EFFECTS OF THE AGENTS AFFECTING CYCLIC-NUCLEOTIDE METABOLISM ON THE BRADYKININ-INDUCED AND DES-ARG(9)-BRADYKININ-INDUCED RELAXATIONS AND CONTRACTIONS IN ISOLATED RAT DUODENUM

1. Bradykinin and related kinins possess two different types of action (consisting of relaxation and contraction) in the isolated rat duoden um via their specific receptors. However, the mechanisms of these acti ons have not been fully elucidated. The present study was undertaken t o investigate the effects of the agents affecting cyclic nucleotide me tabolism on bradykinin-induced relaxations and on bradykinin- and des- Arg(9)-bradykinin-induced contractions. 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 concent rations of bradykinin caused a dose-dependent relaxation in this tissu e. 3. Nicotinic acid, an inhibitor of adenylate cyclase, inhibited the relaxation of rat duodenum induced by bradykinin at low concentration s in a non-competitive manner. However, the inhibitory efficacy of nic otinic acid against bradykinin was limited by 39.9% and this inhibitio n was not further increased by higher concentrations of nicotinic acid up to 10(-3) M. 4. Imidazole, an activator of cyclic nucleotide phosp hodiesterase, caused a slight inhibition of the relaxant responses to low concentrations of bradykinin and of the contractile responses to d es-Arg(9)-bradykinin and high concentrations of bradykinin in isolated rat duodenum. These inhibitions were also limited in efficacies and n ot increased by higher concentrations of imidazole. 5. Methylene blue, an agent that inhibits soluble guanylate cylase, suppressed the contr actions of rat duodenum induced by des-Arg(9)-bradykinin and high conc entrations of bradykinin in a non-competitive manner. Again, these inh ibitions were limited and further increase in the inhibitory efficacy was not observed in spite of increasing the methylene blue concentrati ons. 6. These findings strongly suggested that cyclic nucleotides are not essential to trigger contractile and relaxant responses of rat duo denum to bradykinin and des-Arg(9)-bradykinin, although they may play a role in the contractile process elicited by these kinins.