Identification and characterization of histamine H1- and H2-receptors in guinea-pig left atrial membranes by [3H]-mepyramine and [3H]-tiotidine binding

Pharmacological characterization of the zebrafish Hrh2a histamine H2 receptor

The zebrafish, Danio rerio, is a widely adopted in vivo model that conserves organs such as the liver, kidney, stomach, and brain, being, therefore, suitable for studying human diseases, drug discovery and toxicology. The brain aminergic systems are also conserved and the histamine H1, H2 and H3 receptors were previously cloned and identified in the zebrafish brain. Genome studies identified another putative H2 receptor (Hrh2) with ∼50% sequence identity with H2 receptor orthologs. In this study, we recombinantly expressed both zebrafish H2 receptor paralogs (hrh2a and hrh2b) and compared their pharmacology with the human H2 receptor ortholog. Our results showed that both zebrafish receptors conserve all the class A GPCR motifs. However, in contrast with the Hrh2a paralog, the Hrh2b does not possess all the amino acid residues shown to participate in histamine binding. The zebrafish Hrh2a receptor displays high affinity for [3H]-tiotidine with a binding profile for H2 receptor ligands similar to that of the human H2 receptor. The zebrafish Hrh2a receptor couples to GαS and Gαq/11 proteins, resulting in cAMP accumulation and activation of several reporter genes linked to the Gαq/11 pathway. Additionally, this receptor shows high constitutive activity, with histamine potency in the low nanomolar range for cAMP accumulation and the micromolar range for the activation of the NFAT response element. Moreover, dimaprit and amthamine seem to preferentially activate GαS over Gαq/11 proteins via the zebrafish Hrh2a receptor. These results can contribute to clarifying the functional roles of the H2 receptor in zebrafish.

Function and Role of Histamine H1 Receptor in the Mammalian Heart

Histamine can change the force of cardiac contraction and alter the beating rate in mammals, including humans. However, striking species and regional differences have been observed. Depending on the species and the cardiac region (atrium versus ventricle) studied, the contractile, chronotropic, dromotropic, and bathmotropic effects of histamine vary. Histamine is present and is produced in the mammalian heart. Thus, histamine may exert autocrine or paracrine effects in the mammalian heart. Histamine uses at least four heptahelical receptors: H₁, H₂, H₃ and H₄. Depending on the species and region studied, cardiomyocytes express only histamine H₁ or only histamine H₂ receptors or both. These receptors are not necessarily functional concerning contractility. We have considerable knowledge of the cardiac expression and function of histamine H₂ receptors. In contrast, we have a poor understanding of the cardiac role of the histamine H₁ receptor. Therefore, we address the structure, signal transduction, and expressional regulation of the histamine H₁ receptor with an eye on its cardiac role. We point out signal transduction and the role of the histamine H₁ receptor in various animal species. This review aims to identify gaps in our knowledge of cardiac histamine H₁ receptors. We highlight where the published research shows disagreements and requires a new approach. Moreover, we show that diseases alter the expression and functional effects of histamine H₁ receptors in the heart. We found that antidepressive drugs and neuroleptic drugs might act as antagonists of cardiac histamine H₁ receptors, and believe that histamine H₁ receptors in the heart might be attractive targets for drug therapy. The authors believe that a better understanding of the role of histamine H₁ receptors in the human heart might be clinically relevant for improving drug therapy.

The Roles of Cardiovascular H2-Histamine Receptors Under Normal and Pathophysiological Conditions

This review addresses pharmacological, structural and functional relationships among H₂-histamine receptors and H₁-histamine receptors in the mammalian heart. The role of both receptors in the regulation of force and rhythm, including their electrophysiological effects on the mammalian heart, will then be discussed in context. The potential clinical role of cardiac H₂-histamine-receptors in cardiac diseases will be examined. The use of H₂-histamine receptor agonists to acutely increase the force of contraction will be discussed. Special attention will be paid to the potential role of cardiac H₂-histamine receptors in the genesis of cardiac arrhythmias. Moreover, novel findings on the putative role of H₂-histamine receptor antagonists in treating chronic heart failure in animal models and patients will be reviewed. Some limitations in our biochemical understanding of the cardiac role of H₂-histamine receptors will be discussed. Recommendations for further basic and translational research on cardiac H₂-histamine receptors will be offered. We will speculate whether new knowledge might lead to novel roles of H₂-histamine receptors in cardiac disease and whether cardiomyocyte specific H₂-histamine receptor agonists and antagonists should be developed.

