Extracellular pH affects platelet aggregation associated with modulation of store-operated Ca2+ entry

The pH dependence of store-operated Ca2+, influx (SOCI)into human platelets, as well as its physiological 2 consequence, aggregation, was studied. In Ca2+-free medium, thapsigargin (1 muM) induced a small increase in intracellular free-Ca2+ ([Ca2+](i)), which was not affected by changes in extracellular pH. The addition of Ca2+ (0.5-3 mM) after Ca2+ store depletion caused by thapsigargin resulted in concentration-dependent increases in [Ca 2,]i (SOCI), which were strongly inhibited by SKF-96365 (100 muM), an inhibitor of receptor-mediated Ca2+ entry. SOCI were inhibited by acidosis (pH 6.9) and augmented by alkalosis (pH 7.9). The addition of Ca2+ (0.5-3 mM) to platelets, which were kept in Ca2+-free medium, slightly but significantly increased [Ca2+]i. This Ca2+ leak entry was also decreased and increased by extracellular acidosis (pH 6.9) and alkalosis (pH 7.9), respectively, but not affected by SKF-96365. Neither thapsigargin (1 muM) stimulation in Ca2+-free solution nor elevation of extracellular Ca2+ alone was sufficient to induce platelet aggregation. In contrast, the addition of Ca2+ (1 mM) to platelets activated by thapsigargin resulted in aggregation, which was markedly inhibited by SKF96365 (100 muM). Platelet aggregation associated with SOCI was also inhibited by extracellular acidosis (pH 6.9) and augmented by extracellular alkalosis (pH 7.9). These results suggest that acidosis-induced inhibition, as well as alkalosis-induced promotion of platelet aggregation, involve pH effects on SOCI. (C) 2001 Elsevier Science Ltd. All rights reserved.