The mode of action of 4-aminopyridine on the chronotropic and inotropic responses in the isolated, blood-perfused dog heart

The effects of 4-aminopyridine (4-AP, 0.3 to 300 micrograms) on atrial rate and atrial muscle contractile force were investigated in the isolated, blood-perfused dog atrial preparation. Low doses (less than 30 micrograms) of 4-AP injected into the sinus node artery caused a dose-dependent positive inotropic response and a negligible positive chronotropic response. A higher dose (300 micrograms) of 4-AP induced a transient positive chronotropic response followed by a negative response and positive and negative inotropic responses. The negative cardiac responses to 4-AP were completely inhibited by an adequate dose of atropine and were potentiated by physostigmine. TTX or C6 suppressed partly, but significantly the negative responses induced by 4-AP. On the other hand, the negative responses to electrical stimulation (ES) were inhibited by atropine, C6 and TTX and those to ACh were blocked by atropine but not by C6 or TTX. The positive responses to 4-AP were not affected by propranolol but were potentiated by atropine. The positive inotropic response to 4-AP was significantly suppressed by a calcium channel blocker, diltiazem. From these results, it is concluded that the cardiac responses to 4-AP are composed of direct positive responses and negative responses which are related to activation of parasympathetic ganglionic neurotransmission.     

Anti-nicotinic and anti-muscarinic actions of eperisone in the isolated canine atrium

The effects of eperisone, an antispastic agent, on the chronotropic and inotropic responses to acetylcholine, nicotine or stimulation of intracardiac autonomic nerves were evaluated in isolated, blood-perfused canine atrium. Eperisone (10-300 micrograms) injected into the sinus node artery of the isolated atrium produced dose-related negative chronotropic and inotropic effects, which were not affected by atropine. In the same doses, eperisone inhibited the negative chronotropic and inotropic responses to an injection of acetylcholine and intracardiac parasympathetic stimulation. Eperisone also suppressed the negative followed by positive cardiac responses to nicotine, but did not modify the positive responses to intracardiac sympathetic stimulation or norepinephrine. The inhibitory effect persisted much longer for the responses to nicotine or parasympathetic stimulation than for those to acetylcholine. These results suggest that eperisone at doses that induce direct cardiac depressant effects exerts its blocking action on nicotinic receptors at parasympathetic ganglia and sympathetic nerve terminals and on muscarinic receptors at the effector cells in the dog heart.     

Cardiac effects of vecuronium and its interaction with autonomic nervous system in isolated perfused canine hearts.

The chronotropic and inotropic effects of vecuronium bromide and its interaction with the autonomic nervous system were investigated in the isolated, cross-circulated right atrial and left ventricular preparations of the dog. Vecuronium, injected into the external jugular vein of the support dog, induced dose-dependent decreases in heart rate and arterial blood pressure, and increased atrial contractile force with no change in sinus rate in isolated atrial preparations. Vecuronium (1-3,000 micrograms), injected into the sinus node artery of the isolated atrium, induced dose-dependent increases in atrial contractile force with small increases in sinus rate. Vecuronium also increased the ventricular contractile force in a dose-dependent manner. The positive inotropic effect was attenuated in part by propranolol, but not by either tetrodotoxin or imipramine. Vecuronium inhibited in a dose-related manner the negative chronotropic and inotropic responses to parasympathetic nerve stimulation and carbachol (CCh) and the negative followed by positive cardiac responses to nicotine, but did not attenuate the positive responses to sympathetic nerve stimulation. The ID50s for the responses to parasympathetic stimulation, CCh, and nicotine were not significantly different. Vecuronium enhanced the positive chronotropic and inotropic responses to sympathetic nerve stimulation, tyramine, norepinephrine, and isoproterenol. These results indicate that (a) vecuronium causes the positive inotropic responses mediated by nonadrenergic mechanisms and beta-adrenoceptors, (b) vecuronium blocks ganglionic and presynaptic nicotinic and postsynaptic muscarinic receptor-mediated responses similarly, and (c) vecuronium enhances beta-adrenoceptor-mediated responses in the dog heart.     

