Purified rat pancreatic islet cells express somatomedin receptors which are identified by their affinity for insulin-like growth factor (IGF)-I, IGF-II, and insulin.

Binding of [¹²⁵I]IGF-I to islet A cells was half-maximally inhibited by 7.10^-10 M IGF-I, while IGF-II, insulin, and proinsulin were respectively 10-, 500-, and 10,000-fold less potent displacers of IGF-I binding. Unrelated hormones such as glucagon or GH did not compete with [¹²⁵I] IGF-I binding to A cells. The concentration of IGF-I receptors on A cells was estimated at 5000 IGF-I binding sites per cell with affinity constant (Ka) of 2 × 10⁹ M^-1.

Islet B cells were found to exhibit a reversible time- and temperature-dependent binding with [¹²⁵I]IGF-I. Specificity and affinity of IGF-I binding sites were identical for islet A and B cells. Linear Scatchard plots of competitive binding data on B cells suggest 1 single class of IGF-I receptors in a concentration of 12,000 sites per cell. The presence of high affinity receptors for IGF-I on adult islet A and B cells provides a molecular basis for this growth factor to influence growth, survival, and/or function of these endocrine cell types. Their low affinity for insulin should be considered as a potential mechanism for this hormone to influence, at high concentration, the function of islet A and B cells. (Endocrinology121: 1784–1788, 1987)