THE EFFECT OF DIGOXIN ON THE COLD BLOODED HEART AND ITS BEARING ON THE MECHANISM OF DIGITALIS ACTION

¹ From the Department of Physiology, School of Medicine and Dentistry, University of Rochester

The effect of digoxin has been studied on spontaneously beating strips of auricle from the frog, Rana pipiens, and the turtle, Pseudomys elegans, and rhythmically driven strips of ventricle from the turtle. The spontaneous rhythm was usually slowed markedly by the drug. Measurements were made on the driven strips of the Q-T interval, the conduction time, the maximal tension, the duration of systole, the effect of a single short diastole on the ensuing Q-T, the effect of different rates on Q-T, the relation of Q-T to the duration of systole and variations of the threshold to electric stimuli. Mechanical systole is proportional to electrical systole and both are invariably shortened by the drug. The shortening is less at higher rates of beating because the Q-T of a given tissue cannot be shortened beyond a certain limit, either by single early beats or rapid driving and this limit is the same with or without the drug. Since the refractory period which is equal to Q-T is shortened by the drug and the threshold does not regularly change markedly, the slowing action of the drug is ascribed to the slowing of the development of excitation at the pacemaker region. Slowing of conduction usually occurred but later than Q-T changes. Tension increases were irregular and when seen occurred early. The amplitude of contraction may be independent of the duration of systole and the diastolic interval. The possible significance of the shortening of systole for the therapeutic action of digitalis was discussed briefly. It seems reasonable to suppose that the final action of digitalis is related to the lengthened diastole which it produces. There is reduction in the maintenance energy requirement, and longer time for recovery from contraction is given.