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.     

Beta-adrenoceptor blocking effects of a selective beta 2-agonist, mabuterol, on the isolated, blood-perfused right atrium of the dog.

1. Effects of (+/-)-1-(4-amino-3-chloro-5-trifluoromethyl-phenyl)-2-tert.- butylamino-ethanol hydrochloride (mabuterol) on pacemaker activity and atrial contractility were investigated in the isolated and blood-perfused right atrium of the dog. 2. Mabuterol, injected into the sinus node artery of the isolated atrium, dose-dependently increased atrial rate and contractile force at doses of 0.01-10 nmol but the responses to over 10 nmol of mabuterol gradually decreased and mabuterol at higher doses induced biphasic cardiac responses, i.e., negative followed by positive cardiac responses. 3. The maximal increases in atrial rate and contractile force induced by mabuterol were 41.4% and 12.9%, respectively, of the maximal chronotropic and inotropic effects of isoprenaline. 4. Positive chronotropic and inotropic responses to mabuterol were dose-dependently inhibited by a selective beta 2-adrenoceptor antagonist, ICI 118,551. These responses were only slightly attenuated by atenolol. 5. Mabuterol (1-300 nmol) dose-dependently inhibited both dobutamine- and procaterol-induced positive chronotropic and inotropic responses. 6. These results indicate that mabuterol causes weak positive chronotropic and inotropic effects on the perfused canine right atrium by activating beta 2-adrenoceptors, and that higher concentrations non-selectively block both beta 1-and beta 2-adrenoceptors.     

Effects of tetrodotoxin and imipramine on the cardiac responses to nicotine in isolated, blood-perfused canine heart preparations.

The effects of nicotine on the sinus rate and atrial or left ventricular contractile force were investigated in the isolated, blood-perfused dog right atrium and left ventricle. Nicotine (3-300 nmol) in the right atrium induced dose-dependent negative followed by positive chronotropic and inotropic responses, whereas nicotine caused only a positive inotropic response in the left ventricle. The negative responses to nicotine were blocked by atropine, hexamethonium (C6), and tetrodotoxin (TTX). The positive effects of nicotine were abolished by propranolol and C6. TTX significantly inhibited the positive responses to nicotine by about 50% in the atrial preparation and totally suppressed them in the left ventricle. Imipramine inhibited the positive cardiac responses to nicotine and tyramine, but potentiated the responses to noradrenaline (NA) in atrial and ventricular preparations. These results suggest that (a) nicotine induces negative and positive cardiac effects mediated by parasympathetic ganglionic nicotinic receptors and presynaptic nicotinic receptors of the postganglionic sympathetic nerves, respectively, in the dog heart; (b) there are few parasympathetic ganglionic cells in the dog left ventricle; (c) the positive cardiac responses to nicotine are caused by both TTX-sensitive and TTX-insensitive NA release mechanisms; and (d) imipramine inhibits the positive cardiac responses to nicotine at the presynaptic nicotinic receptor sites of the postganglionic sympathetic nerves.     

Protective effect of KB-R7943, a novel Na+/Ca2+ exchange inhibitor, on ischemic acute renal failure in rats

The effects of KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate), a novel Na+/Ca2+ exchange inhibitor, on ischemic acute renal failure (ARF) in rats were examined. ARF was induced by clamping the left renal pedicle for 45 min followed by reperfusion, 2 weeks after contralateral nephrectomy. Renal function was markedly diminished in ARF rats. Pretreatment with KB-R7943 (10 mg/kg, i.v.) markedly attenuated the ARF-induced renal dysfunction. Histopathological examination of the kidney of ARF rats revealed severe renal damage, which was suppressed by KB-R7943. Activation of the reverse mode of Na+/Ca2+ exchange seems to play an important role in the pathogenesis of ARF. A selective Na+/Ca2+ exchange inhibitor may be useful in cases of ARF.     

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.     

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.     

