Regulation of the human histamine H1 receptor stably expressed in Chinese hamster ovary cells

Histamine Receptors: Ex Vivo Functional Studies Enabling the Discovery of Hits and Pathways

Histamine receptors (HRs) are G-protein-coupled receptors involved in diverse responses triggered by histamine release during inflammation or by encounters with venomous creatures. Four histamine receptors (H1R-H4R) have been cloned and extensively characterized. These receptors are distributed throughout the body and their activation is associated with clinical manifestations such as urticaria (H1R), gastric acid stimulation (H2R), regulation of neurotransmitters in neuronal diseases (H3R), and immune responses (H4R). Despite significant homologous overlap between H3R and H4R, much remains unknown about their precise roles. Even though some drugs have been developed for H1R, H2R, and H3R, not a single H4R antagonist has been approved for clinical use. To enhance our understanding and advance innovative therapeutic targeting of H1R, H2R, H3R, and H4R, we established a robust ex vivo functional platform. This platform features the successful heterologous expression of H1R-H4R in Xenopus laevis oocytes, utilizing an electrophysiological readout. Our findings contribute to a deeper understanding of the function and pharmacological properties of the histamine receptors. Researchers can benefit from the utility of this platform when investigating the effects of histamine receptors and exploring potential therapeutic targets. In doing so, it broadens the horizon of drug discovery, offering new perspectives for therapeutic interventions.

HBK-15, a Multimodal Compound, Showed an Anxiolytic-Like Effect in Rats

Anxiety is a common mental disorder, and its prevalence has lately increased because of the COVID-19 pandemic. Unfortunately, the available anxiolytics are often ineffective, and most possess addictive potential. Thus, searching for novel compounds is essential. In our previous studies, we selected a multimodal compound, HBK-15, which showed a fast antidepressant-like effect in animal models of depression. HBK-15 demonstrated a high affinity for serotonin 5-HT_1A receptors and moderate for 5-HT₇, dopamine D₂, and α₁-adrenoceptors. Based on the receptor profile and preliminary studies, we aimed to investigate the anxiolytic potential of HBK-15 using the conditioned-response rat model of anxiety, i.e., the Vogel drinking test. We performed hot plate and free-drinking tests to exclude false positive results in the Vogel test. Using radioligand binding studies, we also investigated the affinity of the compound for the selected biological targets, which play a role in anxiety. Our experiments revealed that HBK-15 showed an anxiolytic-like effect in rats (5 mg/kg) without influencing the pain threshold or the amount of water consumed in the free-drinking test. Furthermore, the tested compound did not show a significant affinity for the selected biological targets, which suggests that its anxiolytic-like mechanism of action could be connected with the interaction with other receptors. This study indicates that multimodal compounds with a receptor profile similar to HBK-15 could be an attractive therapeutic option for patients with a generalized anxiety disorder. However, more studies are required to determine the exact mechanism of action of HBK-15 and its safety profile.

The Novel Pimavanserin Derivative ST-2300 with Histamine H3 Receptor Affinity Shows Reduced 5-HT2A Binding, but Maintains Antidepressant- and Anxiolytic-like Properties in Mice

The therapy of depression is challenging and still unsatisfactory despite the presence of many antidepressant drugs on the market. Consequently, there is a continuous need to search for new, safer, and more effective antidepressant therapeutics. Previous studies have suggested a potential association of brain histaminergic/serotoninergic signaling and antidepressant- and anxiolytic-like effects. Here, we evaluated the in vivo antidepressant- and anxiolytic-like effects of the newly developed multiple-active ligand ST-2300. ST-2300 was developed from 5-HT_2A/2C inverse agonist pimavanserin (PIM, ACP-103) and incorporates a histamine H₃ receptor (H₃R) antagonist pharmacophore. Despite its parent compound, ST-2300 showed only moderate serotonin 5-HT_2A antagonist/inverse agonist affinity (K_i value of 1302 nM), but excellent H₃R affinity (K_i value of 14 nM). In vivo effects were examined using forced swim test (FST), tail suspension test (TST), and the open field test (OFT) in C57BL/6 mice. Unlike PIM, ST-2300 significantly increased the anxiolytic-like effects in OFT without altering general motor activity. In FST and TST, ST-2300 was able to reduce immobility time similar to fluoxetine (FLX), a recognized antidepressant drug. Importantly, pretreatment with the CNS-penetrant H₃R agonist (R)-α-methylhistamine reversed the antidepressant-like effects of ST-2300 in FST and TST, but failed to reverse the ST-2300-provided anxiolytic effects in OFT. Present findings reveal critical structural features that are useful in a rational multiple-pharmacological approach to target H₃R/5-HT_2A/5-HT_2C.

Molecular Signaling and Transcriptional Regulation of Histamine H1 Receptor Gene

Histamine-activated histamine H₁ receptor (H1R) signaling regulates many gene expressions, mainly through the protein kinase C (PKC)/extracellular signal-regulated kinases (ERK) signaling. Involvement of other signaling, including NF-κB, Wnt, RUNX-2, and Rho A signaling was also demonstrated. In addition, cAMP production through the activation of H1R signaling was reported. H1R gene itself is also up-regulated by the activation of H1R signaling with histamine. Here, we review our recent findings in the molecular signaling and transcriptional regulation of the H1R gene. Stimulation with histamine up-regulates H1R gene expression through the activation of H1R in HeLa cells. The PKCδ/ERK/poly(ADP)ribosyl transferase-1 (PARP-1) signaling was involved in this up-regulation. Heat shock protein 90 also plays an important role in regulating PKCδ translocation. Promoter analyses revealed the existence of two promoters in the human H1R gene in HeLa cells. H1R-activated H1R gene up-regulation in response to histamine was also observed in U373 astroglioma cells. However, this up-regulation was mediated not through the PKCδ signaling but possibly through the PKCα signaling. In addition, the promoter region responsible for histamine-induced H1R gene transcription in U373 cells was different from that of HeLa cells. These findings suggest that the molecular signaling and transcriptional regulation of the H1R gene are different between neuronal cells and non-neuronal cells.

