size (relative to skeletal muscle), residual cross-links between actin and myosin (205, 292, 311). It is difficult to extend individual sarcomeres beyond about 2.7 pm (434) although the muscle can be elongated further. It seems clear that there is no simple 1:1 relationship between overlap and force generation at lengths beyond Lax and Sonnenblick (394) suggests that internal slippage or fiber rearrangement might occur. It seems appropriate to consider briefly the in situ function of papillary muscles. There has been a tendency to believe that the papillary muscles would start to shorten before the onset of isovolumetric contraction but the results of Cronin, Armour, and Randall (98) and those of Semafuko and Bowie (379) show otherwise. In particular the direct in situ study of force and length changes in horse papillary muscle by Semafuko and Bowie (379) showed that a papillary muscle lengthens during the whole of the isovolumetric phase of systole and for the early part of ejection; the observed response is the sum of papillary muscle force development and externally applied force owing to ventricular stretch. Different results have been reported by Karas and Elkins (245) from cineradiographic analysis of metal markers on the chordae tendinae of the mitral valve and the left ventricular apex. These authors report that during ventricular systole papillary muscles contract isometrically. Their results have been challenged by Grimm, Lendrum, and Lin (170), who used the same technique but implanted two metal markers on the anterior papillary muscle of the left ventricle of dogs. No force measurements were made but there is some difference in cycle relationships as the authors reported that shortening was in progress within 70 ms of the QRS complex and reached its maximum 60-130 ms after the T wave. Grimm et al. (170) were surprised to record an average 22.8% shortening of a papillary muscle; this much shortening, as should be evident from the length-tension data cited above, would take a muscle from the peak of the ascending limb of its length-tension curve right to its base where it could not generate any active tension. Since it is apparent from the results of Cronin et al. (98) that in situ dog papillary muscles exert considerable force throughout ejection, Grimm et al. (170) were unable to explain the phenomenon. The results of Semafuko and Bowie (379) might resolve this difficulty if the muscles are stretched prior to shortening. Thus papillary muscle shortening would be 10% per cycle if the shortening is referred to the end-

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182 diastolic length of from the stretched relaxation length, reported by Grimm

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the papillary muscle, but if the shortening is measured length, i.e., peak systolic length to end-isovolumetric then the total shortening is of the order of 20% as et al. (170).

2. Mechanics

Over the last 15 years there has been considerable disagreement between the main investigators in the field of cardiac mechanics. In part controversy has arisen because of some of the unusual features of cardiac muscle relative to skeletal muscle but in addition there has been a reluctance to recognize the effects of species differences and by a desire to interpret cardiac muscle behavior within the framework of the classical Hill analogue of muscle (192) at a time when in skeletal mechanics the classical two-component model was being seriously questioned. The first detailed mechanical experiments with papillary muscles were undertaken by Abbott and Mommaerts (3) and in 1962 Sonnenblick (390) published hyperbolic force-velocity data observed at different muscle lengths and under differing physiological conditions. By the late 1960’s Sonnenblick and colleagues had also described the behavior of the series-elastic element (334,391). The active state was being measured (392, 393) and the maximum shortening velocity, Vmax, was being suggested as an index of cardiac contractility since this was said to be insensitive to muscle length but changed on inotropic intervention. As early as 1965, however, Brady (35) reported that he could not find a hyperbolic relation between force and the velocity of contraction for rabbit papillary muscle and that it was impossible to measure the active state of cardiac muscle with the quick-release technique. He also showed that, compared with skeletal muscle, there was a and that the behavior of cardiac muscle preparaslow onset in contractility tions varied in such a way that the most appropriate model was in certain cases a three-component Hill or Maxwell model and in others a threecomponent Voigt model (36, 37). These experiments and the clear conflict between the two schools of thought initiated a spate of experiments generally with different preparations and with increasing technical sophistication. Both Brady (38) and Edman and Nilsson (114) emphasized the need to determine myocardial mechanical parameters at a constant contractile element (CE) length and Edman and Nilsson reported that if force-velocity measurements were made at such a length then hyperbolic force-velocity relationships could be obtained. Hefner and Bowen (183) and Noble, Bowen, and Hefner (324) reported force-velocity relations that were not hyperbolic and concluded that Vmax occurs for loads appreciably greater than zero. In 19’70, Pollack (341) reevaluated Sonnenblick’s data (390), using both the Voigt and Maxwell models, and claimed that Vmax of the contractile element was dependent on initial muscle length. In 1972 Pollack, Huntsman, and Verdugo (342) used quick-release and quick-stretch experiments to show that the elastic modulus of the series-elastic component (SEC) depended on

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the time during the contractile cycle at which it was measured. They concluded that the concept of the SEC as a passive elastic element was unsatisfactory and questioned the validity of the Hill model for cardiac muscle. In 1969 Brutsaert and Sonnenblick (45) introduced a technique for the analysis of muscle shortening in terms of relationships between force, velocity, and length. The technique allows the influence of time on these relationships to be studied during a twitch contraction. Velocity versus instantaneous muscle length is plotted for any given constant total load (phase-plane analysis). Because the loads are generated electrically it is even possible to unload the muscle during shortening so that shortening velocity can be measured at practically zero load. Using this technique experiments were performed by Brutsaert, Claes, and Sonnenblick (46, 47) in which resting length either was varied while total load remained constant or was abruptly altered during shortening (load clamp). These authors state that if active state is measured as the unloaded velocity of shortening there is a quick rise of active state -quite unlike the slow onset measured when force generation is the criteria (35, 36, 114, 115, 393). Moreover these investigators claim that it is possible to identify an active-state plateau and that there is a unique force-velocity-length relation for the CE during the plateau of the active state [see also Donders and Beneken (log)]. Fortunately some investigators have tried to find unifying concepts that might integrate the divergent points of view of the main protagonists (136, 447). In 1970 Fung (137) proposed a model of cardiac muscle based on the sliding-filament theory and Hill’s characteristic equation that would account for nonhyperbolic force-velocity curves. He later showed that if series elasticity is assumed to be dependent on muscle length then the Voigt and Maxwell models of cardiac muscle are equivalent. In an important paper in 1973, Pinto and Fung (337) conclude that the force-velocity relationship of rabbit papillary muscle cannot be represented by a single hyperbola, that the SEC can bear compressive loads, and that tension in the SEC depends on the overlap between actin and myosin and the extension of the serieselastic element in a sarcomere. In a recent review Julian and Moss (239) examine the active-state concept for striated muscle, comparing the classical data of Hill and colleagues with computer predictions based on the kinetic models that have recently evolved. They point out that the time course of force generation in a simulated isometric contraction is influenced by the sum off + g, where f and g are rate constants for the respective making and breaking of crossbridge attachments in the Huxley (217) model (see sect. VIII and Fig. 4). They emphasize that even if activation rose instantaneously to its full value in an isometric twitch, the formation of new crossbridges would be rate limited by the term (f + g), which is not very large for positive values of x (where x is th e d’ist ante of the actin active site from the equilibrium position of a myosin crossbridge). Therefore the rate of rise of force or the capacity to bear tension does not increase rapidly, whereas Vmax is very much influenced

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by the magnitude of g,, which is the value of g for negative values of X. If the magnitude of g2 is dependent on the level of activation (see sect. VIII), then the situation will arise where force development or resistance to stretch appears to rise slowly whereas the capacity of muscle to shorten at high speeds rises rapidly. This interesting comment may make it possible to resolve the apparent deadlock between the different groups of investigators studying cardiac muscle mechanics, although it should be kept in mind that Brady, Edman, and their colleagues have used a different animal species from Sonnenblick and his colleagues and this may also be a factor. Julian and Moss (239) make it quite clear that they question the classical activestate concept on grounds similar to those outlined by Simmons and Jewel1 (385). The high series compliance measured in papillary muscles has created difficulties; strains of 510% of Lmax have been regularly reported (19, 334, 391). In skeletal muscles values of about 3% are found for whole-muscle preparations and values less than 1.5% for isolated single muscle fibers (222). If the series elasticity of cardiac muscle were to reside solely in the crossbridges, the Huxley and Simmons model (223) would have to be severely modified to account for the predicted crossbridge travel. This problem seems to have been resolved by the recent light-diffraction experiments of Krueger and Pollack (262) demonstrating that most of the series elasticity resides in nonstriated regions near the clamped ends of papillary muscles. In other words, marked sarcomere shortening (516% of the resting sarcomere length) occurs because it is impossible with conventional techniques to prevent the ends of the tissue from stretching. The above information is reassuring since it allows the same mechanism, at the molecular level, to account for both skeletal and cardiac muscle behavior. There are still many problems to be resolved for cardiac muscle, however, and it may be dangerous to push the analogy between cardiac and skeletal muscle too far. Recently Nasser, Manring, and Johnson (314), working at 20-22°C and using a light-diffraction technique and frog atrium, reported the startling observation that sarcomere shortening velocity, which was high compared with that reported in other cardiac preparations, appeared to be independent of sarcomere length and of load. Thus the velocity of shortening appeared to remain relatively unchanged throughout most of the period of tension development; in addition, Krueger and Pollack (262) recently reported the same phenomenon in rat papillary muscle, where once again the observed velocity was high (l-2 muscle lengths/s at 30°C). Biochemical reasons for high shortening velocities in rats comes from experiments by Hoh and Hale (208), who have shown that in juvenile rats there are three isozymes of myosin in ventricular tissue and that rats are peculiar in that the fastest form eventually dominates in ventricular tissue whereas in other species the least active isozyme normally becomes dominant in the ventricle. These biochemical data are supported by recent mechanical measurements. Although there are difficulties in measuring Vmax in cardiac muscle (see above), there apparently are quite striking species differences in

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this parameter. Henderson et al. (188) showed that Vmax for rat heart was much higher than that for guinea pig, which in turn was about twice as high as Vmax for dog and rabbit. Obviously at present there is no universally accepted model for cardiac muscle. There seems to be some consensus that the force-velocity relationship is not hyperbolic, but it is not clear whether differences in the activation pattern of different animal species are responsible for some of the variability. Obviously active-state measurements are fraught with h azard. If there are problems for this concept in skeletal muscle experiments, it will probably be even less rewarding in relation to cardiac muscle. For readers who wish to further investigate the current status of the mechanical models of cardiac muscle Contractile Behaviour of the Heart (421) and Ciba Found. Symp. The Physiological Basis of Starling’s Law of the Heart (347) are recommended. For thoughtful commentaries on the current status of muscle mechanics the recent brief reviews by Abbott and Gordon (1) and Julian and Moss (239) should be consulted. Some of these mechanical problems are considered further in section VIII. III.

ENERGY

COMPARTMENTALIZATION

In this section the various components of muscle energy production are examined according to the framework developed by A. V. Hill and colleagues. The adoption of such a format does not imply a belief that the basis for such a subdivision is interpretable at a molecular level. It seems likely that at best these equations are phenomenological and may obscure rather than enhance thermodynamic understanding of how muscle functions (148). The myothermic material included in this section is necessarily based largely on skeletal muscle studies and on data obtained with rabbit papillary muscles. Fortunately, since the early 1960’s several authors (83, 276, 295, 427) have recorded the oxygen consumption of isolated papillary muscles and trabeculae, usually from cats, and there have been some biochemical studies on these and other cardiac preparations, allowing independent comparisons to be made. A. Initial:Recovery

Heat

Cardiac muscle, like skeletal muscle, depends for its immediate source of energy on the free energy made available when ATP is hydrolyzed; the supply of ATP is buffered by the creatine kinase reaction, which can rephosphorylate ADP at the expense of creatine phosphate. In skeletal muscle it is possible, at least for brief tetani at 20°C or below, to show that the energy expenditure that accompanies the hydrolysis of high-energy phosphates is not dependent on the presence of oxygen. This was originally demonstrated by Weizsacker (425) and was subsequently confirmed by Hill (191). This oxygen-independent energy that accompanies contraction is

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designated the initial energy and, as outlined below, can essentially be regarded as the summed activity of events that 1) cause the release and retrieval of calcium ions and 2) cause chemomechanical transduction at the crossbridges. In amphibian muscle at low temperature this initial phase of energy metabolism is followed by an oxygen-dependent phase of heat production (recovery heat) that is held to be the thermal accompaniment of intermediary metabolism and oxidative phosphorylation (178, 191, 203, 266). This phase of heat production takes some 20-30 min at 0°C and has an energy magnitude rather similar to that of the initial metabolism, resulting in the ratio of total heat in oxygen:heat in nitrogen having a value of about is raised the clear temporal separation of the 2.07 (1 91) As the temperature initial anh recovery metabolism becomes lost. In mammal .ian skeletal muscles at 27OC the two start to overlap even in a 2-s tetanus and all the recovery heat is produced within 2-3 min (152). In the absence of oxygen, but after the mechanical event, there is a slow phase of heat production that probably can be attributed to the restitution of PC stores via anaerobic glycolysis. This phenomenon should be insignificant in well-oxygenated cardiac muscle but in pathological conditions of ischemia and anoxia there is considerable capacity to generate high-energy phosphates by such a process (see sect. VI). The anaerobic d.elayed heat of skeletal muscle is considered in some detail by Hi1 l(201, chapt . 3) and there are some interesting speculations about its magnitude and temperature dependence in Woledge’s review (443). The time course and magnitude of recovery heat in skeletal muscle are thoroughly reviewed by Hill (201, chapt. 4 and 5) and Woledge (443) has drawn attention to some problems that arise when myothermic data from skeletal muscle are compared with established biochemical predictions. In cardiac muscle the discrepancy between biochemical theory and heat data is less apparent. This may be because there is a much smaller literature to draw on but hopefully it relates to the fact that cardiac muscle never accumulates a significant oxygen debt because of its very high mitochondrial content (30-40% of cell volume). This extraordinary energy-generating capacity, manifest at even very low oxygen tensions, has two major consequences. First, it merges the initial and recovery metabolism so that even at 20°C they become indistinguishable. Second, it ensures that even under physiological conditions where energy demands may triple there will be minimal use of anaerobic metabolism to provide additional amounts of high-energy phosphates. In the first myothermic studies with papillary muscle (155, 357) it was noticed that in a single cardiac twitch there was a fast phase of heat production associated with the contractile event, followed by a much slower phase of heat production; this slow phase had a magnitude about 20% of the fast-phase heat production (see Fig. 1). There was a similar small slow phase of heat production after a series of lo-15 contractions at 0.25 Hz. The question arose as to whether all the recovery heat was present and if so what was its magnitude. Subsequently isometric and isotonic experiments were carried out under a) aerobic conditions, b) hypoxic conditions induced l

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15 set

I

lsec FIG. 1. Heat production (upper trace) and tension development (lower trace) averaged for 16 twitch contractions recorded at 1-min intervals in a Krebs solution (glucose substrate). Note difference in time scale for the 2 traces. There is a rapid burst of heat production that accompanies the contractile event, then a much slower phase of heat production. Temperature, from Chapman and Gibbs (70).] 22°C; lmax 9 5.0 mm; muscle mass, 4.0 mg. [Data redrawn

by a 20-min exposure to a PO, in the 8- to 20,torr range, and c) anoxic conditions induced by a 30-min exposure to 0.5 mM iodoacetic acid (IAA) followed by a lO-min exposure to a PO, less than 2 torr (145): It was immediately clear that although mechanical performance was reduced by about l/4 to l/3 under conditions b and c there was more than the predicted fall in heat production over a series of 15 contractions. Thus for a given level of tension development much less heat was produced, particularly in the WA/N,-treated muscles (condition c). The extent of the fall can be estimated from the changes in the isometric heat coefficient (PZIH) where P is the twitch tension, Z is the muscle length, and H is the heat produced in a twitch. Under aerobic conditions this dimensionless ratio had a value of 3.6 but after the WA/N, treatment the ratio rose to 6.7. This result suggests that recovery heat is present but is not temporally separated from the initial heat; indeed a value of 6.7 implies that the recovery heat accounts for some 45% of the total observed heat in oxygen. The errors that can arise working with . meta .bolically inhib ited preparations are considerable and in general they will cause an overestimation of the heat so that a recovery heat:initial heat ratio of 0.8 may even be too high. The data listed above were obtained for a series of contractions but when the heat traces associated with single isometric contractions, recorded in oxygen or in nitrogen, were superimposed there was little difference in the magnitude and time course over the first few seconds. This result could have meant that there was no recovery metabolism in response to a single stimulus. One possible explanation was that in response to a single stimulus

