Atypical Protease-Activated Receptor Mediates Endothelium-Dependent Relaxation of Human Coronary Arteries

Abstract

Abstract —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 A 2 mimetic U46619. These relaxations were dependent on the catalytic activity of each enzyme and were inhibited by the NO synthase inhibitor N G -nitro- l -arginine (100 μmol/L) and the NO scavenger oxyhemoglobin (20 μmol/L). The synthetic PAR-1 tethered ligand sequence SFLLRN-NH 2 (0.01 to 10 μmol/L) also caused endothelium-dependent relaxation of U46619-contracted human coronary arteries; however, the equivalent PAR-2 ligand SLIGKV-NH 2 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-NH 2 . Therefore, we conclude that human coronary artery endothelial cells possess a PAR-1–like receptor that is potently activated by thrombin, trypsin, and SFLLRN-NH 2 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.