The effect of histamine-H1 receptor antagonism with terfenadine on concentration-related AMP-induced bronchoconstriction in asthma

Extracellular cAMP-Adenosine Pathway Signaling: A Potential Therapeutic Target in Chronic Inflammatory Airway Diseases

Adenosine is a purine nucleoside that, via activation of distinct G protein-coupled receptors, modulates inflammation and immune responses. Under pathological conditions and in response to inflammatory stimuli, extracellular ATP is released from damaged cells and is metabolized to extracellular adenosine. However, studies over the past 30 years provide strong evidence for another source of extracellular adenosine, namely the “cAMP-adenosine pathway.” The cAMP-adenosine pathway is a biochemical mechanism mediated by ATP-binding cassette transporters that facilitate cAMP efflux and by specific ectoenzymes that convert cAMP to AMP (ecto-PDEs) and AMP to adenosine (ecto-nucleotidases such as CD73). Importantly, the cAMP-adenosine pathway is operative in many cell types, including those of the airways. In airways, β₂-adrenoceptor agonists, which are used as bronchodilators for treatment of asthma and chronic respiratory diseases, stimulate cAMP efflux and thus trigger the extracellular cAMP-adenosine pathway leading to increased concentrations of extracellular adenosine in airways. In the airways, extracellular adenosine exerts pro-inflammatory effects and induces bronchoconstriction in patients with asthma and chronic obstructive pulmonary diseases. These considerations lead to the hypothesis that the cAMP-adenosine pathway attenuates the efficacy of β₂-adrenoceptor agonists. Indeed, our recent findings support this view. In this mini-review, we will highlight the potential role of the extracellular cAMP-adenosine pathway in chronic respiratory inflammatory disorders, and we will explore how extracellular cAMP could interfere with the regulatory effects of intracellular cAMP on airway smooth muscle and innate immune cell function. Finally, we will discuss therapeutic possibilities targeting the extracellular cAMP-adenosine pathway for treatment of these respiratory diseases.

Enhancement of Mast Cell Degranulation Mediated by Purinergic Receptors' Activation and PI3K Type δ

Mast cells express multiple metabotropic purinergic P2Y receptor (P2YR) subtypes. Few studies have evaluated their role in human mast cell (HMC) allergic response as quantified by degranulation induced by cross-linking the high-affinity IgE receptor (FcεRI). We have previously shown that extracellular nucleotides modify the FcεRI activation-dependent degranulation in HMCs derived from human lungs, but the mechanism of this action has not been fully delineated. This study was undertaken to determine the mechanism of activation of P2YRs on the degranulation of HMCs and elucidate the specific postreceptor pathways involved. Sensitized LAD2 cells, a human-derived mast cell line, were subjected to a weak allergic stimulation (WAS) using a low concentration of Ag in the absence and presence of P2YR agonists. Only the metabotropic purinergic P2Y11 receptor (P2Y11R) agonist, adenosine 5'-(3-thio)triphosphate (ATPγS), enhanced WAS-induced degranulation resulting in a net 7-fold increase in release (n = 4; p < 0.01). None of the P2YR agonists tested, including high concentrations of ATPγS (1000 μM), enhanced WAS-induced intracellular Ca²⁺ mobilization, an essential component of activated FcεRI-induced degranulation. Both a PI3K inhibitor and the relevant gene knockout decreased the ATPγS-induced enhancement. The effect of ATPγS was associated with enhanced phosphorylation of PI3K type δ and protein kinase B, but not the phosphoinositide-dependent kinase-1. The effects of ATPγS were dose dependently inhibited by NF157, a P2Y11R antagonist. To our knowledge, these data indicate for the first time that P2YR is linked to enhancement of allergic degranulation in HMC via the PI3K/protein kinase B pathway.

The role of antihistamines in asthma management

Histamine is an important mediator in airway inflammation. It is elevated in the airways of asthmatic patients and is responsible for many of the pathophysiological features in asthma. Antihistamines block the actions of histamine and also have effects on inflammation which is independent of histamine-H(1)-receptor antagonism. Antihistamines have been shown to have bronchodilatory effects, effects on allergen-, exercise-, and adenosine-monophosphate-challenge testing, and also to prevent allergen-induced nonspecific airways hyperresponsiveness. Clinical studies have shown mixed results, and some studies have reported beneficial effects of azelastine, cetirizine, desloratadine, and fexofenadine on asthma symptoms or physiological measures in patients with asthma. The combination of an antihistamine and a leukotriene receptor antagonist has been shown to have additive effects in certain studies. Antihistamines have also been shown to delay or prevent the development of asthma in a subgroup of atopic children. These data suggest that antihistamines may have beneficial effects in the management of asthma.

