cAMP activation of chloride and fluid secretion across the rabbit alveolar epithelium

Active Na⁺ transport by alveolar epithelial cells has been demonstrated to contribute significantly to alveolar fluid clearance. However, the contribution of transepithelial Cl^- movement to the reabsorption of isosmotic fluid across the alveolar epithelium in vivo has not been elucidated. We hypothesized that Cl^- transport could be increased across the alveolar epithelium in vivo and across cultured alveolar type II cells by agents that increase intracellular cAMP (e.g., forskolin). In studies where 5% albumin in sodium methane-sulfonate (a Cl^--free solution) was administered into the lung, forskolin administration significantly increased intracellular influx of Cl^- and fluid into the alveolar space. In vitro studies with cultured rabbit alveolar type II cell monolayers in Ussing chambers demonstrated that elevations in intracellular cAMP increase short- circuit current by increasing both Cl^- secretion and Na⁺ reabsorption. The cystic fibrosis transmembrane conductance regulator channel blocker glibenclamide and the loop diuretic bumetanide partially decreased the forskolin-induced increase in short-circuit current. These data may explain the failure of agonist that stimulated intracellular cAMP to increase alveolar fluid clearance in the rabbit. Moreover, the data suggest that in the event Na⁺ absorptive pathways are damaged, transepithelial Cl^- secretion and the consequent intra-alveolar fluid influx may be upregulated.