The Interaction between the Effects of Insulin and Ouabain on the Activity of Na Transport System in Frog Skeletal Muscle

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Abstract
The effects of insulin and ouabain on 22Na efflux and net Na loss were examined in frog sartorius muscles. After removal of ouabain, 22Na efflux which was inhibited by ouabain remained at the inhibited level for more than 2 h. In ouabain-free solution, insulin stimulated the Na efflux from ouabain-preincubated muscles. The stimulating effect of insulin on 22Na efflux was completely blocked by ouabain. Insulin shifted the Na efflux vs. intracellular Na concentration [[Na]i] relationship toward the region of lower Na concentrations, and made the slope of log (Na efflux) vs. log [Na]i relationship less steep. Ouabain shifted the Na efflux vs. [Na]i relationship toward the region of higher Na concentration, and made the slope of the log (Na efflux) vs. log [Na]i relationship steeper. The relationship between insulin-stimulated Na efflux and [Na]i obtained from ouabain-preincubated muscle was located in the region of higher Na concentrations than that obtained from control muscle. These findings suggest that insulin increases an apparent affinity of binding sites for Na ions, and the possibility that insulin increases the Vmax of Na transport system may be excluded. Ouabain may decrease the apparent affinity of binding site for Na ions.