Rat histamine N-methyltransferase. Quantification, tissue distribution, purification, and immunologic properties

Histamine, Neuroinflammation and Neurodevelopment: A Review

The biogenic amine, histamine, has been shown to critically modulate inflammatory processes as well as the properties of neurons and synapses in the brain, and is also implicated in the emergence of neurodevelopmental disorders. Indeed, a reduction in the synthesis of this neuromodulator has been associated with the disorders Tourette's syndrome and obsessive-compulsive disorder, with evidence that this may be through the disruption of the corticostriatal circuitry during development. Furthermore, neuroinflammation has been associated with alterations in brain development, e.g., impacting synaptic plasticity and synaptogenesis, and there are suggestions that histamine deficiency may leave the developing brain more vulnerable to proinflammatory insults. While most studies have focused on neuronal sources of histamine it remains unclear to what extent other (non-neuronal) sources of histamine, e.g., from mast cells and other sources, can impact brain development. The few studies that have started exploring this in vitro, and more limited in vivo, would indicate that non-neuronal released histamine and other preformed mediators can influence microglial-mediated neuroinflammation which can impact brain development. In this Review we will summarize the state of the field with regard to non-neuronal sources of histamine and its impact on both neuroinflammation and brain development in key neural circuits that underpin neurodevelopmental disorders. We will also discuss whether histamine receptor modulators have been efficacious in the treatment of neurodevelopmental disorders in both preclinical and clinical studies. This could represent an important area of future research as early modulation of histamine from neuronal as well as non-neuronal sources may provide novel therapeutic targets in these disorders.

Hydralazine is involved in tele-methylhistamine metabolism by inhibiting monoamine oxidase B in pregnancy-associated hypertensive mice

Hypertensive disorders of pregnancy globally affect 6-8% of gestation and remain a major cause of both foetal and maternal morbidity and mortality. However, the antihypertensive medications for the patients of this disease are strictly limited due to the teratogenic potentials. Here, we found that tele-methylhistamine (tMH) increased in response to the administration of hydralazine (Hdz), a vasodilative agent, in the pregnancy-associated hypertensive (PAH) mice. Hdz abrogated the degradation of tMH catalyzed by monoamine oxidase B (MAO-B) in vitro. These results suggested that Hdz inhibited the MAO-B activity and consequently tMH increased in the maternal circulation of PAH mice.

Polyamines as Snake Toxins and Their Probable Pharmacological Functions in Envenomation

While decades of research have focused on snake venom proteins, far less attention has been paid to small organic venom constituents. Using mostly pooled samples, we surveyed 31 venoms (six elapid, six viperid, and 19 crotalid) for spermine, spermidine, putrescine, and cadaverine. Most venoms contained all four polyamines, although some in essentially trace quantities. Spermine is a potentially significant component of many viperid and crotalid venoms (≤0.16% by mass, or 7.9 µmol/g); however, it is almost completely absent from elapid venoms assayed. All elapid venoms contained larger molar quantities of putrescine and cadaverine than spermine, but still at levels that are likely to be biologically insignificant. As with venom purines, polyamines impact numerous physiological targets in ways that are consistent with the objectives of prey envenomation, prey immobilization via hypotension and paralysis. Most venoms probably do not contain sufficient quantities of polyamines to induce systemic effects in prey; however, local effects seem probable. A review of the pharmacological literature suggests that spermine could contribute to prey hypotension and paralysis by interacting with N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, nicotinic and muscarinic acetylcholine receptors, γ-Aminobutyric acid (GABA) receptors, blood platelets, ryanodine receptors, and Ca²⁺-ATPase. It also blocks many types of cation-permeable channels by interacting with negatively charged amino acid residues in the channel mouths. The site of envenomation probably determines which physiological targets assume the greatest importance; however, venom-induced liberation of endogenous, intracellular stores of polyamines could potentially have systemic implications and may contribute significantly to envenomation sequelae.

Monoclonal antibodies for human and porcine histamine N-methyltransferase (HMT) facilitate protein expression and localization studies

Objective

The lack of suitable antibodies for the histamine inactivating enzyme histamine N-methyltransferase (HMT) has so far prevented the direct analysis of HMT proteins in man and other mammals.

Methods

A series of monoclonal antibodies was produced by immunizing mice with human and porcine HMT expressed in vitro. Antibodies were characterized by immunoblotting and immunohistochemical staining.

Results

Six different monoclonal antibodies specific for human HMT and four different monoclonal antibodies specific for porcine HMT were obtained that can detect HMT with up to tenfold greater sensitivity than the most sensitive enzymatic assays currently available. Using these antibodies allowed us to confirm the expression and cellular localization of HMT in various human and porcine tissues, where the presence of the enzyme had previously been deduced from activity measurement and HMT mRNA analysis. Immunohistochemical staining of human and porcine tissue sections clearly showed that HMT is a cytosolic protein, which is localized in specific cells of most mammalian tissues.

Conclusions

The new monoclonal antibodies not only allow a comprehensive quantitative evaluation of the expression of HMT at the cellular level in man and other mammals but will also facilitate sensitive analyses of disease-associated alterations of this protein.

Molecular identification of carnosine N-methyltransferase as chicken histamine N-methyltransferase-like protein (hnmt-like)

Anserine (beta-alanyl-N(Pi)-methyl-L-histidine), a naturally occurring derivative of carnosine (beta-alanyl-L-histidine), is an abundant constituent of skeletal muscles and brain of many vertebrates. Although it has long been proposed to serve as a proton buffer, radicals scavenger and transglycating agent, its physiological function remains obscure. The formation of anserine is catalyzed by carnosine N-methyltransferase which exhibits unknown molecular identity. In the present investigation, we have purified carnosine N-methyltransferase from chicken pectoral muscle about 640-fold until three major polypeptides of about 23, 26 and 37 kDa coeluting with the enzyme were identified in the preparation. Mass spectrometry analysis of these polypeptides resulted in an identification of histamine N-methyltransferase-like (HNMT-like) protein as the only meaningful candidate. Analysis of GenBank database records indicated that the hnmt-like gene might be a paralogue of histamine N-methyltransferase gene, while comparison of their protein sequences suggested that HNMT-like protein might have acquired a new activity. Chicken HNMT-like protein was expressed in COS-7 cells, purified to homogeneity, and shown to catalyze the formation of anserine as confirmed by both chromatographic and mass spectrometry analysis. Both specificity and kinetic studies carried out on the native and recombinant enzyme were in agreement with published data. Particularly, several compounds structurally related to carnosine, including histamine and L-histidine, were tested as potential substrates for the enzyme, and carnosine was the only methyl group acceptor. The identification of the gene encoding carnosine N-methyltransferase might be beneficial for estimation of the biological functions of anserine.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

