Differential involvement of thrombin receptors in Ca 2+} release from two different intracellular stores in human platelets
Résumé
Physiological agonists increase cytosolic free Ca 2+} concentration ([Ca 2+}] c}) to regulate a number of cellular processes. The platelet thrombin receptors, PAR-1, PAR-4 and GPIb-IX-V have been described as potential contributors of thrombin-induced platelet aggregation. Platelets present two separate Ca 2+} stores, the dense tubular system (DTS) and acidic organelles, differentiated by the distinct sensitivity of their respective SERCAs to thapsigargin and 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ). However, the involvement of the thrombin receptors in Ca 2+} release from each Ca 2+} store remains unknown. Depletion of the DTS, using ADP, which releases Ca 2+} solely from the DTS, in combination with 10 nM TG, to selectively inhibit SERCA2 located on the DTS, reduced Ca 2+} release evoked by the PAR-1 agonist, SFLLRN, and the PAR-4 agonist, AYPGKF, by 80 and 50 %, respectively. Desensitisation of PAR-1 and PAR-4 or pre-treatment with the PAR-1 and PAR-4 antagonists SCH 79797 and tcY-NH 2} reduced Ca 2+} mobilisation induced by thrombin, and depletion of the DTS after desensitisation or blockade of PAR-1 and PAR-4 had no significant effect on Ca 2+} release stimulated by thrombin through the GPIb-IX-V receptor. Converse experiments reported that depletion of the acidic stores using TBHQ reduced Ca 2+} release evoked by SFLLRN or AYPGKF, by 20 and 50 %, respectively, and abolished thrombin-stimulated Ca 2+} release through the GPIb-IX-V receptor when PAR-1 and PAR-4 had been desensitised or blocked. Our results indicate that thrombin-induced activation of PAR-1 and PAR-4 evokes Ca 2+} release from both Ca 2+} stores, while activation of GPIb-IX-V by thrombin releases Ca 2+} solely from the acidic compartments in human platelets.
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