Pharmacological properties of cardiovascular histamine H1 receptor binding sites: characterisation with 2-phenylhistamines.

We determined and compared the molecular properties of histamine H1 receptor binding sites in bovine thoracic aorta smooth muscle and guinea pig myocardial membranes from ventricles with saturation and inhibition binding assay, using 3H-mepyramine to label the receptor and specific and selective H1 receptor agonists of the 2-phenylhistamine group as displacers of specific 3H-mepyramine binding. 3H-mepyramine binds in a saturable manner to a homogenous population of binding sites with a K(D) of 5.6 nM and a Bmax of 57 fmol/mg of protein in bovine aorta vascular smooth muscle membranes, whereas in the guinea pig myocardium high and low affinity 3H-mepyramine binding sites exist having the following molecular characteristics: a K(D) of 1.6 nM and a Bmax of 99 fmol/mg of protein (high affinity site) and a K(D) 15.0 nM and a Bmax of 466 fmol/mg of protein (low affinity site). Halogenated 2-phenylhistamines: 2-(3-fluoro-) (2-FPH), 2-(3-trifluoromethyl-) (2-triFMPH), 2-(3-chloro-) (2-CPH), 2-(3-bromo-) (2-BPH) and 2-(3-iodophenyl)histamine (2-IPH), which showed high selectivity and potency for H1 receptors in the functional pharmacological studies, were potent inhibitors of specific radioligand binding in comparison with histamine and parent nonhalogenated 2-phenylhistamine (2-PH). Their rank order of potencies and affinities differ significantly for the vascular and cardiac H1 receptor binding sites: Specific 3H-mepyramine binding to H1 receptors in bovine vascular smooth muscle membranes was displaced in a biphasic manner by 2-(3-fluoro-), 2-(3-trifluoromethyl-), 2-(3-chloro-), 2-(3-bromo-), 2-(3-iodophenyl)histamine and histamine. In guinea pig ventricular myocardium the rank order was 2-(3-iodo-), 2-(3-bromo-), histamine, 2-(3-chloro-), and 2-(3-fluorophenyl)histamine showing better correlation with the lipophilicity of the derivatives than in vascular tissue (order of lipophilicity: 2-triFMPH >2-IPH >2-BPH >2-CPH >2-FPH >>2-PH). Displacement of the radioligand binding to myocardial H1 receptor by the above drugs is (except for 2-(3-fluorophenyl)histamine), better fitted to a two-site model. 2-phenylhistamine, which acted as a moderate agonist in functional studies, displaced the radioligand in a monophasic manner and was the weakest displacer of specific radioligand binding in both model systems (pKi = 5.76--vascular and 5.57--cardiac tissue). The agonistic nature of the halogenated 2-phenylhistamine derivatives was confirmed on the molecular level, since their interaction with the H1 receptor is regulated by guanine nucleotides. GTP (0.1 mM) significantly lowered the affinities of all tested halogenated 2-phenylhistamines and histamine for H1 receptor binding site converting biphasic displacement curves, to monophasic ones, whereas GTP had no effect on the affinity of 2-PH. The results of this study support the conclusions that bovine vascular and guinea pig myocardial histamine H1 receptors differ in their molecular properties. Selective and potent H1 receptor agonists of 2-phenylhistamine class can discriminate between vascular and cardiac receptor.