Guinea Pig Ventricular Papillary Muscles Research Articles

Contribution of ATP-Mediated Positive Feedback to Sympathetic Nerve-Induced Positive Inotropy in Guinea Pig Ventricular Myocardium.

The functional role of ATP released from sympathetic nerve terminals was examined in isolated guinea pig ventricular papillary muscles. The contractile force of papillary muscles was increased by field electrical stimulation of sympathetic nerve endings. This increase was attenuated by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) or suramin, blockers of the P2X receptor, and was abolished by propranolol and prazosin. PPADS, suramin, and ATP affected neither the basal contractile force nor the positive inotropic effect of noradrenaline. These results provide functional evidence that ATP released from sympathetic nerve terminals enhances noradrenaline release and contributes to sympathetic nerve-induced inotropy.

Nonclinical Cardiovascular Studies of Prucalopride, a Highly Selective 5-Hydroxytryptamine 4 Receptor Agonist.

Patients with chronic constipation benefit from treatment with 5-hydroxytryptamine 4 (5-HT4) receptor agonists. However, the first-generation 5-HT4 receptor agonists cisapride and tegaserod were withdrawn from the market owing to rare cardiovascular adverse events that were not 5-HT4-receptor-related but due to the lack of selectivity of these drugs. Here we report the nonclinical cardiovascular profile of the selective 5-HT4 receptor agonist prucalopride. To assess its non-5-HT4 receptor-mediated effects on cardiovascular electrophysiological parameters, in vitro studies were performed in human ether-à-go-go-related gene-transfected cells, guinea pig ventricular myocytes and papillary muscle preparations, rabbit and dog Purkinje fibers, and the Langendorff rabbit heart. In vivo experiments were performed in a rabbit model for drug-induced proarrhythmogenesis, in anesthetized guinea pigs, and anesthetized and conscious dogs. In addition, human platelet aggregation and coronary artery contraction were studied to exclude interactions that have been suggested to mediate the cardiovascular effects of tegaserod. Effects at 5-HT4 receptors were evaluated in piglet and human atrial myocardium, and in anesthetized pigs. Finally, cardiovascular endpoints were investigated in chronic, repeated-dose toxicology studies at very high prucalopride doses in rats and dogs. No relevant effects were observed in any of the cardiovascular studies at concentrations at least 50 times the therapeutic plasma level. Only in pigs were minor and transient increases in heart rate and blood pressure noted upon first exposure to prucalopride, at plasma levels at least 10 times higher than human therapeutic plasma levels. Prucalopride may thus provide therapeutic benefit without the cardiovascular risks reported for other 5-HT4 receptor agonists.

Rate-dependent activation failure in isolated cardiac cells and tissue due to Na+ channel block.

While it is well established that class-I antiarrhythmics block cardiac sodium channels, the mechanism of action of therapeutic levels of these drugs is not well understood. Using a combination of mathematical modeling and in vitro experiments, we studied the failure of activation of action potentials in single ventricular cells and in tissue caused by Na(+) channel block. Our computations of block and unblock of sodium channels by a theoretical class-Ib antiarrhythmic agent predict differences in the concentrations required to cause activation failure in single cells as opposed to multicellular preparations. We tested and confirmed these in silico predictions with in vitro experiments on isolated guinea-pig ventricular cells and papillary muscles stimulated at various rates (2-6.67 Hz) and exposed to various concentrations (5 × 10(-6) to 500 × 10(-6) mol/l) of lidocaine. The most salient result was that whereas large doses (5 × 10(-4) mol/l or higher) of lidocaine were required to inhibit action potentials temporarily in single cells, much lower doses (5 × 10(-6) mol/l), i.e., therapeutic levels, were sufficient to have the same effect in papillary muscles: a hundredfold difference. Our experimental results and mathematical analysis indicate that the syncytial nature of cardiac tissue explains the effects of clinically relevant doses of Na(+) channel blockers.

