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

Modulation of myeloid cells by adenosine signaling

Hypoxia, metabolic activity, cell death and immune responses influence the adenosine concentrations in the extracellular space. Cellular responses to hypoxia and inflammation in myeloid cells promote activation of adenosine sensing circuit, which involves increased expression of ectoenzymes that converts phospho-nucleotides such as ATP to adenosine and increased expression of G protein-coupled adenosine receptors. Adenosine sensing circuitry also involves feedforward signaling, which leads to increased expression of hypoxia-inducible factor 1-alpha (HIF1 and feedback signaling, which leads to the suppression of inflammatory transcription factor, the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation. In this review we will discuss how different subsets of myeloid cells sense adenosine accumulation and how adenosine sensing by myeloid cells influence progression of different immune-related conditions including cancer.

Diagnostic value of bronchoprovocation challenge with adenosine monophosphate versus exercise testing in early diagnosis of asthma

Background

Airway hyperresponsiveness (AHR) is a characteristic feature of bronchial asthma and is diagnosed using direct and indirect bronchoprovocation tests. The diagnosis of AHR is a challenge in symptomatic patients with a normal baseline prebronchodilator spirometry and postbronchodilator spirometry. Exercise-induced asthma or exercise-induced bronchoconstriction (EIB) is a distinct form of AHR. There is no single test that is sufficient to exclude AHR in symptomatic military personnel with normal spirometry. This study was conducted to compare the diagnostic value of indirect bronchoprovocation test using inhaled adenosine monophosphate (AMP) and exercise challenge test (ECT) in the diagnosis of EIB.

Methods

A crossover study was conducted with consecutive sampling of patients presenting with symptoms suggestive of asthma and with normal spirometry results who were subjected to both ECT and bronchoprovocation test using inhaled AMP on separate days.

Results

Forty participants were recruited (the mean age: 25 yrs, 100% male). The commonest presentation was breathlessness on exercise (55%). With exercise alone, 40% tested positive for AHR, while with AMP alone, the positivity increased to 53%, and the difference was statistically significant (p= 0.03). Exercise alone failed to detect 8 cases that tested positive for AHR by inhaled AMP challenge.

Conclusion

Indirect bronchoprovocation test using inhaled AMP may be used to diagnose AHR in conditions in which exercise challenge testing is not available or the patient is unable to complete ECT.

Does airway hyperresponsiveness monitoring lead to improved asthma control?

The current guidelines recommend an approach to asthma management based on asthma control, rather than asthma severity. Although several specific questionnaires have been developed and control criteria have been established based on clinical guidelines, the evaluation of asthma control is still not optimal. In general, these indicators provide adequate assessment of current control, but they are more limited when estimating future risk. There is much evidence demonstrating the persistence of airway inflammation and airway hyperresponsiveness (AHR) in patients with total control. Therefore, the objective of this review was to analyse the possible role of AHR monitoring as an instrument for assessing asthma control. We will evaluate its capacity as an indicator for future risk, both for estimating the possibility of clinical deterioration and loss of lung function or exacerbations. Furthermore, its relationship with inhaled corticosteroid treatment will be analysed, while emphasizing its capacity for predicting response and adjusting dosage, as well as information about the capability of AHR for monitoring treatment. Last of all, we will discuss the main limitations and emerging opportunities of AHR as an assessment instrument for asthma control.

Thymoquinone, the main constituent of Nigella sativa, affects adenosine receptors in asthmatic guinea pigs

OBJECTIVES

For determining the mechanism of anti-asthmatic effect of thymoquinone, this investigation evaluated the effect of thymoquinone in the presence of selective A2A and A2B adenosine receptor antagonists (ZM241385 and MRS1706, respectively).

MATERIALS AND METHODS

Seventy guinea pigs were randomly divided to 7 groups; control (C), sensitized with ovalbumin (S), sensitized groups pretreated with thymoquinone (S+TQ), ZM241385 (S+Anta A2A), MRS1706 (S+Anta A2B), thymoquinone and antagonists (S+Anta A2A+TQ and S+Anta A2B+TQ). Thymoquinone and each of these antagonists with 3 mg/kg dose were injected i.p. on 10(th) day of sensitization protocol. Tracheal responsiveness (TR) to methacholine and ovalbumin (OA), and total and differential cell count in lung lavage fluid (LLF) in different groups were measured.

RESULTS

Increased EC50 and LLF neutrophil count and decreased TR to methacholine and OA, LLF eosinophil and basophil counts were observed in S+TQ group compared to S group (P<0.001 to P<0.05). Significant decrease in EC50 (P<0.01), LLF neutrophil, lymphocyte and monocyte count (P<0.001 for all) and significant increase in TR to OA (P<0.01), LLF total WBC (P<0.01) and eosinophil count (P<0.001) were observed in S+A2A group compared to S+TQ group. There was significant increase in LLF eosinophil and monocyte counts in S+Anta A2B group compared with S+TQ group (P<0.001 for both). In S+TQ+Anta A2A group, there was significant increase in LLF eosinophil (P<0.001) and significant decrease in LLF neutrophil (P<0.01) and monocyte (P<0.001) counts compared with S+TQ group.

CONCLUSION

Thymoquinone affects adenosine receptors, which suggest that some of its anti-inflammatory effects may be mediated by these receptors.

The Effect of Adenosine A2A and A2B Antagonists on Tracheal Responsiveness, Serum Levels of Cytokines and Lung Inflammation in Guinea Pig Model of Asthma

PURPOSE

Nowadays adenosine is specified as an important factor in the pathophysiology of asthma. For determining the effect of different A2 receptors, in this investigation the effect of single dose of selective adenosine A2A and A2B antagonists (ZM241385 and MRS1706) on different inflammatory parameters; tracheal responsiveness to methacholine and ovalbumin, total and differential cell count in bronchoalveolar lavage (BAL), blood levels of IL-4 and IFN-γ and lung pathology of guinea pig model of asthma were assessed.

METHODS

All mentioned parameters were evaluated in two sensitized groups of guinea pigs pretreated with A2A and A2B antagonists (S+Anta A2A, S+Anta A2B) compared with sensitized (S) and control (C) groups.

RESULTS

The tracheal responsiveness to methacholine and OA, total cell and eosinophil and basophil count in BAL, blood IL-4 level and pathological changes in pre-treated group with MRS1706 (S+Anta A2B) was significantly lower than those of sensitized group (p<0.01 to p<0.05). In pretreated group with Anta A2A(S+Anta A2A), all the above changes were reversed.

CONCLUSION

These results showed a preventive effect of A2B antagonist (MRS1706) on tracheal responsiveness to methacholine and OA, total and differential cell count in bronchoalveolar lavage, blood cytokines and pathological changes. Administration of ZM241385, selective A2A antagonist, deteriorated the induction effect of ovalbumin.

