Subthreshold concentration of endothelin-1-enhanced, capsaicin-induced bronchoconstriction in anaesthetized guinea-pigs

An increasing number of studies have been performed to address a possible role for endothelin-l (ET-1) as a significant mediator in asthma, However, the effects of subthreshold concentrations of ET-1, which cannot elicit bronchial smooth muscle contraction itself, in asthma has yet to be determined. This study determined these effects of ET-1 on capsaicin-induced bronchoconstriction in anaesthetized guinea-pigs. Aerosolized ET-1 administered at doses of 10(-9) M and higher induced a dose-dependent increase in pulmonary resistance, but ET-1 at 10(-10) M did not have any bronchoconstrictive effect, However, this subthreshold concentration of ET-1 potentiated capsaicin-induced bronchoconstriction. In addition, the potentiation of capsaicin-induced bronchoconstriction by this subthreshold concentration of ET-1 was completely abolished by BQ788 (ETB receptor antagonist), but not BQ123 (ETA receptor antagonists). Immunoreactive substance P (SP) levels in bronchoalveolar lavage fluid after capsaicin administration were significantly higher than those after solvent administration, However, ET-1 alone did not significantly stimulate immunoreactive SP release and ET-1 (10(-10) M) did not potentiate capsaicin-induced immunoreactive SP release. In contrast, ET-1 (10(-10) M) potentiated exogenous neurokinin A- and SP-induced bronchoconstriction, These findings suggest that a subthreshold concentration of endothelin-l does not potentiate the tachykinin release induced by capsaicin but the airway smooth muscle contraction through endothelin-B receptors.