Nitric Oxide in Exhaled Air: Relevance in Inflammatory Lung Disease

Nitric oxide (NO) is produced by many cells within the respiratory tract and endogenous NO may play an important signalling role in the physiological control of airway function and in the pathophysiology of airway diseases [1–3]. All three isoforms of NO synthase (NOS) exist within the respiratory tract [4–6]. The endothelial constitutive isoform (eNOS) is localised to bronchial endothelial cells and to epithelial cells [7] and the neuronal isoform (nNOS) to parasympathetic nerves and to epithelial cells [8, 9]. Inducible NOS (iNOS) may be localised to several cell types, including epithelial cells and macrophages [10–12] and may be expressed even in the normal human respiratory tract (Fig. 1).