Effect of citalopram on ouabain-induced arrhythmia in isolated guinea-pig atria

The effect of citalopram (CTP) a selective serotonin reuptake inhibitor agent was studied on ouabain-induced arrhythmia in spontaneously beating isolated guinea-pig atria. CTP (2-32 microg/ml) produced a dose-dependent decrease in the force of contractions (7-62%), and in the rate of contractions (11-72%). Pre-administration of the atria with CTP inhibited the ouabain-induced arrhythmia in isolated atria. Ouabain alone (1.2 microg/ml) produced arrhythmia at 4.5 min, and asystole at 20.7 min. Pretreatment with CTP (8 microg/ml) significantly increased the time of onset of arrhythmia to 9.5 min. In addition CTP prolonged the beating of atria (survival time) to more than 56 min, and inhibited the occurrence of asystole. These findings indicate that CTP produces direct cardiac action, probably due to the inhibition of cardiac Na(+) and Ca(2+) channels. Moreover our results suggest that CTP may reduce the membrane conductance through inhibition of ionic channels which decrease ouabain-induced arrhythmia.     

The protective action of R56865 against ouabain-induced intoxication in rat heart isolated atria and ventricles

The action of R56865 has been examined on the contractile effects produced by ouabain concentrations interacting with high (3 microM) and low (300 microM) affinity digitalis receptors on electrically stimulated ventricular strips isolated from rat. R56865 1 microM reduced the increase in resting tension produced by ouabain 300 microM and left unalterated the inotropic effect evoked by ouabain 3 and 300 microM that was reduced by higher concentrations (3 and 6 microM) of R56865. The action of R56865 was also studied on ouabain-induced intoxication in electrically stimulated and spontaneously beating atria of rat. On electrically stimulated (3 Hz) whole atria, R56865 0.3 microM reduced the maximal increase in resting tension produced by ouabain 300 microM and delayed the time to onset of the ouabain-induced arrhythmias but did not affect ouabain's inotropic effect. Higher concentrations of R56865 were required to reduce the inotropic effect of ouabain. The protective action of R56865 against ouabain-induced intoxication was most pronounced on spontaneously beating atria where it reduced spontaneous rate of beats. Experiments in electrically driven left atria indicated that only a part of the protective effect of R56865 could be related to its bradycardic action. The effect of R56865 was also examined on ouabain-induced inhibition of sodium pump in human red blood cells. R56865 6 microM did not modify the inhibition produced by ouabain (from 0.3 to 10 nM), this indicates that the protective action of R56865 against ouabain-induced intoxication is not due to an interaction with the inhibitory effect of ouabain on sodium pump.     

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.     

Effects of purified endogenous inhibitor of the Na+/Ca2+ exchanger on ouabain-induced arrhythmias in the atria and ventricle strips of guinea pig

Previous studies demonstrated that the purified endogenous inhibitor (NCX(IF)) of the cardiac Na(+)/Ca(2+) exchanger (NCX1) has the capacity to modulate cardiac muscle contractility. Here, we tested the effects of purified NCX(IF) on arrhythmias induced by ouabain in the atria and ventricle strips of guinea pig. For the sake of comparison NCX(IF) was compared to lidocaine and KB-R7943. In the ventricle strip, NCX(IF) ( approximately 10 U/ml) results in rapid, complete and stable inhibition of ouabain-induced arrhythmias (the inhibition of arrhythmia is not followed by revival of irregular contractions). Under similar experimental conditions the atria strips require somewhat higher doses of NCX(IF) (25-50 U/ml) for complete suppression of arrhythmia. In the atria strip, NCX(IF) (10-25 U/ml) increases the threshold dose (1 microM) of ouabain for arrhythmia onset 2.2+/-0.5-fold (n=5, p<0.05) as well as prolongs the lag-phase for arrhythmia appearance 4.0+/-0.5-fold (n=5, p<0.01). The lag period for arrhythmia onset was also lengthened (2.0+/-0.4-fold) by NCX(IF) in the ventricle strips (n=6, p<0.002). At low frequency of pacing (1 Hz), all three tested substances, lidocaine, KB-R7943, and NCX(IF) can effectively suppress the ouabain-induced arrhythmia. However, at higher frequency (2 Hz), lidocaine is ineffective in suppressing arrhythmia, whereas KB-R7943 becomes pro-arrhythmic. In contrast to reference drugs, NCX(IF) retains its anti-arrhythmic capacity at high frequencies, either in the atria (n=6, p<0.01) or ventricle (n=5, p<0.05) strips. In conclusion, NCX(IF) results in rapid, effective and stable suppression of arrhythmia both in the atria and ventricle preparations under conditions at which the reference drugs become ineffective.     