Reducing Auditory Nerve Excitability by Acute Antagonism of Ca2+-Permeable AMPA Receptors

Hearing depends on glutamatergic synaptic transmission mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). AMPARs are tetramers, where inclusion of the GluA2 subunit reduces overall channel conductance and Ca²⁺ permeability. Cochlear afferent synapses between inner hair cells (IHCs) and auditory nerve fibers (ANFs) contain the AMPAR subunits GluA2, 3, and 4. However, the tetrameric complement of cochlear AMPAR subunits is not known. It was recently shown in mice that chronic intracochlear delivery of IEM-1460, an antagonist selective for GluA2-lacking AMPARs [also known as Ca²⁺-permeable AMPARs (CP-AMPARs)], before, during, and after acoustic overexposure prevented both the trauma to ANF synapses and the ensuing reduction of cochlear nerve activity in response to sound. Surprisingly, baseline measurements of cochlear function before exposure were unaffected by chronic intracochlear delivery of IEM-1460. This suggested that cochlear afferent synapses contain GluA2-lacking CP-AMPARs alongside GluA2-containing Ca²⁺-impermeable AMPA receptors (CI-AMPARs), and that the former can be antagonized for protection while the latter remain conductive. Here, we investigated hearing function in the guinea pig during acute local or systemic delivery of CP-AMPAR antagonists. Acute intracochlear delivery of IEM-1460 or systemic delivery of IEM-1460 or IEM-1925 reduced the amplitude of the ANF compound action potential (CAP) significantly, for all tone levels and frequencies, by > 50% without affecting CAP thresholds or distortion product otoacoustic emissions (DPOAE). Following systemic dosing, IEM-1460 levels in cochlear perilymph were ~ 30% of blood levels, on average, consistent with pharmacokinetic properties predicting permeation of the compounds into the brain and ear. Both compounds were metabolically stable with half-lives >5 h in vitro, and elimination half-lives in vivo of 118 min (IEM-1460) and 68 min (IEM-1925). Heart rate monitoring and off-target binding assays suggest an enhanced safety profile for IEM-1925 over IEM-1460. Compound potency on CAP reduction (IC₅₀ ~ 73 μM IEM-1460) was consistent with a mixture of GluA2-lacking and GluA2-containing AMPARs. These data strongly imply that cochlear afferent synapses of the guinea pig contain GluA2-lacking CP-AMPARs. We propose these CP-AMPARs may be acutely antagonized with systemic dosing, to protect from glutamate excitotoxicity, while transmission at GluA2-containing AMPARs persists to mediate hearing during the protection.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

Cherry-picked ligands at histamine receptor subtypes

Histamine, a biogenic amine, is considered as a principle mediator of multiple physiological effects through binding to its H1, H2, H3, and H4 receptors (H1-H4Rs). Currently, the HRs have gained attention as important targets for the treatment of several diseases and disorders ranging from allergy to Alzheimer's disease and immune deficiency. Accordingly, medicinal chemistry studies exploring histamine-like molecules and their physicochemical properties by binding and interacting with the four HRs has led to the development of a diversity of agonists and antagonists that display selectivity for each HR subtype. An overview on H1-R4Rs and developed ligands representing some key steps in development is provided here combined with a short description of structure-activity relationships for each class. Main chemical diversities, pharmacophores, and pharmacological profiles of most innovative H1-H4R agonists and antagonists are highlighted. Therefore, this overview should support the rational choice for the optimal ligand selection based on affinity, selectivity and efficacy data in biochemical and pharmacological studies. This article is part of the Special Issue entitled 'Histamine Receptors'.

Development of Fluorine-18 Labeled Metabolically Activated Tracers for Imaging of Drug Efflux Transporters with Positron Emission Tomography

Increased activity of efflux transporters, e.g., P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), at the blood-brain barrier is a pathological hallmark of many neurological diseases, and the resulting multiple drug resistance represents a major clinical challenge. Noninvasive imaging of transporter activity can help to clarify the underlying mechanisms of drug resistance and facilitate diagnosis, patient stratification, and treatment monitoring. We have developed a metabolically activated radiotracer for functional imaging of P-gp/BCRP activity with positron emission tomography (PET). In preclinical studies, the tracer showed excellent initial brain uptake and clean conversion to the desired metabolite, although at a sluggish rate. Blocking with P-gp/BCRP modulators led to increased levels of brain radioactivity; however, dynamic PET did not show differential clearance rates between treatment and control groups. Our results provide proof-of-concept for development of prodrug tracers for imaging of P-gp/BCRP function in vivo but also highlight some challenges associated with this strategy.

The affinity of histamine for Gq protein-coupled histamine H(1)-receptors is predominantly regulated by their internalization in human astrocytoma cells

We examined the regulatory mechanisms of the affinity of Gq protein-coupled histamine H(1)-receptors for histamine after histamine pretreatment in intact human U373 MG astrocytoma cells. In control cells, the displacement curves for histamine against the binding of 5 nM [(3)H]mepyramine, a radioligand for H(1)-receptors, showed the presence of two binding sites for histamine, that is, high and low affinity sites. Pretreatment with 0.1 mM histamine for 30 min at 37°C induced a significant reduction in the percentage of high affinity sites for histamine and a concomitant increase in the percentage of low affinity sites with no change in their pIC(50) values. These histamine-induced changes were insensitive to 30 µM KN-62, an inhibitor of Ca(2+)/calmodulin-dependent protein kinase II, but they were completely inhibited either by 0.4 mM ZnCl(2), an inhibitor of G protein-coupled receptor kinases (GRKs), or under hypertonic conditions, where clathrin-mediated endocytosis is known to be inhibited. These results suggest that histamine-induced conversion of high to low affinity sites for histamine is predominantly regulated by GRK/clathrin-mediated internalization of H(1)-receptors in human astrocytoma cells.

Pharmacological characterization of GSK1004723, a novel, long-acting antagonist at histamine H(1) and H(3) receptors

BACKGROUND AND PURPOSE

Preclinical pharmacological characterization of GSK1004723, a novel, dual histamine H(1) and H(3) receptor antagonist.

EXPERIMENTAL APPROACH

GSK1004723 was characterized in vitro and in vivo using methods that included radioligand binding, intracellular calcium mobilization, cAMP production, GTPγS binding, superfused human bronchus and guinea pig whole body plethysmography.

