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Differential effects of extra- and intracellular calcium chelation on human platelet function and glycoprotein IIb-IIIa complex stability.

The presence of extracellular ionized calcium (Ca2+) is important to support the aggregation of human blood platelets and for the stability of the platelet fibrinogen receptor, the glycoprotein (GP) IIb-IIIa complex. This study was performed in order to determine the effects of intracellular calcium chelation on human platelet functions, on the expression of the fibrinogen receptor and on the stability of the GP IIb-IIIa complex. Intracellular Ca2+ of intact human platelets was extensively chelated by incorporation of high amounts (14-50 mmol/L) of the specific Ca2+ chelator quin2 after incubation of platelets with its lipophilic acetomethoxylester form (quin2-AM). We have investigated the effects of intracellular Ca2+ chelation with quin2 on platelet aggregation and fibrinogen binding induced by ADP and human alpha-thrombin and on the stability of the GP IIb-IIIa complex studied by crossed immunoelectrophoresis of Triton X-100 platelet lysates. The results were compared with control experiments performed with intact platelets treated with the impermeant Ca2+ chelator quin2 and with EDTA or EGTA. This study shows that high intracellular concentration of quin2 inhibits human platelet aggregation and fibrinogen binding but does not induce the dissociation of the GP IIb-IIIa complex. This study adds evidence for the role of external Ca2+ to maintain the integrity of a non-dissociated GP IIb-IIIa complex and suggests that intracellular Ca2+ is not involved in the stabilization of the GP IIb-IIIa complex.