Interrelation between Oxygen Tension and Nitric Oxide in the Respiratory System

To understand the relationship between oxygen tension and nitric oxide (NO) function, one animal and two human studies were designed. In the animal study, the effect of NO in inducing the relaxation of aortic specimens was significantly lower by 68% under 480 mm Hg of oxygen tension than under 28 mm Hg, indicating that oxygen tension has an important role in determining the biological effects of NO. In a clinical analysis with nonsmokers (n 5 23), the alveolar-to-arterial difference for oxygen ( A -aD O 2 ) was reciprocally correlated with exhaled NO concentrations (r 5 0.53). Because NO concentration in the lower respiratory zone depends partly on the amount of inspirable NO originating in the upper airway, a wellventilated area, requiring much perfusion, could receive greater amounts of NO than a poorly ventilated one. Thus, the reciprocal relation of A -aD O 2 with the concentration of exhaled NO is not necessarily incompatible with the effect of hypoxic pulmonary vasoconstriction in ventilation-to-perfusion (V 9 A /Q 9 ) imbalance. In our third experiment, with nonsmokers (n 5 21), pure oxygen inhalation during mechanical ventilation significantly decreased the concentration of exhaled NO and enhanced A -aD O 2 , indicating a relationship between NO and oxygen similar to that observed in the animal experiment. These findings led us to conclude that a positive relation between exhaled NO and blood oxygenation efficiency exists in the respiratory system, and further, that oxygen might affect this relationship. Thus, the relative balance of NO and oxygen concentrations may be another factor for consideration in respiratory function.