Presynaptic inhibitory actions of lignocaine in canine isolated, blood-perfused atrial preparations

1. Cardiac effects of lignocaine on sinoatrial nodal pacemaker activity and atrial contractility were investigated in five canine isolated, blood-perfused right atria that were perfused with heparinized blood from support dogs. The effects of lignocaine on responses to intracardiac nerve stimulation and administered acetylcholine and noradrenaline were also examined. 2. Lignocaine was injected into the support dog intravenously or administered selectively to the sinus node artery of the isolated atrium. At doses that did not produce significant depressor action (0.3, 1.0 and 3.0 mg/kg), lignocaine produced no significant changes in heart rate. A large dose of 10 mg/kg lignocaine caused significant depressor effects and slight bradycardia. Direct administration of lignocaine (0.3, 1.0, 3.0, 10.0 and 30.0 mumol) into the sinus node artery of the isolated atrium consistently caused slight negative chronotropic and rather marked negative inotropic effects. 3. After treatment with a relatively large dose of lignocaine, electrical stimulation-induced negative chronotropic and inotropic responses were significantly inhibited in a dose-related manner, but positive chronotropic and inotropic responses were slightly depressed only at an extremely high dose of lignocaine (10.0 mumol). 4. Noradrenaline-induced positive chronotropic and inotropic effects were not modified by any doses of lignocaine used (0.3, 1.0, 3.0 and 10.0 mumol). Acetylcholine-induced negative chronotropic and inotropic effects were slightly, but significantly, depressed by 10 mumol lignocaine. 5. These results suggest that a relatively large dose of lignocaine has a dominant presynaptic inhibitory action, particularly on the parasympathetic component.     

Suppression of strychnine on the chronotropic and inotropic effects in the isolated, blood-perfused canine atrium

In the isolated, blood-perfused dog atrium, strychnine evoked dose-dependent decreases in atrial rate and contractile force, and the negative cardiac effects were not influenced by atropine. Strychnine suppressed dose-dependently the negative chronotropic and inotropic responses to intramural parasympathetic nerve stimulation, but not the response to acetylcholine. These results suggest that strychnine has direct cardiac depressant properties and inhibits the release of acetylcholine from parasympathetic nerve terminals in isolated dog atrium.     

Effects of 4-aminopyridine on the fade response to vagal nerve stimulation in the isolated dog atrium

Intramural parasympathetic nerve stimulation (5 Hz, 2 min) after propranolol treatment evoked negative chronotropic and inotropic responses in the isolated dog atrium. The cardiac responses were not maintained during stimulation; they reached maximum values and then faded back toward their control values ("fade" response). 4-Aminopyridine increased the maximum values of both responses, and it increased the fade of the chronotropic response but not that of the inotropic response. These results suggest that the fade response to parasympathetic nerve stimulation is modulated at the parasympathetic nerve terminals of the isolated dog atrium.     

The effects of neuropeptide Y on myocardial contractility and coronary blood flow.

1. This study was designed to assess the effects of exogenous neuropeptide Y (NPY) on cardiac contractility and coronary blood flow (CBF) in anaesthetized dogs and to evaluate the effect of NPY on the responses to sympathetic and parasympathetic stimulation and to inotropic agents. 2. Bolus doses of NPY (500 micrograms), administered via the femoral vein, increased mean arterial pressure. The pressor effect was associated with reductions in heart rate, CBF and cardiac contractility. 3. The effects of NPY (500 micrograms) on contractility and CBF were compared with that of vasopressin (VP) (1 unit). For similar reductions in CBF, NPY and VP had similar negative inotropic effects. Thus, it is likely that the negative inotropic response to NPY is not due to a direct effect of NPY on the heart muscle but is largely due to coronary vasoconstriction. 4. In the presence of NPY (500 micrograms, i.v.), there was an inhibition of the positive inotropic response to stimulation of the left cardiac sympathetic nerve (2 or 5 Hz, 0.05 ms). NPY also inhibited the negative inotropic response and chronotropic response to vagal stimulation (2, 3 or 5 Hz, 0.05 ms). 5. Dose-response curves were obtained for the inotropic, chronotropic and pressor responses to cumulative infusions of noradrenaline (n = 6) and dobutamine (n = 6). NPY had no effect on these dose-response curves. 6. The effect of NPY on the responses to salbutamol and impromidine was assessed. NPY did not alter the positive inotropic, chronotropic or depressor responses to these agonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of ketanserin on the pacemaker activity and contractility in the isolated, blood-perfused dog atrium

