Background: Lung, kidney and small intestine are involved in fetal volume regulation and amniotic fluid secretion and play a pivotal role in the transition from intrauterine to extrauterine life. Objective: This study was performed to determine the ontogeny of mineralocorticoid receptors (MR) and glucocorticoid receptors (GR), and of MR- and GR-regulated genes and proteins, serum and glucocorticoid-induced kinase (Sgk-1), epithelial sodium channel (ENaCα), and Na,K-ATPase α1. Methods: Lung, renal cortex and medulla, and small intestine were collected from fetuses at 80, 100, 120, 130 and 145 days’ gestation and from day 1 and 7 neonatal lambs. Real-time PCR was performed to determine mRNA concentration for MR, GR, the 11β-hydroxysteroid dehydrogenases (11β-HSD1 and 2), Sgk-1, ENaCα, and Na,K-ATPase α1. Protein expression of ENaCα and Na,K-ATPase α1 in whole cell and membrane fractions was determined by immunoblotting. Results: Expression of corticosteroid-induced genes in renal cortex increases at term; in small intestine the induction occurs postnatally. In contrast, in lung expression of MR and GR mRNAs were greater at 100 days to term than postnatally and 11β-HSD1 peaked at 145 days; the corticosteroid-induced genes also increased prenatally: Sgk-1 and ENaCα increased by 120 days, peaking at 145 days, and Na,K-ATPase α1 was greatest at 130 days. Conclusions: The expression of high levels of MR and 11β-HSD1 in preterm fetal lung suggest low endogenous fetal cortisol may exert actions at the high affinity MR in vivo, leading to increases in expression of sodium channels important in the regulation of lung liquid secretion and reabsorption.

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