Effect of insulin upon membrane‐bound (Na+ + K+)‐ATPase extracted from frog skeletal muscle.

Abstract

Insulin stimulated the activity of membrane-bound ATPase isolated from frog skeletal muscle and from rat brain. The increase in activity of the membrane-bound ATPase system isolated from frog ranged from 9.8-53% at concentrations of Na+ (25 mM), K+ (10 mM) and ATP (2 mM) similar to those in previous in vivo experiments. The increased activity of the membrane-bound ATPase is at least as great as the insulin-induced increase in Na efflux (10-38%) from intact cells. If the concentration of Na+ is lowered to 4 mM and that of ATP lowered to 0.5 mM, the increase in ouabain-inhibitable ATPase activity can reach as high as 400%. Ouabain, at a concentration (10-3 M) sufficient to inhibit stimulation of the frog ATPase by increasing Na from 4-25 mM, completely blocked the stimulation of ATPase activity due to insulin. At 2 mM-ATP, 100 mM,Na+, and 20 mM-K+, conditions which maximally activate the (Na+ + K+)-ATPase, insulin did not increase the ATPase activity. Stimulation was consistently seen at 10 mM-K+, 0.5 mM-ATP and either 4 mM or 25 mM-Na+. Interaction of insulin with its receptor upon the plasma membrane somehow stimulates the (Na+ + K+)-ATPase system (ouabain-sensitive, ATP phosphohydrolase, EC 3.6.1.3. Insulin probably does not affect Vmax of the (Na+ + K+)-ATPase system, but may increase the affinity of the enzyme system to 1 or more effectors, most likely Na+, ATP and perhaps K+. Oxidized glutathione (2.7 .times. 10-6 M), 10-6 M albumin, and 10-6 M, 10-7 M and 10-8 M cyclic AMP did not affect the ATPase activity. The Na pump may be intimately involved with the physiological action of insulin and may be a transducer between the binding of insulin to its receptor on the plasma membrane and the cellular actions of insulin.