Effects of endurance exercise on isomyosin patterns in fast- and slow-twitch skeletal muscles.

Effects of endurance exercise on isomyosin patterns in fast- and slow-twitch skeletal muscles DANIEL P. FITZSIMONS, GARY M. DIFFEE, ROBERT E. HERRICK, AND KENNETH M. BALDWIN Department

of Physiology

and Biophysics,

University

FITZSIMONS, DANIEL P., GARY M. DIFFEE, ROBERT E. HERRICK, AND KENNETH M. BALDWIN. Effects of endurance exerskeletal cise on isomyosin patterns in fast- and slow-twitch muscles. J. Appl. Physiol. 68(5): 1950-1955, 1990.-Although

endurance training has been shown to profoundly affect the oxidative capacity of skeletal muscle, little information is available concerning the impact of endurance training on skeletal muscle isomyosin expression across a variety of muscle fiber types. Therefore, a lo-wk running program (1 h/day, 5 days/ wk, 20% grade, 1 mile/h) was conducted to ascertain the effects of endurance training on isomyosin expression in the soleus, vastus intermedius (VI), plantaris (PLAN), red and white medial gastrocnemius (RMG and WMG), and red and white vastus lateralis muscles (RVL and WVL). Evidences of training were noted by the presence of a resting and a submaximal exercise bradycardia, as well as an enhancement in peak 02 consumption in the trained rodents relative to the nontrained controls. No evidence for skeletal muscle hypertrophy was observed subsequent to training when muscle weight was normalized to body weight. Shifts in the isomyosin profile of the trained VI, RMG, RVL, and PLAN were seen relative to the nontrained controls. Specifically, training affected the slow myosin (SM) composition of the VI by decreasing the relative content of the SMB isoform by 14% while increasing that of the SM1 isoform (P c 0.05). In addition, training elicited various degrees of a fast to slower myosin transformation in the RMG, RVL, and PLAN. All three muscles showed a significant reduction in the fast myosin 2 isoform (P c 0.05), with significant increases in intermediate myosin in the RVL and PLAN along with elevations in SMZ in the RMG and PLAN (P C 0.05). The isomyosin profiles of the WMG and WVL were nominally affected by training. These data suggest that those muscles of the rodent hindlimb musculature normally recruited during locomotor activity possess the ability to modify their isomyosin distribution toward a greater degree of intermediate myosin expression. It is speculated that this adaptation is designed to economize contractile activity without altering force production during repetitive limb movements. plantaris; medial gastrocnemius; soleus; vastus intermedius; vastus lateralis; fast myosin; intermediate myosin; slow myosin

CONTRACTILE PROTEIN myosin has been shown to exist as a large family of protein isotypes encoded by a highly conserved multigene family. To date, six native isomyosins have been identified and characterized into their subunit composition in the adult rodent hindlimb skeletal musculature (17). Those muscles expressing predominantly slow myosin (SM) have relatively low adenosinetriphosphatase (ATPase) specific activities,