Therapeutic applications

The current treatments offered to patients with chronic respiratory diseases are being re-evaluated based on the loss of potency during long-term treatments or because they only provide significant clinical benefits to a subset of the patient population. For instance, glucocorticoids are considered the most effective anti-inflammatory therapies for chronic inflammatory and immune diseases, such as asthma. But they are relatively ineffective in asthmatic smokers, and patients with chronic obstructive pulmonary disease (COPD) or cystic fibrosis (CF). As such, the pharmaceutical industry is exploring new therapeutic approaches to address all major respiratory diseases. The previous chapters demonstrated the widespread influence of purinergic signaling on all pulmonary functions and defense mechanisms. In Chap. 8, we described animal studies which highlighted the critical role of aberrant purinergic activities in the development and maintenance of chronic airway diseases. This last chapter covers all clinical and pharmaceutical applications currently developed based on purinergic receptor agonists and antagonists. We use the information acquired in the previous chapters on purinergic signaling and lung functions to scrutinize the preclinical and clinical data, and to realign the efforts of the pharmaceutical industry.

IL-4 amplifies the pro-inflammatory effect of adenosine in human mast cells by changing expression levels of adenosine receptors

Adenosine inhalation produces immediate bronchoconstriction in asthmatics but not in normal subjects. The bronchospastic effect of adenosine is largely mediated through adenosine-induced mast cell activation, the mechanism of which is poorly understood due to limitations in culturing human primary mast cells. Here, we show that human umbilical cord blood -derived mast cells incubated with the Th2 cytokine IL-4 develop increased sensitivity to adenosine. Potentiation of anti-IgE- induced and calcium ionophore/PMA-induced degranulation was augmented in mast cells cultured with IL-4, and this effect was reduced or abolished by pre-treatment with A_2BsiRNA and selective A_2B receptor antagonists, respectively. IL-4 incubation resulted in the increased expression of A_2B and reduced expression of A_2A adenosine receptors on human mast cells. These results suggest that Th2 cytokines in the asthmatic lung may alter adenosine receptor expression on airway mast cells to promote increased responsiveness to adenosine.

Methylxanthines in asthma

Methylxanthines represent a unique class of drugs for the treatment of asthma. The methylxanthine theophylline has demonstrated efficacy in attenuating the three cardinal features of asthma - reversible airflow obstruction, airway hyperresponsiveness, and airway inflammation. At doses achieving relatively high serum levels in which toxic side effects are sometimes observed, direct bronchodilatory effects of theophylline are recognized. At lower serum concentrations, theophylline is a weak bronchodilator but retains its capacity as an immunomodulator, anti-inflammatory, and bronchoprotective drug. Intense investigation into the molecular mechanisms of action of theophylline has identified several different points of action. Phosphodiesterase inhibition and adenosine receptor antagonism have both been implicated in promoting airway smooth muscle relaxation and bronchodilation. Similar mechanisms of action may explain the inhibitory effects of theophylline on immune cells. At lower concentrations that fail to inhibit phosphodiesterase, effects on histone deacetylase activity are believed to contribute to the immunomodulatory actions of theophylline. Since anti-inflammatory and immunomodulatory effects of methylxanthines are realized at lower serum concentrations than are required for bronchodilation, theophylline's predominant role in asthma treatment is as a controller medication for chronic, persistent disease.

