Characterization of ETB receptors mediating contractions induced by endothelin‐1 or IRL 1620 in guinea‐pig isolated airways: effects of BQ‐123, FR139317 or PD 145065

1 We have characterized the receptors mediating contractions to endothelin‐1 (ET‐1) or IRL 1620, an ETB receptor selective agonist, in isolated strips of tissue prepared from different parts of the guinea‐pig airways. We used as antagonists BQ‐123 and FR139317 (ETA receptor‐selective) and PD 145065 (ETA/ETB receptor non‐selective). 2 ET‐1 and IRL 1620 (10−10 m to 10−6 m) caused similar concentration‐dependent contractions of strips of guinea‐pig trachea and upper bronchus. In the guinea‐pig trachea without epithelium or lung parenchyma, IRL 1620 was less potent than ET‐1. 3 In the trachea, contraction to ET‐1 (< 10−8 m) was preceded by a transient relaxation which was inhibited by BQ‐123 (10−5 m) or FR 139317 (10−5 m) or by the removal of the epithelium. The concentration‐response curve to ET‐1 in the trachea was shifted to the right by PD 145065 (10−5 m to 10−4 m). PD 145065 (10−4 m) also inhibited the response to ET‐1 (3 × 10−7 m) by 55%. Contractions induced by IRL 1620 were not affected by BQ‐123 (10−6 m) or FR139317 (10−6 m) but were significantly attenuated by 10−5 m of either antagonist. PD 145065 at 10−6 m strongly attenuated and at 10−5 m abolished contractions induced by IRL 1620. 4 In the trachea, removal of the epithelium potentiated the effects of both agonists. BQ‐123 (10−5 m) had no effect on contractions of the trachea without epithelium induced by ET‐1, but FR139317 (10−5 m) caused a significant inhibition. PD 145065 (10−5 m to 10−4 m) caused a shift to the right of the ET‐1 concentration‐response curve without affecting the contractile effect at 3 × 10−7 m. All three antagonists inhibited contractions induced by IRL 1620. 5 In the upper bronchus, BQ‐123 (10−5 m) did not affect contractions induced by ET‐1, while FR139317 (10−5 m) attenuated (20–26%) only contractions induced by 1–3 × 10−7 m ET‐1. PD 145065 (10−5 m to 10−4 m) caused a shift to the right of the ET‐1 concentration‐response curve. The contractions induced by IRL 1620 were inhibited by BQ‐123 or FR139317 (10−5 m to 10−4 m). PD 145065 (10−6 m) strongly inhibited contractions induced by IRL 1620 and PD 145065 (10−5 m) totally abolished them. 6 The contractile action of ET‐1 in the lung parenchyma was significantly and similarly attenuated by BQ‐123 (10−5 m) or indomethacin (10−5 m), while FR139317 (10−5 m) was less effective. PD 145065 (10−6 to 10−5 m) inhibited contractions to ET‐1. IRL 1620, which is less potent than ET‐1 in this preparation, was antagonized by PD 145065 (10−5 to 10−6 m) but unaffected by BQ‐123 (10−6 m to 10−5 m) or FR139317 (10−6 m). 7 Thus, ETB receptors mediate contractions to ET‐1 in all four guinea‐pig airway preparations. In addition, contractions to ET‐1 in the trachea and lung parenchyma are mediated in part by ETA receptors. In the latter tissue, these ETA receptors mediate contraction through the release of cyclo‐oxygenase metabolites. Similarly, ETA receptors located on the epithelial cells also mediate the release of prostanoids in the trachea with epithelium but they are responsible for transient relaxations. Interestingly, contractions induced by IRL 1620 were more susceptible to inhibition by the different antagonists, most probably because it binds to the endothelin receptors in a reversible manner. High concentrations (10−5 m) of ETA‐selective antagonists also inhibit responses to IRL 1620, most probably by an effect at ETB receptors in both the trachea and the upper bronchus.