Alveolar Type 1 Cells Express the 2 Na,K-ATPase, Which Contributes to Lung Liquid Clearance

The alveolar epithelium is composed of alveolar type 1 (AT1) and alveolar type 2 (AT2) cells, which represent 95% and 5% of the alveolar surface area, respectively. Lung liquid clearance is driven by the osmotic gradient generated by the Na,K-ATPase. AT2 cells have been shown to express the 1 Na,K-ATPase. We postulated that AT1 cells, because of their larger surface area, should be important in the regulation of active Na transport. By immunofluorescence and electron microscopy, we determined that AT1 cells express both the 1 and 2 Na,K-ATPase isoforms. In isolated, ouabain-perfused rat lungs, the 2 Na,K-ATPase in AT1 cells mediated 60% of the basal lung liquid clearance. The -adrenergic agonist isoproterenol increased lung liquid clearance by preferentially upregulating the 2 Na,K-ATPase protein abundance in the plasma membrane and activity in alveolar epithelial cells (AECs). Rat AECs and human A549 cells were infected with an adenovirus containing the rat Na,K-ATPase 2 gene (Ad 2), which resulted in the overexpression of the 2 Na,K-ATPase protein and caused a 2-fold increase in Na,K-ATPase activity. Spontaneously breathing rats were also infected with Ad 2, which increased 2 protein abundance and resulted in a 250% increase in lung liquid clearance. These studies provide the first evidence that 2 Na,K-ATPase in AT1 cells contributes to most of the active Na transport and lung liquid clearance, which can be further increased by stimulation of the -adrenergic receptor or by adenovirus-mediated overexpression of the 2 Na,K-ATPase. (Circ Res. 2003;92: 453-460.)