Elevated Airway GSH Resynthesis Confers Protection to Clara Cells from Naphthalene Injury in Mice Made Tolerant by Repeated Exposures1

Abstract

Repeated exposures to Clara cell cytotoxicants, such as naphthalene (NA), render target cell populations resistant to further acute injury. Previous studies suggest that alterations in bioactivation enzymes in target sites (bronchioles) of tolerant mice are insufficient to account for the marked reduction in susceptibility. Mice were made tolerant by seven daily injections of NA. GSH in the terminal airways was 2.7-fold greater in tolerant mice than in vehicle controls and a NA (300 mg/kg) challenge dose did not produce injury. Tolerant mice, allowed to recuperate for 96 h after the seventh NA injection, were again susceptible to NA injury, and terminal airway GSH levels had declined to control levels. To determine whether alterations in GSH resynthesis account for tolerance, the activity of γ-glutamylcysteine synthetase (γ-GCS) was measured or mice were treated with a combination of buthionine sulfoximine (BSO), a γ-GCS inhibitor, and NA. γ-GCS activity was elevated in resistant airways of tolerant mice. Tolerant mice treated with both BSO and NA appeared as susceptible to injury as NA-challenged controls. We conclude that GSH is critical for Clara cell resistance to NA injury in tolerant mice because: 1) GSH levels in target airways from NA-tolerant animals are elevated; 2) after a 96-h recuperation period, tolerant mice had lower GSH levels and are again susceptible to NA injury; 3) alterations in the activity of γ-GCS correspond with changes in susceptibility to NA injury; and 4) inhibition of γ-GCS with BSO increases susceptibility to NA injury in tolerant mice.