Alterations of cardiac NaK-ATPase by the thyroid state in the rat.

Total and specific activity of cardiac NaK-ATPase (EC 3.6.1.3) in the rat varied parallel with the thyroid state. The functional state of the thyroid altered the number of NaK-ATPase molecules in the cell; the catalytic activity (turnover number) of individual enzyme molecules did not change. Sensitivity of NaK-ATPase to its specific inhibitor, ouabain, (as estimated from the half-maximal inhibitory concentration of ouabain) was lower in hypothyroidism but remained unchanged in hyperthyroidism relative to the euthyroid state. Action of thyroid hormones appeared to be selective for NaK-ATPase as shown by the behavior of another enzyme of the sarcolemma, adenyl cyclase. Adenyl cyclase of the heart was not affected by hyperthyroidism but was significantly elevated in hypothyroidism relative to euthyroid controls.