Effects of Cardiac Glycosides upon the Electrical Activity of Single Ventricular Fibers of the Frog Heart, and Their Relation to the Digitalis Effect of the Electrocardiogram

Using microelectrodes having an external diameter of less than 1, &mgr;, potential variations of single cardiac fibers were obtained from frog ventricular muscle in situ. The normal membrane potential consists of a rapid depolarization and slow recovery, the latter occurring in three distinct phases. Digitalis glycosides alter profoundly the slopes of these various phases and drastically shorten the entire period of recovery. The characteristic cardiac action potential is thus converted into a spike which resembles the electrical events recorded for skeletal muscle or nerve. These changes occur independently of mechanical systole which remains essentially unaltered. The deformation of the action potential occurs uniformly throughout the ventricular muscle. They are related to the RS-T segment depression of the surface electrocardiogram which in this instance cannot be explained as being caused by a failure of the diastolic repolarization of the endocardial surface but must be an expression of uniform shortening of the entire process of recovery.