Differing electrophysiological effects of class IA, IB and IC antiarrhythmic drugs on guinea-pig sinoatrial node

Standard microelectrode techniques were used to study the effects of class IA (quinidine, disopyramide, procainamide), IB (lignocaine, mexiletine, tocainide) and IC (flecainide, encainide, lorcainide) antiarrhythmic drugs on action potentials in spontaneously beating sino-atrial node cells from guinea-pigs. The IA drugs all produced significant slowing of spontaneous rate in therapeutic concentrations. The IB agents did so only in concentrations well above therapeutic levels and the IC drugs were of intermediate potency. All nine drugs markedly slowed the repolarization rate and this was the major mechanism of sinus slowing for the IA and IC compounds. The IB drugs shared this effect but prolongation of phase 4 by reduction of the slope of diastolic depolarization was also a prominent feature of their action.