Electrophysiological characterization of BRL-32872 in canine Purkinje fiber and ventricular muscle. Effect on early after-depolarizations and repolarization dispersion.

Amongst the different pharmacological approaches to the treatment of cardiac arrhythmias, compounds with multiple electrophysiological activities appear to exhibit a reduced adverse effect profile. BRL-32872 (N-(3,4-dimethoxyphenyl)-N-[3[[2-(3,4-dimethoxyphenyl) ethyl] propyl]-4-nitrobenzamide hydrochloride) is a typical example of an antiarrhythmic agent with combined K(+) and Ca(2+) blocking actions. In this study, we investigated the effects of BRL-32872 on early after-depolarizations and on dispersion of repolarization. Action potentials were recorded either in canine cardiac Purkinje fibers alone or in preparations containing both ventricular muscle and the attached Purkinje fibers. In Purkinje fibers, BRL-32872 (0. 3-10 microM) induced a bell-shaped concentration-dependent increase in action potential duration. At 90% of repolarization, the action potential was prolonged at concentrations up to 1 microM and was shortened when the concentration of BRL-32872 was further increased. In all 17 experiments, BRL-32872 did not cause early after-depolarizations in Purkinje fibers. On the contrary, BRL-32872 (3 microM) systematically suppressed early after-depolarizations induced by clofilium (4-chloro-N, N-diethyl-N-heptylbenzenebutanaminium tosylate, 1 microM), a selective inhibitor of the delayed rectifier K(+) current. A similar effect was observed once with 1 microM BRL-32872, a concentration able to prolong Purkinje fiber action potentials. Simultaneous recording of action potentials in ventricular and Purkinje preparations showed that increasing concentrations of BRL-32872 (0. 3-10 microM) induced a limited increase in the difference of repolarization time between the two tissues. The selective K(+) channel inhibitor E-4031 (N-(4-(1-[2-(6-methyl-2-pyridyl) ethyl]-4-piperidyl)-carbonyl] phenyl) methanesulfonamide dihydrochloride dihydrate) exhibited a significant concentration-dependent increase in dispersion of repolarization. We conclude from the present results that the Ca(2+) blocking activity of BRL-32872 (i) prevents the occurrence of early after-depolarizations associated with action potential prolongation and (ii) limits an excessive increase in action potential duration heterogeneity. These electrophysiological features might represent the basis for antiarrhythmic compounds with reduced proarrhythmic profile.

Combined potassium and calcium channel antagonistic activities as a basis for neutral frequency dependent increase in action potential duration: comparison between BRL-32872 and azimilide.

OBJECTIVE: The effects of BRL-32872, azimilide and a selective blocker of the delayed rectifier potassium current, E-4031, were measured at two different basic cycle lengths (BCL), 300 and 1000 ms. Calcium channel antagonists of sarcolemmal (verapamil and nitrendipine) and sarcoplasmic reticulum (ryanodine) membranes were used to investigate whether the inhibition of the calcium current or the calcium release from the sarcoplasmic reticulum could alter the reverse-rate dependence of E-4031 on action potential duration (APD). METHODS: Guinea pig isolated papillary muscles were superfused with a Tyrode solution maintained at 37 degrees C and stimulated at a BCL of 300 or 1000 ms. The standard microelectrode technique was used to record action potential parameters and to study the effects of azimilide, BRL-32872 and E-4031. E-4031 was superfused at increasing concentrations (0.01, 0.03, 0.1 and 0.3 microM) in the absence or in the presence of verapamil (0.3 microM), nitrendipine (0.03 microM) or ryanodine (0.1 microM). RESULTS: BRL-32872 and azimilide induced a self-limited concentration-dependent increase in APD. The effect of BRL-32872 was not dependent on the stimulation frequency whereas the effect of azimilide was significantly reduced at the shorter BCL. E-4031 induced a concentration-dependent increase in APD at both stimulation BCL. The increase in APD was significantly more pronounced in fibres stimulated at a BCL of 1000 ms than in fibres stimulated at a BCL of 300 ms, characterising the reverse-frequency dependent effect of class III antiarrhythmic agents. The reverse-frequency dependence in action potential prolongation induced by E-4031 was significantly reduced in the presence of a low concentration of verapamil (0.3 microM), nitrendipine (0.03 microM), or ryanodine (0.1 microM. CONCLUSION: The results show that BRL-32872, in contrast to azimilide, does not induce the reverse-rate dependency of action potential prolongation typically produced by class III antiarrhythmic agents such as E-4031. Our results also show that reverse-rate dependency induced by E-4031 can be reduced by the simultaneous administration of a low concentration of a calcium channel antagonist or an inhibitor of the release of calcium from the sarcoplasmic reticulum. It is thus suggested that compounds with a suitable balance of potassium and calcium antagonistic activities may have less adverse effects than purely selective potassium channel blockers.