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the ADP load presented to the mitochondria was too small to trigger oxidative metabolism. The spectrophotometric and oxygen-consumption measurements of Eisenberg and Ramirez (117, 352) argued against this proposition, and some years later Chapman (68), using the fluorometric technique of Chance and Williams (63), showed for rabbit papillary muscle at 20-22OC that recovery metabolism is fully activated within a single contraction as in one of a series of repetitive contractions. As the data pointed to the presence of recovery heat even in response to a single stimulus, a method was developed to derive the time course of recovery heat at 20°C (70). The method was the converse of that used by Wilkie (432) to deduce the number of phosphorylations arising during recovery metabolism in skeletal muscle. It was calculated that in cardiac muscle the recovery:initial energy ratio is 0.72, which is not too different from the experimentally observed value of 0.8. The higher experimental value could easily be due to some anaerobic glycolysis contributing to heat production. With averaged data from 10 contractions occurring at 0.25 Hz in four different substrates, it was found that, with glucose as substrate, only 27% of the recovery heat evolved 4 s after the first stimulus (in experimental terms this corresponds to a 12% change in totaL heat). At the end of the 10th contraction 65% of the expected recovery heat had appeared, while it took almost an additional minute for all the recovery heat to be evolved. In confirmation of fluorescence data (68) it was shown that the rate of evolution of recovery heat was dependent on substrate. When pyruvate rather than glucose was used, recovery heat production was much more rapid: within 4 s of the 1st and 10th contractions the respective recovery heat percentages were 50 and 79% and all the recovery heat was produced within 20 s of the 10th contraction. It was suggested that the early rapid phase of recovery heat production, most noticeable in pyruvate, was associated with oxidation of the respiratory chain during phosphorylation of ADP, whereas the subsequent slower phase of recovery heat might be associated with regeneration of the resting reduction level of the respiratory chain by oxidation of substrates in intermediary metabolism. There was a close correlation between the time course of the recovery heat and the time course of fluorescence transients under the same substrate conditions. The model with its underlying biochemical assumptions seems to have fitted the experimental data although further testing is obviously desirable, particularly in the light of current biochemical problems (see sect. VII). B. Resting

Heat Production

Basal metabolism is considered in sections IV and VI and only myothermic, biochemical, and fluorescence data are considered here. In heat studies at 20°C (155) it was reported that the resting heat rate was 25 meal/g muscle per min or 1.8 mW/g with 5 mM glucose used as the substrate. This represents an oxygen consumption of close to 2.5 ml OJlOO g per min when

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extrapolated to 37OC. It was also reported that the response of the muscles to stretch (increased resting tension) was quite variable: in some muscles insignificant changes in resting heat production accompanied elevations of resting tension, whereas in other muscles the resting heat rate could be doubled and this effect did not correlate with muscle diameter. Similar results were reported by Pool and Sonnenblick (346). In metabolically inhibited (IAA/N,) cat papillary muscle preparations these authors reported a linear dependence of ATP and CP breakdown on resting tension. It is not possible in myothermic experi .ments to make heat measurements has been . mounted on a thermopile until at least 30 min after the preparation and often the earliest measurements are made 1-2 h after dissection. In observation that the resting myotherm ic studies it has been a consistent the day this decline is normally not heat rate declines sl .owly throughout linked with a decline in contractility, which is usually stable or increases (70). The possibility seems to exist that myothermic measurements on isolated muscles may be made at a time when the basal metabolism has already noticeably declined: such a conclusion is supported by the wholeheart oxygen-consumption studies of Arnold and Lochner (11). Using the fluorometric technique Chapman (68) reported that the resting aerobic level of reduction of mitochondrial pyridine nucleotide was very dependent on the respiratory substrate in the bathing medium and in subsequent studies [Chapman and Gibbs (70)] it has been shown that the resting heat rate can be almost doubled by changing the substrate from 10 mM glucose to 10 mM pyruvate. Acetate is almost as effective as pyruvate in stimulating resting metabolism and lactate occupies an intermediate position. These observations confirm the findings of Lee, Yu, and Burstein (278), who r eported that the oxygen consumption of cat papillary muscles doubled when pyruvate was substituted for glucose. The resting heat rate of cardiac muscle seems to be about 4 times higher than that of mammalian skeletal muscle at the same temperature. The difference in mitochondrial content in the two tissues is more than fourfold but the question arises as to why the heart has need of such a high basal metabolism. The high surface-to-volume ratio may provide part of the explanation particularly if regions of the cardiac sarcolemma are permeable to ions and require a higher than normal rate of Na-K-ATPase activity (378). There is increasing evidence that amino acid transport is linked to such activity and Lesch, Gorlin, and Sonnenblick (280) have shown that passive stretch accelerates amino acid uptake. Rabinowitz (349) has drawn 1n cardiac must le and to the short attention to the rate of protein synthesis half-time for contrac tile protein turnover In response to constriction of the ascending aorta he reports that a 30-50% increase in muscle mass can occur within 24-48 h. Rapoport and Bidinger (353) have shown that stretch (2030% Z,) of frog sartorius muscles at room temperature markedly stimulates the sodium pump and these authors suggest that at Z0 (the in situ muscle length) the sodium pump would utilize about a third of a muscle’s resting high-energy phosphate consumption to promote active Na efWx. Calcula-

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tions for cardiac muscle (148) suggest that active Na transport would account for less than 10% of the resting metabolism. This calculation is supported by the lack of effect of cardiac glycosides on resting heat production (149). Disturbances of intracellular calcium cycling by agents such as caffeine also produce minimal resting heat changes (73) and it seems that considerably more experimentation is needed before an explanation will be found for the high cardiac resting heat rate. C. Activation

Heat

Hill (193) originally defined the activation heat for frog sartorius muscles as “the heat we should observe on stimulation if shortening could be completely obviated” and he showed that this heat starts at its maximum rate and antecedes the mechanical response (194). Recently Homsher et al. (209) have proposed a different definition, namely, that “activation heat is the thermal accompaniment of the liberation of calcium into the sarcoplasm, its movements to and from the myofibrillar binding sites and its return to its storage site by an ATP dependent transport system in the sarcoplasmic reticulum.” This definition is accepted here as operative when the terms activation or tension-independent heat are used. Over a complete contraction cycle any entropic processes - i.e., processes such as the binding and release of ions to and from proteins that do not require the use of the “free” energy made available by ATP or PC hydrolysis-should thermally cancel so that the net activation heat production would derive from the hydrolysis of ATP associated with calcium pumping (422). The definition adopted above makes it clear that this heat component is of different origin than the heat associated with actomyosin ATPase activity. As Woledge (443), Homsher et al. (209), and Smith (387) have all pointed out the early methods of measuring activation heat (156, 193, 195) all had some drawbacks in the sense that a proportion of the measured heat could be chemomechanical transduction heat, i.e., heat related to actomyosin ATPase activity. This defect has largely been done away with for frog semitendinosus muscles, which can be stretched to lengths where there is no overlap of the thick and thin filaments and where the heat released after a single stimulation is unlikely to be seriously contaminated with heat arising from actomyosin ATPase activity. However, the heat probably has several origins and it is by no means certain that the amount of calcium released in a fully stretched muscle is the same as that released when the muscle is at more physiological lengths (359). Recent experiments (209, 387) have established that the activation heat is released in two phases: a temperature-insensitive “fast” phase and a temperature-sensitive “slow” phase (with a QIO of 2.8). This heat, which constitutes about l/3 of the heat in an isometric twitch at Z0 (4 mJ/g), was accounted for in terms of PC or ATP hydrolysis [see also Davies, Kushmerick, and Larson (102)] and is followed by a normal recovery heat cycle. The ratio of recovery:initial heat was 0.94.

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A paper on activation heat by Chaplain and Pfister (67) has been quite often overlooked. The authors measured the heat liberated in a preshortened frog sartorius muscle in response to a single stimulus. This method of determining activation heat has a disadvantage in that there is the possibility of some heat coming from crossbridge cycling; however, if the heat:tension relationship is extrapolated to zero tension, the error introduced is probably less than 20%. Besides making myothermic and biochemical measurements, the murexide technique of Jobsis and O’Connor (235) was used to measure the change in free intracellular calcium. The relationship between the first time derivative of the activation heat and the rate of release and reabsorption of calcium is very striking (cf. Fig. 1; 67). The ATP usage was measured at 70 and 400 ms after a stimulus in 32 muscle pairs. There was no detectable ATP breakdown in the first 40-70 ms but at 400 ms there was a breakdown of 0.072 pmol ATPIg. Their finding that no ATP was split early on (equivalent temporally to the fast phase referred to above) supports the idea that this temperature-independent phase relates to a physical process; Chaplain and Pfister suggested conformational changes as a result of Ca binding to the myofilaments. The latter phase almost certainly represents the ATPdependent Ca-pump activity (179). It has been argued elsewhere (69) that this heat should be about 4 mJ/g per twitch for skeletal muscle if 0.2 pmol of Ca2+ ions/g muscle is released Tom storage sites and is transported back into the sarcoplasmic reticulum (SR) with a stoichiometry of 2 Ca2+ per molecule of ATP hydrolyzed (422). In a brief abstract Yamada (453) reports that the enthalpy change (M) of the calcium-troponin reaction is about -46 kJ/mol at pH 7. Such a reaction may account for the fast component of the activation heat. Recently, Fraser (133) has suggested that the binding of calcium to the troponin-tropomyosin system is of slight thermal importance. Fraser’s conclusion was based on infrared temperature measurements with amphibian skeletal muscle. He found that in response to a second stimulus applied at the peak of the first twitch there was scarcely any increase in the rate of heat production. This occurred in spite of the fact that a considerable amount of calcium is released by a second stimulus (see 14, 235). In the light of recent investigations Fraser’s results could be reinterpreted and it might be inferred that Ca2+ release from the SR is not a source of heat production or alternatively that the Ca2+ released in response to a second stimulus is released very slowly; the latter conclusion is not in accord with the experimental evidence (see Fig. 9; 14). The probable cause of the lack of an increment in heat rate might be that most of the troponin sites still have Ca2+ bound to them at the time of the second stimulus in spite of the falling sarcoplasmic free-calcium level. The suggestion that the release of Ca2+ from the SR and its rebinding to the SR are probably thermoneutral is supported by calorimetric experiments with isolated SR accumulating calcium (157). If ATP was omitted from a reaction medium then very little heat was produced when calcium ions and fragmented SR were mixed together. When ATP was present calcium was accumulated, ATP was hydrolyzed, and the expected amount of heat was produced.

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What is the situation in cardiac muscle? The calcium-release cycle differs significantly from that of skeletal muscle so that the contractile proteins are not normally saturated with calcium (120a, 269). If data from Katz (246) and from Solar0 et al. (389) are used it seems that to maximally activate cardiac muscle 50-100 pmol Ca2+/kg wet wt are needed. Assuming an in vivo M ATP of -46 kJ/mol this would predict an activation heat between 2.0 and 4.0 mJ/g. These estimates include the recovery heat (see above) and assume a 2:l Ca2+:ATP stoichiometry (423). It is a feature of cardiac muscle that its calcium release in vivo is being continually modulated by the effects of rate, length, neurotransmitters, and pharmacological agents. Unlike skeletal muscles such as frog semitendinosus, cardiac muscle cannot be stretched to lengths where there is minimum overlap of thick and thin filaments without substantial decrement of subsequent mechanical responses. For this reason the activation heat, or tension-independent heat, has been measured by shortening cardiac muscle (papillary muscles) to lengths where no active tension is developed in response to a stimulus. The heat liberated under such conditions has been taken to be the activation heat and its recovery heat counterpart. In rabbit papillary muscle the activation heat has a magnitude of about 2 mJ/g muscle at 20°C and a stimulus rate of 0.25 Hz and of 1.2 mJ/g at 30°C and 0.5 Hz. It has been shown that pharmacological agents or physiological procedures that increase contractility increase this heat component (143, 150, 153, 158). It is clear from these papers that the relationship is complex. The tension-independent or activation heat component is sensitive to extracellular calcium levels; at 32°C exposure of papillary muscles to calcium-free solutions, or to physiological solutions without calcium and with a chelating agent present, lowers the activation heat to 0.4 mJ/g muscle (158). There is evidence that in cardiac muscle the sodium pump may contribute to the overall tensionindependent heat (71) as decreasing extracellular sodium and calcium concentrations lowers its magnitude; these responses are additive. Langer (269) has calculated that calcium transport by the SR in a heart beating 80 times/min would account for 15% or more of the total cardiac energy output. In 1974 Rich and Brady (358) measured high-energy phosphate changes occurring during maintained contractions (high KCl) using a rabbit interventricular septal preparation. Glycolysis and oxidative phosphorylation were inhibited separately or in combination and in all cases potassium contractures increased high-energy phosphate utilization. For a given tension-time unit the energy cost of a KC1 contracture was 1.6- to 4.0-fold that in a physiologically induced contraction. This showed that the primary cause of the increased energy cost was independent of tension. Langer and Brady (270) had previously shown that K+ contracture increases Ca2+ influx and Rich and Brady (358) attributed the high energy-consumption rate to large increases in intracellular calcium that had to be handled by the SR calcium pump. It should be emphasized here that if the calcium load presented to the myofibrils is supramaximal the activation-heat component can become so

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large that mechanical efficiency, defined as work/(work + heat), falls in spite of peak mechanical response. In cardiac muscle, caffeine and isoprenaline in combination will produce just such an effect (73). The measurement of activation heat in cardiac muscle is necessarily imprecise because tension development can only be abolished by maximally preshortening muscles before stimulation. If the activation process in cardiac muscle is as length dependent as recent data suggest (120, 120a, 216a, 231a, 232) then the activation heat at normal lengths may be greater than present estimates, although the calcium flux studies of Langer and Serena (271) made on muscles contracting either isometrically at different lengths or isotonically seem to suggest the effect will not be large. It should be apparent from this section that at a biochemical and physical level the origin of activation heat is far from satisfactorily understood. Definite progress will probably only come when calorimetric studies can be made to determine the heats of association and dissociation when calcium is bound to or is released from various cellular components, e.g., troponin, SR, mitochondria, myosin. The activation-heat concept is reviewed by Abbott and Howarth (2), and several authors have speculated on its possible origins (69, 209, 387). D. Tension and Heat Production In muscle physiology the linear relationship between heat production and tension development is an old observation (118, 190). This relationship can be demonstrated by stretching or shortening a muscle to vary its mechanical output. In skeletal muscles stretched beyond Z0 the decline in tension and heat production can almost certainly be attributed to the reduced overlap between thick and thin filaments. It seems likely that the decline in tension at lengths below Z0 will be explained by a number of additional factors such as interactions between thin filaments, increased interfilament distance, abutment of the z disk against the thick filaments, and effects of sarcomere length on activation. Cardiac muscle normally works over the ascending limb of the lengthtension relationship: that is, at lengths below Lmax. Using isolated preparations several authors have shown an essentially linear relationship between energy production and peak developed tension (86, 155, 295). In cardiac muscle the heat-versus-tension relationship is curvilinear at lower temperatures (20°C) but becomes linear as temperature is raised (152). The most interesting feature of the energy:tension relationship for cardiac muscle is its steepness and low intercept. At 30°C the relationship can be adequately described by an equation of the form: E=A+kT

(3)

where A = activation heat, k is a constant, and T is the tension. As early as 1921 Hartree and Hill (177) investigated the relationship between heat production and the duration of the stimulus in tetanic skeletal

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muscle contractions. They found that after the first few stimuli there was a linear relationship between the heat:tension ratio (HIpI,) and stimulus duration of the form: w%

