Insulin-like growth factor-I receptors in the rat adrenals

This study investigated the binding characteristics of insulin-like growth factor-I (IGF-I) to rat adrenal cells. The binding constants in two regions of the adrenal glands (glomerulosa and fasciculata-reticularis-medulla) were studied separately. In membrane fractions prepared from either region, the binding of iodine 125-labeled IGF-I was demonstrated to be protein and temperature dependent, saturable, and specific. Unlabeled IGF-I was more effective than IGF-II or insulin in displacing 125I-labeled IGF-I binding to the receptor. Fifty percent of the binding was displaced when 2.2, 77.2, or 433.2 nmol/L IGF-I, IGF-II, or insulin, respectively, were added. Receptor numbers and affinity in membrane fractions obtained from either region of the adrenal glands were similar (1821 +/- 188 fmol/mg 1689 +/- 211 fmol/mg protein and a dissociation constant of 1.54 +/- 0.16 nmol/L versus 1.61 +/- 0.13 nmol/L for the glomerulosa and fasciculata-reticularis-medulla regions, respectively). When labeled IGF-I was cross-linked to the binding subunit under reducing conditions, a predominant band with a molecular weight of 135 kd was noted in either region of the adrenal glands. The data indicate that both regions of the rat adrenal glands have specific IGF-I receptors. The binding and the molecular weight characteristics are similar to the "classical" type I IGF receptor. The data are consistent with recent reports suggesting a modulating role for IGF-I in regulating adrenal cell growth and function.