Development of tolerance to Clara cell necrosis with repeat administrationof coumarin

Coumarin was identified as a mouse-lung carcinogen following oral gavage ad ministration in a chronic bioassay, and was shown to cause the selective ne crosis of terminal bronchiolar Clara (nonciliated bronchiolar epithelial) c ells in the mouse lung after acute administration. After oral gavage, a sim ilar effect was not observed in the terminal bronchioles of rats, suggestin g that coumarin-mediated Clara cell toxicity is a species-specific effect. Using coumarin dosages (50 and 200 mg/kg) and a dosing schedule modeled aft er the chronic bioassay, the current study examined the effects of repeated coumarin administration in mouse lung. A single dosage of coumarin (200 mg /kg) caused swelling of Clara cells and necrosis in mouse-lung terminal bro nchioles. However, after 5 consecutive oral doses of coumarin (200 mg/kg), the mouse lung became tolerant to coumarin, and although areas of bronchiol ar epithelial flattening and hyperplasia were noted, Clara cell necrosis wa s not observed. After 10 doses of coumarin, mouse lungs appeared nearly nor mal. Coumarin-mediated Clara cell injury is thought to result from the cyto chrome P450-catalyzed formation of coumarin 3,4-epoxide and Western analysi s of whole mouse lung microsomal P450 content indicated that, commensurate with Clara cell necrosis, many P450s were decreased. However, P450 levels a ppeared qualitatively normal in lung microsomes from tolerant mice. Similar ly, coumarin epoxidation and 7-hydroxylation rates in whole lung microsomes from tolerant animals were similar to controls. To determine if animals to lerant to coumarin were tolerant to other Clara cell toxicants, a single to xic dose of naphthalene (200 mg/kg) was administered to coumarin-tolerant m ice. Coumarin pretreatment reduced naphthalene-mediated Clara cell toxicity , supporting the hypothesis that tolerance may result from general biochemi cal and molecular changes and not exclusively from alterations in chemical metabolism.