POTENTIATION OF THE NEGATIVE CHRONOTROPIC AND INOTROPIC EFFECTS OF ADENOSINE BY 2-PHENYLAMINOADENOSINE

1. Effects of 2-phenylaminoadenosine on SA nodal pacemaker activity and atrial contractility were studied, using eleven isolated, blood-perfused dog atrial preparations. The compounds were administered via the cannulated sinus node artery of the isolated atrium. 2. 2-Phenylaminoadenosine induced negative chronotropic and inotropic effects and was 100 times less potent than adenosine in this action. 3. The interaction between adenosine and 2-phenylaminoadenosine was studied. 2-Phenylaminoadenosine potentiated the effect of adenosine on atrial muscle, but not that of acetylcholine.     

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.     

KB-R7943, a selective Na+/Ca2+ exchange inhibitor, protects against ischemic acute renal failure in mice by inhibiting renal endothelin-1 overproduction

We investigated whether the preischemic or postischemic treatment with KB-R7943, a novel and selective Na+/Ca2+ exchange inhibitor, has renal protective effects in mice with ischemic acute renal failure (ARF). Ischemic ARF was induced by clamping the left renal pedicle for 45 min followed by reperfusion, 2 weeks after contralateral nephrectomy. Renal function was markedly diminished 24 h after reperfusion. Preischemic treatment with KB-R7943 attenuated the ARF-induced renal dysfunction. The ischemia/reperfusion-induced renal dysfunction was also overcome by postischemic treatment with KB-R7943. Histopathologic examination of the kidneys of ARF mice revealed severe renal damage such as tubular necrosis, proteinaceous casts in tubuli, and medullary congestion. Histologically evident damage and Ca2+ deposition in necrotic tubular epithelium were improved by preischemic treatment with KB-R7943. In addition, preischemic treatment with KB-R7943 significantly suppressed the increment of endothelin-1 (ET-1) content in the kidney at 2, 6, and 24 h after reperfusion. These findings suggest that Ca2+ overload via the reverse mode of Na+/Ca2+ exchange, followed by renal ET-1 overproduction, plays an important role in the pathogenesis of the ischemia/reperfusion-induced ARF. KB-R7943 may prove to be an effective therapeutic agent for cases of ischemic ARF in humans.     

Influence of a Na+-H+ exchange inhibitor ethylisopropylamiloride, a Na+-Ca2+ exchange inhibitor KB-R7943 and their combination on the increases in contractility and Ca2+ transient induced by angiotensin II in isolated adult rabbit ventricular myocytes

In rabbit, ventricular myocytes loaded with indo-1/AM, angiotensin II (0.1 nM-0.1 microM) exerted a positive inotropic effect with a significant increase in the amplitude of Ca2+ transients. For a given increase in cell shortening, the increase in Ca2+ transients induced by angiotensin II was less than that induced by elevation of extracellular Ca2+ concentration ([Ca2+]0) or isoprenaline, an indication that both the increase in mobilization of intracellular Ca2+ ions and myofibrillar sensitivity to Ca2+ ions contribute to the positive inotropic effect of angiotensin II. The effects of angiotensin II on Ca2+ transients and cell shortening were inhibited by the AT1 receptor antagonist losartan. A Na+ -H+ exchange inhibitor EIPA [5-(N-ethyl-N-isopropyl)amiloride] at 1 and 3 microM did not affect the Ca2+ transients and cell shortening, but it inhibited the angiotensin-II-induced responses in a concentration-dependent manner more effectively than the responses to elevation of [Ca2+]0, indicating that EIPA elicited a selective inhibitory action on the effects of angiotensin II. The observation that EIPA at 10 microM abolished the positive inotropic effect of angiotensin II without a significant depression of the inotropic response to elevation of [Ca2+]0 supports the selective action of EIPA at the high concentration on the response to angiotensin II. A novel selective Na+ -Ca2+ exchange (reverse mode) inhibitor KB-R7943, 2-[2-[4-(-nitrobenzyloxy)phenyl]ethyl] isothiourea methanesulphonate, at 0.3 and 1 microM inhibited also the responses to angiotensin II more effectively than the response to elevation of [Ca2+]0; however, over the same concentration range it suppressed significantly the amplitude of Ca2+ transients and cell shortening. Combination of EIPA (3 microM) and KB-R7943 (0.3 microM), each of which attenuated partially the angiotensin-II-induced responses, abolished the positive inotropic effect and the increase in Ca2+ transients induced by angiotensin II with much less depressant effect on the responses to elevation of [Ca2+]0. Thus, these ion exchange inhibitors exerted selective actions on the respective targets. The results with these selective inhibitors indicate that the activation of Na+ -H+ exchanger and subsequent modulation of the activity of Na+ -Ca2+ exchanger may be responsible for the increase in [Ca2+]i and the myofilament Ca2+ sensitization induced by stimulation of AT1 receptors by angiotensin II in rabbit ventricular myocytes.     

