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On the mechanism of the coronary dilator effect of serotonin in the dog
European Journal o f Pharmacology, 36 (1976) 1--5 © North-Holland Publishing Company, Amsterdam -- Printed in The Netherlands
1
ON THE MECHANISM OF THE C O R O N A R Y D I L A T O R E F F E C T OF SEROTONIN IN THE DOG MARCO AURELIO MENA and HORACIO VIDRIO
Instituto Miles de Terap~utica Experimental, M~xico 22, D.F., M~xico Received 22 July 1975, revised MS received 17 September 1975, accepted 24 September 1975
M.A. MENA and H. VIDRIO, On the mechanism o f the coronary dilator effect o f serotonin in the dog, European J. Pharmacol. 36 (1976) 1--5. In experiments designed to determine the nature of the coronary dilator effect of serotonin the influence of intracoronary administration of the amine on coronary perfusion pressure, heart rate and ventricular contractile force was assessed in anesthetized, open-chest dogs in which the left coronary artery was perfused with blood at a constant rate. Serotonin elicited dose-related decreases in coronary perfusion pressure and increases in contractile force, and lowered heart rate slightly. The dilator response was antagonized by methysergide, slightly potentiated by practolol and unaffected by reserpine. The inotropic effect was partially antagonized by methysergide and completely blocked by practolol and reserpine. It is concluded that serotonin induces coronary dilatation by direct stimulation of specific receptors, that this effect is independent of the cardiac stimulation produced by the amine, and that the latter response is mediated through/31 -adrenoceptors activated by released norepinephrine. Catecholamine release Serotonin receptors
Myocardial contraction Coronary dilatation
1. Introduction It has been repeatedly shown that serotonin produces coronary dilatation in the dog (Schofield and Walker, 1953; Schneider and Yonkman, 1954; Maxwell et al., 1959; Griggs and Case, 1960; Saxena et al., 1971; Vyden et al., 1974). Since some of these studies indicate that the amine concomitantly produces important changes in other cardiac parameters, such as heart rate, contractility and oxygen consumption, observed coronary dilatation could well be secondary to these effects. On the other hand, some cardiovascular actions of serotonin are known to be mediated in part through adrenergic or cholinergic mechanisms, rather than through specific serotonin receptors exclusively (Garattini and Valzelli, 1965). The present study was therefore conducted to determine the nature of the coronary dilator effect of serotonin in the dog in reference to its mediation through serotonin receptors or
Serotonin
~-Adrenoceptors
through catecholamine release as well as its dependence on other cardiac variables affected by the amine. To this effect, the influence of intracoronary administration of serotonin on coronary perfusion pressure, heart rate and ventricular contractile force was determined in normal dogs and in animals pretreated with the serotonin antagonist methysergide, the /3adrenergic blocking agent practolol and the catecholamine depletor reserpine.
2. Materials and methods Experiments were carried out in 24 mongrel dogs of either sex, weighing between 15 and 25 kg. The animals were anesthetized i.v. with 30 mg/kg of pentobarbital sodium, supplemented by additional doses as needed. Artificial respiration was maintained with a Palmer pump adjusted to deliver approximately 15 ml/kg of room air through a tracheal cannula. The left
2
femoral artery and vein wdre cannulated for recording of systemic blood pressure and for drug administration, respectively. The thorax was opened at the fourth left intercostal space and the pericardium cut and sutured to the chest wall to form a craddle for the heart. An Eckstein brass cannula (Eckstein et al., 1951), approximately 11 cm long and 0.5 cm in diameter, was introduced in the left coronary ostium via the left subclavian artery and was fixed in place with a metal clip attached externally to the left coronary trunk. Heparin, 4 mg/kg, was injected i.v. to prevent clotting. Blood obtained from the left c o m m o n carotid artery was delivered at a constant rate to the left coronary bed with a Model T6S Sigmamotor pump and pressure in the perfusion system recorded with a Statham P23AC transducer connected to a side arm at the o u t p u t portion of the pump. Perfusion pressure, which was taken as a measure of coronary resistance, was adjusted to coincide with the level of systemic blood pressure observed once the thorax had been opened and before administration of any drug; this level was maintained throughout the experiment within 10 mm Hg o f the initial value, irrespective of any subsequent changes in systemic blood pressure. Contractile force was measured with a Walton--Brodie strain gauge arch sutured to the anterior left ventricular wall, approximately 2 cm from the apex. Heart rate was recorded with a Grass tachograph triggered by the R wave of the precordial electrocardiogram. All parameters were recorded on a Grass Model 7 polygraph. After adequate stabilization of all variables for at least 20 min, responses to serotonin were obtained by injecting the amine into the rubber tubing of the perfusion system immediately upstream to the Eckstein cannula. Doses of 0.1, 1.0, 10 and 100 pg were administered at 10 min intervals to groups of 6 dogs receiving either no pretreatment, 0.5 mg/kg of methysergide, or 1.0 mg/kg of practolol. The serotonin and fl-adrenergic antagonists were administered i.v. 20 min before applying the first serotonin dose. An additional group of 6 animals was pretreated with reserpine, 0.5 mg/kg, administered i.p.
