Evidence for H2-receptor-mediated inhibition of histamine release from isolated rat mast cells

Impromidine, an H₂-receptor agonist, inhibited the release of histamine from isolated purified rat mast cells evoked by compound 48/80 and acetylcholine. Pyridilethyl-amine (PEA), an H₁-receptor agonist, on the other hand, was only slightly effective at very high concentrations. The inhibitory effect of impromidine was blocked by preincubating the cells with cimetidine, but not by chlorpheniramine.The existence of an H₂-mediated inhibitory feed-back regulation of histamine release was also suggested by the demonstration of specific binding sites for [³H]-cimetidine in rat mast cell membranes.     

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 then _H 0.50–0.72, whereas antagonists were devoid of this effect (steeper slopes of the inhibition curvesn _H1). 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.     

Sexual dimorphism modulates brain histamine functions at multiple sites

Sexual dimorphism has been demonstrated in rat brain and the ovarian steroids affect H₁ and H₂ histaminergic binding sites as demonstrated with [³H]-mepyramine and [³H]-histamine, respectively. The evaluation of histamine release, related to the presynaptic H₃ receptors, from cortical slice preparations reveals hyposensitive releasing activity in the female rat compared to the male, both after KCl-induced histamine release and after inhibition of histamine release by (R)-methylhistamine (H₃ selective agonist). Therefore, we may suggest a global hyposensitivity of the histaminergic neural system in female rats under the modulation of ovarian sexual steroids.     

Additional data on the function of hypothalamic histamine

In a preliminary study, the stimulatory effect of histamine on an adenylate cyclase system in a solubilized cell-free preparation of the rat hypothalamus was established. The effect was dose dependent, and the histamine concentration required for half-maximal activation (K _a) was determined at 0.1 M. At a 10-fold higher concentration, both chloropyramine, the classical histamine H₁ antagonist, and metiamide, the selective H₂-receptor blocker, partially blocked this action. Experiments carried out in hypothalamic slices showed a stimulatory effect of both the H₁-agonist, 2-(2-pyridyl)-ethylamine, and the H₂-antagonist, dimaprit, on adenylate cyclase in the range of histamine action. These effects could be reversed completely by the H₁-antagonist, mepyramine, and the H₂-receptor blocker, cimetidine.In an additional study, histamine, histamine agonists and antagonists were tested on the spontaneous and the potassium-activated outflow of³H-noradrenaline from rat hypothalamic slices. Histamine did not modify this outflow significantly, whereas the H₁-agonist, 2-(2-pyridyl)-ethylamine, produced a marked, dose-related increase in both the spontaneous and the potassium-stimulated release of noradrenaline. The H₂-receptor blocker, cimetidine, also exerted a moderate but statistically significant stimulatory effect in this system.In combination studies, the noradrenaline-releasing action of these compounds could not be reversed by the selectively acting histaminic or antihistaminic agents, showing that this effect does not relate to the histaminic or antihistaminic property of the compound.It is becoming clear that histamine exerts a direct stimulatory effect on hypothalamic adenylate cyclase. The noradrenaline-releasing potency of some histaminic and antihistaminic agents showed that these compounds might modify the clear histamine effects through the release of other transmitter amines.     

Effects of the H2-receptor agonist dimaprit on lymphocyte responsivenessin vitro

The histamine H₂-agonist dimaprit was found to increase the response of rat spleen cells to the T-cell mitogen Concanavalin A, when present at concentrations of 10^–5 and 10^–4 M. Higher concentrations of dimaprit were cytotoxic. The enhanced response seemed to be associated with an inhibitory effect of dimaprit on T-suppressor cell activity rather than with a direct mitogen-like stimulation of lymphocyte proliferation or with an interference with monocyte/macrophage functions. The stimulatory effects of dimaprit were not reversed by the H₂-receptor antagonist, cimetidine, nor by the -receptor antagonists metoprolol and H 35/25. Addition of the H₁-receptor antagonist, mepyramine, further increased the stimulatory effect of dimaprit on lymphocyte responsiveness.     

A postsynaptic inhibitory histamine H2-receptor in the mouse isolated vas deferens

The effect of histamine on adrenergic neurotransmission in the mouse isolated vas deferens preparation was investigated. Concentrations of histamine ranging from 0.2 to 650 M depressed, in a dose-related manner, not only the contractile response elicited by field stimulation but also the response caused by the addition of exogenous noradrenaline and acetylcholine. However, the release of [³H]-NA evoked by field stimulation or by high K⁺ remained unchanged in the presence of these concentrations of histamine. The inhibitory effect of histamine on the contractile responses caused by various stimuli was reduced or completely antagonized by cimetidine, a histamine H₂-receptor antagonist but not by mepyramine, a conventional antihistamine. The inhibitory effect of histamine was found to be inversely proportional to both the Ca²⁺ concentration in the bathing medium and to the frequency of field stimulation. Further, the inhibitory effect of histamine was markedly reduced when Mg²⁺ was omitted from the bathing medium. It is concluded that the mouse vas deferens preparation contains a post-junctional inhibitory H₂-receptor. The stimulation of H₂-receptors by histamine inhibits the contractile response of the vas deferens, possibly by decreasing the availability of Ca²⁺ required for contraction by depressing the influx of Ca²⁺.     

