The acute respiratory distress syndrome is characterized by impairment of the alveolar-capillary barrier. Our laboratory has shown that distal lung epithelial cell (DLEC) amiloride-sensitive Na ϩ transport is impaired by in vitro coculture with endotoxin (lipopolysaccharide)-stimulated alveolar macrophages (AM) through an L-arginine-dependent mechanism. To investigate the effect of this model on mRNA levels of the rat epithelial Na ϩ channel, mature fetal rat DLEC monolayers were incubated for 16 h with rat AM (1 ϫ 10 7 ) and lipopolysaccharide (10 g/mL), or the cell-free supernatant of lipopolysaccharide-stimulated rat AM. Such exposure resulted in a profound decrease in mRNA expression for all subunits (␣, ␤, and ␥) of the rat epithelial Na ϩ channel, without affecting 18S RNA levels. This effect was prevented by the antioxidant N-acetylcysteine. In separate experiments, confluent DLEC monolayers were exposed to lipopolysaccharide-stimulated AM supernatant for 16 h with or without N-acetylcysteine and DTT and studied in Ussing chambers. As previously demonstrated in our laboratory, AM supernatant resulted in a significant (p Ͻ 0.05) impairment of DLEC Na Abbreviations AM, alveolar macrophages ARDS, adult respiratory distress syndrome ALI, acute lung injury DLEC, distal lung epithelial cells ENaC, epithelial sodium channel FBS, fetal bovine serum GSH, glutathione I sc , short-circuit current LPS, lipopolysaccharide MEM, minimum essential medium NAC, N-acetyl cysteine NAME, N -nitro-L-arginine methyl ester NO, nitric oxide PD, transepithelial potential difference rENaC, rat epithelial sodium channel ROS, reactive oxygen species ARDS, or ALI, represents an important cause of mortality in critically ill patients (1). Both direct lung injury and distant insults (1), most commonly nonpulmonary sepsis, can initiate a process resulting in a fundamental derangement of the intrinsic barrier property of the lung, manifested clinically as highpermeability pulmonary edema, which is one characteristic of ARDS. It is widely held that these devastating changes most commonly arise because of widespread and uncontrolled activation of inflammatory cells, including neutrophils and macrophages, with the release of a wide range of potentially injurious mediators, such as cytokines, proteolytic enzymes, biologically active lipids, and ROS (2). Activation of inflammatory cells could, in turn, have deleterious effects on the ability of alveolar epithelia to clear edema fluid.The failure of the alveolar-capillary membrane, the structure responsible for preserving a fluid-free alveolar space vital for adequate gas exchange, has been the focus of increasing attention in continuing efforts to understand the basic mechanisms