Antiplatelet activity of green tea catechins is mediated by inhibition of cytoplasmic calcium increase

We have previously reported that green tea catechins (GTC) display a potent antithrombotic activity, which might be due to antiplatelet rather than an ticoagulation effects. In the current study, we investigated the antiplatel et mechanism of GTC. We tested the effects of GTC on the aggregation of hum an platelets and on the binding of fluorescein isothiocyanate-conjugated fi brinogen to human platelet glycoprotein (GP) IIb/IIIa. GTC inhibited the co llagen-, thrombin-, adenosine diphosphate (ADP)-, and calcium ionophore A23 187-induced aggregation of washed human platelets, with 50% inhibitory conc entration values of 0.64, 0.52, 0.63, and 0.45 mg/ml, respectively. GTC sig nificantly inhibited fibrinogen binding to human platelet surface GPIIb/III a complex but failed to inhibit binding to purified GPIIb/IIIa complex. The se results indicate that the antiplatelet activity of GTC may be due to inh ibition of an intracellular pathway preceding GPIIb/IIIa complex exposure. We also investigated the effects of GTC on intracellular calcium levels. wh ich are critical in determining the activation status of platelets and on i nduction of platelet aggregation by thapsigargin, which is a selective inhi bitor of the Ca2+-ATPase pump. Pretreatment of human platelets with GTC sig nificantly inhibited the rise in intracellular Ca2+ concentration induced b y thrombin treatment, and GTC significantly inhibited the thapsigargin-indu ced platelet aggregation. We also examined the effect of GTC on the second messenger, inositol 1,4,5-triphosphate (IP3). GTC significantly inhibited t he phosphoinositide breakdown induced by thrombin. Taken together, these ob servations suggest that the antiplatelet activity of GTC is be mediated by inhibition of cytoplasmic calcium increase, which leads to the inhibition o f fibrinogen-GPIIb/IIIa binding via the activation of Ca2+-ATPase and inhib ition of IP3 formation.