Effects of EGIS-7229 (S 21407), a novel class III antiarrhythmic drug, on myocardial refractoriness to electrical stimulation in vivo and in vitro

Effects of ibutilide on canine cardiac pacing threshold and on induction rates of atrial fibrillation

This study aims to observe the effects of ibutilide on canine cardiac pacing threshold and on induction rates of atrial fibrillation. Eighteen mongrel dogs were anesthetized and administrated with ibutilide. The pacing thresholds and induction rates of atrial fibrillation were measured with and without ibutilide (10-min infusion dose was 0.10 mg kg(-1), followed by a maintaining dose of 0.01 mg min(-1) 30 min later). This study found that ibutilide increases pacing thresholds in dogs. Moreover, there were significant differences between pacing thresholds with and without ibutilide (P < 0.05). Further, ibutilide significantly reduces the induction rates of atrial fibrillation (P < 0.05). Our findings indicate that pacing voltage changes should be closely monitored in patients taking anti-arrhythmic drugs, who are treated with cardiac stimulation or have undergone pacemaker implantation. We also found that ibutilide is an effective drug in preventing or controlling atrial fibrillation.

Effects of Dofetilide and EGIS-7229, an Antiarrhythmic Agent Possessing Class III, IV, and IB Activities, on Myocardial Refractoriness in Hyperkalemia, Hypokalemia, and During β-Adrenergic Activation in the Rabbit Papillary Muscle In Vitro

Lengthening of the effective refractory period (ERP) by EGIS-7229, a class III/Ib/IV drug, and by dofetilide, a selective I(Kr) blocker, was compared in normokalemia (NK), hypokalemia (LK), and hyperkalemia (HK) in right ventricular papillary muscles of rabbits paced at 0.5, 1, and 2 Hz, in vitro, and also during beta-adrenergic activation. In NK, EGIS-7229 (3 and 10 microM) and dofetilide (30 and 100 nM) similarly lengthened ERP in a steeply reverse frequency-dependent manner. The two compounds produced smaller ERP prolongations at 0.5 Hz in HK and LK, so rate-dependence of ERP changes decreased. EGIS-7229 lengthened ERP more at 2 Hz than at 0.5 Hz at 10 microM in LK, that is, the effect of EGIS-7229 turned into positive frequency-dependence from 3 to 10 microM. Furthermore, EGIS-7229 lengthened ERP at 10 microM more than dofetilide at 100 nM at 2 Hz stimulation rate (P<0.05). Isoproterenol (30 nM) eliminated the effect of dofetilide on ERP, while EGIS-7229 prolonged ERP during beta-adrenergic activation. In conclusion, efficacy of EGIS-7229 was superior to that of dofetilide in LK and during beta-adrenergic stimulation, suggesting improved antiarrhythmic action for EGIS-7229 under certain conditions in the patient.

Beta-adrenoceptor activation plays a role in the reverse rate-dependency of effective refractory period lengthening by dofetilide in the guinea-pig atrium, in vitro

1. Blockers of the rapid component of the delayed rectifier potassium current (I(Kr)) prolong cardiac action potential duration (APD) and effective refractory period (ERP) in a reverse rate-dependent manner. Since activation of beta-adrenoceptors attenuates prolongation of APD evoked by I(Kr) blockers, rate-dependent neuronal noradrenaline liberation in the myocardium may contribute to the reverse rate-dependent nature of the effects of I(Kr) blockers. In order to test this hypothesis, we studied the effects of dofetilide, a pure I(Kr) blocker, on ERP after activation or blockade of beta-adrenoceptors and after catecholamine depletion in guinea-pig left atrial myocardium paced at 3, 2 and 1 Hz, in vitro. 2. Dofetilide (100 nM) lengthened ERP in a reverse rate-dependent manner in the left atrial myocardium of guinea-pigs. Strong activation of beta-adrenoceptors using 10 nM isoproterenol abolished the dofetilide-induced lengthening of ERP at all pacing rates. 3. Blockade of the beta-adrenoceptors with metoprolol (1 micro M), atenolol (3 micro M) or propranolol (300 nM) increased the dofetilide-evoked prolongation of ERP at 3 and 2 Hz, but not at 1 Hz. As a consequence, metoprolol attenuated while propranolol and atenolol fully eliminated the reverse rate-dependent nature of the dofetilide-induced ERP lengthening. In catecholamine-depleted atrial preparations of the guinea-pig (24 h pretreatment with 5 mg kg(-1) reserpine i.p.), the effect of dofetilide on ERP was not frequency dependent, and propranolol did not alter the effects of dofetilide. 4. In contrast to results obtained in guinea-pig atrial preparations, propranolol failed to change the reverse rate-dependent effect of dofetilide on ERP in the right ventricular papillary muscles of rabbits and guinea-pigs. 5. As an indication of the functional consequences of rate-dependent noradrenaline liberation, propranolol decreased twitch tension at 3 and 2 Hz but not at 1 Hz in the atrial myocardium of control guinea-pigs, whereas no such effect was detected in catecholamine-depleted atrial preparations. Propranolol failed to change contractility of ventricular myocardium in guinea-pigs and rabbits. 6. It is concluded that rate-dependent noradrenaline release and the ensuing beta-adrenoceptor activation contributed to the reverse rate-dependent nature of ERP prolongation caused by I(Kr) blockers in isolated guinea-pig atrial myocardium.