Background Platelet-vessel wall interaction plays an important role in acute cardiovascular disorders. Thrombin is a potent platelet activator but also has profound effects on the endothelium. Endothelial cells possess antithrombotic activity by releasing nitric oxide and prostacyclin, both potent vasodilators and platelet inhibitors. We studied the role of thrombin as a regulator of platelet-vessel wall interaction in isolated human arteries suspended in organ chambers for isometric tension recording.Methods and Results In arteries with endothelium, thrombin (0.01 to 1 U/mL) induced endothelium-dependent relaxations, which were reduced by the nitric oxide synthase inhibitor NG-nitro-l-arginine methyl ester (L-NAME; 10-4mol/L) and/or indomethacin (10-5mol/L). Human platelets (75 000/microliters) evoked only marginal contractions in arteries with endothelium (3+-3% of the contraction to KCl 100 mmol/L; NS), which were markedly enhanced by endothelial removal (22+-4%; P<.05). Thrombin (1 U/mL) did not affect the response to platelets in arteries with (6+-5%; NS) but induced a huge contraction in rings without endothelium (53+-6%; P<.01 versus control without endothelium). The potent contraction to thrombin-activated platelets (1000 to 75 000/microliters) in arteries without endothelium was markedly inhibited by the thromboxane A2synthetase/receptor antagonist ridogrel (10-5mol/L; P<.005 versus control) and the single-acting thromboxane receptor blocker SQ-30741 (10-7mol/L; P<.01 versus control).Conclusions Thus, thrombin directly stimulates platelets to release thromboxane A2, inducing potent vasoconstriction, which is prevented by the simultaneous thrombin-induced release of prostacyclin and nitric oxide from endothelial cells. In arteries devoid of functional endothelial cells, as occurs in patients with coronary artery disease, a combined inhibition of thromboxane production and action provides a potent therapeutic tool to interfere with the thrombin-induced activation of platelet-vessel wall interaction. (Circulation. 1994;89:2266-2272.)