Selective activation of excitation‐contraction coupling pathways by ET A and ET B receptors in guinea‐pig tracheal smooth muscle

Signalling events responsible for endothelin A (ET A ) and ET B receptor‐induced contraction were examined in epithelium‐denuded guinea‐pig tracheal smooth muscle strips. Selective stimulation of each subtype was achieved by a combination of ET‐1 (100 n M ) and ET A and ET B receptor‐selective antagonists, BQ‐123 (10 μ M ) and BQ‐788 (3 μ M ), respectively. Both ET A and ET B receptors induced long‐lasting contraction that was totally dependent on Ca 2+ influx. Stimulation of ET A receptor induced both transient and sustained (Ca 2+ ) i increases whereas that of ET B receptor induced only a sustained increase. Suppression of the transient (Ca 2+ ) i increase by U73122 (3 μ M ) did not affect the ET A ‐induced sustained (Ca 2+ ) i increase and tension development. Stimulation of ET A receptor, but not ET B , induced phosphoinositide breakdown and protein kinase C (PKC). The activated PKC contributed to the contraction by increasing the Ca 2+ ‐sensitivity of the contractile apparatus. Thus, ET A receptor is coupled both with phospholipase C/Ca 2+ /PKC signalling and Ca 2+ influx pathways whereas ET B receptor was coupled only with the latter. Stimulation of ET B receptor, but not ET A , caused membrane depolarization measured with a fluorescent indicator, bis‐(1,3 dibutylbarbituric acid)‐trimethine oxonol. Both nifedipine (1 μ M ) and verapamil (10 μ M ) abolished ET B ‐induced Ca 2+ influx and contraction, while they barely affected ET A ‐induced responses. Therefore, the Ca 2+ influx pathways activated by each subtype appeared to be completely different; ET A and ET B receptors opens voltage‐independent Ca 2+ channels and L‐type voltage‐dependent Ca 2+ channels, respectively.British Journal of Pharmacology (1999) 126 , 893–902; doi: 10.1038/sj.bjp.0702365