Regulation of the Cardiovascular System by Histamine

Histamine mediates a wide range of cellular responses, including allergic and inflammatory reactions, gastric acid secretion, and neurotransmission in the central nervous system. Histamine also exerts a series of actions upon the cardiovascular system but may not normally play a significant role in regulating cardiovascular function. During tissue injury, inflammation, and allergic responses, mast cells (or non-mast cells) within the tissues can release large amounts of histamine that leads to noticeable cardiovascular effects. Owing to intensive research during several decades, the distribution, function, and pathophysiological role of cardiovascular H₁- and H₂-receptors has become recognized adequately. Besides the recognized H₁- and H₂-receptor-mediated cardiovascular responses, novel roles of H₃- and H₄-receptors in cardiovascular physiology and pathophysiology have been identified over the last decade. In this review, we describe recent advances in our understanding of cardiovascular function and dysfunction mediated by histamine receptors, including H₃- and H₄-receptors, their potential mechanisms of action, and their pathological significance.

Molecular and cellular analysis of human histamine receptor subtypes

The human histamine receptors hH(1)R and hH(2)R constitute important drug targets, and hH(3)R and hH(4)R have substantial potential in this area. Considering the species-specificity of pharmacology of H(x)R orthologs, it is important to analyze hH(x)Rs. Here, we summarize current knowledge of hH(x)Rs endogenously expressed in human cells and hH(x)Rs recombinantly expressed in mammalian and insect cells. We present the advantages and disadvantages of the various systems. We also discuss problems associated with the use of hH(x)R antibodies, an issue of general relevance for G-protein-coupled receptors (GPCRs). There is much greater overlap in activity of 'selective' ligands for other hH(x)Rs than the cognate receptor subtype than generally appreciated. Studies with native and recombinant systems support the concept of ligand-specific receptor conformations, encompassing agonists and antagonists. It is emerging that for characterization of hH(x)R ligands, one cannot rely on a single test system and a single parameter. Rather, multiple systems and parameters have to be studied. Although such studies are time-consuming and expensive, ultimately, they will increase drug safety and efficacy.

H₂receptor-mediated positive inotropic effect of histamine in neonatal guinea-pig left atria

The receptor type mediating the positive inotropic effect of histamine was examined in left atria from neonatal guinea pigs. The positive inotropic effect of histamine, as well as its action potential prolonging effect, was antagonized by ranitidine, but not by chlorpheniramine or thiperamide. The positive inotropic effect was enhanced by isobutylmethylxantine. Receptor binding studies revealed the presence of both H₁ and H₂ receptor types. These results suggest that the positive inotropic effect of histamine in the neonatal guinea-pig atrium is mediated by H₂ receptors.

A comparison of the binding profiles of dextromethorphan, memantine, fluoxetine and amitriptyline: Treatment of involuntary emotional expression disorder

We compared the binding profiles of medications potentially useful in the treatment of involuntary emotional expression disorder at twenty-six binding sites in rat brain tissue membranes. Sites were chosen based on likelihood of being target sites for the mechanism of action of the agents in treating the disorder or their likelihood in producing side effects experienced by patients treated with psychoactive agents. We used radioligand binding assays employing the most selective labeled ligands available for sites of interest. Concentrations of labeled ligand were used at or below the K(i) value of the ligand for the target site. Compounds were initially screened at 1 muM. For compounds that competed for greater than 20-30% of specific binding at target sites of interest, full concentration curves were constructed. Dextromethorphan, amitriptyline and fluoxetine competed for binding to sigma(1) receptors and to serotonin transporters with high to moderate affinity. Of the target sites tested, these are the most likely to contribute to the therapeutic benefit of the various agents. In addition, all three drugs showed some activity at alpha(2) and 5-HT(1B/D) sites. Of the drugs tested, dextromethorphan bound to the fewest sites unlikely to be target sites. Although the mechanism of action of dextromethorphan or any drug that has been used in the treatment of involuntary emotional expression disorder is currently unknown, our data support that the affinity of the drug for sigma(1) receptors is consistent with its possible action through this receptor type in controlling symptoms of the disorder.

