Allergen-induced airway reactions in the pig in vivo : Inflammatory mediators as targets for asthma therapy
Abstract: A pig model for studies of allergen-induced airway reactions has previously been developed. These pigs are actively sensitized to Ascaris suum and challenged with this antigen in the airways under anesthesia. In the present study, pharmacological interventions were used to assess the importance of four inflammatory mediators in the allergen- induced airway reaction in the pig: cysteinyl leukotrienes (cysLTs), endothelins (ETs), tryptase and bradykinin (M), with focus on airway obstruction, mediator release and bronchial blood flow. MK-571, a CysLT1-receptor antagonist, did not affect the acute allergen-induced bronchoconstriction in sensitized pigs, indicating that cysLTs are probably not of major importance in this reaction in the pig. Nonetheless, exogenous LTD4 induced bronchoconstriction and also bronchial vasoconstriction. MK-571 pre-treatment led to an elevated bronchial vascular conductance (BVC) upon allergen challenge compared to BVC in control pigs, seemingly due to blockade of a vasoconstrictive effect mediated by CysLT1 receptors. To investigate the role of ETs, ET agonists (ET-1, ET-3 and sarafotoxin) were administered as i.v. injections and as aerosols. Since only the selective ETB-receptor agonist sarafotoxin induced a significant increase in Raw, ET-induced bronchoconstriction in the pig seems to be primarily ETBmediated. Bosentan, a non-selective ET-receptor antagonist, blocked the ET-induced bronchoconstriction but did not affect the allergen-induced airway response. All agonists induced a significant rise in BVC when given i.v. due to NO-mediated vasodilatation. Aerosol challenges with ET agonists resulted in a decrease in BVC, probably due to direct ETA/ETB2-mediated effects on vascular smooth muscle. This direct vasoconstrictive effect seems to dominate in the anesthetized pig, as bosentan pre-treatment resulted in an increase in baseline BVC, and a lowered mean arterial pressure. The tryptase inhibitor APC-366 reduced the acute allergen-induced airway obstruction and histamine release significantly. APC-366 also blocked the increase in BVC after allergen challenge, apparently via an indirect inhibition of histamine release from mast cells. The BK B2 receptor antagonist NPC-567 blocked both the acute and late-phase allergen-induced airway obstructions. NPC-567 also affected the late but not the acute rise in BVC after allergen challenge. No increase in urinary histamine concentration was seen after allergen challenge in the pigs treated with NPC-567. NPC-567 probably inhibits histamine release via indirect effects on the mast cell. In conclusion, LTD4 and ETs are bronchoconstrictive when given i.v. but have opposite effects on the BVC: ETs induce vasodilatation via NO formation while LTD4 is vasoconstrictive. Blockage of CysLT1 receptors or ETA/ETB receptors did not affect the acute allergen-induced bronchoconstriction. A tryptase inhibitor and a BK B2-receptor antagonist were effective in reducing this response, probably due to indirect effects on histamine release from mast cells, as both drugs reduced the allergeninduced acute elevation of histamine in urine.
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