The Effect of Adenosine A_2A and A_2B Antagonists on Tracheal Responsiveness, Serum Levels of Cytokines and Lung Inflammation in Guinea Pig Model of Asthma

Abstract

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 A_2A and A_2B 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 A_2A and A_2B antagonists (S+Anta A_2A, S+Anta A_2B) 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 A_2B) was significantly lower than those of sensitized group (p<0.01 to p<0.05). In pretreated group with Anta A_2A(S+Anta A_2A), all the above changes were reversed.

Conclusion: These results showed a preventive effect of A_2B 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 A_2A antagonist, deteriorated the induction effect of ovalbumin.

Introduction

Asthma is a chronic disease characterized by a variety of features including reversible airways obstruction, airway inflammation and an increased airway responsiveness.¹ Evidence has increasingly implicated adenosine in the pathophysiology of asthma.² Adenosin is the breakdown product of ATP via endogenous ecto-ATPases and is also present at cell surface in cultured airway epithelial cells.³ Adenosine in a signaling nucleoside is eliciting many physiological responses. Elevated levels of adenosine have been found in bronchoalveolar lavage, blood and exhaled breath condensate of patients with asthma. 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.⁴

It has become clear that biological functions of adenosine are mediated by four distinct subtypes of receptors (A₁, A_2A, A_2B, and A₃) and that biological responses are determined by the different pattern of receptors distribution in specific cells. Adenosine receptors are ubiquitously expressed throughout the body, with virtually all cells expressing one or more adenosine receptor subtype. With respect to the lung, little is known about the relative expression of adenosine receptor subtypes; however, binding studies in healthy peripheral lung tissue have suggested that A₂ receptor subtypes are much more abundant than the A₁ and A₃ receptor subtypes.⁵

For determining the effect of different A₂ receptors in pathophysiology of asthma, in this investigation the effect of selective adenosine A_2A and A_2B antagonists (ZM241385 and MRS1706) on tracheal responsiveness to methacholine and ovalbumin (OA), total and differential cell count in bronchoalveolar lavage, blood levels of IL-4 and IFN-γ and lung pathology of guinea pig model of asthma were assessed.

Materials and Methods

Animal sensitization and animal groups

Forty male adult Dunkin-Hartley guinea pigs (400–700 g) were used throughout the study. They were allowed to acclimatize to the new situation for ten days. The animals were group-housed in individual cages in climate-controlled animal quarters and were given water and food ad libitum, while a12-h on/12-h off light cycle was maintained.

Animals randomly divided to four groups; Control group (C), Sensitized group with ovalbumin (OA, S), sensitized groups pretreated with selective A_2A antagonist (ZM241385) and selective A_2B antagonist (MRS1706) (S+Anta A_2A and S+Anta A_2B groups). Each of these antagonists (Tocris bioscience, UK) with 3 mg/kg dose was injected i.p. on day 10 of induction protocol.^6,7

Sensitization of animals to OA (Grade II Sigma Chemical Ltd., UK) was performed according to our previous study.⁸ Briefly, on the first day, 100 mg of OA, dissolved in saline, injected intraperitoneally and other 100 mg of OA subcutaneously. A week later, subsequent 10 mg of OA was injected intraperitoneally. Then from day 14, sensitized animals were exposed to an aerosol of 4% OA for 18 ± 1 days, 4 min daily. The aerosol was administered in a closed chamber with dimensions 30 × 20 × 20 cm. Control animals were treated similarly but saline was used instead of OA solution. The study was approved by the ethical committee of the Tabriz University of Medical Sciences.

Tissue preparation

Guinea pigs were killed by a blow on the neck and the trachea was removed. In each animal, after separation of the trachea from adjacent tissues, one tracheal chain was prepared as follows: The trachea was cut into 10 rings (each containing 2–3 cartilaginous rings) and sutured together to form a tracheal chain. Then all the rings (except terminal rings) were cut open opposite the trachealis muscle to clarify the muscular response. Finally tissue was suspended in a 20-mL organ bath (Schuler organ bath type 809, Germany) containing Krebs-Henseliet solution of the following composition (mM): NaCl; 120, NaHCO₃; 25, MgSO₄; 0.5, KH₂PO₄; 1.2, KCl; 4.72, CaCl₂; 2.5 and dextrose 11. The Krebs solution was maintained at 37 °C and gassed with 95% O₂ and 5% CO₂. Tissue was suspended under isotonic tension of 1 g and allowed to equilibration for at least 1 hour while it was washed with Krebs solution every 15 min.⁹

Responses were measured using an isometric transducer (ADInstruments, spain) with a sensitivity range of 0–25 g. These responses after amplifying with ML/118 quadribridge amplifier (March-Hugstetten, Germany) were recorded on a powerlab (ML-750, 4 channel recorder; March-Hugstetten, Germany).

