THE ACTION OF CARDIAC GLYCOSIDES ON THE EXCITABILITY AND CONDUCTION VELOCITY OF THE MAMMALIAN ATRIUM

¹ Department of Pharmacology, Instituto Nacional de Cardiología, México, D. F., México

The actions of K-strophanthoside and lanatoside C on the following physiological properties of atrial muscle were studied: basal excitability, conduction velocity, and refractory period as determined by the excitability recovery curve.

The glycosides reduced basal or diastolic excitability. The strength-duration curves were totally displaced; i.e., the threshold was elevated at all values of stimulus duration. During the course of digitalis intoxication the depression of excitability began promptly after the administration of about 30 per cent of the lethal dose; i.e., at dose levels corresponding to the therapeutic range.

Conduction velocity was slowed, beginning at about 35 to 40 per cent of the lethal dose.

These effects on excitability and conduction velocity show that the atria are more sensitive than the ventricles to the action of the glycosides.

The changes of excitability and conduction velocity produced by the glycosides are not affected by vagal innervation, since stimulation of the cut vagi at intensities which caused cardiac slowing comparable to that produced reflexly by digitalis itself in innervated preparations did not modify these properties.

The cardiac glycosides accelerate the recovery of excitability when vagal reflex activity is maintained; but in the denervated heart they cause a considerable delay in the recovery process. The delay involves almost exclusively the relatively refractory period, with little or no change in the duration of the absolutely refractory phase, indicating that proper study of the action of any drug upon the refractory period of a tissue requires a determination of the whole excitability recovery curve.

The mechanism by which vagal section or atropinization confers an antifibrillatory action upon the cardiac glycosides was discussed.