KEY RESULTS

In cell membranes over-expressing human recombinant H(1) and H(3) receptors, GSK1004723 displayed high affinity, competitive binding (H(1) pKi = 10.2; H(3) pKi = 10.6). In addition, GSK1004723 demonstrated slow dissociation from both receptors with a t(1/2) of 1.2 and 1.5 h for H(1) and H(3) respectively. GSK1004723 specifically antagonized H(1) receptor mediated increases in intracellular calcium and H(3) receptor mediated increases in GTPγS binding. The antagonism exerted was retained after cell washing, consistent with slow dissociation from H(1) and H(3) receptors. Duration of action was further evaluated using superfused human bronchus preparations. GSK1004723 (100 nmol·L(-1) ) reversed an established contractile response to histamine. When GSK1004723 was removed from the perfusate, only 20% recovery of the histamine response was observed over 10 h. Moreover, 21 h post-exposure to GSK1004723 there remained almost complete antagonism of responses to histamine. In vivo pharmacology was studied in conscious guinea pigs in which nasal congestion induced by intranasal histamine was measured indirectly (plethysmography). GSK1004723 (0.1 and 1 mg·mL(-1) intranasal) antagonized the histamine-induced response with a duration of up to 72 h.

CONCLUSIONS AND IMPLICATIONS

GSK1004723 is a potent and selective histamine H(1) and H(3) receptor antagonist with a long duration of action and represents a potential novel therapy for allergic rhinitis.

In vitro characterisation of the duration of action of the histamine-1 receptor antagonist azelastine

Azelastine is a selective antagonist at the human histamine-1 receptor and is used clinically in the treatment of allergic rhinitis. In this study we have investigated its duration of action in vitro in an effort to characterise the receptor and tissue components involved. Chinese hamster ovary cell membrane fragments were used to determine the kinetics of azelastine at the H₁ receptor in a radioligand binding assay. Further duration of action studies were completed in tissue preparations using guinea-pig trachea and human bronchus. In radioligand binding studies, azelastine reached steady state at the H₁ receptor after approximately 41 min and exhibited a significantly slower dissociation rate constant from the receptor than the first generation antihistamine, diphenhydramine. In washout studies completed in guinea-pig and human airway in vitro tissue preparations, azelastine continued to antagonise the effects of histamine at the H₁ receptor for at least 18 h post-washout of the antagonist. This outcome was reversed following removal of the epithelium from guinea-pig isolated tracheal strips. These studies indicate there is a tissue component contributing to azelastine's duration of action, in addition to its direct H₁ receptor binding, with evidence suggesting a role for the epithelial layer.

Agonist-promoted homologous desensitization of human airway smooth muscle bitter taste receptors

Bitter taste receptors (TAS2Rs) were shown to be expressed in human airway smooth muscle (ASM). They couple to specialized [Ca(2+)](i) release, leading to membrane hyperpolarization, the relaxation of ASM, and marked bronchodilation. TAS2Rs are G-protein-coupled receptors, known to undergo rapid agonist-promoted desensitization that can limit therapeutic efficacy. Because TAS2Rs represent a new drug target for treating obstructive lung disease, we investigated their capacity for rapid desensitization, and assessed their potential mechanisms. The pretreatment of human ASM cells with the prototypic TAS2R agonist quinine resulted in a 31% ± 5.1% desensitization of the [Ca(2+)](i) response from a subsequent exposure to quinine. No significant change in the endothelin-stimulated [Ca(2+)](i) response was attributed to the short-term use of quinine, indicating a homologous form of desensitization. The TAS2R agonist saccharin also evoked desensitization, and cross-compound desensitization with quinine was evident. Desensitization of the [Ca(2+)](i) response was attenuated by a dynamin inhibitor, suggesting that receptor internalization (a G-protein coupled receptor kinase [GRK]-mediated, β-arrestin-mediated process) plays an integral role in the desensitization of TAS2R. Desensitization was insensitive to antagonists of the second messenger kinases protein kinase A and protein kinase C. Using intact airways, short-term, agonist-promoted TAS2R desensitization of the relaxation response was also observed. Thus these receptors, which represent a potential novel target for direct bronchodilators, undergo a modest degree of agonist-promoted desensitization that may affect clinical efficacy. Collectively, the results of these mechanistic studies, along with the multiple serines and threonines in intracellular loop 3 and the cytoplasmic tail of TAS2Rs, suggest a GRK-mediated mode of desensitization.

The antinociceptive and anti-inflammatory action of the CHCl3-soluble phase and its main active component, damnacanthal, isolated from the root of Morinda citrifolia

Morinda citrifolia (Rubiaceae, Noni) is a traditional medicine with various pharmacological activities. We investigated if the MeOH-, CHCl(3)- and BuOH-soluble phase and its main active component, damnacanthal, isolated from the Noni root, have antinociceptive and anti-inflammatory actions in mice. The CHCl(3)-soluble phase (3 g/kg, per os (p.o.)) significantly reduced pain-related behavior observed in the formalin test. These effects were not suppressed by pretreatment with naloxone (1 mg/kg, intraperitoneally (i.p.)), an opioid receptor antagonist. The CHCl(3)-soluble phase (3 g/kg, p.o.) significantly reduced histamine-induced paw edema. The MeOH- and BuOH-soluble phase had no effect in either test. Furthermore, damnacanthal (10-100 mg/kg, p.o.) exerted an antinociceptive effect on chemical nociceptive stimuli, and decreased histamine-induced paw edema. Damnacanthal was weakly bound to the histamine H(1) receptor. These data suggest that the CHCl(3)-soluble phase of the Noni root has antinociceptive and anti-inflammatory effects. Furthermore, these effects of damnacanthal isolated from the Noni root is mediated in part by the histamine H(1) receptor.

Potential utility of histamine H3 receptor antagonist pharmacophore in antipsychotics

Histamine H3 receptor (H3R) antagonists have some antipsychotic properties although the clear molecular mechanism is still unknown. As actually the most effective and less side effective antipsychotics are drugs with multiple targets we have designed typical and atypical neuroleptics with an additional histamine H3 pharmacophore. The 4-(3-piperidinopropoxy)phenyl pharmacophore moiety has been linked to amitriptyline, maprotiline, chlorpromazine, chlorprothixene, fluphenazine, and clozapine. Amide, amine and ester elements have been used generally to maintain or slightly shift affinity at dopamine D(2)-like receptors (D2 and D3), to decrease affinity at histamine H(1) receptors, and to obtain H3R ligands with low nanomolar or subnanomolar affinity. Change of effects at D(1)-like receptors (D1) and (D5) were heterogeneous. With these newly profiled compounds different antipsychotic properties might be achieved.

