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

The potential mediator role of the prostanoid PGE2 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. 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 ∼amp; SFFLRN‐NH2>SFNGGP‐NH2. SFFLRN‐NH2, SLIGRL‐NH2 and GYPGKF‐NH2, but not SFNGGP‐NH2, induced significant PGE2 release that was abolished by indomethacin. Like that for relaxation, the relative order for mean PGE2 release was SLIGRL‐NH2>GYPGKF‐NH2>SFFLRN‐NH2>SFNGGP‐NH2. In dose‐response studies, SLIGRL‐NH2 induced concentration‐dependent increases in PGE2 release (EC50=20.4 μM) and smooth muscle relaxation (EC50=15.8 μM). The selective COX‐2 inhibitor, nimesulide, but not the COX‐1 inhibitor valeryl salicylate, significantly attenuated SLIGRL‐NH2‐induced smooth muscle relaxation and PGE2 release. Exogenously applied PGE2 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. 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, PGE2. PAR‐stimulated PGE2 release appears to be generated preferentially by COX‐2 rather than COX‐1, and induces relaxation via activation of the EP2 receptor. British Journal of Pharmacology (2001) 132, 93–100; doi:10.1038/sj.bjp.0703776