Role of PGE2 in protease-activated receptor-1,-2 and-4 mediated relaxation in the mouse isolated trachea

1 The potential mediator role of the prostanoid PGE(2) in airway smooth muscle relaxations induced by peptidic and proteolytic activators of PAR-1, PAR-2, PAR-3 and PAR-4 was investigated in carbachol-precontracted mouse isolated tracheal segments. 2 The tethered ligand domain sequences of murine PAR-1 (SFFLRN-NH2), PAR-2 (SLIGRL-NH2) and PAR-4 (GYPGKF-NH2), but not PAR-3 (SFNGGP-NH2), induced smooth muscle relaxation that was abolished by the non-selective cyclo-oxygenase (COX) inhibitor, indomethacin. The relative order for mean peak relaxation was SLIGRL-NH2 > GYPGKF-NH2 approximate to SFFLRN-NH2 > SFNGGP-NH2. 3 SFFLRN-NH2, SLIGRL-NH2 and GYPGKF-NH2, but not SFNGGP-NH2, induced significant PGE(2) release that was abolished by indomethacin. Like that for relaxation, the relative order for mean PGE(2) release was SLIGRL-NH2>GYPGKF-NH2>SFFLRN-NH2 >SFNGGP-NH2. 4 In dose-response studies, SLIGRL-NH2 induced concentration-dependent increases in PGE(2) release (EC50 = 20.4 muM) and smooth muscle relaxation (EC50 = 15.8 muM). 5 The selective COX-2 inhibitor, nimesulide, but not the COX-1 inhibitor valeryl salicylate, significantly attenuated SLIGRL-NH2-induced smooth muscle relaxation and PGE(2) release. 6 Exogenously applied PGE(2) induced potent smooth muscle relaxation (EC50 = 60.3 nM) that was inhibited by the mixed DP/EP1/EP2 prostanoid receptor antagonist, AH6809. SLIGRL-NH2-induced relaxation was also significantly inhibited by AH6809. 7 In summary, the results of this study strongly suggest that PAR-mediated relaxation in murine tracheal smooth muscle is dependent on the generation of the spasmolytic prostanoid, PGE(2). PAR-stimulated PGE(2) release appears to be generated preferentially by COX-2 rather than COX-1, and induces relaxation via activation of the EP2 receptor.