Insulin Does Not Act by Causing a Change in Membrane Potential or Intracellular Free Sodium and Potassium Concentration of Adipocytes

The influence of insulin on the intracellular free sodium and potassium ion concentrations ([Na⁺]_i, [K⁺]_i) and resting membrane potential of rat epididymal adipocytes was examined to determine its potential for mediating insulin's action on other cellular processes. Direct intracellular measurements of [Na⁺]_i, [K⁺]_i, and the resting membrane potential were made using ionselective and conventional microelectrodes. The use of these microelectrodes enabled us to continuously monitor these parameters in the same cell before, during, and after periods of insulin stimulation of up to 20-min duration. The electrical potentials measured in these experiments remained unchanged when the cells were stimulated with insulin (0.01, 0.1, or 1.0 mU/ml). Varying the extracellular glucose concentration had no effect on these results. Our results provide the first direct measurement of [Na⁺]_i and [K⁺]_i in adipose tissue and clearly demonstrate that the response to insulin's association with its receptors on the cell surface and subsequent action on hexose transport and cellular metabolism does not involve a change in membrane potential or intracellular sodium and potassium ions.