Two bradykinin binding sites with picomolar affinities

Bradykinin (BK) and related peptides exert a wide range of effects on several organ systems. We have attempted to sort out these effects by studying the binding interaction of [3H]BK at the membrane level with in vitro receptor binding techniques. High specific activity [3H]BK and an enzyme inhibitor cocktail has enabled us to label two BK binding sites with different affinity and peptide specificity in several guinea-pig tissues. In the guinea-pig ileum the high-affinity site has an equilibrium dissociation constant (Kd) for [3H]BK of 13 pM and a maximal number of binding sites of 8.3 pmol/g of tissue wet weight. The low-affinity guinea-pig ileum site displays a Kd of 910 pM, a maximum number of binding sites of 14 pmol/g of tissue wet weight and shows a greater selectivity for BK analogs over Lysyl-BK analogs. Two similar sites can also be discriminated in kidney and heart. The potencies of a series of BK analogs at the high-affinity guinea-pig ileum site correlate well with their potencies in contracting ileal smooth muscle. The binding of [3H]BK in the guinea-pig ileum is inhibited by physiological concentrations of monovalent and divalent cations