Evidence for two high-affinity bradykinin binding sites in the guinea-pig lung

We examined the binding of [3H]bradykinin ([3H]BK) to the guinea-pig lung which was saturable. Scatchard analysis indicated the existence of two binding sites, one with a high affinity (KD = 15 pM) and one with a low one (KD = 570 pM) with maximum number of binding sites of 12 and 45 fmol/mg protein, respectively. Kinetic studies confirmed the presence of these two types of binding sites and their affinity ranges. Neither the B1 agonist des-Arg9-BK, nor the B1 antagonist des-Arg9-[Leu8]BK displaced [3H]BK, demonstrating the absence of B1 receptors in the guinea-pig lung. Current B2 antagonists fully displaced the [3H]BK binding. Their potencies differed slightly according to the concentration of [3H]BK, suggesting a specificity of current B2 antagonists for the lower affinity site as opposed to the higher one. Altogether, these results do not allow us to confirm the occurrence of putative B3 receptors in guinea-pig lung.