Electrophysiological effects of a new antiarrhythmic agent, lorcainide, on the isolated cardiac muscle fiber as compared with disopyramide.

Electrophysiological effects of a new antiarrhythmic agent, lorcainide, were studied in the isolated guinea pig ventricular myocardium and were compared with those of disopyramide; some of the experiments were also made by using the canine ventricular myocardium. Both lorcainide and disopyramide selectively depressed the maximum rate of rise of the action potential with little effect on the resting and overshooting potential. The action potential duration tended to be shortened slightly, and the prolongation of the refractory period produced by these agents was only slight. Both agents did not produce the substantial modification of the slow response produced by isoproterenol in the depolarized muscle at the concentrations enough to depress the maximum rate of rise of the action potential. Both agents also produced the marked depression of the rate of the action potential of the canine ventricular muscle, and the conduction velocity measured in the canine false tendon was markedly decreased by both agents. It was concluded that both lorcainide and disopyramide are typical Class I agents according to the classification proposed by Vaughan Williams and that lorcainide is about 10 times (at least 3 times) more potent than disopyramide in this respect.