THE

1950

0161-7567/90

$1.50 Copyright

of California,

Irvine,

California

92717

whereas muscles with a predominance of fast myosin (FM) possess a relatively high ATPase specific activity (16). Interestingly, in those muscles expressing a predominance of intermediate myosin (IM), the heavy chain of which is encoded by the fast II,-MHC gene and contains a mixture of fast and slow light chains (Fig. 1 and Ref. 17), the ATPase activity is bracketed between those muscles possessing myosin with a predominance of either fast IIb or slow (type I) heavy chains (16). Furthermore, previous findings clearly show that the isomyosins are distributed in a characteristic fashion among rodent hindlimb muscles, thereby implying a functional role for the various isoforms (16, 17). The SM isoforms (SM2 and SM1) typically predominate the isomyosin profile of the slow-twitch postural muscles, such as the soleus (SOL) and vastus intermedius (VI) (17). In contrast, fast-twitch white muscles, normally recruited only for high power activity (e.g., vertical jumping or sprinting), primarily express only the three FM isoforms (FM1, FM2, and FM3) (17, 19). However, fast-twitch red muscle regions, principally recruited during prolonged locomotor activity (3), express the full complement of adult FMs and SMs, including a relatively high content of the IM isoform (17). This observation has led us to propose that the IM isoform may be most suitable for activities such as repetitive locomotion (17, 18). Endurance training has been shown to induce a number of peripheral adaptations in skeletal muscle, including enhancement in both mitochondrial oxidative capacity (13) and blood flow capacity (15) to those regions of muscle participating in locomotor activity (2, 3). Likewise, after high-intensity training both the metabolic and fiber type profiles would suggest a transformation in the functional properties of fast-twitch red muscles (9, 10). In light of these cardiovascular and biochemical adaptations to endurance training, it was of interest to determine the extent of isomyosin plasticity after the imposition of a moderate-intensity training regimen. Therefore, the goal of the present study was to ascertain whether moderate-intensity endurance training exerts a significant impact on the isomyosin distribution in a broad range of rodent hindlimb skeletal muscles with varying muscle fiber types [i.e., slow oxidative (SO), fast oxidative glycolytic (FOG), and fast glycolytic (FG)] (1). In view of our previous findings suggesting that IM 1) is highly expressed in regions of rodent muscle heavily recruited during locomotion and 2) is thought to possess ATPase activity intermediate to that of FM and SM (16,

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Physiological

Society

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MYOSIN

DISTRIBUTION

IN TRAINED

ABCDEFGHIIK

200 116 97 66

43

31

-SIX, -FLCl

22

1 i

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-

4-FLC2

14

SKELETAL

1951

MUSCLE

(Nembutal). Skeletal muscle was obtained from both the lower and upper hindlimbs of each rodent. The quadriceps muscle group was removed from the upper portion of each limb and dissected to obtain the vastus lateralis (VL) and VI muscles. The lower rodent hindlimb was used to obtain the SOL, plantaris (PLAN), and medial gastrocnemius (MG) muscle groups. Additional dissection of the MG and VL samples was performed to partition both muscles into their respective predominantly red and white regions (1, 3). Care was taken to sample the VL and MG muscles on clearly defined regions to separate the “white” portions. In both muscles, equal portions (-75-100 mg) of “red” muscle were removed from the core of the red region for both experimental groups (3,4). All muscles were trimmed free of connective tissue, blotted dry, and weighed. Muscle samples were stored in cold glycerol at -20°C until analyzed for native isomyosin distribution.

4-FLCl

1. Myosin light chain profile of rodent hindlimb skeletal muscle. Myosin light chain separation in T and SED muscle was performed in 14% acrylamide gels according to method of Laemmli (14). Numbers on the far left of gel represent molecular mass (in kDa) of molecular weight standard (MWS) (Bio-Rad). Myosin light chains are identified by arrows on the far right: SLCi, slow light chain 1; FL&, fast light chain 1; SLC?, slow light chain 2; FL&, fast light chain 2; and FL&, fast light chain 3. Samples are as follows: MWS (lane A), VI [lanes B (T) and C @ED)], RMG [lanes D (T) and E @ED)], RVL [lanes F (T) and G (SED)l. PLAN llunes H (T) and Z (SED)l. and WMG Ilanes J (T) and K (SED)]. Note decrease in relative content of SLC, in’trained VI (lane B). FIG.

17), we hypothesized that the IM isoform expression would be optimized in those muscle regions repeatedly used extensively during locomotor training. The findings reported here are in agreement with this notion. METHODS

Animal

Care and Experimental

Protocol

Female Sprague-Dawley rodents initially weighing 175-200 g were obtained from Taconic Farms (Germantown, NY). The animals were housed in groups of five rats per cage and supplied with food and water ad libitum. The rodents were randomly assigned into one of two groups designed normal sedentary (SED) and endurance trained (T). These animals participated in a lo-wk endurance training running program. Briefly, the rodents were conditioned 5 days/wk, utilizing an exercise program involving both progressive intensity and duration. Animals were initially run-trained on a rodent treadmill (model 42-15, Quinton) at 0.5 mph (13 m/min) and 20% grade for 15 min. By the end of 5 wk, the rodents were running at 1.0 mph and 20% grade for 1 h/day. All animals were maintained at this final level of intensity and duration for the remainder of the training program.