=A+Bt

(4

where A and B are constants, P is the tetanic tension, and t is the stimulus duration. The first term on the right is a measure of the energy cost of tension development and relaxation; the second term is a measure of the cost of tension maintenance. Hartree and Hill (177) investigated the relationship between isometric heat production and “tension-time” in the hope of finding that the mechanical response would be proportional to heat production beginning with the first stimulus. They showed that tension-time was approximately linearly related to stimulus duration for temperatures between 0 and 15°C and that linearity and steepness of slope increased with temperature (see their Fig. 7). When normalized heat production was plotted against normalized tension-time the relationships became linear after the first 0.5 s; the slope was steepest at high temperatures (see their Fig. 8). These experiments were all on tetanized skeletal muscles. Recently it has been shown that cardiac muscle can be tetanized if exposed to high concentrations of caffeine and calcium (131). Cooper (88) has reported a linear relationship in cat papillary muscles between oxygen consumption, measured polarographically, and tension-time and this result has been confirmed in myothermic investigations with cat, rat, and guinea pig cardiac preparations (154). A similar result, however, can be obtained even when twitch contractions are studied; thus in myothermic studies of cardiac muscle it has been shown that there is usually a linear energy:tension-time relationship (150, 151) even when the energyqeak-tension relationship is somewhat curvilinear. A schematic representation is given in Figure 2. The mechanical response was altered either by varying muscle length or by manipulating the basic stimulus rate. As mentioned at the time, a linear relationship between heat and tension-time does not necessarily make tension-time the best mechanical predictor of isometric energy expenditure. Tension-time and peak tension are related differently to energy expenditure because they are curvilinearly related to each other (see Fig. 2; 151). The degree of curvature depends on the physiological circumstances, e.g., temperature, stimulus rate, or catecholamine levels. Undoubtedly there are other mechanical parameters, e.g., rate of tension development, that will also predict energy expenditure. For reasons outlined elsewhere (146) peak tension seems to be the best mechanical index to use because its predictive value is less tiected by changes in physiological and pharmacological factors. Nevertheless the duration of a contraction must be important. Intuitively this conclusion is easily reached by considering a tetanized skeletal muscle: with repetitive stimulation tension soon plateaus, while energy expenditure continues, albeit at a reduced rate. Under such conditions “tension-time”

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FIG. 2. Schematic drawing showing isometric enthalpy:peak tension (P) relationship for normal contractility (Al-& solid line) and for increased contractility (A& solid line). Note that curves are parallel to one another but are displaced by an increased activation-energy component (A ,-A I). These curves are usually obtained by varying initial muscle length to alter isometric tension development. Relationship between isometric enthalpy and tensiontime index (TTI) is also shown (dotted lines). Note that this relationship is linear for a given set of conditions but that the slope of the relationship changes with contractility so that A ,-B 1 is not parallel to AZ-D. These curves show that peak tension estimates energy expenditure more accurately than ‘IT1 when cardiac contractility changes. In the example shown when peak tension doubles the energy expenditure goes from 23, to E,. However, TTI does not change much because cardiac contractile duration usually decreases under such conditions and this compensates for the increase in peak tension. Thus energy expenditure doubles while there is only a modest increase in ‘ITI. [Data extrapolated from results in Gibbs and Gibson ww.1

must be the correct index, and in a simulation study Taylor (412) has shown that the energy output is proportional to the calcium load presented to the contractile proteins and ultimately to the calcium pump. It is very doubtful whether the tension-time integral of the relaxation phase is energetically equivalent to the tension-time integral of the contractile phase. Hill (199) postulated a tension feedback heat that operated mainly during the relaxation phase and accounted for 15-20% of the heat seen in a skeletal twitch contraction. Subsequent experiments by other investigators have not confirmed its existence (211, 293) and the relevant biochemical data are conflicting (101, 225, 306). In cardiac muscle, experiments by Monroe (307), using an isovolumic dog heart preparation, have shown that 91% of the oxygen cost of a contraction is incurred by the time peak aortic pressure is reached.

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Surprisingly little attention has been paid to determining the relationship between oxygen consumption and tension development; this is in contrast to whole-heart oxygen consumption, where considerable study has gone into the relationship (see sect. 1123). The most relevant study is that by McDonald (295) with cat papillary muscles. In this study the preload was usually altered so that initial length was not constant. However, the peak developed tension and the developed tension area clearly correlated well with oxygen consumption (measured above the basal level). In one set of experiments at a fixed initial length, tension development was varied by changing rate; over a restricted tension range, 50-100% of the maximum, there was a close relationship between developed tension and oxygen consumption (see 295, Fig. 4). This result has been confirmed in myothermic experiments and shows that it is developed tension and not initial length per se that determines the energy cost of a contraction (1%). Over a more restricted tension range Coleman, Sonnenblick, and Braunwald (86) have also obtained a linear relationship. As in the heat studies referred to above, they altered tension development by varying the initial length of the muscle. No mention has been made of thermoelastic heat. This heat component has been studied in skeletal muscle by Hill (196) and Woledge (441) but no comparable studies have been made for cardiac muscle. Over a complete contraction cycle it can be ignored because the heat absorbed during tension development will be liberated during relaxation. Before concluding this section on tension-related heat it seems reasonable to ask where the heat comes from. It is clear from the Huxley (217) model and the subsequent experimental development of it by Huxley and Simmons (223) that during the rising phase of tension development there is Even when the initial internal a gradual recruitment of crossbridges. shortening stabilizes there will still be some turnover of crossbridges, and everytime a crossbridge is broken and reformed a molecule of ATP will be used and some fraction of the free energy available will be used to stretch the elastic element of the crossbridge [possibly the S-2 subunit (219>]. The free energy not used in elastic storage must be degraded to heat and will sum with the obligatory or entropic component of ATP hydrolysis. The elastic storage constitutes the internal work and such mechanically stored energy will be converted to heat. During the rising phase of tension development it is obvious, particularly in cardiac muscle, that there is considerable internal sarcomere shortening, possibly at quite high velocities (262, 314). Under such conditions there will be a rapid turnover of crossbridges, a much smaller average amount of mechanical energy storage per crossbridge cycle, and hence greater energy dissipation.

E. Shortening

Heat

Since the reviews of Mommaerts (303) and Abbott and Howarth (2), the shortening-heat controversy still has not been satisfactorily resolved. How-

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ever, there is now agreement that in tetanic contractions of amphibian muscle at 0°C there is extra heat liberated and this heat can be measured both during shortening and even after relaxation is complete (17,106). As most workers in the field have emphasized, the problem in measuring shortening heat or its biochemical equivalent is a problem of base lines, i.e., what is to be subtracted from what and when. As early as 1962, Mommaerts, Seraydarian, and Marechal (305), on the basis of biochemical investigations, suggested that, depending on the size of the load, there might be redistribution of the various energy components making up the total energy output of a muscle such that it would be difficult to separate out a shortening-heat component. These concepts were reinforced by the investigations of Hill (197,200) in which it was reported that shortening heat was load dependent and that within twitches or brief tetani there could be changes in the magnitudes of other heat components that would obscure the measurement of a shortening-heat component. In 1967 Gibbs, Mommaerts, and Ricchiuti (155), working with rabbit cardiac muscle, used a balance sheet made up of four parameters according to Hill (200) such that E = A + W + f(P, t) + ax

(5)

where A = activation heat, W = external work, cyx = shortening heat, and f (P, t) = tension-dependent heat. It was possible to “extract” a load-dependent shortening-heat component if the activation- and tension-dependent heats and external work were subtracted from total energy production. In subsequent skeletal muscle experiments (210, 211, 303) this position has been maintained and extended and it has been shown that the normalized value of the force-dependent shortening-heat coefficient (cu) is the same in either a twitch or a tetanus. In the view of Mommaerts, Homsher, and colleagues, shortening heat represents degraded free energy and hence must have a chemical counterpart in terms of ATP or PC breakdown. Woledge (443) seems to come to the same conclusion, although he is apparently willing to accept the biochemical studies of Kushmerick and colleagues (264, 265) as evidence that there is no chemical change that will account for the observed heat if PC or ATP measurements are made at the time of the increased heat production. Woledge postulates an unidentified reaction that must be reversed later in the contraction cycle and he concludes that the reverse process is thermally neutral and is paid for by PC splitting. Indeed Woledge reinterprets data from Kushmerick et al. (265) to support his position. Lebacq (273) investigated the heat of shortening in repeated tetanic contractions of normal and l-fluoro=2,4=dinitrobenzene (FDNB)-poisoned muscles; under these conditions PC cannot rephosphorylate ADP. He found that the heat of shortening did not change significantly with repeated tetanizations but after FDNB poisoning it was larger than normal and then

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declined rapidly in subsequent tetani. In these experiments the muscles were released during the plateau of an isometric tetanus and the velocity and extent of shortening were controlled; subsequent to the shortening, tension redeveloped, although to a slightly smaller plateau value. In the following year Dickinson and Woledge (106) confirmed the existence of additional heat in a tetanus where shortening occurred but they also reported that this shortening heat was greater if measured before relaxation took place than afterward. They found that the difference was larger than could be accounted for in terms of any accompanying mechanical changes. There was some tension deficit after shortening that leads to reduced thermoelastic heat and work dissipation but even when allowance was made for these effects the magnitude of the shortening heat had fallen. Dickinson and Woledge suggest that the reduction in heat output during relaxation is due to the occurrence of a reaction in which the exothermic process causing the shortening heat is reversed. In a further paper Dickinson and Woledge (107) used essentially. the technique of Lebacq (273), but employed a series of brief tetani at 5-s intervals. They found that the size of the shortening-heat component fell during the series to 66% of the initial value by the 3rd contraction and to 15% by the 10th. This reduction did not occur if the interval between tetani was increased to 40 s. They interpreted their results to show that: “the process producing shortening heat is not, as had been thought, essential for shortening or the performance of work.” This is an important point and should stimulate further research into this elusive component. The question of “shortening heat” is not yet resolved even for skeletal muscle. In cardiac muscle the situation is more complex. At least in skeletal muscle it can be demonstrated that in an isotonic contraction the rate of heat production during a period of shortening exceeds the rate of heat production during the rising phase of isometric tension development. This experimental result has not been achieved in cardiac muscle: when heat traces are superi mposed the rate of isometri .c heat production always leads the isotonic trace regardless of the load (151). In itself this does not constitute evidence against additional heat being liberated when a muscle shortens; the strong length dependence of the activation mechanism, the high series and the poorer time resolution (because of muscle size and the elasticity, volume of adhering fluid) may make it dificult to detect such a component. As mentioned above indirect evidence for such a component can be obtained by use of the heat in an “equivalent” isometric contraction as the base line (see Fig. 3, stippled area). It is then possible to show that more heat is produced in an afterloaded isotonic contraction than in an equivalent is produced contraction where the same amount of tension or tension-time (155, 210). This conclusion is not secure if allowa rice is made for the recoveryheat counterpart of the external work [Gibbs and Gibson (l51)] since it can be argued that the work term should be approximately doubled before subtraction from the total energy (see Fig. 3). Thus a shortening-heat component can only be demonstrated in cardiac muscle if it is tentatively

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FIG. 3. Relationship between total active energy production (E) and load obtained in afterloaded isotonic contractions. Any particular load (P) is expressed as a fraction of the isometric force for the prevailing physiological conditions. Energy production is shown subdivided into activation, tensiondependent, and work terms; stippled area may represent shortening metabolism or alternatively it may predominantly be the recovery counterpart of the work term. [Based on data from Gibbs et al. (151, 155).]

8-O

ACTIVATION HEAT

0

1 0.2

1 o-4

I O-6

1 08

I 1-o

PIP,

identified as the difference between the isotonic heat and the equivalent isometric heat. With this procedure a shortening-heat component can be measured that has the load-dependent properties found in skeletal muscle (151, 155). In some studies made with papillary muscles it has been implied (428) or explicitly stated that there is no additional energy production incurred when a muscle shortens, apart from the work done (86, 344). The design of such experiments has been similar to that used by Carlson, Hardy, and Wilkie (59) for twitch skeletal muscle and it is therefore possible on statistical analysis to reach the conclusion that there is no shortening-heat component; in reality it is a base-line problem (151). Clearly the present method of measuring shortening heat in cardiac muscle is very different from Hill’s 1938 method (192) and indeed it raises another problem. How is equivalence to be measured? Should peak developed tension, tension-time, or some other mechanical parameter be used? In a recent paper Curtin et al. (100) raise this problem at a kinetic crossbridge level. They calculate that the cycle time of a crossbridge in an isometric contraction is about 3 times greater than in an isotonic contraction, where maximum work output is occurring, and therefore argue that it is dangerous and kinetically incorrect to equate different types of contractions. However, it could be argued that by the very nature of the force generator, i.e., the

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crossbridge cycle, the equivalence is built in. It has been accepted since the Huxley model that in shortening there is a more rapid turnover of crossbridges. Thus if the simplifying assumption is made that a crossbridge has a linear stress:strain relationship then it follows that to achieve a certain tension development while a muscle shortens the crossbridge turnover frequency must increase compared with the isometric case where the muscle is developing the same force. The increased crossbridge turnover should lead to more ATP being hydrolyzed. In this sense shortening heat can be regarded as having two origins: 1) the additional dissipational or waste heat accompanying the work cycle, since it is thermodynamically very unlikely that such a process can be conducted with unit enthalpy efficiency [see Mommaerts (303)], and hence less work will be extracted per cycle at faster shortening velocities; and 2) the increased bridge turnover required to maintain a given level of force generation when shortening takes place. Nevertheless, an explanation along these lines obviously would offer no insight into the results reported by Dickinson and Woledge (107) and their results, obtained with repetitive stimulation, would seem to more closely mimic the situation for cardiac muscle in vivo. F. Work and Efficiency External work is measured easily and it is not too difficult to obtain an estimate of muscle efficiency defined as work/energy expended. However, the above definition can be misleading and misuse of the term efficiency presents difficulties that have been discussed in detail over recent years (145, 303, 431, 433). In practice it has been common to define mechanical efficiency as work work -- W work + heat - enthalpy change - - AH

(6)

Neither the molar enthalpy in vivo (i.e., M) or more importantly the molar free-energy change in vivo accompanying ATP hydrolysis is known with any degree of certainty. As Wilkie (431, 433) has pointed out physiologists would really prefer to know how much of the free energy, made available when ATP is hydrolyzed, is turned into useful work: in other words they would like to measure the thermodynamic efficiency defined as: work W free energy change = - AG If both the initial and recovery metabolism are measured, which has not usually been the case in skeletal muscle studies, and if glycogen or glucose is underwriting metabolism there is biochemical evidence that m = AG for the complete oxidation cycle (49) so that mechanical and thermodynamic

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efficiencies are then similar. Even if this assumption is correct for cardiac muscle, which is uncertain because fatty acids underwrite much of cardiac metabolism, the efficiency so determined does not relate solely to the transduction step: there are other heat-generating reactions both during the initial and recovery phases. Several authors have suggested that the thermodynamic efficiency of the transduction step is probably quite high (39, 146, 264, 333). Since the experiments of Fenn (124) the bell-shaped work-versus-load curve has been obtained in a variety of muscles. Although external work is zero in isometric contractions there is a considerable amount of internal work being done and there is an internal work component in all isotonic contractions; its magnitude increases with the load. In the past this internal work was assumed to represent the elastic energy stored in the stretched series-elastic element. These days it would be attributed to crossbridge energy storage and to the stretch of mechanically more compliant regions of muscle (262). In many oxygen-consumption studies mechanical data from Parmley and Sonnenblick (334, 391) have been used to calculate the internal work. On integration these data give a very high series-elastic extension so that in several papers the calculated internal work in an isometric contraction approached the maximum value of external work recorded for loads of about 0.5 P, (84, 295, 344), where P, is the isometric force for the prevailing physiological conditions. It seems likely that the internal work in an isometric contraction at L,,, is equal to 20-25% of the maximum external work. Thus a value of about 0.5 mJ/g in an isometric contraction liberating 10 mJ/g of heat would be reasonable; this is still high in comparison with skeletal muscle (59). Wong (449) has criticized the way in which some authors have used the exponential load-extension relationship of the SEC to calculate the internal work in cardiac muscle, arguing that the mathematical derivation is incorrect so that internal work is overestimated. The maximum external work differs in magnitude from one preparation to the next, depending on both intrinsic and extrinsic factors. In cardiac muscle as in skeletal muscle, maximum work output occurs when loads are close to 0.5 P, . In cardiac muscle the size of the work term closely reflects the prevailing level of contractility, so that a muscle developing a peak tension of 80 mN/mm2 will have a much greater work output than one developing half this value. In experiments at 23OC with rabbit papillary muscles being stimulated at 0.25 Hz and developing an average force near 50 mN/mm2, the mean maximum external work was about 2.0 mJ/g per contraction (151). The active mechanical efficiency (basal metabolism subtracted from denominator) of cardiac muscle might seem high in comparison with values obtained with skeletal muscle. In experiments on tetanized skeletal muscle at O"C, Hill (198) reported values of about 45% for initial mechanical efficiency, which means that the totaL mechanical efficiency would fall to about 22% if allowance was made for recovery heat production. This value can be considered to be near maximal for skeletal muscle; indeed much