Intranasal administration of NECA can induce both anti-inflammatory and pro-inflammatory effects in BALB/c mice: evidence for A 2A receptor sub-type mediation of NECA-induced anti-inflammatory effects

The role of adenosine in allergic inflammation is unclear. This study investigated the effects of the non-selective adenosine receptor agonist, 5-N-ethylcarboxamidoadenosine (NECA), on immunized only and immunized and airway challenged mice. The adenosine receptor sub-type(s) mediating the NECA effects and the A(2A) receptor mRNA expression were also investigated. In mice that were only immunized, intranasal NECA (1 mM) administration caused a significant increase in bronchoalveolar lavage total cell count (TCC), neutrophils and eosinophils (>1.5-, >6 and >60-fold, respectively). Two and four intranasal ovalbumin (OVA) challenges induced a significant (P < 0.05) increase in TCC (>2.1- and >4-fold, respectively) and eosinophils (>350- and >1700-fold, respectively). Real-time PCR analysis showed that the A(2A) receptor sub-type mRNA was significantly increased (P < 0.05) in the lung tissue of immunized mice following both two and four OVA challenges. NECA (0.3 mM) treatment caused a significant reduction in the increase induced by the two and four OVA challenges in the TCC by 46.1% and 56.6%, respectively, eosinophils by 70.1% and 75.6%, respectively, and in the A(2A) receptor sub-type mRNA by 43.2% and 41.0%, respectively. Treatment with the A(2A) receptor antagonist, 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine), SCH-58261, completely reversed both the NECA-mediated reduction in TCC and eosinophilia. Moreover, OVA challenge of immunized mice, over 2 consecutive days, resulted in a significant (P < 0.05) increase in TCC (4.5-fold) and eosinophils (>2000-fold) that was detected 72 h later. NECA (0.3 mM) treatment, at 24 and 48 h post OVA challenge, significantly reduced the increase in both TCC and eosinophils by 45.0% and 74.8%, respectively. Our data show that in immunized, but not OVA-challenged mice, high dose of NECA (1 mM) induces an inflammatory airway response. In contrast, in models of inflammation, NECA, at mainly 0.3 mM, induces a significant anti-inflammatory effect when administered prior to the induction of airway inflammation or therapeutically following its establishment. The data also indicate that the anti-inflammatory action of NECA seems to be mediated via the A(2A) receptor sub-type and hence the use of selective A(2A) receptor agonists as potential therapeutic agents in the treatment of inflammatory diseases such as asthma should be investigated further.

Adenosine induces airway hyperresponsiveness through activation of A3 receptors on mast cells

BACKGROUND

The mechanisms responsible for the development of airway hyperresponsiveness in asthma are poorly understood. Adenosine levels are high in the lungs of patients with asthma, but a role for adenosine in the development of this cardinal feature of asthma has not been previously reported.

OBJECTIVE

To determine the capacity of adenosine to induce airway hyperresponsiveness, and to investigate the mechanisms behind these effects of adenosine on airway physiology.

METHODS

Wild-type C57BL/6 mice were exposed to aerosolized adenosine analog adenosine-5' N-ethylcarboxamide (NECA), and subsequent hyperresponsiveness to methacholine was investigated by measuring airway mechanics after anesthesia and tracheostomy. Similar experiments were conducted with A(1)-deficient, A(3)-deficient, and mast cell-deficient mice, as well as with mast cell-deficient mice engrafted with wild-type (wt) or A(3)(-/-) mast cells. The effect of NECA on methacholine-induced tension development in ex vivo tracheal rings was also examined.

RESULTS

Exposure of wt mice to NECA resulted in the robust induction of airway hyperresponsiveness. NECA failed to induce hyperresponsiveness to methacholine in tracheal ring preps ex vivo, and NECA-induced airway hyperresponsiveness in vivo was not affected by the genetic inactivation of the A(1) adenosine receptor. In contrast, NECA-induced airway hyperresponsiveness was abolished in A(3) adenosine receptor-deficient mice and in mice deficient in mast cells. Reconstitution of mast cell-deficient mice with wt mast cells restored hyperresponsiveness, whereas reconstitution with A(3) receptor-deficient mast cells did not.

CONCLUSION

Adenosine induces airway hyperresponsiveness indirectly by activating A(3) receptors on mast cells.

Involvement of A1 adenosine receptors and neural pathways in adenosine-induced bronchoconstriction in mice

High levels of adenosine can be measured from the lungs of asthmatics, and it is well recognized that aerosolized 5'AMP, the precursor of adenosine, elicits robust bronchoconstriction in patients with this disease. Characterization of mice with elevated adenosine levels secondary to the loss of adenosine deaminase (ADA) expression, the primary metabolic enzyme for adenosine, further support a role for this ubiquitous mediator in the pathogenesis of asthma. To begin to identify pathways by which adenosine can alter airway tone, we examined adenosine-induced bronchoconstriction in four mouse lines, each lacking one of the receptors for this nucleoside. We show, using direct measures of airway mechanics, that adenosine can increase airway resistance and that this increase in resistance is mediated by binding the A(1) receptor. Further examination of this response using pharmacologically, surgically, and genetically manipulated mice supports a model in which adenosine-induced bronchoconstriction occurs indirectly through the activation of sensory neurons.

