Oxygen therapy often rescues and reduces the mortality resulting from acute respiratory distress syndrome, chronic obstructive pulmonary diseases, exposure to toxic fumes, and drowning (1). However, prolonged exposure to supra-physiological concentrations of oxygen, referred to as hyperoxia, causes extensive damage to the alveolar-capillary barrier resulting in increased permeability and decreased lung function (2). Although the molecular mechanisms of hyperoxia-induced lung injury and cell death are complex, recent studies suggest that the generation of excessive reactive oxygen species (ROS), 1 loss of antioxidant defense pathways, cytokine-mediated inflammation, and modulation of signal transduction may regulate pulmonary edema and apoptosis/necrosis of endothelial and epithelial cells (3). The vascular endothelium has long been recognized to generate superoxide (O 2 . ), hydrogen peroxide (H 2 O 2 ), hydroxyl radical ( ⅐ OH), and nitric oxide (NO) via enzymatic and nonenzymatic reactions. In endothelial cells (ECs), in addition to the mitochondrial electron transport, other potential enzymatic pathways of ROS production include cyclooxygenase/lipoxygenase, cytochrome P450, xanthine oxidase, NADPH oxidase, NO synthase, and peroxidase. In the lung, the vascular NADPH oxidase seems to play an important role in excessive production of O 2 . in atherosclerosis, ischemic lung, pulmonary hypertension, and ventilator-associated lung injury (4 -9). NADPH oxidase catalyzes the one-electron reduction of molecular oxygen to O 2 . by using NADPH or NADH as an electron donor (9). Activated NADPH oxidase is a multimeric protein complex consisting of at least three cytosolic subunits of p47 phox , p67 phox , and p40 phox ; a regulatory small molecular weight G-protein of either Rac1 or Rac2 and a membraneassociated cytochrome b 558 reductase made up of p22 phox and gp91 phox . We and others (10, 11) have shown that most of the subcomponents of phagocytic NADPH oxidase are expressed in vascular ECs. ECs exhibit a low output in of O 2. production under basal conditions, and stimulation by TNF-␣, pulsatile stretch, hypoxia reoxygenation, and phorbol ester enhanced the
