FC-GAMMA-II RECEPTOR-MEDIATED PLATELET ACTIVATION-INDUCED BY ANTI-CD9MONOCLONAL-ANTIBODY OPENS CA2+ CHANNELS WHICH ARE DISTINCT FROM THOSEASSOCIATED WITH CA2+ STORE DEPLETION

Anti-human platelet CD9 mAb, NNKY1-19, induced platelet activation in a Fc gamma RII-dependent manner in terms of aggregation and secretion of intracellular granule contents. These responses were considerably s uppressed by aspirin. [Ca2+](i) elevation in the absence of extracellu lar Ca2+ (([Ca2+](e)), which represents the amount of Ca2+ released fr om intracellular Ca2+ ([Ca2+](i)) stares, was also greatly reduced, wh ereas Ca2+ influx was sustained at similar levels. We thus investigate d the mechanism that leads to the opening of Ca2+ channels in platelet s incubated with aspirin. IP3 production and Ca2+ efflux were below de tectable levels. ,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic ac id loading of platelets to chelate [Ca2+](i) did not reduce Ca2+ influ x, as assessed by Ca-45(2+) measurement. These findings suggested that NNKY1-19 induces Ca2+ channels to open without [Ca2+](i) mobilization or by depleting the [Ca2+](i) stores. The magnitude of Ca2+ influx wa s evaluated by adding [Ca2+](e) to a platelet suspension activated by various agonists in the absence of [Ca2+](e). The dose dependence of t he Ca2+ influx on [Ca2+](e) concentrations differed according to the m ode of activation. The ED(50) value of Ca2+ after thrombin or thapsiga rgin stimulation was 0.6 mM, whereas that of NNKY1-19 activation was a bout 3 mM. The addition of anti-Fc gamma RII mAb, IV.3, even 10 min af ter the initiation of platelet activation induced by NNKY1-19, inhibit ed the Ca2+ influx. These findings suggest that the Fc gamma RII-depen dent activation of platelets induced by NNKY1-19 directly opens Ca2+ c hannels, which are distinct from those opened by thrombin or thapsigar gin.