Adenosine receptors: promising targets for the development of novel therapeutics and diagnostics for asthma

Interest in the role of adenosine in asthma has escalated considerably since the early observation of its powerful bronchoconstrictor effects in asthmatic but not normal airways. A growing body of evidence has emerged in support of a proinflammatory and immunomodulatory role for the purine nucleoside adenosine in the pathogenic mechanisms of chronic inflammatory disorders of the airways such as asthma. The fact that adenosine enhances mast cell allergen-dependent activation, that elevated levels of adenosine are present in chronically inflamed airways, and that adenosine given by inhalation cause dose-dependent bronchoconstriction in subjects with asthma emphasizes the importance of adenosine in the initiation, persistence and progression of these common inflammatory disorders of the airways. These distinctive features of adenosine have been recently exploited in the clinical and research setting to identify innovative diagnostic applications for asthma. In addition, because adenosine exerts its multiple biological activities by interacting with four adenosine receptor subtypes, selective activation or blockade of these receptors may lead to the development of novel therapies for asthma.

Transcriptional up-regulation of histamine receptor-1 in epithelial, mucus and inflammatory cells in perennial allergic rhinitis

BACKGROUND

Histamine receptors play an important role in the pathogenesis of nasal allergy. Activation of histamine receptor 1 (H1R) and 2 (H2R) can cause allergic symptoms which can be blocked effectively by antihistamines. H1R and H2R transcript levels have been found to be up-regulated in perennial - but not in seasonal - allergic rhinitis (AR). The present study aimed to explore H1R and H2R expression in complex tissues of the nasal mucosa of perennial allergic rhinitis (PAR).

METHODS

Ten patients with PAR and 13 non-AR subjects were recruited for the study by medical history, physical examination and laboratory screening tests. In this study, we have analysed single cells dissected from the nasal mucosa biopsies by laser-assisted microdissection. H1R mRNA expression was analysed in different cell types such as epithelial, endothelial, mucus and inflammatory cells isolated from the nasal mucosa of PAR in comparison with non-AR subjects.

RESULTS

H1R mRNA gene expression level was significantly increased in the nasal mucosa of PAR in comparison with non-AR (P<0.0001). H1R mRNA was significantly elevated in epithelial (P<0.001) and mucus cells (P<0.05) of PAR in comparison with non-AR whereas H1R gene expression levels in endothelial cells between both groups were not changed (P=0.23). Interestingly, inflammatory cells in the nasal mucosa of PAR patients were also strongly expressed H1R mRNA (P<0.001).

CONCLUSION

The present study indicates that PAR alters the expression of H1R mRNA in epithelial, mucus and inflammatory cells of the nasal mucosa and but not in endothelial cells. Therefore, epithelial, mucus and inflammatory cells may play an important role in histamine-mediated allergic airway inflammation in PAR.

Adenosine in the airways: implications and applications

Adenosine in a signaling nucleoside eliciting many physiological responses. Elevated levels of adenosine have been found in bronchoalveolar lavage, blood and exhaled breath condensate of patients with asthma a condition characterized by chronic airway inflammation. In addition, inhaled adenosine-5'-monophosphate induces bronchoconstriction in asthmatics but not in normal subjects. Studies on animals and humans have shown that bronchoconstriction is most likely due to the release of inflammatory mediators from mast cells. However a number of evidences suggest that adenosine modulates the function of many other cells involved in airway inflammation such as neutrophils, eosinophils, lymphocytes and macrophages. Although this clear pro-inflammatory role in the airways, adenosine may activate also protective mechanisms particularly against lung injury. For many years this dual role of adenosine in the respiratory system has represented an enigma, and only recently it has become clear that biological functions of adenosine are mediated by four distinct subtypes of receptors (A1, A2A, A2B, and A3) and that biological responses are determined by the different pattern of receptors distribution in specific cells. Therefore, pharmacological modulation of adenosine receptors, particularly A2B, may represent a novel therapeutic approach for inflammatory diseases. Moreover, as bronchial response to adenosine strictly reflects airway inflammation in asthma, bronchial challenge with adenosine is considered a valuable clinical tool to monitor airway inflammation, to follow the response to anti-inflammatory treatments and to help in the diagnostic discrimination between asthma and chronic obstructive lung disease.