Simultaneous fluorometric measurement of histamine and tele-methylhistamine levels in rodent brain by high-performance liquid chromatography

An improved high-performance liquid chromatography (HPLC) method was developed for simultaneous analysis of histamine (HA) and tele-methylhistamine (tele-MHA) levels in mouse and rat brain. The method consists of a solid-phase extraction (SPE) and subsequent HPLC with postcolumn derivatization of the amines with o-phthalaldehyde. The recovery rates of HA and tele-MHA during the SPE procedure were 82.8+/-3.4 and 86.0+/-1.7%, respectively. The detection limits for HA and tele-MHA were 8 and 12pg, respectively, with sufficient linearity up to 30pg. Using this newly developed system, we observed that the brain tele-MHA levels in H3 receptor knockout mice were significantly higher than those of wild-type mice by 2.1-fold. Furthermore, we also observed that the brain HA and tele-MHA levels in Zucker rats were significantly lower than those of lean rats by 76.6+/-5.3 and 77.8+/-5.0%, respectively. These observations coincided well with those of previous studies using radioimmunoassay or HPLC with precolumn OPA derivatization, confirming the utilization of the assay system.

Search for Histamine H3Receptor Ligands with Combined Inhibitory Potency at HistamineN-Methyltransferase: ω-Piperidinoalkanamine Derivatives

In an effort to design new hybrid compounds with dual properties, i.e. binding affinity at histamine H(3) receptors and inhibitory potency at the catabolic enzyme histamine N(tau)-methyltransferase (HMT), a novel series of 1-substituted piperidine derivatives was synthesized. This alicyclic heterocycle is structurally linked via aminoalkyl spacers of variable lengths to additional aromatic carbo- or hetero-cycles. These new hybrid drugs were pharmacologically evaluated regarding their binding affinities at recombinant human H(3) receptors, stably expressed in CHO cells, and in a functional assay for their inhibitory potencies at rat kidney HMT. All compounds investigated proved to be H(3) receptor ligands with binding affinities in the micro- to nanomolar concentration range despite significant differences in the type of the aromatic moiety introduced. The most potent compound in this series was the quinoline derivative 20 (K(i) = 5.6 nM). Likewise, all new ligands studied showed impressive HMT inhibitory activities. Here, compounds 5, 10, 14 and 18-20 exhibited submicromolar potencies (IC(50) = 0.061-0.56 microM). The aminomethylated quinoline 19 showed almost the same, well balanced nanomolar activities on both targets. In this study, new hybrid compounds with a dual mode biological action were developed. These pharmacological agents are valuable leads for further development and candidates for treatment of histamine-dependent disorders.

Imidazole derivatives as a novel class of hybrid compounds with inhibitory histamine N-methyltransferase potencies and histamine hH3 receptor affinities

In this study, a novel series of imidazole-containing compounds with dual properties, that is, inhibitory potency at the enzyme histamine N(tau)-methyltransferase (HMT) and antagonist potency at histamine H(3) receptors was designed and synthesized. Pharmacologically, these new hybrid drugs were evaluated in functional assays for their inhibitory potencies at rat kidney HMT and for their antagonist activities on synaptosomes of rat cerebral cortex. For selected compounds, binding affinities at recombinant human histamine H(3) receptors were determined. The first compounds (1-10) of the series proved to be H(3) receptor ligands of high potency at rat synaptosomes or of high binding affinity at human H(3) receptors, respectively, but of only moderate activity as inhibitors of rat kidney HMT. In contrast, aminoquinoline- or tetrahydroacridine-containing derivatives 11-17 also displayed HMT inhibitory potency in the nanomolar concentration range. Preliminary data from molecular modeling investigations showed that the imidazole derivative 15 and the HMT inhibitor quinacrine possess identical binding areas. The most interesting compound (14) is simultaneously a highly potent H(3) receptor ligand (K(i)=4.1nM) and a highly potent HMT inhibitor (IC(50)=24nM), which makes this derivative a valuable pharmacological tool for further development.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

The role of hypothalamic histamine in leptin-induced suppression of short-term food intake in fasted rats

OBJECTIVE

Leptin suppresses food intake; however, the precise mechanism is not fully understood. Histamine (HA), which acts as a neurotransmitter in the central nervous system, has also been shown to be involved in feeding and exerts an inhibitory effect through activation of H(1) receptors. Therefore, we studied the possible role of HA in short-term leptin-induced suppression of food intake.

METHODS

We studied the 6-h feeding response of overnight-fasted adult (200 g) male Wistar rats to leptin and the HA synthesis inhibitor alpha-fluoromethylhistidine (alpha-FMH). Levels of transcription for neuropeptide Y (NPY) and corticotropin-releasing hormone (CRH), as well as hypothalamic content of HA and the HA metabolite telemethyl-HA were investigated.

RESULTS

Central administration of leptin (3, 5 and 10 microg at 09:00 h) in fasted rats caused a decrease in food intake. In contrast, central administration of alpha-FMH (11, 22 and 112 microg at 09:00 h) increased food intake. Prior administration of alpha-FMH prevented the leptin-induced decrease in food intake. Leptin decreased hypothalamic histamine content, while increasing the ratio between telemethyl-HA and HA, indicating that leptin reduces HA metabolism. Finally, alpha-FMH suppressed basal and leptin-induced CRH expression while stimulating NPY expression in fasted rats.

CONCLUSION

Histamine is involved in leptin-induced inhibition of food intake. The role of histamine may be mediating, i.e. leptin may directly activate and/or change the metabolism of the histaminergic system. Alternatively, the histaminergic system may be involved in a permissive manner.

A comparison of the airway response to segmental antigen bronchoprovocation in atopic asthma and allergic rhinitis

BACKGROUND

Patients with allergic asthma and those with allergic rhinitis (without asthma) share many immunopathologic features but differ in the presence of lower airway symptoms in response to antigen.

OBJECTIVES

We sought to compare the airway inflammatory response to antigen in patients with atopic asthma and allergic rhinitis.