Biochemical and Electrophoretic of Skeletal Isomyosins

Analysis

Myosinpurification. Skeletal muscle samples, weighing -150-500 mg, were homogenized in 20 vol of 300 mM KCl, 150 mM K2HP04, 10 mM ATP, 2 mM EDTA, and 2 mM dithiothreitol, pH 6.8. The resulting homogenate was gently stirred at 4°C for 30 min. Nonsoluble particulate matter was removed from the homogenates by centrifugation at 48,000 g for 15 min followed by a 2-h spin at 160,000 g. The resulting supernatant fraction was dialyzed overnight in 1,000 vol of 10 mM imidazole, pH 6.8, and subsequently centrifuged at 48,000 g for 15 min. The resulting pellet was resuspended in 150 mM KC1 and 30 mM tris(hydroxymethyl)aminomethane (Tris), pH 7.0, with protein concentration measured by the biuret method (8). Myosin was suspended to a final concentration of 1 mg/ml in a solution containing 50% glycerol, 100 mM Na4P207, pH 8.80, 5 mM EDTA, and 2 mM 2-mercaptoethanol. Myosin stored in this fashion is stable for performing native myosin separation for -1 yr (unpublished observations). Nondissociating

polyacrylamide

gel

electrophoresis.

Skeletal muscle isomyosin distribution was determined by a nondissociating electrophoretic procedure (native polyacrylamide gel electrophoresis) based on the techniques of Hoh et al. (12) described in detail previously (16,17). For resolving all isoforms in the muscle samples studied, l-5 pg were first electrophoresed and examined for representative T and SED muscles. For best resolution and quantitation, -2-3 pg of myosin were loaded onto the gels and electrophoresed for 22 h at 90 V constant voltage. After electrophoresis, the gels were fixed for 30 min in 15% trichloroacetic acid and stained for 1 h in 0.1% Coomassie Brilliant Blue R-250, 30% isopropanol, and 10% glacial acetic acid. All gels were destained by diffusion in 15% isopropanol and 10% glacial acetic acid.

Tissue Sampling

Sodium dodecyl sulfate-polyacrylumide gel electrophoresis. Skeletal muscle myosin light chain gel patterns

Approximately 4 h after participation in an exercise test to assess work capacity (6), the rodents were killed after an intraperitoneal injection of pentobarbital sodium

were analyzed using a modification of the sodium dodecyl sulfate-polyacrylamide gel electrophoresis technique of Laemmli (14) as described in detail previously (16, 17).


1952

MYOSIN

DISTRIBUTION

IN TRAINED

Representative myosin light chain pat tern s for the various trained and nontrained muscles are presented in Fig. 1.

SKELETAL

MUSCLE

1. Body weight and skeletal muscle weights after endurance training TABLE

Sedentary

Quantification

of Native

Skeletal Muscle Isomyosins

Native skeletal muscle myosin isoforms were analyzed densitometrically from a permanent film record of the gel. A 35mm film negative was made of the backlit gels on Kodak Technical Pan 2415 film exposed at ASA 25 and f/16. The film was processed in Kodak Pota developer, with the resulting 35-mm negative enlarged to full frame on Kodak 4127 film. The Kodak 4127 film (4 x 5 in.) was processed in Kodak HC 110 developer. The film records were scanned at 630 nm on a Zeineh soft laser densitometer (BioMedical Instruments, Fullerton, CA). The densitometer was interfaced to an IBM AT computer equipped the necessary software to perform peak density analysis. The relative proportion of the individual isomyosins was determined by first summating the peak densities of each isoform to a value representing 100% of the total myosin. The relative percent of each isomyosin was estimated by expressing its peak density relative to the total peak densities for all the skeletal isomyosins. Statistical

Analysis

All data, reported as means & SE, were analyzed with a GB-STAT statistical package (version 1.5; New England Software, Greenwich, CT). All data, examined for homogeneity of variance by the Bartlett’s test (20), were found to be homogenous. A Student’s two-tailed t test for independent samples was used as the post hoc test. Statistical significance was set at P < 0.05. RESULTS

