INFLUENCE OF ENDOTHELIN-1 ON CHOLINERGIC NERVE-MEDIATED CONTRACTIONS AND ACETYLCHOLINE-RELEASE IN RAT ISOLATED TRACHEAL SMOOTH-MUSCLE

The aim of this study was to assess the influence of endothelin-1 (ET- 1) on cholinergic nerve-mediated contractions in rat isolated tracheal smooth muscle by use of electrical-field stimulation (EFS) and [H-3]c holine efflux studies. EFS (80 V, 0.5 ms, 0.1-30 Hz for 10 s) evoked t ransient, frequency-dependent contractions of isolated tracheal prepar ations. Contractions were abolished in the presence of atropine or tet rodotoxin, which suggests they were mediated by acetylcholine (ACh) re lease from cholinergic nerves. The ET(B) receptor-selective agonist sa rafotoxin S6c (1 nM) augmented EFS (0.6-1 Hz)-induced contractions by 179%. These effects were significantly attenuated in the presence of t he ET(B) receptor-selective antagonist eucyl-D-1-methoxycarbonyltrypto phanyl-D-norleucine (BQ-788; 1 mu M) ET-1 (1 nM) also markedly potenti ated EFS-induced contractions (153%). This was apparently not a postju nctional effect, because ET-1 did not alter contractile responses to e xogenously applied ACh. Cyclo[D-Trp-D-Asp-L-Pro-D-Val-L-Leu](BQ-123; 3 mu M) and BQ-788 when used alone, failed to inhibit ET-l-induced pote ntiation of EFS-evoked contractions. However, in their combined presen ce, BQ-123 and BQ-788 significantly attenuated ET-l-induced potentiati on of EFS responses. EFS (100 V, 0.5 ms, 3 Hz for 2 min) applied to tr acheal preparations preloaded with [H-3]choline, caused airway smooth muscle contraction and an efflux of radioactivity. Both sarafotoxin S6 c (10 nM) and ET-1 (10 nM) significantly enhanced the EFS-induced H-3- efflux and the latter was abolished only in the combined presence of B Q-123 and BQ-788. These data indicated that ET-1 enhances cholinergic nerve-mediated contractions in rat isolated trachea via activation of prejunctional ET(A) and ET(B) receptors that were linked to increased ACh release from cholinergic nerves.