Ca2+ accumulation into acidic organelles mediated by Ca2+- and vacuolar H+-ATPases in human platelets

Most physiological agonists increase cytosolic free [Ca2+](c) (cyto-solic free Ca2+ concentration) to regulate a variety of cellular processes. How different stimuli evoke distinct spatiotemporal Ca2+ responses remains unclear, and the presence of separate intracellular Ca2+ stores might be of great functional relevance. Ca2+ accumulation into intracellular compartments mainly depends on the activity of Ca2+- and H+-ATPases. Platelets present two separate Ca2+ stores differentiated by the distinct sensitivity to thapsigargin and TBHQ [2,5-di-(t-butyl)-1,4-hydroquinone]. Although one store has long been identified as the dense tubular system, the nature of the TBHQ-sensitive store remains uncertain. Treatment of platelets with GPN (glycylphenylalanine-2-naphthylamide) impaired Ca2+ release by TBHQ and reduced that evoked by thrombin. In contrast, GPN did not modify Ca2+ mobilization stimulated by ADP or AVP ([arginine] vasopressin). Treatment with nigericin, a proton carrier, and bafilomycin A I, an inhibitor of the vacuolar H+-ATPase, to dissipate the proton gradient into acidic organelles induces a transient increase in [Ca2+], that was abolished by previous treatment with the SERCA (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase) 3 inhibitor TBHQ. Depleted acidic stores after nigericin or bafilomycin A1 were refilled by SERCA 3. Thrombin, but not ADP or AVP, reduces the rise in [Ca2+], evoked by nigericin and bafilomycin A1. Our results indicate that the TBHQ-sensitive store in human platelets is an acidic organelle whose Ca2+ accumulation is regulated by both Ca2+ and vacuolar H+-ATPases.