Restoration of insulin-like growth factor I action in skeletal muscle of old mice.

This study examined the effects of long-term chronic voluntary wheel exercise on the ability of insulin-like growth factor I (IGF-I) to stimulate rates of protein synthesis in the soleus muscle of old C57Bl/6 mice. Factors contributing to any changes in hormone action were analyzed at the level of hormone receptor binding, protein content, and gene expression. Chronic exercise resulted in an increased skeletal muscle mass (10-22%) and a 56% increase in IGF-I-stimulated rates of protein synthesis (P < 0.05). IGF-I receptor mRNA was increased 46%, IGF-I receptor protein was increased 65%, and the binding capacity of the IGF-I high-affinity site was increased sixfold (P < 0.05) with chronic wheel exercise. Insulin receptor protein content was decreased 35% (P < 0.05), whereas GLUT-4 content was increased 47% with chronic exercise (P < 0.05). This study demonstrates that old animals retain a plasticity for IGF I receptor and glucose transporter expression that may have valuable physiological consequences.