This study examined the effects of human umbilical vein endothelial cells (ECs) and calcium channel blockers (Ca2+ blockers), diltiazem, verapamil, and nicardipine, on platelet aggregation in vitro. ECs markedly inhibited the platelet aggregation induced by ADP, collagen, thromboxane A2 (TXA2), or thrombin. When platelets were incubated with ECs, the antiaggregatory activity reached a plateau within 5 to 10min. As the number of ECs added to the platelet-rich plasma was increased, platelet

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... ation declined progressively. The antiaggregatory activity of ECs was attenuated considerably by the addition of aspirin. All three Ca2+ blockers inhibited platelet aggregation in a dose-dependent manner. The combination of ECs and a Ca2+ blocker resulted in more potent inhibition of platelet aggregation than either alone, but the effects were not synergistic. Both ECs and Ca2+ blockers inhibited the synthesis of TXA2 during platelet aggregation. However, Ca2+ blockers did not significantly influence the production of prostaglandin I2 (PGI2) by ECs during incubation and aggregation. These results suggest that PGI2 is an important factor in endothelial antiaggregatory activity. Ca2+ blockers directly inhibit platelet aggregation by suppressing TXA2 formation, but do not appear to enhance the antiaggregatory activity of ECs.