Influence of Enzyme Induction and Exposure Profile on Liver Injury Due to Chlorinated Hydrocarbon Inhalation

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Abstract
Rats were exposed for four weeks either to air or to vapours of chloroform, carbon tetrachloride or 1,1‐dichloroethylene given either as a constant concentration (continuous profile) or as repeated exposures for 6 hr per day, 5 days per week (fluctuating profile). Vapour concentrations were used such that the total exposure (concentration × time) was the same for the two profiles. Within each group, some animals received the enzyme‐inducing agents, phenobarbitone or 1,3‐butanediol, in their drinking water. Separate experiments were conducted to determine the influence of enzyme inducers and vapour concentration on chlorocarbon uptake and metabolism. In the case of chloroform, hepatic injury was more severe in animals exposed to constant vapour concentration, while dichloroethylene was more toxic when given as a fluctuating profile, especially in butanediol‐treated rats. Carbon tetrachloride hepatotoxicity was similar in the two exposure profiles but was exacerbated by butanediol treatment. Butanediol‐treated animals in the fluctuating profile group showed evidence of developing cirrhosis. These results could not be fully explained on the basis of the effect of enzyme inducers and exposure profile on amount of agent metabolized. Both the amount of toxic metabolites and the temporal pattern of their formation appear to be important determinants of liver injury.
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