M.A. MENA, H. VIDRIO
for 2 days prior to the experiment. These dogs were anesthetized with a mean dose of 16 mg/kg of pentobarbital and coronary perfusion pressure was adjusted at 100 mm Hg, irrespective of their systemic blood pressure, which was consistently below this value. The adequacy of reserpinization was assessed by administering 0.1 mg/kg of tyramine i.v. and verifying that blood pressure, contractile force and heart rate responses were almost absent. Doses of methysergide maleate (Sandoz Pharmaceuticals), practolol hydrochloride (Ayerst Laboratories) and tyramine hydrochloride (Nutritional Biochemicals) refer to the respective salts. Doses of reserpine and serotonin creatinine sulfate (both obtained from Nutritional Biochemicals) refer to the base. All drugs, except reserpine, were dissolved in isotonic NaC1 solution; propylene glycol was used to dissolve the alkaloid. Statistical significance between control and pretreated groups were determined by Student's t-test.
3. Results Baseline values for blood pressure, coronary perfusion pressure and heart rate in the diverse groups of dogs are shown in table 1. Figures for left ventricular contractile force are not presented in the table, since this parameter was recorded by adjusting the amplitude of the tracing to a predetermined value, and no steps were taken to calibrate the strain gauge. 'Before treatment' values were similar in the control, methysergide and practolol groups, and only heart rate was decreased after treatment with either drug, significantly so in the case of methysergide. As expected, reserpinized dogs had lower blood pressure and heart rate than did control animals. Injection of serotonin into the left coronary artery produced dose-related decreases in perfusion pressure and increases in contractile force. Heart rate was slightly decreased at the two highest doses (fig. 1). These effects were apparent within 10 sec after administration of the amine. The fall in coronary resistance was
CORONARY DILATOR E F F E C T OF SEROTONIN TABLE
%A
w,A
1
Baseline values for blood pressure, coronary perfusion pressure and heart rate in control dogs and in dogs treated with methysergide, practolol or reserpine. Figures are means + S.E.M. of 6 experiments.
Group
3
30
S
20 t0"
O"
Blood pressure (ram Hg)
-iO -
Before treatment
After treatment -20-
Control Methysergide Practolol Reserpine
104.2 ± 6.76 106.7 ± 6.03 103.0 ± 6.67 --
Group
Perfusion pressure (mm Hg)
-
100.5 ± 8.54 106.3 ± 7.15 83.3 ± 6.67**
-30 "
o'.,
,.~
,~
,Go
~,
SEROTON}N DOSE (racg)
,:o
,b
,Go
,b
,Go
% o,
Before treatment
After treatment
Control Methysergide Practolol Reserpine
102.1 ± 7.97 106.0 -+ 5.30 105.3 -+ 5.55 --
105.8 ± 5.93 103.0 ± 4.62 100.8 -+ 0.54
Group
Heart rate (beats/min) Before treatment
Control Methysergide Practolol Reserpine
METHYSERGIDE
-
174.2 -+ 11.29 175.0 -+ 8.21 168.5 ± 11.01 --
-1o, -20
After treatment
141.7 ± 10.64" 142.0 -+ 9.42 122.2 ± 6.60**
* Significantly different from pretreatment value (p < 0.05). ** Significantly different from control values (p < 0.01).
