Oxygen regulation of the epithelial Na channel in the collecting duct

Husted RF, Lu H, Sigmund RD, Stokes JB. Oxygen regulation of the epithelial Na channel in the collecting duct. Am J Physiol Renal Physiol 300: F412-F424, 2011. First published December 1, 2010; doi:10.1152/ajprenal.00245.2010.-The PO2 within the kidney changes dramatically from cortex to medulla. The present experiments examined the effect of changing PO2 on epithelial Na channel (ENaC)-mediated Na transport in the collecting duct using the mpkCCD-c14 cell line. Decreasing ambient O-2 concentration from 20 to 8% decreased ENaC activity by 40%; increasing O-2 content to 40% increased ENaC activity by 50%. The O-2 effect required several hours to develop and was not mimicked by the acid pH that developed in monolayers incubated in low-O-2 medium. Corticosteroids increased ENaC activity at each O-2 concentration; there was no interaction. The pathways for O-2 and steroid regulation of ENaC are different since O-2 did not substantially affect Sgk1, alpha-ENaC, Gilz, or Usp2-45 mRNA levels, genes involved in steroid-mediated ENaC regulation. The regulation of ENaC activity by these levels of O-2 appears not to be mediated by changes in hypoxia-inducible factor-1 alpha or -2 alpha activity or a change in AMP kinase activity. Changes in O-2 concentration had minimal effect on alpha- or gamma-ENaC mRNA and protein levels; there were moderate effects on beta-ENaC levels. However, 40% O-2 induced substantially greater total beta- and gamma-ENaC on the apical surface compared with 8% O-2; both subunits demonstrated a greater increase in the mature forms. The alpha-ENaC subunit was difficult to detect on the apical surface, perhaps because our antibodies do not recognize the major mature form. These results identify a mechanism of ENaC regulation that may be important in different regions of the kidney and in responses to changes in dietary NaCl.