Effects of ouabain on Na efflux in high internal Na and insulin-preincubated muscles

To gain an insight into the mechanism of stimulation by insulin of Na efflux, the effect of high concentrations of internal Na on the inhibition by ouabain of Na efflux was compared to the inhibition by ouabain of insulin-stimulated Na efflux. The rate coefficient of 22Na efflux from "high-Na" muscle exposed to ouabain was lower than that from "low-Na" muscle exposed to ouabain. Similarly, the rate of net Na loss from the "high-Na" muscle which had been exposed to ouabain was lower than that frm the "low-Na" muscle. These findings indicate that Na transport units whose internal Na-binding sites have been occupied by Na is susceptible to ouabain. The insulin-stimulated Na efflux was inhibited by ouabain to a larger extent than was the Na efflux from the control muscle, although the final level of the rate coefficient of 22Na efflux from the muscle exposed to both insulin and ouabain was not always lower than that from the muscle exposed only to ouabain. The similarity between the effects of ouabain in "high-Na" muscle and in insulin-preincubated muscle suggests that insulin increases the fraction of Na transport units whose internal Na-binding sites have been occupied by Na.