PHARMACOLOGICAL PROPERTIES OF CARDIOVASCULAR HISTAMINE H-1 RECEPTOR-BINDING SITES - CHARACTERIZATION WITH 2-PHENYLHISTAMINES

We determined and compared the molecular properties of histamine H-1 r eceptor binding sites in bovine thoracic aorta smooth muscle and guine a pig myocardial membranes from ventricles with saturation and inhibit ion binding assay, using H-3-mepyramine to label the receptor and spec ific and selective H-1 receptor agonists of the 2-phenylhistamine grou p as displacers of specific H-3-mepyramine binding. H-3-mepyramine bin ds in a saturable manner to a homogenous population of binding sites w ith a K-D of 5.6 nM and a B-max of 57 fmol/mg of protein in bovine aor ta vascular smooth muscle membranes, whereas in the guinea pig myocard ium high and low affinity H-3-mepyramine binding sites exist having th e following molecular characteristics: a K-D of 1.6 nM and a B-max of 99 fmol/mg of protein (high affinity site) and a K-D 15.0 nM and a B-m ax of 466 fmol/mg of protein (low affinity site). Halogenated 2-phenyl histamines: 2-(3-fluoro-) (2-FPH), 2-(3-trifluoromethyl-) (2-triFMPH), 2-(3-chloro-) (2-CPH), 2-(3-bromo-) (2-BPH) and 2-(3-iodophenyl)hista mine (2-IPH), which showed high selectivity and potency for H-1 recept ors in the functional pharmacological studies, were potent inhibitors of specific radioligand binding in comparison with histamine and paren t nonhalogenated 2-phenylhistamine (2-PH). Their rank order of potenci es and affinities differ significantly for the vascular and cardiac H- 1 receptor binding sites: Specific H-3-mepyramine binding to H-1 recep tors in bovine vascular smooth muscle membranes was displaced in a bip hasic manner by 2-(3-fluoro-), 2-(3-trifluoromethyl-), 2-(3-chloro-), 2-(3-bromo-), 2-(3-iodophenyl)histamine and histamine. In guinea pig v entricular myocardium the rank order was 2-(3-iodo-), 2-(3-bromo-), hi stamine, 2-(3-chloro-), and 2-(3-fluorophenyl)histamine showing better correlation with the lipophilicity of the derivatives than in vascula r tissue (order of lipophilicity: 2-triFMPH >2-IPH >2-BPH >2-CPH >2-FP H much greater than 2-PH). Displacement of the radioligand binding to myocardial H-1 receptor by the above drugs is (except for 2-(3-fluorop henyl)histamine), better fitted to a two-site model. 2-phenylhistamine , which acted as a moderate agonist in functional studies, displaced t he radioligand in a monophasic manner and was the weakest displacer of specific radioligand binding in both model systems (pK(i) = 5.76 - va scular and 5.57 - cardiac tissue). The agonistic nature of the halogen ated 2-phenylhistamine derivatives was confirmed on the molecular leve l, since their interaction with the H-1 receptor is regulated by guani ne nucleotides. GTP (0.1 mM) significantly lowered the affinities of a ll tested halogenated 2-phenylhistamines and histamine for H-1 recepto r binding site converting biphasic displacement curves, to monophasic ones, whereas GTP had no effect on the affinity of 2-PH. The results o f this study support the conclusions that bovine vascular and guinea p ig myocardial histamine H-1 receptors differ in their molecular proper ties. Selective and potent H-1 receptor agonists of 2-phenylhistamine class can discriminate between vascular and cardiac receptor.