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lower values are measured in twitch contractions at higher temperatures (125, 147). It is well known, however, that the human heart in vivo functions with a mechanical efficiency of about 10% (53, 129); this value includes the basal metabolism and hence underestimates active efficiency [Bing (27)]. Under the conditions of vigorous exercise efficiencies in the ZO-30% range can be achieved. In the original myothermic studies with rabbit papillary muscle (155) a maximum average active mechanical efficiency of 12% was recorded, and in subsequent studies (149, 151) somewhat higher values (14-16%) were obtained at room temperature. At 27-3O”C, with higher stimulus rates (0.51.0 Hz), values in the 15-25% range have been recorded (72, 149). In these experiments it was by no means certain that the maximum mechanical output had been achieved because stimulus rates were kept low to allow for adequate oxygenation. At the moment there must be some reservations about the efficiency values determined in cardiac myothermic experiments, since the calibration of heat measurements is less secure than for skeletal muscle. Nevertheless in some papers where the oxygen consumption of isolated papillary muscles has been measured, enough additional data have been provided to allow the mechanical efficiency to be calculated and it appears to be quite high. Thus if the data for cat papillary muscle of Coleman, Sonnenblick, and Braunwald (86, Fig. 1) are examined at a load level of 0.5 PO, the calculated mechanical efficiency is 27%. It is not too difficult to identify some of the reasons for the higher efficiency values of cardiac muscle. Because the initial calcium release is much less than in skeletal muscle the activation heat is smaller: 15% rather than 25% of the total. The relatively long contraction time of cardiac muscle probably results from the slower retrieval of the released calcium and this must allow a longer time, relative to skeletal twitch muscles, for crossbridge interactions; thus in spite of its lower ATPase activity cardiac muscle can probably shorten further than mammalian skeletal muscle in a twitch contraction at a comparable temperature before the calcium concentration drops below a critical level. Another factor that might increase cardiac mechanical efficiency is the possibility that the recovery-heat component may be about 0.7-0.8 rather than 1.1 times the initial heat (145). In rabbit papillary muscle there is a steep fall in total energy production, as load decreases, and hence the peak mechanical efficiency usually occurs in the range of 0.2-0.4 PO (see Fig. 3). Do these high mechanical efficiencies suggest that cardiac muscle is different from skeletal muscle regarding energy transduction? The slope of the normalized heat:tension relationship is similar in both cardiac and skeletal muscle, suggesting a similar mechanism. It seems possible that skeletal muscle efficiency is being reduced by the ATP cost of its higher calcium release. Brandt and Orentlicher (39) have pointed out that the transduction mechanical efficiency defined as work/(work + shortening heat) for frog sartorius muscle is really about 77% (initial cycle) or 38% including recovery. Interestingly this value is not far from the overall

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mechanical eficiency measured by the isometric heat rate is about muscle. The data of Dickinson and falls drastically in brief repetitive efficiency, defined as above, of frog

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Woledge (442) for tortoise muscle, where 50 times smaller than in frog skeletal Woledge (107) where the shortening heat tetani would push the transduction sartorius muscles into the 90% range.

G. Fenn Effect

There has been considerable discussion of the Fenn effect in recent years; the reviews of Mommaerts (303) and Woledge (433) are helpful and the specific article devoted to describing and interpreting this effect [Mommaerts (304)] provides a good summary of present knowledge. Under his experimental conditions Fenn found that less heat was liberated in an isometric contraction than in any contraction where shortening took place. It appeared that the extra energy was approximately equal to the work done, although as Mommaerts (304) has pointed out this point was not sufficiently established in the paper and suggestions of a calibration error (202) make for some uncertainty. However, the work of Carlson et al. (59) supported the proposition that the extra energy liberated was approximately equal to the work done. Unfortunately such a conclusion only holds for amphibian muscles briefly tetanized at low temperatures (125), and this result has often distracted physiologists from the really important conclusion that the energy flux of a muscle is regulated by the mechanical constraints under which it shortens. Indeed if Fenn’s result is interpreted to mean that work is performed at unit enthalpy efficiency the result is thermodynamically misleading, as Mommaerts has argued (304). Figure 3 clearly shows that, if the heat in an isometric contraction at I, is taken as the reference level above which “extra” energy is to be defined, it would not be possible to show a Fen .n effec t in cardiac muscle. Nevertheless several authors (see sect. IHE) have concluded that the same phenomenon is present in cardiac muscle and have proposed that the “extra” energy in an isotonic contraction should be taken as the energy above that measured ,in an isometric contraction in which the developed tension equals the isotonic load (86, 155, 304). Mommaerts has proposed a new definition of the Fenn effect that would encompass both cardiac and skeletal muscle experiments under all physiological conditions. He suggests: “a muscle doing work mobilizes over and above that needed for activation and the main .tenance of tension, energy, accounting for the work and for the d.issipation of energy accompanying the work process.” Although this is a reasonable working definition, explanations as to how a load regulates energy production at the molecular level are still unsatisfactory. Mommaerts considers two possibilities: 1) that activation is not maximal and that the sensing of force and other functions feed back positively on the calcium-release mechanism to produce further activation and 2) the self-regulatory property resides in the chemomechanical transduc-

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tion process itself. The first suggestion cannot be ruled out but most recent evidence suggests that when a muscle shortens mechanical deactivation occurs (81, 113, 359, 416). Fuchs (135) has pointed out that data emerging about cooperative interactions between myofibrillar proteins, particularly the varying affinity of troponin for calcium under different physiological conditions, may provide the answer (42, 213, 411, 424). H. Hypertrophy

and Energy

Production

In some experiments with papillary muscles from hypertrophied hearts the mechanical performance has been reported as being normal or even better than normal (89, 127, 169). On the other hand numerous studies (26, 90, 91, 176, 402) have shown pronounced depression of myocardial contractility whether measured as decrements in tension generation per cross-sectional area or maximal shortening velocity. The reasons for these divergent results are not clear but several factors probably contribute -e.g., the type of hypertrophy (volume or pressure induced), the duration of the induction period, animal species, etc. The studies of Coleman’s group (89, 90, 176) are especially interesting as they have made detailed oxygen-consumption measurements on papillary muscles. In pressure-load hypertrophy they report that resting oxygen consumption doubles and that the oxygen consumption per gram of developed tension is also approximately doubled. The oxygen consumption of afterloaded contractions was not significantly different when comparisons were made between a control and a hypertrophied/cardiac failure group. It was suggested that the decline in the external work and velocity factors might have offset the increased cost of tension development. The increased slope of the relationship of oxygen consumption versus tension was unexpected, particularly in light of the linear relationships between ATPase activity and tension found in hypertrophied rabbit heart by Henry et al. (189). Coleman and associates suggest that their own results may be brought about by a change in the intracellular ionic milieu rather than in the intrinsic properties of the contractile apparatus, and in particular they favor changes in myocardial calcium metabolism affecting predominantly state 4 mitochondrial nonphosphorylating respiration (90). IV.

WHOLE-HEART

A. Resting

OXYGEN

CONSUMPTION

Oxygen Consumption

The oxygen consumption of nonbeating cardiac muscle has been measured many times by several different methods and the data have been reviewed by Bing (27) and by Lochner, Arnold, and Muller-Ruchholtz (286). The latter review provides values ranging from 1.2 to 4.8 ml/l00 g wet wt per min for a variety of hearts and a mean of about 2.5 ml/100 g per min

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thus would be a reasonable average. An interesting study by McKeever, Gregg, and Canney (298) using dogs showed that if cardiac arrest were caused by vagal stimulation or intracoronary potassium injection the oxygen consumption fell to about 2 ml/l00 g per min. Using a rapid hemorrhage procedure where the heart rate remained essentially unchanged but arterial blood pressure and cardiac output were effectively zero, a value of 3.4 ml/100 g per min was reached; during ventricular fibrillation, naturally or artificially induced, the resting oxygen usage was 2.8 ml/100 g per min. In rabbit heart, Rohde (362) reported values of 2.1-2.7 ml/100 g per min using a potassiumand calcium-deficient perfusate. Jardetzky, Greene, and Lorber (228), using isolated dog hearts, reported a value of 2.20 ml/100 g per min. In guinea pigs, Jordan and Lochner (236) reported a value of 3.27 ml/100 g per min in KCl-arrested heart (20 mM KCl) and subsequently a value of 3.4 ml/ 100 g per min for rat hearts [Arnold and Lochner (ll)]. In these studies, and those of other workers, there is a gradual decline in oxygen consumption with time; thus in the Znd-3rd h after cardiac arrest Jordan and Lochner reported a value of 1.84 ml/100 g per min. This effect thus parallels the myothermic data referred to previously. Arnold and Lochner (11) published a series of curves showing the decline in the resting oxygen consumption with time of KCl-arrested rat hearts at 4, 14, 24, and 34°C. The curves deserve attention because if they are extrapolated back to the time of arrest the resting oxygen consumption rates would be higher than the usually accepted values-at 34°C a value of about 5 ml/100 g per min might be obtained. Arnold and Lochner also report low Q10 values: 1.44 and 1.85 as the temperature was decreased from 34 to 14°C. Lochner, Arnold, and Muller-Ruchholtz (286) present evidence that the arresting agent (KC1 13-50 mM) does not stimulate the resting myocardial metabolism of hearts already arrested by lowering the temperature (14°C). These authors show that the level of oxygen consumption of a heart at the moment of arrest influences the subsequently determined resting uptake values. The higher the active oxygen utilization, the higher will be the arrest values. As discussed in the review of Lochner et al. (286), it is possible to measure the oxygen consumption of hearts arrested by reducing the extracellular calcium level to zero and several authors have reported that under such conditions the oxygen consumption is higher than in the KCl-arrested heart (181, 335). It is important to realize that under such conditions electrical activity is maintained and there may still be a sizable activationheat component unless chelating agents are used (158). The energy cost of electrical activity is probably not important since Klocke, Braunwald, and Ross (257) have shown that the amount of oxygen required for electrical activity of the heart is less than 1% of the total oxygen consumption of the normally working heart. If the activation-heat level dropped to 0.6 mJ/g per beat (158) a rat heart beating 300 times/min would consume an extra 200 mJ/g per min of energy: i.e., it would utilize 1 ml O,/lOO g per min above the basal rate. If there were even a very small amount of residual mechanical

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activity left after calcium-free perfusion then even higher figures could be achieved. However, other explanations are possible and some are discussed by Penparkgul and Scheuer (335). In particular the findings of Challoner (60) suggest that part of the heart’s oxygen consumption, possibly as much as 13%, is not tied to mitochondrial oxidative phosphorylation. This conclusion rests largely on the basis of experiments with oligomycin [(Challoner and Steinberg (61)], a potent inhibitor of oxidative phosphorylation. These experiments on rat hearts were supported by the studies of Penparkgul and Scheuer (335). Before accepting these results unreservedly, however, it would be desirable to see experiments with other animal species and a clear demonstration that the effect of oligomycin is the same in intact cardiac tissue as it is on isolated mitochondria. Some authors have made use of the fact that the relationship between oxygen consumption and heart rate is essentially linear (provided arterial blood pressure and end-diastolic volume are kept constant); hence by extrapolation to zero frequency the resting oxygen consumption can be estimated (207, 418, 420). Such extrapolation procedures measure basal oxygen consumption PLUSthe activation component of energy production, and if procedures are used that alter the inotropic state of the muscle false conclusions can be reached about the resting oxygen consumption rate. In the paper by Van Citters et al. (418) the extrapolation procedure produced two quite different results: in an empty beating dog heart a value of about 1.8 ml/100 g per min was obtained, whereas the same heart working against a constant load produced a resting value of about 3 ml/l00 g per min. Perhaps this result can be explained by the observation of Lochner et al. (286) that the oxygen consumption of the arrested heart is dependent on its previous metabolic rate. Van der Veen and Willebrands (420) have shown that at normal (2.6 mM) calcium concentrations the extrapolation procedure predicts a resting rate near 5 ml O,/lOO g per min. They report that lowering the calcium concentration to 0.5 mM had little effect on the estimated basal oxygen uptake but raising calcium to 8.4 mM increased the uptake by about 4 ml. This result may not be secure if the extrapolation procedure also measures the activation metabolism, as discussed above. B. Determinants of Active Oxygen Consumption The determinants of myocardial oxygen consumption have fascinated physiologists for well over 60 years. In this review consideration is limited primarily to papers published since the mid 1950’s; the choice of this time period is not arbitrary, since there are several reviews (165, 250, 282, 445) that cover the earlier period. Wollenberger (445) gives a good historical account of some of the controversies that have existed. There are also more recent publications dealing with the determinants of oxygen consumption either directly or peripherally (27, 40, 85, 146, 148, 396). It would be unfair, however, if several of the key early papers in this

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field were not referred to, particularly since recent experiments have confirmed many of the early observations made under conditions of considerable technical difficulty. As early as 1912, Rohde (363), using perfused hearts from rabbits and cats, showed that more oxygen was consumed if the (119), using the dog arterial pressure was increased; Evans and Matsuoka heart-lung preparation, demonstrated that 1) increasing arterial pressure increases oxygen consumption, 2) increasing venous return raises cardiac output and oxygen consumption increases up to some maximal level before falling off, 3) a given increment in cardiac output is performed more efficiently at low arterial pressures (i.e., pressure work is more expensive than volume work), and 4) there is usually a parallel relationship between oxygen usage and heart volume. Unfortunately, out of Evans and Matsuoka’s excellent paper a difference of opinion developed as to what was the major mechanical determinant of cardiac energy expenditure. Subsequently Starling and Visscher (404), using the same preparation, varied arterial pressure, cardiac output, and diastolic volume. They concluded that cardiac oxygen consumption was solely determined by the diastolic volume. Similar results were reported by Hemingway and Fee (187) and by Clark and White (79) using frog heart. Nevertheless when diastolic volume was controlled or held constant the effects of elevated systolic pressure on oxygen consumption were always evident, as in the experiments of Luscher (288) using frog ventricle. A paper by Stella (407) has never received due recognition. These experiments with tortoise ventricle at 12-18OC clearly show that at constant diastolic volume the energy expenditure parallels the work done; work was altered by varying the arterial resistance. Stella’s total energy-versus-load (arterial resistance) curves (see his Fig. 3) are very similar to those found in present-day experiments (see Fig. 3 of this review) and his mechanical efficiencies are similar to present-day estimates. In the same study Stella examined the effects of increasing diastolic volume on work and energy output at constant and so concluded arterial resistance and found a fairly linear relationship that length did have a considerable effect on energy liberation. GollwitzerMeier, Kramer, and Kruger (160, 161) showed that if cardiac output is increased by increasing arterial resistance the energy cost is greater than when a similar increase is produced by augmenting venous return. These results were confirmed by Katz et al. (251), although their emciencies were quite low compared with values reported by previous workers. Thus by the 1940’s there were two schools of thought about cardiac oxygen consumption. One group was strongly influenced by the volume concept of Starling; the other group, possibly influenced by the skeletal muscle data of Hill and Fenn, was convinced that developed pressure-and hence wall tension (stress)-was the prime determinant of energy expenditure. As early as 1892, Woods (451), by making postmortem analyses of human hearts and by measuring the principal curvatures at different points had shown that the law of Laplace applies on the surface of both ventricles,

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and that wall thickness varies with wall curvature and with the magnitude of the intraventricular pressure. Not much use of this observation was made until the 1950’s, when several papers were published that examined the problem of cardiac geometry and the importance of shape and size of the heart (48, 52, 285). These considerations eventually allowed the two schools of thought to be reconciled. Subsequently investigators have usually tried to keep diastolic volume or pressure constant while examining the effects of varying intraventricular pressure (308,309,384) or else the reverse procedure has been adopted (360,364,383,414). Some investigators have even examined the effects of both parameters on myocardial oxygen consumption (Mvo2) with encouraging success (111, 364). In 1956 Laurent et al. (272) suggested that cardiac oxygen consumption could be predicted from the equation: Mire,

= MAP

x HR

e9

where MAP = mean arterial pressure and HR = heart rate. In the same year Rodbard and Williams (360) assumed that the heart could be approximated by a thin-walled sphere at the commencement of systole. In experiments with dogs, using a variation of the heart-lung preparation, these authors showed that increases in aortic pressure produced almost equivalent increases in coronary flow and MvoZ but that at constant aortic pressure a doubling of the stroke volume produced only a small increment in oxygen consumption. Using the Laplace relationship to calculate wall tension they showed that the MVo, could be approximated by the sum of a basal and a tension-dependent term. These papers served as a starting point for subsequent investigations into the mechanical parameters that influence Mvo,.

