Summary
In order to elucidate the significance of intracellular alkalinization in signal transduction
of platelets, we investigated the effects on capacitative Ca2+ entry (CCE) of intracellular alkalinization that was induced by NH4Cl. Addition of NH4Cl (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 Ca2+ to platelets in which Ca2+ stores had been depleted by stimulation with thapsigargin in nominally Ca2+-free medium. NH4Cl markedly augmented CCE and subsequent platelet aggregation, both of which were
abolished in the presence of SKF-96365, an inhibitor of capacitative Ca2+ entry in non-excitable cells such as platelets. The augmentation of CCE and subsequent
aggregation by NH4Cl 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.
Keywords
Alkalosis - calcium channels - phosphatidylinositol - platelet function - signal transduction
