Intracellular alkalinization augments platelet aggregation due to increase in cytosolic free-Ca 2+

Abstract

Intracellular pH is known to increase during agonist-induced platelet activation. In order to elucidate the role of intracellular alkalinization in platelet activation, the effects of NH 4 Cl, as a tool to induce intracellular alkalinization, on ionomycin-induced platelet activation were investigated. NH 4 Cl (2.5-10 mM) concentration-dependently induced intracellular alkalinization. Platelet aggregation induced by ionomycin (0.1 w M) was augmented by treatment with NH 4 Cl (2.5-10 mM). Ionomycin-induced platelet aggregation in the absence of extraplatelet Ca 2+ , which was markedly attenuated compared to that in the presence of extraplatelet Ca 2+ , was also augmented by NH 4 Cl. NH 4 Cl treatment increased the number of large aggregates after ionomycin stimulation, while it decreased the number of small aggregates. Both transplasmalemmal Ca 2+ entry and intracellular Ca 2+ release induced by ionomycin were increased by treatment with NH 4 Cl (10 mM). SKF-96365 (100 w M), an inhibitor of receptor-operated Ca 2+ channels, did not affect ionomycin-induced Ca 2+ entry but abolished the effect of NH 4 Cl on Ca 2+ entry. Thus, NH 4 Cl augments receptor-operated Ca 2+ channels and intracellular Ca 2+ release. These findings suggest that intracellular alkalinization plays a significant role in agonist-induced platelet activation.