Progress in allergy signal research on mast cells: up-regulation of histamine signal-related gene expression in allergy model rats

Brown Norway allergy model rats sensitized to toluene 2,4-diisocyanate (TDI) were developed. Histamine H(1) receptor mRNA level was elevated in nasal mucosa of allergy model rats after the provocation with TDI, which was followed by H(1)-receptor up-regulation. Elevation of histamine H(1) receptor mRNA was partially suppressed by d-chlorpheniramine and olopatadine, antihistamines. Histamine induced increase in histamine H(1) receptor gene expression in vitro, and the protein kinase C-delta isoform was suggested to mediate the gene expression. On the other hand, elevation of histamine H(1) receptor mRNA was completely suppressed by dexamethasone in allergy model rats. Provocation with TDI also induced mRNA elevation of histidine decarboxylase, a sole histamine-forming enzyme, followed by the increase of both HDC activity and histamine content in nasal mucosa of allergy model rats. HDC mRNA elevation and increase in both HDC activity and histamine level were almost completely suppressed by dexamethasone. These observations suggest that histamine H(1) receptor up-regulation and increase in histamine level play an important role in allergy through the regulation of histamine signaling.

Histamine receptor H1 is required for TCR-mediated p38 MAPK activation and optimal IFN-gamma production in mice

Histamine receptor H1 (H1R) is a susceptibility gene in both experimental autoimmune encephalomyelitis (EAE) and experimental autoimmune orchitis (EAO), 2 classical T cell-mediated models of organ-specific autoimmune disease. Here we showed that expression of H1R in naive CD4+ T cells was required for maximal IFN-gamma production but was dispensable for proliferation. Moreover, H1R signaling at the time of TCR ligation was required for activation of p38 MAPK, a known regulator of IFN-gamma expression. Importantly, selective reexpression of H1R in CD4+ T cells fully complemented both the IFN-gamma production and the EAE susceptibility of H1R-deficient mice. These data suggest that the presence of H1R in CD4+ T cells and its interaction with histamine regulates early TCR signals that lead to Th1 differentiation and autoimmune disease.

[Role of therapeutics for allergic diseases in targeting histamine H1 receptor gene expression]

Histamine H(1) receptors are down-regulated as one step in receptor desensitization. Five phosphorylation sites of the H(1) receptor seem to play a key role in receptor down-regulation. In contrast, an increase in the H(1) receptor expression level following its mRNA elevation was found in the nasal mucosa in hypersensitivity model rats. Up-regulation of the H(1) receptor was induced by the direct stimulation of the H(1) receptor. H(1) receptor up-regulation was suppressed by pretreatment with antiallergic agents.

The histamine H1 receptor activates the nitric oxide pathway at fertilization

Sperm fusion with the egg initiates a signaling cascade that releases intracellular calcium (Ca(i) (2+)) from the endoplasmic reticulum (ER). In sea urchins, Ca(2+) is released as a single, large transient via two distinct pathways. The first depends on inositol 1,4,5-triphosphate (IP(3)) production and triggers the initial phase of Ca(2+) release, while the second depends on nitric oxide (NO) production and is thought to maintain the duration of the Ca(2+) wave. We identified a sea urchin homolog of the seven trans-membrane G protein-coupled receptor for histamine (suH(1)R) on the egg cell surface that activates NO production. Treatment with histamine (HA) causes fluctuations in the resting levels of NO in the egg, while antagonists or antibodies of H(1)R inhibit the rise of NO normally observed at fertilization. Inhibition of suH(1)R function decreases the maintenance, but not the amplitude, of the Ca(2+) transient and suggests that it is an integral part of the overall pathway leading to egg activation at fertilization in sea urchins.

Recent advances in molecular pharmacology of the histamine systems: regulation of histamine H1 receptor signaling by changing its expression level

Histamine H1 receptor (H1R) signaling is regulated by changing its expression level. Two mechanisms are involved in this regulation. One is down-regulation through receptor desensitization. Receptor phosphorylation seemed crucial because stimulation of the mutant H1R lacking five putative phosphorylation sites did not show down-regulation. The phosphorylation level of the mutant receptor was much smaller than that of the wild type ones by several protein kinases. The other is up-regulation through activation of receptor gene expression. Protein kinase C (PKC) signaling was suggested to be involved in this up-regulation. Regulation of H1R expression level was mediated not only through H1R but also autonomic nerve receptors. Stimulation of M3 muscarinic receptors (M3R) induced both down-regulation and up-regulation of H1R. Down-regulation of M3R-mediated H1R seemed not to be mediated by PKC activation, although PKC activation induced H1R phosphorylation. Elevation of H1R expression was induced by the stimulation of M3Rs. PKC was suggested to be involved in this up-regulation. Stimulation of beta2-adrenergic receptors induced H1R down-regulation through several mechanisms. One of them is enhanced receptor degradation after desensitization and another is suppression of receptor synthesis that includes the suppression of receptor gene expression and enhanced degradation of the receptor mRNA. Protein kinase A was suggested to be involved in enhanced degradation and the activation of the receptor gene expression. Elevation of both H1R expression and its mRNA was observed in nasal mucosa of nasal hypersensitivity allergy model rat after toluene diisocyanate provocation. These results suggest that activation of H1R gene expression plays an important patho-physiological role in allergy. Elevation of the mRNA was partially but significantly suppressed by antihistamines.

Preclinical Pharmacology of Bilastine,??a New Selective Histamine??H1 ??Receptor Antagonist

OBJECTIVE

This study aimed to establish the receptor selectivity and antihistaminic activity of bilastine, a new selective antihistamine receptor antagonist.

DESIGN AND METHODS

In vitro experiments were conducted using a receptor binding screening panel and guinea-pig and rat tissues. Antihistaminic activity was determined using H1 receptor binding studies and in vitro H1 antagonism studies conducted in guinea-pig tissues and human cell lines. Receptor selectivity was established using a receptor binding screening panel and a receptor antagonism screening conducted in guinea-pig, rat and rabbit tissues. Inhibition of inflammatory mediators was determined through the Schultz-Dale reaction in sensitised guinea-pig ileum.

