Corticosterone and triiodothyronine control of myocardial Na+-K+-ATPase activity in rats.

Experiments were carried out to examine the effects of corticosterone and triiodothyronine on rat myocardial Na+-K+-ATPase activity. Corticosterone increased Na+-K+-ATPase activity when administered intravenously in multiple doses, a single dose, and different dosages. The possibility that enhancement of myocardial Na+-K+-ATPase activity by corticosterone is mediated by early changes in intracellular Na+ and K+ was investigated. Following a single dose of corticosterone (1 mg/100 g body wt), the increase in Na+-K+-ATPase activity preceded the changes in intracellular Na+ and K+. Corticosterone did not change the transition temperature and activation energy of the reaction in myocardial Na+-K+-ATPase. This implies that corticosterone does not act by altering the lipid matrix in the microenvironment of the Na+-K+-ATPase system. The interaction between the effects of corticosterone and triiodothyronine on myocardial Na+-K+-ATPase activity was examined. Our studies indicated that corticosterone and triiodothyronine regulate Na+-K+-ATPase activity via parallel independent pathways.