The role of α-, β-, and γ-ENaC subunits in distal lung epithelial fluid absorption induced by pulmonary edema fluid

Edema fluid (EF) increases epithelial Na+ transport by rat fetal distal lung epithelia (FDLE) and induces net lung fluid absorption in fetal mouse lung explants [Rafii B, Gillie DJ, Sulowski C, Hannam V, Cheung T, Otulakowski G, Barker PM, O'Brodovich H. J Physiol (Lond) 544: 537-548, 2002]. We now show that EF increases fluid absorption across monolayers of rat FDLE in a dose-dependent manner. To study the role of subunits of the epithelial Na+ channel (ENaC) in the phenomena, we cultured explants from the distal lungs of 16-day gestational age wild-type (WT) or alpha-, beta-, or gamma- ENaC knockout or heterozygote (HT) mice. WT explants cultured in media continuously expanded over time as a result of net fluid secretion. In contrast, when explants were exposed to EF for 24 h, net fluid absorption occurred. EF-exposed explants had normal histology, but marked changes were seen after Triton X-100 or staurosporine exposure. Transmission electron microscopy showed EF promoted lamellar body formation and abundant surfactant in the explants' lumens. EF-induced changes in explant size were similar in alpha- ENaC knockout, WT, and HT littermate fetal lung explants (P > 0.05). In contrast, EF's effect was attenuated in beta- and gamma- ENaC knockouts (P < 0.05) vs. WT and HT littermate fetal lung explants. EF exposure slightly decreased or had no effect on mRNA levels for alpha- ENaC in various mouse genotypes but decreased expression of beta- and gamma- ENaC subunit mRNAs (P < 0.01) across all genotype groups. We conclude that beta- and gamma-, but not alpha-, ENaC subunits are essential for EF to exert its maximal effect on net fluid absorption by distal lung epithelia.