Changes in sarcolemmal adenosine triphosphatase activity and in ouabain sensitivity of rat myocardium in isoproterenol-induced cardiac hypertrophy.

Sodium, potassium-activated adenosine triphosphatase (ATPase) activity and the sensitivity of rat myocardium to ouabain was studied in isoproterenol (IPR)-induced cardiac hypertrophy. IPR in a dose of 5 mg/kg was administered to rats intraperitoneally, once daily, for seven days. Left ventricular trabeculae originating from IPR-treated rats were significantly less sensitive than controls to ouabain-induced positive inotropy. In crude homogenate and sarcolemmal fractions the ATP hydrolysing activity both in the presence of Mg++ (basic) and Mg++, Na+, K+ (total) was significantly reduced in the heart of IPR-treated rats. The difference between the total and basic ATPase, i.e. the Na+, K(+)-stimulated portion of the activity was slightly reduced, but the sensitivity of Na+, K(+)-ATPase to ouabain remained unchanged. The results indicate that the well-known relation of sodium pump inhibition to positive inotropy in the heart of IPR-treated rats may not be valid.