Nitric oxide and eicosanoids : Significance and interactions during antigen-induced responses in peripheral lung tissue

Abstract: Asthma means difficulty in breathing and is described as a chronic, inflammatory disorder that produces narrowing of the lower respiratory tract. The allergen-induced asthmatic bronchoconstriction is primarily caused by an IgE-mediated release of the mast cell mediators, histamine and eicosanoids (leukotrienes and prostanoids). Asthmatics have elevated levels of nitric oxide (NO) in the exhaled air, which been proposed as a sign of airway inflammation. In the airways, mast cells represent a major source of NO. This formed NO may act in an autocrine fashion to suppress mast cell function, including release of histamine and leukotriene synthesis, and thereby be a regulator of allergen-induced responses. Nevertheless, the function of NO in the peripheral lung is not clear. The aim of this thesis was therefore to establish the role of nitric oxide and eicosanoids during early allergic airway responses in the peripheral lung. Antigen-induced contractions to the allergen ovalbumin were studied in the lung parenchyma obtained from actively sensitized guinea pigs, an in vitro-model for mast cell driven antigen-induced contractions. The peripheral lung is a complex tissue with airway smooth muscle, bronchioles, vessels and connective tissue. To further understand and characterize the contractile responses obtained to allergen or agonists in lung parenchymal tissue, studies in guinea pig precision cut lung slices (GP PCLS) were established and performed. Another aim of this thesis was also to compare species differences during the early allergic airway response in the PCLS. Inhibition of nitric oxide synthase (NOS) enhanced the contractions to cumulative doses of ovalbumin, whereas addition of the different NO donors SNP and NCX 2057 attenuated the antigen-induced contractions. The action of NO was however not relaxation of airway smooth muscle, since NO potently dilated precontracted vascular preparations and weakly relaxed precontracted tracheal rings, while there was no effect on precontracted GPLP. Instead, NO act as inhibitor of allergen-induced mediator release in the peripheral lung. Inhibition of endogenous NO increased the release of leukotrienes, whereas SNP and NCX 2057 distinctly inhibited the release of histamine or leukotrienes during antigen challenge. In conclusion, the findings support that endogenous NO has a protective role in the peripheral lung as a beneficial immunomodulator of the early allergic airway response. The findings also indicate that different NO donors may have selective and protective anti-inflammatory effects in the peripheral lung tissue. The GP PCLS was established and represents now a new in vitro model to simultaneously measure airway and vascular responses under cell culture conditions. The study showed that the pharmacology of the guinea pig PCLS most closely resembled that of the corresponding human tissues. In guinea pigs and humans, leukotrienes and prostanoids were primary mediators of the antigen-induced bronchoconstriction. In contrast, the contractile response to antigen in rat PCLS was mainly mediated by serotonin and modulated by locally formed prostanoids, in particular COX-2 derived PGE2, acting at EP1 receptors. Thus, mechanisms by which eicosanoids contribute to the early allergic airway response differ among species.