David H. Rutschman

Alveolar Type 1 Cells Express the α2 Na,K-ATPase, Which Contributes to Lung Liquid Clearance

Abstract— 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 &agr;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 &agr;1 and &agr;2 Na,K-ATPase isoforms. In isolated, ouabain-perfused rat lungs, the &agr;2 Na,K-ATPase in AT1 cells mediated 60% of the basal lung liquid clearance. The &bgr;-adrenergic agonist isoproterenol increased lung liquid clearance by preferentially upregulating the &agr;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 &agr;2 gene (Ad&agr;2), which resulted in the overexpression of the &agr;2 Na,K-ATPase protein and caused a 2-fold increase in Na,K-ATPase activity. Spontaneously breathing rats were also infected with Ad&agr;2, which increased &agr;2 protein abundance and resulted in a ≈250% increase in lung liquid clearance. These studies provide the first evidence that &agr;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 &bgr;-adrenergic receptor or by adenovirus-mediated overexpression of the &agr;2 Na,K-ATPase.

Role of the Na,K-ATPase α2 subunit in lung liquid clearance

An important function of alveolar epithelial cells is to keep the airspace free of liquid and preserve gas exchange. Previous studies have suggested that Na,K-ATPase may play a role contributing to alveolar epithelial vectorial sodium transport and thus lung liquid clearance1-3. In the lungs the αl and β 1 isoforms have been localized to alveolar type 2 (AT2) cells 4,5. The α2 isoform has been described in whole lung homogenates from adult rats6,7. The normal lung has two types of alveolar epithelial cells. Alveolar type 2 cells (AT2) are cuboidal cells that occupy about 5% of the alveolar surface area, and synthesize and secrete pulmonary surfactant 8. Alveolar type 1 cells (ATI) are large flattened cells that cover >90% of the alveolar surface and are thought to be terminally differentiated8. Expression of Na,K-ATPase αl, α2 and βl subunits was investigated in rat AT2 cells cultured for seven days, a period during which they lose their phenotypic markers and differentiate toward an alveolar type 1-like cell phenotype.