[I-125] PIP HOE-140, A HIGH-AFFINITY RADIOLIGAND FOR BRADYKININ-B2 RECEPTORS

A high affinity radioligand for bradykinin B2 receptors was prepared by coupling an activated ester of [I-125]4-iodobenzoic acid to the amino terminus nitrogen of the potent B2 antagonist HOE 140. The ligand, [I-125]para-iodophenyl HOE 140 ([I-125]PIP HOE 140), bound to a homogeneous set of sites in guinea pig ileal membranes with an equilibrium dissociation constant of 15 pM and a maximal binding density of 193 fmole/mg protein. Competition studies with a number of BK-related peptides indicated that the ligand specifically labeled B2 receptors in the preparation. The results suggest that [I-125]PIP HOE 140 will be a useful tool for future studies of B2 receptors.