An experimental study of mediators in allergic bronchoconstriction with focus on eicosanoids

Abstract

Mast cells have a central role in the inflammatory response in airways of both atopic and non-atopic asthmatics. The mast cell-derived mediators such as histamine, leukotriene C4 and prostaglandin D2, released upon activation of the cell, produce acute bronchoconstriction that is characteristic of the early asthmatic response (EAR), and contribute to the development of airway inflammation and hyperresponsiveness following the initial reaction. There is a great need for relevant experimental in vitro models to study the airway effects of the mast cell-derived mediators. The overall aim of the experimental studies presented in this thesis was accordingly an evaluation of the relevance of the isolated perfused and ventilated guinea pig lung (IPL) as an experimental model to study the EAR.

The reactivity of the IPL to histamine, cysteinyl-leukotrienes (CysLTs: leukotriene (LT) C4, D4 and E4) and prostanoids, established by intravascular challenge of the normal lungs with these mediators, was closely similar to that of human airways. The cyclooxygenase (COX)-dependent modulation of the responses to bronchoconstrictive agonists in IPL followed two different patterns. The responses to histamine and LTD4 were amplified by agonist-induced secondary release of bronchoconstrictive COX products. Both COX isoenzymes, COX-1 and COX-2 participated in the LTD4-induced generation of COX products. In contrast, the response to PGD2 did not actively promote the agonist-induced release of COX products, but was down-modulated by relaxant COX products presumably tonically formed in airways.

Investigation of the pharmacology of the PGD2 response in IPL by the use of COX inhibitors gave rise to the hypothesis that diclofenac also was a thromboxane (TP) receptor antagonist. Further investigation of the effects of diclofenac and its structural analogue lumiracoxib on the TP-mediated responses in standard airway and vascular pharmacology in vitro models and in human platelets confirmed that these drugs, in addition to being COX unselective and highly COX-2 selective inhibitors, respectively, also were competitive TP receptor antagonists.

The IPL was demonstrated to provide a model where a specific, dose-dependent and reproducible bronchoconstriction was obtained in response to intravascular challenge with antigen (ovalbumin) in lungs actively sensitised to ovalbumin. The mediators of the antigen-induced bronchoconstriction were investigated by pharmacological interventions with an antihistamine, an antileukotriene or COX inhibitors as well as by measurements of the antigen-induced mediator release. It was established that similarly to the situation in human airways, histamine, CysLT and prostanoids mediated the antigen response in IPL. Furthermore, both COX-1 and COX-2 contributed to the immediate antigen-induced generation of prostanoids. The results of the inhibition of two classes of mediators at a time disclosed interactions among the mediators at several levels. The complete inhibition of the antigen response produced by inhibition of all the three mediator classes at a time demonstrated that the concerted actions of histamine, CysLTs and prostanoids could solely explain the immediate antigen-induced bronchoconstriction in this model. In conclusion, the IPL is considered a relevant experimental model for studies of the EAR, and it is proposed to test the concept of inhibition of the three mediator classes in subjects with asthma.