Synergy between A2B adenosine receptors and hypoxia in activating human lung fibroblasts

Chronic inflammatory airway diseases, such as asthma, chronic obstructive pulmonary disease and pulmonary fibrosis, are associated with subepithelial fibroblast activation, myofibroblast hyperplasia, hypoxia, and increase in interstitial adenosine concentrations. The goal of this study was to determine the effect of adenosine and its receptors on activation of human lung fibroblasts under normoxia (21% O2) and hypoxia (5% O2). Under the normoxic condition, adenosine and its stable analog, 5'-(N-ethylcarboxamido)-adenosine, via activation of A2B adenosine receptors, increased the release of interleukin (IL)-6 by 14-fold and induced the differentiation of human lung fibroblasts to myofibroblasts. This latter effect of 5'-(N-ethylcarboxamido)-adenosine was abolished by an IL-6-neutralizing antibody. Hypoxia increased the release of IL-6 by 2.8-fold, and there was a synergy between hypoxia and activation of A2B adenosine receptors to increase the release of IL-6 and to induce differentiation of fibroblasts into myofibroblasts. Hypoxia increased the expression of A2B adenosine receptors by 3.4-fold. Altogether, these data suggest that hypoxia amplifies the effect of adenosine on the release of IL-6 and cell differentiation by upregulating the expression of A2B adenosine receptors. Our findings provide a novel mechanism whereby adenosine participates in the remodeling process of inflammatory lung diseases.

Mast cell mediator release in nonasthmatic subjects after endobronchial adenosine challenge

BACKGROUND

Adenosine 5'-monophosphate (AMP) has been shown to cause bronchoconstriction in atopic subjects but to have no effect on nonatopic nonasthmatic subjects. Endobronchial AMP challenge has previously been shown to cause mast cell mediator release in asthmatic subjects, but it is unknown whether a similar response occurs in atopic nonasthmatic and nonatopic nonasthmatic control subjects who have no response to inhalation AMP challenge.

OBJECTIVE

This study examined the change in mast cell-derived products after endobronchial saline challenge and AMP challenge in subjects with and without a positive inhalation response to AMP.

METHODS

Inhalation challenge with AMP challenge was performed in normal, atopic nonasthmatic, and atopic asthmatic subjects. Levels of mast cell mediators were measured after endobronchial adenosine challenge and after placebo endobronchial saline challenge.

RESULTS

There were significant increases in histamine, tryptase, protein, and prostaglandin D2 levels (P=.02, P=.02, P=.01, and P=.01, respectively) after AMP challenge compared with after saline challenge in nonatopic nonasthmatic subjects. There was no significant increase in any mediator in either of the other 2 groups.

CONCLUSION

This study suggests dissociation between mediator release and bronchoconstriction in response to AMP.

Targeting adenosine receptors: novel therapeutic targets in asthma and chronic obstructive pulmonary disease

Adenosine, an endogenous signaling nucleoside that modulates many physiological processes has been implicated in playing an ever increasingly important role in the pathogenesis of asthma and chronic obstructive pulmonary disease (COPD). All cells contain adenosine and adenine nucleotides and the cellular production of adenosine is greatly enhanced under conditions of local hypoxia as may occur in inflammatory conditions such as asthma and COPD. In 1983, it was first reported that inhaled adenosine causes dose-related bronchoconstriction in patients with both allergic and non-allergic asthma but not in healthy volunteers. This hyperresponsiveness was also reported in patients with COPD, with those patients who smoked exhibiting a significantly greater response. This bronchoconstrictor effect of adenosine is orchestrated through the stimulation of specific cell membrane receptors and involves an important inflammatory cell, the mast cell. There is substantial evidence which suggests that mast cell activation is central to this unique response to adenosine. Mast cell mediator release makes a significant contribution towards airflow obstruction and the consequent symptoms in patients with asthma. Over the last two decades, researchers have investigated the effect of mast cell inhibitors as well as mast cell mediator receptor antagonists and their role in attenuating the bronchoconstrictor response to inhaled adenosine 5'-monophosphate (AMP). Promising results have been shown using mast cell stabilizers, histamine H1 receptor antagonists, selective cysteinyl leukotriene-1 receptor antagonists and inhibitors of 5-lipoxygenase and cyclo-oxygenase. Through these findings, the mast cell has been recognized as being a critical inflammatory cell in the adenosine-induced response in patients with asthma and COPD. To date, four subtypes (A1, A2A, A2B, A3) of adenosine receptors have been cloned each with a unique pattern of tissue distribution and signal transduction. Activation of these receptors has pro- and anti-inflammatory consequences making the development of agonists and/or antagonists at these receptor sites a novel approach in the treatment of patients with asthma and COPD. This review highlights the importance of adenosine in the pathophysiology of asthma and COPD, the critical role of the mast cell and the potential to target the adenosine receptor subtype in patients with asthma and COPD. The complete characterization of these adenosine receptor subtypes in terms of their distribution in humans and the development of selective agonists and antagonists, holds the key to our complete understanding of the role of this important mediator in asthma and COPD.

