Bronchiolar epithelial damage and impairment of pulmonary microsomal monooxygenase activity in mice by naphthalene

Abstract

Injection of a single dose of naphthalene into C57BL/6J mice (225 mg/kg, ip) produced a significant (30–70%) and prolonged (8–15 days) impairment in pulmonary microsomal monooxygenase activities without altering these activities in liver microsomes. The time course of naphthalene-induced morphologic damage to bronchiolar epithelium paralleled compromises in pulmonary monooxygenase activity. No concomitant alterations in hepatic morphology were observed. Five microsomal enzymes were studied: benzphetamine N-demethylase, aryl hydrocarbon hydroxylase, NADPH cytochrome c reductase, 7-ethoxyresorufin O-deethylase (a cytochrome P-448-dependent enzyme), and styrene epoxide hydrolase (a cytochrome P-450-independent enzyme). In general, the time course of the inhibition of these pulmonary enzymes was similar but the magnitude of the inhibition varied somewhat. Maximum inhibition of enzyme activity occurred about 3 days after naphthalene administration; 7-ethoxyresorufin O-deethylase activity was reduced to about 30% of control values whereas benzphetamine N-demethylase declined to about 70% of control. The remaining enzymes clustered midway between these extremes at about 50% of control values. Inhibited activities remained at relatively constant levels between Days 3 and 8 and by Day 15, there was a clear trend returning toward controls. Despite this trend, three of the six pulmonary enzyme activities examined remained significantly below control levels 15 days after a single dose of the hydrocarbon. Histologically, the pulmonary nonciliated bronchiolar epithelial (Clara) cell was the primary target of naphthalene toxicity. At early time points and at low magnifications, it appeared as if the entire bronchiolar epithelium was undergoing necrosis and sloughing into the lumen. However, higher magnifications revealed residual ciliated epithelium. The distribution of Clara cell damage appeared to vary considerably. One could find bronchioles that appeared completely denuded of epithelium and others in the same section whose Clara cells had been spared or, alternatively, were in the process of regeneration. The results are discussed in relation to recent work which has shown selective covalent binding of naphthalene to pulmonary Clara cells.