Effects of thyroxine treatment on the hepatic plasma membrane ATPase activity in rats.

The effects of T4 and T3 upon ATPase activity of plasma membrane preparations produced by different methods has been studied. Media without Na+ or K+ was used to identify Mg++ activated ATPase, and media containing 100-120 mM Na+ and 20-30 mM K+ in addition to Mg++ was used to identify the augmented activity of Na-K ATPase. We also utilized media containing Na+, K+, and Mg++ in the presence and absence of ouabain to identify the Na+-K+ activation. We found no effect of doses of thyroxine in the physiologic range upon the Na+-K+ ATPase activity. A dose-response study indicated that high doses of T4 may stimulate Mg++ activated ATPase. In the P2 preparation of the Touster method we found minimally enhanced Na+-K+ ATPase activity. The quantity of T4 needed to cause such enhancement is approximately 50 times the physiologic replacement dose needed to restore thyroidectomized animals to a euthyroid state. We conclude that thyroid hormones do not stimulate Na+-K+ ATPase of plasma membranes when administered in a physiologic quantity, but may enhance Mg++ atpase activity when administered in a pharmacologic or toxic quantity.