1. Tension-time

index

In 1958, one of the most influential papers in the field of cardiac energetics was published by Sarnoff et al. (372). These authors used isolated dog hearts metabolically supported by arterial blood from a donor dog. Continuous coronary flow measurements were made and oxygen utilization was determined. Three variables were controlled: aortic pressure (outflow resistance), cardiac output (inflow resistance), and heart rate. The primary determinant of oxygen consumption was found to be the total tension developed by the myocardium as indicated by the area beneath the systolic pressure curve, the so-called tension-time index (TTI). It was clearly established in this paper that raising aortic pressure while maintaining cardiac output constant produced a much higher oxygen demand than when a similar increase in work output was achieved by raising cardiac output at a constant aortic pressure. Sarnoff et al. (372) also showed that increasing heart rate at a fixed aortic pressure increased myocardial oxygen usage and

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that MVoZ bore little relation to external work of the heart per se. They also reported that ITI was a better index of myocardial oxygen usage than peak systolic pressure. It is worth emphasizing that wall tension was not measured in these experiments; the mechanical parameter was intraventricular pressure. Subsequently many workers have confirmed that stroke work is a poor index of cardiac energy expenditure (18, 44, 297, 308, 315, 364, 384); indeed there has been some danger that cardiac physiologists might neglect the factor completely. Fortunately some studies have shown that there is an oxygen equivalent of the work term (51, 414). Although TTI is still regarded as an important predictor of MVo, it has become apparent that under certain physiological circumstances changes take place in MVo, that are not accurately predicted by changes in the TTI. Thus during sympathetic stimulation (354), hypothermia (309), paired electrical stimulation, and norepinephrine or calcium infusions (365, 397) clear discrepancies between ‘IT1 and MVo, have been established. It is important, however, that this index should not be neglected, as its MVoz-predictive value is high. Instead physiologists should accept the concept that there exists a family of ?TI:MVo, curves in the same way that there exists a family of Starling cardiac output curves (371). A schematic illustration of why these discrepancies occur is shown in Figure 2. Because of the problems with ITI as an index of MVo, physiologists examined various other mechanical indices. 2. Work and contractile

element work

In 1964 Britman and Levine (44) published a paper in which they reported that contractile element work (CEW, the sum of external work and internal work) gave a better correlation with MVo, than did pressure-time per minute, force-time per minute, or stroke work. These experiments on dogs involved simultaneous measurements of cardiac output, left ventricular (LV) and aortic pressures, heart rate, LV volume, LV coronary blood flow, and 0, extraction for a wide range of hemodynamic interventions. The ventricle was assumed to be a sphere and the circumferential fiber-shortening rate was calculated as was myocardial tensile force: F = v2P, where P = intraventricular pressure in dynes per square centimeter. The shortening velocity of the CE was put equal to the sum of the fiber-shortening rate and the lengthening velocity of the SEC (dZ/dt) according to Hill (192). In order to measure d.l/dt use was made of the relationship: dF -- - dF -Xdt dl

dl dt

where dF/& could be measured experimentally and where the stiffness of the SEC (dF/dZ) was taken from Sonnenblick’s cat papillary data (391). In

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retrospect this was a major source of error as it is now apparent that papillary muscle data greatly overestimate cardiac muscle series elasticity. Indeed by 1966 Forwand et al. (132) had shown that for dog ventricle the average modulus of active stiffness was 50% lower than that reported for cat papillary muscle. Although later workers have been unable to confirm the importance of CEW as a determinant of myocardial oxygen usage (84, 151, 297, 364) this paper represents the first serious attempt to take Hill’s three-component model and his myothermic data and apply the analysis to cardiac muscle. These authors were also the first to estimate an activation-heat component in cardiac muscle. They obtained a value of 0.02 ml/beat per 100 g for the activation-heat component and a value of 1.93 ml/100 g per min for basal metabolism. They did not discuss the reason for the rather poor correlation coefficients obtained with pressure-time per minute and force-time per minute. 3. Shortening,

shortening

velocity, and inotropic

state

In 1965 Ross et al. (365) published a paper primarily concerned with the effects of paired electrical stimulation on the ventricular performance of dog hearts (right-heart-bypass preparations). As part of that study, however, oxygen consumption and tension-time index were measured during paired stimulation and in eight experiments there was an average 35% increase in MVo2 but an average 14% decrease in TIT. In order to explain this result the authors suggested that the enhanced speed of contraction, caused by paired stimulation, might be an important determinant of MVo,. This concept was further developed in later papers (77, 397). With the same preparation and HR, stroke volume, and aortic pressure kept constant, the velocity of contraction (the rate of LV ejection) was augmented by 1) paired stimulation, 2) norepinephrine infusion, or 3) calcium infusion. With the three procedures the ejection rate was increased by 50.3, 52.9, and 55.l%, respectively; MVoz increased 39.8, 34.8, and 29.2% but TTI decreased 12.4, 15.9, and 12.4%. The authors concluded that since MVo, always rose while TTI fell, tension could not be the sole mechanical determinant and velocity of contraction must also be important. They cited Hill’s data (192, 197) showing that maximum rates of energy expenditure occur at low loads and high shortening velocities. Graham et al. (163) examined the effects of the prevailing level of contractility (inotropic state) on MVo, using isovolumetrically contracting dog hearts paced at a constant heart rate. Peak developed tension was varied by altering ventricular volume and MVo, was measured at different tensions. Norepinephrine was then infused and the tension:MVo, relationship was redetermined. Thus for any particular tension level, oxygen consumption could be compared under control conditions or in the presence of norepinephrine. Graham et al. reported a 40% increment in oxygen consumption and interpreted this increment to be associated with an incre-

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ment in the estimated Vmax. They concluded that the oxygen cost of augmentation of contractility is substantial and is independent of any change in fiber shortening. However, although changes in inotropic state definitely alter Mvo, (30), it is still not proven that the increased MVo, results from changes in shortening velocity or Vmax. Similar results can be obtained myothermically but the interpretation is different (see sect. v). 4. Peak tension and rate of tension development As mentioned earlier Rodbard and colleagues had approximated the heart by a thin-walled sphere and found a relationship between Mvoz and peak wall tension (360, 361). In 1966 McDonald, Taylor, and Cingolani (297) published a careful study of the Mvo, of a canine right-heart-bypass preparation and concluded that of four determinants examined -1) developed tension x HR, 2) developed tension area to valve closure x HR, 3) developed tension area to peak tension x HR, and 4) peak developed tension x HRthat the latter was the best index (r = 0.87). When CEW was examined a correlation coefficient of only 0.77 was obtained. McDonald et al. calculated wall tension using a thick-walled spherical model of the left ventricle. Since the 1960’s more realistic models of the heart and more particularly of the left ventricle have steadily developed. Nevertheless the simple spherical models seem capable of estimating wall stress and hence lead to reasonably accurate predictions of Mvo,. The more sophisticated models are mentioned later. It is of interest that Hefner et al. (184) have shown that the force that cardiac muscle develops perpendicular to a given plane through the ventricle is almost identical to the product of intracavity pressure times the area of the cavity and that this is true regardless of the thickness of the wall or the shape of the ventricle. It seems clear from the results of several authors that peak wall stress is a major mechanical index of Mvo,. Therefore cardiac oxygen consumption is expected to be predicted with somewhat less accuracy by related measurements such as peak intraventricular pressure and mean arterial pressure, and the literature seems to support this suggestion (272, 322). The maximum rate of pressure development @P/h,,) is one other parameter that has been proposed as a sensitive index of contractility and energy production. The use of dP/haX or variations of it (e.g., dP/ch&&KP, where K is a constant and P is the ventricular pressure) is controversial (93, 291, 415, 419). There are reasons for believing that dP/haX will reflect the rate at which calcium becomes bound to troponin (248). Provided there is a simple relationship between the rate of calcium release and the magnitude of calcium release, this mechanical measurement may incorporate information about the size of the activation-heat component as well as wall stress. In 1967 Neely et al. (315) reported the development of an isolated perfused rat heart preparation that could perform graded external work. Pressure development or the ‘IT1 was the major determinant of nonbasal

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oxygen consumption. In experiments with these working heart preparations, when left atria1 pressure was varied and cardiac output was kept constant, the oxygen consumption correlated well with the increment in peak systolic pressure but not with work. Under such conditions oxygen consumption increased from 1.6 to 2.65 mmol/g dry wt per h (10.6-17.6 ml/l00 g per min). When aortic pressure was kept essentially constant but cardiac output was varied over a threefold range, there was practically no change in cardiac oxygen consumption (see their Table 5). Their highest external mechanical efficiency value reached 26% and extrapolation of the relationship between pressure and oxygen consumption to zero pressure indicated that quiescent rat heart would use 0.7 mmol/g per h. The suitability of isolated rat hearts for studies on the relationship between Mvo, and mechanical indices is further emphasized in a paper by Gamble et al. (138). Besides showing that this preparation matched the mechanical performance of rat hearts in vivo the authors also measured oxygen consumption and reported a linear relationship between left ventricular systolic pressure and Mvo,, whether expressed per beat or measured over a period of minutes. The authors reported that systolic pressure gave a better correlation than developed pressure and in an interesting discussion they calculated the energy cost of rat heart contractions and compared their data with values obtained in dogs in other studies. At a systolic pressure of 130 mmHg they obtained a value of about 0.06 ml/l00 g per beat for rats and a value of 0.08 for dogs (they averaged the canine data of several workers). These values include basal metabolism and, if a correction is made for this factor, the energy cost per beat is about 10 mJ/g per beat in rats and 14 mJ/g per beat in dogs. Thus the whole-heart oxygen-consumption studies seem to have clearly identified the most important of the several vari ables that determine Mvo, . These include heart rate, wal .l stress or related parameters, external work, and activation or inotropic state. The importance of shortening per se remains to be established. C. Wall Stress In order to predict cardiac oxygen consumption, or to relate it to events of the cardiac cycle, it i .s necessary to measure the stress within the walls of the heart as accurately as possible. In recent years several models have been developed in an attempt to evaluate stress and strain within the chambers of the heart and more particularly within the left ventricle. There are several books that deal very thoroughly with such models (8, 302,421) and it is encouraging to find that even geometrically simple models can give the experts problems (347, p. 193-208). In spite of the development of more sophisticated models, Mirsky (301) has concluded that the application of Laplace’s law to the simpler models is satisfactory in allowi w mean wall stresses to be calculated and that such values can be used to 1) predic t qua litative changes in the stress respon .se of

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ENERGETICS

ventricles under different loading conditions, 2) predict oxygen consumption in normal and pathological conditions, and 3) calculate force-velocity parameters. He also outlines the additional data needed to improve the existing models. Fortunately several groups of workers are becoming interested in the technical problems of recording wall stresses in vivo (50, 283, 284). In the study of Burns et al. (50) auxotonic strain gauges were implanted into the major and minor axes of the left ventricle of dogs. Major and minor axis forces were measured and aortic flow rate and left ventricular pressure were monitored. Wall-force recordings were corrected for myocardial wall thickness by three different methods. Wall stress estimated on the basis of spherical and ellipsoidal models was calculated and compared with the directly measured values. At control levels of end-diastolic pressure (3 mmHg), the measured peak systolic wall stress was 97 g/cm2, whereas the spherical and ellipsoid models predicted 79 and 119 g/cm2, respectively. At higher end-diastolic pressures these values increased threefold. The average value, when dynamic wall thickness was considered, was 192 g/cm2. Hood, Rackley, and Rolett (216) calculated a peak wall stress of 329 g/cm2 in man at an end-diastolic pressure of 12 mmHg. Burns et al. (50) suggest that at the usual physiological end-diastolic pressure a peak wall stress of about 200 g/cm2 can be expected, whereas Hood (215) shows normal circumferential peak stress values of 312 g/cm2 at end-diastolic stresses of 32 g/cm2, ranging from an endocardial value of about 340 g/cm2 to an epicardial value of 250 g/ cm2. Apparently these peak wall stress values are low compared with the maximum tension that can be developed by isolated papillary muscles. Indeed when studies are made with in situ papillary muscles the forces per cross-sectional area in spontaneous extrasystolic beats (409) have reached values 3-4 times higher than the data reported above. It seems reasonable to believe , however that wall stress would normally only be a fraction of the maxim urn possib 1e stress an .d that the complex geometry of the cardiac wall would prevent stress measured i .n a particular plane from approaching that obtainable in a papillary muscle where the cells are aligned approximately in parallel. D. Coronary

Blood Flow:Oxygen

Tension

There are several good reviews of coronary blood flow and the factors that regulate it (23, 24, 165, 250). Cardiac oxygen consumption is determined by the product of coronary blood flow (CBF) and the arteriovenous (AV) oxygen difference. For a wide range of hemodynamic conditions the AV difference changes only slightly. The ratio of CBF to oxygen consumption is approximately constant and as Katz (250) pointed out this means that experimentally the ratio CBF/(HR* BP) approaches a constant under much BP). This led the same physiological conditions as does the ratio MVo,/(HR to the development of an index of cardiac effort by Katz and colleagues (121, l

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142), where Mvo,, and hence coronary flow, was related to the product of HR and mean aortic pressure. In experiments with isolated supported hearts, Sarnoff et al. (374) showed that if cardiac activity was kept constant oxygen consumption did not vary much over a wide range of coronary blood flows. When CBF was held constant and the activity varied, then myocardial 0, consumption did vary in proportion to the ‘ITI. In a series of in vivo studies Gregg and colleagues (166, 255, 354) used implanted flowmeters to measure instantaneous coronary flow together with other parameters (heart rate, cardiac output, aortic blood pressure, and oxygen consumption) in unanesthetized greyho und dogs. Measurements were m .ade under resti .ng conditions and the effects of exercise and excitement were examined. They reported that at rest the CBF rates and MVo, values were considerably less than those usually found in open-chest or anesthetized dogs: their range of 4.4-8.6 ml/100 g per min can be compared with literature values of 8-10 ml/100 g per min. In moderate-to-severe exercise, where cardiac output and coronary flow increased 350-400%, the relationship between Mvo, and ITI broke down. In 1965 Opie (328), using the isolated rat heart perfused by a modified LangendorE technique, reported that coronary flow was related to perfusion pressure and that contractility and total oxygen uptake increased up to perfusion pressures of 160-180 mmHg. Since these pressures were clearly unphysiological, the results suggested that contractility might normally be limited by oxygen delivery to the tissues, although Opie showed that norepinephrine and 2,ddinitrophenol (DNP) could further increase pyruvate oxidation. The idea that contractility was partly controlled by coronary flow received some support from Fisher et al. (128) working with canine right ventricular papillary muscles in situ perfused by their coronary circulation. These authors reported that contractility fell much more rapi .dly when coronary flow was totally stopped than when coronary perfusion continued at the same rate with oxygen-deficient Tyrode. One interpretation of the result could be that metabolites built up in the stopped-fl .ow situation, and the au thors were careful to point out that under the usual conditions of flow and oxygen availability it was impossible to conclude that the rate of coronary flow was an independent determinant of contractility and Mvo,. Downey (110) has measured the contractile force of deep and superficial fibers of dog heart and shown that if stringent criteria are used contractile force is relatively independent of CBF at flow levels above that chosen by autoregulation. At CBF values below the appropriate autoregulatory flow, contractile force is flow dependent. In a recent series of papers Arnold, Morgenstern, and colleagues (10, 12, 167, 310) have demonstrated that increasing the pressure in the coronary perfusion vessels produces a positive inotropic effect and thus increases metabolism. However, this effect is not related in any way to overcoming tissue hypoxia. The authors ascribe this effect to coronary artery distension and show that this distension is a determinant of the pressure-volume relationship of the heart. Over the 70- to 170,mmHg range of coronary

