LOCALIZED INFUSION OF IGF-I RESULTS IN SKELETAL-MUSCLE HYPERTROPHY INRATS

Insulin-like growth factor I(IGF-I) peptide levels have been shown to increase in overloaded skeletal muscles (G. R. Adams and F. Haddad. J. Appl. Physiol. 81: 2509-2516, 1996). In that study, the increase in I GF-I was found to precede measurable increases in muscle protein and w as correlated with an increase in muscle DNA content. The present stud y was undertaken to test the hypothesis that direct IGF-I infusion wou ld result iu an increase in muscle DNA as well as in various measureme nts of muscle size. Either 0.9% saline or nonsystemic doses of IGF-I w ere infused directly into a non-weight-bearing muscle of rats, the tib ialis anterior (TA), via a fenestrated catheter attached to a subcutan eous miniosmotic pump. Saline infusion had no effect on the mass, prot ein content, or DNA content of TA muscles. Local IGF-I infusion had no effect on body or heart weight. The absolute weight of the infused TA muscles was similar to 9% greater (P < 0.05) than that of the contral ateral TA muscles. IGF-I infusion resulted in significant increases in the total protein and DNA content of TA muscles (P < 0.05). As a resu lt of these coordinated changes, the DNA-to-protein ratio of the hyper trophied TA was similar to that of the contralateral muscles. These r esults suggest that IGF-I may be acting to directly stimulate processe s such as protein synthesis and satellite cell proliferation, which re sult; in skeletal muscle hypertrophy.