antagonized by methysergide, unaffected by reserpine, and somewhat potentiated by practolol, while the positive inotropic response was blocked partially by the antiserotonin agent and completely by the other two drugs. All three agents reduced the effect on heart rate. A s s h o w n in t a b l e 2, t h e e f f e c t s o f s e r o t o nin on systemic blood pressure were negligible in the control, methysergide and reserpine groups, and no dose-related pressor or depressor responses were apparent. Dogs pretreated with practolol, however, showed consistent f a l l s in b l o o d p r e s s u r e , t h e r e s p o n s e s o b s e r v e d
-30 -40
RESERPINE
-50
,£,
G, ,;o ,'o SEROTONIN ,Go DOSE (mcg)
Fig. 1. Effects of serotonin on coronary perfusion pressure (PP), left ventricular contractile force (CF) and heart rate (HR) in the perfused dog heart in situ. Each point corresponds to the mean of 6 experiments; vertical lines indicate standard errors. Shown are serotonin responses in control dogs and in animals pretreated with 0.5 mg/kg of methysergide or 1.0 mg/kg of practolol administered i.v. 20 rain before the experiment, or with 0.5 mg/kg of reserpine injected i.p. for 2 days. Asterisks indicate responses significantly different from control (p < 0.05). Abscissae denote intracoronary doses of serotonin in pg; ordinates, per cent changes in the diverse parameters.
at 100 pg differing significantly from those of control animals.
4. D i s c u s s i o n The present results confirm the coronary dilator and positive inotropic effects of serot o n i n in t h e d o g s h o w n b y o t h e r s in d i v e r s e in v i v o a n d in v i t r o p r e p a r a t i o n s f r o m t h i s species. H o w e v e r , a n d in c o n t r a s t w i t h t h e s e
4
M.A. M E N A , H. V I D R I O
TABLE 2 S y s t e m i c b l o o d pressure e f f e c t s o f i n t r a c o r o n a r y injections o f s e r o t o n i n in c o n t r o l dogs a n d in dogs t r e a t e d with m e t h y s e r g i d e , practolol or reserpine. Figures are m e a n s -+ S.E.M. o f 6 e x p e r i m e n t s . Group
% Change in b l o o d pressure 0.1 pg
1.0 p g
Control Methysergide Practolol Reserpine
--0.7 t 0.67 0 0 0
--0.7 --0.3 --3.7 --3.7
Group
% Change in b l o o d pressure 10 p g
Control Methysergide Practolol Reserpine
--3.2 --2.1 --8.2 --4.5
± 2.32 ± 1.09 +- 1.31 ± 1.82
100 p g ± 6.28 ± 2.77 -+ 1.88 +- 3.48
+1.7 +2.8 --22.3 0
± 8:07 ± 4.47 ± 6.34* +- 8.21
* Significantly d i f f e r e n t f r o m c o n t r o l ( p < 0.05).