Effects of ketanserin on the pacemaker activity and atrial contractility and on 5-hydroxytryptamine (5-HT)-induced cardiac responses were investigated in the isolated, blood-perfused dog atrium. Ketanserin (1-300 micrograms) injected into the sinus node artery evoked a transient positive followed by a negative chronotropic effect, and a dose above 30 micrograms of ketanserin produced a dose-dependent negative inotropic effect with a little positive one. Ketanserin-induced negative chronotropic and inotropic effects were not affected by atropine in a dose which blocked ACh-induced responses. Propranolol inhibited positive inotropic responses to ketanserin and norepinephrine and significantly augmented the negative chronotropic and inotropic responses to ketanserin. Imipramine did not affect the transient positive followed by negative chronotropic and the negative inotropic responses to ketanserin but it induced the positive cardiac responses following the negative ones. Tetrodotoxin, phentolamine, and diphenhydramine did not modify the effects of ketanserin. From these results, it is concluded that ketanserin might induce the direct negative chronotropic and inotropic effects and the indirect effects by catecholamine release. Ketanserin-induced catecholamine release might not be due to tyramine-like or nicotine-like action. Ketanserin significantly inhibited a low dose (3 micrograms) of 5-HT-induced negative chronotropic and inotropic effects, suggesting the possibility of 5-HT2 receptors in the isolated dog atrium.     

Inhibition by glibenclamide of negative chronotropic and inotropic responses to pinacidil, acetylcholine, and adenosine in the isolated dog heart.

The blocking effects of glibenclamide on the chronotropic and inotropic responses to K+ channel openers pinacidil (ATP-sensitive) and acetylcholine (ACh) or adenosine (receptor-operated) were investigated in the isolated, blood-perfused canine atrium or ventricle. Glibenclamide (0.1-3 mumol) induced no significant cardiac effects. Cumulative administration of pinacidil (0.03-3 mumol) dose-dependently decreased sinus rate much less than the contractile force of the atrial and ventricular muscles. Glibenclamide similarly inhibited the negative chronotropic and inotropic responses to pinacidil in a dose-related manner. A high dose of glibenclamide (3 mumol) slightly but significantly attenuated the negative chronotropic and inotropic responses to ACh and adenosine but not to verapamil. These results demonstrate that glibenclamide inhibits the negative chronotropic and inotropic responses to the ATP-sensitive K+ channel opener pinacidil and the receptor-operated K+ channel openers ACh and adenosine but more selectively antagonizes the responses to pinacidil in the dog heart and suggest that in contrast to ACh and adenosine, an ATP-sensitive K+ channel opener has a greater effect on the ventricle than on the sinoatrial node.     

Muscarinic suppression of the nicotinic action of acetylcholine on the isolated,blood-perfused atrium of the dog

1. The isolated canine right atrium perfused through the sinus node artery at a constant pressure of 100 mm Hg with blood led from a support dog was suspended in a bath filed with blood and kept at constant temperature. This preparation maintained its constant tension development and rate over 5 hrs in all 5 control experiments. 2. A relatively small amount of acetylcholine (ACh) induced a negative inotropic effect at 0.01 mug and a negative chronotropic effect at 0.1 mug. 3. A relatively large dose of ACh induced a biphasic inotropic response, i.e., initially a negative inotropic response and secondarily a long-lasting positive tropic response. These positive chronotropic and inotropic responses were abolished by propranolol, hexamethonium or tetrodotoxin. 6. In the papillary muscle and atrial muscle preparations isolated from one canine heart, ACh caused negative and positive inotropic effect in both paced papillary and atrial muscle preparations. In contrast to the results obtained with atria, the positive inotropic response of the papillary muscle preparation was completely blocked by treatment with propranolol or tetrodotoxin. 7. From these results, it is suggested that in the canine atrium muscarinic mechanisms predominate over nicotinic ones. This may well be due to the known inhibition of nicotinic responses by stimulation of muscarinic receptors.     

Pharmacologic analysis of ketamine-induced cardiac actions in isolated, blood-perfused canine atria

Effects of ketamine were investigated on atrial rate and contractile force in the isolated, blood-perfused canine atrium. When a relatively small dose (3 micrograms) of ketamine was injected into the sinus node artery, positive chronotropic and inotropic responses were consistently observed. With doses of 10-300 micrograms i.a., biphasic (i.e., negative followed by positive) chronotropic and inotropic responses were induced. An extremely large dose of ketamine frequently produced biphasic chronotropic and only negative inotropic responses. The negative effects of ketamine were not affected by atropine pretreatment. After treatment with propranolol or imipramine, the positive effects were significantly suppressed. The effects were not influenced by tetrodotoxin. These results suggest that ketamine has (a) indirect cardioexcitatory properties mediated by a release of catecholamines which is due to a tyramine-like action, and (b) direct cardioinhibitory properties by which high doses depress the contractility rather than the pacemaker activity.     