Assessment of tracheal response to Methacholine

In each experiment, a concentration-response curve of the tracheal chain was obtained. Consecutive concentrations of methacholine hydrochloride (Sigma Chemical Ltd., UK); including 10⁻⁷ to 10⁻² M, dissolved in saline; were added every 3 minutes. The contraction due to each concentration was recorded at the end of 3 minutes and the effect reached a plateau in all experiments. Then the percentage of the tracheal smooth muscle contraction due to each concentration of methacholine in proportion to the maximal contraction obtained by its final concentration was plotted against log concentration of methacholine. A concentration-response curve of methacholine was performed in the tracheal chain of each studied animal. The effective concentration of methacholine causing 50% of maximum response (EC50) was measured from the methacholine response curve in each experiment using 50% of the maximum response in the Y axis and measuring the dose of methacholine causing this response in the X axis. The contractility response to 10 μM methacholine as the magnitude of contraction was also measured.

Measurement of tracheal response to Ovalbumin (OA)

The tracheal response of all animals to a 0.1% solution of OA was measured in each studied animal as follows: 0.5 mL of 4% OA solution (dissolved in saline) was added to the 20-mL organ bath and the degree of tracheal chain contraction was recorded after 10 min and was expressed as a proportion (in percentage) to the contraction obtained with 10 μM methacholine.

The measurements of tracheal response to methacholine and OA were performed in random order.

Lung lavage and its white blood cell count

Coincident with preparing the tracheal chain, a cannula was located into the remaining trachea and the lungs were lavaged with 5mL of saline 4 times (total: 20 mL). One mL of lung lavage fluid (LLF) was stained with Turk solution and counted in duplicate in a hemocytometer (in a Burker chamber). The Turk solution consisted of 1mL of glacial acetic acid, 1 mL of gentiac violet solution 1% and 100 mL of distilled water.

The remaining LLF was centrifuged at 2500 × g at 4 °C for 10 min. The supernatant was removed. The smear was prepared from the cells and stained with Wright-Giemsa. According to staining and morphological criteria, differential cell analysis was carried out under a light microscope by counting 100 cells twice and the percentage of each cell type calculated.¹⁰

Measurement of blood interleukin 4 (IL-4) and interferon-g (IFN-γ) levels

Ten milliliters of peripheral blood was obtained immediately after sacrificing the animals and placed at room temperature for 1 hour. The samples were then centrifuged at 3500g 4 °C for 10 min. The supernatant was collected and immediately stored in deep freezer at –70°C until analyzed. Finally, blood IL-4 and IFN-γ levels were measured using the sandwich ELISA method.¹¹

Pathological evaluation

Guinea pigs were sacrificed by a cervical dislocation, and their lungs and trachea were removed and placed into 10% buffered formalin (37%, Merck, Germany).

Seven days later, the tissues were dried using an Autotechnicon apparatus by passage of the tissues through 70% ethanol and xylol to clear the tissues and then paraffin block the tissues. The specimens were cut into 4-μm slices and stained with hematoxylin and eosin (H&E stain). The tissues were then evaluated under a light microscope. For each specimen, at least 10 airways and vessels were evaluated.⁸

The following pathological changes in the lungs of the sensitized groups were observed: vascular and airway membrane hyperplasia, mucosal plug, local epithelial denudation, eosinophil and lymphocyte infiltration and emphysema. These changes were scored as follows: no pathologic changes = 0, patchy changes =1, local changes =2, scattered changes =3 and severe changes =4.

