European Journal of Pharmacology, 31 (1975) 287--291 © North-Holland Publishing Company, Amsterdam -- Printed in The Netherlands
EFFECT OF SODIUM NITROPRUSSIDE ON MYOCARDIAL PERFORMANCE AND VENOUS TONE RICHARD GMEINER, J O R G R I E D L and H O L G E R B A U M G A R T N E R
Department of Internal Medicine and Department of Pharmacology, University of Innsbruck, Innsbruck, Austria Received 16 July 1974, revised MS received 9 December 1974, accepted 6 January 1975
R. GMEINER, J. R I E D L and H. BAUMGARTNER, Effect of sodium nitroprusside on myocardialperformance and venous tone, European J. Pharmacol. 31 (1975) 287--291. The action of sodium nitroprusside upon the mechanical performance of the isolated, working rat heart and on isolated bull veins has been studied. Cumulative concentrations of 1--1000 ng sodium nitroprusside per ml had no effect on left ventricular pressure, end-diastolic left ventricular pressure, dp/dtmax, cardiac output and calculated wall stiffness of isolated rat hearts with constant rate, preload and afterload. Isolated strips of bull veins contracted with noradrenaline showed a dose dependent relaxation by sodium nitroprusside in concentrations of 5--1000 ng per ml. This study shows that sodium nitroprusside has no direct effect on myocardial contractility and causes venous relaxation. Myocardial contractility Vasodilating substances
Bull veins, isolated
1. Introduction Sodium nitroprusside has been employed in the t r e a t m e n t of hypertensive states for m a n y years (Ahearn and Grim, 1974; Mani, 1971; Page et al., 1955; Schlant et al., 1962). Recently, its therapeutic application was extended to the t r e a t m e n t of left ventricular failure complicating acute myocardial infarction (Chatterjee et al., 1973a; Franciosa et al., 1972), mitral regurgitation due to dysfunction of the subvalvular apparatus (Chatterjee et al., 1973b) and chronic left ventricular failure (Guiha et al., 1974). In all patients studied, a fall in left ventricular filling pressure was noted (Chatterjee et al., 1973a; Chatterjee et al., 1973b; Franciosa et al., 1972; Guiha et al., 1974). Cardiac o u t p u t rose only in patients with low cardiac index (Chatterjee et al., 1973a; Franciosa et al., 1972). The latter effect can be explained by the decreased afterload due to the dilating action of sodium
Nitroprusside
Rat heart, isolated
Venous tone
nitroprusside on the arterial bed (Cohn, 1973; Johnson, 1929). The fall in left ventricular filling pressure cannot be explained by reduced afterload exclusively since this effect was even apparent when no alteration in arterial pressure was observed, suggesting additional actions of sodium nitroprusside. A positive inotropic effect, an increase in myocardial compliance or reduced venous return due to venodilation would offer explanations for the fall in enddiastolic pressure (Franciosa et al., 1972). The present experiments were designed to study these possibilities and to separate the effects of sodium nitroprusside on the myocardium from its effects on the venous tone. Isolated preparations were employed for this purpose. Increasing doses of sodium nitroprusside were added to isolated working rat hearts with constant rate, preload and afterload and to isolated bull veins contracted with noradrenaline.