Myocardial Depressant Effects of Desflurane

The authors determined whether desflurane altered myocardial excitation-contraction coupling and electrophysiologic behavior in the same manner as isoflurane and sevoflurane. The effects of desflurane on isometric force in guinea pig ventricular papillary muscles were studied in modified standard and in 26 mM K(+) Tyrode solution with 0.1 microm isoproterenol. Desflurane effects on sarcoplasmic reticulum Ca(2+) release were also determined by examining its actions on rat papillary muscles, guinea pig papillary muscles in low-Na(+) Tyrode solution, and rapid cooling contractures. Normal and slow action potentials were recorded using a conventional microelectrode technique. Ca(2+) and K(+) currents of guinea pig ventricular myocytes were examined. Desflurane (5.3% and 11.6%) decreased peak force to approximately 70% and 40% of the baseline, respectively, similar to the effects of equianesthetic isoflurane concentrations. With isoproterenol in 26 mM K(+) Tyrode solution, desflurane markedly depressed late peaking force and modestly depressed early peak force. The rested state contractions of rat myocardium or guinea pig myocardium in low-Na(+) Tyrode solution were modestly depressed, whereas rapid cooling contractures were virtually abolished after desflurane administration. Desflurane significantly prolonged the action potential duration. Desflurane reduced L-type Ca(2+) current and the delayed outward K(+) current but did not alter the inward rectifier K(+) current. Myocardial depression by desflurane is due to decreased Ca(2+) influx, whereas depolarization-activated sarcoplasmic reticulum Ca(2+) release is modestly depressed, similar to the actions of isoflurane and sevoflurane. Desflurane depressed the delayed outward K(+) current associated with significant lengthening of cardiac action potentials.

Behavior of Ca(2+) waves in multicellular preparations from guinea pig ventricle.

Ca(+) waves have been implicated in Ca(2+) overload-induced cardiac arrhythmias. To deepen understanding of the behavior of Ca(2+) waves in a multicellular system, consecutive two-dimensional Ca(2+) images were obtained with a confocal microscope from surface cells of guinea pig ventricular papillary muscles loaded with fluo 3 or rhod 2. In intact muscles, no Ca(2+) waves were detected under the resting condition, whereas they were frequently observed during the rest immediately after high-frequency stimulations where cytoplasmic Ca(2+) concentration and Ca(2+) stored in the sarcoplasmic reticulum (SR) were gradually decreasing. The intervals of Ca(2+) waves increased as they occurred later, their amplitudes and velocities remaining unchanged. A SERCA inhibitor reversibly prolonged the wave intervals. In Na(+)-free/Ca(2+)-free medium where neither Ca(2+) influx nor Na(+)/Ca(2+) exchange took place, recurrent Ca(2+) waves emerged at constant intervals in each cell. These results are consistent with the conclusion that the loading level of the SR is critical for induction of Ca(2+) waves. Each cell independently exhibited its own regular rhythm of Ca(2+) wave with a distinct interval. These waves propagated in either direction along the longitudinal axis within a muscle cell, but seldom beyond the cell boundary. In contrast, in partially damaged muscles that showed spontaneous Ca(2+) waves at rest in normal Krebs solution, their propagation often was unidirectional, decreasing in frequency. In these cases, however, Ca(2+) waves rarely moved beyond the cellular boundary. The gradient of the cytoplasmic Ca(2+) concentration was suggested to be the cause of the one-way propagation.

Direct Myocardial Depressant Effect of Desflurane: Mechanical and Electrophysiological Actions in Vitro

Background: Desflurane actions on myocardial contractility and cellular electrophysiologic behavior were studied in isolated guinea pig and rat right ventricular papillary muscles. Methods: The isometric force of isolated guinea pig ventricular papillary muscles was studied in normal and 26 mM Tyrode's solution at various stimulation rates. Experiments using rat papillary muscles under normal Tyrode's solution at the rested-state (RS) and using guinea pig papillary muscles under low Tyrode's solution (25 mM) were performed to evaluate the effect of release from the sarcoplasmic reticulum (SR). Effects of desflurane on SR function in situ were examined by its effect on rapid cooling contractures (RCCs). Normal and slow action potentials (APs) were evaluated by using a conventional microelectrode technique. Finally, currents of isolated guinea pig ventricular myocytes were examined using the whole cell patch clamp technique. Results: 1 MAC (minimum alveolar concentration: 6%) and 2 MAC desflurane were applied. 1 MAC and 2 MAC desflurane depressed guinea pig myocardial contractions by 30% and 60%, respectively, from RS to 3 Hz stimulation rates. 1 MAC (1.15%) and 2 MAC isoflurane depressed peak force by 25% and 45%, respectively. Contractile force after rest in rat and guinea pig myocardium under low Tyrode's solution showed modest depression. In the partially depolarized, adrenergically stimulated myocardium, 1 MAC and 2 MAC desflurane caused a marked depression of the late peak force (1 MAC:60%, 2 MAC:80%) with moderate changes of the early peak force (1 MAC: 20%, 2 MAC: 40%). RCCs were abolished at 1 MAC desflurane. Desflurane did not alter the peak amplitude or maximum depolarization rate of normal and slow APs, however, AP duration was significantly prolonged. In isolated guinea pig myocytes at room temperature, 1 MAC and 2 MAC desflurane caused a 28% and 55% decrease in currents, respectively. Conclusions: These results indicate that desflurane causes a dose-dependent depression of contractile force in isolated myocardium, which is comparable to that of isoflurane. The depression seems to be related, at least in part, to its ability to reduce inward currents through the cardiac membrane. Therefore, it is likely that various methods employed to enhance inward current may improve the hemodynamic depression induced by desflurane.

