Human vascular kinin receptors of the B2 type characterized by radioligand binding

1. The human umbilical vein responds to bradykinin (BK) with contractions that are mediated by B2 receptors. In the present study, the corresponding vascular smooth muscle B2 binding sites have been investigated. 2. [3H]-BK, a full agonist labelled ligand, was used to demonstrate a single binding site giving a Kd value of 0.51+/-0.02 nM and a Bmax of 24+/-1 fmol mg(-1) protein. Scatchard plots were linear (r=0.98) in the 0.05-5 nM range of concentrations. Non-specific binding was found to be 30% of total binding. 3. Competition binding curves gave the following order of potency for various B2 receptor agonists: BK-[Hyp3]-BK > or = Lys-BK >> [Aib7]-BK >>> [desArg9]-BK, which is typical of B2 receptors. There was no binding to B1 receptors since the selective B1 receptor ligand, Lys-[desArg9]BK was inactive up to 10 microM (n=4). 4. Characterization of the binding site with antagonists, performed with three chemically distinct series of peptide and non-peptide compounds, revealed a high affinity of Hoe 140 (D-Arg-[Hyp3,Thi5,D-Tic7,Oic8]-BK) (Ki 0.17 nM; n=4) which was more potent that FR 173657 ([(E)-3-(6-acetamido-3-pyridyl)-N-[N-[2,4-dichloro-3-[(2-methyl-8-quinol inyl)oxymethyl]phenyl]-N-methylaminocarbonylmethyl] acrylamide]) (Ki 1.94 nM; n=4), D-Arg-[Hyp3,D-Phe7,Leu8]-BK (Ki 256 nM; n=4) and Win 64338 (phosphonium, [[4[[2[[bis(cyclohexylamino)methylene]amino]-3-(2-naphthalenyl)-1-oxopro pyl]amino]phenyl]methyl]tributyl, chloride, monohydrochloride) (Ki 1,450 nM; n=4). 5. The present study describes and characterises B2 receptor binding sites in the vascular smooth muscle of the human umbilical vein. The binding assay appears to be suitable for studying new agonists or antagonists designed to activate or block the B2 receptor class that mediate the majority of the physiopathological effects of kinins in man.