Intracellular alkalinization augments capacitative Ca2+ entry in platelets.

In order to elucidate the significance of intracellular alkalinization in signal transduction of platelets, we investigated the effects on capacitative Ca(2+) entry (CCE) of intracellular alkalinization that was induced by NH(4)Cl. Addition of NH(4)Cl (10 mM) to the medium resulted in an elevation of intracellular pH by about 0.35, which was eliminated by simultaneous addition of propionate (20 mM), an inducer of intracellular acidification, to the medium. CCE was induced by an extracellular addition of Ca(2+) to platelets in which Ca(2+) stores had been depleted by stimulation with thapsigargin in nominally Ca(2+)-free medium. NH(4)Cl markedly augmented CCE and subsequent platelet aggregation, both of which were abolished in the presence of SKF-96365, an inhibitor of capacitative Ca(2+) entry in non-excitable cells such as platelets. The augmentation of CCE and subsequent aggregation by NH(4)Cl was not observed in the presence of propionate or SKF-96365. Extracellular alkalosis induced by Tris also markedly augmented CCE and subsequent aggregation. These augmenting effects of extracellular alkalosis by Tris were significantly but incompletely inhibited by simultaneous addition of propionate (20 mM), which completely eliminated elevation of intracellular pH elicited by Tris. Thus, the augmenting effect of extracellular alkalosis on CCE was in part mediated by intracellular alkalosis. These findings suggest that intracellular alkalinization is a potent signal that augments CCE in platelets.