THROMBIN RECEPTOR-MEDIATED VASCULAR RELAXATION DIFFERENTIATED BY A RECEPTOR ANTAGONIST AND DESENSITIZATION

The vasorelaxant actions of the serine protease, alpha-thrombin, are s electively blocked by the thrombin active site inhibitors, suggesting that proteolytic cleavage is required for alpha-thrombin-induced relea se of nitric oxide. Whether these relaxations are caused by interactio n with a thrombin receptor was evaluated using a prototype thrombin re ceptor antagonist, a decapeptide analogue of the tethered ligand throm bin receptor ropionyl-Phe-Cha-Cha-Arg-Lys-Pro-Asn-Asp-Lys-amide (c186- 65)]. In rings of pig coronary arteries with endothelium contracted su bmaximally with U-46619, the relaxation caused by extremely low concen trations of alpha-thrombin were mimicked by the synthetic thrombin rec eptor-activating peptide (TRAP-7: SFLLRNP). These relaxations were inh ibited by C186-65. In contrast, C186-65 had no effect on the relaxatio ns caused by bradykinin and serotonin. Exposure of arteries to a-throm bin or TRAP-7 caused heterologous desensitization to subsequent stimul ation by alpha-thrombin or TRAP-7 but not by bradykinin. These studies support the hypothesis that alpha-thrombin-induced endothelium-depend ent relaxations occur by activation of the cloned ''tethered-ligand'' thrombin receptor in vascular endothelium.