A review of HNS-32: a novel azulene-1-carboxamidine derivative with multiple cardiovascular protective actions.

HNS-32 [N(1),N(1)-dimethyl-N(2)-(2-pyridylmethyl)-5-isopropyl-3,8-dimethylazulene-1- carboxamidine] (CAS Registry Number: 186086-10-2) is a newly synthesized azulene derivative. Computer simulation showed that its three dimensional structure is similar to that of the class Ib antiarrhythmic drugs, e.g., lidocaine or mexiletine. HNS-32 potently suppressed ventricular arrhythmias induced by ischemia due to coronary ligation and/or ischemia-reperfusion in dogs and rats. In the isolated dog and guinea pig cardiac tissues, HNS-32 had negative inotropic and chronotropic actions, prolonged atrial-His and His-ventricular conduction time and increased coronary blood flow. In the isolated guinea pig ventricular papillary muscle, HNS-32 decreased maximal rate of action potential upstroke (Vmax) and shortened action potential duration (APD). These findings suggest that HNS-32 inhibits inward Na+ and Ca2+ channel currents. In the isolated pig coronary and rabbit conduit arteries, HNS-32 inhibited both Ca2+ channel-dependent and -independent contractions induced by a wide variety of chemical stimuli. HNS-32 is a potent inhibitor of protein kinase C (PKC)-mediated constriction of cerebral arteries. It is likely to block both, Na+ and Ca2+ channels expressed in cardiac and vascular smooth muscles. These multiple ion channel blocking effects are largely responsible for the antiarrhythmic and vasorelaxant actions of HNS-32. This drug may represent a novel approach to the treatment of arrhythmias.

Intracellular chloride activity increases in guinea pig ventricular muscle during simulated ischemia.

We investigated the effects of simulated ischemia on intracellular Cl- activity ([Cl-]i) in isolated guinea pig ventricular papillary muscles using ion-selective microelectrode techniques. Simulated ischemia in ventricular muscles was produced by stopping the flow of superfusion and immersing preparations in mineral oil as previously described [B. Vanheel, L. Leybaert, A. De Hemptinne, and I. Leusen. Am. J. Physiol. 257 (Cell Physiol. 26): C365-C379, 1989; Z. F. Lai, J. Liu, and K. Nishi. Jpn. J. Pharmacol. 72: 161-174, 1996]. When preparations were exposed to paraffin oil for 15 min, [Cl-]i markedly increased and the peak magnitude of [Cl-]i reached 55.3 +/- 2.5 mM from 18.7 +/- 3.5 mM, whereas membrane potentials (Vm) depolarized from -82.5 +/- 1.1 to -54.7 +/- 2.4 mV (n = 6 muscles from 6 animals). SITS (0.5 mM), a known blocker of the Cl-/HCO-3 exchanger, suppressed the ischemia-induced depolarization of Vm and delayed the onset of the ischemia-induced increase in [Cl-]i but did not suppress the magnitude of the increase of [Cl-]i. Under Cl--free conditions created by replacing Cl- with equimolar gluconate, the increase in [Cl-]i during ischemia was transient and suppressed by >60% compared with that in normal-Cl- conditions (peak value was 20. 3 +/- 1.7 mM, n = 6 muscles from 6 animals). The present results provide direct evidence that [Cl-]i in ventricular muscle increases in ischemic conditions in quiescent guinea pig ventricular muscle, suggesting that activation of the Cl-/HCO-3 exchanger by ischemia would partially contribute to the elevation of [Cl-]i during the initial stage of ischemia.