Evidences

of Physical

Training

The effects of a moderate-intensity lo-wk training program on adult isomyosin expression were examined in skeletal muscles obtained from rodents used in a previous study (6). Briefly, we observed a 10% increase in peak O2 consumption in the T rodents (T: 96.0 t 1.2 vs. SED: 88.5 t 2.7 ml 02. kg-’ min-l). Both total run time (T: 26.8 t 2.0 vs. SED: 14.4 t 1.1 min) and speed at peak O2 consumption (T: 2.25 t 0.17 vs. SED: 1.31 t 0.07 mph) were nearly doubled in the T rodents (6). These indexes clearly indicated an augmented endurance capacity of the T rodents. Further confirmation of a training effect was noted during a submaximal stress test, in which the T rodents exhibited a marked bradycardia both at rest and at running speeds up to 1.25 mph (6). For example, during steady-state running after 25 min at 0.75 mph the heart rates of SED rats were 543 t 15 vs. 478 t 12 beats/min in T rats (P < 0.05). Submaximal exercise bradycardia is believed to be a hallmark of a cardiovascular training effect (5). l

Body Weight and Muscle Mass

We observed no significant difference in body weight between the T and SED controls (Table 1). Likewise,

Trained

BW, g

262t8 SOL, mg 114.1t5.0 103.8t5.7 VI, w PLAN, mg 293.11t14.4 593.3H6.8 MG, w 943.6t22.6 VL, mg 0.43kO.03 somw, w/g 0.4OkO.02 WBW w/g PLAN/BW, mg/g 1.12t0.07 2.27kO.08 Mww wdg 3.61t0.12 WBK w/g Values are means t SE; n = 6 animals/group. = 0.05.

2. Skeletal muscle isomyosin in trained and nontrained rodents

TABLE

267kll 121.2Ik5.1 111.7t7.0 342.3t,12.7* 667.4&21.1* 980.2t42.1 0.45t0.03 0.42kO.03 1.29t0.06 2.52&O. 13 3.62&O. 16 BW, body weight.

distribution

SK

SM

T VI

85.1t2.8 82.0t2.9

5.3kl.O 7.720.8

5.3tl.l 5.3t0.7

4.4t1.8 5.1t1.4

SED T RMG SED

61.7t2.4 52.6t4.4

5.8t1.9 12.4t2.5*

25.8t0.7 28.9k2.3

6.7t1.2 6.2t0.8

23.6kO.5

RFL

26*gt0*5*

SOL SED

SED

T

PLAN SED

T

IM

*p

FM,

FM,

FM,

38.45fr1.7 25.1-c-0.9 41.1kl.l 24.6t1.6

12.9tl.l 7.3&0.9*

9.6H.9 9.0t0.9

23.6t2.6* 38.0&2.0*

32.4t3.3 27.8k1.4

25.8t2.1 16.9&2.5*

8.6k1.2 7.4M.l

4.4t0.2 6.6&0.3*

31.3k2.1 40.0tl.O*

34.3k3.7 33.9k1.8

22.3k2.7 14.6t1.7*

7.71t1.9 5.6k1.3

40.8tl.l 36.2t1.4*

31.6t2.5 38.7t2.8

27.5k1.9 25.2kl.9

43.7k2.2 40.2t2.5

43.0t1.7 45.3t1.8

13.320.8 14.6kl.4

WMG SED T WVL SED T

Values are means k SE expressed as a fraction of total myosin pool; n = 6 animals/group. Clear detection of SM1 isoform was not possible in RMG, RVL, and PLAN samples. Therefore, SM band observed after native polyacrylamide gel electrophoresis was arbitrarily labeled SM2. Statistical significance: SED vs. T, * P < 0.05.

