Flow-independent improvement by diltiazem of ischemia-induced conduction delay in porcine hearts.

The calcium channel blocking agent, diltiazem, improves ischemia-induced conduction delays in the canine heart. It is not known, however, if the improvement of myocardial blood flow caused by diltiazem participates in this response. Accordingly, ischemia-induced conduction delay was measured during brief coronary artery occlusion before and after administration of diltiazem in nine anesthetized pigs with fixed heart rate. Acute coronary occlusion prolonged subendocardial (mean +/- standard error of the mean, 39.9 +/- 3.9 ms) and subepicardial (41.6 +/- 4.1 ms) conduction times (time to peak of the bipolar electrogram in each region) by 51 +/- 4 and 58 +/- 5%, respectively. Regional myocardial blood flow at the ischemic electrode sites was 0.006 +/- 0.002 ml/min per g and was unaffected by diltiazem. Intravenous diltiazem pretreatment (0.01, 0.1, 0.3 and 1.0 mg/kg) 5 minutes before coronary occlusion significantly reduced the ischemia-induced conduction delay in both subendocardial and subepicardial regions during coronary occlusion. The pigs in which ventricular fibrillation occurred within 10 minutes showed a significantly longer conduction delay than that observed in pigs in which ventricular fibrillation occurred later (greater than 10 minutes). Thus, the data suggest that the reduction of ischemia-induced conduction delay produced by diltiazem is independent of blood flow changes and, therefore, that diltiazem may have a beneficial antiarrhythmic action.

Prevention of some hypothermia induced electromechanical changes by calcium channel blockade.

OBJECTIVE: Cooling induces electromechanical changes in the heart. The aim of the study was to examine how the calcium channel blocker, nisoldipine (NIS), altered these changes compared to those induced by other drugs that shorten action potential duration such as tetrodotoxin and nicorandil. METHODS: Guinea pig papillary muscle action potentials and developed force were recorded using the conventional microelectrode technique and a force transducer. Restitution of action potential duration was determined by introducing extrastimuli at progressively longer diastolic intervals from 40 to 9000 ms. Preparations were divided into four groups: (1) no drug (control); (2) 1 microM tetrodotoxin, a sodium channel blocker; (3) 1 mM nicorandil, an ATP sensitive potassium channel activator; and (4) 1 microM nisoldipine (n = 6 in each group). Action potential duration and developed force were recorded after addition of drug at 37 degrees C, and at each 1 degree C change in temperature during cooling to 27 degrees C. The restitution protocol was performed at 37 degrees C and 27 degrees C. RESULTS: Tetrodotoxin had no effect on action potential duration at 90% of repolarisation (APD90) while nisoldipine and nicorandil greatly shortened APD90. Cooling from 37 degrees to 27 degrees C with nisoldipine produced less hypothermia induced lengthening in APD90 than in the other group. Developed force did not increase with reduction in temperature in the presence of nisoldipine. The range of premature action potential durations was defined as the difference in APD90 at diastolic interval of 40 and 100 ms. This range decreased with nisoldipine in contrast to the marked increases that occurred in the other groups during cooling. CONCLUSIONS: Increased intracellular Ca2+ might be responsible for the hypothermia induced increase in APD90, developed force, and range of premature action potential durations, since calcium channel blockade, which prevents an increase in intracellular Ca2+, greatly reduced these changes. The reduced range of premature action potential durations may reduce dispersion of ventricular refractoriness, and hence be expected to decrease hypothermia induced arrhythmias.

Class III antiarrhythmic action by potassium channel blockade: dofetilide attenuates hypoxia induced electromechanical changes.

