Peptide and non-peptide bradykinin B2 receptor agonists and antagonists: a reappraisal of their pharmacology in the guinea-pig ileum.

We have compared the pharmacology of different antagonists, Icatibant (H-DArg-Arg-Pro-Hyp-Gly-Thi-Ser-DTic-Oic-Arg-OH), MEN 11270 (H-DArg-Arg-Pro-Hyp-Gly-Thi-c(Dab-DTic-Oic-Arg)c(7 gamma-10 alpha)), and FR173657 ((E)-3-(6-acetamido-3-pyridyl)-N-[N-[2, 4-dichloro-3-[(2-methyl-8-quinolinyl)oxymethyl]phenyl]-N-methyl aminocarbonylmethyl]acrylamide) at bradykinin B2 receptors expressed in the guinea-pig ileum by using bradykinin and the non-peptide FR190997 ((8-[2,6-dichloro-3-[N-[(E)-4-(N-methylcarbamoyl)cinnamidoacety l]-N -methylamino]benzyloxy]-2-methyl-4-(2-pyridylmethoxy)quinoline) as agonists. In organ bath experiments, Icatibant and FR173657 exerted a non-competitive antagonism (pKB 9.5 and 9.2, respectively) of the contractile response to bradykinin, whereas MEN 11270 showed competitive antagonism (pKB 8.3, slope -0.90). The profile of action and apparent affinities of the three antagonists did not change if contact time was prolonged. The inhibition by the three antagonists of the contractile response to bradykinin was differently reverted by washout (MEN 11270 <30 min, Icatibant <60 min, FR173657 >60 min). The non-peptide ligand FR190997 acted as partial agonist if applied cumulatively to the bath (pD2 8.06, Emax 43% of maximal contractility), but as a full agonist when a maximally effective concentration was added (Emax 83%). FR173657 produced non-competitive antagonism of the response to FR190997 with apparent affinity similar to that measured toward bradykinin. On the contrary, Icatibant and MEN 11270 (300 nM both) competitively antagonized the contractile activity exerted by FR190997 with lower apparent pA2 value (6.9 and 7.2, respectively). In radioligand binding experiments, MEN 11270 and Icatibant displaced the [3H]bradykinin binding with pKi of 10.2 and 10.5 (Hill slope not different from unity), respectively. The non-peptide ligands displaced the [3H]bradykinin binding with similar affinity, their pKi being 8.7 and 8.6 for FR173657 and FR190997, respectively (both Hill slopes <1). The present study indicates the difference in the antagonism type (competitive vs. non-competitive) by Icatibant, MEN 11270, and FR173657, as mainly ascribable to their different reversibility from the bradykinin B2 receptor, and affected by the kinetic of the response induced by the different agonists. Results are discussed in view of a different interaction of peptide and non-peptide agonist at the receptor.