EVIDENCE FOR THE PRESENCE OF A PROTEINASE-ACTIVATED RECEPTOR DISTINCTFROM THE THROMBIN RECEPTOR IN VASCULAR ENDOTHELIAL-CELLS

The thrombin receptor was the first cloned G protein-coupled receptor reported to be activated by proteolytic cleavage of its extracellular amino terminus. A second proteinase-activated receptor (PAR-2) was clo ned recently and expressed in Xenopus laevis oocytes. PAR-2 was activa ted by trypsin and by a peptide (SLIGRL) derived from the new amino te rminus. Since PAR-2 mRNA was detected in highly vascularized organs, w e compared the physiological functions of the thrombin receptor and PA R-2 in vascular endothelium. Thrombin and trypsin both elicited endoth elium-dependent relaxations in prostaglandin F-2 alpha (PGF(2 alpha))- contracted strips of porcine coronary artery. Whereas high doses of bo th thrombin or trypsin (10 U/mL) caused homologous desensitization, tr ypsin caused further relaxation of thrombin-desensitized tissues. Thro mbin and PAR-2-derived peptides (SFLLRN and SLIGRL) both induced endot helium-dependent relaxations in PGF(2 alpha)-contracted porcine corona ry arteries. SFLLRN or SLIGRL (30 mu mol/L) also showed homologous des ensitization but not cross desensitization. In the presence of the NO synthase inhibitor N-G-monomethyl-L-arginine (1 mmol/L), both SFLLRN- and SLIGRL-induced relaxations were partially inhibited. SFLLRN elicit ed weak contraction in coronary arteries without endothelium, whereas SLIGRL had no effect. Intravenous injection of SFLLRN (1 mg/kg, bolus) into anesthetized rats elicited a transient depressor response follow ed by pronounced presser response. In contrast, intravenous administra tion of SLIGRL (1 mg/kg, bolus) produced only a marked depressor respo nse. Consistent with the in vivo data, SFLLRN contracted the endotheli um-rubbed rat aortic rings and aggregated human platelets in vitro, wh ereas SLIGRL had no effect. The finding that both trypsin and SLIGRL i nduced endothelium-dependent relaxations indicates the presence of PAR -2 on endothelial cells. In addition, both trypsin and SLIGRL elicited relaxations in thrombin- or SFLLRN-desensitized tissue, suggesting th at PAR-2 is distinct from thrombin receptor in vascular endothelium. T he lack of PAR-2-mediated platelet aggregation or smooth muscle contra ction suggested it might not share the pathogenic properties associate d with the thrombin receptor in the vasculature.