Effects of dopamine on sinus rate and ventricular contractile force of the dog heart in vitro and in vivo.

1 Experiments were carried out on dog isolated papillary muscle and sinus node preparations perfused with arterial blood from a donor dog. The chronotropic and inotropic effects of dopamine were analysed by using reserpine, desmethylimipramine (DMI), cocaine and phenoxybenzamine, and the relative chronotropic and inotropic effects defined, the results being compared with those for noradrenaline (NA). 2 The effects of dopamine administered intra-arterially into the isolated preparations were reduced by pretreatment of animals with reserpine both in the papillary muscle and sinus node. The chronotropic effects, however, were affected less by pretreatment with reserpine than were the inotropic effects. 3 Desmethylimipramine (DMI) reduced the inotropic and chronotropic effects of dopamine, and enhanced the effects of NA and nerve stimulation; the chronotropic effects of the amines were less affected than the inotropic effects. 4 Cocaine enhanced considerably the inotropic and chronotropic effects of NA, and decreased the inotropic but not the chronotropic effect of dopamine. 5 Phenoxybenzamine enhanced the inotropic effects of dopamine, NA and nerve stimulation, but did not affect the chronotropic effects of the amines. 6 When dopamine (1 to 300 mug/kg) was administered intravenously to the donor dog, it increased preferentially the contractile force of the ventricular myocardium with a comparatively small change of the sinus rate in the isolated preparations as well as in the heart in vivo. NA (0.1 to 10 mug/kg) caused effects similar to those of dopamine. The maximal inotropic responses to these catecholamines were reached with lower doses than the chronotropic ones. 7 It is concluded that both the positive inotropic and the positive chronotropic responses to dopamine are mediated partially by a direct and partially by an indirect stimulant effect on beta-adrenoceptors in the dog heart. The present results suggest that the difference in activity of dopamine and NA between the ventricular myocardium and the sinus node may be ascribed to the unequal innervation with adrenergic nerve fibres of the atrium and the ventricle (Furnival, Linden & Snow, 1971). The sinus node which is densely innervated by adrenergic nerve fibres may inactivate noradrenaline and dopamine more effectively than the ventricular myocardium through the uptake into the nerve and thereby be less sensitive to the exogenous catecholamines.     

Indirekter Nachweis parasympathischer Nervenfasern im Ventrikelmyokard des Menschen

Summary The release of endogenous acetylcholine (ACh) has been investigated in 9 human papillary muscles. In the isolated preparation, transmitter substances were released by high voltage stimulation and simultaneous application of trains of stimuli during the abolute refractory period of the myocardium.In the pharmacologically untreatet human heart muscle the negative inotropic effect of ACh was superimposed on the positive inotropic effect of simultaneously released catecholamines. After treatment with guanethidine, high voltage stimulation reduced the amplitude of isometric contraction by an average of 14%. After contractility had been increased by noradrenaline, isoprenaline, orciprenaline or theophylline high voltage stimulation reduced the amplitude of contraction by 23%. These negative inotropic effects were increased up to 47% by neostigmine, diminished by hemicholinium and prevented by atropine, Blockade of intramural nerve endings by tetrodotoxin abolished completely the positive as well as negative inotropic effects of high voltage stimuli.These findings indicate the possibility of a release of endogenous ACh from intramural nerve endings also in human ventricular myocardium. The negative inotropic effect of endogenous ACh is most pronounced during positive inotropic effects of sympathominetic agents or of theophylline.

Ein Teil der Ergebnisse wurde anläßlich der 39. Tagung der Deutschen Physiologischen Gesellschaft als vorläufige Mitteilung publiziert (Sieber et al., 1972).     