288 2. Materials and methods 2.1. Isolated rat hearts 2.1.1. Perfusion system and perfusion medium The perfusion system is a recirculating, isolated, working rat heart apparatus of the type described by Neely et al. (1967}. In this apparatus the heart is perfused through the left atrium and ejects fluid against a hydrostatic pressure. The atrial reservoir was 10 cm, the aortic reservoir 70 cm above the heart. The perfusion medium was a modified Krebs-Henseleit buffer (2.5 mM CaCl: and 0.5 mM EDTA Ca • Na: ) (Krebs and Henseleit, 1932}. Continuous gassing with 95% 0 2 - - 5 % CO2 led to oxygen tensions near 500 mm Hg and a pH close to 7.4 at 37°C. The buffer used for preperfusion contained no substrate, the recirculated buffer 5.5 mM glucose. 2.1.2. Experimental procedure Male Sprague--Dawley rats which had free access to food and water were anesthetized with 60 mg/kg pentobarbital ( N e m b u t a l ° ) , prior to removal of their hearts. The b o d y weight ranged from 320 to 480 g, the heart dry weight from 0.183 to 0.233 g. Following a 5 min preperfusion with substrate-free buffer the hearts were perfused for an additional 60 min in the recirculation system. All hearts were paced from the right atrium. When a steady state performance was achieved, cumulative doses of sodium nitroprusside (Merck AG, Darmstadt) were added to the atrial reservoir at final concentrations of 1, 10, 100 and 1000 ng per ml perfusate. Finally, 1 pg glucagon (Lilly) per ml perfusate was added. All substances were dissolved in the perfusion medium. Left ventricular pressure was monitored via a 20 cm fluid-filled stiff polyethylene catheter which pierced the wall of the left ventricle and was attached to a Statham transducer. The rate of pressure rise (dp/dt) was obtained by an RC-differentiating circuit (Elema-Sch~nander EMT 63). Aortic flow was estimated by collecting the
R. GMEINER ET AL. overflow from the aortic reservoir, coronary flow was obtained b y collecting the fluid dripping out of the pulmonary artery. Aortic flow plus coronary flow formed cardiac output. Wall stiffness (Ap/AV) was calculated by the difference between enddiastolic and initial diastolic pressure divided by the stroke volume. Details of the procedure and the calculations can be found elsewhere (Gmeiner, 1974}. 2.2. Isolated bull veins 2.2.1. Experimental procedure Segments of the metacarpal veins were removed from bulls, placed in Krebs--Henseleit (Krebs and Henseleit, 1932) solution at ambient temperature immediately after death and transported to the laboratory. They were then stored at 4°C for no longer than 24 hr until helical strips approximately 40 mm long and 5 mm wide were prepared according to the m e t h o d of Furchgott and Bhadrakom (1953). The strips were then placed in a 10 ml organ bath filled with Krebs--Henseleit buffer, bubbled with 95% O2--5% CO2 at 37°C. For isotonic recordings one end of the strip was attached to a fixed glass pin in the bath and the other to a lever. The isotonic lever was of the ink writing gravity type counterweighed to exert 1 g tension on the strip (Furchgott and Bhadrakom, 1953; O'Mahony, 1963). An equilibration period of 60 min was allowed prior to the addition of drug solutions. In preliminary experiments 1000 ng sodium nitroprusside per ml produced only inconsistent relaxation of the strips. Therefore in all consecutive experiments the effect of each sodium nitroprusside dose was tested after contraction of the strip with a standard dose of 100 ng noradrenaline (British Drug House, Ltd.) per ml. Following the addition of sodium nitroprusside the organ bath was flushed repeatedly. A time allowance of 15 min each was given for contraction, for each sodium nitroprusside dose and for washing out. Sodium nitroprusside doses of 5, 10, 20,
NITROPRUSSIDE, HEART FUNCTION AND VENOUS TONE
289
40, 80, 160 and 1000 ng per ml bathing solution were given at r a n d o m t h r o u g h o u t each experiment.
6O
3. Results
40
Fig. 1 shows the mechanical performance of the isolated rat heart experiments. The values presented are the means of 8 hearts -+ the standard error of the mean during the control period (C), and 5 min after the addition of sodium nitroprusside (NP) and glucagon (G). Left ventricular pressure, d p / d t and heart rate were monitored continuously, cardiac o u t p u t was determined during the fifth
developed I.v. pressure
::Em:Hgi 5,O
mmHg/sec t 3000 2000L z i 450 beats/rain 40Ot 4000.
max.
heart rate
cardiac
ootput
350t
300L z I 70 ml/rnin 6054 I T I
i
i
/
I
I
T
Zo
~P/~'v
5,o6'f ° z ko
I C
I 1
'
-3
-1
1
3
5
10 100 ~ I ng/ml NP
~00 " I
I
G
Fig. 1. Dynamic performance of isolated rat hearts (n = 8). Values are means ± S.E.M. of controls (C), 1 to 1000 ng/ml sodium nitroprusside (NP) and 1 ug/ml glucagon (G).