Comparison of direct and indirect bronchoprovocation testing using ventilator-acquired pulmonary mechanics in healthy cats and cats with experimental allergic asthma

Airway hyperresponsiveness (AHR) is a key feature of asthma and can be measured using bronchoprovocation. Direct (methacholine, MCh) or indirect (adenosine-5-monophosphate, AMP; or mannitol) bronchoprovocants are used in human patients, the latter inducing AHR only with pre-existing airway inflammation. The present study compared the responses to direct (MCh) and indirect (mannitol, AMP) bronchoprovocation in healthy and asthmatic cats (n=6/group). The order of bronchoprovocant was randomized using a published table of random numbers and there was a 1-month washout before crossover to the next treatment. Pulmonary mechanics were measured in anesthetized and mechanically ventilated cats using a critical care ventilator. Saline at baseline and increasing doses of each bronchoprovocant were aerosolized for 30 s, followed by 4 min of data collection between doses. The endpoint for each bronchoprovocant was reached when airway resistance exceeded 200% of baseline values (EC200Raw). There was a significant difference (P<0.001) in the airway response of asthmatic vs. healthy cats over the range of MCh concentrations, despite there being no significant difference in the EC200Raw between the groups. Response to MCh was significantly greater (P<0.05) in asthmatic than in healthy cats at MCh concentrations as low as 0.0625 mg/mL. For AMP, a small subset of asthmatics (n=2/6) responded at low concentrations; four asthmatic cats and all healthy cats failed to respond even to the highest concentrations of AMP. One asthmatic cat but no healthy cats responded to mannitol. In conclusion, MCh discriminated asthmatic from healthy cats but neither AMP nor mannitol was an effective bronchoprovocant in this model.

Adenosine A(1) and prostaglandin E receptor 3 receptors mediate global airway contraction after local epithelial injury

Epithelial injury and airway hyperresponsiveness are prominent features of asthma. We have previously demonstrated that laser ablation of single epithelial cells immediately induces global airway constriction through Ca(2+)-dependent smooth muscle shortening. The response is mediated by soluble mediators released from wounded single epithelial cells; however, the soluble mediators and signaling mechanisms have not been identified. In this study, we investigated the nature of the epithelial-derived soluble mediators and the associated signaling pathways that lead to the L-type voltage-dependent Ca(2+) channel (VGCC)-mediated Ca(2+) influx. We found that inhibition of adenosine A1 receptors (or removal of adenosine with adenosine deaminase), cyclooxygenase (COX)-2 or prostaglandin E receptor 3 (EP3) receptors, epidermal growth factor receptor (EGFR), or platelet-derived growth factor receptor (PDGFR) all significantly blocked Ca(2+) oscillations in smooth muscle cells and airway contraction induced by local epithelial injury. Using selective agonists to activate the receptors in the presence and absence of selective receptor antagonists, we found that adenosine activated the signaling pathway A1R→EGFR/PDGFR→COX-2→EP3→VGCCs→calcium-induced calcium release, leading to intracellular Ca(2+) oscillations in airway smooth muscle cells and airway constriction.

Purinergic signaling in the airways

Evidence for a significant role and impact of purinergic signaling in normal and diseased airways is now beyond dispute. The present review intends to provide the current state of knowledge of the involvement of purinergic pathways in the upper and lower airways and lungs, thereby differentiating the involvement of different tissues, such as the epithelial lining, immune cells, airway smooth muscle, vasculature, peripheral and central innervation, and neuroendocrine system. In addition to the vast number of well illustrated functions for purinergic signaling in the healthy respiratory tract, increasing data pointing to enhanced levels of ATP and/or adenosine in airway secretions of patients with airway damage and respiratory diseases corroborates the emerging view that purines act as clinically important mediators resulting in either proinflammatory or protective responses. Purinergic signaling has been implicated in lung injury and in the pathogenesis of a wide range of respiratory disorders and diseases, including asthma, chronic obstructive pulmonary disease, inflammation, cystic fibrosis, lung cancer, and pulmonary hypertension. These ostensibly enigmatic actions are based on widely different mechanisms, which are influenced by the cellular microenvironment, but especially the subtypes of purine receptors involved and the activity of distinct members of the ectonucleotidase family, the latter being potential protein targets for therapeutic implementation.

Novel 1,3-dipropyl-8-(3-benzimidazol-2-yl-methoxy-1-methylpyrazol-5-yl)xanthines as potent and selective A₂B adenosine receptor antagonists

Molecular modeling studies, including the comparative molecular field analysis (CoMFA) method, on 52 antagonists of the A(2B) adenosine receptor with known biological activity were performed to identify the three-dimensional features responsible for A(2B) adenosine receptor antagonist activity. On the basis of these and previous results on the potent antagonist effect of 8-pyrazolyl-xanthines at human A(2B)AR, a new series of compounds was synthesized and evaluated in binding studies against the human A(1), A(2A), A(3), and A(2B)ARs. A remarkable improvement in selectivity with respect to the previous series, maintaining the potency at human A(2B) receptor, was achieved, as exemplified by the 8-[3-(4-chloro-6-trifluoromethyl-1H-benzoimidazol-2-yl-methoxy)-1-methyl-1H-pyrazol-5-yl]-1,3-dipropyl-3,7-dihydro-purine-2,6-dione derivative 66: K(i) A(2B) = 9.4 nM, IC(50) hA(2B) = 26 nM hA(1)/hA(2B) = 269, hA(2A)/hA(2B) > 106, hA(3)/hA(2B) >106. This study also led to the identification of a series of pyrazole-xanthine compounds with a simplified structure, exemplified by 8-(3-hydroxy-1-methyl-1H-pyrazol-5-yl)-xanthine 80 displaying very high affinity at A(2B)AR with good selectivity over AR subtypes (K(i) = 4.0 nM, IC(50) hA(2B) = 20 nM hA(1)/hA(2B) = 183, hA(2A),hA(3)/hA(2B) > 250).

Adenosine receptors in health and disease

The adenosine receptors A(1), A(2A), A(2B), and A(3) are important and ubiquitous mediators of cellular signaling, which play vital roles in protecting tissues and organs from damage. In particular, adenosine triggers tissue protection and repair by different receptor-mediated mechanisms, including an increase of oxygen supply/demand ratio, preconditioning, anti-inflammatory effects, and stimulation of angiogenesis. Considerable advances have been recently achieved in the pharmacological and molecular characterization of adenosine receptors, which have been proposed as targets for drug design and discovery. At the present time, it can be speculated that adenosine A(1), A(2A), A(2B), and A(3) receptor-selective ligands may show utility in the treatment of pain, ischemic conditions, glaucoma, asthma, arthritis, cancer, and other disorders in which inflammation is a feature. This chapter documents the present state of knowledge of adenosine receptors' role in health and disease.

Pharmacological characterization of adenosine receptors on isolated human bronchi

Adenosine induces airways obstruction in subjects with asthma, but the receptor subtype responsible remains unknown. The objectives of this study were to determine the pharmacological profile of adenosine receptor subtypes mediating contraction and to investigate the mechanism in normal and passively sensitized human airway tissues. Contraction of bronchial rings isolated from resected lung tissue of patients with lung carcinoma was measured in response to nonselective adenosine receptor agonists, 5-AMP and 5'-(N-Ethylcarboxamido)adenosine, and A(1) receptor agonist, N(6)-cyclopentyladenosine, in the absence and presence of selective adenosine receptor antagonists. Pharmacological antagonists, chemical ablation of airway sensory nerves using capsaicin, and passive sensitization of tissue with serum from subjects with atopy and asthma was used to investigate the mechanism of contraction. Human bronchial tissue contracted in a concentration-dependent manner to adenosine agonists that showed a rank order of activity of A(1) > A(2B) >> A2(A) = A3. The maximum contractile response to N(6)-cyclopentyladenosine (231.0 ± 23.8 mg) was significantly reduced in tissues chemically treated with capsaicin to desensitize sensory nerves (desensitized: 101.6 ± 15.2 mg; P < 0.05). Passive sensitization significantly augmented the contraction induced by adenosine A(1) receptor activation (sensitized: 389.7 ± 52.8 mg versus nonsensitized; P < 0.05), which was linked to the release of leukotrienes, and not histamine (MK571: 25.5 ± 1.7 mg; epinastine 260.0 ± 22.2 mg versus control; P < 0.05). This study provides evidence for a role for adenosine A(1) receptors in eliciting human airway smooth muscle constriction, which, in part, is mediated by the action of capsaicin sensitive sensory nerves.