METHODS

Segmental bronchoprovocation with saline or ragweed antigen was performed in 9 patients with atopic asthma and 9 patients with allergic rhinitis without asthma. The antigen dose used in segmental bronchoprovocation was 10% of the dose that caused a 20% decrease in FEV1 in response to inhalation challenge. Bronchoalveolar lavage (BAL) was performed from the saline- and antigen-challenged segments at 5 minutes and 48 hours after challenge. BAL fluid was analyzed for cell count and differential, distribution of lymphocytes, and concentration of soluble factors (histamine, IL-5, matrix metalloproteinase 9, tissue inhibitor of metalloproteinase 1, and fibronectin). In addition, BAL cells were cultured ex vivo, and IL-5, IFN-gamma, and IL-10 generation was measured.

RESULTS

Antigen challenge led to similar patterns of cellular recruitment, mediator levels, and BAL cell cytokine generation in both groups; however, the dose of antigen required to promote comparable responses in the airway was significantly less in patients with asthma.

CONCLUSION

These data suggest that the pattern of acute airway inflammation in response to allergen does not by itself explain antigen-induced lower airway obstruction and asthma symptoms. We speculate that other factors, such as increased airway sensitivity to allergen or preexisting airway injury and remodeling, might explain why patients with asthma and rhinitis differ in their clinical and physiologic response to antigen exposure.

Development of a new class of nonimidazole histamine H(3) receptor ligands with combined inhibitory histamine N-methyltransferase activity

In search of novel ways to enhance histaminergic neurotransmission in the central nervous system, a new class of nonimidazole histamine H(3) receptor ligands were developed that simultaneously possess strong inhibitory activity on the main histamine metabolizing enzyme, histamine N-methyltransferase (HMT). The novel compounds contain an aminoquinoline moiety, which is an important structural feature for HMT inhibitory activity, connected by different spacers to a piperidino group (for H(3) receptor antagonism). Variation of the spacer structure provides two different series of compounds. One series, having only an alkylene spacer between the basic centers, led to highly potent HMT inhibitors with moderate to high affinity at human histamine H(3) receptors. The second series possesses a p-phenoxypropyl spacer, which may be extended by another alkylene chain. This latter series also showed strong inhibitory activity on HMT, and in most cases, the H(3) receptor affinity even surpassed that of the first series. One of the most potent compounds with this dual mode of action is 4-(4-(3-piperidinopropoxy)phenylamino)quinoline (34) (hH(3), K(i) = 0.09 nM; HMT, IC(50) = 51 nM). This class of compounds showed high antagonist potency and good H(3) receptor selectivity in functional assays in guinea pig on H(1), H(2), and H(3) receptors. Because of low or missing in vivo activity of two selected compounds, the proof of concept of these valuable pharmacological tools for the supposed superior overall enhancing effect on histaminergic neurotransmission failed to appear hitherto.

Two polymorphic forms of human histamine methyltransferase: structural, thermal, and kinetic comparisons

BACKGROUND

Histamine plays important biological roles in cell-to-cell communication; it is a mediator in allergic responses, a regulator of gastric acid secretion, a messenger in bronchial asthma, and a neurotransmitter in the central nervous system. Histamine acts by binding to histamine receptors, and its local action is terminated primarily by methylation. Human histamine N-methyltransferase (HNMT) has a common polymorphism at residue 105 that correlates with the high- (Thr) and low- (Ile) activity phenotypes.

RESULTS

Two ternary structures of human HNMT have been determined: the Thr105 variant complexed with its substrate histamine and reaction product AdoHcy and the Ile105 variant complexed with an inhibitor (quinacrine) and AdoHcy. Our steady-state kinetic data indicate that the recombinant Ile105 variant shows 1.8- and 1.3-fold increases in the apparent K(M) for AdoMet and histamine, respectively, and slightly (16%) but consistently lower specific activity as compared to that of the Thr105 variant. These differences hold over a temperature range of 25 degrees C-45 degrees C in vitro. Only at a temperature of 50 degrees C or higher is the Ile105 variant more thermolabile than the Thr105 enzyme.

CONCLUSIONS

HNMT has a 2 domain structure including a consensus AdoMet binding domain, where the residue 105 is located on the surface, consistent with the kinetic data that the polymorphism does not affect overall protein stability at physiological temperatures but lowers K(M) values for AdoMet and histamine. The interactions between HNMT and quinacrine provide the first structural insights into a large group of pharmacologic HNMT inhibitors and their mechanisms of inhibition.

Novel histamine H(3)-receptor antagonists with carbonyl-substituted 4-(3-(phenoxy)propyl)-1H-imidazole structures like ciproxifan and related compounds

Novel histamine H(3)-receptor antagonists possessing a 4-(3-(phenoxy)propyl)-1H-imidazole structure generally substituted in the para-position of the phenyl ring have been synthesized according to Mitsunobu or S(N)Ar reactions. With in vitro and in vivo screening for H(3)-receptor antagonist potency, the carbonyl-substituted derivatives proved to be highly active compounds. A number of compounds showed in vitro affinities in the subnanomolar concentration range, and the 4-hexanoyl (10) and 4-acetyl-3-methyl (29) substituted derivatives showed in vivo antagonist potencies of about 0.1 mg/kg after po administration. Many proxifans were also tested for their affinities at other histamine receptor subtypes thereby demonstrating their pronounced H(3)-receptor subtype selectivity. Since the cyclopropyl ketone derivative 14 (ciproxifan) had high affinity in vitro as well as high potency in vivo, it was selected for further studies in monkeys. It showed good oral absorption and long-lasting, dose-dependent plasma levels making it a promising compound for drug development.

Inhaled budesonide decreases airway inflammatory response to allergen

To define the mechanisms by which inhaled glucocorticosteroid regulates allergen-induced airway inflammation, a double-blind, placebo-controlled, cross-over study with inhaled budesonide was conducted in 14 subjects with allergic asthma. After baseline bronchoscopy and bronchoalveolar lavage (BAL), subjects were randomized to budesonide (400 microgram, twice daily) or placebo treatment for 4 wk. At the end of each treatment phase, whole-lung allergen inhalation challenge was performed, followed by BAL 48 h later. Budesonide treatment improved the FEV(1), attenuated both the immediate- and late-phase response to allergen, and prevented the increase in bronchial hyperresponsiveness after allergen challenge. Budesonide treatment also decreased allergen-induced airway eosinophilia. To determine the effects of budesonide on airway cell function, BAL cells were stimulated ex vivo with the T cell mitogen PHA, and cytokine generation was measured by ELISA. Budesonide decreased ex vivo generation of IL-5 and IFN-gamma by BAL cells. Ex vivo IL-5 production was significantly correlated with the number of airway eosinophils (r(s) = 0.61), and levels of eosinophil-derived neurotoxin (EDN) (r(s) = 0.57) and IL-5 (r(s) = 0.52) in BAL fluid. Moreover, PHA-induced IL-5 generation correlated with FEV(1) fall during the late-phase response to allergen (r(s) = 0.60). Budesonide decreased circulating eosinophils and serum levels of IL-5, but did not reduce IL-5 generation by peripheral blood mononuclear cells. The reduction in circulating eosinophils correlated with the decrease in levels of EDN (r(s) = 0.61) in BAL fluid and late response to inhaled allergen (r(s) = 0.51). These findings suggest that long-term treatment with inhaled budesonide reduces airway cell generation of cytokines, specifically IL-5, which then decreases circulating eosinophils and their availability for recruitment to the airway after allergen exposure.

