Role of endogenous nitric oxide in asthma

Endogenous nitric oxide (NO) is an ubiquitous signaling molecule with impor tant regulatory functions such as regulation of blood pressure, neurotransm ission, and host and immune defense. In the respiratory tract, NO is formed and released by various sources including endothelial and epithelial cells , nerves, airway smooth muscle, and inflammatory cells. Recent evidence sug gests that endogenous NO is the neurotransmitter of the nonadrenergic nonch olinergic inhibitory (iNANC) system, the only bronchorelaxant neural pathwa y of human airways. A number of studies also suggest that in some species e pithelium-derived NO accounts for the functional bronchoprotective sole of the so-called epithelium-derived relaxing factor. In human airways, endogen ous NO counteracts the bronchoconstriction induced by pharmacologic stimuli such as bradykinin, histamine, and methacholine. On the basis of these and other observations, it is suggested that a reduced synthesis and/or activi ty of endogenous NO may contribute to the pathogenesis of airway hyperrespo nsiveness that characterizes asthma and other respiratory disorders. This s hort paper summarizes the activities of endogenous NO in the airways of exp erimental animals and man, and discusses the evidence supporting the view t hat NO confers bronchoprotection.