I
Fig. 2. Relaxation of isolated bull vein strips (n = 12). The strips were contracted by 100 ng/ml noradrenaline and relaxed with different doses of sodium nitroprusside. The ordinate gives the % relaxation of noradrenaline contraction. The abscissa represents doses of sodium nitroprusside after logarithmic transformation. A linear regression was performed with the logarithmic doses and found to be valid (p =
0.05).
endd~astolic Z0I I.v.pressure 6,0
dp/dt
-5
min after the addition of each dose of sodium nitroprusside. Developed left ventricular pressure, end-diastolic left ventricular pressure, dp/dtmax, the paced heart rate, cardiac output and AP/AV were n o t altered by sodium nitroprusside over the dose range tested. Glucagon, however, increased the developed pressure, dp/dtm ax and reduced the end-diastolic left ventricular pressure. Parasystoles appeared on the pressure tracings after the addition of glucagon. After turning o f f the artificial pacemaker the endogenous heart rate was markedly increased. Within 20 sec after the administration all glucagon effects were evident. Fig. 2 shows the effects of sodium nitroprusside on isolated bull vein strips. Sodium nitroprusside exerted a dose-related diminution of the contraction induced by noradrenaline. In addition to the concentrations shown in fig. 2 a dose of 1000 ng sodium nitroprusside per ml was tested and found to induce a relaxation of 88.0 + 6.7% (n = 12). This value seems to fit well to an extension of the linear regression line (see legend to fig. 2).
290
4. Discussion Our experiments were designed to evaluate the direct effects of sodium nitroprusside on the mechanical performance of the heart and on the venous tone. Isolated preparations were employed for this purpose in order to avoid neurohumoral counterregulations which might be expected after the administration of a p o t e n t hypotensive agent to the whole animal. A baroreceptor-mediated increase in sympathetic nervous tone and increased levels of catecholamines might affect heart rate, myocardial contractility and venous distensibility. In addition alterations of preload and afterload might obscure direct myocardial effects. In our isolated rat hearts the rate was kept constant by right atrial pacing. Under these conditions no direct positive chronotropic action of sodium nitroprusside was observed. Glucagon increased the endogenous rate above the pacing rate. Afterload was maintained constant by the overflow design of the perfusion apparatus {Gmeiner, 1974). The difficulty of assessing preload in this type of experiment has recently been discussed in more detail (Gmeiner, 1974). However, some inferences are possible from our data. Each heart served as its own control. This rules out major variations in initial diastolic volume. The volume change occurring during each diastole can be assumed to be equal to stroke volume under steady state conditions. This volume change was related to the change in pressure and used as a measure of wall stiffness (Gmeiner, 1974). This index remained almost constant during the administration of sodium nitroprusside. Furthermore, at a constant atrial filling pressure sodium nitroprusside did n o t alter the end-diastolic left ventricular pressure. These arguments strongly suggest that the preload remained constant. At these conditions of constant rate, preload and afterload dp/dtmax is a valid parameter of contractility (Mason, 1969}. A wide dose range of sodium nitroprusside did n o t influence the maximum dp/dt. The sensitivity