Increased histamine release and granulocytes within the thalamus of a rat model of Wernicke's encephalopathy

The current study examined the possible role of increased histamine release and granulocyte activity in the vascular changes that precede the onset of necrotic lesions with the thalamus of the pyrithiamine-induced thiamine deficiency (PTD) rat model of Wernicke's encephalopathy (WE). An increase in histamine release and the number of granulocytes was observed in lateral thalamus on day 9 and in medial thalamus on day 10 of PTD treatment, a duration of thiamine deficiency associated with perivascular edema in this brain region. Within the hippocampus, histamine release was significantly increased on day 9, declined to control levels on days 10-12, and was significantly elevated on days 12-14. No granulocytes were observed in hippocampus of either PTD or control rats. These observations suggest that the release of histamine from nerve terminals and histamine and other vasoactive substances from granulocytes may be responsible for thiamine deficiency-induced vascular breakdown and perivascular edema within thalamus.

The effect of segmental bronchoprovocation with allergen on airway lymphocyte function

We hypothesized that allergen-induced airway eosinophilia is linked to activation or recruitment of T cells in the airway and generation of interleukin-5 (IL-5). To evaluate this hypothesis, we performed bronchoscopy with segmental antigen bronchoprovocation in 12 atopic subjects. Bronchoalveolar lavage (BAL) was done 5 min and 48 h after challenge with saline or antigen. Airway cells were isolated and then stimulated ex vivo with a T-cell mitogen, phytohemagglutinin (PHA), and cytokine release was determined. Cells retrieved from the saline-challenged segment secreted principally interferon-gamma (IFN-gamma) and IL-2. In contrast, cells obtained 48 h after allergen challenge secreted high levels of IL-5 and small but increased amounts of IL-4, IL-10, and granulocyte-macrophage colony-stimulating factor (GM-CSF). Although CD4+ T cells were a major source of IL-5, there were no significant changes in the relative proportion of CD4+ cells in response to bronchoprovocation. Additionally, ex vivo secretion of IL-5 by airway cells correlated closely with amounts of IL-5 and eosinophils present in the bronchoalveolar lavage fluid (BALF). These observations suggest that following exposure to allergen, airway T cells are functionally but not phenotypically different from resident airway T cells, and that T cells within the airway contribute to eosinophilic airway inflammation through the secretion of IL-5.

HMT regulates histamine-induced glycoconjugate secretion from human airways in vitro

To determine whether histamine N-methyltransferase (HMT) regulates mucus glycoprotein (MGP) secretion from airways, we examined the effect of an HMT inhibitor, SKF 91488, on MGP secretion from human airways in vitro. MGP secretion from human airway explants (with epithelium) and isolated submucosal glands was estimated by measuring trichloroacetic acid (TCA) precipitable glycoconjugates using secretory indices. Histamine induced significant MGP secretion from both explants and isolated glands. Pretreatment with SKF 91488 significantly inhibited histamine-induced secretion from explants, while it did not alter significantly the secretion from isolated glands. H1-blocker significantly reversed the inhibition by SKF 91488 of the secretion from explants, while H2-blocker abolished histamine-induced secretion from both explants and isolated glands. Prostaglandin E2 (PGE2) significantly inhibited histamine-induced secretion from isolated glands. The inhibitory action of SKF 91488 in airway explants was blocked by indomethacin and was significantly reduced by a prostanoid EP4 receptor antagonist (AH23848B). These findings suggest that HMT regulates MGP secretion from human airway submucosal glands through an interaction with epithelial cells which involves the release of PGE2.

Similar deficits of central histaminergic system in patients with Down syndrome and Alzheimer disease

In order to study whether Alzheimer-like neuropathological changes involve the central histaminergic system we measured the concentration of histamine, its precursor histidine as well as the activity of histidine decarboxylase (HDC) and histamine-N-methyl-transferase (HMT) in frontal cortex of aging Down syndrome (DS) patients, Alzheimer patients and control individuals. The study populations were also investigated for choline acetyltransferase (ChAT) activity, since reduced ChAT activity is an established biochemical hallmark in DS and Alzheimer disease (AD). HDC and ChAT activity were reduced in brains of both DS and Alzheimer patients versus control patients. Additionally, we observed a significant decrease of histamine levels in the DS group. Histamine levels in AD brains tended to be decreased. Histidine concentrations and HMT activities were comparable between the three groups. Thus, our results for the first time show histaminergic deficits in brains of patients with DS resembling the neurochemical pattern in AD. Neuropathological changes may be responsible for similar neurochemical alterations of the histaminergic system in both dementing disorders.

Late allergic airway response to segmental bronchopulmonary provocation in allergic subjects is related to peripheral blood basophil histamine release

BACKGROUND

Basophil histamine release has been found to correlate with the presence and severity of allergic disease. However, it remains to be established whether airway response to antigen is related to basophil involvement and its release of histamine.

OBJECTIVE

The purpose of this study was to investigate whether the intensity of airway response to an inhaled antigen challenge in allergic subjects is related to IgE-dependent peripheral blood basophil histamine release.

METHODS

The response to segmental bronchoprovocation with antigen was examined in 34 subjects with allergic rhinitis. Bronchoalveolar lavage samples were obtained 5 minutes (immediate response) and 48 hours (late response) after allergen challenge. Peripheral blood maximal basophil histamine release (MBHR) in response to in vitro antigen stimulation was determined in each subject before segmental bronchoprovocation.

RESULTS

Bronchoalveolar lavage samples obtained during immediate response showed an increase in histamine, whereas the late response was noted for a marked enhancement in airway cells, particularly eosinophils. Interestingly, a significant correlation (r = 0.73, p < 0.0001, Spearman Rank test) was noted between MBHR and intensity of bronchoalveolar lavage eosinophilia at 48 hours. Furthermore, subjects with high (> or = 20%) MBHR had significantly higher total cells and eosinophils in bronchoalveolar lavage fluid 48 hours after antigen segmental bronchoprovocation when compared with subjects with low (< 20%) MBHR.

