Mouse models of asthma: can they give us mechanistic insights into the role of nitric oxide?

New clinical practice guidelines for patients with asthma include the recommendation to monitor exhaled breath nitric oxide (NO) levels. NO concentrations in exhaled breath are increased in asthmatics and increased NO levels correlate with worsening airway inflammation and asthma symptoms. The multiple roles of NO in the lung have not been delineated clearly. Clinical trials are being performed presently that test the apparently conflicting hypotheses that either donors or inhibitors of NO in the lung are effective strategies for treating asthma. These strategies evolved, in part, from results of pre-clinical studies performed in mice and other animal models. This review evaluates the existing literature with regard to mouse models of asthma and explores the often conflicting data on the role of NO, the nitric oxide synthase (NOS) enzymes, and the arginase enzymes in allergic airway inflammation. While we will emphasize the ovalbumin exposure mouse model, we will also examine other models. Where inconsistencies are identified among the studies, we attempt to determine whether such inconsistencies arise from methodological differences or alternative mechanisms. Ultimately, we address whether the allergen-exposed mouse is a suitable model for identifying promising new drugs for the treatment of human asthma. While a consensus is building that NO is beneficial or protective in subsets of asthmatics, results from studies using mouse models to investigate the individual roles of NO and the NOS enzymes in airway inflammation are often contradictory. Further research efforts with this model will allow us to distinguish which asthma patients may benefit best from NO donors and which may benefit from NO inhibitors.