Mepyramine, a histamine H1 receptor inverse agonist, binds preferentially to a G protein-coupled form of the receptor and sequesters G protein

Accurate characterization of the molecular mechanisms of the action of ligands is an extremely important issue for their appropriate research, pharmacological, and therapeutic uses. In view of this fact, the aim of the present work was to investigate the mechanisms involved in the actions of mepyramine at the guinea pig H(1) receptor stably expressed in Chinese hamster ovary cells. We found that mepyramine is able to decrease the basal constitutive activity of the guinea pig H(1) receptor, to bind with high affinity to a G(q/11) protein-coupled form of the receptor and to promote a G protein-coupled inactive state of the H(1) receptor that interferes with the G(q/11)-mediated signaling of the endogenously expressed ATP receptor, as predicted by the Cubic Ternary Complex Model of receptor occupancy. The effect of mepyramine on ATP-induced signaling was specifically neutralized by Galpha(11) overexpression, indicating that mepyramine is able to reduce G protein availability for other non-related receptors associated with the same signaling pathway. Finally, we found a loss of mepyramine efficacy in decreasing basal levels of intracellular calcium at high Galpha(11) expression levels, which can be theoretically explained in terms of high H(1) receptor constitutive activity. The whole of the present work sheds new light on H(1) receptor pharmacology and the mechanisms H(1) receptor inverse agonists could use to exert their observed negative efficacy.

Histamine H1 and H2 receptor gene and protein levels are differentially expressed in the hearts of rodents and humans

Histamine is highly concentrated in the heart of animals and humans. Excessive release in pathophysiological conditions, such as immediate hypersensitivity and septic shock, causes cardiac dysfunction and arrhythmias. Previous pharmacological studies revealed that H(1) and H(2) receptors mediate these effects. Yet, an accurate estimate of the distribution and molecular characteristics of cardiac histamine receptors is missing. Recently, the genes encoding H(1) and H(2) receptors have been cloned, and the amino acid sequence and protein structure have been elucidated. Accordingly, we analyzed gene and protein expression levels of H(1) and H(2) receptors in atria and ventricles of guinea pig, rabbit, rat, and human hearts. With immunocytochemical techniques, we examined the regional expression of H(1) and H(2) receptor proteins in the sinoatrial and atrioventricular nodes and surrounding myocardium of the guinea pig heart. Northern and Western blot studies revealed that cardiac histamine H(1) and H(2) receptors are variably distributed among different mammalian species and different regions of the heart, whereas H(2) receptors are abundantly expressed in human atrial and ventricular myocardium. These findings agree with those of previous pharmacological studies, clearly demonstrating that the responses of the heart to histamine depend on the expression level of H(1) and H(2) receptors. The highly abundant expression of H(2) receptors in the human heart substantiates histamine arrhythmogenicity in various disease states. The new knowledge of a differential distribution of histamine receptor subtypes in the human heart will foster a better understanding of histamine roles in cardiovascular pathophysiology and may contribute to new therapeutic approaches to histamine-induced cardiac dysfunctions.

Histamine H(2) -like receptors in chick cerebral cortex: effects on cyclic AMP synthesis and characterization by [(3) H]tiotidine binding

In this study, histamine (HA) receptors in chick cerebral cortex were characterized using two approaches: (1) analysis of the effects of HA-ergic drugs on the cAMP-generating system, and (2) radioreceptor binding of [(3) H]tiotidine, a selective H(2) antagonist. HA was a weak activator of adenylyl cyclase in a crude membrane preparation of chick cerebrum. On the other hand, HA (0.1-1000 microm) potently and concentration dependently stimulated cAMP production in [(3) H]adenine pre-labelled slices of chick cerebral cortex, displaying an EC(50) value (concentration that produces 50% of maximum response) of 2.65 microm. The effect of HA was mimicked by agonists of HA receptors with the following rank order of potency: HA >or= 4-methylHA (H(2)) >or= N alpha,N alpha-dimethylHA (H(3) >> H(2) = H(1)) >> 2-methylHA (H(1)) >> 2-thiazolylethylamine (H(1)) >or= R alpha-methylHA (H(3)) >> amthamine, dimaprit (H(2)), immepip (H(3), H(4)). The HA-evoked increase in cAMP production in chick cerebral cortex was antagonized by selective H(2) receptor blockers (aminopotentidine >or= tiotidine > ranitidine >> zolantidine), and not significantly affected by mepyramine and thioperamide, selective H(1) and H(3) /H(4) receptor blockers, respectively. A detailed analysis of the antagonistic action of aminopotentidine (vs. HA) revealed a non-competitive mode of action. The binding of [(3) H]tiotidine to chick cortical membranes was rapid, stable and reversible. Saturation analysis resulted in a linear Scatchard plot, suggesting binding to a single class of receptor binding site with high affinity [equilibrium dissociation constant (K (d)) = 4.42 nm] and high capacity [maximum number of binding sites (B (max) ) = 362 fmol/mg protein]. The relative rank order of HA-ergic drugs to inhibit [(3) H]tiotidine binding to chick cerebrum was: antagonists - tiotidine >> aminopotentidine = ranitidine >or= zolantadine >> thioperamide - triprolidine; agonists - HA >or= 4-methylHA >> 2-methylHA >or=R alpha-methylHA - dimaprit. In conclusion, chick cerebral cortex contains H(2) -like HA receptors that are linked to the cAMP-generating system and are labelled with [(3) H]tiotidine. The pharmacological profile of these receptors is different from that described for their mammalian counterpart. It is suggested that the studied receptors represent either an avian-specific H(2) -like HA receptors or a novel subtype of HA receptors.

