n(6)-cyclopentyladenosine and Bronchial-Hyperreactivity

The effect of the adenosine triphosphate sensitive K+ (K(ATP)) channel opener (3S,4R)-3,4-dihydro-3-hydroxy-2,2-dimethyl-4-(2-oxo-1-piperidinyl)-N-phenyl-1-benzopyran-6-sulphonamide (KCO912) on airway hyperresponsiveness induced using either a combination of allergen immunization (i.p.) followed by aerosol allergen challenge or immunization alone was investigated. Rabbits were immunized with Alternaria tenuis for the first 3 months of life. Airway responsiveness to histamine was measured 24 h before and after A. tenuis aerosol challenge. Fifteen minutes before the second challenge, rabbits were pre-treated with 10 microg of KCO912 or vehicle by inhalation. Allergen challenge induced airway hyperresponsiveness in vehicle pre-treated rabbits and pre-treatment with KCO912 abolished the airway hyperresponsiveness. The effect of KCO912 (10 microg) or vehicle on baseline airway hyperresponsiveness to the adenosine A(1) receptor agonist, cyclopentyl adenosine (CPA), induced by immunization with A. tenuis alone, was also assessed. Rabbits, immunized with A. tenuis alone, exhibited baseline airway hyperresponsiveness as demonstrated by an increase in airway resistance to CPA. Treatment with KCO912 did not alter the allergen-induced airway responsiveness to CPA. This study demonstrates that KCO912 can inhibit allergen-induced exacerbations of airway hyperresponsiveness.

Topics: Adenosine; Adenosine Triphosphate; Allergens; Alternaria; Animals; Antigens, Fungal; Benzopyrans; Bronchial Hyperreactivity; Disease Models, Animal; Dose-Response Relationship, Drug; Hypersensitivity; Piperidines; Potassium Channels; Rabbits

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.

Topics: Adenosine; Anesthesia; Animals; Body Weight; Bradycardia; Bronchial Hyperreactivity; Capsaicin; Dose-Response Relationship, Drug; Infusions, Intravenous; Lung; Male; Nerve Fibers, Unmyelinated; Rats; Rats, Sprague-Dawley; Receptor, Adenosine A1; Receptors, Purinergic P1; Xanthines

1. New Zealand White (NZW) rabbits were immunized within 24 h of birth with Alternaria tenuis in aluminium hydroxide (Al (OH)3) (i.p.) or sham immunized (saline plus Al (OH)3 i.p.) and subsequently injected with the allergen (i.p.) or sham-immunized for the next 3 months. At 3 months of age, baseline airway responsiveness was assessed using cyclo-pentyl adenosine (CPA). Bronchoalveolar lavage (BAL) was performed in all animals and samples of peripheral blood were collected from some animals for estimation of dexamethasone levels. In some animals, blood was collected at the end of the experiment and cellular function was assessed by measurement of ex vivo proliferation of mononuclear cells in response to phytohaemagglutinin (PHA). 2. Allergen immunization significantly increased baseline airway responsiveness to inhaled CPA (P<0.05) in comparison with sham-immunized animals, at 3 months after immunization. Dexamethasone (0.5 mg kg(-1) day(-1)) treatment for 1 month did not modify this established airway hyper-responsiveness to CPA. Dexamethasone treatment did not affect either total or differential cell numbers in BAL fluid during the 4 week period, although significant plasma levels of dexamethasone were achieved in dexamethasone treated animals. 3. Treatment of rabbits with dexamethasone (0.1 mg kg(-1) i.p.), 6 h prior to each allergen injection from the neonatal stage, significantly reduced baseline airway hyper-responsiveness to CPA measured at 3 months (P<0.05). There was no significant difference in either total or differential cell numbers in BAL fluid, or any difference in mitogen-induced proliferation of mononuclear cells between dexamethasone and vehicle treated rabbits. 4. These results suggest that introduction of glucocorticosteroids in early life can prevent baseline airway hyper-responsiveness to inhaled CPA in allergic rabbits. However, once established, such underlying airway hyper-responsiveness is difficult to resolve, even with prolonged treatment with glucocorticosteroids.

Topics: Adenosine; Animals; Bronchial Hyperreactivity; Bronchoalveolar Lavage Fluid; Cell Count; Cell Division; Dexamethasone; Disease Models, Animal; Female; Glucocorticoids; Hypersensitivity; Leukocytes, Mononuclear; Male; Phytohemagglutinins; Purinergic P1 Receptor Agonists; Rabbits