R. G M E I N E R ET AL.
of our set-up to inotropic interventions was established by the marked effects of glucagon. Other parameters of left ventricular performance such as developed left ventricular pressure and cardiac o u t p u t were not influenced by sodium nitroprusside. Hence sodium nitroprusside did n o t alter the contractile state of the heart. Thus our studies extend and support experiments in isolated cat papillary muscles (Chatterjee et al., 1973a). Doses of sodium nitroprusside which were ineffective on the contractility of the isolated rat heart relaxed strips of isolated bull veins contracted with noradrenaline. Without noradrenaline pretreatment only feeble actions of sodium nitroprusside were observed. This observation is in agreement with similar results reported in an investigation of other vein dilating substances (Sutter, 1965). Before contraction with noradrenaline, isolated strips of bull veins appear to be almost completely relaxed and are therefore less suitable to demonstrate any additional dilating effect. However, increased sympathetic nervous tone or elevated levels of catecholamines are present in acute myocardial infarction and in left ventricular failure (Chidsey and Braunwald, 1966; McDonald et al., 1969). These are the main indications for sodium nitroprusside therapy. Therefore the contraction of isolated vein strips by noradrenaline may simulate the venous contraction in pathologic states. Our study shows that sodium nitroprusside relaxes the contracted vein strips in a dose-dependent manner. The clinical implications of these experiments remain a matter of speculation. It is not known whether venodilatation is important for the therapeutic action of nitroprusside in left ventricular failure (Guiha et al., 1974). However it is conceivable that nitroprusside may increase venous pooling by relaxing systemic veins and thereby reduce venous return to the right heart. This would diminish the o u t p u t of the right ventricle ultimately reducing pulmonary artery and capillary pressure. It has n o t been shown to our knowledge whether nitroprusside directly
NITROPRUSSIDE, HEART FUNCTION AND VENOUS TONE
relaxes the pulmonary arteries and veins as well. Peripheral venodilatation may significantly reduce central blood volume and thereby alleviate the pulmonary congestion of left heart failure.
Acknowledgement This study was supported by the ' F o n d s zur Forderung der wissenschaftlichen Forschung in Osterreich'.
References Ahearn, D.J. and C.E. Grim, 1974, Treatment of malignant hypertension with sodium nitroprusside, Arch. Intern. Med. 133, 187. Chatterjee, K., W.W. Parmley, W. Ganz, J. Forrester, P. Walinsky, C. Crexalls and H.J.C. Swan, 1973a, Hemodynamic and metabolic responses to vasodilator therapy in acute myocardial infarction, Circulation 48, 1183. Chatterjee, K., W.W. Parmley, H.J.C. Swan, G. Berman, J. Forrester and H.S. Marcus, 1973b, Beneficial effects of vasodilator agents in severe mitral regurgitation due to dysfunction of subvalvar apparatus, Circulation 47, 684. Chidsey, C.A. and E. Braunwald, 1966, Sympathetic activity and neurotransmitter depletion in congestive heart failure, Pharmacol. Rev. 18, 685. Cohn, J.N., 1973, Blood pressure and cardiac performance, Amer. J. Med. 55, 351. Franciosa, J.A., N.H. Guiha, C.J. Limas, E. Rodriguera and J.N. Cohn, 1972, Improved left ventricular function during nitroprusside infusion in acute myocardial infarction, Lancet 25, 650. Furchgott, R.F. and S. Bhadrakom, 1953, Reactions
291
of strips of rabbit aorta to epinephrine isoproterenol, sodium nitrite and other drugs, J. Pharm. Exptl. Therap. 108, 129. Gmeiner, R., 1974, Effect of nitroglycerin on the mechanical and metabolic performance of the isolated aerobic and hypoxic rat heart, European J. Cardiol. 2/1, 47. Guiha, N.H., J.N. Cohn, E. Mikulic, J.A. Franciosa and C.J. Limas, 1974, Treatment of refractory heart failure with infusion of nitroprusside, N. Engl. J. Med. 291, 587. Johnson, C.C., 1929, The actions and toxicity of sodium nitroprusside, Arch. Intern. Pharmacodyn. Therap. 35, 480. Krebs, H.A. and K. Henseleit, 1932, Untersuchungen i]ber die Harnstoffbildung im TierkSrper, HoppeSeyler's Z. Physiol. 210, 33. Mani, M.K., 1971, Nitroprusside revisited, Brit. Med. J. 3,407. Mason, D.T., 1969, Usefulness and limitations of the rate of rise of intraventricular pressure (dp/dt) in the evaluation of myocardial contractility in man, Amer. J. Cardiol. 23, 516. McDonald, L., C. Baker, C. Bray, A. McDonald, N. Restieaux, 1969, Plasma catecholamines after cardiac infarction, Lancet 2, 1021. Neely, J.R., H. Liebermeister, E.J. Battersby and H.E. Morgan, 1967, Effect of pressure development on oxygen consumption" by isolated rat heart, Amer. J. Physiol. 212, 804. O'Mahony, 1963, The pharmacology of isolated venous muscle, Irish J. Med. Sci. 6, 401. Page, I.H., A.C. Corcoran, H.P. Dustan and T. Koppanyi, 1955, Cardiovascular actions of sodium nitroprusside in animals and hypertensive patients, Circulation 11,188. Schlant, R.C., T.S. Tsagiris and R.J. Robertson, 1962, Studies on acute cardiovascular effects of intravenous sodium nitroprusside, Amer. J.Cardiol. 9, 51. Sutter, M.C., 1965, The pharmacology of isolated veins, Brit. J. Pharmacol. 24, 742.
