Nitric oxide and upper airway

Nitric oxide is formed in cells by the enzyme known as NO synthase (NOS). Three isoforms of NOS are now known to exist. They are nNOS, iNOS and eNOS. In normal persons, a large NO production takes place in the epithelium of paranasal sinuses and this NO contributes considerably to the levels of NO found in nasally exhaled air. The high local NO concentrations in nasal airways and the sinuses may help to protect the airway against airborne infectious agent. Thus, airborne NO may represent first line of defence in the airways, possible acting on pathogens, even before they reach the mucosa. On the other hand, inhaled endogenous NO, derived from the upper airway, may be involved in regulation of pulmonary function in human.