To gain an insight into the mechanism of stimulation by insulinof Na efflux, the effect of high concentrations of internal Na on the inhibition by ouabain of Na efflux was compared to the inhibition by ouabainof insulin-stimulated Na efflux.The rate coefficient of 22Na efflux from “high-Na” muscle exposed to ouabain was lower than that from “lowNa” uscle exposed to ouabain.Similarly, the rate of net Na loss from the “high-Na” e which had been exposed to ouabain was lower thanthat from the “low-Na”muscle.findings indicate that Na transport units whose internal Na-binding sites have been occupied by Na issusceptible to ouabain.The insulin-stimulated Na efflux was inhibitedby ouabain to a larger extent than was the Na efflux from the controlmuscle, although the final level of the rate coefficient of 22 Na efflux fromthe muscle exposed to both insulin and ouabain was not always lower thanthat from the muscle exposed only to ouabain.The similarity betweenthe effects of ouabain in “high-Na” muscle and in insulin-preincubatedmuscle suggests that insulin increases the fraction of Na transport unitswhose internal Na-binding sites have been occupied by Na.