studies, no positive chronotropic response to the amine could be demonstrated, possibly because all injections were made into the left coronary artery, thus preventing adequate amounts of the drug from reaching the pacemaker tissue of the right atrium, which is mainly irrigated by the right coronary branch. This has been shown to be the case by Lluch and Fillion (1970), who found that serotonin produced tachycardia when injected i.v. or into the right coronary artery, but not when injected into the left branch. The fact that coronary dilatation was effectively blocked by the serotonin antagonist methysergide, b u t not by practolol or reserpine, indicates that this response is mediated by specific serotonin receptors and is not dependent on intact ~-adrenoceptors or catecholamine stores. This finding agrees with that of Saxena et al. (1971), who reported that serotonin-induced coronary dilatation could be blocked by the antiserotonin agent mianserin. On the other hand, the stimulant effect of the amine on left ventricular contractility does not appear to be mediated entirely by the
same mechanism, since it was completely abolished by practolol or reserpine and only partially inhibited by methysergide. This effect could therefore be attributable to norepinephrine released by serotonin and acting on myocardial ~,-adrenergic receptors, in a situation similar to that occurring in the rabbit atrium, in which stimulation by serotonin is abolished by reserpine or by dichloroisoproterenol (Trendelenburg, 1960). The fact that serotonin has been shown to release I norepinephrine from the dog heart (Fillion et al., 1971) supports the above contention. The dissociation of the dilator and inotropic responses to serotonin achieved through selective pharmacological blockade demonstrates, in addition, that the former is not a consequence of increased oxygen demand induced by cardiac stimulation, b u t that it is a direct effect of serotonin on the coronary vasculature. The fact that the slight lowering of heart rate produced by serotonin was significantly inhibited by methysergide, practolol and reserpine suggests a non-specific effect exerted by the three drugs, rather than a true pharmacological antagonism. In this connection, it should be pointed out that all pretreated groups had lower basal heart rates than did the control animals, and that this difference could have masked the slight bradycardia induced by the amine. An interesting observation was the apparent potentiation of serotonin-induced coronary dilatation by practolol. No plausible explanation for this phenomenon can be offered, since if any interaction between the blocker and the amine t o o k place, a reduced response to the latter would be expected, in view of the reported ability of dichloroisoproterenol to antagonize serotonin vasodilatation in the dog hind limb and kidney (McCubbin et al., 1962), and of propranolol, practolol and oxprenolol to block serotonin contractions in the isolated rat stomach and uterus (Schechter and Weinstock, 1974). It is possible that the blockade by practolol of the inotropic effect of serotonin could lead to an enhanced
CORONARY DILATOR EFFECT OF SEROTONIN v a s o d i l a t a t i o n b y eliminating the increased int r a m y o c a r d i a l t e n s i o n g e n e r a t e d b y this response, and w h i c h n o r m a l l y w o u l d t e n d to restrict c o r o n a r y flow. Such an e x p l a n a t i o n a p p e a r s u n t e n a b l e since reserpine, w h i c h also b l o c k e d t h e i n o t r o p i c effect, did n o t a f f e c t the d i l a t o r response. A n alternative possibility is t h a t practolol, b y b l o c k i n g m y o c a r d i a l illr e c e p t o r s c o u l d m a k e m o r e o f t h e norepin e p h r i n e w h i c h is released b y s e r o t o n i n available f o r s t i m u l a t i o n o f the u n b l o c k e d coron a r y adrenergic receptors. A c c o r d i n g to s o m e a u t h o r s (Parrat and W a d s w o r t h , 1 9 7 0 ; Broadley, 1970), these r e c e p t o r s are o f t h e fi:-type. T h e result w o u l d be a p o t e n t i a t i o n o f t h a t portion of the dilatation due to norepinephrine release. This h y p o t h e s i s , h o w e v e r , w o u l d n o t explain the e n h a n c e m e n t o f the s y s t e m i c d e p r e s s o r response to s e r o t o n i n observed in t h e present s t u d y .
References Broadley, K.J., 1970, An analysis of the coronary vascular responses to catecholamines, using a modified Langendorff heart preparation, Brit. J. Pharmacol. 40, 617. Eckstein, R.W., J.A. McEachen, J. Detaining and W.B. Newberry, Jr., 1951, A special cannula for determination of blood flow in the left common coronary artery of the dog, Science 113, 385. ~ Fillion, G.M.B., S. Lluch and B. Uvn~s, 1971, Release of noradrenaline from the dog heart in situ after intravenous and intracoronary administration of 5-hydroxytryptamine, Acta Physiol. Scand. 83, 115.
Garattini, S. and L. Valzelli, 1965, Serotonin (Elsevier, Amsterdam).