Airway responsiveness to indirect challenges in COPD

Patients with chronic obstructive pulmonary disease (COPD) demonstrate airway hyperresponsiveness to a number of indirect stimuli. Hyperresponsiveness to cold air hyperventilation, exercise, and drugs like propranalol and methoxamine seem to be able to distinguish patients with COPD from those with asthma, whereas hyperresponsiveness to stimuli like adenosine 5-monophosphate (AMP) and hypertonic saline seem unable to do so. The relationship of airway responsiveness to indirect stimuli and airway inflammation has received little study. The clinical relevance of hyperresponsiveness to an indirect challenge, including the impact on the natural history, relation to types of bronchitis, baseline airway calibre, and response to treatment need to be studied.

Airway hyperresponsiveness to adenosine 5'-monophosphate in feline chronic inflammatory lower airway disease

Airway hyperresponsiveness is a key feature of human asthma and chronic bronchitis and response to the indirectly acting agonist adenosine 5'-monophosphate (AMP) is thought to reflect underlying airway inflammation. To examine whether airway responsiveness testing (ART) with AMP may be used to differentiate healthy cats from those with asthma (FA) and chronic bronchitis (CB), 24 cats (9 FA, 6 CB, 9 controls) underwent ART with AMP at concentrations of 0.1, 1, 10, 100 and 500mg/mL using barometric whole body plethysmography. The defined endpoint of ART, an increase in enhanced pause (Penh) exceeding 300% of the post-saline value (baseline), was reached in 9/15 patients (7 FA, 2 CB), but in none of the controls. Mean Penh (±SD) at baseline (BL) was 0.49±0.16 for cases, and 0.54±0.16 for controls, and was significantly increased after AMP challenge in clinical cases (2.62±2.20), but not in controls (0.63±0.30, P<0.05). After separating responder (R) and non-responder (NR) cases, a more pronounced difference after challenge was found (R: 3.96±1.84, NR: 0.6±0.21, P<0.001). The provocative concentration of the agonist that increased Penh to 300% of BL (PC Penh 300) in R cases was 52.98±48.04mg/mL AMP. Age had no influence on the responder status or PC Penh 300. It was concluded that AMP challenge may offer a new method for the identification of cats with lower inflammatory airway disease, and possibly for monitoring disease progression or response to therapy.

Isolated Bronchi of Patients with COPD Show Decreased Histamine Responsiveness Compared to Smokers with Normal Lung Function

This study compared airway responsiveness in vitro, as measured in isolated bronchi, with responsiveness in vivo in patients with COPD and smokers with normal lung function. In 9 patients with COPD (mean (range) FEV, 55 (30-78) %predicted) and 8 smokers with normal lung function (FEV1 101 (89-117) %predicted), who underwent surgery for lung cancer, responses to inhaled histamine and salbutamol were assessed before surgery. Bronchial specimens of 1-4 mm internal diameter were studied in the organ bath and histamine concentration-response curves assessed. All patients with COPD and none of the control individuals were hyperresponsive to inhaled histamine. Five patients with COPD and no control patient showed a bronchodilator response to salbutamol. Opposite to these findings, bronchial rings in the organ bath demonstrated a rightward shift of histamine concentration-response curves in COPD compared to controls, (p < 0.005). Accordingly, pED50 but not Emax differed statistically (p = 0.0016) between groups, mean+/-SEM values of pED50 in COPD (controls) being 4.67+/-0.08 (5.29+/-0.15) and of Emax 672+/-86 (772+/-120) mg. Patients with COPD showing hyperresponsiveness to inhaled histamine demonstrated lower responsiveness of their isolated bronchi compared to smokers with normal lung function. This suggests that in vivo hyperresponsiveness is based on other mechanisms than alterations in smooth muscle physiology.

Adenosine induces a cholinergic tracheal reflex contraction in guinea pigs in vivo via an adenosine A1 receptor-dependent mechanism

Adenosine induces dyspnea, cough, and airways obstruction in asthma, a phenomenon that also occurs in various sensitized animal models in which a neuronal involvement has been implicated. Although adenosine has been suggested to activate cholinergic nerves, the precise mechanism has not been established. In the present study, the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) induced a cholinergic reflex, causing tracheal smooth muscle contraction that was significantly inhibited by the adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 100 microg/kg) (P < 0.05) in anesthetized animals. Furthermore, the adenosine A(2) agonist 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680) induced a small reflex, whereas the A(3) selective agonist N(6)-(3-iodobenzyl)-5'-N-methylcarbamoyladenosine (IB-MECA) was without effect. The tracheal reflex induced by CPA was also inhibited by recurrent nerve ligation or muscarinic receptor blockade (P < 0.001), indicating that a cholinergic neuronal mechanism of action accounted for this response. The cholinergic reflex in response to aerosolized CPA was significantly greater in passively sensitized compared with naive guinea pigs (P < 0.01). Chronic capsaicin treatment, which inhibited sensory nerve function, failed to inhibit CPA-induced reflex tracheal contractions in passively sensitized guinea pigs, although the local anesthetic lidocaine inhibited CPA-induced tracheal contractions. The effects of CPA on the reflex response was not dependent on the release of histamine from tissue mast cells or endogenous prostaglandins as shown by the lack of effect of the histamine H(1) receptor antagonist pyrilamine (1 mg/kg) or the cyclooxygenase inhibitor meclofenamic acid (3 mg/kg), respectively. In conclusion, activation of pulmonary adenosine A(1) receptors can stimulate cholinergic reflexes, and these reflexes are increased in allergic guinea pigs.

Sulfur-containing malodorant vapors enhance responsiveness to the sensory irritant capsaicin

The nose is innervated with both odor responsive olfactory (cranial nerve I) and irritant responsive trigeminal (cranial nerve V) nerves. The nature and extent of any interactions between these two nerves is poorly understood. The aim of the current study was to determine if two sulfur-containing malodorants, ethyl sulfide and t-butyl sulfide, modulated responsiveness to the trigeminal C fiber stimulant capsaicin using female C57Bl/6J mice as an experimental model. Cessation or marked slowing of flow at the onset of each expiration (termed braking) was used as a biomarker for trigeminal nerve stimulation. Aerosolized capsaicin solution (100 microg/ml) increased the time of braking from baseline levels of 8 ms to an average of 69 ms. At an exposure concentration of 100 ppm the malodorants induced only a minimal time of braking response (< 35 ms); the time of braking response in animals exposed to either malodorant plus capsaicin was 2.5-fold greater than in animals exposed to capsaicin alone (p < 0.01). In a subsequent experiment the time of breaking response to capsaicin was doubled (281 vs. 146 ms) by concomitant exposure to a no effect level of ethyl sulfide (11 ppm) and the modulation of capsaicin responsiveness was nearly abolished by inclusion of the adenosine antagonist theophylline in the aerosol formulation (time of braking 184 ms, p > or = 0.05 compared with capsaicin alone). These results suggest trigeminal nerve responsiveness is enhanced by exposure to malodorants through a theophylline-sensitive paracrine signaling pathway between olfactory and trigeminal nerves.

