INCREASED PROTEIN-SYNTHESIS AFTER ACUTE IGF-I OR INSULIN INFUSION IS LOCALIZED TO MUSCLE IN MICE

The purpose of the present study was to determine the effect of acute administration of insulin-like growth factor I(IGF-I) or insulin on in vivo protein synthesis in muscle and other organs in fasted mice and to compare this response with that produced by feeding. Recombinant IG F-I (3.3 nmol prime, 3.33 nmol/h) or insulin (0.056 nmol/h) was infuse d intravenously for 60 min along with glucose to prevent hypoglycemia. Fractional rates of tissue protein synthesis (FSR) were determined by injection of [H-2(5)]phenylalanine (25 mg/100 g body wt, 40% enriched ). Both IGF-I and insulin caused a 25% increase in FSR of heart (P < 0 .001) and soleus muscle (P < 0.05) and a 65% increase in gastrocnemius and plantaris muscle (both P < 0.001), thus restoring rates to those seen in fed animals. A fivefold lower dose of IGF-I also stimulated pr otein synthesis in gastrocnemius muscle and heart (both P < 0.05) but not in soleus muscle. No significant effects of IGF-I on FSR were dete cted in liver, kidney, spleen, proximal small intestine, colon, lung, or brain. The results indicate that the ability of an overnight fast t o decrease protein synthesis and the acute effects of insulin and IGF- I to stimulate protein synthesis are restricted to skeletal and cardia c muscles.