5 Griggs, D.M. and R.B. Case, 1960, Effect of serotonin on coronary vascular resistance and myocardial oxygen consumption, Circulation 22, 758. Lluch, S. and G. Fillion, 1970, Cardiovascular responses to intravenous and intracoronary administration of 5-hydroxytryptamine, Acta Pharmacol. Toxicol. 28, Suppl. I, 66. Maxwell, G.M., C.A. Castillo, J.E. Clifford, C.W. Crumpton and G.G. Rowe, 1959, Effect of serotonin (5-hydroxytryptamine) on the systemic and coronary vascular bed of the dog, Amer. J. Physiol. 197,736. McCubbin, J.W., Y. Kaneko and I.H. Page, 1962, Inhibition of neurogenic vasoconstriction by serotonin. Vasodilator action of serotonin, Circulation Res. 11, 74. Parrat, J.R. and R.M. Wadsworth, 1970, The effect of 'selective' ~-adrenoceptor blocking drugs on the myocardial circulation, Brit. J. Pharmacol. 39, 296. Saxena, P.R., P. van Houwelingen and I.L. Bonta, 1971, The effects of mianserin hydrochloride on the vascular responses evoked by 5-hydroxytryptamine and related vasoactive substances, European J. Pharmacol. 13, 295. Schechter, Y. and M. Weinstock, 1974, ~-Adrenoceptor blocking agents and responses to adrenaline and 5-hydroxytryptamine in rat isolated stomach and uterus, Brit. J. Pharmacol. 52, 283. Schneider, J.A. and F.F. Yonkman, 1954, Species differences in the respiratory and cardiovascular response to serotonin (5-hydroxytryptamine), J. Pharmacol. Exptl. Therap. 111, 84. Schofield, B.M. and J.M. Walker, 1953, Perfusion of the coronary arteries of the dog, J. Physiol. (London) 122, 489. Trendelenburg, U., 1960, The action of histamine and 5-hydroxytryptamine on isolated mammalian atria, J. Pharmacol. Exptl. Therap. 130, 450. Vyden, J.K., D. Lent, K. Nagasawa, M. Carvalho, A. Serruya and E. Corday, 1974, The effects of serotonin on regional hemodynamics in the vascular system, J. Clin. Pharmacol. 14, 434.
1
ON THE MECHANISM OF THE C O R O N A R Y D I L A T O R E F F E C T OF SEROTONIN IN THE DOG MARCO AURELIO MENA and HORACIO VIDRIO
Instituto Miles de Terap~utica Experimental, M~xico 22, D.F., M~xico Received 22 July 1975, revised MS received 17 September 1975, accepted 24 September 1975
M.A. MENA and H. VIDRIO, On the mechanism o f the coronary dilator effect o f serotonin in the dog, European J. Pharmacol. 36 (1976) 1--5. In experiments designed to determine the nature of the coronary dilator effect of serotonin the influence of intracoronary administration of the amine on coronary perfusion pressure, heart rate and ventricular contractile force was assessed in anesthetized, open-chest dogs in which the left coronary artery was perfused with blood at a constant rate. Serotonin elicited dose-related decreases in coronary perfusion pressure and increases in contractile force, and lowered heart rate slightly. The dilator response was antagonized by methysergide, slightly potentiated by practolol and unaffected by reserpine. The inotropic effect was partially antagonized by methysergide and completely blocked by practolol and reserpine. It is concluded that serotonin induces coronary dilatation by direct stimulation of specific receptors, that this effect is independent of the cardiac stimulation produced by the amine, and that the latter response is mediated through/31 -adrenoceptors activated by released norepinephrine. Catecholamine release Serotonin receptors
Myocardial contraction Coronary dilatation
1. Introduction It has been repeatedly shown that serotonin produces coronary dilatation in the dog (Schofield and Walker, 1953; Schneider and Yonkman, 1954; Maxwell et al., 1959; Griggs and Case, 1960; Saxena et al., 1971; Vyden et al., 1974). Since some of these studies indicate that the amine concomitantly produces important changes in other cardiac parameters, such as heart rate, contractility and oxygen consumption, observed coronary dilatation could well be secondary to these effects. On the other hand, some cardiovascular actions of serotonin are known to be mediated in part through adrenergic or cholinergic mechanisms, rather than through specific serotonin receptors exclusively (Garattini and Valzelli, 1965). The present study was therefore conducted to determine the nature of the coronary dilator effect of serotonin in the dog in reference to its mediation through serotonin receptors or
Serotonin
~-Adrenoceptors
through catecholamine release as well as its dependence on other cardiac variables affected by the amine. To this effect, the influence of intracoronary administration of serotonin on coronary perfusion pressure, heart rate and ventricular contractile force was determined in normal dogs and in animals pretreated with the serotonin antagonist methysergide, the /3adrenergic blocking agent practolol and the catecholamine depletor reserpine.