[Respiratory function assessment in cooperative patients. Part II]

Analysis of bronchial hyperresponsiveness using bronchial provocation tests are a key feature in the diagnosis of asthma, as well as a valid tool for monitoring disease severity, clinical course, and treatment response. We review non-specific bronchial challenge tests, including pharmacological stimuli (methacholine, adenosine) and physical stimuli (exercise, hypertonic saline, cold air hyperventilation). Although there is some correlation among responses to the distinct tests, individual responses are also observed. The indication for a single test will depend on whether the procedure will be used for diagnostic or epidemiologic purposes, and on experience of its use. Frequently, complementary information will be obtained. Indirect airway challenges tests such as physical stimuli and adenosine are more specific for asthma diagnosis.

AMP challenge induces a decrease in FE(NO) in asthmatic subjects modulated by nedocromil

BACKGROUND

Allergen challenge results in an immediate reduction in exhaled nitric oxide (FE(NO)) followed by a long-term increase. To study mast cell activation in relation to nitric oxide (NO), the study investigated the effect of inhaled adenosine monophosphate (AMP) as a mast cell activator and mast cell stabilizer - nedocromil sodium - on FE(NO). The NO synthase (NOS) iso-enzyme involved was studied by the NOS inhibitor aminoguanidine.

MATERIALS AND METHODS

A double-blind, placebo-controlled, cross-over study was performed in two parts. Part I: eight atopic asthmatic subjects inhaled nedocromil or placebo before the AMP challenge. Spirometry and FE(NO) were measured at intervals over a 24-h period. Part II: seven subjects inhaled aminoguanidine before an identical protocol was used, as in Part I.

RESULTS

Part I: AMP challenge caused a significant decrease from baseline FE(NO)[placebo, 28.9 (20.3-37.4)%, P < 0.002 and nedocromil, 20.9 (8.2-33.6)%, P < 0.01]. Nedocromil gave partial protection against this decrease in FE(NO). The time-FE(NO) curve (AUC(0-24)) differed significantly between nedocromil and placebo: 2.7% (-3.6 to -9) vs. -6.6% (-12 to -1.3) FE(NO) changes h(-1), P < 0.002, respectively. Nedocromil protected against AMP-induced bronchoconstriction (AMP PC(20)) [nedocromil 182 (72.5-291) mg mL(-1) vs. placebo 21.7 (10.7-33) mg mL(-1), P < 0.002]. Part II: nebulized aminoguanidine resulted in a significant reduction in FE(NO) from baseline and was greater than after AMP alone (P = 0.006). Nedocromil increased AMP PC(20), but no longer protected against the late decrease in FE(NO).

CONCLUSIONS

The AMP challenge caused a reduction in FE(NO) as a result of prior treatment with nedocromil. Aminoguanidine abolished the nedocromil-induced protection on the late reduction in FE(NO), but not on AMP PC(20). Inducible NOS was implicated in the late FE(NO) decrease after the AMP challenge.

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.

E-NTPDases in human airways: Regulation and relevance for chronic lung diseases

Chronic obstructive lung diseases are characterized by the inability to prevent bacterial infection and a gradual loss of lung function caused by recurrent inflammatory responses. In the past decade, numerous studies have demonstrated the importance of nucleotide-mediated bacterial clearance. Their interaction with P2 receptors on airway epithelia provides a rapid ‘on-and-off’ signal stimulating mucus secretion, cilia beating activity and surface hydration. On the other hand, abnormally high ATP levels resulting from damaged epithelia and bacterial lysis may cause lung edema and exacerbate inflammatory responses. Airway ATP concentrations are regulated by ecto nucleoside triphosphate diphosphohydrolases (E-NTPDases) which are expressed on the mucosal surface and catalyze the sequential dephosphorylation of nucleoside triphosphates to nucleoside monophosphates (ATP → ADP → AMP). The common bacterial product, Pseudomonas aeruginosa lipopolysaccharide (LPS), induces an acute reduction in azide-sensitive E-NTPDase activities, followed by a sustained increase in activity as well as NTPDase 1 and NTPDase 3 expression. Accordingly, chronic lung diseases, including cystic fibrosis (CF) and primary ciliary dyskinesia, are characterized by higher rates of nucleotide elimination, azide-sensitive E-NTPDase activities and expression. This review integrates the biphasic regulation of airway E-NTPDases with the function of purine signaling in lung diseases. During acute insults, a transient reduction in E-NTPDase activities may be beneficial to stimulate ATP-mediated bacterial clearance. In chronic lung diseases, elevating E-NTPDase activities may represent an attempt to prevent P2 receptor desensitization and nucleotide-mediated lung damage.

Cross-talk between G(s)- and G(q)-coupled pathways in regulation of interleukin-4 by A(2B) adenosine receptors in human mast cells

Human mast cells express functional A(2A) and A(2B) adenosine receptors. However, only stimulation of A(2B), not A(2A), leads to secretion of interleukin (IL)-4, an important step in adenosine receptor-mediated induction of IgE synthesis by B-cells. In this study, we investigate intracellular pathways that link stimulation of A(2B) receptors to IL-4 up-regulation in HMC-1 mast cells. Both A(2A) and A(2B) receptors couple to G(s) proteins and stimulate adenylate cyclase, but only A(2B) stimulates phospholipase Cbeta through coupling to G(q) proteins leading to activation of protein kinase C and calcium mobilization. Inhibition of phospholipase Cbeta completely blocked A(2B) receptor-dependent IL-4 secretion. The protein kinase C inhibitor 2-{8-[(dimethylamino)-methyl]-6,7,8,9-tetrahydropyrido[1,2-a]indol-3-yl}-3-(1-methyl-1H-indol-3-yl)maleimide (Ro-32-0432) had no effect on A(2B) receptor-mediated IL-4 secretion but inhibited phorbol 12-myristate 13-acetate-stimulated IL-4 secretion. In contrast, chelation of intracellular Ca(2+) inhibited both A(2B) receptor- and ionomycin-dependent IL-4 secretion. This Ca(2+)-sensitive pathway probably includes calcineurin and nuclear factor of activated T cells, because A(2B) receptor-dependent IL-4 secretion was blocked with cyclosporin A or 11R-VIVIT peptide. G(s)-linked pathways also play a role in the A(2B) receptor-dependent stimulation of IL-4 secretion; inhibition of adenylate cyclase or protein kinase A attenuated A(2B) receptor-dependent IL-4 secretion. Although stimulation of adenylate cyclase with forskolin did not increase IL-4 secretion on its own, it potentiated the effect of Pasteurella multocida toxin by 2-fold and ionomycin by 3-fold. Both forskolin and stimulation of A(2B) receptors up-regulated NFATc1 protein levels. We conclude that A(2B) receptors up-regulate IL-4 through G(q) signaling that is potentiated via cross-talk with G(s)-coupled pathways.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

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.

