THE MECHANISM OF RABBIT PLATELET-AGGREGATION INDUCED BY 2,5-DI-(TERT-BUTYL)-1,4-BENZOHYDROQUINONE, AN INHIBITOR OF ENDOPLASMIC-RETICULUM CA2-ATPASE()

2,5-Di-(tert-butyl)-1,4-benzohydroquinone (tBuBHQ), an inhibitor of en doplasmic reticulum Ca2+-ATPase, caused aggregation accompanied by a m arked increase in intracellular Ca2+ concentrations ([Ca2+](i)) in was hed rabbit platelets. Treatment of platelets with SK&F96365 or removal of extracellular Ca2+ inhibited both Ca2+ influx and platelet aggrega tion induced by tBuBHQ. These responses of platelets to tBuBHQ were al so inhibited by genistein. Western blots using antibody against phosph orylated amino acid residues revealed that tBuBHQ activated tyrosine k inase independently of extracellular Ca2+ These results suggest that t BuBHQ induces Ca2+ influx into platelets probably through activation o f tyrosine kinase, resulting in platelet aggregation.