2. Materials and methods Experiments were carried out in 24 mongrel dogs of either sex, weighing between 15 and 25 kg. The animals were anesthetized i.v. with 30 mg/kg of pentobarbital sodium, supplemented by additional doses as needed. Artificial respiration was maintained with a Palmer pump adjusted to deliver approximately 15 ml/kg of room air through a tracheal cannula. The left
2
femoral artery and vein wdre cannulated for recording of systemic blood pressure and for drug administration, respectively. The thorax was opened at the fourth left intercostal space and the pericardium cut and sutured to the chest wall to form a craddle for the heart. An Eckstein brass cannula (Eckstein et al., 1951), approximately 11 cm long and 0.5 cm in diameter, was introduced in the left coronary ostium via the left subclavian artery and was fixed in place with a metal clip attached externally to the left coronary trunk. Heparin, 4 mg/kg, was injected i.v. to prevent clotting. Blood obtained from the left c o m m o n carotid artery was delivered at a constant rate to the left coronary bed with a Model T6S Sigmamotor pump and pressure in the perfusion system recorded with a Statham P23AC transducer connected to a side arm at the o u t p u t portion of the pump. Perfusion pressure, which was taken as a measure of coronary resistance, was adjusted to coincide with the level of systemic blood pressure observed once the thorax had been opened and before administration of any drug; this level was maintained throughout the experiment within 10 mm Hg o f the initial value, irrespective of any subsequent changes in systemic blood pressure. Contractile force was measured with a Walton--Brodie strain gauge arch sutured to the anterior left ventricular wall, approximately 2 cm from the apex. Heart rate was recorded with a Grass tachograph triggered by the R wave of the precordial electrocardiogram. All parameters were recorded on a Grass Model 7 polygraph. After adequate stabilization of all variables for at least 20 min, responses to serotonin were obtained by injecting the amine into the rubber tubing of the perfusion system immediately upstream to the Eckstein cannula. Doses of 0.1, 1.0, 10 and 100 pg were administered at 10 min intervals to groups of 6 dogs receiving either no pretreatment, 0.5 mg/kg of methysergide, or 1.0 mg/kg of practolol. The serotonin and fl-adrenergic antagonists were administered i.v. 20 min before applying the first serotonin dose. An additional group of 6 animals was pretreated with reserpine, 0.5 mg/kg, administered i.p.
M.A. MENA, H. VIDRIO
for 2 days prior to the experiment. These dogs were anesthetized with a mean dose of 16 mg/kg of pentobarbital and coronary perfusion pressure was adjusted at 100 mm Hg, irrespective of their systemic blood pressure, which was consistently below this value. The adequacy of reserpinization was assessed by administering 0.1 mg/kg of tyramine i.v. and verifying that blood pressure, contractile force and heart rate responses were almost absent. Doses of methysergide maleate (Sandoz Pharmaceuticals), practolol hydrochloride (Ayerst Laboratories) and tyramine hydrochloride (Nutritional Biochemicals) refer to the respective salts. Doses of reserpine and serotonin creatinine sulfate (both obtained from Nutritional Biochemicals) refer to the base. All drugs, except reserpine, were dissolved in isotonic NaC1 solution; propylene glycol was used to dissolve the alkaloid. Statistical significance between control and pretreated groups were determined by Student's t-test.
3. Results Baseline values for blood pressure, coronary perfusion pressure and heart rate in the diverse groups of dogs are shown in table 1. Figures for left ventricular contractile force are not presented in the table, since this parameter was recorded by adjusting the amplitude of the tracing to a predetermined value, and no steps were taken to calibrate the strain gauge. 'Before treatment' values were similar in the control, methysergide and practolol groups, and only heart rate was decreased after treatment with either drug, significantly so in the case of methysergide. As expected, reserpinized dogs had lower blood pressure and heart rate than did control animals. Injection of serotonin into the left coronary artery produced dose-related decreases in perfusion pressure and increases in contractile force. Heart rate was slightly decreased at the two highest doses (fig. 1). These effects were apparent within 10 sec after administration of the amine. The fall in coronary resistance was
CORONARY DILATOR E F F E C T OF SEROTONIN TABLE
%A
w,A
1
Baseline values for blood pressure, coronary perfusion pressure and heart rate in control dogs and in dogs treated with methysergide, practolol or reserpine. Figures are means + S.E.M. of 6 experiments.
