Mechanism of Insulin Resistance Induced by Sustained Levels of Cytosolic Free Calcium in Rat Adipocytes*

Abstract

We have recently provided evidence that elevated levels of cytosolic free Ca 2+ ([Ca 2+ ]i) decreased insulin-stimulated glucose uptake in isolated rat adipocytes. To investigate the mechanism of Ca 2+ action, we examined the effects of elevated levels of [Ca 2+ ]i on insulin binding, autophosphoryla-tion, and tyrosine kinase activity (TKA) of insulin receptors as well as basal and insulin-stimulated cellular distribution of glucose transporters. The latter was assessed by cytochalasin-B binding to plasma membrane and cytosolic fractions. Elevated concentrations of [Ca 2+ ]i were maintained by incubating adipo-cytes with a depolarizing concentration of K + (40 mM). Basal nonstimulated glucose uptake was not altered by increased levels of [Ca 2+ ]i. Adipocytes with higher [Ca 2+ ]i (220 ± 15 nM) showed 30% reduction in insulin-stimulated 2-deoxyglucose uptake compared with control cells ([Ca 2+ ]i, 140 ± 18 nM). Moreover, adipocytes with higher levels of [Ca 2+ ]i demonstrated an approximately 10% reduction in autophosphorylation and TKA of insulin receptors without a change in insulin binding. Both basal and insulin-stimulated distributions of glucose transporters were unaffected by sustained levels of [Ca 2+ ]i. The effects of elevated [Ca 2+ ]i were not mimicked by protein kinase-C activation. These observations suggest that 1) elevated or sustained levels of [Ca 2+ ]i impair insulin-stimulated glucose uptake; and 2) Ca 2+-induced impairment appears to reside at the postbinding steps of insulin action and probably interferes with the TKA of insulin receptors and the intrinsic activity of glucose transporters. (Endocrinology 125: 2341-2349, 1989)