Synthesis, electrophysiological properties and analysis of structural requirements of a novel class of antiarrhythmic agents with potassium and calcium channel blocking properties.

Class III antiarrhythmic agents have been shown to prevent reentrant arrhythmias but also to be responsible for initiating arrhythmias characterised by afterdepolarizations and triggered activities. By combining potassium and calcium channel antagonistic actions, as with BRL-32872 (1), it might be possible to reduce the incidence of proarrhythmias albeit retaining antiarrhythmic efficacy. In the present study we synthesised and tested for their electrophysiological activity in guinea pig papillary muscle a wide panel of analogues of BRL-32872. Some qualitative relationships between compound structure and the inhibitory effect on the rapidly activating component of the delayed rectifier potassium current and/or the L-type calcium current will be presented. New derivatives depicting bell-shaped dose-response curves on action potential duration may therefore represent novel agents for improved antiarrhythmic therapy.

Electrophysiological effect of BRL-32872, a novel antiarrhythmic agent with potassium and calcium channel blocking properties, in guinea pig cardiac isolated preparations.

The effects of N-(3,4-dimethoxyphenyl)-N-[3[[2-(3,4-dimethoxyphenyl) ethyl] propyl]-4-nitrobenzamide hydrochloride (BRL-32872), a novel antiarrhythmic agent, were studied in guinea pig cardiac preparations using standard microelectrode and patch-clamp techniques. In papillary muscle, BRL-32872 did not change resting membrane potential and maximum rate of depolarization but prolonged action potential duration (APD) by 24% +/- 2% at 1.0 microM. When the concentration was increased to 3.0 and 10.0 microM, the effect on APD was not further enhanced, and a bell-shaped dose-response curve resulted. Patch-clamp experiments in isolated myocytes showed that BRL-32872 inhibited the rapidly activating component of the delayed rectifier potassium current (EC50 = 0.028 microM) and the L-type calcium current (EC50 = 2.8 microM) but had a limited effect on the inward rectifier potassium current. In papillary muscles stimulated at 300, 500, 1000 and 2000 msec, the effect of BRL-32872 in prolonging APD did not vary (P = .717). By contrast, N-(4-(1-[2-(6-methyl-2-pyridyl)ethyl]-4-piperidyl)- carbonyl]phenyl)methanesulfonamide dihydrochloride dihydrate (E-4031), a pure class III antiarrhythmic agent, increased APD more at slower than at faster stimulation rates (P = .001), which illustrated the reverse frequency-dependence of this agent. Among the 35 experiments performed with BRL-32872, only one fiber showed early afterdepolarizations (EADs), and these, which occurred at 1.0 microM, were suppressed at higher concentration (3.0 microM). Moreover, EADs induced by E-4031 were suppressed by BRL-32872 (3.0 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of zatebradine, a specific bradycardic agent, on ischemia-induced arrhythmias in anesthetized rabbits.

The effects of the specific bradycardic agent, zatebradine (UL-FS 49), on ventricular arrhythmias occurring during an acute ischemia were compared to those of verapamil. Anesthetized rabbits were submitted to a ligation of the left circumflex coronary artery for 20 min. Zatebradine (150 and 750 micrograms/kg, i.v.) dose-dependently reduced heart rate, but changed neither the left ventricular pressure nor the (+)dp/dtmax. In comparison, verapamil (150 and 750 micrograms/kg, i.v.) reduced heart rate, systemic blood pressure, left ventricular pressure and (+)dp/dtmax. The incidence of ventricular premature beats occurring during acute ischemia was changed neither by zatebradine nor by verapamil. Ventricular fibrillation, occurring in 36% of the saline-treated rabbits, was reduced to 18% in the presence of 750 micrograms/kg of zatebradine and 0% with verapamil (750 micrograms/kg, p < 0.05). The action of zatebradine on ischemia-induced ventricular fibrillation, albeit limited, was completely reversed by atrial pacing to the predrug heart rate. To further investigate the mechanisms involved in the antiarrhythmic potential of both zatebradine and verapamil, their electrophysiological actions were compared in canine Purkinje fibers. Both zatebradine and verapamil induced a dose-dependent increase in action potential duration (APD) measured at 90% repolarization. The APDs measured during the plateau level (APD30) and at 50% of the repolarization (APD50) were shortened by verapamil and increased by zatebradine showing that at the concentrations used, zatebradine did not exhibit any calcium antagonistic activity when compared to verapamil. The results of the present study suggest that the specific bradycardic agent zatebradine showed a beneficial action mainly because of its anti-ischemic properties. However, the present studies performed in anesthetized rabbits suggest that in this species pure reduction in heart rate is not sufficient to entirely prevent ischemic arrhythmias.

