Hypoxia compared with normoxia alters the effects of nitric oxide in ischemia-reperfusion lung injury

Because both the biosynthesis of nitric oxide NO . and its metabolic fate are related to molecular O-2, we hypothesized that hypoxia would alter the effects of NO . during ischemia-reperfusion (IR) in the lung. In this study, buffer-perfused lungs from rabbits underwent either normoxic IR (AI), in which lungs were ventilated with 21% O-2 during ischemia and reperfusion, or hypoxic IR (NI), in which lungs were ventilated with 95% N-2 during ischemia followed by reoxygenation with 21% O-2. Lung weight gain (WG) and pulmonary artery pressure (P-pa) were monitored continuously, and microvascular pressure (P-mv) was measured after reperfusion to calculate pulmonary vascular resistance. We found that both AI and NI produced acute lung injury, as shown by increased WG and P-pa during reperfusion. In AI, where perfusate PO2 was >100 mmHg, the administration of the NO . synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME) before ischemia worsened WG and P-pa. P-mv also increased, suggesting a hydrostatic mechanism involved in edema formation. The effects of L-NAME could be attenuated by giving L-arginine and exogenous NO . donors before ischemia or before reperfusion. Partial protection was also provided by superoxide dismutase. In contrast, lung injury in NI at perfusate PO2 of 25-30 mmHg was attenuated by L-NAME; this effect could be reversed by L-arginine. Exogenous NO . donors given either before ischemia or before reperfusion, however, did not increase lung injury. NO . production was measured by quantifying the total nitrogen oxides (NOx) accumulating in the perfusate. The average rate of NOx accumulation was greater in AI than in NI. We conclude that hypoxia prevented the protective effects of NO . on AI lung injury. The effects of hypoxia may be related to lower NO . production relative to oxidant stress during LR and/or altered metabolic fates of NO .-mediated production of peroxynitrite by hypoxic ischemia.