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Am J Physiol Lung Cell Mol Physiol (August 29, 2003). doi:10.1152/ajplung.00342.2002
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Submitted on October 15, 2002
Accepted on August 26, 2003

Postnatal Glucocorticoids Induce {alpha}-ENaC Formation and Regulate Glucocorticoid receptors in the Preterm Rabbit Lung

Shamimunisa B. Mustafa1*, Robert J. DiGeronimo2, Jean A. Petershack1, Joseph L. Alcorn3, and Steven R. Seidner1

1 Department of Pediatrics, University of Texas health science center, San Antonio, TX, USA
2 Department of Pediatrics, Wilford hall USAF MC, San Antonio, TX, USA
3 Department of Pediatrics, University of Texas-Houston Medical School, Houston, TX, USA

* To whom correspondence should be addressed. E-mail: mustafa{at}uthscsa.edu.

At the time of birth clearance of lung fluid is coupled to Na+ transport through epithelial Na+ channels (ENaC) located in the distal lung epithelium. The developmental expression of the {alpha}-subunit of ENaC is regulated by maternal glucocorticoids (GC) and may be delayed if birth occurs prematurely. We evaluated the effect of postnatal GC treatment on lung expression of {alpha}-ENaC in preterm 29 day gestational age (GA) rabbits. In control preterm rabbits, {alpha}-ENaC mRNA expression was detectable in lung and isolated fetal distal lung epithelial (FDLE) cells by 27 days GA. {alpha}-ENaC protein levels in whole lung progressively increased until full term at 31 days GA. Postnatal treatment of preterm 29 day GA rabbits with 0.5mg/kg dexamethasone (dex), i.v. resulted in a 2- and 22-fold increase in lung {alpha}-ENaC mRNA expression compared to saline-treated fetuses after 8 and 16h respectively. Lung {alpha}-ENaC protein levels in dex-treated fetuses at the same time points were also elevated compared to saline-treated counterparts. The extravascular lung water/dry lung tissue weight ratios of preterm 29 day GA rabbits treated with either saline or dex decreased over a period of 24h following delivery compared to that observed immediately after birth; however at 24h the extravascular lung water/dry lung tissue weight ratios of saline and dex-treated fetuses were similar. Dex-induced {alpha}-ENaC mRNA and protein levels were attenuated by the glucocorticoid receptor antagonist RU-486 in FDLE cells isolated from 29 day GA rabbits, indicating that GC-dependent augmentation of lung {alpha}-ENaC requires the presence of functional glucocorticoid receptors (GCR). In the fetal lung GC regulate the cellular level of GCR. Lung GCR mRNA expression and protein levels were elevated in 29 day GA fetuses compared to fetuses at earlier GA. Exposure of preterm 29 day GA rabbits to dex for 16h caused a 2.1-fold increase in GCR mRNA expression. In contrast lung GCR protein levels were markedly decreased in dex-treated rabbit fetuses over a period of 24h. We conclude that postnatal GC treatment results in an elevation of fetal lung {alpha}-ENaC that is accompanied by an autoregulation of pulmonary GCR.




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