Different effects of IGF-I on insulin-stimulated glucose uptake in adiposetissue and skeletal muscle

The effect of insulin-like growth factor I (IGF-I) on insulin-stimulated gl ucose uptake was studied in adipose and muscle tissues of hypophysectomized female rats. IGF-I was given as a subcutaneous infusion via osmotic minipu mps for 6 or 20 days. All hypophysectomized rats received L-thyroxine and c ortisol replacement therapy. IGF-I treatment increased body weight gain but had no effect on serum glucose or free fatty acid levels. Serum insulin an d C-peptide concentrations decreased. Basal and insulin-stimulated glucose incorporation into lipids was reduced in adipose tissue segments and isolat ed adipocytes from the IGF-I-treated rats. In contrast, insulin treatment o f hypophysectomized rats for 7 days increased basal and insulin-stimulated glucose incorporation into lipids in isolated adipocytes. Pretreatment of i solated adipocytes in vitro with IGF-I increased basal and insulin-stimulat ed glucose incorporation into lipids. These results indicate that the effec t of IGF-I on lipogenesis in adipose tissue is not direct but via decreased serum insulin levels, which reduce the capacity of adipocytes to metaboliz e glucose. Isoproterenol-stimulated lipolysis, but not basal lipolysis, was enhanced in adipocytes from IGF-I-treated animals. In the soleus muscle, t he glycogen content and insulin-stimulated glucose incorporation into glyco gen were increased in IGF-I-treated rats. In summary, IGF-I has opposite ef fects on glucose uptake in adipose tissue and skeletal muscle, findings whi ch at least partly explain previous reports of reduced body fat mass, incre ased body cell mass, and increased insulin responsiveness after IGF-I treat ment.