Effects of the agents affecting cyclic nucleotide metabolism on the bradykinin-and des-Arg 9-bradykinin-induced relaxations and contractions in isolated rat duodenum

Abstract

1. 1. Bradykinin and related kinins possess two different types of action (consisting of relaxation and contraction) in the isolated rat duodenum via their specific receptors. However, the mechanisms of these actions have not been fully elucidated. The present study was undertaken to investigate the effects of the agents affecting cyclic nucleotide metabolism on bradykinin-induced relaxations and on bradykinin-and des-Arg 9-bradykinin-induced contractions. 2. 2. Des-Arg 9-bradykinin, B 1 receptor agonist, and high concentrations of bradykinin elicited dose-dependent contractile responses in the rat duodenum, while low concentrations of bradykinin caused a dose-dependent relaxation in this tissue. 3. 3. Nicotinic acid, an inhibitor of adenylate cyclase, inhibited the relaxation of rat duodenum induced by bradykinin at low concentrations in a non-competitive manner. However, the inhibitory efficacy of nicotinic acid against bradykinin was limited by 39.9% and this inhibition was not further increased by higher concentrations of nicotinic acid up to 10 −3M. 4. 4. Imidazole, an activator of cyclic nucleotide phosphodiesterase, caused a slight inhibition of the relaxant responses to low concentrations of bradykinin and of the contractile responses to des-Arg 9-bradykinin and high concentrations of bradykinin in isolated rat duodenum. These inhibitions were also limited in efficacies and not increased by higher concentrations of imidazole. 5. 5. Methylene blue, an agent that inhibits soluble guanylate cylase, suppressed the contractions of rat duodenum induced by des-Arg 9-bradykinin and high concentrations of bradykinin in a non-competitive manner. Again, these inhibitions were limited and further increase in the inhibitory efficacy was not observed in spite of increasing the methylene blue concentrations. 6. 6. These findings strongly suggested that cyclic nucleotides are not essential to trigger contractile and relaxant responses of rat duodenum to bradykinin and des-Arg 9-bradykinin, although they may play a role in the contractile process elicited by these kinins.