Antiplatelet effect of marchantinquinone, isolated from Reboulia hemisphaerica, in rabbit washed platelets

Platelet activation is involved in serious pathological situations, includi ng atherosclerosis and restenosis. It is important to find efficient antipl atelet medicines to prevent fatal thrombous formation during the course of these diseases. Marchantinquinone, a natural compound isolated from Reboulia hemisphaerica, inhibited platelet aggregation and ATP release stimulated by thrombin (0.1 units mL(-1)), platelet-activating factor (PAF; 2 ng mL(-1)), collagen (10 mu g mL(-1)), arachidonic acid (100 mu M), or U46619 (1 mu M) in rabbit wa shed platelets. The IC50 values of marchantinquinone on the inhibition of p latelet aggregation induced by these five agonists were 62.0+/-9.0, 86.0+/- 7.8, 13.6+/-4.7, 20.9+/-3.1 and 13.4+/-5.3 mu M, respectively. Marchantinqu inone inhibited thromboxane B-2 (TxB(2)) formation induced by thrombin, PAF or collagen. However, marchantinquinone did not inhibit TxB(2) formation i nduced by arachidonic acid, indicating that marchantinquinone did not affec t the activity of cyclooxygenase and thromboxane synthase. Marchantinquinon e did inhibit the rising intracellular Ca2+ concentration stimulated by the five platelet-aggregation inducers. The formation of inositol monophosphat e induced by thrombin was inhibited by marchantinquinone. Platelet cAMP and cGMP levels were unchanged by marchantinquinone. The results indicate that marchantinquinone exerts antiplatelet effects by inhibiting phosphoinositide turnover.