Group
3
30
S
20 t0"
O"
Blood pressure (ram Hg)
-iO -
Before treatment
After treatment -20-
Control Methysergide Practolol Reserpine
104.2 ± 6.76 106.7 ± 6.03 103.0 ± 6.67 --
Group
Perfusion pressure (mm Hg)
-
100.5 ± 8.54 106.3 ± 7.15 83.3 ± 6.67**
-30 "
o'.,
,.~
,~
,Go
~,
SEROTON}N DOSE (racg)
,:o
,b
,Go
,b
,Go
% o,
Before treatment
After treatment
Control Methysergide Practolol Reserpine
102.1 ± 7.97 106.0 -+ 5.30 105.3 -+ 5.55 --
105.8 ± 5.93 103.0 ± 4.62 100.8 -+ 0.54
Group
Heart rate (beats/min) Before treatment
Control Methysergide Practolol Reserpine
METHYSERGIDE
-
174.2 -+ 11.29 175.0 -+ 8.21 168.5 ± 11.01 --
-1o, -20
After treatment
141.7 ± 10.64" 142.0 -+ 9.42 122.2 ± 6.60**
* Significantly different from pretreatment value (p < 0.05). ** Significantly different from control values (p < 0.01).
antagonized by methysergide, unaffected by reserpine, and somewhat potentiated by practolol, while the positive inotropic response was blocked partially by the antiserotonin agent and completely by the other two drugs. All three agents reduced the effect on heart rate. A s s h o w n in t a b l e 2, t h e e f f e c t s o f s e r o t o nin on systemic blood pressure were negligible in the control, methysergide and reserpine groups, and no dose-related pressor or depressor responses were apparent. Dogs pretreated with practolol, however, showed consistent f a l l s in b l o o d p r e s s u r e , t h e r e s p o n s e s o b s e r v e d
-30 -40
RESERPINE
-50
,£,
G, ,;o ,'o SEROTONIN ,Go DOSE (mcg)
Fig. 1. Effects of serotonin on coronary perfusion pressure (PP), left ventricular contractile force (CF) and heart rate (HR) in the perfused dog heart in situ. Each point corresponds to the mean of 6 experiments; vertical lines indicate standard errors. Shown are serotonin responses in control dogs and in animals pretreated with 0.5 mg/kg of methysergide or 1.0 mg/kg of practolol administered i.v. 20 rain before the experiment, or with 0.5 mg/kg of reserpine injected i.p. for 2 days. Asterisks indicate responses significantly different from control (p < 0.05). Abscissae denote intracoronary doses of serotonin in pg; ordinates, per cent changes in the diverse parameters.
at 100 pg differing significantly from those of control animals.
4. D i s c u s s i o n The present results confirm the coronary dilator and positive inotropic effects of serot o n i n in t h e d o g s h o w n b y o t h e r s in d i v e r s e in v i v o a n d in v i t r o p r e p a r a t i o n s f r o m t h i s species. H o w e v e r , a n d in c o n t r a s t w i t h t h e s e
4
M.A. M E N A , H. V I D R I O
TABLE 2 S y s t e m i c b l o o d pressure e f f e c t s o f i n t r a c o r o n a r y injections o f s e r o t o n i n in c o n t r o l dogs a n d in dogs t r e a t e d with m e t h y s e r g i d e , practolol or reserpine. Figures are m e a n s -+ S.E.M. o f 6 e x p e r i m e n t s . Group
% Change in b l o o d pressure 0.1 pg
1.0 p g
Control Methysergide Practolol Reserpine
--0.7 t 0.67 0 0 0
--0.7 --0.3 --3.7 --3.7
Group
% Change in b l o o d pressure 10 p g
Control Methysergide Practolol Reserpine
--3.2 --2.1 --8.2 --4.5
± 2.32 ± 1.09 +- 1.31 ± 1.82
100 p g ± 6.28 ± 2.77 -+ 1.88 +- 3.48
+1.7 +2.8 --22.3 0
± 8:07 ± 4.47 ± 6.34* +- 8.21
* Significantly d i f f e r e n t f r o m c o n t r o l ( p < 0.05).