there existed no significant difference in absolute (mg) or normalized (muscle wt/body wt) muscle mass between groups for the SOL, VI, and VL muscles (Table 1). However, we did observe a significant increase in absolute muscle mass for the PLAN (+17%) and MG (+l2%) muscles of T rodents (Table 1). Nevertheless, when the MG and PLAN muscle weights were normalized for body weight, no evidence for significant skeletal muscle hypertrophy was seen (Table 1). Native Isomyosin Skeletal Muscle

Distribution

in Rodent Hindlimb

SOL and VI. Nondenaturing gel electrophoresis revealed a partial SM to IM shift in the myosin pool of the T VI muscle (Table 2). The relative distribution of the SM2 isoform was decreased from 62 to 53% of the total myosin pool, and this shift was accounted for by an increase in the SM1 isoform in the T animals (P < 0.05).


MYOSIN

DISTRIBUTION

IN TRAINED

Although mean levels of IM were slightly elevated after training, no statistical difference existed between the groups. Although the SOL and VI shared similar qualitative shifts in isomyosin profile after endurance training, none of the isomyosin transitions in the SOL were significant at the 0.05 level (Fig. 2). Red medial gastrocnemius (RMG), red vastus lateralis (RVL), and PLAN. In all three of the fast-twitch red

muscles examined, endurance training elicited a significant shift in the myosin isoform profile, revealing an enhanced expression of either IM or SO (Table 2). Endurance training induced a significant reduction in the relative content of the FM2 isoform in the RMG, RVL, and PLAN muscles (P < 0.05). Moreover, levels of the SM2 isoform were affected by training, in that slight increases were seen in the RMG and PLAN, respectively (P < 0.05). Prominent elevations in the relative IM content were found in the RVL and PLAN muscle (P < 0.05). Although the slight increase in IM content of the trained RMG was nonsignificant, there nevertheless existed a trend across all three mixed red muscles toward a greater expression of the IM isoform (Table 2). White medial gastrocnemius (WMG) and white vastus lateralis (WVL). Endurance training exerted relatively

little affect on the isomyosin profile in these fast-twitch white muscles. If there existed any directional shifts in isomyosin content, it would appear that training reduced levels of the FM3 isoform, while slightly increasing the relative content of the FM2 isoform. The functional significance of these isomyosin shifts, if any, remain unclear. Moreover, a confounding problem to interpreting adaptations in the white regions of the MG and VL muscles is the likelihood that continued recruitment of fibers in these regions was most likely attenuated as the training program progressed (3).

A T

T

S

VI

S

T

s

PLAN

umG

B T

S

T

S

T

S

sM2 Im

FM3

FM2 w RVL

wmG

WVL

2. Native isomyosin distribution in rodent hindlimb skeletal muscle after endurance training. Representative native gels of T and SED animals for VI, PLAN, and RMG (A) and RVL, WMG, and WVL (B). Isomyosin bands are designated with nomenclature used by Tsika et al. (17) and are identified by arrows on the far left and right in order of increasing electrophoretic mobility. In T, note I) increases in SM, content of the VI. PLAN. and RVL and IM content in PLAN and 2) decrease in FM2 content df RVL and RMG. FIG.