OBJECTIVE: The aim was to examine the electromechanical effects of dofetilide, a new class III antiarrhythmic agent, in isolated guinea pig ventricular muscle during hypoxia. METHODS: Hypoxia was induced by superfusing guinea pig right ventricular papillary, muscles with Tyrode's solution gassed with 95% N2 + 5% CO2 [PO2 = 5.3(SEM 1.3) kPa]. Prior to hypoxia, the preparations were either pretreated for 30 min with 0.1 microM dofetilide (n = 6) or with 100 microM glibenclamide (a blocker of ATP sensitive K+ channels, n = 6), or not pretreated (n = 6). Sixteen additional preparations were exposed to 1 mM nicorandil (an activator of ATP sensitive K+ channels) in the absence (n = 6) and presence of dofetilide (n = 6) or glibenclamide (n = 4). Transmembrane action potentials and developed force were recorded using conventional microelectrode techniques and a force transducer. RESULTS: During normoxia, dofetilide markedly increased APD90 from 236(SEM 6) ms to 298(7) ms (p < 0.05) and the effective refractory period (ERP) from 248(5) ms to 315(6) ms (p < 0.05). In the drug free group, 60 min hypoxia decreased APD90 by 47(5)% (p < 0.05), ERP by 48(4)% (p < 0.05) and developed force by 71(6)% (p < 0.05) of baseline, respectively. These hypoxia induced effects were significantly attenuated after pretreatment with dofetilide or glibenclamide. Nicorandil decreased APD90 by 45(5)% (p < 0.05), ERP by 44(6)% (p < 0.05), and developed force by 69(10)% (p < 0.05) of baseline, respectively. Pretreatment with dofetilide or glibenclamide also significantly attenuated the nicorandil induced decreases in APD90, ERP, and developed force. CONCLUSIONS: Dofetilide, like glibenclamide, effectively attenuates hypoxia and nicorandil induced action potential shortening and the associated reduction in contractile force. Thus dofetidile would be expected to retain its antiarrhythmic efficacy during myocardial hypoxia or ischaemia.

Effects of temperature on cycle length dependent changes and restitution of action potential duration in guinea pig ventricular muscle.

OBJECTIVE: The aim was to investigate the effects of temperature on cycle length dependent changes of action potential duration and on restitution of action potential duration. METHODS: Guinea pig papillary muscle action potentials were recorded using conventional microelectrode techniques. Action potential duration was measured at cycle lengths ranging from 500 to 2000 ms at both 27 degrees C and 37 degrees C. Restitution of action potential duration was determined by introducing an extra stimulus at progressively longer diastolic intervals from 40 to 9000 ms at pacing cycle lengths of 500, 1000, and 2000 ms. RESULTS: At 37 degrees C, action potential duration measured at 90% of repolarisation (APD90) during continuous pacing and the maximum value of APD90 achieved during restitution (APD90pl) decreased by 18(SEM 6) ms (n = 7) and 24(7) ms (n = 6), respectively, when pacing cycle length was reduced from 2000 to 500 ms. At 27 degrees C, the magnitude of the shortening of APD90 and APD90pl observed when pacing cycle length was similarly reduced was greater than at 37 degrees C, ie, 143(21) ms (n = 6) and 115(11) ms (n = 6), respectively. Thus the relation for restitution of action potential duration shifted downwards with reduction in pacing cycle length, and the magnitude of this shift was greater at 27 degrees C than at 37 degrees C. The difference between APD90 at the shortest diastolic interval (40 ms) and at diastolic interval of 100 ms (range of premature action potential durations) was much greater at 27 degrees C than at 37 degrees C at all three pacing cycle lengths. CONCLUSIONS: Reduction in temperature magnifies the cycle length dependent changes in action potential duration both during abrupt changes in cycle length, as with an extra stimulus, and during changes of steady state cycle length. This may indicate a greater dispersion of premature action potential durations during hypothermia, and hence predispose to hypothermia induced arrhythmias.

Pharmacology of calcium antagonists.

Although the calcium antagonists verapamil, nifedipine, diltiazem and bepridil are structurally diverse, they share, to a variable extent, several pharmacologic properties. These effects are presumably the result of dose-related inhibition of transmembrane calcium ion flux through the slow channel. In diseased tissue, other routes of calcium entry may also be inhibited, and intracellular sites of action also are now strongly suspected. The calcium antagonists tend to relax vascular smooth muscle in a dose-dependent and site-specific manner. Effective coronary vasodilation is found with each agent; peripheral vasodilation is most pronounced with nifedipine, followed, in descending order of potency, by verapamil, diltiazem and bepridil. Atrioventricular conduction is also inhibited by diltiazem, bepridil and verapamil, whereas nifedipine paradoxically has no effect at therapeutic doses. The calcium antagonists also reduce muscle contractile force, but again in variable degrees. Negative inotropy is significant with verapamil and minimal with diltiazem and bepridil. Nifedipine often causes a reflex increase in contractility and heart rate. At therapeutic doses, bepridil has additional properties: it appears to affect sodium and perhaps potassium channels, producing a quinidine-like effect, and it prolongs the refractory period. Experimentally, bepridil has also been found to extend the duration of the action potential, raise the ventricular fibrillation threshold and possess both class I and class IV antiarrhythmic activity at relatively small doses. If documented clinically, bepridil may prove to be an effective antiarrhythmic as well as antianginal agent.

