Efffect of noscapine, the antitussive opioid alkaloid, on bradykinin-induced smooth muscle contraction in the isolated ileum of the guinea-pig.

Bradykinin (BK) and related kinins are autocoid peptides that play integral roles in many pathophysiological processes such as cough. In this study, the inhibitory effect of noscapine, the antitussive opioid alkaloid, on BK receptors, was tested in the guinea-pig ileum. Contractions of the isolated ileum of the guinea-pig in response to BK were inhibited by noscapine (10-1,000 nM) in a concentration-dependent manner. Concentration-response curves (CRCs) to BK were slightly shifted to the right with a concomitant decrease in the maximum effect. A pA2 value of 6.68 was calculated for noscapine. The slope of the Schild plot of the antagonism was found to be 0.56. Noscapine had no effect on contractions induced by KCl, acetylcholine, histamine, 5-hydroxy tryptamine or angiotensin II. In conclusion, noscapine has a specific antagonistic effect on BK receptors and the mode of inhibition was found to be non-competitive.