Effects of chronic phenobarbital (PB) administration (75mg/kg, p.o. for 59 days) on certain hepatic mixed function oxidases (HMFO) were investigated in male Sprague-Dawley rats to determine whether maximal induction is maintained throughout this period or whether further alterations occur in the inductive effect. As demonstrated by measurements of 14CO2 excretion rates in breath after a singl i.p. injection of [dimethylamine-14C]aminopyrine ([14C]aminopyrine), PB in the above dose for 7 days accelerated hepatic aminopyrine demethylation 92 per cent above control values. However, after 59 days of PB administration, 14CO2 excretion was only 5 per cent above control values; thus, initial PB-induced enhancement of aminopyrine N-demethylation declined with time. Phenobarbital administration did not affect total (unlabeled) CO2 output, indicating that decreased 14CO2 excretion did not result from potential PB-induced reduction of CO2 output. In contrast, total cytochrome P450 content and aniline hydroxylase activity remained elevated throughout the 59 days of PB treatment. Ethylmorphine N-demethylase activity and aminopyrine N-demethylase activity (using a 3.0 mM substrate concentration) remained elevated until 28 days of PB administration, after which time these activities decreased significantly (P < 0.05), finally reaching 71 and 83 per cent respectively, of peak activity. Kinetic analyses of aminopyrine N-demethylation by hepatic microsomes revealed the Km values were unchanged from 4 to 59 days of PB administration, but that Vmax increased to a maximum of 15.5 on day 28, after which it decreased to 6.60 on day 59. This pattern of change for Vmax of aminopyrine N-demethylase was similar to that for enzyme activity obtained using a constant 3.0 mM concentration of aminopyrine. Although aminopyrine N-demethylase activity (using a 3.0 mM substrate concentration) peaked after 2 days of PB treatment, 7 days of PB treatment reduced this activity to 14 per cent below peak values. After 59 days of PB treatment, aminopyrine N-demethylase activity (at a 0.3 mM substrate concentration) declined to 61 per cent of the peak value attained on day 2. Another chronic PB study was performed and different time points selected to determine the reproducibility of these differential alterations of MFO activity. Results in the second study were similar to those obtained in the first study. Appreciable loss with time of the inductive effects produced by PB on aminopyrine metabolism both in vitro and in vivo suggests a heterogeneous response of hepatic MFO to PB; metabolism of aniline remained at peak inductive values throughout the 59 days of PB administration, whereas ethylmorphine metabolism declined only slightly.
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