Irreversible binding of acetylethylcholine mustard to cardiac cholinergic muscarinic receptors

The interaction of acetylethylcholine mustard (Aech-M) with cardiac muscarinic receptors was studied with radioligand binding techniques and with isolated beating atria. Aech-M and acetylcholine competed about equally for (-)-[3H]quinuclidinyl benzilate (QNB)-binding sites and 5'-guanylylimidodiphosphate reduced the ability of both compounds to compete by 30-fold. Pretreatment of cardiac membranes with 0.25 microM Aech-M, followed by washing, reduced the [3H]QNB binding capacity by 60% without changing the ligand affinity of the receptors left. In the same membranes, a similar fraction of receptors was lost as measured by [3H]N-methyoscopolamine, but a greater fraction was lost when measured by [3H]oxotremorine-M. The loss of [3H]QNB binding capacity was dose, time, and temperature dependent, blocked by atropine and carbachol, and modulated by several guanine nucleotides but not affected by membrane pretreatment with several group-selective reagents. Superfusion of spontaneously beating atria with Aech-M (100 microM) initially reduced the beating rate which returned to control values by 17 min. Atropine blocked the initial reduction in beating rate. In contrast, both carbachol and acetylcholine produced sustained decreases in the beating rate. After pretreatment of atria for 30 min with 100 microM Aech-M alone or 10 microM Aech-M plus 10 microM edrophonium followed by washing, the EC50 value for carbachol inhibition of the beating rate was increased 15.8- and 10.3-fold, respectively, with no change in the ability of isoproterenol to increase the spontaneous beating rate. In addition, there was a 46-47% and a 37-41% reduction in the binding capacity of both [3H]QNB and [3H]oxotremorinel-M in the 100 and 10 microM Aech-M-pretreated atria, respectively. The data indicated that Aech-M is a muscarinic agonist which appears to irreversibly bind to the muscarinic receptor.     

Interaction of amrinone with endogenous adenosine in guinea-pig atria.

In spontaneously beating atria from reserpine-treated guinea-pigs, amrinone (10 microM to 2 mM) induced a positive inotropic and chronotropic effect that was preceded by a transient reduction in contractile force and in frequency. Both the positive and negative effects were concentration-dependent. The inotropic action of amrinone was antagonized by low concentrations of 8-phenyltheophylline that compete with adenosine at R-receptors on plasma membrane without significantly influencing phosphodiesterase activity. Cumulative concentrations of amrinone (1 mM) antagonized the reduction of rate of contraction and amplitude induced by dipyridamole 1 microM in spontaneously beating atria and restored the maximum contractile effect reached in the absence of dipyridamole. In spontaneously beating preparations incubated in the presence of adenosine deaminase (1 u ml-1), amrinone lost its positive effects on the atria and only reduction of rate and contractile force was evident. Both effects were antagonized by scopolamine 1 mM thus indicating their cholinergic nature. Adenosine at 0.1 microM and 0.5 microM significantly inhibited the inotropic effect induced by amrinone (0.03 to 3 mM) and the concentration-effect curves of amrinone obtained in the absence and presence of adenosine clearly indicate a competitive antagonism between the two drugs. Thus the contractile activity of amrinone in spontaneously beating atria from reserpine-treated guinea-pigs originates from a displacement of adenosine from its R-receptor sites in the cardiac cell.     