The effect of group selective reagentsN-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, andN-ethylmaleimide (NEM), a proteinthiol 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 M), histamine shows a higher affinity for [³H]-mepyramine binding (K _i 0.35 M) than in the untreated membranes (K _i 3.7 M). 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.     

Lipolytic responses induced by intracerebroventricular administration of histamine in the rat

Histamine (10–50 g) administered intraventricularly in conscious rats induced an increase in serum-free fatty acids. The maximum, significant increase appeared 30–60 min after administration. Histamine H₁-receptor antagonists, mepyramine and chloropyramine, when injected 2 h prior to histamine, abolished considerably hyperlipaemic responses to histamine. H₂-Receptor antagonists, metiamide and cimetidine, given i.c.v. only moderately diminished the histamine-induced hyperlipaemia. Histamine injected i.c.v. also increased serum corticosterone levels considerably. This elevation was prevented significantly by the H₁-receptor antagonist, mepyramine, but not by the H₂-receptor blocker, cimetidine. It seems likely that histamine given i.c.v. induces lipolysis through the release of ACTH, one of the known lipid-mobilizing hormones. The central lipid-mobilizing mechanism after histamine depends more on activation of H₁- than H₂-receptors.     

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: B_max=34.6 fmol [³H]mepyramine/mg protein, K_D=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.     

Labelling of non-H1-receptor binding sites by [3H]-mepyramine on the rat liver plasma membrane

In the present study we characterized [³H]-mepyramine binding to rat liver plasma membranes. Binding of [³H]-mepyramine proved to be of high affinity (K _d =7.7±0.4 nM) and saturable, resulting in a B_max-value of 70.4±9.5 pmol/mg protein. However, displacement studies revealed that this binding site was different from other H₁-receptor systems. The two stereoisomers of chlorpheniramine were rather ineffective in displacing [³H]-mepyramine and showed a stercospecificity in favour of thel-isomer. Also several H₁-receptor agonists were not potent in displacing [³H]-mepyramine from rat liver plasma membranes. Morcover, the histamine metabolite imidazole-4-acetic acid was about as potent as the H₁-agonists, whereas imidazole was even more potent. These data strongly suggest that [³H]-mepyramine labels a non-H₁-receptor binding site on the rat liver plasma membrane.     

Desensitization by histamine of H2 receptor activity in HGT-1 human cancerous gastric cells

Histamine produced a time-dependent (half-life: 20 min at 37°C), temperature-dependent (no effect at 20°C) and homologous desensitization of histamine H₂ receptor activity (H₂ R) in HGT-1 cells. Maximal and half-maximal desensitization were respectively observed at 10^–5 and 2×10^–7 M histamine. Decline of responsiveness in intact cells was related to a remarkable loss in histamine efficacy (from 15- to 2-fold stimulation in control and treated cells). The affinity of the H₂R for histamine (EC₅₀=10^–5 M) did not change during desensitization. Paradoxically, histamine treatment is associated with increased [³H] histamine binding capacity in intact HGT-1 cells, and no change in H₂ receptor antagonist binding ([³H]-tiodine and [³H]-SKF 93479). Desensitization process was preferentially mimicked by H₂ receptor agonists (impromidine > histamine > AET > PEA) and preferentially reversed by simultaneous addition of H₂ receptor antagonists (cimetidine > DPH). We suggest that the desensitization of H₂R activity by histamine presented here may be involved in the pathophysiological regulation and pharmacological control of gastric cell function in man.     

Influence of histamine H3-antagonists on human leukocytes

To determine the role of the histamine H₃-receptor on basophils, different specific H₃-antagonists were investigated. Incubation of washed leukocytes with N-acylated histamine-derivatives (N-ahd) induced elevated histamine levels. This process turned out to be dependent on dose, time and temperature, but independent of Ca²⁺ and Mg²⁺ ions. IgE-mediated histamine release was not modulated. [³H]-l-histidine was not decarboxylated into [³H]-histamine in spite of the observed histamine increase. Highly purified basophils did not show any histamine elevation but purified neutrophils and eosinophils were found to have increased histamine levels even after disintegration and subsequent incubation with N-ahd. It seems that the increased histamine levels result from the cleavage of the applied histamine amides. Other potent H₃-antagonists (e.g. thioperamide) neither produced increased histamine levels nor influenced IgE-mediated release from basophil leukocytes. The existence of H₃-receptors on human basophils therefore seems unlikely.This work was supported by Grant No. KI 622/1-1 from the Deutsche Forschungsgemeinschaft, Bonn, FRG.     