Electrical and mechanical effects of a novel antihypertensive quinazoline derivative, 3-[[4-(2-methoxyphenyl) piperazin-1-ly]methyl]-5-(methylthio)-2,3-dihydroimidazo [1,2-c]quinazoline, on guinea pig ventricular muscles.

Electrical and mechanical effects of 3-[[4-(2-methoxyphenyl)piperazin-1-ly]methyl]-5-(methylthio)-2,3-+ ++dihydroimidazo[1,2-c]quinazoline (DL-017), a new synthesized antihypertensive agent, were studied in guinea-pig ventricular papillary muscles. In muscle fibers driven at 1 Hz, DL-017 decreased the twitch force in a concentration-dependent manner and significantly increased the action-potential duration and decreased intracellular Na+ activity (ai(Na)) and maximal rate of upstroke of action-potential (Vmax) when concentrations were greater than 1 microM. Phenylephrine in the presence of 1 microM propranolol produced a concentration-dependent positive inotropy. DL-017 (0.01 microM) antagonized the positive inotropic effect of phenylephrine and shifted the concentration-response curve to the right. In K+-depolarized muscle fibers, 0.1 microM DL-017 significantly decreased the contractile force without changing the slow action potential. In low-[K+]o and high-[Ca2+]o solutions, a train of stimuli triggered a spontaneous rhythm that could be abolished by 3 microM DL-017. Our results suggest that DL-017 not only exhibits an alpha1-antagonistic effect but also induces negative inotropy by a decrease in myofibrillar calcium sensitivity and inhibits Na+ channels at higher concentrations, contributing to the drug's negative inotropic and type-I antiarrhythmic effects.

Myocardial Depressant Effects of Sevoflurane

PARK, WYUN KON; PANCRAZIO, JOSEPH J.; SUH, CHANG KOOK; III, CARL LYNCH Author Information

Myocardial Depressant Effects of Sevoflurane

The effects of anesthetic concentrations of sevoflurane were studied in isolated myocardial tissue to delineate the mechanisms by which cardiac function is altered. Isometric force of isolated guinea pig ventricular papillary muscle was studied at 37 degrees C in normal and 26 mM K+ Tyrode's solution at various stimulation rates. Normal and slow action potentials were evaluated using conventional microelectrodes. Effects of sevoflurane on sarcoplasmic reticulum function in situ were also evaluated by its effect on rapid cooling contractures, which are known to activate Ca2+ release from the sarcoplasmic reticulum, and on concentrations of rat papillary muscle. Finally, Ca2+ and K+ currents of isolated guinea pig ventricular myocytes were examined using the whole-cell patch clamp technique. Sevoflurane equivalent to 1.4% and 2.8% depressed guinea pig myocardial contractions to approximately 85 and approximately 65% of control, respectively, although the maximum rate of force development at 2 or 3 Hz and force in rat myocardium after rest showed less depression. In the partially depolarized, beta-adrenergically stimulated myocardium, sevoflurane selectively depressed late peak force without changing early peak force, whereas it virtually abolished rapid cooling contractures. Sevoflurane did not alter the peak amplitude or maximum depolarization rate of normal and slow action potentials, but action potential duration was significantly prolonged. In isolated guinea pig myocytes at room temperature, 0.7 mM sevoflurane (equivalent to 3.4%) depressed peak Ca2+ current by approximately 25% and increased the apparent rate of inactivation. The delayed outward K+ current was markedly depressed, but the inwardly rectifying K+ current was only slightly affected by 0.35 mM sevoflurane. These results suggest that the direct myocardial depressant effects of sevoflurane are similar to those previously described for isoflurane. The rapid initial release of Ca2+ from the sarcoplasmic reticulum is not markedly decreased, although certain release pathway, specifically those induced by rapid cooling, appear to be depressed. Contractile depression may be partly related to the depression of Ca2+ influx through the cardiac membrane. The major electrophysiologic effect of sevoflurane seems to be a depression of the delayed outward K+ current, which appears to underlie the increased action potential duration.

