Several studies suggest that low ambient P_O2 is a factor that limits epithelial Na⁺ transport (e.g. Ramminger et al. 2000), but the mechanism remains unclear. Nitric oxide-evoked inhibition of epithelial Na⁺ transport contributes to pulmonary oedema in lung injury (Pittet et al. 2002), and by implication, may repress Na⁺ transport in the fetal lung. We therefore sought to determine whether fetal pseudoglandular lung tissue exhibits a suitable distribution and activity of nitric oxide synthase (NOS) isoforms to effect repression of epithelial Na⁺ transport.

Lung sections from day 16 (G16) rat fetuses were immunostained using antibodies against inducible (i) or endothelial (e) NOS. Both isoforms localised to airway structures with iNOS confined to the distal columnar epithelium and eNOS in the basement membrane region of the proximal airways. Total NOS enzyme activity in G16 and adult lung was 0.17 ± 0.07 (mean ± S.E.M.; n = 4) and 0.71 ± 0.21 fmol citrulline (mg protein)^-1 min^-1 (fmol mg^-1 min^-1), respectively. To determine the effect of P_O2 on NOS activity, G16 lung explants were cultured at either fetal (23 mmHg) or ambient (142 mmHg) P_O2 for 24 h with or without 50 mg ml^-1 E. coli lipopolysaccharide (LPS). Basal NOS activity at fetal P_O2 was 1.6 ± 0.95 fmol mg^-1 min^-1, rising 2.3-fold with LPS. Basal NOS activity was markedly lower at ambient P_O2 (0.49 ± 0.33 fmol mg^-1 min^-1) and was not raised by LPS. Epithelial NO production (as nitrite) was determined using the Griess assay in a human (H441) lung epithelial cell line maintained in RPMI medium containing 10 % dialysed fetal calf serum. At fetal P_O2, basal nitrite production was 21.6 ± 0.9 mM and was 1.5-fold greater than that at ambient P_O2 (P < 0.05; ANOVA post-hoc Tukey’s HSD, n = 4). LPS increased (P < 0.05; n = 4) nitrite release at fetal P_O2 which was blocked by 1 mM dexamethasone (Dex) or 15 mM of the p44/42 mitogen-activated protein kinase (MAPK) inhibitor, PD98059. These effects were not observed at ambient P_O2. p44/42 MAPK, an activator of NO synthesis, was inactive in untreated H441 cultures at ambient P_O2, but was consistently active at fetal P_O2; this was blocked on addition of 50 nM Dex. We conclude that fetal P_O2 values are requisite for NO production in fetal rat lung tissue and H441 epithelial cells. This activity is sustained by p44/42 MAPK phosphorylation and can be attenuated by dexamethasone.

This work was supported by The Wellcome Trust and the MRC.

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