Comparison of the effect of amiloride and its analogue dichlorobenzamil on cardiac chronotropic responses to ouabain in myocardial cell aggregates in culture

The purpose of this study was to compare the effects of amiloride, an inhibitor of Na(+)-H+ exchange, and its analog 3',4'-dichlorobenzamil, a more specific inhibitor of Na(+)-Ca2+ exchange on the response of cardiac myocytes to ouabain. Cardiac myocyte aggregates were prepared from myocytes obtained from 7-day-old chick embryo hearts. Ouabain at 10(-6) M produced a marked and significant (p less than 0.05) reduction in contractile frequency. Amiloride, at 10(-7) to 10(-5) M produced a definite, significant (p less than 0.05) and dose-dependent reduction in this effect of ouabain. In contrast, dichlorobenzamil, 10(-7) to 10(-6) M, significantly (p less than 0.05) accentuated this effect of ouabain. Thus, amiloride and its analog dichlorobenzamil have different effects on the cardiac responses to ouabain presumably because of the differences in the specificity of their inhibition of Na+/H+ and Na+/Ca2+ exchange. Thus to the extent that the effects of amiloride and dichlorobenzamil are mediated through, respectively Na(+)-H+ and Na(+)-Ca2+ exchange, these data suggest that ouabain-induced reduction in contractile frequency is mediated through Na(+)-H+ exchange while Na(+)-Ca2+ exchange acts to minimize this action of ouabain. Amiloride may be useful to oppose the negative chronotropic effect of ouabain while dichlorobenzamil accentuates this effect of ouabain.     

Cardiotoxic and inotropic effects of ouabain on atria isolated from rabbits of different ages.

1 In sino-atrial node-right atrial preparations and left atrial preparations obtained from rabbits of different ages (2 to 360 days old), cardiotoxic and inotropic effects of ouabain were compared. Tachyarrhythmias were produced in immature and mature rabbit atria exposed for 6 to 8 min to ouabain (5 x 109(-6) M). The total number of contractions before the arrhythmias developed varied inversely with age (2 to 90 days old). Percentage reductions in the resting membrane potential, overshoot and maximum rate of rise induced by ouabain were greater in mature rabbit atria than in immature rabbit atria. 2 Reduction of Ca2+ in the bathing media suppressed the development of tachyarrhythmias. 3 In electrically-driven left atria isolated from immature and mature rabbits, similar frequency-contractile force curves were obtained, maximum contractions being attained at a frequency of 120/min. Ouabain at 2 x 10(-7) M shifted the curve upward and at 10(-6) M greatly potentiated the contractile force developed at low frequencies (6 to 30/min). Increase in contractions induced by ouabain relative to pre-drug concentrations at a frequency of 60/min were greater in immature rabbit atria (15 days old) but the positive inotropic effect of 2 x 10(-7) M ouabain relative to the effect seen at 10(-6) M was appreciably less. 4 It may be concluded that atria isolated from immature rabbits tolerate higher concentrations of ouabain than those isolated from mature rabbits.     

Influence of highly unsaturated phosphatidylcholine on the effects of ouabain and some cardioactive drugs on cardiac contractile force and Na+, K+-ATPase activity.

Isolated left guinea pig auricles and cardiac Na,K-ATPase from calf heart were incubated with highly unsaturated phosphatidylcholine (PC) for 2 hr. Thereafter the actions of ouabain on Na, K-ATPase, and of ouabain, digoxin, digitoxin, isoproterenol, acetylcholine, pentobarbital and different extracellular Ca²⁺ concentrations on contractile force of the auricles were investigated. PC itself altered neither the ATPase activity nor the intracellular ionic homeostasis, nor contractile force of the auricles. Similarly, the staircase phenomenon, the contractile response to different extracellular Ca²⁺ concentrations, to pentobarbital and ouabain, and the extent of ouabain-induced inhibition of the ATPase were not influenced. In addition the toxicity of ouabain was not altered. However, the rate of development of the ouabain-induced ATPase inhibition, as well as of the positive inotropism and toxic effects of ouabain, digoxin and digitoxin was markedly reduced. The maximum inotropic effect of digoxin and digitoxin, and the toxicity of digitoxin were significantly enhanced. Remarkably, the binding of [³H]digitoxin was significantly diminished in PC-pre-incubated auricles, binding of [³H]ouabain, however, remained unchanged. The dose-response curves to isoproterenol and acetylcholine, the latter also in the presence of physostigmine, were markedly parallel-shifted to the right. The modifying effect of highly unsaturated PC on the action of the drugs studied is suggested to be due to an altered physico-chemical state (fluidity) of the outer surface of the cellular membrane.     

