Transport properties of isolated type II alveolar epithelial cells.

Type II cells are granular cells located in the alveolar epithelium. In addition to the synthesis and secretion of surfactant, these pneumocytes exhibit several other interesting properties. Although type II cells possess a high permeability to sodium, they maintain a low free intracellular sodium concentration by the presence of a Na-K pump. The activity of the Na-K pump is high and can result in substantial net movement of solute and water. Therefore, type II cells may employ this pumping capacity to play a significant role in the transepithelial transport of water. Type II cells are also relatively resistant to oxidant damage and play a role in the regeneration of the alveolar epithelium after oxidant injury. Ascorbate is a known antioxidant that is accumulated by type II cells via a specialized transport system for the uptake of ascorbate and sodium. The presence of this specialized transport system in type II cells and alveolar macrophages may explain in part why these cells are more resistant to oxidant injury than other pneumocytes.