ROLE OF B-1 AND B-2 RECEPTORS AND OF NITRIC-OXIDE IN BRADYKININ-INDUCED RELAXATION AND CONTRACTION OF ISOLATED RAT DUODENUM

Bradykinin (BK) and its analogues induce a typical biphasic response (relaxation followed by contraction) in the isolated rat duodenum. We studied the role of B-1 and B-2 BK receptors and nitric oxide (NO) in relaxation and contraction of the isolated rat duodenum. Both effects are concentration-dependent: BK has shown an EC(50) (contraction) of 3.8 +/- 1.9 x 10(-7) M and an IC50 (relaxation) of 3.0 +/- 0.7 x 10(-9). Similar results were obtained with the selective B-2 receptor agonists [Hyp(3),Tyr(Me)(8)]-BK and [Phe(8) Psi(CH2-NK)Arg(9)]-BK, showing an EC(50) of 9.6 +/- 1.9 x 10(-7) M and 5.6 +/- 2.9 x 10(-7) M and an IC50 of 3.5 +/- 0.6 x 10(-10) M and 6.8 +/- 1.7 x 10(-10) M, respectively. Furthermore, the effects induced by these three agonists were not altered when tissues were treated with 42.1 mu M Mergetpa, a carboxypeptidase N inhibitor. While the relaxant and contractile effects elicited by BK were significantly inhibited in the presence of Hoe 140 (0.7 mu M), a selective B-2 receptor antagonist, those induced by the selective B-1 receptor agonist desArg(9)-BK were not. Furthermore, [Leu(8)]-desArg(9)-BK (2.6 mu M), which is both a pure and selective B-1 receptor antagonist, acted as an agonist on the rat duodenum, inducing a biphasic relaxant and contractile effect. These relaxant and contractile effects were not altered by drugs that inhibit or stimulate NO production, such as L-NAME (200 mu M), a combination of L-NAME (200 mu M) and indomethacin (2.5 mu M), L-arginine (1 mM), or superoxide dismutase (20 U/ml). However, the contractile effect was significantly reduced when tissues were preincubated with methylene blue (100 mu M), which inhibits activation of guanylate cyclase. We conclude that 1) BK and its analogues selectively activate a B-2 receptor, producing a biphasic effect (relaxation and contraction); 2) DesArg(9)-BK may either acts via a different receptor which might be another B-1 receptor subtype or a typical B-1 receptor where [Leu(8)]-desArg(9)-BK acts as a partial agonist; and 3) neither NO nor the prostaglandin pathway mediates BK-induced relaxation in the isolated rat duodenum.