Induction of gastric histamine synthesis by H2-receptor antagonists: Potentiation of their antisecretory activity by histidine decarboxylase inhibitors

The activation of histamine (HA) formation in rat stomach was measured after repeated administrations of histamine H₂-receptor antagonists and the potentiation of their antisecretory activity was examined in histidine decarboxylase inhibitor (HDI)-pretreated rats. 40 mg/kg of metiamide, given intraperitoneally (i.p.) 24, 16 and 2 h prior to the examination, produced approximately 50% increase in the amount of¹⁴C-histamine, formed from¹⁴C-histidine in the stomach, and an almost equal enhancement in the gastric histidine decarboxylase (HD) activity. An equal dose of the compound did not influence the endogenous histamine level in the glandular stomach whereas it caused a significant increase in the serum histamine content. By similar treatment, 10 mg/kg of cimetidine enhanced the newly formed histamine in the rat stomach by 57%. The potent HDI, 2-hydroxy-5-carbomethoxy-benzyloxyamine (GYKI-11 121) suppressed the metiamide- and eimetidine-induced increases in histamine synthesis to slightly above or below the control values. In pharmacological studies, the antisecretory activity of histamine H₂-receptor blockers could markedly be potentiated by HDI. In GYKI-11 121 and NSD-1055-pretreated rats, the inhibiting potency of metiamide and cimetidine on pentagastrin-stimulated gastric acid secretion, increased to approximately twice that of the original effect. Neither GYKI-11 121 nor NSD-1055 produced significant inhibition on pentagastrin-stimulated gastric acid secretion in the applied doses. These findings provided evidence for the feedback stimulation of gastric HA synthesis by H₂-receptor blockers and confirmed the role of HA in the gastric acid secretion. Potentiation of the antisecretory activity of H₂-receptor antagonists by HDI would be useful in the therapeutic application of these compounds.     

Mast cell receptors controlling histamine release: Influences on the mode of action of drugs used in the treatment of adverse drug reactions

Summary In drug-induced allergic diseases of the immediate type (anaphylactic and anaphylactoid reactions), the primary target cells are tissue mast cells, which discharge their granular content upon interaction with different secretagogues (immunological releasers; histamine liberators) on specific plasma membrane receptors.Experiments are reviewed here which report that IgE-mediated histamine release from mast cells, and the secretion of histamine induced by non-immunological secretagogues (dextran; compound 48/80; acetylcholine) are blocked by beta-adrenoceptor and H₂-receptor agonists, their inhibiting effect being surmountable by beta-adrenoceptor blocking drugs and by anti-H₂-antihistamines. Specific radioligands ([³H]-dihydroalprenolol; [³H]-cimetidine) binding to rat mast cell membranes points to the possibility that inhibition of histamine release is brought about by the activation of mast cell beta-adrenoceptors and H₂-receptors.Drugs used in therapy of anaphylactic or anaphylactoid reactions may act either on tissue receptors, competing with released mediators, or by inhibiting the release of allergic mediators from mast cells, on activation of specific receptors located in mast cell plasma membranes.     

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.     

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

The antagonist-sensitive binding of [³H]mepyramine to beef aortic membranes was as expected for binding to histamine H₁-receptors. [³H]mepyramine binds rapidly and in saturable fashion to the specific receptor sites, specific binding reaching equilibrium in 3 min at 37°CScatchard's analysis of the binding data gave a dissociation constant of 3.0 nM for the radioligand-receptor complex and maximal number of binding sites: 31 fmol/mg protein. In the competition studies histamine H₁-antagonists are more potent inhibitors of radioligand binding than H₂-antagonist. They inhibit [³H]mepyramine binding in the following order: mepyramine >triprolidine     

Pharmacological properties of the hydroxylated histamine, (±)-4(5)-(2-amino-1-hydroxyethyl)-imidazole

The pharmacological effects of the -hydroxylated histamine, 4(5)-(2-amino-1-hydroxyethyl)-imidazole, on smooth muscle contraction of the ileum (H₁-receptor activity) and gastric acid secretion (H₂-receptor activity) of the guinea-pig were investigated and compared with those of histamine. Although -hydroxy histamine contracted the ileum with the same maximal response as hstamine, the concentration response curve was shifted to the right by approximately three orders of magnitude. At submaximal concentrations, co-administration of -hydroxy histamine with histamine revealed only additive effects. This H₁-activity was competitively inhibited by diphenhydramine. Similarly, the hydroxylated analogue also increased intracellular cyclic AMP level and [¹⁴C] aminopyrine accumulation as a marker of acid secretion in the parietal cells. However, the EC₅₀ was approximately ten fold that of histamine. This H₂-receptor activity was inhibited completely by cimetidine. These results suggest that -hydroxy histamine possesses nearly full intrinsic activities at both H₁ and H₂-receptors and that the introduction of a hydroxyl group at the -carbon reduces and dissociates these activities.     