Effects of Stilbene Derivatives SITS and DIDS on Development of Intracellular Acidosis during Ischemia in Isolated Guinea Pig Ventricular Papillary Muscle In Vitro

Using ion-selective microelectrode techniques, we investigated the effects of 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), which are known as Cl−-HCO3− exchange blockers, on action potentials and intracellular pH (pHi) in guinea pig ventricular papillary muscles subjected to simulated ischemia. Simulated ischemia was produced by stopping the flow of superfusing solution and then covering the preparations with mineral oil. Simulated ischemia induced a progressive decrease in the maximum upstroke rate and resting membrane potentials, shortened action potential duration, and resulted in cessation of action potentials within 10–12 min after the onset of simulated ischemia. The pHi-measurements revealed progressive intracellular acidosis during the period of simulated ischemia. SITS (0.5 mM) or DIDS (0.1 mM) delayed the onset of ischemia-induced deterioration of action potentials and prolonged the time to cessation of action potentials. SITS or DIDS (0.1–0.5 mM) induced an increase in pH in HCO3−-buffered solution and suppressed the development of intracellular acidosis during ischemia. Under the external CP-free condition, the time to cessation of action potentials caused by ischemia was significantly delayed, and the development of intracellular acidosis during ischemia was attenuated. The present results indicate that activation of the Cl−-HCO3− exchange system would be involved, in part, in the development of intracellular acidosis during cardiac ischemia.

Electromechanical effects of zatebradine on isolated guinea pig cardiac preparations.

The effects of zatebradine on rate and contractile force and transmembrane action potentials were studied in isolated guinea pig atria and ventricular papillary muscles. In spontaneously beating right atria, zatebradine, 10(-7)-10(-4) M, produced a negative chronotropic effect (IC50 = 6.5 +/- 3.0 x 10(-6) M) and prolonged the recovery of the sinus function. In addition, it produced a biphasic effect on the atrial contractility, so that at concentrations up to 10(-5) M, it exerted a positive inotropic effect, whereas at higher concentrations, a negative inotropic effect was observed (IC50 = 9.0 +/- 0.3 x 10(-5) M). In electrically driven left atria, zatebradine produced a negative inotropic effect, though no changes were observed in the total contraction time or the time to peak tension. In papillary muscles, zatebradine > or = 5 x 10(-6) M caused a significant decrease in the maximum upstroke velocity (Vmax) without altering the resting membrane potential and exerted biphasic effects on the action potential duration (APD). At concentrations up to 5 x 10(-5) M, it prolonged the APD, whereas at higher concentrations, it shortened the APD. In addition, zatebradine, 10(-4) M, significantly reduced the amplitude and Vmax of the slow action potentials elicited by isoproterenol in K(+)-depolarized papillary fibres. In the presence of zatebradine, trains of stimuli at rates between 0.5 and 3 Hz led to an exponential decline in Vmax (frequency-dependent Vmax block), which was augmented at higher rates of stimulation. The time constant for the recovery of Vmax from the frequency-dependent block was 2.9 s.(ABSTRACT TRUNCATED AT 250 WORDS)

A Xanthine Derivative Denbufylline Inhibits Negative Inotropic Response to Verapamil in Guinea Pig Ventricular Papillary Muscles, Independent of Its Phosphodiesterase Inhibitory Activity

A phosphodiesterase (PDE) III inhibitor, amrinone, inhibited both the negative inotropic actions of verapamil and nicardipine in guinea pig ventricular papillary muscle; this effect was canceled by the protein kinase A inhibitor H-89. The PDE IV inhibitor 1,3-di-n-butyl-7-(2′-oxopropyl)xanthine (denbufylline), which elicited a negative inotropic action by itself, attenuated the action of verapamil up to 10 μM, without any interaction with nicardipine. The attenuation by denbufylline was not influenced by H-89. This suggests that in the ventricular papillary muscle, denbufylline acts on some verapamil-sensitive site(s) in the membrane and interferes with the calcium channel function without involvement of its PDE inhibitory activity.

