Nitric oxide production in the lung and its regulation by oxygen

Nitric oxide (NO) in exhaled breath is primarily derived from the airway. NO is endogenously synthesized by nitric oxide synthases (NOS) which convert L-arginine to L-citrulline and NO. Three NOS isoforms (type I, II and III) have been identified in the human lung. Oxygen is a substrate for all NOS enzymes and it also regulates NOS expression and the enzyme activity in the lung. Rapid response of endogenous NO in direct proportion to inspired oxygen strongly supports a role for oxygen in regulating NO production in the lung through a kinetic mechanism.