Histamine increases anti-CD3 induced IL-5 production of TH2-type T cells via histamine H2-receptors

Besides its proinflammatory functions histamine released from basophils and mast cells during immediate-type hypersensitivity reactions is known to inhibit several lymphocyte functions like IL-2 and -IFN production. Recently, it has been shown that T helper cells of type 2 phenotype (T_H2) represent the T cell fraction which may play a pivotal role in the promotion of the allergic inflammatory eosinophilic late-phase reaction by secretion of cytokines, especially IL-4 and IL-5. We have investigated the effect of histamine on anti-CD3 induced IL-4 and IL-5 production by T_H2 cells. Histamine in concentrations between 10^–7 and 10^–5 mol/l concentration-dependently increased anti-CD3 induced IL-5 production up to 120%, whereas IL-4 production was not affected. The activity of histamine in increasing IL-5 production was mimicked by the H₂-receptor agonist dimaprit. Histamine induced increase in IL-5 production was inhibited by histamine H₂-receptor antagonists, but remained unaffected by H₁- or H₃-receptor antagonists. Administration of forskolin which directly stimulates the production of cAMP, the second messenger of the H₂-receptor, also resulted in an increase in anti-CD3 induced IL-5 production. These results indicate that the histamine-mediated increase in anti-CD3 induced IL-5 production is mediated via H₂-receptors. Consequently, histamine released from mast cells and basophils during the early-phase allergic reaction may act as an important stimulatory signal for the initiation of the allergic inflammatory late-phase reaction by increasing local IL-5 production of allergen triggered T_H2 cells.     

Species differences in histamine receptors in the vas deferens

Histamine, specific H₁-and H₂-receptor agonists in conjunction with specific H₁-and H₂-receptor antagonists and other types of classical antagonists were used to characterize histamine receptors in the vasa deferentia of mice, rats and guinea pigs. The H₁-receptor mediates contraction while the H₂-receptor produces inhibition. There were marked qualitative and quantitative differences in the distribution of the two types of histamine receptors in the vas deferens of different species. Results indicate that mouse and rat vas deferens contain an inhibitory H₂-receptor, but virtually no excitatory H₁-receptor. In contrast, guinea pig vas deferens contained an excitatory H₁-receptor but was essentially devoid of an inhibitory H₂-receptor. The rank order of relative potencies of various agonists as well as the calculated pA ₂ values of cimetidine in the mouse and rat vas deferens suggest that the two species probably have the same H₂-receptor. High concentrations of histamine and 2-methyl histamine have a stimulant action in the mouse and rat vas deferens which was secondary to release of endogenous noradrenaline rather than to the stimulation of an excitatory H₁-receptor.     

Histamine H1- and muscarinic receptor antagonist activity of cimetidine and tiotidine in the guinea pig isolated ileum

The histamine H₂-receptor antagonists cimetidine and tiotidine were compared as antagonists for H₁- and muscarinic receptors in the guinea pig isolated ileum. Their interactions with histamine were evaluated on the phasic and tonic components of the histamine response. For carbachol, only the phasic component of the response was monitored.For their interactions with histamine, the type of antagonism, surmountable or nonsurmountable, depended upon the choice of response metameter. Evaluation of the antagonism produced by either tiotidine or cimetidine for histamine concentration-response curves based on the phasic response metameter showed an initial dextral shift with surmountable antagonism. With increasing antagonist concentrations the concentration-response curves were further shifted to the right and the maximum response was depressed. When this antagonism was evaluated for histamine concentration-response curves based on the tonic response metameter, cimetidine produced dextral shifts and nonsurmountable antagonism, and tiotidine exhibited only nonsurmountable antagonist activity. pA₂ values for cimetidine antagonism of the phasic and tonic components of the histamine response were 3.2 and 3.7 respectively, and pD₂ values for the phasic and tonic components were 1.8 and 2.3 respectively. The pA₂ value for tiotidine antagonism of the phasic component was 3.9 and the pD₂ values for the phasic and tonic components were 2.6 and 3.3 respectively.Cimetidine and tiotidine at concentrations which did not depress the phasic component of the histamine response produced surmountable antagonism to the contractile action of carbachol. The pA₂ values for cimetidine and tiotidine antagonism of carbachol were 3.8 and 3.6 respectively.