Arrhythmogenic, antiarrhythmic and inotropic properties of opioids. Effects of piritramide, pethidine and morphine compared on heart muscle isolated from rats

Since the effects in the intact organism are complicated by central as well as peripheral effects, we compared the direct cardiac effects of three commonly used opioids on isolated heart muscle. Concentration-response curves for electrophysiological and inotropic effects of piritramide, pethidine and morphine (10(-6)-10(-4) mol/l) on spontaneously beating and electrically stimulated rat atria were obtained. Piritramide decreased spontaneous frequency, induced arrhythmia and cardiac arrest. It had no significant effect on effective refractory period and electrical threshold for excitation, but decreased contractile force. Pethidine increased effective refractory period, had no effect on electrical threshold and increased contractile force. Morphine induced no significant electrophysiological effects, but decreased contractile force slightly. These results indicate direct negative chronotropic and arrhythmogenic actions of piritramide, possible class III antiarrhythmic action of pethidine and lack of major direct cardiac effects of morphine.     

Interaction of the cardiotonic agent DPI 201-106 with cardiac Ca2+ channels

The cardiotonic agent DPI 201-106 was investigated for its effects on (a) contractile force in guinea pig left atria, (b) membrane currents in isolated guinea pig cardiac myocytes, and (c) [3H]nitrendipine binding in guinea pig cardiac membranes. The compound elicited a positive inotropic effect in normally polarized (5.9 mM extracellular KCl) and a negative inotropic effect in partially depolarized (20 mM KCl) isolated, electrically stimulated left atria. This decrease in contractile force was probably caused by inactivation of the fast Na+ inward current and concomitant blockade of the inward Ca2+ current. The blocking effect on Ca2+ channels was directly shown in voltage-clamp experiments using isolated cardiocytes. Further evidence for interaction of DPI 201-106 with Ca2+ channels was obtained from the [3H]nitrendipine binding studies. Thus, Ca2+ antagonism contributes to the complex pharmacologic profile of DPI 201-106, and is probably responsible for the bradycardia and lowering of systemic vascular resistance observed in vivo.     

Antagonism between (-)-N6-phenylisopropyladenosine and the calcium channel facilitator Bay K 8644, on guinea-pig isolated atria.

Antagonism between (-)-N6-phenylisopropyladenosine (PIA) and the dihydropyridine calcium channel facilitator Bay K 8644 was investigated in guinea-pig spontaneously beating or electrically driven isolated atria, taken from normal and from reserpine-treated animals. PIA (3-100 nM) produced a dose-dependent decrease in contractile tension and frequency in spontaneously beating atria being more effective in reserpinized preparations. Bay K 8644 (5-200 nM) produced an increase in contractile tension in both normal and reserpinized atria. In electrically driven left atria the positive inotropic effect of Bay K 8644 was similar to that in spontaneously beating preparations. The positive chronotropic effect of Bay K 8644 was slight and variable. PIA produced a rightward parallel shift of the concentration-response curves for the positive inotropic effects of Bay K 8644 in all experimental conditions. In spontaneously beating atria from normal guinea-pigs, the Schild regression plot was linear and its slope near to unity; pA2 of PIA 8.63 +/- 0.05 (IC50 2.35 +/- 0.25 nM). In electrically driven atria the antagonism by PIA of the effects of Bay K 8644 was apparently competitive, and the IC50 of PIA was 18.6 +/- 0.4 nM. PIA antagonized the positive chronotropic effect of Bay K 8644 in spontaneously beating preparations, both from normal and from reserpine-treated animals. Carbachol did not modify the positive inotropic effects of Bay K 8644. These data indicate that PIA may interact with Bay K 8644 at the level of the slow calcium channels, and may decrease the transmembrane calcium flux into the cell.     

