Abstract 1858 Poster Session IV, Tuesday, 5/4 (poster 290)

For many years, antenatal steroids have been used in humans and animals to accelerate lung maturation. However, steroids do not only promote surfactant secretion by type II pneumocytes, they also affect lung morphology by thinning the alveolar septa, thus mimicking normal post-natal development. It was recently shown that programmed cell death contributes to postnatal lung development (Am J Resp Cell Mol Biol 1998; 18: 786-793). Therefore, we hypothesized that the morphological change observed with antenatal steroids could be a result of programmed cell death, and that the effect is modulated by an increase expression of p53 protein, which possesses glucorticoid response elements. Six pregnant rabbits received 0.2 mg/kg of betamethasone or saline injection on days 25 and 26 of gestation (term 32 days) and were sacrificed on day 27. Lungs of the fetuses were fixed with 10% formalin instillation and embedded in paraffin. Tissue sections were studied by terminal transferase-mediated dUTP nick end labeling (TUNEL) assay and 4', 6-diamidine-2-phenylindole-dihydrochloride (DAPI). The number of apoptotic nuclei was counted over 35 random fields per animal using computer aided image analysis. The mean number of apoptotic nuclei per field (mean ± SE) was 4.6 ± 0.3 in the steroid treated group (n = 8) and 9.0 ± 0.3 in the saline treated group (n = 9). The saline group had a significantly higher apoptosis per field (P < 0.0001) based on a Student t-test. In the 2 groups, most of the TUNEL positive cells were alveolar cells, either pneumocytes or underlying fibroblasts. There was no staining over airways or blood vessels, p53 mRNA was assayed by Northern blot. No difference was found between the two groups. Therefore, the results do not support the original hypothesis: antenatal steroid administration inhibits normal apoptosis otherwise observed during fetal lung development. We speculate that these observations are related to the decrease in lung growth associated with the treatment.

Support: Fonds de Recherche en sante du Quebec, and Fondation de la recherche sur les maladies infantiles