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The effects of methyltestosterone administration (100 mg/kg for four days) on the hepatic microsomal drug-metabolizing system from old male rats was investigated. Age-related decreases in cytochrome P-450 content, cytochrome c reductase activity and benzphetamine N-demethylase activity were unaffected by methyltestosterone treatment. Administration of the androgen induced nitroanisole O-demethylase and aniline hydroxylase activities, resulting in a restoration of the latter to levels found in young-adult animals.

Hepatic microsomal monooxygenase induction was investigated in young-adult and middle-aged male Fischer 344 rats. Monooxygenase components and drug metabolism activities were determined in liver microsomes prepared from rats treated with phenobarbital (PB, beta-naphthoflavone (BNF) or methyltestosterone (MT) and compared with values from untreated rats. PB and BNF effects on cytochrome P-450 concentration and cytochrome c reductase activity were similar in young-adult and middle-aged animals. However, the extent of cytochrome P-450 induction by MT was less in the older animals. The age-related changes in induction of drug metabolism activities differed with different substrates for the monooxygenase system. In contrast to the inducibility of benzphetamine N-demethylation and aniline hydroxylation, which was diminished in the older rats, the inducibility of nitroanisole O-demethylation was enhanced. The results imply that qualitative changes in the microsomal enzyme system occurred as the animals progress from young to middle adulthood.