Cardiac glycoside receptors in cultured heart cells--I. Characterization of one single class of high affinity receptors in heart muscle cells from chick embryos

Binding of (3H)-ouabain and ouabain-induced inhibition of the sodium pump and of the (Na+ + K+)-ATPase have been characterized in cultured cardiac muscle and non muscle cells, as well as in cardiac cell membranes--all obtained from chick embryos. In both cell types, ouabain binds to a single type of binding sites in a temperature-dependent manner. The association rate but not the dissociation rate, is lowered by K+; specific binding is lost after heat-denaturation of the cells. Binding parameters (association and dissociation rate constants, activation energies for association and dissociation) are similar in muscle and non muscle cells. The dissociation constant of specific ouabain binding is 1.5 X 10(-7)M in cardiac muscle cells, and 1.9 X 10(-7)M in cardiac non muscle cells, the binding capacity being 2.6 and 2.1 pmoles/mg protein respectively. Specific binding of ouabain to the cells is coupled to inhibition of the sodium pump, as can be seen from ouabain-induced inhibition of active (86Rb+ + K+)-uptake, decrease in cellular K+, and increase in cellular Na+ (EC50 = 10(-7)-10(-6)M). The data obtained with cardiac cells are in good agreement with results found for ouabain binding (dissociation constant 4.3 X 10(-7)M) and (Na+ + K+)-ATPase inhibition (EC50 = 1.4 X 10(-6)M) in cardiac cell membranes prepared from the same tissue. Due to the experimental evidence it is concluded that the binding site for ouabain is identical with the cardiac glycoside receptor of these cells. In cardiac non muscle cells, binding of ouabain to its receptor is strictly coupled to inhibition of active K+-transport in a stoichiometric manner. In cardiac muscle cells, however, active K+-transport is inhibited by less than 10% when up to 40% of cardiac glycoside receptors have bound ouabain. It is assumed that this non-stoichiometric coupling of receptor occupancy and sodium pump inhibition in cardiac muscle cells may prevent substantial changes of Na+- and K+-contents in the heart in the presence of therapeutic levels of cardiac glycosides.     

A comparison of the effects of ouabain, noradrenaline and nifedipine on the contractile force of the isolated rat atrium at different calcium levels

The effects of ouabain 5 x 10-5 M, noradrenaline 10-7 M and nifedipine 100 mug/1 on the contractile force of the isolated rat left atrium were tested and compared at varying concentrations of calcium in the Ringer solution. The effect of ouabain was small, developed slowly and almost independently of the calcium concentration. Noradrenaline, which increases Ca++ influx during excitation, caused an increase in the contractile force which was complete within 2 min. The percentage as well as the absolute increase in contractile force was pronounced at lower, but small at higher calcium concentrations. Nifedipine, which reduces Ca++ influx during excitation, caused a decrease in contractile force which was complete within 2-4 min. The nifedipine-induced depression in contractile force decreased with a rise in the calcium concentration. It is assumed that the ouabain-induced increase in contractile force in the rat, is not mediated by an increase in the magnitude of the inward calcium current, and other modes of action for the inotropic effect of glycosides are discussed.     

Embelin reduces cutaneous TNF-α level and ameliorates skin edema in acute and chronic model of skin inflammation in mice

Ursolic acid is reported to have beneficial effects on the regulation of cardiovascular homeostasis. However, the effects of ursolic acid on cardiac hormone secretion are yet to be defined. The present study was designed to test the effects of ursolic acid on the secretory and contractile functions of the atria. Experiments were conducted in isolated perfused beating rabbit atria. We measured the changes in atrial dynamics, pulse pressure, stroke volume, cAMP efflux, as well as the secretion of atrial natriuretic peptide (ANP). Ursolic acid increased ANP secretion and mechanical dynamics in a concentration-dependent manner. The inhibition of L-type Ca(2+) channels with nifedipine attenuated the ursolic acid-induced increase in ANP secretion but not mechanical dynamics. The inhibition of K(+)(ATP) channels with glibenclamide attenuated the ursolic acid-induced increase in ANP secretion-but not atrial dynamics-in a concentration-dependent manner. The selective Na(+)-K(+)-ATPase inhibitor ouabain blocked the ursolic acid-induced increase in atrial dynamics but not ANP secretion. These findings show that ursolic acid increases ANP secretion via its activation of K(+)(ATP) channels and subsequent inhibition of Ca(2+) entry through L-type Ca(2+) channels in rabbit atria. These data also suggest that ursolic acid increases atrial dynamics via its inhibition of Na(+)-K(+)-ATPase activity.     

