Differential involvement of thrombin receptors in Ca2+ release from two different intracellular stores in human platelets.
Physiological agonists increase cytosolic free Ca2+ concentration to regulate a number of cellular processes. The platelet thrombin receptors, PAR (protease-activated receptor) 1 PAR-4 and GPIb-IX-V (glycoprotein Ib-IX-V) have been described as potential contributors of thrombin-induced platelet aggregation. Platelets present two separate Ca2+ stores, the DTS (dense tubular system) and acidic organelles, differentiated by the distinct sensitivity of their respective SERCAs (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPases) to TG (thapsigargin) and TBHQ [2,5-di-(tert-butyl)-1,4-hydroquinone]. However, the involvement of the thrombin receptors in Ca2+ release from each Ca2+ store remains unknown. Depletion of the DTS using ADP, which releases Ca2+ solely from the DTS, in combination with 10 nM TG, to selectively inhibit SERCA2 located on the DTS reduced Ca2+ release evoked by the PAR-1 agonist, SFLLRN, and the PAR-4 agonist, AYPGKF, by 80 and 50% respectively. Desensitization of PAR-1 and PAR-4 or pre-treatment with the PAR-1 and PAR-4 antagonists SCH 79797 and tcY-NH2 reduced Ca2+ mobilization induced by thrombin, and depletion of the DTS after desensitization or blockade of PAR-1 and PAR-4 had no significant effect on Ca2+ release stimulated by thrombin through the GPIb-IX-V receptor. Converse experiments showed that depletion of the acidic stores using TBHQ reduced Ca2+ release evoked by SFLLRN or AYPGKF, by 20 and 50% respectively, and abolished thrombin-stimulated Ca2+ release through the GPIb-IX-V receptor when PAR-1 and PAR-4 had been desensitized or blocked. Our results indicate that thrombin-induced activation of PAR-1 and PAR-4 evokes Ca2+ release from both Ca2+ stores, while activation of GPIb-IX-V by thrombin releases Ca2+ solely from the acidic compartments in human platelets.
Jardin
Isaac
I
Cell Physiology Research Group, Department of Physiology, University of Extremadura, 10071 Cáceres, Spain.
Ben Amor
Nidhal
N
Bartegi
Ahgleb
A
Pariente
José A
JA
Salido
Ginés M
GM
Rosado
Juan A
JA
D001792:Blood Platelets / Q000166:cytology / Q000502:physiology*; D002118:Calcium / Q000097:blood*; D002470:Cell Survival; D003600:Cytosol / Q000502:physiology; D006801:Humans; D007700:Kinetics; D018179:Receptors, Thrombin / Q000097:blood / Q000502:physiology*; D013917:Thrombin / Q000502:physiology
D018179:Receptors, Thrombin; D013917:Thrombin; D002118:Calcium