Combined effects of nitric oxide and hyperoxia on surfactant function and pulmonary inflammation

NO and its derivative ONOO^- are potent free radicals that can cause cell damage, especially in the presence of O₂. To determine the potential pulmonary toxicities of nitric oxide (NO) and peroxynitrite (ONOO^-) in vitro, Survanta (2.5 mg/ml) was exposed to ONOO^- (0.3-8 mM) in the presence of two different buffering systems (N-2-hydroxyethylpiperazine-N'-2- ethanesulfonic acid and phosphate buffer) and minimum surface tension (MST) was determined with an oscillating bubble surfactometer. Significant increases in MST were seen only with exposure to 8 mM ONOO^-, indicating that in vitro, high concentrations of ONOO^- can inhibit natural surfactant function. The in vivo effects of NO and hyperoxia were then studied in four groups of newborn piglets ventilated for 48 h with 21% O₂, 100% O₂, 21% O₂ and 100 ppm NO, or with 90% O₂ and 100 ppm NO. Five animals served as an untreated control group. Bronchoalveolar lavage fluid (BAL) obtained at 48 h was subjected to centrifugation and the surfactant pellet was reconstituted to 5 mg phospholipid/ml. Significant increases in MST were seen in surfactant from piglets ventilated with NO and 90% O₂, compared with either untreated controls or piglets ventilated with 21% O₂ for 48 h (P < 0.05, analysis of variance). Significant increases in neutrophil chemotactic activity (NCA) of BAL were also found in the NO and O₂ group (P < 0.05), with significant positive interaction between NO and O₂ found (P < 0.01). These data indicate that inhaled NO, in vivo, in the presence of hyperoxia, causes significant surfactant dysfunction and early evidence of pulmonary inflammation. This suggests that NO therapy may exacerbate pulmonary O₂ toxicity.