O2 can raise fetal pneumocyte Na+ conductance without affecting ENaC mRNA abundance

In fetal pneumocytes, increasing P(O(2)) can raise apical Na(+) conductance (G(Na(+))) and increase the abundance of epithelial Na(+) channel subunit (alpha-, beta-, and gamma-ENaC) mRNA, suggesting that the rise in G(Na(+)), which may be important to the perinatal maturation of the lung, reflects O(2)-evoked ENaC gene expression. However, we now show that physiologically relevant increases in P(O(2)) do not affect alpha-, beta-, and gamma-ENaC mRNA abundance in pneumocytes maintained (approximately 48 h) in hormone-free medium or in medium supplemented with dexamethasone and tri-iodothyronine, although the response does persist in cells maintained in medium containing a complex mixture of hormones/growth factors. However, parallel electrometric studies revealed clear increases in G(Na(+)) under all tested conditions and so it is now clear that O(2)-evoked increases in G(Na(+)) can occur without corresponding increases in ENaC mRNA abundance. It is therefore unlikely that this rise in G(Na(+)) is secondary to O(2)-evoked ENaC gene expression.