Pulmonary disposition of inhaled NO2-nitrogen in isolated rat lungs

Nitrogen dioxide (NO2) is a relatively insoluble, reactive gas that, on inhalation, generates a diverse array of pulmonary toxic effects. Its uptake and transformation in isolated lungs have been shown to be proportional to inspired dose and associated with significant accumulations of the nitrite ion. However, not all absorbed NO2 is directly detectable as soluble nitrite. To further characterize its uptake and chemical disposition, we determined the chemical fate of 15NO2-nitrogen in isolated perfused (red cell-free) rat lungs that were exposed to 20 ppm 15NO2 for 60 min. Total excess 15N (relative to unexposed controls) was determined by isotope ratio mass spectrometry and total nitrogen analysis. Excess 15N was detected in whole lungs and in soluble and insoluble fractions but not in the total lipid pool. Perfusate excess 15N and nitrate correlated and accounted for all absorbed NO2 not detectable in tissue fractions. Exogenously instilled [15N]nitrite distributed within lung tissue, bound to insoluble elements, and diffused to the vascular space similar to NO2-nitrogen. Instilled [15N]nitrate did not distribute or bind like NO2-nitrogen or nitrite. Dialysis (1000 molecular weight cutoff) of cytosol, membranes, and perfusate removed excess 15N and nitrite derived from NO2, nitrite, or nitrate sources. We conclude that in isolated lungs, inhaled NO2 (1) undergoes rapid uptake and transformation in sites accessible to the pulmonary circulation; (2) does not form stable addition products with lipids; and (3) forms small-molecular-weight soluble reaction product(s) that behave similarly to nitrite, most likely indicating predominant univalent reduction of NO2 via initial hydrogen abstraction and subsequent HNO2 dissociation.