Developmental regulation of epithelial sodium channel subunit mRNA expression in rat colon and lung

Abstract

Na+absorption via amiloride-sensitive Na+ channels is of critical importance in the transition between fetal and neonatal life in several tissues, including the colon, lung, and kidney. To characterize and contrast the mRNA expression of each of the three epithelial Na+ channel complex (ENaC) subunits, we conducted RNase protection assays (RPA) and in situ hybridization in colon and lung in fetal (17, 19, 20, and 21 days) and postnatal (1, 3, 9, 15, and 30 days) rats (r). In the colon the α-, β-, and γ-rENaC subunits showed quantitatively different but qualitatively similar expression. All three subunits gradually increased in abundance from fetal day 19 through day 30 of life. The amount of each subunit on day 30 was approximately three times the amount at day 1. In situ hybridization showed that each subunit was localized to the surface epithelial cells with minimal expression in the crypts. The lung showed a completely different pattern. In contrast to the colon, the total amount of α-rENaC mRNA (by RPA) in the lung increased dramatically from fetal day 19 to 21, whereas β- and γ-rENaC showed modest prenatal increases. The amounts of all three mRNAs fell after birth through day 9 (to about 75% of the day 1 value). On days 15 and 30 the amount of mRNA rose to approach the values on day 1. α-rENaC mRNA abundance always exceeded β- and γ-rENaC, and the quantitative expression was different for α- than for β- and γ-rENaC. In situ hybridization studies showed that all three subunits were expressed in epithelial cells of the bronchi, bronchioles, and alveoli and not in blood vessels. These studies show striking developmental heterogeneity in rENaC mRNA expression between lung and colon, probably reflecting different developmental regulatory mechanisms in these organs.

Ca2+-dependent stimulation of alveolar fluid clearance in near-term fetal guinea pigs

“…At birth, amiloride inhibits a large fraction of AFC, and within a few days after birth, the amiloride-inhibited fraction of AFC has decreased to 40-50% (5), which is similar to the adult inhibition (18). This developmental pattern of amiloride sensitivity largely conforms to the development suggested for ENaC in rats (20,32) and guinea pigs (1).…”

“…Marunaka and co-workers (14) reported that a nonspecific cation channel (NSC) can be stimulated by increased [Ca 2ϩ ] i . Several reports suggest that ENaC is expressed in the lung epithelium near birth (1,5,20,32), but no reports are present on its dependence or relation to changes in [Ca 2ϩ ] i in fetal lungs. However, because the amiloride sensitivity was completely abolished after verapamil administration, it appears as if ␤-adrenergic stimulation of ENaC depends on Ca 2ϩ from extracellular spaces in the in vivo situation.…”

“…A significant fraction of AFC is mediated through amiloride-sensitive pathways in fetal (19,23) and newborn (5) animals. The epithelial Na ϩ channel (ENaC), which is amiloride sensitive, has been proposed as one pathway for Na ϩ absorption in near-term or newborn animals (1,5,32). Upregulation of the basolateral Na ϩ -K ϩ -ATPase may be involved in emptying the fetal air spaces of fluid (6).…”

Alveolar epithelial type I cells contain transport proteins and transport sodium, supporting an active role for type I cells in regulation of lung liquid homeostasis

“…Previous studies of ENaC distribution in the lung have reported conflicting results regarding localization of the various subunits to alveoli and, within alveoli, to specific alveolar cells. By using radioisotopic in situ hybridization or immunocytochemistry, investigators have concluded variously that there is no expression of any subunit in alveoli (18), that there is selective expression of only ␣ENaC (43), of both ␣ENaC and ␥ENaC (9), or of all three subunits (44). Although Watanabe et al (44) concluded that ␣ENaC was expressed in TII cells, the relatively low spatial resolution of the methodology does not permit definitive cellular localization from these data.…”

ENaCs and ASICs as therapeutic targets

“…All three main ENaC subunits are expressed in both type I alveolar cells [AT1, involved in defense against oxidative injury (43, 212)] and type II cells [AT2, responsible for surfactant secretion (211,380)]. There is a significant upregulation in ␣ENaC expression in the alveoli around birth, consistent with the switch from the fluid-filled uterine to an air-breathing environment (408). Knockout studies demonstrated that mice lacking ␣ENaC expression failed to clear the liquid that filled the lungs during gestation, dying within 40 h of birth (188).…”