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perfusion pressure, the intracoron .ary bl .ood volu .me increases from 11 to 17.8 ml/100 g wet wt. Although the mean radius of the left ventricle is not changed the thickness of the mocardial wall increases and Arnold and coworkers suggest that this change in geometry is probably responsible for the effect described by Opie (328). If contractility is not normally limited by oxygen delivery what is the critical oxygen tension above which left ventricular performance is independent of oxygen supply? The experiments of Starliner and Lubbers (405, 406) have convincingly shown that the so-called “critical” PO, of cells is not a function of the respiratory properties of cells and mitochondria but depends largely on the measuring device. They have shown that although the rate of respiration of mitochondria is dependent on substrate, ADP concentration, and temperature it is not possible even at a 0.5torr oxygen pressure to measure the critical PO,. This is not to say of course that the oxygen tension of the blood and/or the rate of blood flow cannot cause intracellular PO, to drop below some level requisite for adequate metabolism; indeed they can and some pathological conditions in which this occurs are considered by Rakusan (350). Several studies show there exists a broad spectrum of PO, values within the myocardium. Whalen (429) reports a mean value of 6.9 torr in cat heart muscle and Coburn et. al. (82), using an ingenious method of estimating mean myoglobin oxygen tension from carbon monoxide binding to myoglobin, report values in the range of 4-6 torr. These values tend to be somewhat lower than those obtained by earlier workers but a recent study by Losse, Schuchhardt, and Niederle (287) reports a mean value of 19.3 torr with 60% of the readings below the mean coronary venous PO, of 18.8 torr and with the mode occurring in the 0- to 5-torr range. This same study showed that these values were not altered by moderate levels of arterial hypoxemia (64 torr). Indeed Coburn et al. (82) report that intracellular oxygen tension is not altered until arterial oxygen tension falls below 30-35 torr. Studies by Lee et al. (274, 275) in newborn lambs and cats also suggest that cardiac function is not impaired until arterial PO, reaches the 30- to 45-torr range. E. Whole-Heart Heat Production

The conclusions drawn in section III relied heavily on results that my colleagues and I have obtained measuring heat production of rabbit papillary muscles. There is a danger in having to rely on data obtained primarily in one laboratory and from one species of animal. Fortunately some wholeheart heat-production experiments have been undertaken and the most reliable of these studies have been on rabbit cardiac muscle. Afonso and colleagues (4-6) injected cold saline into the right ventricle (RV) of anesthetized dogs and showed by a temperature-probe technique that the LV myocardial temperature decreased. Part of the heat loss occurred by transit of the blood through the coronary circulation. From a knowledge

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of blood flow and the temperature jump it was possible to calculate cardiac heat production. When thi s was plotted against heart rate there was a linear relationship between the two parameters but a wide scatter not only between dogs but even in the same dog on a repeat determination. In experiments where LV ventricular work was lowered by bleeding, significant decreases occurred in LV work, coronary flow, myocardial oxygen consumption, and LV heat production. Using a somewhat similar procedure Neil1 and Huckabee (320) studied anaerobic heat production using a coronary artery-coronary sinus thermal-gradient system to estimate heat production. Unfortunately it is difficult to compare their results with oxygen-consumption studies because the mechanical indices were not quantitated. In 1968 Boivinet and Rybak (31) measured heat generation from isolated frog hearts using a microcalorimeter. At 27OCthey measured a heat rate of 10.6 J/g per h using a medium aerated with 75% oxygen and 25% nitrogen. With hearts beating at about 72 beats/min the heat production was estimated to be 2.7 mJ/g per beat. This value seems low but since the mechanical conditions were not specified it is difficult to comment. In 1971 McDonald (296) developed an isolated and metabolically supported rabbit heart preparation that was suspended in a vacuum flask. The perfused hearts developed tension isovolumetrically against fluid-filled balloons and the temperature differential between the perfusate inflow and outflow was measured. In experiments at 37°C McDonald showed a linear relation between heat production and tension-time (g . min/cm2). Heat production and tension-time varied approximately in parallel when end-diastolic volume was increased. An infusion of norepinephrine resulted in a greater heat production than could be predicted on the basis of tension changes alone such that at the same developed tension a 15% increment in heat production was measured. From Figure 6 of McDonald’s paper (296) the extrapolated heat production for zero tension development is 630 mJ/g per min and for maximal tension development the value doubles to about 1460 mJ/g per min; the average heart rate was 200 beats/min. The intercept of 630 mJ/g per min includes the resting heat production and if this can be taken to be about 400 mJ/g per min at 37OCit would make the active heat production 1460 - 400 or about 5 mJ/g per beat and would suggest that the activation heat might be about 1.0 mJ/g per beat (not the 3.2 mJ/g per beat calculated by McDonald because the resting heat was included). Working with the same experimental apparatus in Wilkie’s laboratory, Coulson and Rusy (94, 95, 368) have continued experiments with rabbit hearts but have used a lower temperature (25°C) and a lower heart rate (54 beats/min). They have shown a linear relationship between heat production and the TTI. When the heat:tension relationship was extrapolated to zero tension the intercept was 7.2 mW/g (400 mJ/g per min). As this value includes the activation-heat component (probably about 100 mJ/g per min) the resting heat would be about 300 mJ/g min at 25°C a value 2.5 times higher than that obtained with rabbit papillary muscles at 20°C with glucose as substrate (155). If the best mechanical data of Rusy and Coulson (95) are

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used (taken from their Fig. 3) and the activation heat is included it can be calculated that the heat production per contraction is about 11 mJ/g at 25OC. Such a value is very similar to that obtained with isolated rabbit papillary muscles; this is encouraging because although the time resolution of wholeheart calorimetry is poor its calibration by passing a known current through a calibrated resistance, to heat the perfusing fluid, should be quite reliable. Very recently Coulson (94) has reported that heat production and oxygen consumption increased in a similar way with force development when the diastolic size of the heart was increased. More important, he obtained a direct measurement of the caloric equivalent of oxygen in the beating rabbit heart. With pyruvate as substrate there was 20.48 mJ of energy liberated per microliter of 0, used at 25OC. Coulson also reported that anaerobic metabolism was low, always accounting for less than 5% of total cardiac energy liberation. V.

PHARMACOLOGICAL

AGENTS

AND

ENERGY

EXPENDITURE

There are two major classes of pharmacological agents that increase cardiac contractility: the catecholamines and the cardiac glycosides. Only the energetic consequences of their administration are considered here. A. Cardiac

Glycosides

1. Resting

metabolism

The data from isolated cardiac preparations are somewhat conflicting. Using polarographic techniques, Coleman (83) reported that acetylstrophanthidin had no effect on the resting oxygen consumption of cat papillary muscle and Klaus and Krebs (256) also found that di&oxigenin had little or no effect on guinea pig heart. In myothermic experiments ouabain did not alter rabbit papillary muscle resting heat production (149). On the other hand, Lee, Yu and Burstein (278), using cat papillary muscles, have reported a doubling of oxygen uptake after a 120.min exposure to ouabain. As the dosage levels in the different experiments are of the same order the reason for the discrepancy is unclear. 2. Active

metabolism

In experiments on isolated cat papillary muscle at 29°C Coleman (83) showed that acetylstrophanthidin shifted the force-velocity relation to the right, producing about a 30% increment in force development and shortening velocity. If active tension development was made the same as that in the control situation by decreasing muscle length, then in the presence of the glycoside oxygen consumption increased by about 30%. Somewhat similar

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results were obtained myothermically with rabbit papillary muscle at 20 and 30°C (149). Force development and work output were increased at both temperatures but changes in mechanical efficiency were small. The slope of the heat:tension relationship was not significantly altered but the activationheat component was increased by lo-20%. It was concluded that in nonfailing preparations cardiac glycosides do not produce major changes in mechanical efficiency and do not alter the transduction mechanism but do modify the activation level. The small discrepancies between the oxygen consumption and myothermic studies probably relate to species, dosage, and stimulus pattern differences. These observations with isolated nonfailing papillary muscles support the view developed by Sarnoff et al. (375) and Cove11 et al. (96) that cardiac glycosides do not directly influence myocardial efficiency. These authors make the point that if whole-heart efficiency is defined as stroke work:total oxygen consumption, then it is apparent from the clinical literature that when glycosides are administered to patients in cardiac failure the ratio often rises. This result, however, is confounded by changes in cardiac size: in particular the glycosides via their inotropic effects reduce end-diastolic volume, and hence oxygen consumption can drop either due to the resultant fall in wall stress (Laplace relationship) or to a reduction in resting metabolism if this is length dependent. In the isolated metabolically supported canine heart Sarnoff and colleagues (375) kept aortic pressure, heart rate, and stroke volume constant and found that although contractility increased with doses of acetylstrophanthidin there were no changes in oxygen consumption or efficiency. Subsequently Cove11 et al. (96), using a canine right-heart-bypass technique, showed that if end-diastolic volume was kept low the glycosides could in fact be shown to decrease efficiency: i.e., under conditions where work output was kept fairly constant, oxygen consumption was increased. They concluded that digitalis tends to increase MVo, but that its inotropic effect usually results in a reduction of ventricular wall tension that masks the small metabolic increment. The increase in oxygen consumption produced by digitalis was ascribed to the increase in Vmax associated with the inotropic response. A later study using intact canine hearts by Beiser et al. (21) showed that at therapeutic dose levels the cardiac glycosides do not produce inotropic responses as large as those obtained by catecholamine administration. Indeed their moderate inotropic effect is probably one of the reasons why these agents are so successful clinically; glycosides act to reduce heart rate whereas catecholamines, which are energetically more extravagant in their inotropic action, further stimulate cardiac oxygen demand by increasing heart rate. B. Catecholamines 1. Resting

metabolism

Using cat papillary

muscles and a polarographic

technique

Lee and Yu

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(277) reported that epinephrine (or norepinephrine) did not change basal oxygen consumption. In a biochemical study Chandler, Sonnenblick, and Pool (65) showed that in metabolically inhibited (WA/N,) cat papillary muscle preparations norepinephrine did not alter resting high-energy phosphate utilization and Coleman, Sonnenblick, and Braunwald (87) reported that norepinephrine did not alter the resting oxygen consumption of cat papillary muscle. In myothermic studies with rabbit papillary muscles an increment of 16-37% in resting heat production was reported with epinephrine (143) but in subsequent myothermic experiments with isoproterenol [Gibbs and Gibson (153)] no detectable effect was observed. In whole-heart studies, Klocke et al. (258) have reported that isoproterenol, norepinephrine, and epinephrine at high dosage levels produce small increments (g-39%) in the oxygen consumption of potassium-arrested dog hearts. 2. Active metabolism

Lee and Yu (277) reported that epinephrine and norepinephrine did not significantly alter the tissue content of energy-rich phosphate compounds in cat papillary muscle. The ratio of contractile tension:total oxygen consumption was not significantly than .ged by these amines if the preparations had normal contractility. When administered to hypodynamic muscles, this ratio returned to normal. Chandler, Sonnenblick, and Pool (65), using metabolically inhibited cat papillary muscles, found that norepinephrine-treated muscles performing isometric work used 115% as much high-energy phosphates as control muscles while performing only 87% as much work in only 59% as many contractions. They concluded that the “oxygen-wasting” effect of norepinephrine results from the increased utilization of energy associated with an increased contractile state. In 1965, Sonnenblick et al. (397) implicated velocity of contraction as a determinant of Mvo,, as discussed earlier, and showed that catecholamine administration was one of several procedures that would increase peak ejection velocity and raise MVo,. In 1968, Graham et al. (163) used anesthetized open-chest dogs and examin .ed the isovolu .metrically contracting left ventricle with volume 1, heart rate, and systemic perfusion controlled. Peak developed tension was increased at a constant Vmax by increasing volume. Norepinephrine was then infused and peak developed tension was measured at different ventricular volumes and at a higher Vmax. Oxygen consumption was linearly related to peak developed tension in both sets of experiments but for a given level of tension development epinephrine substantially increased the oxygen cost of contraction. In particular their Figure 4 can be compared with data collected in myothermic experiments [Gibbs (144, Fig. l)] and for a schematic representation of this effect see Figure 2 of this review. Obviously several investigators are obtaining the same result but interpreting the effect differently. A myothermic investigation of the effects of epinephrine on isometric contractions of rabbit papillary muscle showed that this agent did not change the slope of the heat:tension relationship but greatly increased the

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acti vation-heat component (144). Subsequently i t was shown that i .soprotereno1 had actions similar to epinephrine (153). In isometric studies, the slope of the heat:tension relationship was unchanged. At control calcium levels (2.5 mM) isoproterenol had no effect on mechanical efficiency, but in hearts rendered hypodynamic by decreasing the extracellular calcium level to 0.6 mM the efficiency values were restored to normal levels. The simplest explanation for the results was that catecholamines altered the amount of calcium delivered to the contractile proteins without necessarily altering the efficiency of either the mechanochemical transduction mechanism or the calcium pump. Coleman, Sonnenblick, and Braunwald (87) measured the oxygen consumption of cat papillary muscles contracting both isometrically and isotonically (afterloaded) in the presence of norepinephrine. Their data are very similar to values obtained in rabbit myothermic studies (144, 153). When developed tension was kept constant by decreasing initial muscle length, MVo, was significantly increased in the presence of norepinephrine. When the change in the velocity of contraction (under a small preload) was plotted against the ch .ange in isometric MVo2 for constant tension developm .ent, a linear relation was obtained alth .ough the scatter was fairly 1.arge (r = 0.66). The authors concluded that this result identifies velocity of contraction as a major determinant of MVo,. They argued that norepinephrine, by increasing the velocity of contraction, increased MVo,. The experiments of Beiser et al. (21), comparing inotropic responses of a catecholamine and a cardiac glycoside, have already been mentioned, and a somewhat similar study was undertaken by Downing et al. (111) using newborn lambs whose aortic blood pressure, cardiac output, and heart rate were kept constant while intravenous infusions of norepinephrine and acetylstrophanthidin were made. There were minimal changes in MVo, even though these agents produced large increases in the rate of rise of LV pressure and large reductions in LV end-diastolic pressure (LVEDP). This paper clearly shows the linear relationship between MVo, and LVEDP pressure. For a given LVEDP norepinephrine increased MVo, by 3 ml/100 g per min and the slopes of the MVo,:LVEDP were closely parallel under control and norepinephrine conditions [Fig. 6 of Downing et al. (111) can be compared with Fig. 3 of Coleman et al. (87) and Fig. 1 of Gibbs et al. (153); the tension axis must be converted into a length or volume axis for the latter two studies]. Although the myothermic and oxygen-consumption techniques show substantially similar results when isolated papillary muscles are exposed to catecholamines and cardiac glycosides there is one major difference in interpretation. In myothermic studies the increment in energy production for a set level of tension development has been ascribed to an increase in the tension-independent or activation heat, whereas in the oxygen-consumption studies the increment in oxygen consumption has been ascribed to the increase in shortening veloci .ty and, by inference, to an increase in Vmax. Both groups of experimenters would agree that these pharmacological agents

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increase intracellular free-calcium levels during contraction. However, on the basis of a kinetic model of the cardiac contractile element Wong (449) has concluded that Vmax cannot be a major energy determinant since during isometric contraction CE Vmax is independent of muscle length and yet Mvo, or cardiac heat production is strongly tension dependent and therefore length dependent. VI.

