Biochemical assessment of acute nitrogen dioxide toxicity in rat lung

The early primary biochemical response of lung to NO2 was studied separately from the later secondary responses of inflammation and proliferation by measuring several biochemical parameters in lungs of rats immediately following a 4-hr exposure to nitrogen dioxide (NO2) at concentrations of 10, 20, 30, and 40 ppm. Cell-free lavage fluid contained elevated amounts of lactate dehydrogenase (LDH), malate dehydrogenase (MDH), isocitrate dehydrogenase (IDH), glucose-6-phosphate dehydrogenase (GDH), acid phosphatase (AP), and aryl sulfatase (AS) after 30 or 40 ppm NO2. Total protein and sialic acid were increased in cell-free lavage after 20, 30, or 40 ppm NO2. The amounts of protein, sialic acid, and acid phosphatase recovered by airway lavage were equal to the amounts found in 0.7 ml of plasma, consistent with transudation of this volume of plasma into airways as a source of these parameters. The plasma activity of the other parameters measured was too low to account for their increase in lavage fluid by plasma leakage into airways. Decrease in the number and enzyme content of lavagable cells indicated damage to free cells in the airways. The amount of the decrease in enzyme content of the lavagable cell fraction was similar to the increase in the cell-free lavage for all of the measured enzymes except acid phosphatase, suggesting the release of these enzymes into airways as a result of damage to free cells. However, the LDH isoenzyme profile in cell-free lavage after exposure is inconsistent with free cells as the source of this enzyme. No changes were observed in the whole-lung homogenate content of protein, DNA, lipid, LDH, MDH, IDH, GDH, AP, AS, glutathione reductase, NADPH cytochrome c, or succinate cytochrome c reductase immediately after NO2 exposure. This study indicates that initial acute damage to lung by NO2 results in translocation of enzymes, proteins, and sialic acid into airways. Plasma is a likely source of translocated protein, sialic acid, and acid phosphatase. The sources of the other enzyme activities remain to be identified, with lung parenchyma and free cells as likely sources.