Fluorescent Ca2+-indicator quin 2 as an intracellular Ca2+ antagonist in platelet reaction

Preincubation of fluorescent Ca2+-indicator quin 2 resulted in inhibition on platelet aggregation and secretion in the absence of extracellular Ca2+. And the mechanism of the inhibition was studied. The inhibition by quin 2 of thrombin stimulated aggregation and ATP secretion of human platelets was dose and incubation time dependent and the inhibition was overcome by an addition of CaCl2 to the suspending buffer. Combination of quin 2 and Ca2+-blockers exerted the complete inhibition of the reaction in the presence of extracellular Ca2+. The inhibitory effect was observed when the intracellular concentration of quin 2 exceeds 3 mM, regardless of the initial dose or the preincubation time. The cellular content of ATP was not reduced by loading platelets with quin 2 in the concentration which exerted an inhibitory effect on the platelet reaction. From these observations, it was postulated that the inhibition is due to chelation of intracellular Ca2+ by quin 2 and the application of this agent as an intracellular Ca2+ antagonist was proposed. Also, we discussed the limitations in the use of quin 2 system as an intracellular Ca2+ indicator.