Protection of ischemic rat spinal cord white matter: Dual action of KB-R7943 on Na+/Ca2+ exchange and L-type Ca2+ channels

The effect of the Na+/Ca(2+)-exchange inhibitor KB-R7943 was investigated in spinal cord dorsal column ischemia in vitro. Oxygen/glucose deprivation at 37 degrees C for 1 h causes severe injury even in the absence of external Ca2+. KB-R7943 was very protective in the presence and absence of external Ca2+ implicating mechanisms in addition to extracellular Ca2+ influx through Na+/Ca(2+)-exchange, such as activation of ryanodine receptors by L-type Ca2+ channels. Indeed, blockade of L-type Ca2+ by nimodipine confers a certain degree of protection of dorsal column against ischemia; combined application of nimodipine and KB-R7943 was not additive suggesting that KB-R7943 may also act on Ca2+ channels. KB-R7943 reduced inward Ba2+ current with IC50 = 7 microM in tsA-201 cells expressing Ca(v)1.2. Moreover, nifedipine and KB-R7943 both reduced depolarization-induced [Ca2+]i increases in forebrain neurons and effects were not additive. Nimodipine or KB-R7943 also reduced ischemic axoplasmic Ca2+ increase, which persisted in 0Ca2+/EGTA perfusate in dorsal column during ischemia. While KB-R7943 cannot be considered to be a specific Na+/Ca2+ exchange inhibitor, its profile makes it a very useful neuroprotectant in dorsal columns by: reducing Ca2+ import through reverse Na+/Ca2+ exchange; reducing influx through L-type Ca2+ channels, and indirectly inhibiting Ca2+ release from the ER through activation of ryanodine receptors.     

Functional role of Na+/H+ exchanger in Ca2+ influx mediated via human endothelin type A receptor stably expressed in Chinese hamster ovary cells

This study examines the functional role of Na+/H+ exchanger (NHE) in Ca2+ influx mediated by human endothelin type A receptor (ET(A)R) expressed in Chinese hamster ovary (CHO) cells. Endothelin-1 (ET-1) increased extracellular acidification rate (ECAR), which was abolished by 5-(N-ethyl-N-isopropyl)amiloride (EIPA), an NHE inhibitor. EIPA and KB-R7943, a Na+/Ca2+ exchanger (NCX) inhibitor, inhibited ET-1-induced sustained increases in intracellular Ca2+ concentration ([Ca2+]i), and EIPA had no effect on [Ca2+]i after KB-R7943 treatment. ET-1-elicited sustained [Ca2+]i increase was suppressed by reducing extracellular Na+ concentration. These results suggest that possible coupling of NHE with NCX via Na+ transport is involved in ET(A)R-mediated sustained [Ca2+]i increase.     

