Radioligand binding characterization of the bradykinin B2 receptor in the rabbit and pig ileal smooth muscle

Several species-related differences have been reported in kinin B2 receptor pharmacology. The present study aimed to evaluate the affinity of the bradykinin B2 receptor antagonist MEN16132 for the rabbit and pig B2 receptor, and radioligand binding experiments using [3H]bradykinin and membranes of rabbit and pig ileum smooth muscle were conducted. The [3H]bradykinin binding was characterized by homologous displacement curves indicating Kd values of 0.65 and 0.33 nM in rabbit and pig, respectively. The B2 receptor specificity of [3H]bradykinin binding was shown by the low affinity (> µM) displayed by agonists ([desArg9]bradykinin and Lys[desArg9]bradykinin) and antagonists [Leu8,desArg9]bradykinin and Lys[Leu8,desArg9]bradykinin) selective for the B1 receptor. The affinity of MEN16132 and other antagonists was determined by inhibition curves (pKi values in the rabbit and pig assay, respectively): MEN16132 (10.4 and 10.3) and peptide compounds such as icatibant (10.1 and 9.9) and MEN11270 (10.3 and 10.1) displayed subnanomolar potency in both assays; the nonpeptide LF16-0687 (8.4 and 8.5) and FR173657 (8.2 and 9.1) exhibited a different affinity pattern, whereas WIN64338 displayed low affinity (5.7 and ≤ 5). Results are discussed focusing on comparisons with previous findings obtained in rabbit and pig vascular functional assays, but also with those obtained in analog guinea pig and mouse assays and at the human B2 receptor. An attempt to highlight differences which can undertake ligands selectivity across the species is presented. In conclusion, the present study indicates MEN16132 as the only nonpeptidic compound which displays an even subnanomolar affinity for the rabbit and pig B2 receptor.