Heterogeneous mechanisms of endothelium‐dependent relaxation for thrombin and peptide activators of protease‐activated receptor‐1 in porcine isolated coronary artery

Mechanisms of protease‐activated receptor‐1 (PAR1)‐ and PAR2‐induced relaxation were investigated in pre‐contracted porcine coronary artery ring preparations. Thrombin (0.01–0.3 u ml −1 ) and the PAR1‐activating peptide SFLLRN (0.1–10 μ M ) caused concentration‐ and endothelium‐dependent relaxation. pEC 50 s (−log u ml −1 for enzymes, −log M for peptides) and maximum relaxations (R max , %) for thrombin were 1.8±0.1 and 93.5±2.8% respectively, and for SFLLRN 6.8±0.1 and 90.8±1.3%. Similar concentration‐ and endothelium‐dependent relaxations occurred with trypsin (pEC 50 2.3±0.2; R max 94.1±1.9%) and the PAR2‐activating peptide SLIGRL (pEC 50 6.5±0.2; R max 92.4±1.6%). Relaxations to thrombin, SFLLRN, trypsin and SLIGRL were significantly inhibited ( P <0.05) to similar extents by the nitric oxide (NO) synthase inhibitor N G ‐nitro‐ L ‐arginine ( L ‐NOARG; 100 μ M ) and the NO scavenger oxyhaemoglobin (20 μ M ), both separately and in combination. In the presence of the L‐type voltage‐operated calcium channel (L‐VOCC) inhibitor nifedipine (0.3 μ M ), K + (67 m M ) abolished the L ‐NOARG‐resistant relaxations to thrombin, SFLLRN, trypsin and SLIGRL. However, nifedipine alone significantly ( P <0.05) reduced the pEC 50 (1.5±0.1) and R max (77.5±7.0%) for thrombin but had no effect on relaxations to SFLLRN, trypsin or SLIGRL. Furthermore, L ‐NOARG‐resistant relaxations to thrombin were abolished by nifedipine, whereas relaxations to SFLLRN, trypsin or SLIGRL were not further inhibited by combined treatment with nifedipine and L ‐NOARG, than they were with L ‐NOARG treatment alone. Similar selective inhibition of the L ‐NOARG‐resistant relaxation to thrombin, but not SFLLRN, occurred with verapamil (1 μ M ) and diltiazem (3 μ M ). Our results suggest heterogeneous mechanisms in the NO‐independent relaxation to thrombin and peptide activators of PAR1 in the porcine coronary artery.British Journal of Pharmacology (2000) 130 , 181–188; doi: 10.1038/sj.bjp.0703146