Enhancement of alveolar epithelial sodium channel activity with decreased cystic fibrosis transmembrane conductance regulator expression in mouse lung

Lazrak A, Jurkuvenaite A, Chen L, Keeling KM, Collawn JF, Bedwell DM, Matalon S. Enhancement of alveolar epithelial sodium channel activity with decreased cystic fibrosis transmembrane conductance regulator expression in mouse lung. Am J Physiol Lung Cell Mol Physiol 301: L557-L567, 2011. First published July 8, 2011; doi: 10.1152/ajplung.00094.2011.-We sought to establish whether the cystic fibrosis transmembrane conductance regulator (CFTR) regulates the activity of amiloride-sensitive sodium channels (ENaC) in alveolar epithelial cells of wild-type, heterozygous (Cftr(+/-)), knockout (Cftr(-/-)), and Delta F508-expressing mice in situ. RT-PCR studies confirmed the presence of CFTR message in freshly isolated alveolar type II (ATII) cells from wild-type mice. We patched alveolar type I (ATI) and ATII cells in freshly prepared lung slices from these mice and demonstrated the presence of 4-pS ENaC channels with the following basal open probabilities (P(o)): wild-type = 0.21 +/- 0.015: Cftr(+/-) = 0.4 +/- 0.03; Delta F508 = 0.55 +/- 0.01; and Cftr(-/-) = and 0.81 +/- 0.016 (means +/- SE; n >= 9). Forskolin (5 mu M) or trypsin (2 mu M), applied in the pipette solution, increased the Po and number of channels in ATII cells of wild-type, Cftr(+/-), and Delta F508, but not in Cftr(-/-) mice, suggesting that the latter were maximally activated. Western blot analysis showed that lungs of all groups of mice had similar levels of alpha-ENaC; however, lungs of Cftr(+/-) and Cftr(-/-) mice had significantly higher levels of an alpha-ENaC proteolytic fragment (65 kDa) that is associated with active ENaC channels. Our results indicate that ENaC activity is inversely correlated to predicted CFTR levels and that CFTR heterozygous and homozygous mice have higher levels of proteolytically processed ENaC fragments in their lungs. This is the first demonstration of functional ENaC-CFTR interactions in alveolar epithelial cells in situ.