Thrombin receptor-mediated vascular relaxation differentiated by a receptor antagonist and desensitization

The vasorelaxant actions of the serine protease, alpha-thrombin, are selectively blocked by the thrombin active site inhibitors, suggesting that proteolytic cleavage is required for alpha-thrombin-induced release of nitric oxide. Whether these relaxations are caused by interaction with a thrombin receptor was evaluated using a prototype thrombin receptor antagonist, a decapeptide analogue of the tethered ligand thrombin receptor [3-mercapto-propionyl-Phe-Cha-Cha-Arg-Lys-Pro-Asn-Asp-Lys-amide (c186-65)]. In rings of pig coronary arteries with endothelium contracted submaximally with U-46619, the relaxation caused by extremely low concentrations of alpha-thrombin were mimicked by the synthetic thrombin receptor-activating peptide (TRAP-7: SFLLRNP). These relaxations were inhibited by C186-65. In contrast, C186-65 had no effect on the relaxations caused by bradykinin and serotonin. Exposure of arteries to alpha-thrombin or TRAP-7 caused heterologous desensitization to subsequent stimulation by alpha-thrombin or TRAP-7 but not by bradykinin. These studies support the hypothesis that alpha-thrombin-induced endothelium-dependent relaxations occur by activation of the cloned "tethered-ligand" thrombin receptor in vascular endothelium.