Validity of (-)-[3H]-CGP 12177A as a radioligand for the 'putative beta4-adrenoceptor' in rat atrium

1. We have recently suggested the existence in the heart of a 'putative beta4-adrenoceptor' based on the cardiostimulant effects of non-conventional partial agonists, compounds that cause cardiostimulant effects at greater concentrations than those required to block beta1- and beta2-adrenoceptors. We sought to obtain further evidence by establishing and validating a radioligand binding assay for this receptor with (-)-[3H]-CGP 12177A ([-]-4-(3-tertiarybutylamino-2-hydroxypropoxy) benzimidazol-2-one) in rat atrium. We investigated (-)-[3H]-CGP 12177A for this purpose for two reasons, because it is a non-conventional partial agonist and also because it is a hydrophilic radioligand. 2. Increasing concentrations of (-)-[3H]-CGP 12177A, in the absence or presence of 20 microM (-)-CGP 12177A to define non-specific binding, resulted in a biphasic saturation isotherm. Low concentrations bound to beta1- and beta2-adrenoceptors (pKD 9.4+/-0.1, Bmax 26.9+/-3.1 fmol mg(-1) protein) and higher concentrations bound to the 'putative beta4-adrenoceptor' (pKD 7.5+/-0.1, Bmax 47.7+/-4.9 fmol mg(-1) protein). In other experiments designed to exclude beta1- and beta2-adrenoceptors, (-)-[3H]-CGP 12177A (1-200 nM) binding in the presence of 500 nM (-)-propranolol was also saturable (pKD 7.6+/-0.1, Bmax 50.8+/-7.4 fmol mg(-1) protein). 3. The non-conventional partial agonists (-)-CGP 12177A (pKi 7.3+/-0.2), (+/-)-cyanopindolol (pKi 7.6+/-0.2), [-]-pindolol (pK1 6.6+/-0.1) and (+/-)-carazolol (pKi 7.2+/-0.2) and the antagonist (-)-bupranolol (pKi 6.6+/-0.2), all competed for (-)-[3H]-CGP 12177A binding in the presence of 500 nM (-)-propranolol at the 'putative beta4-adrenoceptor', with affinities closely similar to potencies and affinities determined in organ bath studies. 4. The catecholamines competed with (-)-[3H]-CGP 12177A at the 'putative beta 4-adrenoceptor' in a stereoselective manner, (-)-noradrenaline (pKiH 6.3+/-0.3, pKiL 3.5+/-0.1), (-)-adrenaline (pKiH 6.5+/-0.2, pKiL 2.9+/-0.1), (-)-isoprenaline (pKiH 6.2+/-0.5, pKiL 3.4+/-0.1), (+)-isoprenaline (pKi< 1.7), (-)-RO363 ((-)-(1-(3,4-dimethoxyphenethylamino)-3-(3,4-dihydroxyphenoxy++ +)-2-propranol)oxalate, pKi 5.5+/-0.1). 5. The inclusion of guanosine 5-triphosphate (GTP 0.1 mM) had no effect on binding of (-)-CGP 12177A or (-)-isoprenaline to the 'putative beta4-adrenoceptor'. In competition binding studies, (-)-CGP 12177A competed with (-)-[3H]-CGP 12177A for one receptor state in the absence (pKi 7.3+/-0.2) or presence of GTP (pKi 7.3+/-0.2). (-)-Isoprenaline competed with (-)-[3H]-CGP 12177A for two states in the absence (pKiH 6.6+/-0.3, pKiL 3.5+/-0.1; % H 25+/-7) or presence of GTP (pKiH 6.2+/-0.5, pKiL 3.4+/-0.1; % H 37+/-6). In contrast, at beta1-adrenoceptors, GTP stabilized the low affinity state of the receptor for (-)-isoprenaline. 6. The specificity of binding to the 'putative beta 4-adrenoceptor' was tested with compounds active at other receptors. High concentrations of the beta 3-adrenoceptor agonists, BRL 37344 ((RR+SS)[4-[2-[[2-(3-chlorophenyl)-2-hydroxy-ethyl]amino]propyl]phenoxy]acetic acid, 6 microM), SR 58611A (ethyl{(7S)-7-[(2R)-2-(3-chlorophenyl)-2-hydroxyethylamino]-5,6,7,8-tetrahydronaphtyl2-yloxy} acetate hydrochloride, 6 microM), ZD 2079 ((+/-)-1-phenyl-2-(2-4-carboxymethylphenoxy)-ethylamino)-ethan-1-ol, 60 microM), CL 316243 (disodium (R,R)-5-[2-[2-(3-chlorophenyl)-2-hydroxyethyl-amino]propyl]- 1,3-benzodioxole-2,2-dicarboxylate, 60 microM) and antagonist SR 59230A (3-(2-ethylphenoxy)-1-[(1S)-1,2,3,4-tetrahydronaphth-1-ylamino]-2S-2-propanol oxalate, 6 microM) caused less than 22% inhibition of (-)[3H]-CGP 12177A binding in the presence of 500 nM (-)propranolol. Histamine (1mM), atropine (1 microM), phentolamine (10 microM), 5-HT (100 microM) and the 5-HT4 receptor antagonist SB 207710 ((1-butyl-4-piperidinyl)-methyl 8-amino-7-iodo-1,4-benzodioxan-5-carboxylate, 10nM) caused less than 26% inhibition of binding. 7.Non-conventional partial agonists, the antagonist (-)bupranolol and catecholamines all competed for (-)[3H]-CGP 12177A binding in the absence of (-)propranolol at beta1-adrenoceptors, with affinities (pKi) ranging from 1.6-3.6 log orders greater than at the 'putative beta 4-adrenoceptor'. 8.We have established and validated a radioligand binding assay in rat atrium for the 'putative beta 4-adrenoceptor' which is distinct from beta1-, beta2- and beta 3-adrenoceptors. The stereoselective interaction with the catecholamines provides further support for the classification of the receptor as 'putative beta 4-adrenoceptor'.

