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The past decade has witnessed an explosion in the interest of biologists in the gas nitric oxide (NO). This highly reactive free radical, first considered only a noxious air pollutant, is produced in mammalian cells by specific enzymes and is believed to play a vital role in many biological events including regulation of blood flow, platelet function, immunity, and neurotransmission.1 2Direct measurement of NO in biological tissues is difficult to perform because this gas reacts rapidly with, for example, haemoglobin or other Fe2+-containing proteins. It is therefore often necessary to rely on indirect measurements in order to detect NO synthesis in vivo. Unlike the situation in most biological tissues where NO is rapidly destroyed, in the gas phase NO is fairly stable at low concentrations.3 NO produced in superficial structures of hollow organs will diffuse into the lumen and thus be detectable in gas collected from such organs.
The presence of NO in exhaled breath of humans was first demonstrated by Gustafsson et al in 1991.4Later studies clearly showed that, in healthy controls at rest, almost all NO found in exhaled air originates from the upper airways with only a minor contribution from the lower respiratory tract and the lungs.5-7
Origin of nasal NO
The exact origin of the NO found in nasal air and the relative contribution from different sources within the nasal airways are not known. There are, however, some indications that favour the paranasal sinuses rather than the mucosa of the nasal cavity as a major source of nasal NO in adult healthy humans.8 Firstly, NO release in the sinuses is markedly reduced by intrasinus instillation of an NO synthase inhibitor (l-NAME) whereas nasal NO concentrations are only slightly reduced following intranasal administration of this inhibitor.9 Secondly, immunohistochemical and in …
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