RESULTS

Bilastine binds to histamine H1-receptors as indicated by its displacement of [3H]-pyrilamine from H1-receptors expressed in guinea-pig cerebellum and human embryonic kidney (HEK) cell lines. The studies conducted on guinea-pig smooth muscle demonstrated the capability of bilastine to antagonise H1-receptors. Bilastine is selective for histamine H1-receptors as shown in receptor-binding screening conducted to determine the binding capacity of bilastine to 30 different receptors. The specificity of its H1-receptor antagonistic activity was also demonstrated in a series of in vitro experiments conducted on guinea-pig and rat tissues. The results of these studies confirmed the lack of significant antagonism against serotonin, bradykinin, leukotriene D4, calcium, muscarinic M3-receptors, alpha1-adrenoceptors, beta2-adrenoceptors, and H2- and H3-receptors. The results of the in vitro Schultz-Dale reaction demonstrated that bilastine also has anti-inflammatory activity.

CONCLUSIONS

These preclinical studies provide evidence that bilastine has H1- antihistamine activity, with high specificity for H1-receptors, and poor or no affinity for other receptors. Bilastine has also been shown to have anti-inflammatory properties.

Homologous and heterologous phosphorylations of human histamine H1 receptor in intact cells

Homologous and heterologous phosphorylations of histamine H1 receptor (H1R) in intact cells were investigated using Chinese hamster ovary cells stably co-expressing c-myc-tagged human histamine H1 and muscarinic M3 receptors. Increase in histamine-induced homologous phosphorylation of H1R was induced in a dose- and time-dependent manner. Maximum phosphorylation of H1R by 8-fold over the basal level was induced 1 min after the stimulation, and the increased phosphorylation level was maintained over 40 min. M3 receptor-mediated heterologous phosphorylation of H1R reached maximum by 2-fold over the basal level at 5 min after the stimulation and then rapidly returned to the basal level by 40 min after the stimulation. Histamine-induced phosphorylation of H1R was partially inhibited by three protein kinase inhibitors including Ro-31-8220 for protein kinase C (PKC), KN-93 for calcium/calmodulin-dependent kinase II (CaMKII), and KT5823 for protein kinase G (PKG), while, M3-receptor-mediated phosphorylation of H1R was completely inhibited by Ro 31-8220. Protein kinase activators including phorbol 12-myristate 13-acetate (PMA), 8-bromo-cyclic GMP (8-Br-cGMP), and 8-bromo-cyclic AMP (8-Br-cAMP) induced increases in H1R phosphorylation. Increased phosphorylation of H1R, by 5-fold over the basal level, induced with a combination of PMA, 8-Br-cGMP, and 8-Br-cAMP was still lower than that with histamine. It was suggested that H1R-mediated H1R phosphorylation involves the activation of PKC, CaMKII, PKG, and other unidentified kinases including G-protein coupled receptor kinases (GRKs) and that PKC is solely involved in M3 receptor-mediated H1R phosphorylation.

Bimodal regulation of the human H1 histamine receptor by G protein-coupled receptor kinase 2

The H1 histamine receptor (H1HR) is a member of the G protein-coupled receptor superfamily and regulates numerous cellular functions through its activation of the G(q/11) subfamily of heterotrimeric G proteins. Although the H1HR has been shown to undergo desensitization in multiple cell types, the mechanisms underlying the regulation of H1HR signaling are poorly defined. To address this issue, we examined the effects of wild type and mutant G protein-coupled receptor kinases (GRKs) on the phosphorylation and signaling of human H1HR in HEK293 cells. Overexpression of GRK2 promoted H1HR phosphorylation in intact HEK293 cells and completely inhibited inositol phosphate production stimulated by H1HR, whereas GRK5 and GRK6 had lesser effects on H1HR phosphorylation and signaling. Interestingly, catalytically inactive GRK2 (GRK2-K220R) also significantly attenuated H1HR-mediated inositol phosphate production, as did an N-terminal fragment of GRK2 previously characterized as a regulator of G protein signaling (RGS) protein for Galpha(q/11). Disruption of this RGS function in holo-GRK2 by mutation (GRK2-D110A) partially reversed the quenching effect of GRK2, whereas deletion of both the kinase activity and RGS function (GRK2-D110A/K220R) effectively relieved the inhibition of inositol phosphate generation. To evaluate the role of endogenous GRKs on H1HR regulation, we used small interfering RNAs to selectively target GRK2 and GRK5, two of the primary GRKs expressed in HEK293 cells. A GRK2-specific small interfering RNA effectively reduced GRK2 expression and resulted in a significant increase in histamine-promoted calcium flux. In contrast, knockdown of GRK5 expression was without effect on H1HR signaling. These findings demonstrate that GRK2 is the principal kinase mediating H1 histamine receptor desensitization in HEK293 cells and suggest that rapid termination of H1HR signaling is mediated by both the kinase activity and RGS function of GRK2.

Two threonine residues and two serine residues in the second and third intracellular loops are both involved in histamine H1receptor downregulation

Human histamine H1 receptor (H1R) contains five possible phosphorylation residues (Thr140, Thr142, Ser396, Ser398 and Thr478) and the substitution of all these five residues to alanine completely impairs agonist-induced receptor downregulation. In the present study, to determine which residue(s) are responsible for receptor downregulation, we used mutant H1Rs in which single or multiple residues were substituted with alanine. The results suggested that two groups, i.e., residues Thr140 and Thr142, and residues Ser396 and Ser398, independently contributed to H1R downregulation. Thr140 and Ser398 mainly contributed to downregulation, and Thr142 or Ser396 had a slight inhibitory effect on Thr140- or Ser398-mediated process, respectively.

Identification of Amino Acid Residues Responsible for Agonist-Induced Down-Regulation of Histamine H1 Receptors

The histamine H(1) receptor (H1R) level is dynamically regulated in vivo under various physiological and pathological conditions. The H1R regulation may consist of various processes, and this study focused on the process of receptor trafficking, that is, receptor internalization to endosomes and the following receptor degradation. First, we identified five possible phosphorylation residues of human H1R, Thr(140), Thr(142), Ser(396), Ser(398), and Thr(478), based on in vitro phosphorylation studies. Then to determine the role of these residues, we constructed a mutant H1R in which all of these five residues were substituted with alanine. Both wild-type and the mutant receptors expressed in Chinese hamster ovary (CHO) cells had similar values of K(d) for [(3)H]mepyramine binding and K(i) for histamine, and these cells showed similar levels of histamine-stimulated inositol phosphate formation. Both types of H1Rs were internalized essentially in the same way upon stimulation with histamine (100 microM) for 30 min. However, down-regulation of the mutant H1R was completely impaired, whereas that of wild-type H1R occurred by approximately 60% by the treatment with 100 microM histamine for 24 h. These results suggest that these residues are responsible for receptor down-regulation but not for receptor internalization. Possibly, phosphorylation of the residues is required for receptor transport from endosomes to lysosomes.