Activation of murine lung mast cells by the adenosine A3 receptor

Adenosine has been implicated to play a role in asthma in part through its ability to influence mediator release from mast cells. Most physiological roles of adenosine are mediated through adenosine receptors; however, the mechanisms by which adenosine influences mediator release from lung mast cells are not understood. We established primary murine lung mast cell cultures and used real-time RT-PCR and immunofluorescence to demonstrate that the A(2A), A(2B), and A(3) adenosine receptors are expressed on murine lung mast cells. Studies using selective adenosine receptor agonists and antagonists suggested that activation of A(3) receptors could induce mast cell histamine release in association with increases in intracellular Ca(2+) that were mediated through G(i) and phosphoinositide 3-kinase signaling pathways. The function of A(3) receptors in vivo was tested by exposing mice to the A(3) receptor agonist, IB-MECA. Nebulized IB-MECA directly induced lung mast cell degranulation in wild-type mice while having no effect in A(3) receptor knockout mice. Furthermore, studies using adenosine deaminase knockout mice suggested that elevated endogenous adenosine induced lung mast cell degranulation by engaging A(3) receptors. These results demonstrate that the A(3) adenosine receptor plays an important role in adenosine-mediated murine lung mast cell degranulation.

Evolving concepts on the value of adenosine hyperresponsiveness in asthma and chronic obstructive pulmonary disease

Adenosine is a purine nucleoside which mediates a variety of cellular responses relevant to asthma and COPD through interaction with specific receptors. Administration of adenosine by inhalation to patients with asthma and COPD is known to cause concentration related bronchoconstriction. Responses elicited by this purine derivative in asthma and COPD should not be considered as a mere reflection of non-specific airways hyperresponsiveness. Evaluation of airways responsiveness by adenosine induced bronchoconstriction may be valuable in differentiating asthma from COPD, monitoring of anti-inflammatory treatment in asthma, surveying disease progression, and assessing disease activity in relation to allergic airways inflammation.

Role of cysteinyl leukotrienes in adenosine 5'-monophosphate induced bronchoconstriction in asthma

BACKGROUND

Adenosine induced bronchoconstriction in patients with asthma is thought to be mediated by the synthesis and release of autacoids from airway mast cells. In vitro, adenosine induced constriction of asthmatic bronchi is blocked by a combination of specific histamine and cysteinyl leukotriene receptor antagonists, but the relative contribution of these mediators in vivo is unclear. We hypothesised that adenosine induced bronchoconstriction in asthmatic patients may be blocked by pretreatment with the orally active selective cysteinyl leukotriene-1 (CysLT(1)) receptor antagonist, montelukast.

METHODS

In a randomised, double blind, crossover study, oral montelukast (10 mg) or placebo was administered once daily on two consecutive days to 18 patients with mild to moderate persistent atopic asthma. Incremental doses of adenosine 5'-monophosphate (AMP) from 0.39 to 400 mg/ml were inhaled by dosimeter and the dose producing a 20% fall in FEV(1) (PC(20)AMP) after AMP inhalation was recorded. Leukotriene E(4) (LTE(4)) urinary concentrations were measured by enzyme immunoassay 4 hours after AMP challenge.

