Evidence for two insulin receptor populations on human erythrocytes

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Abstract
Human erythrocytes specifically bind 125I-insulin in a manner similar to cells in which insulin exerts a physiological response1. In addition, erythrocytes are of practical value for correlating in vitro insulin binding with in vivo carbohydrate intolerance2,3. The competitive binding of labelled and unlabelled insulin to erythrocyte receptors is typically curvilinear when plotted according to Scatchard4. The curvilinear nature of the Scatchard plot describing insulin binding to membrane receptors, although originally attributed to heterogeneous sites5, has been more recently interpreted as negative cooperativity between homogeneous sites6. Evidence reported here, however, suggests that there are two populations of insulin receptors on erythrocytes. Specific concentrations of concanavalin A (Con A), a lectin which mimics insulin activity7, are shown here to inhibit one population of receptors leaving another population unaffected.