EFFECT OF SODIUM NITROPRUSSIDE ON MYOCARDIAL PERFORMANCE AND VENOUS TONE RICHARD GMEINER, J O R G R I E D L and H O L G E R B A U M G A R T N E R
Department of Internal Medicine and Department of Pharmacology, University of Innsbruck, Innsbruck, Austria Received 16 July 1974, revised MS received 9 December 1974, accepted 6 January 1975
R. GMEINER, J. R I E D L and H. BAUMGARTNER, Effect of sodium nitroprusside on myocardialperformance and venous tone, European J. Pharmacol. 31 (1975) 287--291. The action of sodium nitroprusside upon the mechanical performance of the isolated, working rat heart and on isolated bull veins has been studied. Cumulative concentrations of 1--1000 ng sodium nitroprusside per ml had no effect on left ventricular pressure, end-diastolic left ventricular pressure, dp/dtmax, cardiac output and calculated wall stiffness of isolated rat hearts with constant rate, preload and afterload. Isolated strips of bull veins contracted with noradrenaline showed a dose dependent relaxation by sodium nitroprusside in concentrations of 5--1000 ng per ml. This study shows that sodium nitroprusside has no direct effect on myocardial contractility and causes venous relaxation. Myocardial contractility Vasodilating substances
Bull veins, isolated
1. Introduction Sodium nitroprusside has been employed in the t r e a t m e n t of hypertensive states for m a n y years (Ahearn and Grim, 1974; Mani, 1971; Page et al., 1955; Schlant et al., 1962). Recently, its therapeutic application was extended to the t r e a t m e n t of left ventricular failure complicating acute myocardial infarction (Chatterjee et al., 1973a; Franciosa et al., 1972), mitral regurgitation due to dysfunction of the subvalvular apparatus (Chatterjee et al., 1973b) and chronic left ventricular failure (Guiha et al., 1974). In all patients studied, a fall in left ventricular filling pressure was noted (Chatterjee et al., 1973a; Chatterjee et al., 1973b; Franciosa et al., 1972; Guiha et al., 1974). Cardiac o u t p u t rose only in patients with low cardiac index (Chatterjee et al., 1973a; Franciosa et al., 1972). The latter effect can be explained by the decreased afterload due to the dilating action of sodium
Nitroprusside
Rat heart, isolated
Venous tone
nitroprusside on the arterial bed (Cohn, 1973; Johnson, 1929). The fall in left ventricular filling pressure cannot be explained by reduced afterload exclusively since this effect was even apparent when no alteration in arterial pressure was observed, suggesting additional actions of sodium nitroprusside. A positive inotropic effect, an increase in myocardial compliance or reduced venous return due to venodilation would offer explanations for the fall in enddiastolic pressure (Franciosa et al., 1972). The present experiments were designed to study these possibilities and to separate the effects of sodium nitroprusside on the myocardium from its effects on the venous tone. Isolated preparations were employed for this purpose. Increasing doses of sodium nitroprusside were added to isolated working rat hearts with constant rate, preload and afterload and to isolated bull veins contracted with noradrenaline.