CARDIAC

METABOLISM

Several reviews have covered the metabolism of the heart under normal, ischemic, and hypoxic conditions in considerable detail. In particular the articles by Bing (27) Wollenberger and Krause (446), Opie (329-331), Kubler and Spieckerman (263), Neely, Rovetto, and Oram (317), Neely and Morgan (316), and two recent symposia on ischemia, one edited by Braunwald (41) and one by Wildenthal et al. (430), should be consulted for detailed considerations of metabolic regulation. This section deals with 1) the energy sources available to normally functioning cardiac muscle and the size of those energy stores and 2) the likely contribution of aerobic and anaerobic metabolism to the energy supply under normal, ischemic, and hypoxic conditions. It is becoming increasingly clear that the metabolic responses of cardiac muscle to hypoxia and ischemia are distinctly different (229, 230, 367). In this review hypoxia is taken to be an inadequate oxygen supply to cardiac muscle in the presence of an adequate blood or perfusion fluid flow; ischemia implies that blood or perfusion fluid flow has stopped or is inadequate to supply the necessary oxygen and substrates and to remove metabolites. A. Energy Sources and Metabolic Regulation It has been known since the early thirties that isolated frog hearts do not depend on endogenous stores of carbohydrates for their metabolic requirements. Clarke, Gaddie, and Stewart (78) showed that the carbohydrate oxidized by a frog heart, perfused with blood containing glucose plus insulin over a period of 6 h, would account for only about 40% of the total metabolism. A later study (99) pointed to fatty acids as a significant energy source and Bing (27) established that in man fatty acids are the most important fuel of the heart in the postabsorptive state. It is now fairly common to come across the statement that when carbohydrates and lipids are available together the heart will preferentially utilize lipid (27, 316). However, the uncritical acceptance of this conclusion has been queried by Opie (330), who believes that the role of lipid as a fuel for heart metabolism has been overemphasized and is critical of the accuracy of many measurements of triglyceride fatty acid uptake. There are several studies that seem to support Opie’s position (e.g., 206, 254, 281). It does appear well established, however, that most of the commonly available substrates, e.g., free fatty

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acids, short-chain fatty acids, ketones, lactate, acetate, and pyruvate, will all be oxidized before exogenous glucose (317, 382, 435, 436). It also has been clearly shown that the isolated perfused heart can maintain contractility for long periods of time by oxidizing endogenous lipid (104, 126, 327). Several authors have shown that normal, well-oxygenated hearts do not produce lactate (326) or produce it in insignificant quantities (313, 376) such that ATP formed by conversion of hexose to surplus lactate (i.e., aerobic glycolysis) must account for less than 2% of the total ATP supply. On the other hand, according to conventional biochemical theory glycolysis must contribute 2/38 (i.e., 5.3%) of the total ATP supply. The very low or zero level of lactate production must mean that glycolytic pyruvate production normally is precisely adjusted to its aerobic oxidation. There is good evidence from experiments with isolated cardiac mitochondria that P:O ratios (moles of ATP produced per atom of oxygen consumed) of 3 (299, 388, 408) are obtainable if enough attention is paid to preparatory procedures. It may be worth keeping in mind that when the first direct P:O measurement was made for whole skeletal muscles (266, 267) the ratio turned out to be unexpectedly low in comparison with measurements made on isolated mitochondrial suspensions. B. Aerobic

Metabolism

The accepted figure for the oxygen consumption of the normally beating human heart is about 8-10 ml O,/lOO g per min. It is evident from the literature (15) that cardiac outputs six- to eightfold greater than normal can be encountered in champion athletes under conditions of strenuous exercise. It has been argued that under such conditions cardiac efficiency probably doubles (148) but even so oxygen consumption must reach 30-40 ml/100 g per min. This means that aerobic ATP production must reach 15-20 mmol ATP/ 100 g per min (assuming AHATp = -46 kJ/mol). This very high energy demand can probably be handled because of the high concentration of mitochondria. The mechanism of respiratory control has been largely elucidated by Chance and colleagues (62, 64). The rate of oxidative phosphorylation is coupled to the electron transport rate, which in turn is dependent on the level of the phosphate acceptor ADP. It is logical to believe that muscular contraction leads to levels of ADP higher than normal and therefore that the rate of respiration can be closely coup1.ed to the rate of ATP utilization. The role of ADP as a phosphate acceptor and hence as a major factor regulating metabolism has been stressed. It appears, however, that intracellular creatine levels may play an important role in regulating cellular energy production. The end product of contractile activity is creatine and its phosphorylation by mitochondrial creatine phosphokinase (CPK) generates ADP, which stimulates oxidative phosphorylation. There are two isoenzymes of creatine kinase-one in the cytoplasm and one in mitochondria (227, 369,

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377). Several authors have suggested that CPK does not function solely to buffer the supply of ATP to contractile proteins and ion pumps but that it may serve as a high-energy phosphate transport system and transmit the redox energy of respiration across the mitochondrial membrane and hence establish a high-energy phosphate potential within the cardiac cell (227, 369). The important role of creatine and CPK in the energy metabolism of tissue-cultured cardiac and skeletal muscle cells has been shown by Seraydarian, Artaza, and Abbott (380, 381). These authors speculate that crossbridge activity might only be able to take place in the presence of a high This suggestion might explain why in certain local phosphate potential. situations contractile activity ceases in spite of what appear to be adequate reserves of ATP and phosphocreatine (PC). Another outcome of tight energy feedback regulation via the levels of creatine, PC, and ADP would be a blurring of the temporal distinctions between initial and recovery metabolisms in cardiac tissue (see sect. IIIA). This might occur because the mitochondrial CPK levels constitute half of the total cellular CPK activity (377) and because cardiac muscle has such a high mitochondrial content. As mentioned above the consensus view is that fatty acids are the most important cardiac substrate. The control mechanisms involved have been reviewed by Neely and Morgan (316) and Opie (329, 330) and they stress that the oxidation rate will be determined by the availability of fatty acids, the rate of acetyl-CoA oxidation by the citric acid cycle at low rates of metabolism, and the rate of acyl translocation across the inner mitochondrial membrane at high metabolic rates. There is one other problem that might be mentioned now since it forms a useful bridge to the next section. This relates to the compartmentalization of intermediates within cells and in particular the distribution of ATP and PC. Evidence for the specific localization of PC was produced by Hill in 1962 (204) and over recent years the idea of different pools of ATP and other highenergy phosphates has been discussed more frequently (33) and indirect evidence has been accumulating to support such a proposition (25, 105, 141, 175, 185, 452). Any such compartmentalization may have far-reaching consequences not only in regulating metabolism but also in the control of contractility, the interpretation of energy-balance studies, and the value of in vivo free-energy changes. C. Anaerobic Metabolism

The concentration of the primary energy donor, i.e., ATP, in cardiac muscle is similar to its concentration in skeletal muscle; most authors report ATP levels of 3-6 mM/kg wet wt. However, there does seem to be 5-10 times less PC in cardiac tissue, with most values being in the 3- to 8-mM range. If anaerobic energy reserves have to be utilized cardiac muscle is less

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well protected than skeletal muscle since the glycogen content of cardiac muscle is about 5 g/kg wet wt , a third to a quarter of that in skeletal muscle (126, 182). There are large species differences; turtle heart, which can function anaerobically for long periods of time (34, 355, 356), has a glycogen concentration of 30-60 g/kg wet w-t, depending on the time of year. The amazing thing about the heart is how rapidly its performance deteriorates when oxygen delivery ceases; all authors report an obvious decline within 15 s and Kubler and Spieckermann (263), reviewing the literature, suggest that it takes only about eight contractions or 8 s to be noticeable under normal physiological conditions. Obviously such a rapid response cannot be due to depletion of the pool of high-energy phosphates since at most about 0. 4 pmol ATP or PC/g muscle per beat would be utilized. Either the ATP available to the contractile protein compartment is limited or else the rapid buildup of metabolic end products causes adverse side effects. In ischemia or severe hypoxia it seem clear that ATP and PC levels remain unaltered for the first 8-10 s and then PC levels start to fall and intracellular lactate levels to rise. The results of Lai and Scheuer are fairly typical (268). According to Kubler and Spieckermann (263) in the ischemic dog heart the relation between lactate production and the number of split high-energy phosphate bonds is constant at 1.4-1.5. Now as several authors have pointed out the maximum activities of the enzymes of glycolysis are high enough to su .pport the production of 20-40 pm01 lactate/g per min and hence to gen .erate 20-40 pmol ATP/g per min, which would just cover the energy demand of the normally working heart (about 40 pmol ATP/g per min) . Lochner, Arnold, and Muller-Ruchholtz (286) have also studied anaerobic metabolism under resting conditions, with glucose as substrate and insulin present. They report that at rest under aerobic conditions only 4.5 mg/lOO g per m .in (0.5 pm01 ./g per min) of lactate wa s produced .; this cou ld only supply about 4% of the energy consumed by a normally working heart. Under anaerobic conditions the lactate production rate increased to 18.7 mg (after 15 min) and 32.5 mg (at 60 min). To get some idea of the maximum glycolytic capacity DNP (312) was added to a KCl-arrested heart under anaerobic conditions and the lactate production reached 110 mg/lOO g per min or 24 pmol/g per min, an amount energetically equivalent to an oxygen consumption near 7.0 ml/100 g per min. However, the maximal lactate production rates measured in intact rat and dog hearts range from 10 to 20 pmol/g per min [see Muller-Ruchholtz and Lochner (313)], and most authors suggest that physiologically anaerobic glycolysis can only underwrite up to 30% of normal energy demands. Opie (329) and Kubler and Spieckermann (263) draw attention to the fact that the glycolytic enzyme activities in vivo are usually l-10% of the activities measured in vitro, although hexokinase activity can reach 20% of its maximal in vitro value. It therefore is evident that mechanisms must exist that limit the activity of these enzymes in vivo and also interfere with mechanical activity.

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Glycogen is not normally utilized by aerobic hearts but in anaerobically perfused rat heart it has been repored that all the glycogen is utilized within 4-10 min (92, 126). It seems likely, however, that in ischemia glycolysis is inhibited, whereas there are still significant reserves of glycogen left (263). Since the glycogen concentration (expressed as hexose units) is about 35 PM/g, if 7 ,wM/g per min were broken down then 21 pmol of ATP would be generated, which would not be quite adequate to support normal activity (see above)-this rate could continue for 5 min before glycogen supplies would be exhausted. Thorn, Gercken, and Hurter (417) indirectly confirmed this point: when rabbit hearts were perfused with artificial blood, with oxygen but no substrate present, the hearts functioned normally for 40 min before the glycogen level started to decline. In this experimental situation endogenous fat as well as glycogen can be utilized and with oxygen available the ATP yield per mole of glycogen is now 39 rather than 3; accordingly the hearts can function tenfold longer. Neely, Whitmer, and Rovetto (319) have shown that, aside from the stored glycogen, utilization of exogenous glucose can only underwrite about 20% of the normal aerobic ATP generation. In 1963 Neil1 et al. (321) using the coronary AV temperature difference to calculate cardiac heat production showed that intravascular administration of KCN, 3.3 mg/kg, stimulated work output but reduced aerobic metabolism and they concluded that in dogs anaerobic metabolism could be used for work production. Subsequently Neil1 and Huckabee (320) using isolated dog hearts and injecting NaCN showed that whenever the oxygen usage w pas reduced below 72.5 ml/100 g per min there was additional anaerobic heat and the maximaL rate of anaerobic heat production seemed to be about 40 J/100 g per min (see their Fig. 4): this is about 20% of normal cardiac energy expenditure. Technically more reliable measurements by Muller-Ruchholtz and Lochner (313), using isolated working guinea pig hearts and a cyanide concentration of 3 x 10m4 M, showed that for periods of at least 15 min energy production can be maintained when at least 12% of the total energy requiremen .ts must be coming from glycolytic flux. These authors believed that under the appropriate conditions a higher contribution would be obtainable. D. Hypoxia

and Ischemia

The biochemical changes that take place during anoxia or ischemia have been detailed in the reviews of Wollenberger and Krause (446) and Kubler and Spieckermann (263). The effects of anoxia and ischemia on highenergy phosphate levels have been studied for many years (22, 122, 226, 417) and it is becoming accepted that the decline in contractility cannot be ascribed to the depletion of ATP that takes place, unless that depletion occurs within a particular and critical cellular compartment. Over recent years techniques have been developed that have allowed a

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clear distinction to be made between the biochemical effects of ischemia and anoxia. In 1968 Braasch et al. (33) developed a preparation in which the anterior ascending and circumflex coronary arteries of dogs were ligated to produce areas of ischemia. It was possible to compare ischemic with normal areas and it was found that 30 min of ischemia reduced PC levels from 8.0 to 1.4 pmol/g and ATP from 5.8 to 1.5 pmol/g. In later experiments using the same preparation Gudbjarnason, Mathes, and Ravens (175) took biopsy samples from acutely ischemic areas and compared these with samples from noninfarcted areas. Interestingly, they reported that ischemic areas stopped contracting when the ATP levels were only 20% below control values but nonischemic areas still contracted forcibly at ATP levels only 40% of the control. This suggested to them the possibility of compartmentalization of ATP. Experiments by Rovetto et al. (318, 367) with the working rat heart clearly show the difference between hypoxia and ischemia. When coronary flow was maintained but oxygen replaced with nitrogen there was a threefold increase in glucose utilization within 5 min that was maintained for at least 30 min; in addition tissue glycogen dropped by 22% in the first 2 min and by 70% at the end of 4 min. To produce ischemia the authors used a one-way valve to prevent retrograde perfusion of the coronary vessels during diastole; coronary flow was reduced 50% within 30 s and was only 10% of control at 12 min. In such hearts glucose utilization actually decreased within 2 min and was only 50% of the control at the end of 12 min. For the first 2 min the ischemic heart used glycogen at the same rate as the anoxic heart; however, after 4 min the glycogen level was only 44% below control. It appears that in anoxia glycolytic flux may increase 15. to 20-fold for the first 2-3 min and then stabilize at some rate 5-10 times control, but in ischemia a 3-fold increase is the maximum reached and even that declines to less than 2-fold at the end of 30 min. Insulin, which increases glucose transport, did not overcome this ischemic inhibition. A 50% reduction of coronary flow produced ventricular failure within 8 min and there was depletion of high-energy phosphates and accumulation of ADP and AMP. Within 2 min PC and ATP tissue levels had decreased by 75% and 25%, respectively, and ADP levels had increased 100%. After 30 min of anoxia intracellular lactate had only doubled, whereas in ischemic hearts the increase was lo-fold. Recent experiments of Neely and co-workers (319, 366) are particularly interesting. In the first study (319) the authors showed that a 60% reduction in the coronary flow of working rat heart produced a 30% fall in oxygen consumption, accelerated glucose utilization, lowered levels of high-energy phosphate, and increased intracellular levels of lactate and hydrogen ions. Any further flow reduction caused inhibition of glycolysis, larger decreases in high-energy phosphates, and higher tissue levels of lactate and H+ ions. In the second paper (366) it was concluded that the inhibition of glycolysis that occurs with ischemia, but not with anoxia, is due to the high tissue levels of lactate and H+. Indeed if lactate loss was prevented by perfusing either aerobic or anoxic hearts with media containing 20-40 mM lactate

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there was rapid inhibition of glycolysis even though perfusion was maintained. They suggest that the inhibition of glycolysis occurs at the level of glyceraldehyde-3-phosphate dehydrogenase, as indicated by accumulation of intermediates above but not below this point in the glycolytic pathway. Although the NADH:NAD+ ratio increased 16-fold in ischemia a similar change occurred with anoxia, which suggests that a change in this ratio is not the primary event. Glyceraldehyde-3-phosphate dehydrogenase is inhibited by a fall in pH but this effect seems to be more than compensated for by other feedback loops; in particular, increased ADP and AMP levels stimulate rather than inhibit phosphofructokinase. The studies of Neely and colleagues are nicely complemented bY those of Williamson et al. (437) and Opie (332). Williamson et al. (437) studied the effects of respiratory acidosis, pH 6.6, on the isolated perfused working rat heart. External work soon ceased and pressure generation fell by 75% within 5 min. Extensive biochemical analysis accompanied these studies and it was concluded that the effects of ischem .ia were caused by lowering of the intracellular pH, which decreased the rate of energy prod .uction relative to the rate of energy demand. The authors concluded, however, that the primary cause of the decline in mechanical performance was not due to a defect of energy metabolism but due to alterations in the calcium cycle of the heart. Opie (332) has examined the effects of coronary artery ligation in the dog heart and measured the relative rates of aerobic and anaerobic energy production in the central infarct, peripheral infarct, peri-infarct border, and normal tissue zones. Opie discusses the possible cellular mechanisms in ischemic injury, including changes in high-energy phosphate distribution; ATP “wastage”; reduced metabolite washout with the accumulation of protons, lactate, and CO,; and retention of sodium ions. Opie suggested that changes in ATP or PC may not directly impair contractile activity but could impair subcellular calcium transport. The metabolic role of creatine in promoting ATP and PC synthesis and of PC and mitochondrial CPK in transferring the ATP across the mitochondrial membrane makes these agents possible control sites (see above). As yet there has been no universally accepted hypothesis as to the primary factor(s) responsible for the decline in cardiac contractility either in ischemia, hypoxia, or long-term cardiac failure. The literature has been well reviewed in recent symposia (41, 430) and it seems apparent that progress in this field will come only when authors clearly define the type of impaired contractility studied, the animal species involved, the circulatory parameters measured and controlled, the substrate availability, and the energy demand under the existing physiological conditions. The past two decades have seen claim and counterclaim for the factor(s) responsible and, among others, low levels of high-energy phosphates, defects of energy utilization, low P:O ratios, contractile protein defects, mitochondrial abnormalities, and EC coupling defects have all been suggested. Nevertheless it is apparent that more and more of the authors active in this important area are attracted to the hypothesis first explicitly developed

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by Katz and Hecht (249) and further elaborated by Katz (247) that draws attention to the state of acidosis occurring when metabolism shifts from being aerobic to anaerobic (Pasteur effect) and links intracellular acidosis to changes in calcium metabolism. Indeed there is increasing evidence that these effects on intracellular calcium release and reuptake are not the whole story, since the acidosis also produces changes in the cardiac action potential and the inwardly directed calcium flux (75, 294). VII.

