In adult mammals, the nonciliated bronchiolar epithelial (Clara) cell is the principal site for cytotoxicity and metabolism of pulmonary environmental toxicants which require activation via the cytochrome P-450 monooxygenase system, whether these compounds enter the organism via inhalation or ingestion. A number of recent studies suggest that the lungs of fetuses whose mothers have been subjected to environmental toxins also may be at risk as a result of these exposures. The process by which fetal airway cells differentiate into functional Clara cells is both a pre- and postnatal phenomenon. Because there is virtually no information on which factors regulate differentiation of the Clara cell, nor on how bioactivated pulmonary cytotoxins may modulate this process, we have proposed to evaluate the effects on Clara cell differentiation of a series of compounds which inhibit or induce the cytochrome P-450 system. The overall hypothesis we will be testing is that compounds which alter the pulmonary cytochrome P- 450 system in the adult function as stimulators of Clara cell differentiation and maturation in the fetal and neonatal animal. We we evaluate the effects of hormones which have been shown to regulate other aspects of lung development (glucocorticoids and progesterone), inducers of hepatic P-450s in adults (sodium phenobarbital, Arochlor, beta-naphthoflavone), a bioactivated Clara cell-specific toxicant (4-ipomeanol) and inhibitors of cytochrome P-450 activity (SKF 525-A and aminobenzotriazole). The attainment of adult organelle composition by differentiated Clara cells will be assessed by ultrastructural morphometry. In addition, all three major functions of the Clara cell will be assessed: role as progenitor of bronchiolar epithelial cells, site of cytochrome P- 450-mediated metabolism, and source of bronchiolar mucosecretory proteins. 3H-Thymidine incorporation and cell turnover will be estimated by autoradiography, and bronchiolar epithelial population densities will be assessed quantitatively. Immunocytochemical, immunochemical, spectrophotometric and enzymatic activity assays will be used to measure the cytochrome P-450 system. Immunocytochemical and immunochemical methods will be used to evaluate alterations in secretory function. This proposal offers a unique opportunity to assess the degree of hazard faced by neonates and fetuses of mothers exposed to environmental toxins, to define the impact of environmental toxins on a lung cell type which is pivotal for a number of normal lung functions and to identify compounds which may serve as regulators of Clara cell differentiation.