Molecular pharmacological aspects of histamine receptors

In this article, we review the recent developments in the field of histamine research. Besides the description of pharmacological tools for the H1, H2 and H3 receptor, specific attention is paid to both the molecular aspects of the receptor proteins, including the recent cloning of the receptor genes, and their respective signal transduction mechanisms.

Characterization of the binding of the first selective radiolabelled histamine H3-receptor antagonist, [125I]-iodophenpropit, to rat brain

1. The binding of the first selective radiolabelled histamine H3-receptor antagonist [125I]-iodophenpropit to rat cerebral cortex membranes was characterized. 2. [125I]-iodophenpropit, radiolabelled to a high specific activity of 1900 Ci mmol-1, saturably bound to a single class of sites with a KD of 0.57 +/- 0.16 nM (n = 4) and Bmax of 268 +/- 119 fmol mg-1 protein. 3. Specific binding at a concentration below 1 nM represented 50 to 60% of total binding. 4. Binding of [125I]-iodophenpropit to rat cerebral cortex membranes was readily displaced by histamine H3-agonists and antagonists. In contrast, the inhibitory potencies of selective histamine H1- and H2-receptor ligands were very low. 5. [125I]-iodophenpropit was biphasically displaced by the histamine H3-receptor antagonists, burimamide and dimaprit, which may indicate the existence of histamine H3-receptor subtypes. Other histamine H3-receptor antagonists showed a monophasic displacement. 6. Competition binding curves of H3-agonists were biphasic and showed a rightward shift upon the addition of the nonhydrolysable GTP analogue, guanosine 5'-o-(3-thio) triphosphate (GTP gamma S; 100 microM) which implicates the interaction of histamine H3-receptors with G-proteins. The affinities of the H3-receptor antagonists iodophenpropit, thioperamide and burimamide were not altered by GTP gamma S. 7. Histamine competition binding curves were shifted to the right by different nucleotides (100 microM) with a rank order of potency GTP gamma S > Gpp(NH)p, GTP. 8 In vitro autoradiographic studies revealed a heterogeneous distribution of [125I]-iodophenpropitbinding sites in rat brain, with highest densities observed in specific cerebral cortical areas and layers,the caudate-putamen complex, the olfactory tubercles, the hippocampal formation, the amygdala complex, the hypothalamic area and the mammillary bodies.9 It is concluded that the histamine H3-receptor antagonist, [125I]-iodophenpropit, meets the criteria fo ra suitable radioligand for histamine H3-receptor binding studies in rat brain.

