Effect of protease-activated receptor (PAR)-1, -2 and -4-activating peptides, thrombin and trypsin in rat isolated airways

1 Mechanisms of relaxation and contraction to protease-activated receptor- (PAR) tethered ligand peptides (SFLLRN/TFLLR, SLIGRL and GYPGKF (all C-term inally amidated) for PAR1, PAR2 and PAR4, respectively) and enzymes (thromb in and trypsin) were investigated in isolated segments of rat trachea, main and first order intrapulmonary bronchi. 2 In airway segments previously exposed to SLIGRL, SFLLRN caused contractio ns that were potentiated by indomethacin, but were independent of mast cell degranulation. Contractions to TFLLR in the intrapulmonary bronchi were si milarly potentiated by indomethacin. 3 SLIGRL caused epithelium-dependent relaxations which were unaffected by N -G-nitro-L-arginine, 1-H-oxodiazol-[1,2,4]-[4,3-a]quinoxaline-1-one or zinc -protoporphyrin-IX but were abolished by haemoglobin in all three regions o f the airways. Relaxations to SLIGRL were markedly attenuated by indomethac in only in the main and intrapulmonary bronchi. 4 GYPGKF caused epithelium-dependent relaxations in all three regions of th e airway which were only significantly inhibited by indomethacin in the int rapulmonary bronchi. 5 In general, thrombin and trypsin failed to cause any response in the airw ays tested. 6 Intense PAR2-immunoreactivity was observed on airway epithelium. PAR1-imm unoreactivity was faint on airway epithelium and smooth muscle, but was pre valent in mast cells. 7 These findings indicate that PAR2 and possibly PAR4 present on rat airway epithelia mediate smooth muscle relaxation via cyclo-oxygenase-dependent a nd -independent mechanisms. PAR1-mediated contractions were most likely due to activation of smooth muscle receptors. The general failure of thrombin and trypsin to cause responses which may have been due to endogenous protea se inhibitors, highlights the need for caution in assessing pathophysiologi cal roles for PARs if only enzymes are used to activate PARs.