In rainbow trout liver, dexamethasone caused a reduction in cytochrome P450 1A1 proteins but no change in P450 3A proteins as measured by Western blots using specific anti P450 1A1 and P450 3A sera. 2. After dexamethasone administration, the level of P450 1A1 was down to 14% and 8% of the control values at 24 and 48 hr, respectively. 3. There was no change in P450 1A1 mRNA abundance at 24 hr but a decrease to 45% of the control value was observed at 48 hr after dexamethasone. 4. There is, therefore, a differential response between P450 1A1 and P450 3A to dexamethasone in the trout liver. 5. The initial decrease in P450 1A1 protein probably occurs at the translational level while both translational and transcriptional levels are involved 48 hr after dexamethasone administration.
