Role of exhaled nitric oxide in asthma

Nitric oxide (NO), an evanescent atmospheric gas, has recently bren discove red to be an important biological mediator in animals and humans. Nitric ox ide plays a key role within the lung in the modulation of a rude variety of functions including pulmonary vascular tone. nonadrenergic non-cholinergic (NANC) transmission and modification of the Inflammatory response. Asthma is characterized by chronic airway inflammation and Increased synthesis of NO and other highly reactive and toxic substances (reactive oxygen species) . Pro-inflammatory cytokines such as TNF alpha and IL-1 beta are secreted i n asthma and result in inflammatory cell recruitment. but also induce calci um- and calmodulin-independent nitric oxide synthases (iNOS) and perpetuate the inflammatory response within the airways. Nitric oxide is released by several pulmonary cells including epithelial cells. eosinophils and macroph ages, and NO has been shown to be increased in conditions associated with a irway inflammation, such as asthma and viral infections. Nitric oxide can b e measured in the expired air of several species. and exhaled NO can now be rapidly and easily measured by the use of chemiluminescence analysers in h umans. Exhaled NO is increased in steroid-naive asthmatic subjects and duri ng an asthma exacerbation, although it returns to baseline levels with appr opriate anti-inflammatory treatment, and such measurements have been propos ed as a simple non-invasive method of measuring airway inflammation in asth ma. Here the chemical and biological properties of NO are briefly discussed followed by a summary of the methodological considerations relevant to the measurement of exhaled NO and its role in lung diseases including asthma. The origin of exhaled NO is considered and brief mention made of other pote ntial markers of airway inflammation or oxidant stress in exhaled breath.