Insulin improves rat soleus axon function by increasing glucose transport

The effect of varying glucose and insulin concentration on neuromuscular transmission was investigated in rat soleus and extensor digitorum longus (EDL) nerve-muscle preparations using conventional microelectrode techniques. Soleus and EDL axons were similar in that both failed after approximately 70 min of glucose deprivation and often did not recover from glucose deprivation. Soleus and EDL axons differed in the following ways: 1) soleus axons required greater than 7.64 mM of glucose to give sustained function vs. greater than 2.78 mM of glucose for the EDL; 2) 30 microU/ml of insulin improved soleus axon function over a range of glucose concentrations from 4.17 to 9.72 mM vs. no effect of 1,000 microU/ml on EDL axons; 3) resting glucose transport was slow into soleus axons and rapid into EDL axons; and 4) 10 microU/ml of insulin increased glucose transport into soleus axons vs. no effect of 1,000 microU/ml of insulin on glucose transport into EDL axons. These results suggest that insulin improves soleus axon function by increasing glucose transport, whereas insulin has no effect on EDL axons.