L-azetidine-2-carboxylic acid retards lung growth and surfactant synthesis in fetal rats.

Maturation of the pulmonary epithelium during late fetal development is controlled at least in part by the underlying fibroblasts. To further investigate this cellular interdependence and the role of collagen in type 2 cell differentiation, we studied the effects of inhibiting fibroblast function in vivo by injecting the proline analog L-azetidine-2-carboxylic acid (LACA) to timed pregnant rats, and examining changes in cell proliferation and surfactant synthesis in fetal lungs. LACA (200 mg/kg) was injected twice daily for 2 days and rats were killed 2 days later at days 19, 20, 21, and 22 of gestation. Fetal lung weight and DNA content were about 50% of controls, hydroxyproline per dry weight was reduced and by electron microscopy, there appeared to be less fibrillar collagen in the lung. Autoradiography after [3H]thymidine pulse-labeling showed reduced cell proliferation on days 19 and 20 mainly due to lower fibroblast growth with a smaller reduction in epithelial labeling. Lung development in LACA-treated rats was retarded; air sacs were slow to open, epithelial cells retained glycogen longer and fewer cells developed lamellar bodies compared with age-matched controls. There was a reduction in the incidence of epithelial-interstitial cell contacts at day 20 only. Measurements of disaturated phosphatidylcholine showed a 50% reduction per dry weight and a lower disaturated phosphatidylcholine/lipid ratio after LACA. The results indicate that LACA administration in vivo slows fibroblast growth and greatly reduces fibrillar collagen deposition with an accompanying reduction in pulmonary surfactant. This suggests that secreted matrix influences growth and differentiation of the alveolar epithelium.