Na, K-ATPase activity, ATP and Pi concentrations in various regions of monkey heart and their relation to post-overdrive hyperpolarization.

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Abstract
The relationship among Na,K-ATPase activity, ATP and Pi-concentrations in functionally different regions of cardiac and skeletal muscles were studied in the Japanese monkey (Macaca fuscata). The activity of Na,K-ATPase and the concentrations of ATP and Pi were determined by isotachophoresis. The ATP concentrations in several regions of cardiac and skeletal muscles correlated positively with the Na,K-ATPase activities and Pi concentrations, whereas there was no significant correlation between the Na,K-ATPase activities and the Pi concentrations. Therefore, the ATP concentrations in functionally different regions of the tissues seemed to be a feasible measure of Pi concentrations and the Na,K-AtPase activities. The Na,K-ATPase activity was significantly higher in the epicardium than in the endocardium of the ventricle. The post-overdrive hyperpolarization was compared in these 2 tissues, using microelectrode methods. The hyperpolarization which followed overdrive stimulation (3.3-3.8 Hz) in the epicardium exceeded that in the endocardium, and was attributed to the difference in Na,K-ATPase activity. Tissues with a higher Na,K-ATPase activity may have a more potent Na,K-pump activity, in association with higher tissue concentrations of ATP and Pi.