Effects of ouabain on the electrophysiological properties of subendocardial Purkinje fibers surviving in regions of acute myocardial infarction

Abstract We compared the effects of ouabain 2.5 × 10 −8 M. on the electrophysiological properties of minimally-depressed subendocardial Purkinje fibers surviving in regions of acute myocardial infarction with its effects on subendocardial Purkinje fibers in adjacent normal regions and in anatomically identical regions of noninfarcted hearts. Experiments were carried out in vitro on left ventricular tissue preparations dissected from the hearts of dogs 24 hours after coronary artery ligation and from normal canine hearts. Ouabain had no effect on maximum diastolic potential, action potential upstroke, or V max of Purkinje fibers in normal or infarcted regions. It prolonged the action potential duration in infarcted but not in normal regions. In infarcted tissue preparations ouabain increased conduction velocity and failed to alter the effective and functional refractory periods, while in normal tissue preparations the drug did not affect conduction velocity or the effective refractory period but caused an increase in the functional refractory period. Ouabain did not cause spontaneous activity in normal tissue preparations. In infarcted preparations it resulted in spontaneous activity in six of eight preparations studied. This was due to the development of oscillatory afterpotentials in three preparations and phase 4 depolarization in one; in the other two preparations the mechanism was uncertain. Thus, Purkinje fibers surviving in infarcted regions are more sensitive than normal fibers to the direct electrophysiological effects of digitalis.