モルモット乳頭筋におけるPDE阻害薬の特異的作用と非特異的作用

The inotropic actions of xanthine derivatives, having long alkyl chains, were investigated in guinea-pig ventricular papillary muscle. A potent and nonselective phosphodiesterase (PDE) inhibitor, 3-isobutyl-l-methylxanthine, elicited a positive inotropy and inhibited the negative inotropic effects of calcium channel inhibitors, as well as a selective PDE III inhibitor, amrinone, these effects which were canceled by a protein kinase inhibitor, N-[2-(ρ-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89). However, 1, 3-di-n-butyl-7-(2'-oxopropyl)xanthine (denbufylline) and 1-n-butyl-3-n-propylxanthine (XT-044), which possess potent and selective PDE IV inhibitory activities, showed negative inotropic actions which became more potent in the presence of H-89. Denbufylline allowed to disappear late restoration phase induced by ryanodine. This xanthine derivative attenuated the both effects of calcium channel acting agents, Bay K 8644 and verapamil, without interaction with caffeine and dihydropyridine calcium channel inhibitors. A nonxanthine PDE IV inhibitor, Ro 20-1724, did not affect the inotropic actions of calcium channel inhibitors. The attenuation by denbufylline or XT-044 of the negative inotropic action of verapamil was not influenced by pretreatment with H-89. These results suggest that these xanthine derivatives elicit negative inotropy through acting on a verapamil-sensitive site of calcium channel without involving of their PDE inhibitory activitiy, in the ventricular papillary muscle.

Mechanical and electrophysiological effects of protamine on isolated ventricular myocardium: evidence for calcium overload

The aim was to examine in vitro the cellular cardiac effects of protamine, the cationic polypeptide employed to reverse heparin anticoagulation, and to define its mechanisms of action. Isometric contractile force and action potential characteristics after rest and at frequencies up to 3 Hz were recorded in guinea pig ventricular papillary muscle. The actions of protamine (10-300 micrograms.ml-1) were compared to those of heparin (10, 30 units.ml-1), and to heparin (10 units.ml-1) neutralised with equivalent (100 micrograms.ml-1) or excess (200 micrograms.ml-1) protamine. The effects of protamine were also examined using (1) muscle rapid cooling contractures (RCC) to assess intracellular Ca2+ stores and (2) the whole cell voltage clamp method to evaluate K+ and Ca2+ currents of isolated ventricular myocytes. Protamine (100-300 micrograms.ml-1) depressed contractions by 35-65% at 3 Hz, whereas contractions were enhanced by 150-500% at lower rates (resting state, 0.5 Hz), with a concomitant rise in resting force. Protamine caused a resting depolarisation from -90 to -76 mV and depressed action potential amplitude. In contrast, heparin altered contractile or action potential characteristics minimally. In 26 mM K+ solution with 0.1 microM isoprenaline, 30-300 micrograms.ml-1 protamine caused dose dependent depression of late peaking force development and slow action potential prolongation. After 15 min rest, when RCC were not normally elicited, rest RCC became prominent in 100-300 micrograms.ml-1 protamine. Effects of heparin with 100 micrograms.ml-1 excess protamine were similar to those of 100 micrograms.ml-1 protamine alone. Voltage clamp of isolated myocytes revealed that 10 micrograms.ml-1 protamine irreversibly decreased current through inwardly rectifying K+ channels (IK1), increased leakage current, and decreased inward Ca2+ current (ICa). The loss of the normal force-frequency relation, partial depolarisation, rise in resting tension, and appearance of rested state rapid cooling contractures suggest that unbound protamine can lead to excess intracellular Ca2+, mediated by an alteration in membrane ionic conductances.

Effects of altered extracellular potassium and pacing cycle length on the class III antiarrhythmic actions of dofetilide (UK-68,798) in guinea-pig papillary muscle.

