INSULIN STIMULATES THE TRANSLOCATION OF NA+/K+-DEPENDENT ATPASE MOLECULES FROM INTRACELLULAR STORES TO THE PLASMA-MEMBRANE IN FROG SKELETAL-MUSCLE

The mechanism of the stimulation of Na+/K+ transport by insulin in frog skeletal muscle was studied. The ouabain-binding capacity in detergent-treated plasma membranes of insulin-exposed muscles was increased 1.9-fold compared with that of controls. Na+/K+-ATPase activity was found in an intracellular 'light fraction' (fraction II) prepared by using anion-exchange chromatography. Marker enzyme activities for plasma and Golgi membranes were not detected in this fraction. The specific activity of Na+/K+ -ATPase in fraction II from insulin-exposed muscles was 58% of that in an identical fraction from control muscles. No significant difference in the protein yield of the plasma membrane preparation was observed between these two groups. In parallel with the decrease in the Na+/K+ -ATPase activity in fraction II from insulin-exposed muscles, the ouabain-binding capacity in this fraction was also decreased. The addition of saponin to fraction II increased both Na+/K+ -ATPase activity and ouabain binding, indicating that some of the Na+/K+ -ATPase is located in sealed vesicles. These findings support the view that insulin stimulates the translocation of Na+/K+ -ATPase molecules from fraction II to the plasma membrane.