Specific binding of [3H]mepyramine to histamine H1-receptors in vascular smooth muscle membranes

The role of histamine in dural vessel dilation

The pain of migraine is often throbbing suggesting an important role for the cranial blood vessels and their innervation by the trigeminal nerve. It is proposed that clinically effective anti-migraine compounds, such as 5-HT(1B/1D) agonists, have actions that include inhibiting calcitonin gene-related peptide (CGRP) release from trigeminal nerves. Human studies suggest that histamine can induce migraine possibly by activating nitric oxide (NO) synthase to promote endogenous NO production. The present studies investigated the effect of histamine and its antagonists on the cranial blood vessels using intravital microscopy to assess directly the diameter of dural arteries in sodium pentobarbitone anaesthetised rats. Electrical stimulation of a closed cranial window produces, by local depolarisation of nerves, dural vessel dilation that is monitored continuously on-line using video-microscopy and a video dimension analyser. Histamine infusion caused immediate and reproducible dilation of meningeal blood vessels (103.5+/-6%; n=40) that could be blocked by H(1)- (mepyramine) and H(2) (famotidine)-receptor antagonists (P<0.05), as well as a nitric oxide synthase inhibitor (N(G)-nitro-L-arginine methylester; P<0.05). Neurogenic dural vasodilation was not inhibited by H(2)-receptor antagonists, but was significantly inhibited by a H(1)-receptor antagonist at the high dose of 10 mg/kg. The present studies demonstrate that histamine is likely to activate NO synthase to promote NO production. There is also evidence that H(1)-receptors may be present on trigeminal neurones as the H(1)-receptor antagonist inhibited neurogenic vasodilation, albeit at a large dose.

Histaminergic H1-receptors in smooth muscle and endothelium of bovine thoracic aorta

The vascular endothelium modulates relaxation and contraction of blood vessels. Since endothelial cells respond to a variety of vasoactive substances, it was suggested that specific cell membrane receptors exist on the endothelial cells which are responsible for the modulatory role of the endothelium on the blood vessels. We therefore investigated the localization and binding characteristics of histaminergic H1-receptors in the vascular model system of the bovine thoracic aorta. Our earlier binding experiments showed that histaminergic H1-receptor binding sites labelled with [3H]mepyramine are present on the vascular smooth muscle membranes of this tissue. In addition a small number of specific H1-receptor binding sites also exist on the endothelial cells of this tissue with the following binding characteristics: Bmax = 34.6 fmol [3H]mepyramine/mg protein, KD = 2.13 nM. [3H]mepyramine binding is more effectively inhibited by H1- than H2-receptor agonists and antagonists. These results provide evidence for the existence of endothelial histaminergic H1-receptor binding sites in addition to vascular smooth muscle H1-receptors in the bovine thoracic aorta.

Ascorbate increases apparent "specific" binding of [3H]pyrilamine to glass fiber filters

Although filtration through glass fiber filters provides a convenient method for separating bound from unbound radioligand, binding of the ligand to the filters is often a problem. Ascorbate (0.1%) has been shown to decrease binding of serotonergic acids to these filters. In the present study, relatively high binding of [3H]pyrilamine, an H1-antagonist, to glass fiber filters was observed. The addition of ascorbate increased, rather than decreased, binding of [3H]pyrilamine to the filters. Thus, the effect of ascorbate on binding of radioligands to glass fiber filters appears to be dependent upon the particular ligand involved.

Regulation of membrane histamine H1-receptor binding sites by guanine nucleotides, mono- and divalent cations

Agonist interaction with histamine H1-receptor in [3H] mepyramine bovine aortic membranes labeled with [3H] mepyramine is selectively regulated by cations and guanine nucleotides. GTP and his nonhydrolisable analog Gpp(NH)p' markedly decrease histamine affinity for [3H] mepyramine binding sites. The effect of GTP is reversed in the presence of divalent cation, magnesium. Calcium and sodium ions have little effect on histamine binding whereas magnesium ions decrease the affinity of histamine for the radioantagonist binding sites about tenfold. GTP has little effect on [3H] mepyramine binding and the interaction of H1-antagonist triprolidine with histamine H1-receptors. The above results indicate that the effect of guanine nucleotides, mono and divalent cations involves the effect on membrane signal transducing mechanism probably GTP-binding protein(s) cation regulatory site(s) rather than receptor binding site directly.

Characterization of vascular histamine H1-receptors: multiple affinity states of aortic histamine H1-receptor

We have shown that [3H]mepyramine labels histamine H1-receptor-binding sites in bovine aortic membranes. Further characterization of H1-receptors in this tissue was done by the interaction of an unlabelled histamine receptor agonist or antagonist, with the radioantagonist [3H]mepyramine-binding sites. The competition-binding assays have uncovered differences in the characteristics of the agonist/receptor interaction not shared by antagonists. Agonists interact in the heterogeneous manner with the radioantagonist-labelled sites, showing shallow competition curves with the nH 0.50-0.72, whereas antagonists were devoid of this effect (steeper slopes of the inhibition curves nH approximately 1). The results suggest the presence in this tissue of multiple affinity states of histamine H1-receptor, differentiated by high and low affinity for agonists and the same affinity for antagonists.

Specific binding of [3H] mepyramine to histamine H1-receptors in the sarcolemma from porcine aorta and coronary artery

We studied the subcellular distribution and the properties of [3H] mepyramine binding in the porcine vascular smooth muscle. A close correlation was observed between the specific binding activity of [3H] mepyramine and the extent of the enrichment of sarcolemmal marker enzyme in 4 subfractions obtained by sucrose density gradient centrifugation of the aortic microsome. In the binding isotherm in the sarcolemmal fractions from the aorta and coronary artery, there was no difference in the Kd value, but the Bmax of the coronary artery was significantly lower than that of the aorta. Thus, there is a single type of high affinity mepyramine binding site in the porcine vascular smooth muscle sarcolemma and the number of H1-receptors of the coronary artery may be smaller than that of the aorta.

The effect of group selective reagents N-ethylmaleimide and dithiothreitol on histamine H1-receptor binding sites in the vascular smooth muscle membranes

The chemical nature of the histamine H1-receptors of beef aortic membranes has been elucidated by introducing two group selective reagents in the [3H]-mepyramine binding studies: dithiothreitol (DTT), a protein-disulphide group reducing reagent, and N-ethylmaleimide (NEM), a protein-thiol group alkylating agent. In the binding experiments, NEM independently inhibits [3H]-mepyramine binding. The inhibition is time and concentration dependent. DTT on the other hand potentiates the binding of the radioligand to its receptor and changes the affinity of histamine in competing for [3H]-mepyramine binding site. In the DTT-pretreated membranes (100 microM), histamine shows a higher affinity for [3H]-mepyramine binding (Ki 0.35 microM) than in the untreated membranes (Ki 3.7 microM). Comparison of the pharmacological studies on the DTT-treated rabbit aortic strips and above binding studies, revealed a good correlation between the changes in the affinity of histamine for its receptor, when DTT was present. The results suggest an important role of the S-S and SH groups in the function of aortic histamine H1-receptor.