Effects of external K concentration on the electrogenicity of the insulin-stimulated Na,K-pump in frog skeletal muscle.

Insulin hyperpolarized the membrane of frog skeletal muscle by stimulating the electrogenic Na,K-pump. At external K concentrations of 1,2,5 and 10 mM, both the insulin-induced hyperpolarization and the insulin-stimulated ouabain-sensitive Na efflux (an index of Na,K-pump activity) were observed. By increasing the external K concentration, the insulin-stimulated Na efflux increased, but the magnitude of the insulin-induced hyperpolarization decreased; i.e., although the activity of the insulin-stimulated Na,K-pump increased, on the contrary, the magnitude of the hyperpolarization decreased. To clarify the causes of this phenomenon, the specific membrane resistance was measured and found to decrease upon increasing the external K concentration. One of the reasons for the decrease in magnitude of the hyperpolarization is the decrease in the specific membrane resistance. However, the decrease in magnitude of the hyperpolarization with a rise of the external K concentration, which increased the insulin-stimulated Na,K-pump activity, cannot be explained only by the decrease in the specific membrane resistance. It is suggested that the decrease in magnitude of the hyperpolarization is mainly caused by a decrease in the electrogenicity of the insulin-stimulated Na,K-pump upon an increase in the external K concentration. The conclusion of the present study is that the electrogenicity of the insulin-stimulated Na,K-pump in muscles is variable and decreases with increasing the external K concentration.