Endotoxic lipid A induces intracellular Ca2+ increase in human platelets

Abstract

The activation of protein kinase C by endotoxic lipid A was observed with both intact platelets and in a cell-free system [Romano and Hawiger (1990) J. Biol. Chem. 265, 1765-1770]. We have now studied the action of lipid A on intracellular Ca2+ concentration ([Ca2+](i)). Lipid A induced a concentration-dependent rise in [Ca2+](i) in human platelets loaded with fura-2, which reached a maximum at 37.1 ± 3.8 s (t(max)). Maximum [Ca2+](i) levels, observed at 30 μM lipid A, were 432 ± 60 nM. EGTA (2 mM) or NiCl2 (1 mM) each decreased the lipid A-dependent elevation of [Ca2+](i) by 50-60% without significant modification of t(max), but shortening the time for 50% recovery (t50) from >400 s to 113.1 ± 29.1 s and 54 ± 2.1 s, respectively. Quenching of the fura-2 signal was also observed in lipid A-stimulated platelets resuspended with MnCl2 (1 mM), suggesting that both mobilization and external influx of Ca2+ occur. Intracellular Ca2+ mobilization depended on release from Ins(1,4,5)P3-sensitive stores, since Ins(1,4,5)P3 accumulation was detected in lipid A-activated platelets. Staurosporine, an inhibitor of protein kinase C, blocked the [Ca2+](i) rise generated by lipid A in platelets [concn.giving 50% inhibition (IC50) = 0.1 μM], prolonging the t(max) to 54.7 ± 5.1 s, but decreasing the t50 to 157.5 ± 31.8 s. Staurosporine also suppressed InsP3 accumulation (IC50 = 0.15 μM). These results suggest that platelet activation by lipid A involves an interaction between [Ca2+](i) elevation and protein kinase C activation.