Hemodynamic and electrophysiologic effects of amlodipine, a new calcium channel blocker.

The effects of amlodipine (300 micrograms/kg administered intravenously), a new, long-acting, dihydropyridine class, calcium channel blocking drug, were studied in atrially paced (120 beats/min), autonomically blocked dogs. Hemodynamic and electrophysiologic parameters were measured before and up to 3 hours after amlodipine. Coronary blood flow was significantly increased 10 and 30 minutes after drug administration, whereas cardiac output and mean arterial pressure were unaffected. Coronary vascular resistance was decreased but total systemic vascular resistance did not change. Atrioventricular (AV) nodal conduction was slightly prolonged, as reflected by increases in atrial-His bundle and AV conduction times and PR interval, 30 minutes after administration. All parameters returned toward their control values within 3 hours after drug administration. Comparison of coronary vascular resistance and AV conduction changes with those previously reported for other calcium channel blocking drugs where autonomic blockade existed suggests that at equivalent levels of coronary vasodilation, amlodipine's effects more closely resemble the effects of diltiazem or verapamil than other dihydropyridines.

Amlodipine, a long-acting calcium antagonist drug reduces ischemia-induced ventricular conduction delay in pig hearts.

The effects of amlodipine, a novel, long-lasting calcium channel blocking agent, on ischemia-induced myocardial conduction delay was studied in anesthetized pigs paced at a constant heart rate. Acute coronary occlusion (3 minutes) significantly lengthened time to onset, time to peak and duration of bipolar electrograms recorded from both subendocardial and subepicardial left ventricular sites. After intravenous injection of amlodipine (0.3 mg/kg, n = 6), subsequent periods of ischemia greatly reduced (p less than 0.01) all indexes of subepicardial conduction delay. In the subendocardium, amlodipine decreased only time to onset (-25 +/- 4%, p less than 0.01) within the ischemic zone. Significant delays in all indexes were present during repeated ischemic periods in the placebo-treated control group (n = 5). Amlodipine also increased regional myocardial blood flow within the nonischemic myocardium by 25 +/- 10% and decreased mean aortic pressure by 7 +/- 2% without altering flow in the ischemic region. Left atrial pressure remained unchanged. Indexes of ischemia-induced conduction delay were more rapidly restored after reperfusion in amlodipine-pretreated than in control animals. In conclusion, amlodipine produced a beneficial blood flow-independent effect on ischemia-induced injury potentials. The effect may help to reduce the likelihood of development of lethal ventricular arrhythmias in the early stage of myocardial ischemia in the clinical setting.

Differential cardiovascular effects of calcium channel blocking agents: potential mechanisms.

The three major calcium channel blocking agents, diltiazem, nifedipine and verapamil, inhibit calcium entry into excitable cells. Despite this apparent common action at the cell membrane, these drugs produce quantitative and frequently qualitative differences in cardiovascular variables (for example, heart rate, atrioventricular [A-V] conduction and myocardial inotropic state) when evaluated at equieffective vasodilator doses. All three drugs increase coronary blood flow in a dose-dependent fashion (nifedipine greater than diltiazem = verapamil), and produce a negative inotropic effect in vitro in isolated atria and ventricles, also in a dose-dependent manner (verapamil greater than nifedipine greater than diltiazem). However, in conscious dogs nifedipine increases, verapamil decreases and diltiazem has little effect on the inotropic state. A-V conduction is slowed by diltiazem and verapamil but not by nifedipine in anesthetized dogs and in conscious dogs as judged from the P-R interval in the electrocardiogram. Heart rate is slowed in pentobarbital-anesthetized animals but is accelerated in conscious dogs (nifedipine greater than verapamil greater than diltiazem). Nifedipine also appears to interfere significantly with the arterial baroreceptor reflex by an apparent vagolytic action that is less evident with diltiazem and verapamil. Diltiazem, and possibly verapamil and nifedipine as well, appears to retard myocardial damage that accompanies ischemia. The mechanisms and sites of action of these drugs are presumed to be at the cell membrane; however, intracellular sites may also be involved.

