Molecular link between membrane cholesterol and Na +/H + exchange within human platelets

Abstract

Incubation of human platelets with cholesterol-poor, cholesterol-normal and cholesterol-rich liposomes revealed that: (i) acquisition or depletion of platelet membrane cholesterol was highly selective; (ii) variation in membrane cholesterol was highly selective. Variation in membrane cholesterol content (cholesterol-to-phospholipid molar ratio from 0.15–1.2) with respect to values found in unmodified normal platelets, was paralleled by the observed changes in amiloride-sensitive cytoplasmic pH, as well as phospholipase A 2 activity. However, a decrease in cytoplasmic pH was accompanied by an increase in phospholipase A 2 activity; (iii) membrane cholesterol-modulated changes in intra-platelet pH, as well as phospholipase A 2 activity, was completely inhibited when platelets were pretreated with quinacrine (a specific phospholipase A 2 inhibitor) before exposure to various types of liposomes. Although exposure of platelets (pretreated with amiloride) with various types of liposomes resulted in the inhibition of Na +/H + exchange it had no noticeable effect upon the observed phospholipase A 2 activity. Based upon these results we suggest that membrane cholesterol-modulated phospholipase A 2 activity may be the basic mechanism responsible for the nature of Na +/H + exchanger activity observed in cholesterol-enriched platelets, leading these platelets to a hypersensitized state.