EFFECT OF SECRETIN ON PACEMAKER ACTIVITY AND CONTRACTILITY IN THE ISOLATED BLOOD-PERFUSED ATRIUM OF THE DOG

1. The effects of secretin on inotropic and chronotropic activity were investigated in nine isolated canine atrium preparations which were suspended in a bath and perfused with arterial blood from a carotid artery of a heparinized donor dog. 2. Secretin administered into the cannulated sinus node artery in a dose range of 0.1–10 units produced a dose-related positive inotropic and a biphasic chronotropic effect. 3. The positive chronotropic and inotropic responses to secretin were not suppressed by treatment with alprenolol in doses which blocked responses to noradrenaline. 4. The negative chronotropic response to secretin was not blocked by atropine in doses which blocked the response to acetylcholine. 5. After treatment with glucagon, secretin produced dose-related negative chronotropic and a positive inotropic effects. Thus glucagon may antagonize the positive chronotropic effect of secretin. 6. From these results, it is concluded that secretin has a direct effect on atrial rate and contractility.     

Dual effects of alpha-adrenoceptor agonist on contractility of mice isolated atria

Contractility responses of mice atria to alpha-adrenergic agonists (methoxamine and clonidine) were studied. The alpha-adrenergic agonists increased the rate of force development (dF/dt) and decreased contractile frequency. The positive inotropic effect was mediated through cardiac alpha-adrenoceptors, while the negative chronotropic effect involved parasympathetic activation. Blockers of phospholipase C inhibited both the positive inotropic and the negative chronotropic effects of the alpha-adrenergic agonists. Inhibitors of phospholipase A2 and cyclooxygenase activity attenuated the positive inotropic effect of the agonists without modifying the negative chronotropic effect.     

Pharmacological analyses of hydralazine-induced cardiac action in intact dogs and isolated, blood-perfused canine atria

The effects of hydralazine (HYD) on heart rate and blood pressure in the intact dog and on atrial rate and contractile force in the isolated atrium were investigated. HYD (0.1-1 mg) injected into the sinus node artery produced double peaked positive inotropic and negative chronotropic effects in a dose-related manner. The initial positive inotropic and negative chronotropic responses were not affected by propranolol and atropine, respectively. The second positive inotropic response was inhibited by propranolol or reserpine, but it was not suppressed by imipramine or tetrodotoxin. When HYD (0.1-1 mg/kg) was administered intravenously to the donor dog, an initial increase followed by a decrease in blood pressure and an increase in heart rate were observed. In the isolated atrium, an increase in contractile force was induced. The increases of blood pressure and heart rate in the donor dog and the positive inotropic effect in the isolated atrium after HYD treatment were suppressed by reserpine. These results suggest that HYD has direct positive inotropic and negative chronotropic effects and indirect cardiac stimulating effects caused by a release of catecholamines from sympathetic nerve terminals, and that HYD-induced catecholamine release is not mediated by a tyramine-like action or via nerve excitation.     

COMPARATIVE STUDY OF THE ACTION OF ANTIARRHYTHMIC DRUGS ON SINOATRIAL NODAL PACEMAKER ACTIVITY AND CONTRACTILITY IN THE ISOLATED BLOOD-PERFUSED ATRIUM OF THE DOG

1. The effects of five antiarrhythmic drugs (quinidine, procainamide, lignocaine, phenytoin and propranolol) were studied on pacemaker activity in the sinoatrial node and on contractility in twenty-three isolated, blood-perfused atrium preparations of dogs. Each drug was administered directly into the cannulated sinus node artery of an isolated atrium over a period of 4 s. 2. Quinidine produced negative chronotropic and inotropic effects. Occasionally, higher doses induced a biphasic inotropic effect, an initial negative phase being followed by an increase in contractility. 3. Three response patterns to procainamide were observed: negative chronotropic and inotropic effects; biphasic effects, initially negative chronotropic and inotropic effects being followed by positive effects; a biphasic inotropic effect and a negative chronotropic effect. The third pattern was most frequently produced by relatively high doses. 4. Lignocaine, phenytoin and propranolol induced dose-related negative chronotropic and inotropic effects. 5. All five of the antiarrhythmic drugs used caused sinus arrest in high doses. 6. Procainamide-induced positive chronotropic and inotropic effects were significantly inhibited by treatment with alprenolol or nadolol, but not by tetrodotoxin or desipramine. 7. These results indicate that procainamide-induced positive chronotropic and inotropic effects may involve an adrenergic mechanism.     

Pharmacologic analysis of papaverine-induced positive chronotropic and inotropic effects.