The combined electrophysiological effects of lignocaine and sotalol in canine isolated cardiac Purkinje fibres are rate-dependent.

1. The frequency-dependent electrophysiological effects of lignocaine, sotalol, and their combination were studied in dog isolated cardiac Purkinje fibres, both at various constant rates of stimulation and following abrupt changes in pacing cycle length. 2. The combined effect of 18 microM lignocaine and 30 microM sotalol selectively lengthened duration of premature action potentials evoked at a diastolic interval of 40 ms (from 172.2 +/- 5.4 to 201.7 +/- 4.9 ms, n = 6, P less than 0.01) without significantly changing the durations of action potentials evoked at the basic cycle length of 500 ms (259.1 +/- 7.7 vs 251.9 +/- 3.9 ms, n = 11). 3. The combination of lignocaine with sotalol, like lignocaine alone, displayed a use-dependent depression of Vmax and revealed a slow component for a recovery of Vmax (tau = 173.5 +/- 16.0 ms, n = 5). 4. The kinetics for restitution of action potential duration were also slowed by the combination of the two dwo drugs (tau f = 173.6 +/- 16.7, before, vs 268.5 +/- 8.5 ms, after; n = 5, P less than 0.01), while the maximum action potential duration attained in this relation was not increased as it was by sotalol alone. 5. Lignocaine, therefore, appeared to inhibit the sotalol-induced lengthening of action potential duration at slow pacing rates and at long diastolic intervals. The combination of lignocaine with sotalol also completely abolished the occurrence of sotalol-induced early after depolarizations. 6. Finally, sotalol alone moderately increased the range of premature action potential durations, while the combination of the two drugs significantly decreased this parameter. 7. These findings indicate that the combination of lignocaine with sotalol may provide important, and unique, beneficial electrophysiological alterations that might be expected to provide enhanced antiarrhythmic efficacy in patients.

In vitro cardiac models of dog Purkinje fibre triggered and spontaneous electrical activity: effects of nicorandil.

1. The effects of nicorandil (30 microM and 100 microM) on two models of triggered activity [early afterdepolarizations (EADs) and delayed afterdepolarizations (DADs)] and on spontaneous automaticity occurring from both normal and depolarized levels of membrane potential were examined in isolated cardiac Purkinje fibres of the dog. Standard intracellular microelectrode techniques were used. 2. Nicorandil (30 microM) abolished EADs provoked by superfusion with Tyrode solution containing 2.7 mM K+ and 3 mM Cs. 3. DADs were induced by 0.2 microM acetylstrophanthidin in Tyrode solution containing 5.4 mM K+. Nicorandil (30 microM) significantly reduced the amplitude of these DADs from 12.5 +/- 2.5 mV to 5.5 +/- 0.2 mV (P less than 0.02, n = 6), while DADs were fully abolished by 100 microM nicorandil. 4. In unstimulated Purkinje strands, superfused with 2.7 mM K+ containing Tyrode solution having a pH of either 7.4 or 6.8, spontaneous depolarizations developed with a mean maximum diastolic potential (MDP) of -84.6 +/- 1.6 mV (n = 9) or -54.0 +/- 1.2 mV (n = 9), respectively. Nicorandil significantly reduced the frequency of this automatic activity and caused its cessation, at either level of MDP. Nicorandil, however, produced significant hyperpolarization only when automaticity occurred from the depolarized level of potential. 5. These results suggest that nicorandil may exert significant antiarrhythmic actions in vivo by abolishing both spontaneous and triggered electrical activity.

Pharmacological block of the slow component of the outward delayed rectifier current (I(Ks)) fails to lengthen rabbit ventricular muscle QT(c) and action potential duration.

