Thyroid hormone potentiates glucocorticoid-evoked airway Na+ transport without affecting alpha-ENaC transcription.

Glucocorticoid and thyroid hormones (T3) are important for the development of the lungs' Na+ absorbing phenotype, which is essential for the integrated functioning of the respiratory tract. Electrometric studies of H441 airway epithelial cells confirmed that dexamethasone increases apical Na+ conductance (GNa) and demonstrated that T3 facilitates this control over GNa. Assays of transcriptional activity showed that dexamethasone caused concentration-dependent activation of the human alpha-ENaC promoter (EC50 approximately 5 nM) but, despite its clear effect on GNa, T2 had no effect upon the transcriptional response to dexamethasone. The facilitation of Na+ transport may thus reflect control over events downstream to transcription.