RESULTS

Montelukast pretreatment provided highly significant protection against adenosine induced bronchoconstriction, with geometric mean PC(20)AMP values of 52.6 mg/ml (95% CI 35.2 to 78.7) after placebo and 123.9 mg/ml (95% CI 83.0 to 185.0) after montelukast (p=0.006). The geometric mean of the montelukast/placebo PC(20)AMP ratio was 2.4 (95% CI 1.3 to 4.2). Montelukast had no significant effect on 4 hour urinary excretion of LTE(4) compared with placebo.

CONCLUSIONS

Selective CysLT(1) receptor antagonism with montelukast provides highly significant protection against AMP induced bronchoconstriction in patients with atopic asthma, implying that cysteinyl leukotrienes are generated from airway mast cells through preferential activation of their A(2B) receptors.

Adenosine monophosphate and histamine induced bronchoconstriction: repeatability and protection by terbutaline

BACKGROUND

Inhaled adenosine monophosphate (AMP) is thought to cause bronchoconstriction in asthmatic patients indirectly through mast cell mediator release. It may therefore be a more sensitive marker of airway inflammation in asthma and hence more specific for epidemiological surveys of asthma than challenges that act directly on airway smooth muscle such as histamine. There is some uncertainty as to how repeatable the measurement is and this is important if it is to be used for epidemiological studies.

METHODS

The response to histamine and AMP challenges and the protection afforded by terbutaline (500 micrograms) against these two challenges was measured on two occasions two weeks apart in 20 subjects with asthma (19 completed the study). The response to histamine and AMP was measured as the provocative dose causing a 20% fall in forced expiratory volume in one second (PD20) and the protection afforded by terbutaline in doubling doses (DD). Repeatability was assessed as the limits of agreement.

RESULTS

Although terbutaline had a slightly greater protective effect against AMP than histamine on both the first (delta PD20 = 2.66 versus 2.11 DD) and second occasion (2.56 and 2.15 DD), the differences were not statistically significant. The limits of agreement for the two histamine and two AMP challenges after placebo were from 3.06 to -3.5 and from 3.78 to -4.54 DD respectively, and these values did not differ significantly. The agreement limits between the first PD20 histamine and PD20 AMP values after placebo were similar, being from 3.73 to -3.72 DD after allowing for the 17.8-fold higher PD20 values for AMP compared with histamine.

CONCLUSIONS

Terbutaline caused a slightly greater inhibition of the bronchoconstrictor response to AMP than histamine but the differences were small and non-significant. Any differences in repeatability between AMP and histamine challenges are small and in this study were not significant. The fact that the agreement between histamine and AMP PD20 values was similar to the agreement between repeat histamine or repeat AMP PD20 values suggests that, within an asthmatic population, PD20 AMP may not be providing different information from that provided by PD20 histamine.

The effect of inhaled ipratropium bromide alone and in combination with oral terfenadine on bronchoconstriction provoked by adenosine 5'-monophosphate and histamine in asthma

The aim of this study was to investigate the effect of terfenadine, an antihistamine, 180 mg orally, the anticholinergic drug, ipratropium bromide (IB), 0.5 mg nebulized aerosol, the combination of these two drugs, and placebo tablets and aerosol on histamine- and adenosine 5'-monophosphate (AMP)-induced bronchoconstriction in a randomized, double-blind fashion. Airway response was evaluated as FEV1. After placebo, the geometric mean (GM) provocative concentration causing a 20% in FEV1 from the postsaline baseline value (PC20) for histamine and AMP was 0.63 and 5 mg/ml, respectively. Terfenadine displaced the FEV1 concentration-response curves obtained with both histamine (GM PC20 values increasing to 26.92 mg/ml) and AMP (GM PC20 values increasing to 26.7 mg/ml) to the right. IB had a small, but significant, protective effect against the fall in FEV1 produced by histamine and AMP, the GM PC20 values increasing to 1.69 and to 12.6 mg/ml, respectively. Terfenadine and IB in combination produced protection against histamine and AMP that was more than the production produced by either drug alone, the GM PC20 values increasing to 54.76 and 47.7 mg/ml, respectively. There was no correlation between degree of bronchodilatation induced by active treatments and concentration ratios for AMP or histamine. These data suggest that histamine release and vagal reflexes both contribute to AMP-induced bronchoconstriction in clinical asthma in man.