Regulation of insulin-like growth factor I binding in the rat uterus by growth hormone and thyroxine

The insulin-like growth factor-I (IGF-I)-binding sites in the rat uterus were characterized further and the effects of growth hormone and thyroxine examined. The 125I-labelled IGF-I binding sites on uterine membranes demonstrated relative binding affinity of less than 20% for IGF-II, less than 1% for insulin and no affinity for an unrelated peptide, epidermal growth factor, compared with 100% for IGF-I confirming the specificity of these binding sites. Scatchard analysis of the specific binding data revealed the presence of a single class of high-affinity binding sites (Kd = 2.50 +/- 0.68 nmol l-1, with a binding capacity of 1.02 +/- 0.13 pmol mg-1 membrane protein in the uterus of the pituitary-intact ovariectomized rat. After hypophysectomy, the uteri from these rats had significantly (P less than 0.05) increased IGF-I binding sites, without significant changes in their affinity. Administration of growth hormone with or without L-thyroxine reversed this increase in IGF-I binding. Injection of thyroxine alone to the hypophysectomized ovariectomized rats had no significant effects on the uterine IGF-I binding sites. These data show that growth hormone, but not thyroxine, can regulate IGF-I binding sites in the rat uterus, possibly through regulating IGF-I production.