CONCLUSION

Our data suggest that the intensity of airway eosinophilia in response to antigen challenge is correlated with the magnitude of basophil mediator release in allergic subjects.

HMT regulates histamine-induced Cl- secretion across the canine tracheal epithelium

Although histamine N-methyltransferase (HMT), the primary enzyme responsible for the inactivation of histamine, has been shown to exist in the airway epithelium, it is still unknown whether this enzyme regulates ion transport across the airway epithelium. Using an Ussing chamber, we examined the effect of a HMT inhibitor, SKF 91488, on potential difference (PD) and short circuit current (SCC) in epithelial membranes from the posterior portion of canine trachea. SKF 91488 itself did not significantly alter PD or SCC values. Pretreatment with SKF 91488 significantly augmented PD and SCC induced by histamine. Amiloride did not significantly alter the augmentation by SKF 91488 in histamine-induced PD and SCC rises. These findings indicate that HMT regulates Cl- secretion across airway epithelium.

Histamine does not play an essential role in electrocortical activation during waking behavior

Intraperitoneal injection of alpha-fluoromethylhistidine (alpha-FMH; 200 mg/kg), a specific inhibitor of histidine decarboxylase produced a severe depletion of neocortical and hippocampal histamine 3 h later as determined by a radioenzymatic assay. This treatment had no obvious effect on either low voltage fast activity (LVFA) in the neocortex or on rhythmical slow activity (RSA) in the hippocampus during an 8 h recording period during the rats' light cycle. Scopolamine-sensitive LVFA, scopolamine-resistant LVFA and scopolamine-resistant hippocampal RSA all appeared unaffected. This suggests that any contribution histamine makes to electrocortical activation is probably indirect, acting via other transmitter systems.

alpha-Fluoromethylhistidine-induced inhibition of brain histidine decarboxylase. Implications for the CO2-trapping enzymatic method

The actions of S-alpha-fluoromethylhistidine (FMH), an irreversible inhibitor of the histamine biosynthetic enzyme histidine decarboxylase (HD), were studied on rat brain HD, as measured by a recently developed CO2-trapping enzymatic method. As expected, FMH induced a virtually complete inhibition of HD in the hypothalamus both in vivo and in vitro. In the frontal cortex, however, maximal doses of FMH did not maximally inhibit HD, suggesting the existence of an FMH-resistant form of HD. Careful studies of the conditions under which the assays were performed (homogenate dilution, preincubation times, incubation times, temperatures), as well as experiments with inhibitors of other decarboxylases, were unable to provide an explanation for this. When comparable studies of the effects of FMH in these brain regions were performed by alternative methods for measuring HD activity, no evidence for the existence of an FMH-resistant form of HD could be found. Thus, even though the CO2-trapping method appears to be accurate for measuring HD activity in rat hypothalamic homogenates, the present results show that this method may not be specific when studying brain regions other than the hypothalamus.

Tissue distribution of histamine N-methyltransferase-like immunoreactivity in rodents

The rat kidney histamine N-methyltransferase was purified to homogeneity from Escherichia coli transfected with its recombinant cDNA. An antiserum to the enzyme was raised in rabbit by immunization with the purified protein. Western blot analysis of rat tissues with the antiserum revealed a band with identical mobility to that of purified enzyme in the extracts of kidney, jejunum, and brain, where the enzyme activity was detected. The antiserum cross-reacted with a 32K protein in mouse liver, brain, stomach, kidney and lung, and a 33K protein in guinea pig brain, stomach jejunum, spleen, lung, and kidney. The intensity of the staining in western blotting correlated well with the enzyme activity in all the tissues in these three species, suggesting that our antiserum is useful for quantifying histamine N-methyltransferase protein in rodent tissues.

Histamine N-methyltransferase: inhibition by monoamine oxidase inhibitors

Histamine N-methyltransferase (HNMT) catalyzes the N tau-methylation of histamine. N tau-Methylhistamine can then undergo oxidation catalyzed by the mitochondrial enzyme monoamine oxidase (MAO). Addition of an MAO inhibitor such as pargyline to tissue preparations can increase the HNMT activity assayed --presumably as a result of inhibition of N tau-methylhistamine metabolism by MAO. However, pargyline-dependent "activation" of HNMT may also occur in tissue preparations that lack mitochondria. Our experiments were performed to determine whether MAO inhibitors, like many other amine compounds, could directly increase the activity of partially purified HNMT, and, if so, to study the mechanism of activation. Human kidney HNMT was partially purified by sequential ion exchange and gel filtration chromatography. The activity of the purified HNMT was increased approximately 50% in the presence of pargyline. However, enzyme kinetic experiments showed that pargyline, like many other amines, was a competitive inhibitor of HNMT. Apparent activation of the enzyme resulted from sequential shifts of histamine substrate curves to higher Vmax values in the presence of increasing concentrations of pargyline. Other acetylenic MAO inhibitors, clorgyline and the two stereoisomers of deprenyl, were also competitive inhibitors of purified human kidney HNMT. Inhibition kinetic experiments performed in the presence of varying concentrations of histamine demonstrated that Kis values for pargyline, clorgyline, (R)-deprenyl and (S)-deprenyl were 0.126, 0.144, 0.217, and 0.627 mM, respectively. When the concentration of the cosubstrate for the reaction, S-adenosyl-L-methionine, was varied in the presence of variable concentrations of pargyline, inhibition of HNMT by pargyline was noncompetitive with regard to the methyl donor, with Kii and Kis values of 1.23 and 0.95 mM, respectively. Finally, several amine compounds related structurally to pargyline were also found to be inhibitors of HNMT.

