Hyperoxic inhalation (PO2 higher than that in the ambient air) is used in a variety of clinical practices including anaesthesia and post-operative recovery. The duration of hyperoxic ventilation can be prolonged depending on the patient conditions such as peripheral O2 saturation. Also, inpatients with respiratory disorders which cause hypoxemia, long-term oxygen therapy is often used. Although the benefits of O2 supplementation are obvious in clinical situations of fatal hypoxemia, there also are harmful effects of O2. For example, the side effects of increasing concentrations of O2 supplementation are frequently observed as pulmonary injury (1). In neonatal neural tissues such asthe retina, the harmful influence of hyperoxic ventilation has also been observed (2). The cough reflex, a defensive respiratory reflex, is also impaired in hyperoxia, and the inhibition of cough reflex is prevented by dietary antioxidants (3).
It is generally thought that tissue injury caused by O2 is mediated by the formation of reactive oxygen species (ROS), which can react with and damageessential biomolecules via lipid peroxidation, protein sulfhydryl oxidation, and DNA damage (4). Because airway epithelium is constantly and frequently exposed to oxidative stress, it is highly likely that ROS-mediated oxidative stress affects the functions of airway epithelium (5-7). In this context, there is increasing evidence for the protective effects of antioxidant supplementation inrespiratory diseases (3, 4, 8).
Because humans cannot synthesize ascorbic acid, dietary uptake of vitamin C is essential to cope with oxidative stress and to preserve physiological homeostasis. It has been reported that vitamin C is present in airway surface liquid (ASL), a thin (10-30 µm) layer of fluid covering the luminal surface of the airway epithelium (9-11). The physiological role of vitamin C inASL is to stimulate Cl- secretion via cAMP-activated Cl- channels known as cystic fibrosis transmembrane transport regulator (CFTR) in the luminal membrane of the airway epithelium (12). A balanced level of ASL is critical for the protection of the airway epithelium. For transepithelial fluid secretion, an electrogenic Cl- secretion model is regarded as the ionic mechanism in which thecAMP-dependent activations of the luminal Cl- channel (CFTR) and the basolateral K+ channel (potassium voltage-gated channel, KQT-like subfamily, member 1, KCNQ1) are critical steps (11, 13).
While there have been numerous studies on the structural and biochemical changes in respiratory epithelial cells in response to oxidative stress, a direct investigation on the physiological function (i.e., electrolytesecretion) has been rarely conducted. The studies by Cowley and Linsdell showed that exogenous hydrogen peroxide (H2O2, 0.5-2 mM) directly activates the electrogenic Cl- secretion of Calu-3, a cell line model of serous airway epithelial cells (14). In contrast, the oxidative stress caused by pyocyanin, a redox-active phenazine compound, impairs CFTR-dependent Cl- secretion in the bronchialepithelium (15). Consistent with this, vitamin C activates CFTR in primary cultured human airway epithelial cells (12).
Apart from the acute effects, chronic effects of oxidative stress and of vitamin C deprivation on the airway electrolyte transport are very important. To the best of our knowledge, there has been no investigation on the effects of sustained hyperoxic ventilation on the electrolytesecretion of airway epithelium in vivo. Also, the functional changes in the airway epithelium in the vitamin C-deficient animal model have not yet been investigated.
Unlike humans, rodents synthesize vitamin C (ascorbic acid) from glucose in situ. Recently, a mouse line has been generated with a targeted deletion of the gene coding for L-gulono-c-lactone oxidase (Gulo), which catalyzes the final step...