Histamine H1 Receptor Down-Regulation Mediated by M3 Muscarinic Acetylcholine Receptor Subtype

Heterologous down-regulation of histamine H(1) receptor (H1R) mediated by muscarinic acetylcholine receptor subtype was investigated using five kinds of Chinese hamster ovary (CHO) cells stably co-expressing the human H1R and one of the five (M(1) - M(5)) muscarinic acetylcholine receptors, CHO-H1/M1, CHO-H1/M2, CHO-H1/M3, CHO-H1/M4, and CHO-H1/M5 cells. Among the CHO-H1/M1, CHO-H1/M3, and CHO-H1/M5 cells, carbachol treatment of the CHO-H1/M3 cells time-dependently led to remarkable down-regulation of the H1R to 60% of the control level. In contrast, stimulation of CHO-H1/M1 cells by carbachol induced negligible effect on the down-regulation. Stimulation of CHO-H1/M5 cells by carbachol induced significant but only small H1R down-regulation. M(2) and M(4) muscarinic receptors showed negligible effect on the down-regulation. H1R-mediated accumulation of inositol phosphates in CHO-H1/M3 cells with long-term expose to carbachol was decreased to 60% compared with non-treated cells. Heterologous phosphorylation of H1R was induced by the stimulation of each muscarinic receptor. H1R was phosphorylated by about twofold from the basal level through five subtypes of muscarinic receptor. The M(3) muscarinic receptor-mediated phosphorylation of H1R was reversed by the inhibition of protein kinase C. In the present study we demonstrated that the M(3) muscarinic acetylcholine receptor mediated remarkable down-regulation of the H1R with decreased receptor signaling.

A functional assay for G-protein-coupled receptors using stably transformed insect tissue culture cell lines

Insect cells are an underexplored resource for functional G-protein-coupled receptor (GPCR) assays, despite a strong record in biochemical (binding) assays. Here we describe the use of vectors capable of creating stably transformed insect cell lines to generate a cell-based functional GPCR assay. This assay employs the luminescent photoprotein aequorin and the promiscuous G-protein subunit Galpha16 and is broadly applicable to human GPCRs. We demonstrate that the assay can quantitate ligand concentration-activity relationships for seven different human GPCRs, can differentiate between partial and full agonists, and can determine rank order potencies for both agonists and antagonists that match those seen with other assay systems. Human Galpha16 improves signal strength but is not required for activity with some receptors. The coexpression of human and bovine betagamma subunits and/or phospholipase Cbeta makes no difference to agonist efficacy or potency. Two different receptors expressed in the same cell line respond to their specific agonists, and two different cell lines (Sf9 and High 5) are able to functionally detect the same expressed GPCR. Sf9 cells have the capability to produce fully functional human receptors, allied to a low background of endogenous receptors, and so are a valuable system for investigating orphan GPCRs and receptor dimerization.

Application of thin-layer chromatographic data in quantitative structure-activity relationship assay of thiazole and benzothiazole derivatives with H1-antihistamine activity. I

A quantitative structure-activity relationship analysis of H1-antihistamine activity and chromatographic data of 2-[2-(phenylamino)thiazol-4-yl]ethanamine; 2-(2-benzyl-4-thiazolyl)ethanamine; 2-(2-benzhydrylthiazol-4-yl)ethylamine derivative; 2-(1-piperazinyl- and 2-(hexahydro-1H-1,4-diazepin-1-yl)benzothiazole derivatives was made. The RP2 thin-layer chromatography (TLC) plates (silica gel RP2 60F254 silanised precoated), impregnated with solutions of selected amino acid mixtures (L-Asp, L-Asn, L-Thr and L-Lys), were used in two developing solvents as hH1R antagonistic interaction models. Using regression analysis, the relationships between chromatographic and biological activity data were found. The correlations obtained in regression analysis for the examined thiazole and benzothiazole derivatives with H1-antihistamine activity [pA2(H1)] represent their interaction with all the proposed biochromatographic models (S1-S7). Some of the calculated equations can be applied to predict the pharmacological activity of new drug candidates. The best multivariate relationships useful in predicting the pharmacological activity of thiazole and benzothiazole derivatives were obtained under the condition of experiment with RP2 TLC plates using the developing solvent acetonitrile-methanol-buffer (40:40:20, v/v). The log P values of particular compounds are extremely important for this kind of activity.

A novel phenylaminotetralin radioligand reveals a subpopulation of histamine H(1) receptors

Previously, (-)-trans-1-phenyl-3-N,N-dimethylamino-1,2,3,4-tetrahydronaphthalene ([-]-trans-H(2)-PAT) was shown to activate stereospecifically histamine H(1) receptors coupled to modulation of tyrosine hydroxylase activity in guinea pig and rat forebrain in vitro and in vivo. Furthermore, the novel radioligand [(3)H](-)-trans-H(2)-PAT was shown to label selectively H(1) receptors in guinea pig and rat brain with high affinity (K(D), ~0.1 and 0.5 nM, respectively) and a B(max) about 50 and 15%, respectively, of that observed for the H(1) antagonist radioligand [(3)H]mepyramine. In the current study, [(3)H](-)-trans-H(2)-PAT-labeled cloned guinea pig and human H(1) receptors in Chinese hamster ovary (CHO) cell membranes with high affinity (K(D), ~0.08 and 0.23 nM, respectively) and a B(max) about 15% of that observed for [(3)H]mepyramine. The binding of H(2)-PAT to H(1) receptors in both CHO-H(1) cell lines was stereoselective with the (-)-trans-isomer having affinity (K(i), ~1.5 nM) about 4-, 20-, and 50-times higher than the (-)-cis-, (+)-trans-, and (+)-cis-isomers, respectively; the affinity of (-)-trans-H(2)-PAT was unaffected by excess GTP. In functional assays, (-)-trans-H(2)-PAT was a full antagonist of histamine H(1)-mediated stimulation of phospholipase C (PLC) and [(3)H]inositol phosphates (IP) formation in CHO-H(1) cells, a full inverse agonist of constitutively active H(1) receptors in COS-7-H(1) cells, and a full competitive antagonist (pA(2) = 9.2) of histamine H(1)-mediated contraction of guinea pig ileum. It is concluded that (-)-trans-H(2)-PAT is an antagonist at H(1) receptors coupled to PLC/IP formation and smooth muscle contraction. Meanwhile, the observation that [(3)H](-)-trans-H(2)-PAT labels only a subpopulation of H(1) receptors and that (-)-trans-H(2)-PAT activates H(1) receptors coupled to modulation of tyrosine hydroxylase suggests that there may be post-translational H(1) receptor heterogeneity.

