Effect of tranilast on endothelin-induced bronchoconstriction in guinea pigs

Tranilast, an anti-asthmatic drug with anti-allergic properties, inhibited endothelin (ET)-induced asthmatic like respiratory obstruction in guinea pigs when administered orally at 200 mg/kg 1 h prior to the intravenous injection of ET. ET also caused a bronchoconstriction detected as an increase in inflation pressure in Konzett-Rössler apparatus. Tranilast (200 mg/kg, p.o. 1 h before ET) inhibited ET-induced increased inflation pressure. ET-induced bronchoconstriction detected as an increase in inflation pressure was clearly inhibited by the administration of indomethacin (used as a reference drug) at doses of 1 and 5 mg/kg 30 min prior to onset of the reaction. In addition to the above in vivo experiments, tranilast at concentrations between 10(-5) and 10(-4) g/ml inhibited ET-induced contraction of isolated guinea pig tracheal muscle, whereas indomethacin did not affect this in vitro response. In order to elucidate the inhibitory mechanism of tranilast on ET-induced bronchoconstriction, the effects in Ca(2+)-free medium and on ET-induced histamine and prostaglandin F2 alpha release from tracheal muscle were investigated. Consequently, tranilast did not affect ET-induced contraction of guinea pig tracheal muscle in Ca(2+)-free Tyrode's solution and ET induced neither histamine nor PGF2 alpha release. These results suggest that tranilast inhibits ET-induced bronchoconstriction by inhibiting the ET-induced calcium influx into tracheal muscle.