EVIDENCE FOR A VITAMIN-D PARACRINE SYSTEM REGULATING MATURATION OF DEVELOPING RAT LUNG EPITHELIUM

Rat fetal lung is a target tissue for 1 alpha,25-dihydroxyvitamin D-3 [1 alpha,25(OH)(2)D-3]. We have identified the cells that respond to t he hormone and tested the hypothesis that the lung is also a source of 1 alpha,25(OH)(2)D-3. We found that 1) at the end of pregnancy (days 20-21) alveolar type II cells (ATII) bore 1 alpha,25(OH)(2)D-3 recepto rs and responded to the hormone. Incubating these cells with 10(-9) M 1 alpha,25(OH)(2)D-3 for 48 h stimulated the synthesis (87.3 +/- 9.1%) and release (61.7 +/- 6.1%) of disaturated phosphatidylcholine; 2) EB -1213, a 1 alpha,25(OH)(2)D-3 analogue with low calcemic activity, had similar effects on ATII; 3) neither fetal lung fibroblasts nor neonat al ATII (day 2 postpartum) expressed 1 alpha,25(OH)(2)D-3 receptors; a nd 4) in contrast, fetal lung fibroblasts taken on days 19-22 of gesta tion converted [H-3]25(OH)D-3 to [H-3]1 alpha,25(OH)(2)D-3, whereas AT II and skin fibroblasts did not. These findings suggest that 1 alpha,2 5(OH)(2)D-3 is a local mediator of epithelial-mesenchymal cell interac tions in the developing rat lung and that 1 alpha,25(OH)(2)D-3 or EB-1 213 might be therapeutically useful in treating the respiratory distre ss syndrome of premature neonates.