Oxygen regulation of the epithelial Na channel in the collecting duct

Abstract

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 O2 concentration from 20 to 8% decreased ENaC activity by 40%; increasing O2 content to 40% increased ENaC activity by 50%. The O2 effect required several hours to develop and was not mimicked by the acid pH that developed in monolayers incubated in low-O2 medium. Corticosteroids increased ENaC activity at each O2 concentration; there was no interaction. The pathways for O2 and steroid regulation of ENaC are different since O2 did not substantially affect Sgk1, α-ENaC, Gilz, or Usp2–45 mRNA levels, genes involved in steroid-mediated ENaC regulation. The regulation of ENaC activity by these levels of O2 appears not to be mediated by changes in hypoxia-inducible factor-1α or -2α activity or a change in AMP kinase activity. Changes in O2 concentration had minimal effect on α- or γ-ENaC mRNA and protein levels; there were moderate effects on β-ENaC levels. However, 40% O2 induced substantially greater total β- and γ-ENaC on the apical surface compared with 8% O2; both subunits demonstrated a greater increase in the mature forms. The α-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.