SPECIFIC AND FUNCTIONAL INSULIN-RECEPTORS IN RAT RENAL PAPILLA

To investigate a possible action of insulin on the rat kidney papilla, the binding of I-125-insulin to papilla microsomes was examined. This binding was specific to insulin in that it was displaced by increasin g concentrations of unlabelled porcine insulin and to a lesser extent by porcine proinsulin and IGF-I, but not by IGF-II and bGH. Scatchard plot of the binding data was curvilinear consistent with either two cl asses of receptors with different affinities or a single class of rece ptors that showed negative cooperativity. A small fraction of I-125-in sulin (maximum 2 %) was degraded during incubation, but with a Km two order of magnitude higher than the constant of affinity for binding. I nsulin stimulates the incorporation of phosphate to phosphatidylcholin e in a dose-dependent manner, reaching a maximum with 10 nM insulin. T his data showed both the presence of specific insulin receptors in the kidney papilla and an insulin action through the synthesis of phospho lipids by insulin.