Protective activity of calcium entry blockers against ouabain intoxication in anesthetized guinea pigs

Several studies have suggested a central role for calcium in the pathogenesis of digitalis-induced arrhythmias. To test this hypothesis, the effects on ouabain-induced arrhythmia of intraarterial pretreatment with the calcium entry blockers nifedipine, flunarizine, verapamil, diltiazem, and bepridil, the calcium entry promotor Bay K 8644, and CaCl2 were compared with those of the currently applied digitalis antidotes phenytoin and lidocaine in urethane-anesthetized (1.5 g/kg i.p.) guinea pigs. Pretreatment with nifedipine (0.03 and 0.1 mg/kg), flunarizine (1 and 3 mg/kg), and phenytoin (10 mg/kg) doubled the time (from 10-20 to 20-40 min) required to provoke toxic ECG changes. Verapamil, diltiazem, and bepridil caused a slight but significant reduction of ouabain toxicity. Pretreatment with CaCl2 (10 mg/kg) enhanced all toxic effects of ouabain. None of the above-mentioned pretreatments as such changed the ECG parameters. Bay k 8644 (0.03 and 0.1 mg/kg) enhanced the effects of ouabain on ventricular rhythm, but abolished the ouabain-induced impairment of AV conduction. Bay k 8644 as such increased heart rate (from 318 +/- 11 to 376 +/- 6 beats/min at 0.1 mg/kg) and shortened the PR interval. The negative inotropic effects of the calcium entry blockers were quantified in electrically paced (3 Hz) guinea pig isolated left atria 15 min after pretreatment with ouabain (3 X 10(-7) M). The rank order of potency for the negative inotropic effect was nifedipine greater than verapamil greater than bepridil greater than diltiazem greater than flunarizine. In conclusion, nifedipine, flunarizine, and phenytoin showed obvious and equally effective protection against ouabain-induced arrhythmia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adrenergic and cholinergic activity contributes to the cardiovascular effects of lionfish (Pterois volitans) venom.

The aim of the present study was to further investigate the cardiovascular activity of Pterois volitans crude venom. Venom (0.6-18 microg protein/ml) produced dose- and endothelium-dependent relaxation in porcine coronary arteries that was potentiated by atropine (10nM), but significantly attenuated by the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (NOLA; 0.1mM), by prior exposure of the tissue to stonefish antivenom (SFAV, 3 units/ml, 10 min), or by removal of extracellular Ca(2+). In rat paced left atria, venom (10 microg protein/ml) produced a decrease, followed by an increase, in contractile force. Atropine (0.5 microM) abolished the decrease in force and potentiated the increase. Propranolol (5 microM) did not affect the decrease in force but significantly attenuated the increase. In spontaneously beating right atria, venom (10 microg protein/ml) produced an increase in rate that was significantly attenuated by propranolol (5 microM). Prior incubation with SFAV (0.3 units/microg protein, 10 min) abolished both the inotropic and chronotropic responses to venom. In the anaesthetised rat, venom (100 micro protein/kg, i.v.) produced a pressor response, followed by a sustained depressor response. Atropine (1mg/kg, i.v.) potentiated the pressor response. The further addition of prazosin (50 microg/kg, i.v.) restored the original response to venom. Prior administration of SFAV (100 units/kg, i.v., 10 min) significantly attenuated the in vivo response to venom. It is concluded that P. volitans venom produces its cardiovascular effects primarily by acting on muscarinic cholinergic receptors and adrenoceptors. As SFAV neutralised many of the effects of P. volitans venom, we suggest that the two venoms share a similar component(s).     

