Ontogeny of Epithelial Na,K-ATPase and Sodium Channels in Baboon Lung

Abstract 290 Poster Session IV, Tuesday, 5/4 (poster 312)

The switch from lung liquid secretion to absorption near birth is associated with increased lung epithelial sodium transport in several mammalian species. To identify in the primate lung the molecular events contributing to this process, we characterized the ontogeny of lung epithelial cell Na,K-ATPase and lung epithelial amiloride-sensitive sodium channel (ENaC) expression in the baboon. Distal lung epithelial cells were isolated by enzymatic digestion of peripheral lung tissue and density grade centrifugation at multiple developmental time points from 125 to 175 days gestation (term = 180 days) and 1 to 3 days postnatally. Quabain-sensitive ⁸⁶Rb uptake was measured in these cells as an index of Na,K-ATPase activity. mRNA content for the α₁ and β₁ subunits of Na,K-ATPase and for the α,β, and γ ENaC subunits was determined by Northern blot analyses of total cellular RNA isolated from peripheral lung tissue at the same developmental time points. Cellular localization of the Na,K-ATPase α₁ subunit and ENaC channel was identified by immunoflourescence microscopy. We found that Na,K-ATPase activity (nmol/10⁶ cells/h) increased progressively during fetal development from 12 ± 5 (mean ± standard error) at 125 d gestation to 23 ± 3 at 175 d gestation and 34 ± 5 at 1 to 3 days after birth. Lung mRNA content for both α₁ and β₁ Na,K-ATPase subunits increased ∼3 fold near term and peaked at 1 to 3 days after birth. Lung mRNA content for the α, β, and γ ENaC subunits also increased late in gestation, but peaked at 175 d gestation and decreased postnatally. Immunoflourescence microscopy demonstrated that epithelial Na,K-ATPase α₁ subunit was distributed at the basolateral surface throughout development. The ENaC channel localized to the luminal surface of the epithelium. These data indicate that expression of the genes encoding the Na,K-ATPase and ENaC subunits increases in the primate lung near birth. However, the regulation of these genes does not appear to be coordinate as Na,K-ATPase subunit mRNA content increased in the first days after birth while ENaC subunit mRNA content decreased. The developmental patterns of gene expression and cellular distributions of Na,K-ATPase and ENaC in the baboon lung are consistent with their participation in late prenatal absorption of sodium by the lung epithelium. This process would be expected to facilitate removal of liquid from the potential airspaces and contribute to the successful transition to pulmonary gas exchange at birth.

Funded by NIH 1U01 HL56320; HL 52636.