288 2. Materials and methods 2.1. Isolated rat hearts 2.1.1. Perfusion system and perfusion medium The perfusion system is a recirculating, isolated, working rat heart apparatus of the type described by Neely et al. (1967}. In this apparatus the heart is perfused through the left atrium and ejects fluid against a hydrostatic pressure. The atrial reservoir was 10 cm, the aortic reservoir 70 cm above the heart. The perfusion medium was a modified Krebs-Henseleit buffer (2.5 mM CaCl: and 0.5 mM EDTA Ca • Na: ) (Krebs and Henseleit, 1932}. Continuous gassing with 95% 0 2 - - 5 % CO2 led to oxygen tensions near 500 mm Hg and a pH close to 7.4 at 37°C. The buffer used for preperfusion contained no substrate, the recirculated buffer 5.5 mM glucose. 2.1.2. Experimental procedure Male Sprague--Dawley rats which had free access to food and water were anesthetized with 60 mg/kg pentobarbital ( N e m b u t a l ° ) , prior to removal of their hearts. The b o d y weight ranged from 320 to 480 g, the heart dry weight from 0.183 to 0.233 g. Following a 5 min preperfusion with substrate-free buffer the hearts were perfused for an additional 60 min in the recirculation system. All hearts were paced from the right atrium. When a steady state performance was achieved, cumulative doses of sodium nitroprusside (Merck AG, Darmstadt) were added to the atrial reservoir at final concentrations of 1, 10, 100 and 1000 ng per ml perfusate. Finally, 1 pg glucagon (Lilly) per ml perfusate was added. All substances were dissolved in the perfusion medium. Left ventricular pressure was monitored via a 20 cm fluid-filled stiff polyethylene catheter which pierced the wall of the left ventricle and was attached to a Statham transducer. The rate of pressure rise (dp/dt) was obtained by an RC-differentiating circuit (Elema-Sch~nander EMT 63). Aortic flow was estimated by collecting the
R. GMEINER ET AL. overflow from the aortic reservoir, coronary flow was obtained b y collecting the fluid dripping out of the pulmonary artery. Aortic flow plus coronary flow formed cardiac output. Wall stiffness (Ap/AV) was calculated by the difference between enddiastolic and initial diastolic pressure divided by the stroke volume. Details of the procedure and the calculations can be found elsewhere (Gmeiner, 1974}. 2.2. Isolated bull veins 2.2.1. Experimental procedure Segments of the metacarpal veins were removed from bulls, placed in Krebs--Henseleit (Krebs and Henseleit, 1932) solution at ambient temperature immediately after death and transported to the laboratory. They were then stored at 4°C for no longer than 24 hr until helical strips approximately 40 mm long and 5 mm wide were prepared according to the m e t h o d of Furchgott and Bhadrakom (1953). The strips were then placed in a 10 ml organ bath filled with Krebs--Henseleit buffer, bubbled with 95% O2--5% CO2 at 37°C. For isotonic recordings one end of the strip was attached to a fixed glass pin in the bath and the other to a lever. The isotonic lever was of the ink writing gravity type counterweighed to exert 1 g tension on the strip (Furchgott and Bhadrakom, 1953; O'Mahony, 1963). An equilibration period of 60 min was allowed prior to the addition of drug solutions. In preliminary experiments 1000 ng sodium nitroprusside per ml produced only inconsistent relaxation of the strips. Therefore in all consecutive experiments the effect of each sodium nitroprusside dose was tested after contraction of the strip with a standard dose of 100 ng noradrenaline (British Drug House, Ltd.) per ml. Following the addition of sodium nitroprusside the organ bath was flushed repeatedly. A time allowance of 15 min each was given for contraction, for each sodium nitroprusside dose and for washing out. Sodium nitroprusside doses of 5, 10, 20,
NITROPRUSSIDE, HEART FUNCTION AND VENOUS TONE
289
40, 80, 160 and 1000 ng per ml bathing solution were given at r a n d o m t h r o u g h o u t each experiment.