A novel class of cardiotonic agents: synthesis and biological evaluation of 5-substituted 3,6-dihydrothiadiazin-2-ones with cyclic AMP phosphodiesterase inhibiting and myofibrillar calcium sensitizing properties.

As part of a search for new cardiotonic agents significantly sensitising the myocardial contractile proteins to calcium, together with cardiac cyclic AMP-PDE inhibitory activity, we have discovered that novel 5-substituted 3,6-dihydrothiadiazin-2-ones may fulfill both properties. The sensitising effect of the contractile proteins to calcium, assessed by the shift in the calcium sensitivity of canine cardiac myofibrillar magnesium-dependent ATPase, is determined by steric and electronic requirements. The requirements for phosphodiesterase inhibition, especially that of a near-planar arrangement for the phenyl and thiadiazin-2-one ring are consistent with those already described for analogous pyridazinones. The synthesis and structure-activity relationships are discussed.

Ischemia and reperfusion-induced arrhythmias in rabbits with chronic heart failure.

The incidence of ventricular arrhythmias occurring during acute myocardial ischemia and reperfusion was studied in anesthetized rabbits with chronic heart failure. Cardiac failure was induced by volume and pressure overload and was characterized by marked hypertrophy (84%) and lower systolic aortic blood pressure (112 +/- 3 mmHg) than in controls (124 +/- 2 mmHg, P less than 0.01). During the first 20 min postcoronary artery ligation, the incidence and duration of ventricular fibrillation were greater in the heart failure group (76% and 485 +/- 77 s, respectively) compared with the control group (27% and 86 +/- 37 s, respectively, P less than 0.01). Reperfusion-induced arrhythmias after various ischemic durations were also more frequent in the heart failure group than in the control group. Papillary muscles taken from rabbits with heart failure showed a reduced diastolic potential and a prolonged action potential duration (APD90) compared with the control group (by 7 and 46%, respectively), but there was no change in maximum upstroke velocity. The present study established that rabbits with pronounced morphological signs of chronic heart failure have an enhanced susceptibility to ischemia and reperfusion-induced arrhythmias. As already described in situations of uncomplicated cardiac hypertrophy, a delay in the repolarization process could represent an arrhythmogenic mechanism in this model.

Diproteverine (BRL 40015): a new type of calcium antagonist with potential antianginal properties.

The calcium channel-blocking activity and associated cardiovascular effects of diproteverine, a novel compound derived from papaverine, were investigated. Electrophysiological measurements in sheep Purkinje fibres showed diproteverine to reduce the amplitude of the slow action potential (IC30 = 2 microM) and to shorten the duration of the fast action potential at 50% repolarisation (IC30 = 2.5 microM). Higher concentrations (4-5 times) were required to block block the sodium channel, as assessed by a reduction in Vmax of the fast action potential. Papaverine was found to possess marginal membrane channel-blocking activity and to be much more potent than diproteverine as a cAMP-phosphodiesterase inhibitor. The most significant haemodynamic property of diproteverine, seen in anaesthetised dogs and conscious dogs pretreated with atropine, was to cause a reduction in heart rate. This appeared to be a particular feature of diproteverine as the other calcium antagonists studied produced either a smaller decrease in heart rate or tachycardia as a reflex response to hypotension. In a chronic myocardial infarct model in dogs, diproteverine caused a redistribution of the available coronary blood flow, to the benefit of an ischaemic area of the myocardium. Diproteverine resembled diltiazem in its effects on coronary blood flow, with both these agents being preferable to nifedipine and verapamil, which caused coronary steal in this model. The combination of the reduction in heart rate, to lower cardiac oxygen demand, with the beneficial action on coronary blood flow should result in diproteverine being particularly beneficial for the treatment of angina pectoris.