Immunocytochemical and enzyme histochemical localization of Na+,K(+)-ATPase in normal and ischemic porcine myocardium

Na+,K(+)-ATPase is involved in generating transmembrane ion gradients and the associated potential difference necessary for contraction of cardiac myocytes. It is possible that changes in the activity or membrane content of this enzyme may occur under ischemic conditions. To investigate this question, right ventricular (RV) ischemia was produced in closed chest pigs and the RV ejection fraction was measured using a fast response thermistor in the pulmonary artery. Sections of RV collected at 15, 30, 45, and 60 min of ischemia were assayed for changes in sarcolemmal Na+,K(+)-ATPase activity using an enzyme coupled histochemical reaction as well as a biochemical assay. Similar sections were examined for changes in the distribution and content of Na+,K(+)-ATPase using an immunocytochemical procedure. The RV ejection fraction fell significantly from baseline after 15 min of ischemia (62 +/- 3% vs 39 +/- 3% respectively, P less than 0.05, n = 10). A decrease in sarcolemmal Na+,K(+)-ATPase activity was first detected after 30 min of occlusion and a significant reduction in enzyme activity was present at 45 min of ischemia. In contrast no changes were detected in the distribution or content of immunoreactive Na+,K(+)-ATPase in the sarcolemma at any time point. In addition, the amount of Na+,K(+)-ATPase in tissue homogenates showed no significant changes after 45 min of ischemia. These findings show that acute ischemia results in the disruption of sarcolemmal Na+,K(+)-ATPase activity and suggests that the decrease in enzyme activity is not due to the loss or redistribution of sarcolemmal Na+,K(+)-ATPase.