Statistical analysis

The data of tracheal response to methacholine (EC50), tracheal contractility response, tracheal response to OA, total WBC numbers and differential WBC counts are quoted as mean ± SEM. The data of three sensitized groups were compared with controls using one-way analysis of variance (ANOVA) with Tukey-Kramer post-test. Moreover, the data of the sensitized group were compared with control and treated guinea pigs using one-way analysis of variance (ANOVA) with Tukey-Kramer post-test. The data between groups of animals treated with antagonists using the unpaired t-test. Significance was accepted at p<0.05.

Results

Tracheal response to methacholine

Concentration response curves to methacholine in non incubated tissues showed left ward shift of the curve in group S compared to group C. However, the curve of S+Anta A_2B group was shifted to right compared to group S. Pretreatment with Anta A_2A caused left ward shift compared to S group (Figure 1).

Figure 1 .

Cumulative log concentration-response curves of methacholine induced contraction of isolated trachea in control (C), sensitized (S), S treated with Anta A_2A (S+Anta A_2A) and Anta A_2B (S+Anta A_2B) guinea pigs in the organ bath (for each group, n=7).

The mean value of EC₅₀ in tracheal chains of group S (1.50±0.36 μM) was significantly lower than in group C (5.31±0.71 μM, p<0.001, Figure 2). The mean value of EC₅₀ in tracheal chains in pre-treated group with Anta A_2A (1.38±0.24μM) was non significantly lower than sensitized group; but administration of Anta A_2B (3.10±0.54 μM, P<0.01) caused significant improvment compared to the group S (Figure 2). However, the mean value of EC₅₀ in tracheal chains of S+Anta A_2B group was still significantly lower than in group C (P<0.05, Figure 2).

Figure 2 .

Individual values and mean±SEM (big symbols with bars) of tracheal response to methacholine (EC₅₀) in control (C), sensitized (S), S treated with Anta A_2A (S+Anta A_2A) and Anta A_2B (S+Anta A_2B) guinea pigs (for each group, n=6). Statistical differences between control and different groups: +; p<0.05, ++; p<0.01, +++; p<0.001. Statistical differences between sensitized and treated groups: *: p<0.05, **: p<0.01, ***: p<0.001.

Tracheal response to ovalbumin

Tracheal response to OA in tracheal chains of group S (62.889.12%, range 42.80-100%) was significantly higher than in group C (4.853.18%, range 0-23%, p<0.001, Figure 3). Tracheal response to OA in group S+Anta A_2A (64.729.01%, range 45-114.45%) was non significantly higher than that of S group but pre-treatment with Anta A_2B (45.7810.24%, range 12-78%) was significantly improved compared to sensitized group (p<0.05, Figure 3). However, tracheal response to OA in S+Anta A_2B group was still significantly higher than in group C (P<0.01, Figure 3).

Figure 3 .

Individual values and mean±SEM (big symbols with bars) of tracheal response to ovalbumin in control (C), sensitized (S), S treated with Anta A_2A (S+Anta A_2A) and Anta A_2B (S+Anta A_2B) guinea pigs (for each group, n=6). Statistical differences between control and different groups: +; p<0.05, ++; p<0.01, +++; p<0.001. Statistical differences between sensitized and treated groups: *: p<0.05, **: p<0.01, ***: p<0.001.

Contractility

The contractility response of tracheal chains to methacholine of group S (1.63±0.08 g) was significantly higher than that of group C (0.58±0.04 g, p<0.05). The contractility response in treated group with Anta A_2B (1.27±0.05 g) caused significant decrease compared with S group (p<0.01) although this response was still significantly higher than controls (p<0.001). There was no significant difference in the contractility response of S+Anta A_2A group (1.84±0.21 g) with group S (Figure 4).

Figure 4 .

Individual values and mean±SEM (big symbols with bars) of tracheal contractility response to 10 μM methacholine in control (C), sensitized (S), S treated with Anta A_2A (S+Anta A_2A) and Anta A_2B (S+Anta A_2B) guinea pigs (for each group, n=6). Statistical differences between control and different groups: +; p<0.05, ++; p<0.01, +++; p<0.001. Statistical differences between sensitized and treated groups: *: p<0.05, **: p<0.01, ***: p<0.001.

