Evidence for active Na+ transport by cultured monolayers of pulmonary alveolar epithelial cells

Primary cultured type II alveolar epithelial cells grown to confluence on nonporous surfaces form many small fluid-filled hemicysts or domes. These domes are generally thought to result from active transport of solutes from the medium above the cell monolayer to the substratum, with water following passively. We have investigated the characteristics of active transport by primary cultured monolayers of type II alveolar epithelial cells from rat lungs. Changes in dome density were measured after exposure to metabolic inhibitors, Na+ or Cl- transport inhibitors, and low-Na+ or low-Cl- culture media. Metabolic and Na+ transport inhibitors, and low-Na+ medium, lead to disappearance of domes, whereas Cl- transport inhibitors and low-Cl- medium seem to have no effect on dome density. These results suggest the presence of a Na+-dependent active transepithelial transport process across the monolayer, which is responsible for the formation of domes. This finding implies that absorption of fluid by mammalian alveolar epithelium in vivo may be important in the maintenance of normal lung fluid balance.