Stimulation of cholesterol 7 -hydroxylase by phenobarbital in two strains of rats

The effect of phenobarbital administration on the in vitro activity of cholesterol 7alpha-hydroxylase was investigated in two strains of rats. In rats of the Wistar strain the daily injection of phenobarbital (100 mg/kg per day ip for 5 days) produced a 33% increase in hepatic microsomal protein and a sixfold stimulation of specific activity of the enzyme. In rats of the Charles River colony (Sprague-Dawley derived) identical treatment with phenobarbital resulted in a 45% increase in hepatic microsomal protein and no change in the specific activity of cholesterol 7alpha-hydroxylase. Plasma phenobarbital concentrations were three to four times greater in the Wistar rats, suggesting that these strains differ also in their capacity to metabolize phenobarbital.