Effect of immunologic mediators on rat pulmonary Endothelial Cell (EC) and rat lung xanthine dehydrogenase/xanthine oxidase: Role of nitric oxide and oxygen

Purpose: To examine the modulating role of nitric oxide (NO) on the activation of xanthine dehydrogenase/xanthine oxidase (XD/XO) by cytokines, in hypoxia (3% O₂) and normoxia (20% O₂). Methods: We exposed rat pulmonary EC to normoxia or hypoxia, in the presence or absence of LPS (10 μg/ml), IL-1 (0.5 ng/ml), and L-NAME (300 μM), either alone or in combination. In separate experiments, we exposed male adult Sprague-Dawley rats to either a normobaric or hypobaric atmosphere (0.5 atm, simulated hypoxia), and to LPS (1 mg/kg) and Il-1 (100 μg/kg) alone or in combination. Results: In vitro, hypoxia alone and the combination of LPS and IL-1 cause a 2-fold increase in EC XD/XO activity as compared to normoxic cells. Simultaneous treatment of EC with hypoxia and LPS+IL-1 causes a 5-fold increase in activity as compared to normoxic EC unstimulated by cytokines. Inhibition of NO synthase by L-NAME causes a further increase in XD/XO activity in each treatment group. Similarly, animals exposed to hypoxia show a significant increase in lung XD/XO compared to normoxic animals. Simultaneous treatment with LPS and IL-1 significantly increases tissue XD/XO above levels seen with hypoxia alone. Conclusions: Our results indicate that certain immunologic mediators are capable of inducing XD/XO enzymatic activity and that this effect is more pronounced with exposure to hypoxia. In vitro, nitric oxide inhibitors potentiate the stimulatory effect of cytokines, suggesting a modulatory effect of nitric oxide. Clinical Implications: XO has been implicated as a major source for reactive oxygen species, particularly during conditions associated with ischemia-reperfusion or hypoxia-reoxygenation. Our findings suggest an intricate regulatory loop linking XD/XO, O₂, NO, and cytokines which could play a role in the pathophysiology of disorders associated with hypoxia and circulating cytokines, such as the adult respiratory distress syndrome.