1. The effects of I(Ks) block by chromanol 293B and L-735,821 on rabbit QT-interval, action potential duration (APD), and membrane current were compared to those of E-4031, a recognized I(Kr) blocker. Measurements were made in rabbit Langendorff-perfused whole hearts, isolated papillary muscle, and single isolated ventricular myocytes. 2. Neither chromanol 293B (10 microM) nor L-735,821 (100 nM) had a significant effect on QTc interval in Langendorff-perfused hearts. E-4031 (100 nM), on the other hand, significantly increased QTc interval (35.6+/-3.9%, n=8, P<0.05). 3. Similarly both chromanol 293B (10 microM) and L-735,821 (100 nM) produced little increase in papillary muscle APD (less than 7%) while pacing at cycle lengths between 300 and 5000 ms. In contrast, E-4031 (100 nM) markedly increased (30 - 60%) APD in a reverse frequency-dependent manner. 4. In ventricular myocytes, the same concentrations of chromanol 293B (10 microM), L-735,821 (100 nM) and E-4031 (1 microM) markedly or totally blocked I(Ks) and I(Kr), respectively. 5. I(Ks) tail currents activated slowly (at +30 mV, tau=888.1+/-48.2 ms, n=21) and deactivated rapidly (at -40 mV, tau=157.1+/-4.7 ms, n=22), while I(Kr) tail currents activated rapidly (at +30 mV, tau=35.5+/-3.1 ms, n=26) and deactivated slowly (at -40 mV, tau(1)=641.5+/-29.0 ms, tau(2)=6531+/-343, n=35). I(Kr) was estimated to contribute substantially more to total current density during normal ventricular muscle action potentials (i.e., after a 150 ms square pulse to +30 mV) than does I(Ks). 6. These findings indicate that block of I(Ks) is not likely to provide antiarrhythmic benefit by lengthening normal ventricular muscle QTc, APD, and refractoriness over a wide range of frequencies.

Rate and concentration-dependent effects of UK-68,798, a potent new class III antiarrhythmic, on canine Purkinje fibre action potential duration and Vmax.

1. The frequency-dependent electrophysiological effects of UK-68,798 in concentrations of 1, 3, 10 and 30 nM were examined in isolated cardiac Purkinje fibres of the dog at both a number of constant rates of stimulation and following abrupt changes in pacing cycle length. 2. In all concentrations evaluated, UK-68,798 lengthened action potential duration in a concentration- and rate-dependent manner (e.g., at a cycle length = 500 ms, control APD90 = 234.0 +/- 3.3 ms, while after 10 nM UK-68,798, APD90 = 315.0 +/- 5.9 ms). 3. The duration of action potentials evoked following abrupt changes in pacing rate were also increased in a concentration-dependent manner at all diastolic intervals tested. 4. The fast and slow time constants for restitution of APD were not altered by UK-68,798. However, the amplitude terms for this relation were increased. 5. In addition, the maximum upstroke velocity (Vmax) was not significantly affected by exposure to UK-68,798 at any concentration or diastolic interval. The kinetics for recovery of Vmax were thus unaffected. 6. These findings are similar to those previously reported for recognized class III antiarrhythmic agents (e.g., bretylium, clofilium, and sotalol); however, UK-68,798 was 1,000 to 10,000 times more potent. 7. The combined potency and selectivity of this agent seem to make it an ideal tool for the investigation of cardiac potassium channels believed responsible for controlling the duration of the action potential. 8. This potent and highly selective compound may prove extremely useful in the control of cardiac arrhythmias.

Rate-dependent electrophysiological effects of OPC-8212: comparison to sotalol.

The rate-dependent effects of OPC-8212 (6 microM), a new positive inotropic drug which lengthens cardiac action potential duration were studied in dog cardiac Purkinje fibers and compared to the effects of sotalol (30 microM), a class III antiarrhythmic using conventional microelectrode technique. Both OPC-8212 and sotalol lengthened action potential duration in a frequency-dependent manner. Both had a greater effect at slower pacing frequencies than at fast. The maximum rising velocity of the action potential upstroke was not affected by either compound at any of the pacing cycle lengths applied (300-2000 ms). The effects of OPC-8212 and sotalol on action potential duration and maximum rising velocity were also compared following abrupt changes in pacing cycle length (i.e. coupling intervals ranging between approximately 250-3000 ms). The effects of both OPC-8212 and sotalol following abrupt changes in pacing cycle length were similar to one another. Neither OPC-8212 nor sotalol altered the time constants for restitution of action potential duration or the kinetics for recovery of the maximum rising velocity of the action potential upstroke. These results indicate that the rate-dependent electrophysiological effects of OPC-8212 closely mimic those of sotalol and suggest that OPC-8212 may be an effective class III antiarrhythmic in addition to being a positive inotropic compound.

Effects of bepridil on force development and transmembrane electrical activity of adult canine Purkinje strands: comparison with nisoldipine and lidocaine.

