Acidic-store depletion is required for human platelet aggregation

Platelet stimulation with thrombin induces an elevation in cytoplasmic free Ca2+ concentration ([Ca2+](c)) due to Ca2+ release from intracellular stores and entry from the extracellular medium. Two different intracellular Ca2+ stores have been described in human platelets: the dense tubular system and the lysosomal-like acidic stores. In the present study, we investigated the contribution of the acidic stores in thrombin-induced platelet aggregation. We have found that platelet aggregation induced by thrombin is reduced in a Ca2+-free medium. Discharge of the acidic Ca2+ stores by treatment with the sarcoendoplasmic Ca2+-ATPase (SERCA)3 selective inhibitor 2,5-di-(tert-butyl)-1,4-hydroquinone reduced thrombin-evoked platelet aggregation. In the presence of 2,5-di-(tert-butyl)-1,4-hydroquinone, platelet aggregation induced by the protease-activated receptor (PAR)-1 and PAR-4 agonist peptides, SFLLRN and AYPGKF, respectively, was significantly reduced. In cells with depleted acidic stores, activation of GPIb-IX-V by thrombin resulted in reduced or no platelet aggregation in a medium containing 1 mmol/l Ca2+ or in a Ca2+-free medium, respectively. This finding suggests that Ca2+ accumulation in the acidic Ca2+ compartments is required for platelet aggregation induced by activation of the G-coupled PAR-1 and PAR-4 thrombin receptors and, by the occupation of the leucine-rich glycoprotein GPIb-IX-V and provide evidence supporting a functional role of the lysosomal-like acidic Ca2+ stores in human platelets. Blood Coagul Fibrinolysis 20:511-516 (C) 2009 Wolters Kluwer Health vertical bar Lippincott Williams & Wilkins.