Stable expression of human H1-histamine-receptor cDNA in Chinese hamster ovary cells. Pharmacological characterisation of the protein, tissue distribution of messenger RNA and chromosomal localisation of the gene

A cDNA clone for the histamine H1 receptor was isolated from a human lung cDNA library; it encoded a protein of 487 amino acids which showed characteristic features of G-protein-coupled receptors. The percentages of identity of the deduced amino acid sequence with bovine, rat and guinea pig H1 histamine receptors were 82.6%, 79.4% and 73.3%, respectively, whereas these percentages decreased to 74.6%, 66% and 56.7% for the amino acid sequence of the third intracellular loop. The human H1-receptor cDNA was transfected into Chinese hamster ovary cells (CHO) via an eukaryotic expression vector; the receptor protein present on cell membranes specifically bound [3H]mepyramine with a Kd of 3.7 nM. The binding was displaced by H1-histamine-receptor antagonists and histamine. Northern blot analysis indicated the presence of two histamine H1 receptor mRNAs of 3.5 kb and 4.1 kb in various human tissues and an additional mRNA of 4.8 kb restricted to the human brain. Finally, by means of somatic cell hybrids segregating either human or rat chromosomes, the gene for histamine H1 receptor was found to reside on human chromosome 3 and rat chromosome 4.

Histamine receptors mediating a positive inotropic effect in guinea pig and rabbit ventricular myocardium: distribution of the receptors and their possible intracellular coupling processes

The difference in histamine receptor subtypes that are involved in the positive inotropic effect of histamine in guinea pig and rabbit ventricular myocardium was analytically characterized. In guinea pig papillary muscles, the positive inotropic effect of histamine was antagonized by cimetidine but not by mepyramine. The converse was true in rabbit papillary muscles. However, histamine evoked a positive inotropic effect through H1- and H2-receptors after blockade of H2- and H1-receptors in guinea pig and rabbit papillary muscles, respectively. Adenylate cyclase was significantly activated by histamine via H2-receptors in guinea pig but not in rabbit myocardial ventricular membranes. Accumulation of [3H]inositol monophosphate in ventricular strips prelabeled with myo-[3H]inositol was increased by histamine via H1-receptors to a similar extent in rabbits and guinea pigs. Radioligand binding experiments with [3H]mepyramine and [3H]tiotidine showed an increased number of H1-receptors and a decreased number of H2-receptors in guinea pig compared with rabbit ventricular myocardium. These results suggest that the positive inotropic effects of histamine are dominated by an H1-receptor-mediated effect in rabbits and by an H2-receptor-mediated one in guinea pig ventricular myocardium, and the positive inotropic effect manifested by one subtype apparently restricts the expression of the positive inotropic effect mediated by the other subtype. This species difference is not due to a difference in densities of the receptor subtypes, but may be partly related to a difference in the extents of coupling of H2-receptors to adenylate cyclase.

Histamine activates phosphorylase and inositol phosphate production in guinea pig hepatocytes

In guinea pig hepatocytes, histamine increased phosphorylase activity and inositol phosphate production. Similar effects were obtained with 2-(2-aminoethyl)-thiazole, a histamine H1 receptor agonist, but not with dimaprit or impromidine, H2 receptor agonists. These effects of histamine were dose-dependently inhibited by the H1 antihistamines, (+)-chlorpheniramine and mepyramine (pyrilamine) but not by cimetidine or ranitidine, H2 antagonists. (+)-Chlorpheniramine and mepyramine had similar potencies (apparent Ki values approximately 3 nM) when incubated with the cells for 1 min (phosphorylase a assays) but the former was 15-20-fold more potent than the latter at longer incubation times (apparent Ki values approximately 3-4 nM and 45-90 nM, respectively) indicating that mepyramine is actively metabolized by guinea pig hepatocytes. Histamine increased cytosol calcium approximately 2-fold, an effect also mediated through H1 receptors. The actions of histamine were not affected by in vivo ADP-ribosylation by pertussis toxin. Our data clearly indicate that histamine modulates the metabolism of guinea pig hepatocytes via activation of H1 receptors. These receptors are coupled to the phosphoinositide turnover-calcium mobilization signalling pathway through a pertussis toxin-insensitive process.