ReN 1869, a novel tricyclic antihistamine, is active against neurogenic pain and inflammation

The tricyclic compound (R)-1-(3-(10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-ylidene)-1-propyl)-3-piperidine carboxylic acid (ReN 1869) is a novel, selective histamine H(1) receptor antagonist. It is orally available, well tolerated, easily enters the central nervous system (CNS) but no adverse effects are seen in mice at 300 mg/kg. ReN 1869 at 0.01-10 mg/kg is antinociceptive in tests of chemical nociception in rodents (formalin, capsaicin, phenyl quinone writhing) but not in thermal tests (hot plate and tail flick). ReN 1869 amplifies the analgesic action of morphine but does not show tolerance after chronic dosing. Moreover, the compound is effective against inflammation of neurogenic origin (antidromic nerve stimulation, histamine-evoked edema) but not in carrageenan-induced inflammation. We suggest that ReN 1869, via H(1) blockade, counteracts the effect of histamine liberated from activated mast cells and inhibits pain transmission in the dorsal spinal cord. ReN 1869 represents a new class of antihistamines with pain-relieving properties that probably is mediated centrally through histamine H(1) receptors but alternative mechanisms of action cannot be excluded.

Immediate allergic response in small airways

The role of small airways in the immediate allergic response is largely unknown. We therefore used the model of precision-cut lung slices (PCLS) in combination with quantitative videomicroscopy to study the early allergic response to allergen in airways ranging from 50 to 900 microm. After PCLS from untreated Wistar rats had been passively sensitized for 16 h with serum from sensitized Brown Norway rats, exposure to 0.1% ovalbumin resulted in an immediate allergic response. Both extent (r = 0.74, p < 0.0001) and velocity (r = 0.49, p < 0.0001) of the allergen-induced bronchoconstriction increased with decreasing airway size. In addition, we observed that smaller airways not only contracted stronger and quicker, but that they also relaxed faster, suggesting that smaller airways are more reactive in principle. The allergen-induced bronchoconstriction in PCLS was prevented by the serotonin receptor antagonist ketanserin (IC(50) 6 nM), but not by antagonists directed against histamine, acetylcholine, PAF, or endothelin receptors, or by cyclooxygenase or lipoxygenase inhibitors. Like allergen, serotonin provoked responses that were stronger in smaller airways. These findings suggest that the immediate allergic response in rat PCLS depends largely on serotonin and that this response can occur in nearly all airway generations, but is most pronounced in the smallest airways, that is, the terminal bronchioles.

Antihistamines as analgesics

Histamine activates pain-transmitting nerve fibres, releases pain-related neuropeptides, and is painful when injected into the skin. Histamine agonists mimic these effects, suggesting that histamine plays a role in mediating the signal transduction of tissue damage or other painful stimulus. Certain 'antihistamines' (histamine H1 receptor antagonists) and other antihistaminics are 'analgesic' in preclinical or clinical models. Potential sites of action of these agents include the brain and spinal cord and a specific histamine receptor subtype might be involved (three subtypes have been identified). However, it is possible that other mechanisms account for the analgesic effect.

Ca(2+)/calmodulin-mediated regulation of the desensitizing process in G(q) protein-coupled histamine H(1) receptor-mediated Ca(2+) responses in human U373 MG astrocytoma cells

We investigated Ca(2+)/calmodulin (CaM)-mediated regulation of the desensitizing process of the histamine H(1) receptor-mediated increase in intracellular Ca(2+) concentration in human U373 MG astrocytoma cells. The desensitizing process was evaluated by measuring the histamine-induced Ca(2+) responses in cells pretreated with histamine for 15 s-30 min under various conditions. Under normal physiological conditions, desensitization developed with three successive phases : a fast desensitization within 15 s, a transient resensitization at 45 s, and a prompt and sustained redesensitization from 1 to 30 min. Similar processes of desensitization/resensitization occurred even under hypertonic conditions, where histamine-mediated internalization of the histamine H(1) receptor is inhibited. The transient resensitization phase was selectively prevented by deprivation of extracellular Ca(2+) and, even more strikingly, by the presence of W-7 (a CaM antagonist). FK506 and cyclosporin A, Ca(2+)/CaM-dependent protein phosphatase (PP2B) inhibitors, mimicked such effects. In the presence of KN-62, a Ca(2+)/CaM-dependent protein kinase II (CaM kinase II) inhibitor, the early development of desensitization disappeared, allowing a slow and simple development of desensitization. The early processes of desensitization and resensitization were unaffected by W-5, okadaic acid, and KN-04 (less potent inhibitors against CaM, PP2B, and CaM kinase II, respectively) or by GF109203X and chelerythrine (protein kinase C inhibitors). The high-affinity site for histamine was converted to a lower-affinity site by histamine treatment, which also showed a transient restoration phase at 45 s in a manner sensitive to KN-62 and FK506. These results provide the first evidence that Ca(2+)/CaM plays a crucial role in determining the early phase of the desensitizing process via activation of CaM kinase II and PP2B, by regulating agonist affinity for histamine H(1) receptors.

Possible contribution of protein kinase C in the effects of histamine on the visceral nociceptor activities in vitro

To clarify the possible contribution of protein kinase C activation in histamine-induced excitation and sensitization of the heat response of testicular polymodal receptors, the effects of staurosporine, a protein kinase C inhibitor, and phorbol 12,13-dibutyrate, a protein kinase C activating phorbol ester, were studied in visceral polymodal receptors. Single polymodal receptor activities were recorded in vitro from testis-spermatic nerve preparations obtained from deeply anesthetized dogs (pentobarbital sodium, 30 mg/kg, i.v.). Histamine (10 microM)-induced excitation and facilitation of the heat response of polymodal receptors were both suppressed by staurosporine (1 microM), suggesting that activation of protein kinase C is involved in both these effects of histamine. Application of phorbol 12,13-dibutyrate (0.1 microM) mixed with histamine increased the histamine-induced excitation, whereas a 5 min application of phorbol 12,13-dibutyrate before histamine suppressed it. These results suggest that phorbol 12,13-dibutyrate-activated protein kinase C has inactivation as well as activation effects on the intracellular cascade connected to histamine receptors, and that the former has a slower time course.

