The Beneficial Effects of Hypercapnia, and the Detrimental Effects of Peroxynitrite, in Chronic Neonatal Lung Injury

Bronchopulmonary dysplasia (BPD) is a chronic neonatal lung injury (CNLI) affecting infants of < 32 weeks gestation, which has a significant associated morbidity and mortality. The hallmarks of BPD as seen in the current era are arrested alveologenesis and parenchymal thickening. Those most severely affected may develop pulmonary hypertension which worsens the prognosis. No effective preventive therapy exists. Generation of damaging reactive oxygen species is implicated in its development. The more recently recognized reactive nitrogen species may also contribute to this disease. Thus, there is considerable interest in preventive antioxidant therapies, but results to date have not been promising. Newborn rats, exposed to 60% O2 for 14 days, develop a parenchymal injury and pulmonary hypertension that resembles the morphological features of human BPD. Previous studies have shown that following exposure to 60% O2, a pulmonary influx of neutrophils is followed by that of macrophages. Inhibiting the influx of neutrophils prevents the generation of reactive oxygen species, while simultaneously enhancing postnatal lung growth. Other interventions have shown that development of pulmonary hypertension is dependent upon increases in both 8-isoprostane and its downstream regulator of vascular tone, endothelin-1. Gentler ventilation strategies, incorporated to minimize induction of stretch-mediated pro-inflammatory cytokines, have shown benefits of permissive hypercapnia in adult lung injury. Multicentre clinical trials of permissive hypercapnia in neonates have not shown benefit. Therapeutic hypercapnia has been demonstrated to have a protective effect of PaCO2 in both acute studies of ventilator-induced and ischemia-reperfusion injuries in animal models. In the studies reported herein, therapeutic hypercapnia was found to completely protect against CNLI and attenuate 60% O2-induced macrophage-derived protein nitration. The likely nitrating agent was macrophage-derived peroxynitrite. The critical role of peroxynitrite, in the development of chronic neonatal lung injury in this model, was confirmed using a peroxynitrite decomposition catalyst. This protected against the impairments of alveolarization and of pulmonary vascularization induced by 60% O2. These results suggest a more significant role for reactive nitrogen species than previously recognized. Finally, preliminary evidence is presented supporting a role for neutrophil-derived elastase in initiating the macrophage influx in the lungs, required for peroxynitrite generation, during 60% O2-mediated injury.Ph