Mast cell involvement in the adenosine mediated airway hyper-reactivity in a murine model of ovalbumin-induced lung inflammation

Airway hyper-reactivity to inhaled adenosine, mediated via mast cell activation, is a cardinal feature of asthma. Animal models have been developed in several species to mimic this phenomenon, but only in the rat has a mast cell involvement been clearly defined. In this study, a model of ovalbumin-induced adenosine hyper-reactivity was developed in BALB/c mice to determine whether mast cells are involved in this phenomenon. Sensitised mice were challenged one, two or three times, on a daily basis, and airway responses to the stable adenosine analogue NECA (5'-N-ethylcarboxamido adenosine) determined 4 and 24 h after each challenge. Airway hyper-reactivity was observed in ovalbumin-challenged mice 4 h after a single challenge and to a minor extent 24 h after a single challenge and 4 h after two challenges. Cromolyn (20 mg ml(-1)), given by aerosol an hour before the NECA provocation, fully inhibited the airway hyper-reactivity observed 4 h after a single allergen challenge, suggesting a role for mast cells in this response. The airway space cellular inflammation was not affected by cromolyn. As observed in human asthma, an acute treatment with steroid (budesonide 3 mg kg(-1), given an hour before the allergen challenge) inhibited the NECA airway hyper-reactivity and significantly inhibited the airway space cellular inflammation. These data suggest that the ovalbumin-challenged BALB/c mice can be considered as a suitable model to study the adenosine-induced airway hyper-reactivity phenomenon observed in human asthma.

Effects of cadmium chloride inhalation on airflow limitation to histamine, carbachol and adenosine 5'-monophosphate assessed by barometric whole body plethysmography in healthy dogs

The effects of pharmacological bronchoprovocation on airflow patterns and surrogate respiratory parameters assessed by barometric whole body plethysmography (BWBP) were investigated in healthy dogs, previously exposed to cadmium chloride inhalation. BWBP-derived respiratory variables were calculated (1) at baseline and (2) following nebulisation of increasing concentrations of histamine, carbachol and adenosine 5'-monophosphate (AMP) until enhanced pause (PENH) increased to 300% of baseline (PCPENH300). Bronchoalveolar lavage fluid (BALF) cytology before (BCC) and after (ACC) cadmium chloride inhalation revealed cadmium-induced airway inflammation. Neutrophils increased from 6.7 +/- 7.3% (728 +/- 104/microL) BCC to 77.8 +/- 8.6% (3255 +/- 1407/microL) ACC. PCPENH300 for all three agonists significantly decreased ACC (means+/-SD) as follows: PCPENH300(histamine) 0.72 +/- 0.28 mg/mL BCC, and 0.35 +/- 0.31 mg/mL ACC (P<0.02); PCPENH300(carbachol) 0.34 +/- 0.16 mg/mL BCC, and 0.064 +/- 0.032 mg/mL ACC (P<0.02); PCPENH300(AMP) 1000 mg/mL BCC, and 415 +/- 398 mg/mL ACC (P<0.03). The only clinical sign shown was coughing. It was concluded that airway hyperresponsiveness after induced airway inflammation can be determined by BWBP in conscious small sized dogs. BWBP could be a suitable technique to study the respiratory effects of urban environmental pollution in pets.

Effects of aerosolized adenosine 5'-triphosphate vs adenosine 5'-monophosphate on dyspnea and airway caliber in healthy nonsmokers and patients with asthma

STUDY OBJECTIVES

Extracellular adenosine 5'-triphosphate (ATP) causes neurogenic bronchoconstriction, inflammation, and coughs, and may play a mechanistic role in obstructive airway diseases. The aims of this study were to determine the effects of inhaled ATP on airway function, and to compare these effects with those of adenosine 5'-monophosphate (AMP).

DESIGN

Prospective, randomized, double-blind study.

SETTING

Clinical research laboratory of a postgraduate teaching hospital.

METHODS

The effects of inhaled equimolar doses of ATP and AMP on airway caliber, perception of dyspnea quantified by the Borg score, and other symptoms were determined in 10 nonsmokers (age 41 +/- 3 years) and 10 patients with asthma (age 39 +/- 3 years) [+/- SEM].

RESULTS

None of the healthy nonsmokers responded to ATP or AMP. All the patients with asthma responded to ATP, and 90% responded to AMP. The geometric mean of the provocative dose causing a 20% fall in FEV1 (PD20) of ATP was 48.7 micromol/mL and that of PD20 AMP was 113.5 micromol/mL in responsive asthmatics (p < 0.05). In asthmatic patients, the percentage change in FEV1 caused by ATP was greater than that caused by AMP (deltaFEV1 ATP = 29% vs deltaFEV1 AMP = 22%, p < 0.05). Borg score increased significantly in asthmatics after ATP (from 0.1 to 3.3, p < 0.01) and after AMP (from 0.2 to 2.5, p < 0.01). This increase was also greater after ATP than AMP in asthma (deltaBorg ATP = 3.2 vs deltaBorg AMP = 2.3, p < 0.05). ATP induced cough in 16 subjects (80%), while AMP induced cough in 8 subjects (40%) [p < 0.05]; in addition, more subjects had throat irritation after inhalation of ATP than AMP (p < 0.05).

CONCLUSIONS

ATP is a more potent bronchoconstrictor and has greater effects on dyspnea and other symptoms than AMP in asthmatic patients. Therefore, ATP could potentially be used as a bronchoprovocator in the clinical setting.

The Quintiles Prize Lecture 2004. The identification of the adenosine A2B receptor as a novel therapeutic target in asthma

Adenosine is a powerful bronchoconstrictor of asthmatic, but not normal, airways. In vitro studies on isolated human mast cells and basophils revealed that adenosine and selective analogues augmented inflammatory mediator release from mast cells by stimulating A(2) receptors. Pharmacological blockade of mast cell mediator release in vivo also attenuated adenosine-induced bronchoconstriction, as did theophylline, by adenosine A(2) receptor antagonism. Further in vitro studies revealed that the asthmatic response to adenosine is likely to be mediated via the A(2B) subtype which is selectively antagonised by enprofylline. Studies in animal models, especially mice, have shown a close synergistic interaction between adenosine, Th2 and airway remodelling responses. The recent description of A(2B) receptors on human airway smooth muscle cells that mediate cytokine and chemokine release and induce differentiation of fibroblasts into myofibroblasts strengthens the view that adenosine maybe more than an inflammatory mediator in asthma but also participates in airway wall remodelling in this disease. These data have provided a firm basis for developing adenosine A(2B) receptor antagonists as a new therapeutic approach to this disease.