Total white blood cell count

The mean value of total white blood cell (WBC) in LLF of group S (9421.43±169.1) was significantly higher than that of group C (2580±180.02, P<0.01) (Figure 5). The WBC in S+Anta A_2A group (9896.43±288.80) was non significantly higher than sensitized animals. The WBC in S+Anta A_2B group (5950±1035.7) showed significant improvement compared to that of S group (p<0.05, Figure 5). However, the mean value of WBC in this group was still significantly higher than in group C (p<0.01, Figure 5).

Figure 5 .

Individual values and mean±SEM (big symbols with bars) of total LLF WBC number in control (C), sensitized (S),S treated with Anta A_2A (S+Anta A_2A) and Anta A_2B (S+Anta A_2B) guinea pigs (for each group, n=6). Statistical differences between control and different groups: +; p<0.05, ++; p<0.01, +++; p<0.001. Statistical differences between between sensitized and treated groups: *: p<0.05, **: p<0.01, ***: p<0.001.

Differential count of WBC in Lung Lavage fluid

There was a significant decrease in neutrophil, lymphocyte and monocyte but significant increase of eosinophil and non-significant increase of basophil in LLF of group S compared to those of group C (p<0.001 for all cases, Figure 6a-e). Administration of Anta A_2A caused significant decline in lymphocyte (p<0.01) and significant increment in eosinophil count (p<0.05) but the neutrophil, monocyte and basophil counts were not significantly different from S group. However treatment of sensitized animals with Anta A_2B caused significant improvement in all LLF different cell counts (p<0.001 for all cases) but there were still significant differences in eosinophil and lymphocyte count of this group in comparison with those of group C (p<0.05 to P<0.001, Figure 6a-e).

Figure 6 .

The percentages of eosinophil (a), neutrophil (b), lymphocyte (c), monocyte (d) and basophil (e) of lung lavage fluid in control , sensitized (S), S treated with Anta A_2A (S+Anta A_2A) and Anta A_2B (S+Anta A_2B) guinea pigs (for each group, n=15). Statistical differences between control and different groups: +; p<0.05, ++; p<0.01, +++; p<0.001. Statistical differences between sensitized and treated groups: *: p<0.05, **: p<0.01, ***: p<0.001.

Blood IL-4 and IFN-γ levels

The mean value of the blood IL-4 levels in groups S (47.41± 1.98), S + Anta A_2A (49.48 ± 2.74) and S + Anta A_2B (45.24 ± 2.15) were significantly higher than that of group C (39.78± 2.10, p < 0.05, Figure 7a). However, the mean values of the IL-4 in these pretreated groups were not significantly lower than those of group S (Figure 7a).

Figure 7 .

The blood IL-4 (a) and IFN-γ (b) levels (pg/ml) in control, sensitized (S), S treated with Anta A_2A (S+Anta A_2A) and Anta A_2B (S+Anta A_2B) guinea pigs (for each group, n = 8). Statistical differences between the control and the different groups: ns; no significant difference, +; p < 0.05. Statistical differences between sensitized and treated groups: NS; no significant difference,** p < 0.01.

The mean value of the blood IFN-γ levels of group S (117.37 ± 2.7) and S + Anta A_2B (120.02 ± 2.95) was significantly higher than that of group C (104.97 ± 2.70, p < 0.05, Figure 7b). The mean value of IFN-γ in S+Anta A_2A group (101.14±3.24) was significantly lower than in group S (p<0.01, Figure 7b).

Pathological results

All pathological changes in the S and S+A_2A groups, including vascular membrane hyperplasia (3.0±0.3 and 2.90±0.28 respectively), airway membrane hyperplasia (2.54±0.28 and 2.54±0.31 respectively), mucosal plug (2.72±0.14 and 2.27±0.23 respectively), local epithelial denudation (2.18±0.23 and 2.45±0.21 respectively), eosinophil and lymphocyte infiltration (1.55±0.20 and 2.18±0.22 respectively) and emphysema (2.0±0.27 and 2.18±0.23 respectively) were significantly higher than control group (0.33±0.21, 0.17±0.16, 0±0, 0.17±0.16, 0±0 and 0.17±0.16 for vascular and airway membrane hyperplasia, mucosal plug, local epithelial denudation, infiltration and emphysema respectively, p<0.05 for all, Figure 8, 9 a-f).

