Calcium-induced platelet aggregation in washed platelets from guinea pigs.

The present study investigated the effects of extracellular calcium on washed platelets of guinea pigs, rabbits, rats and humans. CaCl2 without agonists induced platelet aggregation in guinea pigs and rabbits, but not in rats or humans. CaCl2 increased platelet [Ca2+]i in Fura-2-loaded guinea pig platelets. Calcium-induced platelet aggregation was inhibited by W-7 (a calmodulin antagonist), aspirin, indomethacin, TMB-8 (an inhibitor of intracellular Ca2+ release) and also nicardipine (a calcium antagonist). These data suggest that CaCl2-induced platelet aggregation is mediated by calmodulin, cyclooxygenase and other, as yet unknown, mechanisms.