[On susceptibility for carbon monoxide exposure. The effect of carbon monoxide exposure on hepatic drug metabolizing enzymes of mice].

Male mice were exposed to three levels of carbon monoxide, i.e. 500, 1,000 and 3,000 ppm for 60, 10 and 7 minutes per day respectively. Durations of the exposure were 1 and 5 days and 3, 6 and 9 weeks. As the effect of carbon monoxide exposure, induction of the drug metabolizing enzyme was observed, which was ascertained by the decrease of sleeping time by pentobarbital and by the increase of activities of liver microsomal p-nitroanisole demethylase and of aniline hydroxylase. These results should be due to an adaptive response to carbon monoxide exposure, but the mechanisms of the enzyme induction were not fully explained in this study. In 5 days' exposure to carbon monoxide, there was a dose-response relationship between the degree of carbon monoxide exposure and the extent of enzyme induction. But in the prolonged exposure to higher concentrations of carbon monoxide (1,000 and 3,000 ppm), the increase of drug metabolism became less pronounced and by 9 weeks' exposure to 1,000 ppm carbon monoxide, little induction of the drug metabolizing enzyme was observed. These results suggest that the induction of the drug metabolizing enzyme by carbon monoxide exposure has a certain limit.