Histamine N-methyltransferase modulates histamine- and antigen-induced bronchoconstriction in guinea pigs in vivo

To examine whether histamine N-methyltransferase (HMT; EC 2.1.1.8) modulates the effects of allergic reaction in vivo, we studied the effects of aerosolized SKF 91488, a specific HMT inhibitor, on the responses to aerosolized histamine in unsensitized guinea pigs and to ovalbumin (OA) antigen inhalation in guinea pigs sensitized to OA. Airway responsiveness was assessed by determining provocation concentrations of histamine and OA aerosols that increased pulmonary resistance to twice the baseline values. SKF 94188 shifted, in a dose-dependent fashion, the dose-response curves to histamine and OA antigen to lower concentrations, and it significantly decreased provocation concentrations of both histamine and OA antigen (p < 0.01). In contrast of SKF 91488, aerosolized aminoguanidine, a specific inhibitor of diamine oxidase (10(-2) M, 90 breaths), did not alter the provocation concentration of histamine (p > 0.20). SKF 91488 (10(-2) M, 90 breaths) caused no significant changes in response to acetylcholine (p > 0.30). HMT activities were observed in the entire airways of the trachea, main bronchi, segmental bronchi and bronchioles, and parenchymal tissues. These findings suggest that HMT modulates the effects of exogenous histamine and endogenously released histamine by antigen challenge on bronchoconstrictor responses in guinea pigs in vivo.

Role of mast cell chymase in the extracellular processing of big-endothelin-1 to endothelin-1 in the perfused rat lung

Previous studies of endothelin-1 (ET) synthesis have shown that some cultured endothelial cells secrete an intermediate product, big-endothelin-1 (bigET), suggesting that the processing of secreted bigET to ET may be physiologically significant. In this study, two pertinent ET converting enzyme activities, mast cell chymase I (EC 3.4.21.39) and a phosphoramidon-sensitive, neutral metalloprotease, were identified in a rat lung particulate fraction. We perfused rat lungs with bigET and chymostatin or phosphoramidon to study the relevance of these two proteases to the processing of extracellular bigET in vivo. Addition of compound 48/80 (a compound which activates mast cells, causing degranulation and release of chymase) to the perfusion buffer greatly increased hydrolysis of exogenously added bigET to ET. ET formation was inhibited completely by 32 microM chymostatin, whereas inhibition by 50 microM phosphoramidon was incomplete and variable. Perfusate histamine levels were used to monitor the extent of mast cell degranulation, and inhibition of ET production by phosphoramidon was attributed to inhibition of degranulation, per se. There was a direct correlation between perfusate ET and histamine levels in both control and phosphoramidon-treated (but not chymostatin-treated) lungs. Our results suggest that chymase from lung mast cells is capable of physiologically relevant extracellular processing by bigET to ET in the perfused rat lung.

Morphine-induced increases of extracellular histamine levels in the periaqueductal grey in vivo: a microdialysis study

The effect of morphine on extracellular histamine levels in two regions of the rat midbrain was studied in vivo by microdialysis. Morphine (5.6 and 12.8 mg/kg, s.c.) significantly and dose-dependently increased extracellular histamine levels in the periaqueductal grey, while no significant effect was observed in the reticular formation. In addition, no significant effect of sequential saline injections was observed on extracellular histamine levels in the periaqueductal grey. Since morphine has no effect on histamine catabolism, these results suggest that morphine increases histamine release in the rat PAG, a site where morphine and histamine are known to have analgesic action. Taken with earlier studies showing the ability of H2 antagonists to block morphine analgesia, these results support the hypothesis that histamine and H2 receptors are important in mediating morphine analgesia in the rat periaqueductal grey. The cellular origin of the extracellular histamine, and the mechanism of this morphine effect remain to be determined.

Distribution of histamine in the rat kidney during pregnancy and development

An antiserum against conjugated histamine and two oligonucleotide probes that detect the mRNA encoding L-histidine decarboxylase (HDC) involved in histamine synthesis were used to study the appearance of histamine and its location in the kidneys of fetal, newborn and young postnatal rats and in the kidneys of pregnant rats. On embryonic days 16 and 18 (E16 and E18), some HA-immunoreactive (HA-ir) cells were found within the largest S-shaped bodies. Histamine was found to appear rapidly between the 18th and 20th embryonic days in the convoluted tubules of the kidneys. On postnatal day 0 (P0), the distal convoluted tubules and collecting ducts exhibited bright fluorescence, the intensity of which decreased quickly so that it was faint on day P4 and absent at later stages. In kidneys of pregnant rats HA-ir was found in the epithelium of both the Bowman's capsule, collecting ducts and in a few cells within the tubules. Nonuniform HA-ir was also detected within glomeruli. No evidence for the presence of L-histidine decarboxylase mRNA in kidneys of fetuses or pregnant rats was seen. It is concluded that distinct structures in the developing rat kidney contain histamine during a period around birth from day E20 to day P4. In the pregnant rat, the epithelium that is in direct contact with the urine flow is immunoreactive for histamine from day 16 to 20 of pregnancy. The results suggest that histamine is not synthesized locally in the kidneys but rather originates from other tissues.

Experimental rhinovirus 16 infection potentiates histamine release after antigen bronchoprovocation in allergic subjects

Viral respiratory infections exacerbate asthma in many patients. We hypothesized that one mechanism by which this effect occurs may include potentiated or altered mediator release by mast cells and/or basophils to favor the development of late-phase asthmatic reaction (LAR). Therefore, we studied eight subjects with allergic rhinitis before and during an experimentally induced rhinovirus 16 (RV16) infection. We determined levels of plasma histamine and tryptase, and we observed the associated patterns of airway obstruction that developed following inhaled antigen challenge. Bronchial responsiveness to histamine, methacholine, and antigen were all significantly increased during the RV16 illness. Further, the incidence of LAR was significantly higher (five of eight) during the infection than before (one of eight; p = 0.014). In addition, in those patients whose pattern of response following antigen challenge converted from an immediate response only before infection to a dual response (immediate + late phase) during infection, plasma histamine concentrations after challenge were significantly greater than in those whose pattern of response did not change. We conclude that one mechanism by which RV16 infection increases the likelihood of LAR could include enhanced mediator release from pulmonary mast cells or from circulating or recruited basophils.

Immediate and late airway response of allergic rhinitis patients to segmental antigen challenge. Characterization of eosinophil and mast cell mediators

Segmental antigen bronchoprovocation was used to define the nature of the inflammatory process in allergic airway disease. Bronchoalveolar lavage fluid obtained from allergic rhinitis patients 12 min after segmental antigen instillation (immediate response) revealed a significant increase in histamine and tryptase, but no cellular response. Repeat segmental lavage 48 h later (late response) showed marked and significant increases in both low and normal density eosinophils as well as striking elevations of eosinophil granular protein levels (major basic protein, eosinophil-derived neurotoxin, eosinophil cationic protein, and eosinophil peroxidase). Leukotriene C4, but not tryptase, concentrations were also consistently elevated in late lavage samples. Further, the late lavage samples showed a significant increase in interleukin-5 concentrations that correlated with the presence of eosinophils and eosinophil granular proteins. Neither eosinophils nor soluble mediators of eosinophils increased when normal subjects were similarly challenged with antigen. These data suggest that eosinophils are attracted to the airway during the late-phase allergic reaction and that IL-5 may produce changes in airway eosinophil density and promote the release of granular proteins to cause airway injury.

