An (18)oxygen inhalation method for determination of total body formation of nitric oxide in humans

The formation of nitric oxide (NO) and the subsequent conversion of the NO formed into nitrate require molecular oxygen. Based on this fact, we have r ecently developed a method using inhalation of the stable oxygen isotope, i .e. O-18(2), to determine total formation of NO in small laboratory animals . The method has now been further developed to be applicable also in humans . Five healthy awake male subjects inhaled a gas mixture of unlabelled and 18-labelled oxygen (approximate ratio 4:1) in nitrogen from a closed breath ing system equipped with eliminators for carbon dioxide and water vapour. T he ratio of unlabelled to 18-labelled oxygen, as well as the total oxygen c oncentration during the inhalation, were monitored. Venous blood samples we re taken before and after the inhalation for analysis of unlabelled and O-1 8-labelled nitrate by gas chromatography/mass spectrometry. The procedure w as repeated with the same protocol on a later occasion, during ongoing trea tment with the NO synthesis inhibitor N-G-monomethyl-L-arginine (L-NMMA). T he average nitrate level in plasma in the absence of L-NMMA was 26 mu mol l (-1). The rate of total synthesis of NO was estimated to be 0.38 +/- 0.06 m u mol kg(-1) h(-1), corresponding to a total body formation of 600-700 mu m ol/24 h in an adult male. Infusion of L-NMMA caused an increase in mean art erial blood pressure from 86 +/- 4 to 99 +/- 5 mmHg (P < 0.05). The average plasma level of nitrate during infusion of L-NMMA was 24 mu mol l(-1). NO formation during infusion of L-NMMA was 0.17 +/- 0.03 mu mol kg(-1) h(-1), i.e. significantly (P < 0.05) lower than in the absence of L-NMMA. We sugge st that the described method allows direct determination of total NO format ion in man. The method may be useful in the study of various experimental a nd pathophysiological conditions affecting NO formation.