Intracellular Na+ and K+ activities during insulin stimulation of rat soleus muscle.

The action of insulin on the resting membrane potential (Em) and intracellular sodium and potassium activities (aNa, aK) of rat soleus muscle fibers was determined by direct intracellular measurements of aNa, aK, and Em using Na-selective, K-selective, and conventional microelectrodes. The use of these microelectrodes allowed us to continuously monitor these parameters in the same fiber. Although we were able to accurately measure aNa and aK and continuously monitor their levels throughout periods of insulin stimulation of up to 20 min duration, we were unable to detect any significant change in Em, aNa, or aK. Varying the concentration of insulin or extracellular glucose failed to alter our observations. These results indicate that the action of insulin on the sarcolemma and subsequent increase in glucose transport must result from some mechanism independent of a change in membrane potential or intracellular sodium or potassium activity.