The plasma membrane Na+/Ca2+ exchange inhibitor KB-R7943 is also a potent inhibitor of the mitochondrial Ca2+ uniporter

BACKGROUND AND PURPOSE: The thiourea derivative KB-R7943, originally developed as inhibitor of the plasma membrane Na(+)/Ca(2+) exchanger, has been shown to protect against myocardial ischemia-reperfusion injury. We have studied here its effects on mitochondrial Ca(2+) fluxes. EXPERIMENTAL APPROACH: [Ca(2+)] in cytosol, mitochondria and endoplasmic reticulum (ER), and mitochondrial membrane potential were monitored using both luminescent (targeted aequorins) and fluorescent (fura-2, tetramethylrhodamine ethyl ester) probes in HeLa cells. KEY RESULTS: KB-R7943 was also a potent inhibitor of the mitochondrial Ca(2+) uniporter (MCU). In permeabilized HeLa cells, KB-R7943 inhibited mitochondrial Ca(2+) uptake with a Ki of 5.5+/-1.3 microM (mean+/-S.D.). In intact cells, 10 microM KB-R7943 reduced by 80% the mitochondrial [Ca(2+)] peak induced by histamine. KB-R7943 did not modify the mitochondrial membrane potential and had no effect on the mitochondrial Na(+)/Ca(2+) exchanger. KB-R7943 inhibited histamine-induced ER-Ca(2+) release in intact cells, but not in cells loaded with a Ca(2+)-chelator to damp cytosolic [Ca(2+)] changes. Therefore, inhibition of ER-Ca(2+)-release by KB-R7943 was probably due to the increased feedback Ca(2+)-inhibition of inositol 1,4,5-trisphosphate receptors after MCU block. This mechanism also explains why KB-R7943 reversibly blocked histamine-induced cytosolic [Ca(2+)] oscillations in the same range of concentrations required to inhibit MCU. CONCLUSIONS AND IMPLICATIONS: Inhibition of MCU by KB-R7943 may contribute to its cardioprotective activity by preventing mitochondrial Ca(2+)-overload during ischemia-reperfusion. In addition, the effects of KB-R7943 on Ca(2+) homeostasis provide new evidence for the role of mitochondria modulating Ca(2+)-release and regenerative Ca(2+)-oscillations. Search for permeable and selective MCU inhibitors may yield useful pharmacological tools in the future.     

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.     

Cardiodepressant actions of the orally active male contraceptive, gossypol, in guinea-pig heart muscle

We have previously shown that gossypol has direct actions on isolated atrial muscle preparations. The possible mechanisms responsible for the transient positive and sustained negative inotropic effects of gossypol were examined under conditions that modify Ca2+ pools involved in contractile activation. In Langendorff preparations obtained from guinea-pig or rat heart, gossypol produced marked negative inotropic and arrhythmogenic effects but failed to produce a positive inotropic effect. Langendorff preparations were significantly more sensitive than atrial muscle preparations. In atrial muscle preparations, the negative inotropic effect of gossypol was not specific to utilization of superficial or intracellular Ca2+ pools; force-staircase phenomenon observed between 0.5 and 3 Hz, contractions elicited by slow action potentials in partially depolarized muscle, the inotropic effect of extracellular Ca2+ and potentiated post-rest contractions were all suppressed by gossypol to the similar extent. Low external Na+ concentrations abolished the positive inotropic effect of gossypol without affecting the negative inotropic effect. A low extracellular Ca2+ concentration enhanced the transient positive inotropic effect and delayed development of the negative inotropic effect. Simultaneous reduction of extracellular Na+ and Ca2+ concentrations abolished the positive inotropic effect and enhanced the negative inotropic effect. Gossypol inhibited ATP-dependent Ca2+ uptake by sarcolemmal vesicles obtained from dog heart. These results indicate that the actions of gossypol on cardiac muscle is not specific to utilization of either the superficial or intracellular Ca2+ pools involved in contractile activation.     