ENERGY

BALANCE

Over any specified period of time the total energy output (heat + work = Q + W) of a muscle should be accounted for by the simultaneous occurrence of chemical reactions such that:

where ni is the number of moles of reactant i that have been used and AE& is the molar enthalpy change for reaction i. Apparently over the entire cycle of muscular contraction, including oxidative recovery, there is a balance between the measured energy output and the chemical processes that underwrite muscle metabolism. Kushmerick and Paul (266, 267) recently have shown in combined measurements of oxygen consumption and chemical change that recovery oxygen consumption is linearly related to net chemical energy utilization. If, however, the mechanical events alone (i.e., contraction and relaxation) are examined then the current picture is confused, with different groups of workers reporting different discrepancies between the measured energy output and the measured chemical changes. Several articles have already been devoted to the problem (290, 434, 443). The first comparisons between energy production measured myothermitally and chemically were made by Wilkie and colleagues (59, 432) and in these experiments, where the chemical assays were made some 40 s after various types of contractions, there seemed to be a linear relationship between the sum of the heat and work terms and the amount of chemical (PC) breakdown. The chemical and myothermic measurements cannot be made on the same muscles so large numbers of animals have to be used to obtain statistically significant results. Wilkie (432) obtained a value of -46 kJ/mol for the enthalpy of PC hydrolysis in vivo and several other groups of workers reported values ranging from -32 to -59 kJ/mol [see Marechal

wN1

l

In 1971 Gilbert, Kretzschmar, and Woledge (159) reported results of tetanic energy-balance studies where, for the first time, heat and chemical changes were measured in amphibian sartorius muscles at selected times from as early as 0.5 s to as long as 15 s after the commencement of a tetanus. They measured changes in PC, ATP, and Pi and reported that there was a greater heat output than could be accounted for in terms of PC breakdown, although after about 2.0 s the discrepancy between energy and

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chemistry seemed to remain constant. Subsequent experiments have shown (101). that the discrepancy continues and even increases during relaxation In 1972 Woledge (444) published calorimetric data that made the discrepancy between energy output and chemical breakdown worse than originally envisaged. He reported that calorimetric determination of PC hydrolysis gave a resultant heat of only 34 kJ/mol (this value includes heat from associated buffer reactions). Wilkie, Woledge, and colleagues therefore have concluded that early in a tetanic contraction an unidentified exothermic reaction occurs and that after the first couple of seconds this reaction, or possibly another unidentified reaction, must run in parallel with PC splitting. Subsequent isometric and isotonic studies reported in 1974 (100) confirmed the earlier isometric studies but the isotonic results allowed the authors to conclude that it was not necessary to postulate that the unidentified reaction(s) was fueling the work output. More recently Homsher et al. (212) measured PC breakdown in unpoisoned frog sartorius muscles at 0°C using immersion-freezing and hammerfreezing techniques (260). In experiments with Rana pipiens 1) the rate of PC splitting was 3 times higher in the 1st s than in the succeeding 4 s, in agreement with the heat measurements; 2) the energy liberation paralleled the PC breakdown so that A&c values measured at different times (0.6, 1.0, and 5.0 s) were the same; 3) when experiments were done with R. temporaria (the species used by Wilkie and colleagues) the measured heat at the end of a 4.8 s tetanus was about 50% higher than with R. pipiens 9 yieldi ng an apparent A&c of -75 kJ/mol. Although Homsher et al. reported a correlation between chemical breakdown and energy output, Wilkie still maintained (434) that the high enthalpy values for PC hydrolysis of both sets of investigations are incompatible with the calorimetric studies of Woledge (444). Indeed this viewpoint has been accepted by Rall, Homsher, Wallner, and Mommaerts (351); in isotonic experiments they reported that only a portion of the observed heat plus work measured during a contraction can be explained by PC splitting. They found that only 0.68 of the measured energy output could be accounted for chemically in muscles contracting 30 times against light loads or in muscle isotonically tetanized for 0.6 s and whose metabolism was arrested 3.0 s after the commencement of stimulation. The 0.68 value i.s obtained by use of the calorimetric value of Woledge for the enthalpy of PC hydrolysis. If, however, the metabolism of these isotonically contracting muscles was arrested at 0.5-0.75 s after the commencement of stimulation the ratio of chemically explained energy to measured output ranged between 0.15 and 0.40, suggesting a temporal dissociation of energy liberation and high-energy phosphate splitting during shortening. This paper shows that energy liberation and high-energy phosphate splitting do not parallel one another and that the relationship depends on the mechanical behavior of the muscle; this result thus differs from their previous study on isometrically tetanized muscle (212). Recently some evidence has been obtained supporting the idea that unidentified chemical reactions are contributing to initial energy production.

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In nuclear magnetic resonance (NMR) studies with living muscle (103, 103a) there is evidence for a resonance peak that cannot be identified chemically and there are three adenine phosphate peaks with the P-peak, which corresponds to ATP, being only half the magnitude of an a-peak, which may conceal another unidentified compound. During recovery the measured changes seem to be those predicted by classical biochemical theory, but during contraction many unexplained discrepancies remain. The resul ts mentioned above are startling enough but the picture becomes more confused when resul .ts obtained at other temperatures are considered. Thus in 1973 Canfield, Lebacq, and Marechal (56) carried out energy-balance studies at 20°C using frog sartorius muscles. In isometric tetani of various durations, O-12 s, these authors measured changes in ATP, ADP, AMP, IMP, and PC. They reported that in metabolically inhibited muscles during the first 2 s the only net reaction was PC breakdown and the observed A&c was -35 kJ/mol. Even in unpoisoned muscles the detected heat over the first 2 s could be accounted for by the observed chemical changes and their molar enthalpies. However, for times greater than 2 s there was more heat being liberated . than predicted on the basis of the chemi .cal changes detected, and these authors believe the extra heat comes from reactions associated with recovery. These results are disturbing not only because they disagree with those obtained at lower temperatures but also because the estimated A&c value is close to the in vitro measurement made by Woledge. Even for skeletal muscle it probably will take many years to sort out these difficulties. The experimental design used in the amphibian studies is not without problems since tetanized muscles often produce additional heat after prolonged rest periods even though the mechanical response is essentially the same as that obtained in a subsequent tetanus (266, 267a). Additional unsolved problems relate to the nature of the chemical reactions between calcium and troponin and the possibility that crossbridges might work out of an energy-storage system (410). It obviously is impossible at present to draw any conclusion that can be extrapolated to mammalian cardiac muscle working repetitively at body temperature. Readers may ask the problem is why th .is problem has been considered at all. Unfortunately central to our understanding of muscle contraction and if there is doubt that all the chemical reactions involved have been identified then clearly there are important consequences for theories of muscle contraction. Indeed as Wilkie and colleagues (100) point out the present results provide great difficulty for most of the current theories of muscle contraction, where it has been customary to explain the energy output of muscle in terms of one molecule of ATP being hydrolyzed per crossbridge cycle. This reservation should be kept in mind when considering the current muscle models. VIII.

MUSCLE

MODELS

In a review

devoted to cardiac muscle it may seem inappropriate

to

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spend much time considering models of muscular contraction that have largely been developed to explain the behavior of amphibian skeletal muscles operating at low temperatures. The adoption of this idea, however, would be a backward step: there are ample reasons for believing that cardiac and skeletal muscle function similarly at the level of crossbridge movement, although there can be no argument that there are substantial differences in the activation and relaxation mechanisms. At the present time models of muscle fall basically into two classes. There is the classical approach, with its origins essentially in mechanical analogues of muscle. Such models dominated the literature until the late 1950’sand may still be used fruitfully to answer certain types of questions. In the simplest case they consist of a contractile element (CE) in series with a series-elastic component (SEC). The behavior of the contractile component is considered to be described by the force-velocity relationship and several empirical equations have been developed that adequately describe the relationship. The SEC has been considered to be passive and to have a nonlinear stress-strain relationship. The development of this type of model owes much to the ingenuity and experimental expertise of A. V. Hill, and a good critique of this approach and an introduction into the second type of model are found in Recent Advances in Physiology [Simmons and Jewel1 (38511. The second type of model is a kinetic one based on the sliding-filament theory of muscle contraction. In this model the elements represent actual structures within the muscle and the behavior of muscle is described in terms of the binding of myosin crossbridges to actin filaments. The pioneering model is that of A. F. Huxley (217) and it subsequently has been developed further by Huxley and Simmons (223) and described in detail by Huxley (219, 220). It may be possible to define a third class of muscle model. Since the mid 1960’s the concepts of “irreversible thermodynamics” have been used to describe mechanochemical coupling. In its most abstract application no assumption is made about a molecular mechanism (57, 58) but its protagonists often use concepts or results taken from both the analogue and kinetic models of muscle. As discussed below, all the current molecular models have shortcomings, a situation that has led to the increasing complexity of postulated crossbridge mechanisms. The molecular basis of contraction is far from being satisfactorily established and authors who occasionally remind us of this fact (325a) render muscle physiologists a service. A. Muscle Models and Energetics In his 1969 review, Mommaerts (303) considered the Huxley (217) model in some detail; it would be inappropriate to cover this material again but readers are referred in particular to the section where modifications to the model were suggested in the light of biophysical and biochemical data available after 1957. Since 1969, however, it has become apparent that the

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original model will not satisfactorily account for the mechanical transients observed when length or load is rapidly altered (76, 221). This has led to a new model being proposed for force generation (223) or to substantial changes in the original one (339). Reasons for developing new models and for rejecting certain alternative theories of muscle contraction are provided by Huxley (219); readers will also find a listing of the main experimental results that theories of muscular contraction must be able to predict. In the original Huxley model (see Fig. 4) a myosin crossbridge (M site) oscillated around an equilibrium position (0) and could become attached to an active site (A) on an actin filament with a rate constant f. This linkage could be broken by the combination of the attached crossbridge with a highenergy phosphate compound, this reaction proceeding with a rate constant g. The rate constants f and g were assumed to depend on x, the distance of the actin active site A from the equilibrium position. The rate constant f was 0 when x was negative, i.e., when A was to the left of 0 (considering the right-hand half of a sarcomere) and increased linearly with x up to a point h, beyond which it was 0. The distance x = h could be taken to be the “reach” of a crossbridge. The detachment rate constant was small but finite when x was positive, and it increased linearly with x (g,). When x was 0, i.e., the crossbridge was perpendicular to the fiber, g was considered to be 0, but as soon as x became negative g became large and constant (g,). Thus the model consists of cycling crossbridges that attach with a moderate rate constant, carry out a work cycle, and then detach with a comparatively high rate constant after the A site moves to the left of the equilibrium position. By the selection of appropriate values for f and g and the insertion of data

A

Ia-

CROSSBRIDGE

EQUILIBRIUM POSITION MY OSIN

0 8 s,(b)

-h0

X

-

FIG. 4. A : schematic illustration of a myosin and actin filament with a myosin crossbridge shown either in its equilibrium position (0) or attached to actin at a distance (x) from its equilibrium position. Crossbridge is on the right-hand side of a sarcomere A band. B: diagram showing dependence of rate constants f and g on X. Rate constant for crossbridge attachment is designated f and for detachment g. Notation f(r) and gz(y) indicate that attachment and detachment may also be functions of the free intracellular calcium level, which varies with time during a contraction. Possible variation of g and g, during muscle activity is indicated by dotted lines. [Based on the Huxley model (217, cf. Figs. 5 and 6) with modifications suggested by Julian and colleagues (239, 240).]

(+I

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describing the stiffness and reach of a crossbridge, it was possible to show that the model would predict 1) an adequate force-velocity relationship and 2) a linear relationship between the rate of extra energy liberation (shortening heat + work) and load as reported by Hill (192). To obtain the energy prediction it was assumed that one ATP molecule was split in each crossbridge cycle, liberating e ergs of energy. This is one reason why the current energy-balance data are so disturbing. Setting this problem aside for the moment, the total rate of energy liberation was predicted by the equation:

of muscle, Z = where m = the number of M sites per cubic centimeter in muscle distance between A sites on actin, V = velocity of shortening lengths per second, 4 = (fi + g&h/s, and s = sarcomere length. This reduces to:

E - me 5 figI 21’fX

(12)

when V = 0, thus giving the tetanic maintenance-heat production. As mentioned earlier Civan and Podolsky (76) were the first to show that there were very complex contraction kinetics after a rapid change in load and to identify these events with the motion of cycling crossbridges. In converse experiments, where a sudden length cha change nge was imposed on a tetanized muscle fiber, Huxley and Simmons (222) reported that during a step length change (less than 0.02 ZO)there was a simultaneous tension drop, a rapid l- to 2-ms phase of early tension recovery, followed by a reduction (or even a reversal) in the recovery rate for 5-20 ms, ms then a phase of gradual recovery to the isometric tension level-this level- this last phase would be equivalent of Gasser and Hill (139). There is general to that seen in the experiments agreement that th .at phase 1 can be interpreted by assuming that there is instantaneous IUS elasticity instantaneo within the crossbridges, probably in the S-2 fragment of the myosin crossbridge. There is less agreement as to how the early tension recovery (phase 2) should be interpreted. Podolsky, Nolan, and Zavaler (338, 339) proposed that during an isometric contraction many crossbridges are not attach attached.ed because their filament sites are not available some of the myosin crossbridges are at the time, but on rapid shortening brought near A sites where they can now attach and exert tension. On the basis of this theory the number of crossbridges during phase 2 should increase and the instantaneous stiffness of the muscle should increase. Using a conditioning and then a second test step length change, Ford, Huxley, and Simmons (130) showed that stiffness actually falls slightly during phase 2 and their data have been supported by Julian and Sollins (243)) who used small-amplitude, high-frequency sinusoidal length changes to measure muscle stiffness.

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Huxley and Simmons (223) have suggested that quick tension recovery is caused by the tendency of the myosin head to rotate to positions of lower potential energy with recovery occurring at a finite speed because of the rate constant for movement of the system from one stable position to the next. It is necessary to consider a series of two to four stable states since mathematical analysis assuming only a single crossbridge movement produced a) about 2 x lo-l2 N force, which may be 50% too low (223, 348), and b) a cycle of work output per crossbridge about 50% too low to account for measured mechanical efficiency, i.e., work/(work + heat). Indeed the force discrepancy in a may even be greater as recent X-ray data suggest (9) that fewer crossbridges are attached at any one time than was originally envisaged. Phases 3 and 4 were accounted for by detachment and reattachment of crossbridges with kinetics similar to those in Huxley’s (217) theory. Detachment comes to an end first since it has a higher rate constant and leaves attachment as the predominant process in phase 4. In order to explain any fall of tension in phase 3 it is necessary to suppose that there is accelerated detachment even when a crossbridge is still in a position (x > 0) where it exerts positive tension; this necessitates some change in the original Huxley model. Such a change has been made by Julian, Sollins, and Sollins (241). They described a model in which crossbridges can attach only while in the first of two stable states. Force is generated by transition to a second state, after which detachment can occur. Crossbridges are assumed to be connected to the thin filaments by an elastic element (presumably S-2) whose extension or compression influences the rate constants f and g. As noted above the original Huxley model would predict both the energetic as well as the mechanical data of A. V. Hill. In particular the rate of energy liberation was practically a linear function of the load. This experimental result was the basis for Hill’s “characteristic” equation relating force and velocity in muscle: (P + a> V = b

Cardiac energetics.

PHYSIOLOGICAL REVIEWS Vol. 58, No. 1, January 1978 Printed m U.S.A. Cardiac Energetics COLIN Department of Physiology, Monash L. GIBBS Universit...
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