Renal bradykinin and vasopressin receptors: Ligand selectivity and classification

We studied the specific binding of radiolabeled bradykinin ([H-3]BK) and vasopressin ([H-3]AVP) to membrane preparations of bovine and porcine kidney medulla. [H-3]BK reversible labeled a single site (K-d = 1.06 nM) in bovine kidney medulla independently of [Mg2+]. The number of BK receptors in bovine kidney medulla, B-max = 122 fmol/mg protein, is markedly (2- to 3-fold) higher than that reported in other tissues. Further characterization by ligand binding indicated that the bovine bradykinin receptor was the B-2a subtype, pharmacologically related to B-2a receptors expressed by human and rabbit tissues. In contrast, the specific binding of [H-3]BK, but not [H-3]AVP, to porcine kidney medulla (K-d = 0.32 nM, B-max = 45 fmol/mg) was dependent upon the presence of enzyme inhibitors to prevent the rapid and selective degradation of bradykinin. Interspecies differences were revealed for renal medulla V-2 vasopressin receptors with respect to their abundance and their affinity for several V-2-selective ligands. In summary (i) bovine kidney medulla is a convenient source of tissue for studying the B-2a bradykinin receptor subtype; (ii) there are significant species-dependent differences in both the abundance of renal medulla B-2a and V-2 receptors and the ligand selectivity of V-2 receptors; and (iii) these findings are significant in relation to the physiological and pathological roles of renal kinins-and their interaction with the neurohypophysial peptide hormone system.