Analysis of Apparent Noncompetitive Responses to Competitive H(1)-Histamine Receptor Antagonists in Fluorescent Imaging Plate Reader-Based Calcium Assays

We have examined the utility of high throughput fluorescent imaging plate reader (FLIPR)-based calcium assays for pharmacological characterization of G-protein coupled receptors (GPCRs) using recombinant and native human H1-histamine receptors (H(1)-HR), expressed in HEK293 and HeLa S3 cells, respectively, as model systems. For stably transfected HEK293 cell lines, the potency of histamine for elevating intracellular calcium increased (pD(2), 7.13 and 7.86) with increased H(1)-HR density (about 0.8 and 14 pmol/mg protein, respectively), though histamine binding affinities were similar. The classic H(1)-HR competitive antagonists diphenhydramine and chlorpheniramine appeared noncompetitive by causing depressions of the maximal histamine responses along with rightward shifts of histamine concentration-response curves, thus precluding Schild analysis. Applying the generalized Cheng-Prusoff equation to antagonist concentration-response curves for inhibition of fixed histamine concentrations yielded apparent pK(b) values that were consistent among recombinant and native receptors at different expression levels. These pK(b) values for diphenhydramine and chlorpheniramine (e.g., 7.83 and 8.77, respectively) were in good agreement with binding pK(i) values (e.g., 7.98 and 8.52, respectively). Apparent antagonist affinities determined from FLIPR calcium and competition binding assays were also consistent for the competitive antagonists mepyramine, tripelennamine, and promethazine. In phosphoinositide hydrolysis assays, chlorpheniramine exhibited insurmountable inhibition of histamine calcium responses, although to a lesser extent than that observed in calcium assays; pK(b) values were similar. These results demonstrate that competitive antagonist potencies can be attained from FLIPR-derived data by application of the generalized Cheng-Prusoff equation, despite apparent noncompetitive antagonism under these assay conditions. Apparent noncompetitive antagonist effects may in part be attributable to a lack of equilibrium of histamine and antagonists with H(1)-HR within the short duration of rapid transient effects of histamine on intracellular calcium.

Discrimination of histamine H1 and muscarinic receptor-mediated signalling pathways by phorbol ester in human astrocytoma cells

1. Histamine H1 receptor-mediated signalling was compared with muscarinic receptor-mediated signalling in 1321N1 human astrocytoma cells. 2. Short-term (2 min) treatment of cells with phorbol 12-myristate 13-acetate (PMA) resulted in a reduction of increases in intracellular Ca2+ ([Ca2+]i) induced by carbachol or histamine. 3. Carbachol-induced increases in [Ca2+]i were 10-fold more sensitive to PMA than the histamine-induced increases. 4. When cells were treated with PMA for 48 or 72 h (long-term treatment), protein kinase C (PKC) was down-regulated and PMA did not inhibit carbachol-induced increases in [Ca2+]i. 5. Histamine-induced increases in [Ca2+]i were significantly reduced by long-term treatment with PMA. 6. These findings suggest that the signalling pathways mediated by histamine H1 and muscarinic receptors can be distinguished by using PKC in 1321N1 human astrocytoma cells.

Regulation of H1-receptor coupling and H1-receptor mRNA by histamine in bovine tracheal smooth muscle

1. Pretreatment of bovine tracheal smooth muscle (BTSM) with histamine (1-100 microM, 1 h) induced a concentration-dependent desensitization of the contractile response to subsequently administered histamine, with a reduction of the maximum response of 72 +/- 8% (n = 5) following pre-exposure to 100 microM histamine. In contrast, concentration-response curves to the muscarinic agonist, methacholine were not affected following histamine pretreatment, indicating a homologous desensitization. Furthermore, concentration-response curves to NaF, a G-protein activator, were not altered following histamine pre-incubation. 2. The histamine H1-receptor (H1R) desensitization could be antagonized by mepyramine (an H1-receptor antagonist, 1 microM) but not by cimetidine (an H2-receptor antagonist, 10 microM), indicating that the desensitization occurred via stimulation of histamine H1-receptors, without evidence for the involvement of histamine H2-receptors. 3. Indomethacin (10 microM) did not block the H1R desensitization, suggesting no involvement of prostaglandins. Furthermore, histamine pre-incubation in calcium free medium still induced a functional uncoupling of H1R. 4. GF 109203X, a protein kinase C (PKC) inhibitor, and H-7, a non-selective kinase inhibitor, did not antagonize the homologous H1R desensitization. 5. The steady-state level of H1R mRNA, assessed by Northern blot analysis, was not affected by prolonged histamine exposure (100 microM, 0.5, 1, 2, 4, 16 and 24 h). 6. These results suggest that histamine induces desensitization of the H1R at the level of the receptor protein, which involves a mechanism independent of PKC, PKA, PKG and calcium influx, suggesting the involvement of a receptor-specific kinase.

Receptor-stimulated phospholipase C activity in human umbilical artery cultured endothelial cells grown in a low oxygen environment

Endothelial cells of the human umbilical blood vessels are widely cultured in an oxygen tension (21%) far above that in which they exist in vivo (3%). This study investigates the effect of the long term culture (ca. 1 month) of human umbilical artery endothelial cells in a reduced oxygen environment (3%: HUAEC3) in comparison to cells grown in a 'normoxic' environment (21%: HUAEC21). Despite reports of altered metabolic pathways and reduced membrane integrity in other cell types, the characteristics of HUAEC3 were found to be similar to those of HUAEC21 with respect to morphology, immunocytochemical profile and in vitro growth rates. Cellular glutathione was maintained in these cells although ATP levels in HUAEC3 were found to be significantly lower than those observed in HUAEC21. The phosphoinositide responses of the HUAEC3 to a variety of agonists were also found to be of similar magnitude to those observed in HUAEC21. In addition, the pharmacological characteristics of the phospholipase C-linked histamine H1 and P2y2 (P2U) receptors were not changed by culture of cells in a low oxygen environment.