SKELETAL

MUSCLE

1953

DISCUSSION

At the present time, there exists little information concerning the plasticity of isomyosin expression in the hindlimb skeletal musculature of adult rodents subjected to a moderate-intensity endurance training program. To more fully investigate the potential effects of training on myosin expression, we have chosen a specific set of skeletal muscles representing the spectrum of fiber types present in striated muscle (i.e., SO, FOG, and FG) (1). The findings in the present study clearly demonstrate that the pattern of adult skeletal isomyosin expression can be modified in response to endurance exercise. This is seen most clearly in fast-twitch muscle regions containing fast red fibers (i.e., RVL and PLAN), and to a lesser extent in muscles expressing a relatively high content of either SM (SOL and VI) or IM (RMG and VI) as well as exclusively fast-type myosin (WMG and VL). Our findings of activity induced plasticity of myosin expression in adult skeletal muscle are consistent with that seen previously in functionally overloaded skeletal muscle (18). For example, Tsika et al. (18) demonstrated that levels of both IM and SM were upregulated after as little as 2-4 wk with increased weight-bearing stress imposed on the PLAN muscle. After prolonged elevations in overload stress (12 wk), there was an eventual shift toward enhancement of SM expression and repression of the FM1 isoform, although the IM isoform still remained the predominant isomyosin species in the overload muscle. Conversely, the pattern was reversed during periods of regression of compensatory overload (18). In the present study, we observed a bidirectional shift in the direction of IM content in the VI, RVL, and PLAN muscles of the treadmill-trained rodents (Table 2). The myosin pool in the VI shifted away from the SM2 isoform and toward SM1. In contrast, in the fast-twitch RVL and mixed PLAN muscle, we saw a different response in .that the myosin pool shifted toward lower amounts of FM thereby increasing the relative levels of IM. Interestingly, in the fast RMG, which normally expresses the highest relative content of IM in the untrained state, adaptative changes in IM content were minimal. In the context of the above discussion, it is curious that the VI muscle, which is normally biased to SM2 expression (Table 2), was uniquely affected by training. For example, its relative content of SM, was reduced nearly 15% in the T rodents corresponding to an increase in the relative content of the SM1 isoform, with minimal impact on the IM isoform (Table 2). The functional significance of the relatively low amounts of the SMI isoform, in both T and SED groups, remains unclear. However, it is evident that training induced elevations in the relative content of this slow isomyosin in a specific muscle with known antigravity function. Why IM was not markedly increased in the mixed RVI muscle remains unclear. It is possible that either 1) a more intense training regimen or 2) longer duration of our moderateintensity program may be necessary to induce a significant increase in the relative levels of IM in the VI muscle. In contrast to the patterns of change seen in the VI muscle, we observed a more complex pattern of isomyosin


1954

MYOSIN

DISTRIBUTION

IN TRAINED

redistribution in the RMG, RVL, and PLAN muscles of the T rodents. The relative content of fast myosins (FM1, FM2, and FM3 combined) was reduced in the RMG (T: 32% vs. SED: 38%), RVL (T: 52% vs. SED: 69%), and PLAN (T: 54% vs. SED: 64%) muscles. Specifically, significant reductions were seen in the FM2 isoform content for all three muscles (Table 2, P < 0.05). Less striking reductions were seen in the FM1 isoform of the trained RVL and PLAN. Commensurate with the alterations in FM content, marked elevations in the IM content were also seen in two of the three fast mixed red muscles (Table 2). In fact, in the endurance-trained rodent, the IM isoform was observed to be the predominant myosin species expressed in the RMG, RVL, and PLAN muscles, whereas in the nontrained muscles, the FM3 isoform was typically most prevalent in those muscle regions with relatively low content of both SM and IM. The finding that exercise training increased the relative IM expression in the PLAN muscle is consistent with that seen by Gregory et al. (11). Consistent with previous observations (l7), it is curious that those muscle regions primarily involved in locomotor activity express relatively high levels of the IM isoform. In the present study we chose to focus our attention on three specific muscles and/or muscle regions previously shown to be heavily recruited during locomotion (3,19). The evidence presented herein demonstrates that moderate-intensity training significantly augmented the relative amount of IM, especially in the RVL and PLAN muscles (Table 2). Those muscles that do express abundant levels of IM have been shown to have ATPase specific activities bracketed between those of slow-twitch red and fast-twitch white muscles (16). It is intriguing that Baldwin et al. (4) found that endurance treadmill training reduced the actomyosin ATPase specific activity of the RVL by -20%. Based on the previous results of Baldwin et al. (4), in the context of the findings in the present study, one might speculate on the functional significance of the IM isoform. The experimental evidence (ATPase data of Refs. 4 and 16) suggests that IM is likely the most suited of all the available adult skeletal isomyosins to perform the weight-bearing cyclical limb movements of locomotion. Thus the energy cost for locomotion could be economized without compromising force production. In contrast to the striking effects on the isomyosin profile of fast-twitch red muscle, endurance training had relatively little impact on the myosin expression in fasttwitch white muscle (Table 2). Although it is well established that the properties of FM and SM are markedly different, it is unclear whether there exists any functional differences among the three fast isomyosins. Thus any changes observed in the fast isomyosin pool of white muscle should be interpreted with caution. Furthermore, it is likely that as a training program progresses, the continued recruitment of fast white muscle becomes doubtful (3). This raises the question as to what patterns of isomyosin adaptations can be induced in the fast white muscle, which does not appear to express either SM or IM. It is felt, therefore, that once the pattern of isomyosin expression is established in adult fast white

