Comparative regulation of hepatic sterol 27-hydroxylase and cholesterol 7alpha-hydroxylase activities in the rat, guinea pig, and rabbit: effects of cholesterol and bile acids.

The regulation of the classic and alternative bile acid synthetic pathways by key hepatic enzyme activities (microsomal cholesterol 7alpha-hydroxylase and mitochondrial sterol 27-hydroxylase, respectively) was examined in bile acid depletion and replacement and cholesterol-feeding experiments with rats, guinea pigs, and rabbits. The bile acid pool was depleted by creating a bile fistula (BF) and collecting bile for 2 to 5 days, and it was replaced by intraduodenal infusion of the major biliary bile acids (taurocholic acid [TCA], glycochenodeoxycholic acid [GCDCA], and glycocholic acid [GCA] in the rat, guinea pig, and rabbit, respectively) at rates equivalent to the measured hepatic flux of the bile acids. To study the effects of cholesterol, the animals were fed for 7 days on a basal diet with and without 2% cholesterol. Cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase activities, measured by isotope incorporation assays, were related to bile acid output and composition and hepatic cholesterol concentrations. Intraduodenal infusion of bile acids increased the output of the tested bile acids, but did not significantly change hepatic cholesterol concentrations and had no effect on sterol 27-hydroxylase activity. Neither bile acid depletion nor replacement affected sterol 27-hydroxylase activity when three different substrates (cholesterol, 5beta-cholestane-3alpha,7alpha-diol, and 5beta-cholestane-3alpha,7alpha,12alpha-triol) were tested. In contrast, feeding 2% cholesterol increased hepatic cholesterol concentrations in rats, guinea pigs, and rabbits threefold, twofold, and eightfold, respectively, and increased hepatic mitochondrial sterol 27-hydroxylase activity (conversion of cholesterol to 27-hydroxycholesterol) in all three animal models. The stimulation and feedback inhibition of cholesterol 7alpha-hydroxylase activity by bile acid depletion and replacement were observed in all three animal models, whereas the effect of cholesterol feeding was species-dependent (cholesterol 7alpha-hydroxylase activity increased in the rat, did not change in the guinea pig, and was inhibited in the rabbit). Thus, in contrast to sterol 27-hydroxylase, which was upregulated by cholesterol but not affected by bile acid depletion and replacement in all three animal models, cholesterol 7alpha-hydroxylase activity was controlled consistently and inversely by the hepatic flux of bile acids, but was species-dependent in its response to a 1-week feeding with 2% cholesterol.