Atypical protease-activated receptor mediates endothelium-dependent relaxation of human coronary arteries

Protease-activated receptors (PARs) are a family of G protein-coupled receptors activated by a tethered ligand sequence within the amino terminal that are revealed by site-specific proteolysis. The thrombin-sensitive PAR-1 and trypsin-activated PAR-2 mediate endothelium-dependent vascular relaxation in a number of species. Because both thrombin and trypsin-like enzymes have been implicated in coronary artery disease, the purpose of this study was to investigate whether similar receptors are present in human coronary arteries. Thrombin (0.001 to 0.1 U/mL) and trypsin (0.001 to 1 U/mL) caused concentration- and endothelium-dependent relaxations of human coronary artery ring segments suspended in organ chambers for isometric tension recording and contracted with the thromboxane A2 mimetic U46619. These relaxations were dependent on the catalytic activity of each enzyme and were inhibited by the NO synthase inhibitor NG-nitro-L-arginine (100 micromol/L) and the NO scavenger oxyhemoglobin (20 micromol/L). The synthetic PAR-1 tethered ligand sequence SFLLRN-NH2 (0.01 to 10 micromol/L) also caused endothelium-dependent relaxation of U46619-contracted human coronary arteries; however, the equivalent PAR-2 ligand SLIGKV-NH2 caused almost no relaxation. In addition, desensitization to either thrombin or trypsin resulted in cross-desensitization to the other enzyme but had only a minimal affect on the response to SFLLRN-NH2. Therefore, we conclude that human coronary artery endothelial cells possess a PAR-1-like receptor that is potently activated by thrombin, trypsin, and SFLLRN-NH2 to cause NO-mediated vascular relaxation. Once cleaved, this receptor is recycled in a truncated form, able to respond to exogenous application of only its tethered ligand sequence, suggesting the presence of another endogenous activator possibly acting independently of receptor cleavage.