6O
3. Results
40
Fig. 1 shows the mechanical performance of the isolated rat heart experiments. The values presented are the means of 8 hearts -+ the standard error of the mean during the control period (C), and 5 min after the addition of sodium nitroprusside (NP) and glucagon (G). Left ventricular pressure, d p / d t and heart rate were monitored continuously, cardiac o u t p u t was determined during the fifth
developed I.v. pressure
::Em:Hgi 5,O
mmHg/sec t 3000 2000L z i 450 beats/rain 40Ot 4000.
max.
heart rate
cardiac
ootput
350t
300L z I 70 ml/rnin 6054 I T I
i
i
/
I
I
T
Zo
~P/~'v
5,o6'f ° z ko
I C
I 1
'
-3
-1
1
3
5
10 100 ~ I ng/ml NP
~00 " I
I
G
Fig. 1. Dynamic performance of isolated rat hearts (n = 8). Values are means ± S.E.M. of controls (C), 1 to 1000 ng/ml sodium nitroprusside (NP) and 1 ug/ml glucagon (G).
I
Fig. 2. Relaxation of isolated bull vein strips (n = 12). The strips were contracted by 100 ng/ml noradrenaline and relaxed with different doses of sodium nitroprusside. The ordinate gives the % relaxation of noradrenaline contraction. The abscissa represents doses of sodium nitroprusside after logarithmic transformation. A linear regression was performed with the logarithmic doses and found to be valid (p =
0.05).
endd~astolic Z0I I.v.pressure 6,0
dp/dt
-5
min after the addition of each dose of sodium nitroprusside. Developed left ventricular pressure, end-diastolic left ventricular pressure, dp/dtmax, the paced heart rate, cardiac output and AP/AV were n o t altered by sodium nitroprusside over the dose range tested. Glucagon, however, increased the developed pressure, dp/dtm ax and reduced the end-diastolic left ventricular pressure. Parasystoles appeared on the pressure tracings after the addition of glucagon. After turning o f f the artificial pacemaker the endogenous heart rate was markedly increased. Within 20 sec after the administration all glucagon effects were evident. Fig. 2 shows the effects of sodium nitroprusside on isolated bull vein strips. Sodium nitroprusside exerted a dose-related diminution of the contraction induced by noradrenaline. In addition to the concentrations shown in fig. 2 a dose of 1000 ng sodium nitroprusside per ml was tested and found to induce a relaxation of 88.0 + 6.7% (n = 12). This value seems to fit well to an extension of the linear regression line (see legend to fig. 2).
290
4. Discussion Our experiments were designed to evaluate the direct effects of sodium nitroprusside on the mechanical performance of the heart and on the venous tone. Isolated preparations were employed for this purpose in order to avoid neurohumoral counterregulations which might be expected after the administration of a p o t e n t hypotensive agent to the whole animal. A baroreceptor-mediated increase in sympathetic nervous tone and increased levels of catecholamines might affect heart rate, myocardial contractility and venous distensibility. In addition alterations of preload and afterload might obscure direct myocardial effects. In our isolated rat hearts the rate was kept constant by right atrial pacing. Under these conditions no direct positive chronotropic action of sodium nitroprusside was observed. Glucagon increased the endogenous rate above the pacing rate. Afterload was maintained constant by the overflow design of the perfusion apparatus {Gmeiner, 1974). The difficulty of assessing preload in this type of experiment has recently been discussed in more detail (Gmeiner, 1974). However, some inferences are possible from our data. Each heart served as its own control. This rules out major variations in initial diastolic volume. The volume change occurring during each diastole can be assumed to be equal to stroke volume under steady state conditions. This volume change was related to the change in pressure and used as a measure of wall stiffness (Gmeiner, 1974). This index remained almost constant during the administration of sodium nitroprusside. Furthermore, at a constant atrial filling pressure sodium nitroprusside did n o t alter the end-diastolic left ventricular pressure. These arguments strongly suggest that the preload remained constant. At these conditions of constant rate, preload and afterload dp/dtmax is a valid parameter of contractility (Mason, 1969}. A wide dose range of sodium nitroprusside did n o t influence the maximum dp/dt. The sensitivity