Stimulatory effect of CO2 on vagal bronchopulmonary C-fiber afferents during airway inflammation

This study investigated 1) whether pulmonary C fibers are activated by a transient increase in the CO2 concentration of alveolar gas; and 2) if the CO2 sensitivity of these afferents is altered during airway inflammation. Single-unit pulmonary C-fiber activity was recorded in anesthetized, open-chest rats. Transient alveolar hypercapnia (HPC) was induced by administering a CO2-enriched gas mixture (25-30% CO2, 21% O2, balance N2) for five to eight breaths, which increased alveolar CO2 concentration progressively to near or above 13% for 3-5 s and lowered the arterial pH transiently to 7.10 +/- 0.05. Our results showed the following. 1) HPC evoked only a mild stimulation in a small fraction (4/47) of pulmonary C fibers, and there was no significant change in fiber activity (change in fiber activity = 0.22 +/- 0.16 imp/s; P > 0.1, n = 47). 2) In sharp contrast, after airway exposure to poly-L-lysine, a cationic protein known to induce mucosal injury, the same challenge of transient HPC activated 87.5% of the pulmonary C fibers tested and evoked a distinct stimulatory effect on these afferents (change in fiber activity = 6.59 +/- 1.78 imp/s; P < 0. 01, n = 8). 3) Similar potentiation of the C-fiber response to HPC was also observed after acute exposure to ozone (n = 6) and during a constant infusion of inflammatory mediators such as adenosine (n = 15) or prostaglandin E2 (n = 12). 4) The enhanced C-fiber sensitivity to CO2 after poly-L-lysine was completely abrogated by infusion of NaHCO3 (1.82 mmol.kg(-1).min(-1)) that prevented the reduction in pH during HPC (n = 6). In conclusion, only a small percentage (<10%) of the bronchopulmonary C fibers exhibit CO2 sensitivity under control conditions, but alveolar HPC exerts a consistent and pronounced stimulatory effect on the C-fiber endings during airway inflammation. This effect of CO2 is probably mediated through the action of hydrogen ions.

Involvement of mast cells in adenosine-mediated bronchoconstriction and inflammation in an allergic mouse model

In allergen-induced asthma, activation of lung mast cells leads to bronchial constriction, increased mucus secretion, and an increase in the localization of inflammatory cells to the airways. The purpose of this study was to explore the role of mast cells in adenosine-mediated airway reactivity and inflammation using the mast cell degranulating agent, compound 48/80 (C48/80). Mice were sensitized and challenged with ragweed (or 0.9% saline) followed by C48/80 administration twice a day in increasing doses for 5 days. Dose-responsiveness to the nonspecific adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) was established, and lung lavage was performed 24 h later for cell differential analysis to evaluate inflammation. At a dose of 375 microg/ml (aerosolized NECA), C48/80 pretreatment resulted in a significant attenuation in airway reactivity when compared with sensitized control mice (330.07 versus 581.57%, respectively). Lung lavage from the C48/80 treated mice showed a decrease in eosinophils (17.7 versus 60.9%, respectively) and an increase in macrophages when compared with the sensitized control group (76.4 versus 30.8%, respectively). These results support the conclusion that mast cell degranulation plays an important role in adenosine receptor-mediated airway hyperresponsiveness and inflammation.

Monitoring response to treatment in asthma management: food for thought

Asthma is a chronic inflammatory disorder of the airways that is characterized by episodic symptoms. In this regard, asthma management has classically involved periodic re-assessment by the health-care provider, during which therapy is altered mainly based on clinical and physiological parameters, such as assessment of symptoms, spirometry and peak expiratory flow monitoring. In this context, various markers of airway inflammation (e.g. eosinophils in the induced sputum, nitric oxide in the exhaled air) have been proposed to assess the severity of asthma and to adjust the therapy accordingly. The evaluation of airway hyper-responsiveness with different stimuli has also been suggested as a new tool to monitor asthma. However, the lack of definite relationships between airway inflammation and asthmatic symptoms strongly limit the use of markers of asthma severity in the clinical setting. Therefore, the need of new tools to assess the severity of asthma is raised. The ideal measurement employed to establish the proper asthmatic therapy should be safe, non-invasive, easy to perform, reproducible and accurate, and have the capability to monitor the changes induced by the therapeutic interventions. A careful review of the available techniques, and the evaluation of their sensitivity and specificity in the clinical setting is warranted.

Adenosine-activated mast cells induce IgE synthesis by B lymphocytes: an A2B-mediated process involving Th2 cytokines IL-4 and IL-13 with implications for asthma

Adenosine provokes bronchoconstriction in asthmatics through acute activation of mast cells, but its potential role in chronic inflammation has not been adequately characterized. We hypothesized that adenosine up-regulates Th2 cytokines in mast cells, thus promoting IgE synthesis by B lymphocytes. We tested this hypothesis in human mast cells (HMC-1) expressing A(2A), A(2B), and A(3) adenosine receptors. The adenosine analog 5'-N-ethylcarboxamidoadenosine (NECA) (10 microM) increased mRNA expression of IL-1beta, IL-3, IL-4, IL-8, and IL-13, but not IL-2 and IFN-gamma. Up-regulation of IL-4 and IL-13 was verified using RT-PCR and ELISA; 10 microM NECA increased IL-13 concentrations in HMC-1 conditioned medium 28-fold, from 7.6 +/- 0.3 to 215 +/- 4 pg/ml, and increased IL-4 concentrations 6-fold, from 19.2 +/- 0.1 to 117 +/- 2 pg/ml. This effect was mediated by A(2B) receptors because neither the selective A(2A) agonist 2-p-(2-carboxyethyl)phenethylamino-NECA nor the selective A(3) agonist N(6)-(3-iodobenzyl)-N-methyl-5'-carbamoyladenosine reproduced it, and the selective A(2B) antagonist 3-isobutyl-8-pyrrolidinoxanthine prevented it. Constitutive expression of CD40 ligand on HMC-1 surface was not altered by NECA. Human B lymphocytes cocultured for 12 days with NECA-stimulated HMC-1 produced 870 +/- 33 pg IgE per 10(6) B cells, whereas lymphocytes cocultured with nonstimulated HMC-1, or cultured alone in the absence or in the presence of NECA, produced no IgE. Thus, we demonstrated induction of IgE synthesis by the interaction between adenosine-stimulated mast cells and B lymphocytes, and suggest that this mechanism is involved in the amplification of the allergic inflammatory responses associated with asthma.

Allergen sensitization and bronchial hyper-responsiveness to adenosine monophosphate in asthmatic patients

BACKGROUND

Indirect bronchoprovocation using adenosine monophosphate (AMP) is related to atopic phenotype expression.

OBJECTIVE

To evaluate the putative relationship between skin prick allergen sensitization and bronchial hyper-responsiveness to AMP in a retrospective cross-sectional database analysis.

METHODS

We retrospectively evaluated two groups of non-smoking asthmatics (forced expiratory volume in 1 s (FEV)1>/=60% predicted) who were reactive (responders) or unreactive (controls) to inhaled AMP. The main outcome measure was the difference in sensitization to individual allergens and the total atopic load according to the presence or absence of bronchial hyper-responsiveness.

RESULTS

We initially identified 180 (44%) non-smoking asthmatics with PC20</=200 mg/mL, while 233 (56%) had PC20>/=1600 mg/mL. For those who had a skin prick test, the responders (n=151) and controls (n=151) were found to be matched for age, sex, inhaled corticosteroid dose and number of patients using inhaled corticosteroids. There were significant differences in the number of responders vs. controls in terms of sensitization to house dust mite (77% vs. 62%, P=0.004), aspergillus (19% vs. 9%, P=0.014), cat (61% vs. 48%, P=0.028), total atopic load (493 vs. 380 positive tests, P<0.001) and forced mid-expiratory flow (60% vs. 68% predicted, P<0.001).