Figure 8.

Photographs of a lung specimen in guinea pigs: a- control normal lung tissues (C); b- sensitized (S) with airway membrane hyperplasia and mucosal plug; c- S treated with Anta A_2A (S+Anta A_2A) with severe emphysema and d- S treated with Anta A_2B (S+Anta A_2B) with local epithelial denudation (magnification for each group; 10´20).

Figure 9 .

The vascular (a) and airway membrane hyperplasia (b), mucosal plug (c), local epithelial denudation (d), eosinophil and lymphocyte infiltration (e) and emphysema (f) of lungs in control, sensitized (S), S treated with Anta A_2A (S+Anta A_2A) and Anta A_2B (S+Anta A_2B) guinea pigs (for each group, n = 8). Statistical differences between the control and the different groups: ns; no significant difference, +; p < 0.05. Statistical differences between sensitized and treated groups: NS; no significant difference,* p < 0.05.

In S+A_2B group, some of pathological changes; the mucosal plug (1.14±0.34), local epithelial denudation (0.86±0.26) and emphysema (1.42±0.30) were significantly higher than controls (p<0.05). however, the pathological changes such as vascular membrane hyperplasia (1.42±0.48) and airway membrane hyperplasia (0.86±0.34), the mucosal plug , local epithelial denudation in group S+A_2B were significantly lower than S group (p<0.05, Figure 8, 9 a-f).

Discussion

In the present study, the effect of ZM241385 (selective adenosine A_2A antagonist) and MRS1706 (selective adenosine A_2B antagonists) on tracheal responsiveness to methacholine and OA, total and differential cell count in bronchoalveolar lavage, blood levels of IL-4 and IFN-γ and lung pathology of guinea pig model of asthma were examined. The results showed increased tracheal responsiveness to methacholine and OA, increased contractility response, increment of LLF total WBC count, eosinophil and basophil number (non-significantly), elevated blood IL-4 and IFN-γ and numerous lung tissue pathological changes but decreased neutrophil, lymphocyte and monocyte in sensitized compared to control animal which was similar to the results of our previous studies.^11,12

Exogenous and endogenous adenosine; a ubiquitous purine nucleoside, has essential role in the pathogenesis of asthma and other lung inflammatory disorders. This concept is based on the fact that adenosine receptors are present in many cell types involved in airway inflammation.¹³ It is now clear that the main mechanism responsible for exogenous adenosine inhalation-induced bronchoconstriction is mediators release from mast cells although there are some evidences for neural pathways activation.¹⁴ In addition to this effect, the increased level of adenosine found in biological fluids, such as bronchoalveolar lavage and exhaled breath condensate of patients with asthma.¹⁵ Although the precise source of adenosine release (mast cells, smooth muscle, epithelial cells) remains uncertain it is likely that adenosine may contribute to the bronchoconstriction induced by other stimuli such as allergens, hypoxia, lung injury and chronic inflammation.¹⁶ However the bio-availability of adenosine is an important determinant of its biological functions, the pattern of expression and distribution of its different receptors (A₁, A_2A, A_2B, A₃) in the anatomical and structural sites of the respiratory system and in immune or inflammatory cells, are responsible for this matter that adenosine may exert either deleterious or protective roles in the lung. However the inflammatory cytokines can regulate the expression of adenosine receptors, adenosine had a role in the inflammatory environment.¹⁷

The data strongly suggest that activation of adenosine A_2A receptors, which are present in most of the inflammatory cells (such as neutrophils, mast cells, macrophages, eosinophils, platelets and T cells) inhibit inflammatory responses via affecting multiple aspects of the inflammatory process, modulating neutrophils activation and degranulation, oxidative species production, adhesion molecules expression, cytokines release and mast cells degranulation.^5,18-20

In this study, single dose administration of ZM241385, selective adenosine A_2A antagonist, caused increased tracheal responsiveness (decreased EC50 and incremental contractility), tracheal response to OA and total WBC count and eosinophil and basophil number in LLF and pathological changes and decrease in neutrophil, monocyte and lymphocyte count in comparison to controls. These changes were more than those of sensitized group. It showed that this A_2A receptor antagonist deteriorated the effect of ovalbumin in inducing asthma in guinea pigs. It has been predicted by exerting inhibitory effects of activation of these receptors on multiple inflammatory cell types mentioned before.