studies, no positive chronotropic response to the amine could be demonstrated, possibly because all injections were made into the left coronary artery, thus preventing adequate amounts of the drug from reaching the pacemaker tissue of the right atrium, which is mainly irrigated by the right coronary branch. This has been shown to be the case by Lluch and Fillion (1970), who found that serotonin produced tachycardia when injected i.v. or into the right coronary artery, but not when injected into the left branch. The fact that coronary dilatation was effectively blocked by the serotonin antagonist methysergide, b u t not by practolol or reserpine, indicates that this response is mediated by specific serotonin receptors and is not dependent on intact ~-adrenoceptors or catecholamine stores. This finding agrees with that of Saxena et al. (1971), who reported that serotonin-induced coronary dilatation could be blocked by the antiserotonin agent mianserin. On the other hand, the stimulant effect of the amine on left ventricular contractility does not appear to be mediated entirely by the
same mechanism, since it was completely abolished by practolol or reserpine and only partially inhibited by methysergide. This effect could therefore be attributable to norepinephrine released by serotonin and acting on myocardial ~,-adrenergic receptors, in a situation similar to that occurring in the rabbit atrium, in which stimulation by serotonin is abolished by reserpine or by dichloroisoproterenol (Trendelenburg, 1960). The fact that serotonin has been shown to release I norepinephrine from the dog heart (Fillion et al., 1971) supports the above contention. The dissociation of the dilator and inotropic responses to serotonin achieved through selective pharmacological blockade demonstrates, in addition, that the former is not a consequence of increased oxygen demand induced by cardiac stimulation, b u t that it is a direct effect of serotonin on the coronary vasculature. The fact that the slight lowering of heart rate produced by serotonin was significantly inhibited by methysergide, practolol and reserpine suggests a non-specific effect exerted by the three drugs, rather than a true pharmacological antagonism. In this connection, it should be pointed out that all pretreated groups had lower basal heart rates than did the control animals, and that this difference could have masked the slight bradycardia induced by the amine. An interesting observation was the apparent potentiation of serotonin-induced coronary dilatation by practolol. No plausible explanation for this phenomenon can be offered, since if any interaction between the blocker and the amine t o o k place, a reduced response to the latter would be expected, in view of the reported ability of dichloroisoproterenol to antagonize serotonin vasodilatation in the dog hind limb and kidney (McCubbin et al., 1962), and of propranolol, practolol and oxprenolol to block serotonin contractions in the isolated rat stomach and uterus (Schechter and Weinstock, 1974). It is possible that the blockade by practolol of the inotropic effect of serotonin could lead to an enhanced
CORONARY DILATOR EFFECT OF SEROTONIN v a s o d i l a t a t i o n b y eliminating the increased int r a m y o c a r d i a l t e n s i o n g e n e r a t e d b y this response, and w h i c h n o r m a l l y w o u l d t e n d to restrict c o r o n a r y flow. Such an e x p l a n a t i o n a p p e a r s u n t e n a b l e since reserpine, w h i c h also b l o c k e d t h e i n o t r o p i c effect, did n o t a f f e c t the d i l a t o r response. A n alternative possibility is t h a t practolol, b y b l o c k i n g m y o c a r d i a l illr e c e p t o r s c o u l d m a k e m o r e o f t h e norepin e p h r i n e w h i c h is released b y s e r o t o n i n available f o r s t i m u l a t i o n o f the u n b l o c k e d coron a r y adrenergic receptors. A c c o r d i n g to s o m e a u t h o r s (Parrat and W a d s w o r t h , 1 9 7 0 ; Broadley, 1970), these r e c e p t o r s are o f t h e fi:-type. T h e result w o u l d be a p o t e n t i a t i o n o f t h a t portion of the dilatation due to norepinephrine release. This h y p o t h e s i s , h o w e v e r , w o u l d n o t explain the e n h a n c e m e n t o f the s y s t e m i c d e p r e s s o r response to s e r o t o n i n observed in t h e present s t u d y .
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