Peptide and non-peptide bradykinin B-2 receptor agonists and antagonists: a reappraisal of their pharmacology in the guinea-pig ileum

We have compared the pharmacology of different antagonists, Icatibant (H-DA rg-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-ac etamido-3-pyridyl)-N-[N-[2,4-dichloro-3-[(2-methyl-8-quinolinyl)oxymethyl]p henyl]-N-methylaminocarbonylmethyl]acrylamide) at bradykinin B-2 receptors expressed in the guinea-pig ileum by using bradykinin and the non-peptide F R190997 ((8-[2,6-dichloro-3-[N-[(E)-4-(N-methyl-carbamoyl)cinnamidoacetyl]- N-methylamino]benzyloxyl]-2-methyl-4-(2-pyridylmethoxy)quinoline) as agonis ts. In organ bath experiments, Icatibant and FR173657 exerted a non-competi tive antagonism (p K-B 9.5 and 9.2, respectively) of the contractile respon se to bradykinin, whereas MEN 11270 showed competitive antagonism (p K-B 8. 3, slope - 0.90). The profile of action and apparent affinities of the thre e antagonists did not change if contact time was prolonged. The inhibition by the three antagonists of the contractile response to bradykinin was diff erently reverted by washout (MEN 11270 < 30 min, Icatibant < 60 min, FR1736 57 > 60 min). The non-peptide ligand FR190997 acted as partial agonist if a pplied cumulatively to the bath (p D-2 8.06, E-max 43% of maximal contracti lity), but as a full agonist when a maximally effective concentration was a dded (E-max 83%). FR173657 produced non-competitive antagonism of the respo nse to FR190997 with apparent affinity similar to that measured toward brad ykinin. On the contrary, Icatibant and MEN 11270 (300 nM both) competitivel y antagonized the contractile activity exerted by FR190997 with lower appar ent pA(2) value (6.9 and 7.2, respectively). In radioligand binding experim ents, MEN 11270 and Icatibant displaced the [H-3]bradykinin binding with p K-i of 10.2 and 10.5 (Hill slope not different from unity), respectively. T he non-peptide ligands displaced the [H-3]bradykinin binding with similar a ffinity, their p K-i being 8.7 and 8.6 for FR173657 and FR190997, respectiv ely (both Hill slopes < 1). The present study indicates the difference in t he antagonism type (competitive vs. non-competitive) by I:Icatibant, MEN 11 270, and FR173657, as mainly ascribable to their different reversibility fr om the bradykinin B-2 receptor, and affected by the kinetic of the response induced by the different agonists. Results are discussed in view of a diff erent interaction of peptide and non-peptide agonist at the receptor. (C) 2 000 Elsevier Science B.V. All rights reserved.