SKELETAL

MUSCLE

skeletal muscle, it is unresponsive to further modification by the typical patterns of locomotion normally imposed on the animal. In the context of these findings on fast white muscle, it is interesting that chronic thyroid treatment, sufficient to exert dramatic effects on cardiac function as well as induce changes in isomyosin expression on slow red and fast red muscle, exerted no effect on fast white muscle (7). This further suggests that fast white muscle is relatively insensitive to the typical stimuli that exert marked effects on other types of muscle. In summary, a lo-wk endurance training program of moderate intensity was able to significantly alter the isomyosin expression of adult rodent hindlimb skeletal musculature. Those muscles primarily recruited during sustained locomotor activity were apparently the most responsive to altering their inherent isomyosin profile, with transitions occurring from the primarily FM pool toward an IM bias. This research was supported by National Arthritis and Musculoskeletal and Skin Diseases Institute Grant AR-30346-07. Present address for D. P. Fitzsimons: Dept. of Physiology, University of Texas Southwestern Medical Center, Dallas, TX 75235. Address for reprint requests: K. M. Baldwin, Dept. of Physiology and Biophysics, Medical Sciences I, Bldg. D, Room 340, University of California, Irvine, CA 92717. Received 22 June 1989; accepted in final form 19 December

1989.

REFERENCES 1. ARIANO, M. A., R. B. ARMSTRONG, AND V. R. EDGERTON. Hindlimb muscle fiber populations of five mammals. J. Histochem. Cytochem. 21: 51-55, 1973. 2. ARMSTRONG, R. B., AND M. H. LAUGHLIN. Exercise blood flow patterns within and among rat muscles after training. Am. J. Physiol. 246 (Heart Circ. Physiol. 15): H59-H68, 1984. 3. BALDWIN, K. M., J. S. REITMAN, R. L. TERJUNG, W. W. WINDER, AND J. 0. HOLLOSZY. Substrate depletion in different types of muscle and in liver during prolonged running. Am. J. Physiol. 225: 1045-1050,1973. 4. BALDWIN, K. M., W. W. WINDER, AND J. 0. HOLLOSZY. Adaptation of actomyosin ATPase in different types of muscle to endurance exercise. Am. J. Physiol. 229: 422-426, 1975. 5. BLOMQVIST, C. G., AND B. SALTIN. Cardiovascular adaptations to physical training. Annu. Reu. Physiol. 45: 169-189, 1983. 6. FITZSIMONS, D. P., P. W. BODELL, R. E. HERRICK, AND K. M. BALDWIN. Left ventricular functional capacity in the endurance trained rodent. J. Appl. Physiol. In press. 7. FITZSIMONS, D. P., R. E. HERRICK, AND K. M. BALDWIN. ISOmyosin distributions in rodent skeletal muscles: effects of altered thyroid state. J. Appl. Physiol. In press. 8. GORNALL, A. G., C. J. BARDAWILL, AND M. M. DAVID. Determination of serum proteins by means of the biuret method. J. Biol. Chem. 177: 751-756,1949. 9. GREEN, H. J., G. A. KLUG, H. REICHMANN, U. SEEDORF, W. WIEHRER, AND D. PETTE. Exercise-induced fiber type transitions with regard to myosin, parvalbumin, and sarcoplasmic reticulum in muscles of the rat. Pfluegers Arch. 400: 432-438,1984. 10. GREEN, H. J., H. REICHMANN, AND D. PETTE. Fiber type specific transformations in the enzyme activity pattern of rat vastus lateralis muscle by prolonged endurance training. Pfluegers Arch. 399: 216-222,1983. 11. GREGORY, P., R. B. Low, AND W. S. STIREWALT. Changes in skeletal-muscle myosin isozymes with hypertrophy and exercise. Biochem. J. 238: 55-63, 1986. 12. HOH, J. F. Y., P. A. MCGRATH, AND P. T. HALE. Electrophoretic analysis of multiple forms of rat cardiac myosin: effects of hypophysectomy and thyroxine replacement. J. Mol. Cell Cardiol. 10: 1053-1076,1978. 13. HOLLOSZY, J. O., AND F. W. BOOTH. Biochemical adaptations to


