ANTIFIBRILLATORY AND PROFIBRILLATORY ACTIONS OF SELECTED CLASS-I ANTIARRHYTHMIC AGENTS

We wished to examine selected class I antiarrhythmic agents for their potential to exhibit proarrhythmic or antifibrillatory actions. Quinidine, lidocaine, aprindine, and flecainide were evaluated in an experimental model that made use of rabbit isolated perfused heart. Hearts were stabilized with oxygenated buffer (95% O2/5% CO2) with or without pinacidil (1.25 muM) and then were subjected to hypoxia (95% N2/5% CO2) for 12 min, followed by 40-min normoxic perfusion (95% O2/5% CO2). Test drugs were added to the perfusion medium 5 min before hypoxia was induced. Prevention of spontaneous ventricular fibrillation (VF) was determined. To characterize the electrophysiologic effects of the selected antiarrhythmic agents, we determined the changes in threshold current and effective refractory period (ERP) before and after drug treatment. Addition of the potassium channel agonist, pinacidil, to the perfusion medium invariably resulted in VF during the hypoxic interval or shortly after reoxygenation. Pretreatment of the heart with glibenclamide prevented pinacidil-induced VF. Quinidine, aprindine, lidocaine, and flecainide each were studied at a single concentration. The respective drug concentrations were selected to produce comparable changes in ventricular refractory period. Of the class I agents selected for study, only quinidine prolonged the ventricular ERP and provided significant protection against pinacidil-induced VF. In contrast, aprindine and lidocaine decreased ventricular ERP and did not prevent VF induced by the combination of pinacidil and hypoxia. Quinidine, aprindine, and lidocaine did not exhibit proarrhythmic effects in the presence of hypoxia when pinacidil was not added to the perfusion medium. Flecainide, when added to the perfusion medium without pinacidil elicited proarrhythmic activity leading to VF when the hearts were made hypoxic. Flecainide-induced VF was antagonized by glibenclamide. The data suggest that VF can be provoked by the potassium channel agonist pinacidil or by flecainide under conditions that reduce intracellular ATP concentration. Glibenclamide, a selective antagonist of the K(ATP) Channel prevented the profibrillatory actions of pinacidil and flecainide. Quinidine, but not lidocaine and aprindine, prevented VF induced by pinacidil and hypoxia.