1. Monolayer cultures of rat fetal distal lung epithelial (FDLE) cells gene
rated larger spontaneous short circuit currents (I-SC) when maintained (48
h) at neonatal alveolar P-O2(100 mmHg) than at fetal P-O2 (23 mmHg). When c
ells were shifted between these atmospheres in order to impose a rise in P-
O2 equivnlent to that seen at birth, no rise in I-SC was seen after 6 h but
the response was fully established by 24 h.
2. Studies of basolaterally permeabilised cells revealed a small rise in ap
ical Na+ conductance (G(Na)) 6 h after P-O2 was raised but no further chang
e had occurred by 24 h. A substantial rise was, however, seen after 48 h.
3. Reporter gene assays showed that no activation of the alpha -ENaC (epith
elial Naf channel alpha -subunit) promoter was discernible 24 h after P-O2
was raised but increased transcriptional activity was seen at 48 h.
4. Studies of apically permeabilised cells showed that a small rise in Napump capacity was evident 6 h after P-O2 was raised and, in common with the
rise in I-SC, this effect was fully established by 24 h. The rise in I-SC
thus develops 6-24 h after P-O2 is raised and is due, primarily, to increas
ed Na+ pump capncity.
5. The increase in G(Na) thus coincides with activation of the alpha -ENaC
promoter but these effects occur after the rise in I-SC is fully establishe
d and so cannot underlie this physiological response. The increased transcr
iption may he an adaptation to increased Na+ transport and not its cause.