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14. LAEMMLI, U. K. Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature Lord. 227: 680-685, 1970. 15. LAUGHLIN, M. H., AND R. B. ARMSTRONG. Muscular blood flow distribution patterns as a function of running speed in rats. Am. J. Physiol. 243 (Heart Circ. Physiol. 12): H296-H306, 1982. 16. THOMASON, D. B., K. M. BALDWIN, AND R. E. HERRICK. Myosin isozyme distribution in rodent hindlimb skeletal muscle. J. Appl. Physiol. 60: 1923-1931, 1986.

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17. TSIKA, R. W., R. E. HERRICK, AND K. M. BALDWIN. Subunit composition of rodent isomyosins and their distribution in hindlimb skeletal muscles. J. Appl. Physiol. 63: 2101-2110, 1987. 18. TSIKA, R. W., R. E. HERRICK, AND K. M. BALDWIN. Time course adaptations in rat skeletal muscle isomyosins during compensatory growth and regression. J. AppZ. Physiol. 63: 2111-2121, 1987. 19. WALMSLEY, B., J. A. HODGSON, AND R. E. BURKE. Forces produced by medial gastrocnemius and soleus muscles during locomotion in freely moving cats. J. Neurophysiol. 41: 1203-1216,1978. 20. ZAR, J. H. Biostatisticd Analysis (2nd ed.). Englewood Cliffs, NJ: Prentice-Hall, 1984, p. 181-183.


Effects of exercise and food restriction on rat skeletal muscles.

Isomyosin distributions in rodent muscles: effects of altered thyroid state.

Comparative changes in antioxidant enzymes and oxidative stress in cardiac, fast twitch and slow twitch skeletal muscles following endurance exercise training.

Effects of zero gravity on myofibril content and isomyosin distribution in rodent skeletal muscle.

Power outputs of slow and fast skeletal muscles of mice.

Facilitating effects of fast and slope walking on paraspinal muscles.

The effects of aerobic exercise on skeletal muscle metabolism, morphology and in situ endurance in diabetic rats.

Effects of endurance exercise on mitochondrial function in mice.

Influence of innervation on molecular forms of acetylcholinesterase in regenerating fast and slow skeletal muscles.

Nuclear factor (erythroid derived 2)-like 2 activation increases exercise endurance capacity via redox modulation in skeletal muscles.

Effects of moderate- and high-intensity chronic exercise on brain-derived neurotrophic factor expression in fast and slow muscles.

Neural influence on the expression of acetylcholinesterase molecular forms in fast and slow rabbit skeletal muscles.

Effects of short-term endurance exercise training on acute doxorubicin-induced FoxO transcription in cardiac and skeletal muscle.

Endurance exercise effects on cardiac hypertrophy in mice.

Fiber types and metabolic potentials of skeletal muscles in sedentary man and endurance runners.

A model preparation for studying fast mammalian skeletal muscles (39871).

LDH isozymes in skeletal muscles of endurance and strength trained athletes.

Effect of endurance exercise on microRNAs in myositis skeletal muscle-A randomized controlled study.

Free radical formation after intensive exercise in thoroughbred skeletal muscles.

MicroRNA-23a has minimal effect on endurance exercise-induced adaptation of mouse skeletal muscle.

Forces and powers of slow and fast skeletal muscles in mice during repeated contractions.

Dynamic properties of fast and slow skeletal muscles in the cat and rat following cross-reinnervation.

Relative capabilities of sarcoplasmic reticulum in fast and slow mammalian skeletal muscles.

Evaluation of RNA polymerase activities in nuclei isolated from slow and fast skeletal muscles.