Oxygen‐evoked Na⁺ transport in rat fetal distal lung epithelial cells

Abstract

Monolayer cultures of rat fetal distal lung epithelial (FDLE) cells generated larger spontaneous short circuit currents (ISC) when maintained (48 h) at neonatal alveolar PO2 (100 mmHg) than at fetal PO2 (23 mmHg). When cells were shifted between these atmospheres in order to impose a rise in PO2 equivalent to that seen at birth, no rise in ISC was seen after 6 h but the response was fully established by 24 h. Studies of basolaterally permeabilised cells revealed a small rise in apical Na+ conductance (GNa) 6 h after PO2 was raised but no further change had occurred by 24 h. A substantial rise was, however, seen after 48 h. Reporter gene assays showed that no activation of the α‐ENaC (epithelial Na+ channel α‐subunit) promoter was discernible 24 h after PO2 was raised but increased transcriptional activity was seen at 48 h. Studies of apically permeabilised cells showed that a small rise in Na+ pump capacity was evident 6 h after PO2 was raised and, in common with the rise in ISC, this effect was fully established by 24 h. The rise in ISC thus develops 6‐24 h after PO2 is raised and is due, primarily, to increased Na+ pump capacity. The increase in GNa thus coincides with activation of the α‐ENaC promoter but these effects occur after the rise in ISC is fully established and so cannot underlie this physiological response. The increased transcription may be an adaptation to increased Na+ transport and not its cause.

Hormonal modulation of Na+ transport in rat fetal distal lung epithelial cells

“…The P O 2 of this gas mixture is ~142 mmHg, which is greater than that found in either the fetal (~23 mmHg) or the neonatal alveolar region (~100 mmHg). The realisation that alveolar Na + transport is intrinsically sensitive to P O 2 (Barker & Gatzy, 1993;Pitkänen et al 1996;Rafii et al 1998) prompted experiments in which FDLE cells were maintained in an atmosphere in which P O 2 was controlled (Haddad & Land, 2000;Ramminger et al 2000;Baines et al 2001;Haddad et al 2001;Collett et al 2002). These studies confirmed that isoprenaline stimulates Na + transport when P O 2 is high (Ramminger et al 2000;Collett et al 2002), but showed that this drug failed to elicit any response in cells maintained at the P O 2 experienced in utero (Ramminger et al 2000).…”

β-Adrenoceptor-mediated control of apical membrane conductive properties in fetal distal lung epithelia

“…This discrepancy between the present observations and those made in the intact fetal lung almost certainly reflects the fact that the present study was undertaken using cells cultured at adult alveolar PO 2 (100 mmHg) rather than at fetal PO 2 (ϳ23 mmHg). We (1,36) and others (2,33) have recently shown that this causes the development of the Na ϩ -absorbing phenotype characteristic of the adult lung. Although the present study was undertaken using cells isolated from fetal animals, we believe that these cells expressed an adult phenotype when used in the experiments but cannot formally exclude the possibility that they may have adopted an intermediate phenotype.…”

“…By this time, the cells had almost invariably (Ͼ90% of cell preparations) become integrated into epithelial sheets with transepithelial resistances (R t) Ͼ200 ⍀cm 2 (see also Refs. 1,37). Although the cells are isolated from fetal animals, previously published work (1,33,36) shows that maintaining these cells in an atmosphere that mimics the PO2 of the postnatal alveolar region (ϳ100 mmHg) causes the development of a Na ϩ -absorbing phenotype typical of the neonatal, rather than the fetal lung.…”

Invited Review: Clearance of lung liquid during the perinatal period

“…These findings could be interpreted as indicating that PO 2 activation of NF-B upregulated the expression of ENaC, increasing I sc and Na ϩ transport at the onset of breathing. However, subsequent experiments using FDLE monolayers (5,59) demonstrated that a shift from fetal (23 Torr) to postnatal alveolar PO 2 (100 Torr), which induces a maximal I sc response, resulted in an immediate increase in NF-B expression and a detectable increase in Na/K ATPase capacity within 6 h, whereas activation of the ␣-ENaC promoter was not seen until after 24 h, reaching a maximum (together with G Na ) at 48 h.…”