R. G M E I N E R ET AL.
of our set-up to inotropic interventions was established by the marked effects of glucagon. Other parameters of left ventricular performance such as developed left ventricular pressure and cardiac o u t p u t were not influenced by sodium nitroprusside. Hence sodium nitroprusside did n o t alter the contractile state of the heart. Thus our studies extend and support experiments in isolated cat papillary muscles (Chatterjee et al., 1973a). Doses of sodium nitroprusside which were ineffective on the contractility of the isolated rat heart relaxed strips of isolated bull veins contracted with noradrenaline. Without noradrenaline pretreatment only feeble actions of sodium nitroprusside were observed. This observation is in agreement with similar results reported in an investigation of other vein dilating substances (Sutter, 1965). Before contraction with noradrenaline, isolated strips of bull veins appear to be almost completely relaxed and are therefore less suitable to demonstrate any additional dilating effect. However, increased sympathetic nervous tone or elevated levels of catecholamines are present in acute myocardial infarction and in left ventricular failure (Chidsey and Braunwald, 1966; McDonald et al., 1969). These are the main indications for sodium nitroprusside therapy. Therefore the contraction of isolated vein strips by noradrenaline may simulate the venous contraction in pathologic states. Our study shows that sodium nitroprusside relaxes the contracted vein strips in a dose-dependent manner. The clinical implications of these experiments remain a matter of speculation. It is not known whether venodilatation is important for the therapeutic action of nitroprusside in left ventricular failure (Guiha et al., 1974). However it is conceivable that nitroprusside may increase venous pooling by relaxing systemic veins and thereby reduce venous return to the right heart. This would diminish the o u t p u t of the right ventricle ultimately reducing pulmonary artery and capillary pressure. It has n o t been shown to our knowledge whether nitroprusside directly
NITROPRUSSIDE, HEART FUNCTION AND VENOUS TONE
relaxes the pulmonary arteries and veins as well. Peripheral venodilatation may significantly reduce central blood volume and thereby alleviate the pulmonary congestion of left heart failure.
Acknowledgement This study was supported by the ' F o n d s zur Forderung der wissenschaftlichen Forschung in Osterreich'.
References Ahearn, D.J. and C.E. Grim, 1974, Treatment of malignant hypertension with sodium nitroprusside, Arch. Intern. Med. 133, 187. Chatterjee, K., W.W. Parmley, W. Ganz, J. Forrester, P. Walinsky, C. Crexalls and H.J.C. Swan, 1973a, Hemodynamic and metabolic responses to vasodilator therapy in acute myocardial infarction, Circulation 48, 1183. Chatterjee, K., W.W. Parmley, H.J.C. Swan, G. Berman, J. Forrester and H.S. Marcus, 1973b, Beneficial effects of vasodilator agents in severe mitral regurgitation due to dysfunction of subvalvar apparatus, Circulation 47, 684. Chidsey, C.A. and E. Braunwald, 1966, Sympathetic activity and neurotransmitter depletion in congestive heart failure, Pharmacol. Rev. 18, 685. Cohn, J.N., 1973, Blood pressure and cardiac performance, Amer. J. Med. 55, 351. Franciosa, J.A., N.H. Guiha, C.J. Limas, E. Rodriguera and J.N. Cohn, 1972, Improved left ventricular function during nitroprusside infusion in acute myocardial infarction, Lancet 25, 650. Furchgott, R.F. and S. Bhadrakom, 1953, Reactions
291
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