In addition, administration of selective adenosine A_2A antagonist, ZM241385, caused increased IL-4 level and decreased IFN-γ in blood. In asthma, the inflammation is regulated by two subsets of CD4+ helper T cells; Th1 and Th2 balance. IFN-γ secretes mostly by Th1 cells whereas IL-4 produces mostly by Th2 cells. In fact, asthma is associated with a shift in immune responses away from a Th1 (IFN-γ) pattern and toward a Th2 (IL-4, IL-5 and IL-13) profile.²¹ So one of proposed mechanisms of selective adenosine A_2A antagonist could be its effect on regulation of T helper cells subtypes. Also the decreased lymphocyte count in this study can support this suggested mechanism.

Expression of adenosine A_2B receptors has been found in bronchial epithelium, cultured human airway smooth muscle, human mast cells, monocytes and fibroblasts.⁴ Increasing evidences suggest that in rodents and man activation of adenosine A_2B receptors modulates mast cells function. Adenosine signaling through the A_2B receptors can stimulate the production of IL-8, IL-4, IL-13 and VEGF from mast cells. This receptors signaling is an important factor of aberrant dendritic cell differentiation and generation of tolerogenic, angiogenic, and pro-inflammatory cells that produce VEGF, IL-8, IL-6, IL-10, COX-2 and TGF-β. In addition, A_2B receptors engagement can promote the production of IL-6 and osteopontin from macrophages; IL-6 and MCP-1 from bronchial smooth muscles; increases in IL-6 release from fibroblasts; induces myofibroblasts differentiation; and induces the expression of fibronectin in type II lung epithelial cells. A_2B receptor signaling also contributes to the maintenance of airway surface liquid height in airway epithelial cells and vascular barrier function in endothelial cells.²² It suggests that adenosine, via A_2B receptors participates in the remodelling process occurring in chronic inflammatory lung diseases.²³ Taken together these evidences suggest that adenosine A_2B receptor are deeply involved in the mechanisms underlying mediators release by mast cells, the major mechanism by which adenosine induces bronchoconstriction and airway inflammation in asthma. Therefore it has been suggested that targeting adenosine receptors might be a possible approach for the development of anti-inflammatory treatments in diseases characterized by chronic airway inflammation such as asthma and COPD. Currently there is an agreement that development of selective adenosine A_2B receptor antagonists might be the most appealing approach.^14,24

Some authors have speculated that the pro- and anti-inflammatory property of adenosine may be dictated by its level in the lung. ^14,16 Lung inflammation determines a hypoxic environment in which adenosine is generated. In the initial stage, low levels of adenosine activate high affinity receptors, such as adenosine A_2A receptors, and this triggers a protective pathway. However higher levels of adenosine are released when lung inflammation is severe, and these, activating the low affinity adenosine A_2B receptors, may trigger deleterious signaling pathways that further exacerbate inflammation.¹⁶

Single dose administration of MRS1706, selective adenosine A_2B antagonist, in this study, improved the changes in tracheal responsiveness, total WBC count and lung pathological changes. This drug ameliorated the variations in differential WBC compared to asthmatic guinea pigs. However it could not prevent completely in comparison to controls. Moreover, this drug caused increase in IFN-γ and decrease in IL-4 level in comparison to sensitized animals although it could not reach those of controls. The lymphocyte count was also higher than sensitized group. These results were in line with previous studies. Mustafa and his colleagues in 2007 showed that other antagonist of adenosine A_2B receptors, CVT-6883, inhibited the airway inflammation.²⁵ These studies confirmed the role of A_2B receptors in the pathophysiology of asthma.

Conclusion

In Conclusion, the results showed that administration of single dose of selective adenosine A_2A (ZM241385) deteriorated the inflammatory changes of ovalbumin induced asthma and single dose prescription of A_2B antagonists (MRS1706) could prevent these changes.

Acknowledgements

This investigation was supported by Tuberculosis and lung research center of Tabriz University of medical sciences as the part of the thesis of Msc student, Miss Pejman.

Conflict of Interest

The authors declare that they have no conflict of interest.