Amiloride and KB-R7943 in outward Na+ /Ca2+ exchange current in guinea pig ventricular myocytes

The reverse mode of Na+/Ca2+ exchange represents an important pathway in inducing Ca2+ overload during ischemia and reperfusion. The inhibitory effects of amiloride and KB-R7943 on Na+/Ca2+ exchange current (INa/Ca) were investigated in guinea pig ventricular myocytes. Whole-cell patch clamp techniques were used under bidirectional ionic conditions and 25 mM of Na+ in pipette solution. At +50 mV, amiloride 10, 30, and 100 microM inhibited the outward INa/Ca by 15, 23, and 41%, respectively; at -80 mV, it inhibited inward INa/Ca by 6, 15, and 23%, respectively. Its inhibitory effect on outward INa/Ca was greater than that on inward INa/Ca. At +50 mV, KB-R7943 1 and 10 microM inhibited the outward INa/Ca by 29 and 61%, respectively; at -80 mV, it inhibited inward INa/Ca by 22 and 57%, respectively. KB-R7943 inhibited both directions of the exchange current with an equal potency. The data suggest that KB-R7943 is not a selective inhibitor on reverse mode of Na+/Ca2+ exchange.     

KB-R7943, a Na+/Ca2+ exchange inhibitor,does not suppress ischemia/reperfusion arrhythmias nor digitalis arrhythmias in dogs

KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate) has been used as a pharmacological tool to block the Ca(2+) influx-mode of the Na(+)/Ca(2+) exchanger, which is thought to contribute to ischemia/reperfusion and digitalis arrhythmias. We examined effects of KB-R7943 on ischemia/reperfusion arrhythmias in beagle dogs anesthetized with sodium pentobarbital. Lead II ECG and BP were measured. KB-R7943 or the solvent (10% DMSO) was injected i.v. as a bolus, and 5 min later, the left anterior descending coronary artery was occluded for 30 min followed by reperfusion. KB-R7943 at 5 or 10 mg/kg increased BP without changing ECG parameters including the heart rate. Although 5 mg/kg KB-R7943 deceased the number of arrhythmic beats during the ischemic period, mortality due to ischemia/reperfusion was not decreased by KB-R7943 (5 and 10 mg/kg). KB-R7943 at 5 mg/kg also did not suppress the ouabain-induced arrhythmias. These negative results suggest that Na(+)/Ca(2+) exchange inhibition may not be a useful strategy of suppressing arrhythmias.     

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.     

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.     

Effect of Na+/Ca2+ exchange inhibitor, KB-R7943 on ouabain-induced arrhythmias in guinea-pigs.

1. We investigated protective effects of KB-R7943, a Na+/Ca2+ exchange (NCX) inhibitor, on ouabain-induced tonotropy and arrhythmias in isolated whole atria and ouabain-induced changes in electrocardiogram (ECG) in the guinea-pig. 2. KB-R7943 (10 and 30 microM) suppressed the tonotropic effect of ouabain, and prolonged the onset time of extra-systole induced by ouabain in isolated atria. 3. The intravenous injection of KB-R7943 (1 and 3 mg kg-1) significantly increased the doses of ouabain required to induce ventricular premature beats (VPB), ventricular tachycardia (VT), ventricular fibrillation (VF) and cardiac arrest (CA) in anaesthetized guinea-pigs. 4. Lidocaine (Na+channel inhibitor) and R56865 (Na+ and Ca2+ overload inhibitor) also suppressed the ouabain-induced tonotropic effect and extra-systole in isolated atria, but Hoe-694 (Na+/H+ exchange inhibitor) or diltiazem (Ca2+ channel inhibitor) did not affect them. 5. Lidocaine also increased the doses of ouabain required to induce VPB, VT, VF and CA in anaesthetized guinea-pigs. 6. From these results, we conclude that KB-R7943 suppresses ouabain-induced arrhythmias through inhibition of the reverse-mode NCX.