Superoxide released from neutrophils causes a reduction in nitric oxide gas

Exhaled nitric oxide (NO) is increased in some inflammatory airway disorder s but not in others such as cystic fibrosis and acute respiratory distress syndrome. NO can combine with superoxide (O-2(-)) to form peroxynitrite, wh ich can decompose into nitrate. Activated polymorphonuclear neutrophils (PM Ns) releasing O-2(-) could account for a reduction in exhaled NO in disorde rs such as cystic fibrosis. To test this hypothesis in vitro, we stimulated confluent cultures of LA-4 cells, a murine lung epithelial cell Line, to p roduce NO. Subsequently, human PMNs stimulated to produce O-2(-) were added to the LA-4 cells. A gradual increase in NO in the headspace above the cul tures was observed and was markedly reduced by the addition of PMNs. An inc rease in nitrate in the culture supernatant fluids was measured, but no inc rease in nitrite was detected. Superoxide dismutase attenuated the PMN effe ct, and xanthine/xanthine oxidase reproduced the effect. No changes in epit helial cell inducible NO synthase protein or mRNA were observed. These data demonstrate that O-2(-) released from PMNs can decrease NO by conversion t o nitrate and suggest a potential mechanism for modulation of NO levels in vivo.