Inhibitory effect of lysophosphatidylcholine and phospholipase A2-treated low density lipoproteins on receptor-dependent regulation of {Ca2+}i in platelets.

Treatment of LDL with phospholipase A from bee venom resulted in formation of lipid-protein particles (pl2 LDL) with increased content of lysophosphatidylcholine (LPC). At the same time, composition of other lipids and protein structure were unaffected. Both pl-LDL and LPC abolished PAF-, ADP- and thrombin-induced Ca2+ elevation in platelets and platelet aggregation, while LDL had no effect on hormone-stimulated increase in the intracellular Ca2+ content ({Ca2+}i). The effect persisted in Ca2+-free medium, indicating that pl-LDL and LPC also abolish Ca2+ mobilisation from intracellular stores. Neither LPC nor pl-LDL changed platelet Ca2+ levels and inhibited platelet aggregation induced by thapsigargin, a specific inhibitor of the endoplasmic reticulum Ca2+ ATPase. The inhibitory effect depended on LPC concentration, incubation time and the structure of LPC: lysophosphatidylethanolamine and phosphatidylcholine produced no inhibitory effect. The half-maximum-effective concentrations were the same for LPC and pl-LDL (2-4 microM). The results obtained indicate that LPC and pl-LDL inhibit the receptor-dependent increase in Ca2+. It can be suggested that the effect of LPC is mediated by redistribution of the plasma membrane integral proteins, which leads to disintegration of the intracellular signalling systems.