Relaxant effect of trapidil on isolated dog bronchus

Relaxant effects of trapidil on dog bronchi were pharmacologically analyzed in vitro. Trapidil concentration-dependently relaxed bronchial strips contracted with methacholine, histamine or potassium. In the strips under methacholine-contracture, theophylline, papaverine, isoproterenol and atropine produced concentration-dependent relaxations. Propranolol significantly decreased isoproterenol-induced relaxations of bronchial strips under methacholine-contracture, but did not trapidil-induced relaxations. Prostaglandin (PG) E1 and PGE2 in high doses slightly relaxed bronchial strips under methacholine-contracture, but PGI2 hardly affected them. Tranylcypromine did not influence relaxant responses of the strips to trapidil. The results indicate that trapidil produces relaxations of bronchial smooth muscle through a non-specific action other than adrenergic beta-receptor activation or endogenous prostaglandin biosynthesis.