Effects of liposome-adipocyte interaction on calcium binding and insulin action

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 calcium 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 calcium concentration as in control cells. These data suggest that the inhibition of insulin-stimulated 2-deoxyglucose uptake in vesicle treated adipocytes may result from the binding of calcium to the exogenously introduced phospholipid at the expense of membrane constituents normally involved in insulin-stimulated calcium binding. This is supported by the observations that 1) the calcium ionophore A23187 increased intracellular calcium levels, yet had no effect on insulin-stimulated hexose uptake; and 2) treatment of intact adipocytes with phospholipase A2 after the cells had been incubated with vesicles resulted in a decrease in membrane associated Ca++ and exogenous phosphatidylserine, which is consistent with a direct role of membrane phosphatidylserine in calcium binding.