Effects of labedipinedilol‐A,third‐generation dihydropyridine‐type calcium blocker,on ouabain‐induced arrhythmia

Labedipinedilol‐A, a novel dihydropyridine‐type calcium antagonist with α/β‐adrenoceptor blocking properties, has been reported to produce a cardioprotective effect against ischemia reperfusion injury in rats. We investigated the protective effects of labedipinedilol‐A on ouabain‐induced tonotropy and arrhythmias in isolated whole atria, and using patch‐clamp techniques to study the underlying mechanism of its antiarrhythmic activity on isolated cardiac myocytes. Labedipinedilol‐A (10 µM) suppressed the tonotropic effect of ouabain significantly and prolonged the onset time of extra‐systole (arrhythmia) induced by ouabain in isolate atria. In the voltage‐clamp study, labedipinedilol‐A (1–100 µM) reduced the peak amplitude of sodium inward current (I_Na) and L‐type calcium current (I_Ca‐L), and shifted the current‐voltage (I‐V) curve upward in a concentration‐dependent manner. In contrast, the addition of labedipinedilol‐A increased transient outward potassium current (I_to) and inward rectifier potassium current (I_K1) significantly. Labedipinedilol‐A (10 µM) also effectively depressed the isoproterenol‐induced increase in the Ca²⁺ current. These results show that labedipinedilol‐A blocks I_Ca‐L and I_Na, and increases I_to and I_K1. These findings indicate that labedipinedilol‐A produces direct cardiac action, probably due to the inhibition of cardiac Na⁺ and Ca²⁺ channels. Our results suggest that labedipinedilol‐A may reduce the membrane conduction through inhibition of ionic channels which decrease ouabain‐induced arrhythmia. Drug Dev Res 69:26–33, 2008 © 2008 Wiley‐Liss, Inc.     

Ouabain-induced contraction of vascular smooth muscle in spontaneously hypertensive rats and the effect of hydralazine

The effect of ouabain (10(-3) M) on contractile responses of SHR (spontaneously hypertensive rat) and WKY (Wistar-Kyoto rat) aortas and mesenteric arteries was studied. Ouabain addition caused a rapid contraction of aortic strips with a steeper rate of rise and a larger maximal force development in strips from SHR than WKY. This difference in contractile response is known to occur in the prehypertensive period of SHR (4-week-old). Phentolamine (10(-6) M) pretreatment had no effect on the ouabain-induced contraction but partially suppressed it in both SHR and WKY aortas when diltiazem (10(-5) M) was also added. The difference in the ouabain-induced contractions of SHR and WKY aortas was more apparent in the residual contraction during suppression by diltiazem. The 45Ca uptake in the presence of ouabain was significantly larger in the early period of incubation in SHR aorta than in WKY aorta. The ouabain-induced contraction of hydralazine-treated SHR aorta from the prehypertensive period was very similar to that of non-treated WKY aorta. These results suggested that the abnormality of the ouabain-induced contraction in SHR arterial smooth muscle could have arisen from an increased Ca2+ movement due to Ca2+ leakage when ouabain inhibited the Na+-pump in the membrane. This abnormality seems to start during the prehypertensive period and continue in the hypertensive stage.     

Effects of canrenone on aorta and right ventricle of the rat

Canrenone is a major active metabolite of spironolactone and, in addition to the antimineralocorticoid effect, shares with the parent compound the action as a partial agonist with respect to ouabain on the Na+-K+ ATPase. We have investigated whether canrenone, through its action on Na+-K+ ATPase, reverses rat aorta contractions induced by ouabain and has vasorelaxant properties unrelated to its interaction with ouabain. Contractile responses of endothelium-deprived aorta to 1 mM ouabain, 0.1 microM phenylephrine, 10 microM serotonin, and 60 mM K+ were relaxed by canrenone (50-250 microM), with maximum inhibitions of 85.3%, 55.3%, 56.7%, and 64.2%, respectively. Canrenone shifted to the right the concentration-response curve for Ca2+ in depolarized aorta and did not affect the response to 10 mM caffeine. In rat right ventricular strips driven at 0.1 Hz, canrenone exerted negative inotropic effect. The relaxation of ouabain-induced contraction may be due, at least in part, to an interaction between canrenone and ouabain on the Na+-K+ ATPase. Inhibition of calcium entry through calcium channels either in aorta or ventricles is the most parsimonious hypothesis of mechanism underlying the effect of canrenone on contractile responses of rat aorta to agonists and high K+ and the negative inotropic effect on ventricular strips.