Superfusion of a bepridil containing normal Tyrode solution produced a concentration-dependent decrease in force development and shortening of action potential duration in isolated Purkinje strands. At a concentration close to the 50% inhibitory concentration for effects on force development and action potential duration, bepridil blocked 'slow' action potentials in potassium depolarized, isoproterenol-restored Purkinje strands. When the concentration-response relations for decreasing force development and shortening action potential duration were compared to those of nisoldipine and other calcium entry inhibitors, bepridil was the least potent. Bepridil also showed a concentration-dependent effect on the maximum rate of rise of the Purkinje fiber action potential upstroke similar to, though less potent than that of lidocaine. In addition, like lidocaine, bepridil shifted the membrane responsiveness curve of the Purkinje strands to the left. These findings indicate that bepridil produced effects that are consistent with its classification as a calcium entry inhibitor (class IV antiarrhythmic), as well as effects that are 'lidocaine-like' (class I antiarrhythmic).

Ionic basis for OPC-8212-induced increase in action potential duration in isolated rabbit, guinea pig and human ventricular myocytes.

Changes in transmembrane ionic currents induced by OPC-8212 (3,4-dihydro-6-[4-(3,4-dimethoxybenzoyl)-1-piperazinyl]-2(1H)-quinoline) , a recently introduced positive inotropic agent which lengthens cardiac action potential duration, were examined using whole-cell voltage-clamp techniques in single rabbit, guinea pig and human ventricular myocytes. In rabbit, OPC-8212 (12 mumol/l) significantly increased membrane action potential duration measured at 90% of repolarization by an average of 88 ms (from 462 +/- 25 to 550 +/- 35 ms, n = 4; P < 0.05). In rabbit this increase in duration was not associated with significant changes in either the inward rectifier or transient outward K+ currents. The magnitude of the secondary inward current evoked from a holding potential of -50 mV was significantly increased by 97 +/- 8% (n = 6; P < 0.01) while a demonstrable delayed rectifier outward current could not be identified in the rabbit myocytes examined at room temperature. In guinea pig ventricular myocytes, where the delayed rectifier was large, 12 mumol/l OPC-8212 significantly depressed the current by 58 +/- 10% (n = 6; P < 0.01). The effects of OPC-8212 in human ventricular myocytes obtained from the explanted heart of a single patient having an idiopathic cardiomyopathy most closely resembled those observed in isolated rabbit ventricular myocytes. Thus, in rabbit and a few human ventricular myocytes examined at room temperature, OPC-8212 appeared to lengthen cardiac membrane action potential duration primarily by increasing the amplitude of the secondary inward current believed to primarily represent current through L-type Ca2+ channels. In guinea pig preparations, OPC-8212 also decreased the delayed rectifier outward K+ current which also would account for an increase in action potential duration. OPC-8212 could not be demonstrated to affect Na+ current inactivation in a manner similar to that produced by 1 mg/l veratrine, a recognized Na+ channel agonist, which dramatically slowed this process.

Effects of the optical isomers of verapamil on electrophysiological properties of the heart in conscious dogs.

We compared the cumulative dose-response relations of verapamil (0.1, 0.2 and 0.4 mg kg(-1)) in different R/S enantiomer ratios (100/0, 90/10, 80/20, 50/50 and 20/80) on the electrophysiological and hemodynamic characteristics of the heart using the conscious dogs. A reduction of mean arterial pressure occurred with 20R/80S producing a 3-times greater decrease than 100R/0S, but an increase in heart rate occurred with 20R/80S producing a 9-times greater increase than 100R/0S. Increased heart rate was concurrent with decreased mean arterial pressure most prevalent with a higher ratio of S-isomer that produced a greater reduction in mean arterial pressure and increase in heart rate at lower overall verapamil doses. Atrio-ventricular conduction time increased 3-5 min after each infusion, with 20R/80S producing a 4-times greater effect than 100R/0S. These results indicate that the peripheral and cardiac electrophysiologic properties of various nonracemic verapamil mixtures are mainly attributable to the concentration of S-isomer.

Cyclic AMP-dependent and cyclic AMP-independent actions of a novel cardiotonic agent, OPC-8212.