Effects of papaverine on sinus rate and atrial contractility were investigated using the isolated atrium preparation of the dog perfused with heparinized arterial blood led from a support dog. When papaverine was injected into the cannulated sinus node artery, positive chronotropic and inotropic responses were dose-relatedly produced from about 3 mug. At a dose level above 100 mug, papaverine induced initially slightly negative chronotropic and inotropic effects followed by marked positive ones. Papaverine-induced positive chronotropic and inotropic effects (P-PCIE) were not influenced by treatment with tetrodotoxin which blocked the action of nicotine. P-PCIE were slightly suppressed by an adrenergic beta-blocking agent, alprenolol, which blocked the action of norepinephrine. They were however potentiated by treatment with desmethylimipramine which completely blocked the action of tyramine. A large amount of MnCl2 suppressed the actions of both papaverine and norepinephrine. Papaverine-induced negative chronotropic and inotropic effects were not influenced by atropine treatment. In addition, papaverine slightly potentiated the negative chronotropic and inotropic responses to adenosine. From these results, it appears that papaverine has a direct stimulating property on sinus rate and atrial contractility which may be due to inhibition of phosphodiesterase and successive accumulation of cyclic AMP. Moreover, papaverine may partially cause catecholamine release from adrenergic nerve fibers and it may inhibit the adenosine uptake mechanism.     

Different antagonism of the positive chronotropic and inotropic responses of the isolated, blood-perfused dog atrium to Bay k 8644 by nicardipine and verapamil

The effects of Bay k 8644, a dihydropyridine calcium agonist, on sinoatrial (SA) node pacemaker activity and atrial contractility and its interaction with the calcium antagonists, nicardipine and verapamil, were investigated in the isolated, blood-perfused dog atrium. Bay k 8644 (0.03-30 micrograms) increased sinus rate and atrial contractility dose dependently. Norepinephrine (NE) and CaCl2 dose dependently increased the sinus rate and contractility but the positive chronotropic effect of CaCl2 was much less than that of the other drugs. The positive chronotropic effect of Bay K 8644 (0.3-1 microgram) was dose dependently depressed by nicardipine at doses of 1-10 micrograms but the inotropic effect was depressed only by a large dose of 10 micrograms. After sinus arrest induced by nicardipine (10-30 micrograms), SA node pacemaker activity was readily restored by Bay k 8644 or NE. Verapamil (1-3 micrograms) also depressed the positive chronotropic effect of Bay k 8644 more effectively than the inotropic effect although it did not attenuate the chronotropic effect of NE. The inotropic interaction could not be evaluated at higher doses of antagonists because of sinus arrest. Propranolol (3 micrograms) suppressed both positive chronotropic and inotropic effects of NE but did not depress the Bay k 8644-induced responses. These results show that the antagonism between Bay k 8644 and calcium antagonists is predominant on SA node pacemaker activity in cardiac tissues.     

Calcium channel subtypes for the sympathetic and parasympathetic nerves of guinea-pig atria.

1. The Ca2+ channel subtypes of the autonomic nerves of guinea-pig atria were elucidated by monitoring the effects of specific Ca2+ channel blockers on the negative and positive inotropic responses associated respectively, with stimulation of the parasympathetic and sympathetic nerves. 2. In left atria paced at 2-4 Hz, the negative inotropic effect induced by field stimulation of parasympathetic nerves (in the presence of propranolol) was abolished by omega-conotoxin MVIIC, a blocker of N-type and OPQ subfamily Ca2+ channels. omega-Conotoxin GVIA (an N-type blocker), omega-agatoxin IVA (a P-type blocker), nifedipine (an L-type blocker) and Ni2+ (a T- and R-type blocker) were much less effective. 3. The positive inotropic response resulting from field stimulation of the sympathetic nerves (in the presence of atropine) was abolished by both omega-conotoxins, while omega-agatoxin IVA, nifedipine and Ni2+ were ineffective. 4. In the spontaneously beating right atria, the early negative inotropic effect produced by 1,1-dimethyl-4-phenylpiperazinium was abolished by omega-conotoxin MVIIC, whereas the late positive inotropic effect was partially reduced, but not abolished, by a high concentration of omega-conotoxin GVIA. 5. None of the peptide toxins affected the chronotropic and the inotropic responses evoked by carbachol and isoprenaline. 6. These results suggested that, under physiological conditions, the release of acetylcholine from parasympathetic nerves is dominated by an OPQ subfamily Ca2+ channel while that of noradrenaline from sympathetic nerves is controlled by an N-type Ca2+ channel. Ligand-induced noradrenaline release appeared to recruit additional type(s) of Ca2+ channel.