Characterization of insulin binding to isolated swine adipocytes

The objectives of this study were to determine if binding sites for insulin were present on isolated swine adipocytes and to characterize insulin receptors in terms of kinetics and specificity if they existed. The binding of purified porcine [¹²⁵I]iodoinsulin to isolated adipocytes at 30 C was rapid, achieving a steady state within 45 min that was maintained for 2 h. Specific binding of [¹²⁵I]iodoinsulin tracer was 2.5–3.0% for 2 × 10⁵ cells/ml after 2 h. Competition for binding was observed with half-maximal displacement of [¹²⁵I]iodoinsulin at insulin concentrations of 4.2 × 10^-10 M. Porcine proinsulin was 16-fold less potent that insulin in displacing [¹²⁵I]iodoinsulin from receptor sites. Glucagon did not cause displacement of [¹²⁵I]iodoinsulin. Scatchard analysis of the data from competitive binding studies indicated the presence of two classes of binding sites. The K_a was approximately 4.5 × 10⁹ M^-1 for high affinity sites and approximately 2.1 × 10^-8 M^-1 for low affinity sites. These findings indicate that 1) receptors which specifically bind insulin are present on swine adipocytes and 2) the affinity and number of the two classes of binding sites are similar to those of adipocytes isolated from the rat, a species in which insulin elicits a much greater in vitro stimulatory effect on glucose metabolism than previously observed for swine adipocytes.