Thyroidal regulation of rat thymic and splenic (Na+ + K+)-adenosine triphosphatase

The present studies concern the effects of triiodothyronine (T3) on rat thymic and splenic NaK-ATPase activities. When hypothyroid rats were given a single injection of T3 (250 micrograms/100 g body weight), thymic NaK-ATPase activity increased by 18% at 24 h (p less than 0.005), and remained at this peak at 72 h. Injection of T3 (250 micrograms/100 g body weight, on alternate days X 3) significantly augmented NaK-ATPase activity from the thymus and spleen of hypothyroid rats by 19 and 49%, respectively. Two weeks of daily injections of T3 (1 microgram/100 g body weight) to hypothyroid rats significantly increased NaK-ATPase activity in the thymus and the spleen by 14 and 28%, respectively. Reverse T3 had no significant effect of thymic and splenic NaK-ATPase activities. Mg-ATPase and 5'-nucleotidase activities were not significantly affected by T3 in either tissue under different thyroid states. The lack of response of thymic and splenic NaK-ATPase to reverse T3, and Mg-ATPase and 5'-nucleotidase to T3, substantiates the specificity of the hormone action on NaK-ATPase. To investigate whether the T3-dependent increase in NaK-ATPase activity is a result of an increase in the number of enzyme units, the effect of T3 on the quantity of phosphorylated intermediate of NaK-ATPase was assessed in the partially purified membrane fraction. In the euthyroid state, NaK-ATPase activity and phosphorylated intermediate units were 33 and 43% higher, respectively than in the hypothyroid state, implying that the T3-dependent increase in NaK-ATPase activity is a result of an increase in the number of NaK-ATPase units.