Direct effects of digoxin and deslanoside on sinoatrial conduction in isolated, blood-perfused dog atria

We studied the direct effects of digitalis on sinoatrial conduction time (SACT), atrial rate (AR) and developed tension (DT) in isolated atrial muscle, using an isolated dog atrial preparation perfused with heparinized blood from the carotid artery of an anesthetized donor dog. After digoxin or deslanoside (100 micrograms or 200 micrograms/Kg) was given intravenously to the donor dog, SACT, AR and DT of the isolated atrium were continuously measured. Following administration of 100 micrograms/Kg of digoxin, an immediate sinus bradycardia followed by various kinds of ventricular arrhythmias was observed in the donor dog, and the DT of the isolated atrium was augmented without any effects on the SACT and AR in all 3 experiments. With 200 micrograms/Kg of digoxin or deslanoside, ventricular fibrillation was induced in all 5 donor dogs within 75 min after digitalis administration. In these cases, the DT of the isolated atrium was immediately increased to over 200% of control DT, but SACT and AR were not affected until 20 min later, at which time increases in SACT and AR were finally induced. From these results, it is concluded that a large amount of digitalis has no distinct direct effect on SA nodal pacemaker activity and SA conductivity. Moreover, extremely large doses of digitalis which cause ventricular fibrillation might also induce a prolongation of SACT in addition to sinus tachycardia.