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We previously reported that J78 (2-chloro-3-[2'-bromo, 4'-fluoro-phenyl]-amino-8-hydroxy-1,4-naphthoquinone), a newly synthesized 1,4-naphthoquinone derivative, exhibited a potent antithrombotic effect, which might be due to antiplatelet rather than anticoagulation activity. In the present study, possible anti-platelet mechanism of J78 was investigated. J78 concentration-dependently inhibited rabbit platelet aggregation induced by collagen (10 microg/ml), thrombin (0.05 U/ml), arachidonic acid (100 microM), and U46619 (9,11-dideoxy-9,11-methanoepoxy-prostaglandin F(2); 1 microM), a thromboxane (TX) A(2) mimic, with IC(50) values of 0.32 +/- 0.01, 0.44 +/- 0.02, 0.50 +/- 0.04, and 0.36 +/- 0.02 microM, respectively. J78 also produced a shift to the right of the concentration-response curve of U46619, indicating an antagonistic effect on the TXA(2) receptor. J78 concentration-dependently inhibited collagen-induced arachidonic acid liberation. In addition, J78 potently suppressed TXA(2) formation by platelets that were exposed to arachidonic acid in a concentration-dependent manner but had no effect on the production of PGD(2), indicating an inhibitory effect on TXA(2) synthase. This was supported by a TXA(2) synthase activity assay that J78 concentration-dependently inhibited TXB(2) formation converted from PGH(2). Furthermore, J78 was also able to inhibit the [Ca(2+)](i) mobilization induced by collagen or thrombin at such a concentration that completely inhibited platelet aggregation. Taken together, these results suggest that the antiplatelet activity of J78 may be mediated by TXA(2) receptor blockade with TXA(2) synthase inhibition and suppression of cytosolic Ca(2+) mobilization.