Epithelial Na channels in fetal guinea pig lungs are located in caveoli and relocate to the general membrane late during gestation

Abstract

Distal lung fluid absorption is mediated by induction of lung epithelial ENaC expression. The cellular source of these Na channels is the focus of this study and we propose that a fraction of ENaC channels comes from caveoli. Western blot studies after sucrose gradient separation of caveolar and general membrane fractions showed αENaC and βENaC subunit localization to the caveolar fractions at 61 days gestation in the guinea pig. At 68 days gestation, ENaC subunits appeared in general membrane fractions, correlating with the appearance of propranolol- and amiloride-sensitive distal lung fluid absorption. Maternal IL-1β-mediated cortisol synthesis and conversion of fluid secretion to fluid absorption at 61 days gestation was associated with a significant appearance of ENaC subunits in general membrane fractions, suggesting that this relocation led to induction of distal lung fluid absorption. IL-1β pretreatment at 68 days gestation involved ENaC subunit relocation, thus potentially explaining the stimulation of distal lung fluid absorption. The use of the caveoli disrupting agent, methyl-β-cyclodextrin mimicked the effects on ENaC seen after increased fetal age and after IL-1β-induction of lung fluid absorption. This evidence supports the hypothesis that caveolar ENaC relocates to general membrane upon stimulation by IL-1β-released cortisol. This concept provides a novel molecular mechanism for how the rapid transition from fetal lung fluid secretion to fluid absorption in near-term and newborn lungs occurs. It also provides a molecular mechanism for the induction of distal lung fluid absorption in 61-day gestation guinea pig fetuses following IL-1β-pretreatment. (Supported by Research Grant No. 6 FY03-64 from the March of Dimes Birth Defects Foundation)