Insulin increases amino acid transport into rat soleus motor axons.

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Hyperosmotic neurosecretion was used to measure basal and insulin-stimulated amino acid and myoinosital transport into rat motor nerve terminals. L-Alanine and alpha(methylamino)-isobutyric acid (a nonmetabolizable system A-specific analog) transport was rapid into motor nerve terminals innervating a fast-twitch muscle, the extensor digitorum longus, and slow into motor nerve terminals innervating the soleus, a slow-twitch muscle. A physiological concentration of insulin, 10 microU/mL, increased L-alanine and alpha(methylamino)-isobutyric acid transport into motor nerve terminals in the soleus. Large doses of insulin, 100 or 1000 microU/mL, had no effect on L-alanine or alpha(methylamino)-isobutyric acid transport into nerve terminals in the extensor digitorum longus. There was negligible basal or insulin-stimulated transport of D-alanine or myoinositol into nerve terminals of the soleus or extensor digitorum longus. These studies show that insulin regulates sterospecific amino acid transport into soleus motor axons, but has no effect on the rapid amino acid transport into extensor digitorum longus motor axons. Differences in basal and insulin-stimulated transport suggest that motor axons differ in their metabolism, and might be selectively vulnerable to disease processes.