High-performance liquid chromatographic determination of histamine N-methyltransferase activity

A method for the determination of histamine N-methyltransferase (HMT) activity by high-performance liquid chromatography based on post-column derivatization with omicron-phthalaldehyde is described. The determination involves the separation of the substrate, histamine, from its product. N tau-methylhistamine, using a weak cation exchanger, followed by on-line derivatization of these imidazoleamines with omicron-phthalaldehyde and their detection and quantitation with a fluorimetric detector. This assay method is suitable for the measurement of HMT activity during enzyme purification.

The role of eosinophils in the pathophysiology of asthma

From current information, a number of conclusions can be drawn. Antigen activation of the allergic reaction in the airways is associated with an immediate rise in mast cell derived mediators, including histamine and tryptase. Associated with antigen activation of the allergic reaction is recruitment of eosinophils to the airways. This can best be seen in the airway lavage 48 hours after challenge with antigen. An increased presence of eosinophils suggests that they are an important contributor to the late allergic reaction and may be one of the major constituents in the development of bronchial inflammation. Although many factors participate in the late allergic inflammatory response, eosinophil-derived proteins are known to cause airway injury. Regulation of eosinophils in this process is not clearly established; however, our findings of increased IL-5 in relationship to the presence of eosinophils and their granular proteins suggests that this cytokine may be an important modulator of eosinophil function and activation following allergen challenge. However, much remains unknown in understanding bronchial inflammation and the eosinophil's role in the process. In conclusion, the eosinophil is a major cellular participant in late phase allergic airway disease. Its presence and known functions suggest that the eosinophil is a significant cellular factor in the development of allergic airways disease in asthma. Further advances in this area will follow continued studies, particularly those which involve biopsy and correlation with airway physiology.

Mouse kidney histamine N-methyltransferase: assay conditions, biochemical properties and strain variation

Histamine N-methyltransferase (HNMT) catalyzes the N tau-methylation of histamine and structurally-related compounds. Levels of HNMT activity in the human red blood cell are regulated by inheritance. The inbred mouse is an ideal laboratory animal in which to study the genetics of inherited traits. Therefore, HNMT activity was measured in renal homogenates of A/J mice to establish optimal assay conditions and to determine the properties of mouse kidney HNMT as a first step toward testing the hypothesis that large strain-related variations in HNMT activity might exist among inbred strains of mice. Apparent Km values for histamine and S-adenosyl-L-methionine, the two cosubstrates for the reaction, were 26 and 1.7 microM, respectively. IC50 values for the inhibition of mouse kidney HNMT by amodiaquine and S-adenosyl-L-homocysteine were 1.67 and 11.8 microM, respectively. HNMT activity levels were then measured under optimal assay conditions in renal preparations from male animals of eleven inbred mouse strains chosen because of the availability of recombinant inbred (RI) animals derived from the parental strains. Average values for renal HNMT activity varied among strains by less than two-fold and ranged only from 26.2 +/- 0.51 (mean +/- SEM) units/mg protein in AKR/J mice to 39.1 +/- 2.58 units/mg protein in C57BL/6J animals. Renal HNMT activities in females of the three strains in which both sexes were studied were 11-13% higher than were those in renal tissue from males of the same strain. In summary, the properties of HNMT in the mouse kidney are similar to those of HNMT in other species, but strain variation in levels of enzyme activity among the 11 inbred mouse strains studied was insufficient for these animals to be used in biochemical genetic experiments.

Radioenzymatic assay for histamine: development and validation

Radioenzymatic assays are sensitive analytic tools that use an enzyme to quantify a substrate for that enzyme. Purified histamine N-methyltransferase has been used as the basis for an assay for histamine. The sensitivity of the procedure is less than 10 fmol. The specificity of the assay is increased when the transferase reaction is carried out at 0-3 degrees C. Data documenting the precision of the assay, the stability of histamine in human plasma, and the gastric secretory rate of histamine are presented along with a chronologic description of the development of the technique.

An assessment of plasma histamine concentrations during documented endotoxic shock

Recent reviews of the literature involving histamine release during sepsis and endotoxemia have reported that the majority of the studies are inconclusive due to inadequate assays or experimental protocols. In a controlled experimental setting we have employed a specific and sensitive radioenzymatic assay to determine plasma histamine concentrations temporally during documented endotoxin-induced shock in the conscious rat. Cardiovascular and metabolic measurements for the control group (n = 7) were normal during the study period. Endotoxin (n = 8, LD/90-24 hrs.) induced an early transient hypotensive episode and increase in systemic vascular resistance and a sustained decrease in cardiac index and central venous pressure and increase in heart and respiratory rates. Hypoglycemia and hyperlacticemia were present at the end of the four-hour study period. The small intestine was severely hemorrhaged in all animals given endotoxin. Histamine concentrations for the control group were unchanged throughout the study period. Contrary to previous reports, this model of endotoxemia revealed unchanging histamine concentrations during the first 30 minutes which were temporally coincident with the characteristic early hypotensive episode evoked by endotoxin. The histamine concentrations at 60 and 240 minutes following endotoxin were increased two and three-fold, respectively, compared to the control group. Three of the 8 rats given endotoxin died before four hours; histamine concentrations in plasma taken when death appeared certain were 42, 91, and 174, compared to the control value of approximately 8 ng/ml. There was no clear association of the increases in plasma histamine with any of the parameters measured in this study: however, established histamine effects may have been masked by the pre-existing effects of other mediators known to be active during endotoxemia. In separate groups of animals endotoxin (n = 5) elicited early increases in plasma concentrations of norepinephrine (5-fold) and epinephrine (8-fold) that remained elevated for the 4-hour study period while the control group (n = 4) remained stable. This study establishes that a) plasma histamine concentrations are increased during endotoxemia, b) plasma histamine is not elevated during the initial hypotension episode following endotoxin, c) plasma histamine increases during the progression of endotoxic shock, and d) plasma histamine concentrations are extremely high prior to death.

