Apparent affinity changes induced by insulin of Na-K transport system in frog skeletal muscle

The dependency of Na efflux on intracellular Na concentration was examined on sartorius muscles of Rana catesbeiana. In normal Ringer, the rate coefficient of 22Na efflux is nearly proportional to the internal Na concentration, i.e., Hill's coefficient in the dependency of Na efflux on internal Na concentration was around two. As long as insulin is present in the bathing solution, the rate coefficient of 22Na efflux retains the stimulated level. Insulin caused a leftward shift of the relationship between Na efflux and the logarithm of internal Na concentration, and concomitantly decreased Hill's coefficient. A model of the coupling of Na transport and hydrolysis of ATP was proposed. The theoretical relation derived from this model accounted for these findings quite satisfactorily. According to this model, insulin shifts as a cofactor the equilibrium between dephosphorylated and phosphorylated states of carriers, whose Na binding sites are not occupied by Na, toward the phosphorylated state.