CONCLUSION

Sensitization to common aeroallergens increased the likelihood of bronchial inflammation as reflected by bronchial hyper-responsiveness to inhaled AMP, independently of both FEV1 or inhaled corticosteroid use. This in turn suggests an association between allergen exposure and AMP responsiveness in asthmatics. Further prospective long-term evaluation is indicated to assess whether allergen avoidance strategies can modify the airway response.

Leukotriene C4 synthase polymorphisms and responsiveness to leukotriene antagonists in asthma

AIM

Cysteinyl leukotrienes are important pro-inflammatory mediators in the pathogenesis of asthma, while leukotriene C4 synthase is a key enzyme in their biosynthesis. Our aim is to evaluate whether responsiveness to leukotriene receptor antagonists was determined by expression of the variant (C) or wild-type (A) polymorphism of this enzyme.

METHODS

We carried out a retrospective analysis of 8 randomised, placebo-controlled trials performed in our department in mild-to-moderate asthmatics. In all trials, effect of leukotriene receptor antagonist was compared to placebo, where the primary outcome was bronchial hyperresponsiveness to adenosine monophosphate or methacholine. Secondary outcomes were forced expiratory volume in 1 second, exhaled nitric oxide and peripheral blood eosinophils.

RESULTS

For the primary outcome of attenuation of bronchial hyperresponsiveness by leukotriene receptor antagonist vs placebo, there were significant effects within each genotype on adenosine monophosphate (AMP) (n = 78): 2.21 and 2.07-fold improvements for AA and AC/CC, respectively; while for methacholine (n = 81) there were 1.39 and 1.36-fold improvements, respectively. There were no significant differences between genotypes (i.e. AA vs AC/CC): geometric mean fold-differences of 1.07 (95%CI 0.63-1.81) and 1.02 (95%CI 0.70-1.50) for AMP and methacholine, respectively. There were also no differences between genotypes for all secondary outcomes.

CONCLUSION

Polymorphisms of leukotriene C4 synthase did not determine responsiveness, in terms of attenuation of bronchial hyperresponsiveness, to leukotriene receptor antagonists in mild-to-moderate asthmatics. Further prospective large pharmacogenetic studies are required in more severe patients, where there may be greater improvements in pharmacodynamic outcome measures such as bronchial hyperresponsiveness and exhaled nitric oxide.

Hypersensitivity of pulmonary C fibers induced by adenosine in anesthetized rats

Compelling clinical evidence implicates the potential role of adenosine in development of airway hyperresponsiveness and suggests involvement of pulmonary sensory receptors. This study was carried out to determine the effect of a low dose of adenosine infusion on sensitivity of pulmonary C-fiber afferents in anesthetized open-chest rats. Infusion of adenosine (40 microg x kg-1x min-1 i.v. for 90 s) mildly elevated baseline activity of pulmonary C fibers. However, during adenosine infusion, pulmonary C-fiber responses to chemical stimulants and lung inflation (30 cmH2O tracheal pressure) were markedly potentiated; e.g., the response to right atrial injection of capsaicin (0.25 or 0.5 microg/kg) was increased by more than fivefold (change in fiber activity = 2.64 +/- 0.67 and 16.27 +/- 3.11 impulses/s at control and during adenosine infusion, n = 13, P < 0.05), and this enhanced response returned to control in approximately 10 min. The potentiating effect of adenosine infusion was completely blocked by pretreatment with 8-cyclopentyl-1,3-dipropylxanthine (100 microg/kg), a selective antagonist of the adenosine A1 receptor, but was not affected by 3,7-dimethyl-1-propargylxanthine (1 mg/kg), an A2-receptor antagonist, or 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (2 mg/kg), an A3-receptor antagonist. This potentiating effect was also mimicked by N6-cyclopentyladenosine (0.25 microg x kg-1 x min-1 for 90 s), a selective agonist of the adenosine A1 receptor. In conclusion, our results showed that infusion of adenosine significantly elevated the sensitivity of pulmonary C-fiber afferents in rat lungs and that this potentiating effect is likely mediated through activation of the adenosine A1 receptor.

Relationship between airway hyperresponsiveness to mannitol and adenosine monophosphate

BACKGROUND

Assessment of airway hyperresponsiveness (AHR) to indirect bronchoconstrictor stimuli is a useful noninvasive tool in the evaluation of asthma and its treatment. We investigated the putative relationship in AHR between inhaled adenosine monophosphate and mannitol.

METHODS

Fifteen mild-to-moderate atopic asthmatics were evaluated. On two separate screening days, the threshold AMP concentration and threshold mannitol dose to provoke a given fall in FEV1 were measured.

RESULTS

For AMP PC20vs. mannitol PD15, the Pearsons correlation coefficient was 0.80, P < 0.001. For AMP PC15vs. mannitol PD15 and AMP PC10vs. mannitol PD10 corresponding values were 0.83, P < 0.001 and 0.68, P = 0.005.

CONCLUSIONS

There was a highly significant association between the threshold concentration of AMP and dose of mannitol causing a given fall in FEV1. Further studies are required to evaluate the relationship between inhaled mannitol and other surrogate inflammatory markers.

Research applications and implications of adenosine in diseased airways

Adenosine, when given by inhalation, initiates the narrowing of airways in subjects with asthma or chronic obstructive pulmonary disease (COPD). The underlying mechanism of this narrowing appears to involve the stimulation of specific mast cell surface adenosine receptors with the subsequent release of mediators and contraction of airway smooth muscle. Although methacholine and histamine have become gold standards as bronchial provocants used to quantify bronchial hyperresponsiveness, the airways response to the indirect stimulus adenosine more closely reflects bronchial inflammation. This distinctive feature of adenosine could be exploited to enable superior diagnostic discrimination between asthma and COPD, allow better monitoring of disease activity and progression, and improve the individual adjustment of long-term asthma management with topical glucocorticosteroids. In this article, we review recent developments in this area of rapidly evolving clinical research, focusing on the putative role of adenosine as a mediator of airway inflammation and as a useful bronchoprovocant in several clinical and research applications.

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.

Monitoring of seasonal variability in bronchial hyper-responsiveness and sputum cell counts in non-asthmatic subjects with rhinitis and effect of specific immunotherapy

Bronchial hyper-responsiveness (BHR) is documented in a proportion of non-asthmatic individuals with allergic rhinitis (NAAR) and reflects inflammatory events in the lower airways. Natural exposure to allergens is known to modulate BHR and the level of airway inflammation in asthma, but less consistently in NAAR. Specific immunotherapy (SIT) attenuates symptoms possibly by reducing BHR and airway inflammation. The influence of natural exposure to Parietaria pollen on BHR and sputum cell counts of NAAR was investigated and the effect of Parietaria SIT examined. Thirty NAAR, monosensitized to Parietaria judaica, participated in a randomized, double-blind, placebo-controlled, parallel group study of the effects of a Parietaria pollen vaccine on symptoms/medication score, BHR to inhaled methacholine and adenosine 5'-monophosphate (AMP), and cell counts in the sputum collected out of and during the pollen seasons for 36 months. Seasonal variation in BHR to inhaled methacholine and AMP and changes in sputum cell counts were documented. Changes were consistent for AMP, but not methacholine, and invariably associated with modifications in sputum eosinophils and epithelial cells. The clinical efficacy of Parietaria SIT was associated with a decline in the seasonal deterioration of BHR to AMP, whereas no significant effect was observed on BHR to methacholine or sputum cell differentials. Between-groups comparison of the seasonal changes in PC15 methacholine values and sputum cell differentials calculated as the AUC were not statistically significant, whereas a significant difference in PC15 AMP was demonstrated throughout the study (P=0.029), the median (inter-quartile range) AUC values being 2478.5 (1153.3-3600.0) and 1545.5 (755.3-1797.9) for the SIT- and placebo-treated group, respectively. Bronchial airways of NAAR exhibit features of active inflammation that deteriorate during natural allergen exposure, particularly with regard to BHR to AMP. The clinical efficacy of Parietaria SIT was exclusively associated with attenuation in seasonal worsening of PC15 AMP, suggesting that AMP may be useful in monitoring changes in allergic inflammation of the airways.