Possible cAMP-dependent and cAMP-independent mechanisms of action for the cardiac effects of OPC-8212, a novel piperazinyl-quinolinone derivative, were evaluated. OPC-8212 was tested for in vitro potency as an inhibitor of soluble bovine cardiac phosphodiesterases using a rapid isolation and assay method involving monoclonal antibodies that distinguish among isozymes. The drug was selective for a low-Km, cGMP-inhibited phosphodiesterase (CGI-PDE) with an IC50 (half-maximal inhibition concentration) of 7.4 mumol/l when measured at a substrate level of 0.35 mumol/l cAMP. Under the conditions used, sulfolane, the solvent for OPC-8212, did not affect CGI-PDE activity. In electrophysiological measurements, OPC-8212 prolonged the action potential duration in canine Purkinje strand preparations up to 148% (APD90) at 10 mumol/l. Concomitantly, OPC-8212 produced a 100% increase in developed force. Both prolongation of the action potential duration and the positive inotropic effect were readily reversed after exposure to tetrodotoxin, 3 mumol/l. Using Na-selective microelectrodes, intracellular Na+ ion activity increased 225% upon exposure to 10 mumol/l OPC-8212. OPC-8212 represents a novel type of positive inotropic agent, possessing both cAMP-dependent (selective PDE isozyme inhibition) and cAMP-independent (activation of intracellular Na+) mechanism of action.

Concentration- and rate-dependent electrophysiological effects of restacorin on isolated canine Purkinje fibres.

The cellular electrophysiological effects of restacorin, a new antiarrhythmic agent were studied using conventional microelectrode techniques in isolated dog cardiac Purkinje fibres. Restacorin (1-30 mumol/l) decreased the maximum rate of rise of the action potential upstroke and action potential amplitude while action potential duration measured at 90% of repolarization was shortened in a concentration-dependent manner during pacing at a constant basic cycle length of 500 ms. The effect of 10 mumol/l restacorin on maximal rate of rise of the action potential upstroke and on action potential duration measured at 90% of repolarization were also studied while varying the constant pacing cycle length between 300 and 5000 ms. The results of these studies indicated a rate-dependent effect of restacorin on the action potential characteristics examined. After abrupt changes in cycle length, 10 mumol/l restacorin slowed the fast component of the relation for restitution of action potential duration from 155.3 +/- 5.2 ms (control, n = 6) to 217.1 +/- 17.8 ms (n = 6, P less than 0.05). In the presence of restacorin (10 mumol/l), a second slow component for recovery of maximal action potential upstroke rising velocity was expressed having a time constants of 8.5 +/- 1.2 s. The range of premature action potential durations was significantly decreased (by 57.1%, P less than 0.01) by 10 mumol/l restacorin. These results indicate that the cellular electrophysiological effects produced by restacorin in dog cardiac Purkinje fibres best resemble those produced by recognized class Ic antiarrhythmic drugs.

Effect of sotalol on transmembrane ionic currents responsible for repolarization in cardiac ventricular myocytes from rabbit and guinea pig.

The effects of sotalol, a beta-adrenoceptor blocker and class III antiarrhythmic agent, on transmembrane ionic currents were examined in single rabbit and guinea pig ventricular myocytes using whole-cell voltage-clamp techniques. In neither of these species did 60 microM sotalol appreciably effect the inward rectifier, the transient outward or the inward calcium currents. In addition, sotalol did not elicit a slowly inactivating component of the sodium current as did 1 microgram/ml veratrine. In guinea pig ventricular myocytes, sotalol also significantly depressed the outward delayed rectifier current. An outward delayed rectifier current was not observed in rabbit ventricular myocytes examined at room temperature; and, under these conditions sotalol did not lengthen action potential duration. Sotalol induced lengthening of cardiac action potential duration can, therefore, be explained by depression the outward delayed rectifier current.

Nonlinear relationship between V+max and h infinity in frog skeletal muscle.

The relationship between the maximum velocity of action potential upstroke (V+max) and steady-state Na+ channel inactivation (h infinity) was studied in frog skeletal muscle during repetitive discharges evoked in the presence of cevadine (1 mumol/l). Conventional microelectrodes and vaseline-gap voltage-clamp techniques were used. A severe degree of nonlinearity was found between (h infinity) and (V+max) especially when the Na+ conductance (gNa) was small. The observed nonlinearity could be explained as a property of the normal Na+ channel gating in skeletal muscle rather than that of cevadine-modified channels. Part of this work has been published in abstract form in Biophys. J. 57: 105A, 1990.