Electrophysiologic properties differ in the ventricular endocardium and epicardium of the Japanese monkey

We measured action potential duration (APD) from the endocardium (Endo) and epicardium (Epi) of the left ventricular free wall in Japanese monkey hearts and found that the APD of Endo is significantly longer than that of Epi at a stimulus cycle length of 1500 msec in normal Tyrode solution (control condition). We then hypothesized that shorter APD of Epi results from greater outward pump current and that the difference in the current may be due to a difference in membrane Na,K-ATPase activity between Endo and Epi. If this were the case, interventions which alter the Na,K-pump activity should alter electrophysiologic characteristics in the endocardium and the epicardium to different degrees. An application of ouabain (10(-6) M), an inhibitor of Na,K-ATPase, produced greater shortening of APD and greater depolarization of the resting potential in Endo as compared with Epi. Shortening of stimulus cycle length shortened the APD more markedly in Endo than Epi, resulting in a significantly longer APD in Epi than Endo at a stimulus cycle length of 200 msec. The hyperpolarization and the APD shortening produced when the tissues were returned to 5.4 mM K+ Tyrode solution from K+-free medium were also more marked in Endo than in Epi. Such findings suggest that endocardial cells are more liable to accumulate Na ions intracellularly and K ions extracellularly when the Na, K-pump is suppressed by ouabain or K+-free perfusion, presumably due to lower activity of Na, K-ATPase in Endo compared to Epi.(ABSTRACT TRUNCATED AT 250 WORDS)