Effects of Liposome-Adipocyte Interaction on Calcium Binding and Insulin Action

Abstract

The interaction of phosphatidylserine (PS)-containing liposomes with isolated rat adipocytes under conditions of liposome-cell fusion or endocytosis was accompanied by an increase in adipocyte membrane Ca binding in both the basal and insulin-stimulated state. In vesicle-treated adipocytes, insulin-stimulated 2-deoxyglucose uptake was inhibited and not as dependent on external Ca concentration as in control cells. The inhibition of insulin-stimulated 2-deoxyglucose uptake in vesicle treated adipocytes may result from the binding of Ca to the exogenously introduced phospholipid at the expense of membrnae constituents normally involved in insulin-stimulated Ca binding. This is supported by the following observations: the Ca ionophore A23187 [calcimycin] increased intracellular Ca levels, yet had no effect on insulin-stimulated hexose uptake; and treatment of intact adipocytes with phospholipase A2 after the cells had been incubated with vesicles resulted in a decrease in membrane associated Ca2+ and exogenous phosphatidylserine, which is consistent with a direct role of membrane phosphatidylserine in Ca binding.