Extracellular pH Affects Platelet Aggregation Associated with Modulation of Store-Operated Ca 2+ Entry

Abstract

The pH dependence of store-operated Ca 2+ influx (SOCI) into human platelets, as well as its physiological consequence, aggregation, was studied. In Ca 2+-free medium, thapsigargin (1 μM) induced a small increase in intracellular free-Ca 2+ ([Ca 2+] i), which was not affected by changes in extracellular pH. The addition of Ca 2+ (0.5–3 mM) after Ca 2+ store depletion caused by thapsigargin resulted in concentration-dependent increases in [Ca 2+] i (SOCI), which were strongly inhibited by SKF-96365 (100 μM), an inhibitor of receptor-mediated Ca 2+ entry. SOCI was inhibited by acidosis (pH 6.9) and augmented by alkalosis (pH 7.9). The addition of Ca 2+ (0.5–3 mM) to platelets, which were kept in Ca 2+-free medium, slightly but significantly increased [Ca 2+] i. This Ca 2+ 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 μM) stimulation in Ca 2+-free solution nor elevation of extracellular Ca 2+ alone was sufficient to induce platelet aggregation. In contrast, the addition of Ca 2+ (1 mM) to platelets activated by thapsigargin resulted in aggregation, which was markedly inhibited by SKF-96365 (100 μM). 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.