The effects of altered extracellular K+ concentrations ([K+]o) and pacing cycle lengths (CLs) on the electrophysiological actions of dofetilide (UK-68,798), a potent class III antiarrhythmic agent, were examined in isolated guinea-pig ventricular papillary muscle. At a normal [K+]o (4 mM) and at CL between 300 and 5000 msec, dofetilide (10 nM) significantly increased the action-potential duration (APD) and the effective refractory period (ERP), whereas other action-potential parameters were unaffected. Elevation of [K+]o to 10 mM reduced membrane diastolic potential (MDP), action-potential amplitude (APA), and the maximum rising velocity of the action-potential upstroke (Vmax). These changes were accompanied by a small shortening of APD90, but with an increase in ERP; i.e., the ERP/APD90 ratio was increased. Dofetilide also significantly lengthened APD90 at 10 mM [K+]o and at each CL. Even at the short cycle lengths (300 and 500 msec), dofetilide-induced increases in APD90 were not attenuated whether [K+]o was at 4 or 10 mM. These results indicate that at various pacing CLs, 10 nM dofetilide increases myocardial APD and ERP to a similar extent without significant reverse use-dependence when the cell membrane is normally polarized or partially depolarized by elevated [K+]o. Dofetilide may, therefore, be expected to be beneficial in the treatment of cardiac tachyarrhythmias related or unrelated to regional myocardial hyperkalemia during myocardial ischemia.

External ATP antagonizes the effect of potassium channel openers in guinea-pig ventricular papillary muscle

Right ventricular papillary muscles of the guinea-pig heart were electrically stimulated. Cromakalim 10–100 μM and Ro 31-6930 3 μM depressed the contractile force and shortened the duration of action potentials. Glibenclamide 0.3–3 μM, ATP 100 μM and α,β-methylene ATP (α,β-meATP) 30 μM antagonized these effects. Suramin 300 μM failed to reverse the α,β-meATP-evoked antagonism of the action of cromakalim. It is concluded that both intra- and extracellular ATP may interfere with potassium channel openers and that extracellular ATP does not act via the known P 2-purinoceptor subtypes.

Effects of ouabain, DBcAMP, caffeine, and high [Ca2+]o on twitch tension, intracellular Na+ activity, and action potential of guinea pig papillary muscles.

To explore ionic mechanisms of the positive inotropic process in guinea pig ventricular papillary muscles, we simultaneously measured twitch tension, intracellular Na+ activity (aiNa) and action potential at a stimulus rate of 60/min. The aiNa was 5.0 +/- 0.1 mM in the steady state. When the Na-K pump was inhibited by ouabain (1 microM), the twitch tension was increased in proportion to the aiNa, and the action potential was decreased at diastolic state and at duration for 90% of repolarization (APD90). While exposed to caffeine (3 mM), the aiNa kept increasing even when the twitch tension achieved a peak level. Concomitantly, the action potential slightly depolarized at diastolic state with a prolonged APD90. An application of DBcAMP (1 mM) or an increase in [Ca2+]o (from 1.8 to 3.6 mM) enhanced the twitch tension with a fall of aiNa and a shortening of APD90. These results suggest that in guinea pig papillary muscles Na-Ca exchange plays an important role in the regulation of contractile force and membrane potential, and that the Na influx should be balanced by the activation of the sarcolemmal Na-K pump, a negative feedback mechanism, to prevent calcium overload and abnormal pacemaker activity in electrical excitation.

Effects of combined use of class I antiarrhythmic agents on V max of guinea-pig ventricular muscles

The effects of the combined use of class-I antiarrhythmic drugs on the resting potentials (RP), amplitude of action potential (AMP), and Vmax of the action potential were investigated in guinea-pig ventricular papillary muscles that were superfused with oxygenated Krebs-Ringer solution at 35 degrees C. Disopyramide (40 microM) reduced Vmax to 68.6 +/- 3.1% (mean +/- SE, n = 5) of the control with minimal changes in RP and AMP when preparations were stimulated at 1 Hz. The addition of mexiletine (20 microM) to the solution containing disopyramide (40 microM) caused a minimal reduction of Vmax (less than 5%) for the stimulation of 1 Hz, but a significant reduction of Vmax (13% p less than 0.05) when stimulation was increased to 2 Hz. This amount of the reduction is compatible with that obtained by mexiletine alone, suggesting a simple additive Na+ channel inhibition by this drug combination. This additive effect was also observed in the recovery process of Vmax from the use-dependent block induced by train stimuli at 1 Hz. Flecainide (5 microM) reduced Vmax to 58.6 +/- 13.3% (n = 5). The addition of mexiletine to the superfusate with flecainide produced a further depression of 14 +/- 2.6% of Vmax, even at 1 Hz. This depression was larger than that produced by mexiletine, suggesting a synergistic action of the two drugs on the Na+ channel. Such information about the interaction of the class I drug combinations with the Na+ channel may be clinically important.