Potentiation by endothelin‐1 of cholinergic nerve‐mediated contractions in mouse trachea via activation of ETB receptors

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Abstract
We have previously shown that endothelin‐1‐induced contraction of mouse isolated tracheal smooth muscle was mediated via both ETA and ETB receptors. In the current study, we have investigated endothelin‐1‐induced potentiation of cholinergic nerve‐mediated contractions in mouse isolated trachea and have characterized pharmacologically the endothelin receptors mediating this response. Electrical field stimulation (EFS; 70 V, 0.5 ms duration, 10 s train, 0.1–60 Hz) of mouse isolated trachea caused frequency‐dependent, monophasic contractions (magnitude of contraction of 60 Hz was 56 ± 4%Cmax (n = 6), where Cmax is the contractile response to 10 μm carbachol). EFS‐induced contractions were abolished by either 0.1 μm atropine or 3 μm tetradotoxin, but were not affected by 1 μm hexamethonium, indicating that they were induced by stimulation of postganglionic cholinergic nerves. In contrast, contractions induced by exogenously applied acetylcholine were inhibited by atropine, but not by either tetrodotoxin or hexamethonium. The ETB receptor‐selective agonist, sarafotoxin S6c, caused marked concentration‐dependent potentiation of EFS‐induced contractions in mouse isolated tracheal segments. At 0.1 nm, sarafotoxin S6c exerted no direct contractile effect, but significantly increased a standard EFS‐induced contraction of 20%Cmax by 8 ± 2%Cmax (i.e. 1.4 fold, n = 5, P < 0.05). At higher concentrations, 10 nm sarafotoxin S6c induced a large, transient contraction (peak response of 74 ± 2%Cmax at 10 min; 3 ± 2%Cmax at 45 min) and enhanced the standard EFS‐induced contraction by 30 ± 4%Cmax (i.e. 2.5 fold, n = 5, P < 0.01). In contrast, 10 nm sarafotoxin S6c did not enhance contractile responses to exogenously applied acetylcholine (n = 6). Endothelin‐1 also modulated EFS‐induced contractions. At 0.1 nm, endothelin‐1 exerted no direct contractile effect, but significantly increased the standard EFS‐induced contraction of 20%Cmax by 7 ± 2%Cmax (i.e. 1.35 fold, n = 5, P < 0.05). At 1 nm, endothelin‐1 induced a small, sustained contraction (16 ± 3% Cmax) and increased the standard EFS‐induced contraction by 19 ± 2%Cmax (i.e. 1.95 fold, n = 5, P < 0.01). Finally, 10 nm endothelin‐1 induced a large, sustained contraction (98 ± 8%Cmax), but the EFS‐induced contraction was significantly reduced from 20%Cmax to 6 ± 4%Cmax (n = 6, P < 0.05). In contrast, in the presence of 3 μm BQ‐123 (ETA receptor‐selective antagonist), 10 nm endothelin‐1 induced a transient contraction mediated via ETB receptors (peak response of 59 ± 10%Cmax at 10 min; 8 ± 2%Cmax at 45 min). Under these conditions, the standard EFS‐induced contraction was increased by 26 ± 1%Cmax (i.e. 2.3 fold, n = 6, P < 0.01). The potentiation of EFS‐induced contractions produced by 1 nm endothelin‐1 was not mediated by ETA receptors, since 3 μm BQ‐123 did not diminish this effect (n = 6). Furthermore, 1 nm endothelin‐1 did not potentiate EFS‐induced contractions in preparations in which the function of the ETB receptor‐effector system had been attenuated by desensitization (n = 6). In summary, endothelin‐1 potentiates cholinergic nerve‐mediated contractions in mouse isolated trachea, apparently by activating prejunctional ETB receptors. This neuronal pathway offers an additional mechanism through which endothelin‐1 may elevate bronchomotor tone.