Inflammatory mediator release on conjunctival provocation of allergic subjects with allergen

To evaluate the role of inflammatory mediators in the pathogenesis of the ocular allergic response, 23 subjects with positive histories of allergies to either cat dander or ragweed pollen and positive skin tests to the appropriate allergen extract were recruited and were subjected to conjunctival provocation. The tear duct of the left eye of each subject was blocked with a collagen plug while the right eye was left unplugged. In all cases, the eye was initially provoked with saline and subsequently with the appropriate allergen extract. Nonallergic subjects, or allergic subjects provoked with nonrelevant allergen, were used as control subjects. After each provocation, symptoms were recorded, and tears were collected with preweighed strips of filter paper (Schirmer strip). Each strip was placed into a tared tube containing fluid appropriate for the optimal preservation of the mediator to be measured. It was therefore possible to calculate the weight of tears collected and to express mediator levels per milliliter of tears. All allergic subjects demonstrated a positive symptomatic response to allergen challenge, whereas the control subjects remained asymptomatic. Blockage of the tear duct did not significantly alter the response. For allergic subjects, the levels of histamine, kinins, prostaglandin D2, albumin, and TAME-esterase activity were all significantly (p less than 0.005 in each case) greater after allergen challenge than after saline challenge. Furthermore, levels of each of these mediators after allergen challenge (expressed as increases above levels after saline provocation) were significantly greater for allergic subjects than for control subjects (p less than 0.005 in each case). Thus, the clinical response to conjunctival provocation with allergen is associated with increases in the levels of inflammatory mediators in tears.

Inhibition of histamine N-methyltransferase (HNMT) in vitro by neuromuscular relaxants

There have been reports of hypotension and flushing following vecuronium administration. The etiology of these symptoms, which are similar to those of histamine release, is not clear. The steroidal neuromuscular relaxants (NMRs), unlike muscle relaxants structurally similar to curare, have been shown not to cause histamine release after the administration of typical clinical doses. Histamine levels in plasma reflect a balance between release and catabolism. In humans, histamine N-methyl-transferase (HNMT) is the enzyme primarily degrading for histamine. Therefore, we performed in vitro kinetic studies of purified HNMT to determine the effects of the steroidal and curare-like NMRs and also of gallamine on histamine catabolism. We demonstrated that all NMRs tested were inhibitors of HNMT in vitro. The inhibition was competitive with respect to the cosubstrate S-adenosyl-L-[3H-methyl] methionine, and noncompetitive with respect to histamine. The rank order of inhibition was vecuronium greater than pancuronium greater than gallamine greater than d-tubocurarine greater than metocurine greater than atracurium greater than pipecuronium, with Ki values ranging from 1.2 to 44.8 microM. Our data suggest that HNMT-based radioenzymatic assays for histamine should be susceptible to inhibition by concurrent use of NMRs, particularly vecuronium.

Interleukin-1 potentiates antigen-mediated arachidonic acid metabolite formation in mast cells

Mast cells were isolated from human lung tissues using density gradient centrifugation and a fluorescence-activated cell sorter. Purified cells were sensitized passively with serum from allergic patients sensitive to grass pollen and then challenged with antigen (grass pollen). When these cells were challenged with antigen, LTC4, and PGD2 (19 +/- 6, and 42 +/- 9 pmol/10(6) cells, respectively) were released during 2 hr of incubation. When mast cells were incubated with interleukin-1 (IL-1) no detectable amount of LTC4 or PGD2 was generated. However, when mast cells were challenged with antigen and IL-1, LTC4 and PGD2 were released (60 +/- 15 and 97 +/- 21 pmol/10(6), respectively) after a 2 hr incubation period. The stimulatory action of IL-1 was both time- and dose-dependent (over a 10-1000 units/ml range). In addition, greater activity was observed if IL-1 was added 5-30 min prior to the antigen. Inhibitors of arachidonic acid metabolic pathways prevented the release of LTC4 and PGD2 from mast cells activated with antigen and IL-1. This study shows that IL-1 does not stimulate arachidonic acid metabolite release by mast cells but potentiates the release induced by antigen.

Quantification of urinary histamine by a new radioenzymatic assay: documentation of assay specificity and establishment of normal excretion rates

The goals of this study were to evaluate the specificity of a new radioenzymatic assay for histamine when it is used for urinary-histamine determinations and to establish normal rates of urinary-histamine excretion for male and female subjects. Specificity of the assay was characterized by reacting urine samples at two different incubation temperatures and with varying amounts of highly purified histamine N-methyltransferase and S-adenosyl-L-[methyl-3H]methionine. The radiolabeled products were then separated by thin-layer chromatography and visualized by fluorescence-enhanced autoradiography. Our results indicate that at least one other substrate for histamine N-methyltransferase in addition to histamine is present in human urine, although the identity of this compound could not be determined. Optimization of enzyme-reaction conditions eliminated the methylation of this unidentified substrate while the methylation of histamine was maintained. The average urinary unidentified substrate while the methylation of histamine was maintained. The average urinary excretion of histamine determined for male subjects (N = 37) and female subjects (N = 111) was 18.7 micrograms/24 hr and 27.3 micrograms/24 hr, respectively. More than 12% of the normal female subjects examined had urinary-histamine excretion rates in excess of 50 micrograms/24 hr, which overlaps into the range previously considered to be indicative of abnormal mast cell secretion. In summary, this is the first radioenzymatic assay for histamine with documented specificity for the measurement of histamine in human urine.

Release of leukotrienes and histamine by the isolated anaphylactic heart

The purpose of this study was to demonstrate the ability of heart tissue to release the mediators of anaphylaxis after antigenic challenges. Guinea pigs were sensitized with ovalbumin. Hearts were excised, perfused in a langendorff apparatus, and challenged with a bolus injection of ovalbumin. Analysis of the perfusates demonstrated the presence of histamine as determined by radioenzymatic assay. Histamine release was observed to be maximum after 2 min (8 +/- 1 nmol) of perfusion, then decreased to baseline level. The heart also released LTB4, LTC4, LTD4, and LTE4 as determined by high performance liquid chromatography and bioassays. The release of LTC4 occurred rapidly, reaching maximum after 2 min (4.2 +/- 1 pmol) and then returned to baseline level. Although the release of LTD4 paralleled the release of LTC4, it reached a maximum after 5 min (7.7 +/- 2 pmol). LTE4 was detected after 10 min and was undetectable after 15 min. Maximum release of LTB4 was observed after 5-10 min (15 +/- 3 pmol) and was no longer detectable after 15 min. These results indicate that the isolated sensitized heart undergoing antigenic challenge releases leukotrienes and histamine suggesting the cardiac anaphylaxis might occur by the locally released mediators.