INDUCTION BY ENDOTHELIN-1 OF EPITHELIUM-DEPENDENT RELAXATION OF GUINEA-PIG TRACHEA IN-VITRO - ROLE FOR NITRIC-OXIDE

1 The purpose of the present experiments was to study the underlying mechanisms responsible for the relaxant action of endothelin-1 (ET-1) in the guinea-pig trachea in vitro. 2 In tracheal strips precontracted (60-70% of the maximum) with carbachol, ET-1 (1-100 nm) evoked slowly developing concentration-dependent relaxations. Preincubation of the tissues with the thromboxane A2/prostaglandin H-2 receptor antagonist, BM 13505 (5 mum) significantly potentiated the relaxant response to ET-1. 3 Removal of the epithelium changed the response of precontracted tracheal preparations to ET-1 from a relaxation to a sustained contraction. 4 ET-1-induced relaxations were abolished by methylene blue (10 mum) and were almost completely attenuated by oxyhaemoglobin (5 mum) and N(G)-monomethyl-L-arginine (L-NMMA, 100 mum), an inhibitor of nitric oxide synthesis, but were not altered by indomethacin (10 mum). 5 In tracheal strips under passive tension, ET-1 (1-100 nm) elicited dose-dependent contractions. The sensitivity of tissues to ET-1 was significantly enhanced by removal of the epithelium (apparent EC50 values were 28.1 +/- 4.1 and 12.5 +/- 0.8 nm in intact and rubbed trachea, respectively, n = 7, P < 0.01). 6 Preincubation of intact tracheal strips with methylene blue, oxyhaemoglobin or L-NMMA did not mimic the effect of epithelium removal on ET-1-induced contractions. 7 There was a concentration-dependent increase in thromboxane A2 but not in PGE2 and prostacyclin release from intact tracheal strips following stimulation with ET-1 (5-100 nm). 8 These results show that ET-1 exerts a dual action on guinea-pig isolated trachea: it evokes contractions at low resting tone, whereas it induces relaxations at higher resting tone. The relaxant action of ET-1 may be mediated by nitric oxide released from epithelial cells and resultant activation of smooth muscle guanylate cyclase.