A proof of concept study to evaluate putative benefits of montelukast in moderate persistent asthmatics

AIMS

Whether chronic dosing with montelukast confers benefit in patients with moderate to severe asthma remains to be fully established. A proof of concept study was performed evaluating putative benefits with montelukast in moderate persistent asthmatics who were taken off inhaled corticosteroids (ICS) and switched to salmeterol. The latter was done to dissociate the effects of montelukast from ICS.

METHODS

Twenty moderate to severe persistent asthmatics completed a randomized double-blind crossover study. Subjects received montelukast 10 mg daily or placebo for 2 weeks each. This was preceded by a 2-week run-in when ICS were discontinued and salmeterol started, and used on a regular basis throughout the study. Measurements were made after run-in and after both randomized treatments.

RESULTS

There were no significant sequence effects for responses as to whether placebo or montelukast were given first or second. Methacholine PD20 values after run-in, first and second placebo were 63 micro g, 60 micro g and 64 micro g, respectively (corresponding to 2, 4 and 6 weeks of ICS washout, respectively). Lung function deteriorated pre vs post run-in, which was significant (P < 0.05) for FEF25-75 % predicted. Montelukast conferred significant (P < 0.05) improvements as change from post run-in compared with placebo in methacholine PD20, FEV1 % predicted, FEF25-75 % predicted, diurnal peak expiratory flow, symptoms and salbutamol use. For the primary outcome of methacholine PD20, this amounted to a 1.6-fold difference (95% CI 1.1, 2.5).

CONCLUSIONS

In moderate persistent asthmatics switched from taking ICS to salmeterol alone, adding montelukast conferred significant benefits on all parameters of asthma control. Further studies are indicated to evaluate whether montelukast exhibits additive effects to ICS/long-acting beta2-adrenoceptor agonist combination inhalers upon clinically important outcomes.

Effects of fluticasone plus salmeterol versus twice the dose of fluticasone in asthmatic patients

OBJECTIVE

Current guidelines advocate adding a long acting beta(2)-agonist (LABA) to an inhaled corticosteroid as an alternative to increasing the dose of the latter. Since it is unclear how this translates into effects on surrogate inflammatory markers, we evaluated the anti-inflammatory activity of fluticasone plus salmeterol in combination versus twice the dose of fluticasone alone.

METHODS

Fifteen mild-to-moderate asthmatics (mean FEV(1) 80% predicted) uncontrolled on inhaled corticosteroids (mean dose 470 microg) were randomised in a single-blind crossover fashion to receive 2 weeks each of fluticasone 250 microg plus salmeterol 50 microg in combination (FP+SM), 1 puff b.i.d., and fluticasone 500 microg (FP), 1 puff b.i.d. Prior to each randomised treatment, there was a 2-week run-in and washout period during which patients used their usual inhaled corticosteroid therapy. Measurements were made before and after randomised treatment periods. The primary outcome was airway hyper-responsiveness to adenosine monophosphate (AMP PC(20)), while secondary endpoints were exhaled tidal nitric oxide (NO), forced expiratory volume in 1 second (FEV(1)) and forced mid-expiratory flow (FEF(25-75)).

RESULTS

For AMP PC(20), FP alone but not FP+SM conferred a significant ( P<0.05) improvement amounting to 3.27 (95% CI 1.46-7.32) and 1.44 (95% CI 0.64-3.23) geometric mean fold shifts, respectively, from baseline, while the difference between treatments was significantly ( P<0.05) greater with FP alone: a 2.26-fold (95% CI 1.01-5.07) difference. Both FP alone and FP+SM conferred significant ( P<0.05) falls in NO from baseline: 2.33 (95% CI 1.71-3.19) and 1.49 (95% CI 1.09-2.03) geometric mean fold changes, respectively, while between treatments the reduction was significantly ( P<0.05) greater with FP alone: a 1.57-fold (95% CI 1.15-2.14) difference. Neither treatment significantly improved FEV(1) or FEF(25-75).

CONCLUSION

Double the dose of FP alone relative to FP+SM conferred superior effects on surrogate inflammatory markers but not on lung function. Long-term studies are required to evaluate whether these improvements on surrogate inflammatory markers translate into commensurate reductions in airway remodelling and exacerbations.

Ecto 5'-nucleotidase and nonspecific alkaline phosphatase. Two AMP-hydrolyzing ectoenzymes with distinct roles in human airways

In human airways, extracellular adenosine regulates epithelial functions supporting mucociliary clearance, an important airway defense mechanism against bacterial infection. Thus, defining the mechanisms of adenosine generation is critical for elucidating the role of this nucleoside in airway homeostasis. In this study, we identified the source of adenosine on the mucosal surface of human airway epithelia. Polarized primary cultures of human nasal or bronchial epithelial cells were assayed for transepithelial transport, cytosolic and cell surface adenosine production. Ussing chamber experiments indicated that serosal 1 microM [(3)H]adenosine was not transported to the mucosal compartment. Messenger RNA for the cytosolic AMP-specific 5'-nucleotidase (CN-I) was not detected in human bronchial epithelial cells, suggesting that mucosal adenosine did not originate from intracellular pools. In contrast, extracellular 0.1 mm ATP was rapidly dephosphorylated into adenosine on the mucosal epithelial surface. We identified two ectonucleotidases that mediated the conversion of AMP to adenosine: ecto 5'-nucleotidase (ecto 5'-NT, CD73) and alkaline phosphatase (AP). Both mucosal and serosal epithelial surfaces displayed ecto 5'-NT activity (K(m) = 14 microM, V(max) = 0.5 nmol x min(-1) x cm(-2)), whereas AP activity was restricted to the mucosal surface (K(m,)(high) = 36 microM, V(max) = 1.2 nmol x min(-1) x cm(-2); K(m,)(low) = 717 microM, V(max) = 2.8 nmol x min(-1) x cm(-2)). In bronchial cultures and tissues, ecto 5'-NT accounted for >80% of total activity toward 0.01 mm AMP, compared with <15% for 5 mm AMP. The proximal airway AP isoform was identified as nonspecific AP (NS AP) by levamisole sensitivity and mRNA expression. The two ectoenzymes presented opposite airway distributions, ecto 5'-NT and NS AP mRNA dominating in higher and lower airways, respectively. Collectively, these experiments support a major role for extracellular nucleotide catalysis and for ecto 5'-NT and NS AP in the regulation of adenosine concentrations on airway surfaces.