Contribution of newly synthesized cholesterol to rat plasma and bile determined by mass isotopomer distribution analysis: bile-salt flux promotes secretion of newly synthesized cholesterol into bile

To quantify the contribution of newly synthesized cholesterol to total plasma and biliary cholesterol under physiological conditions, unrestrained rats were infused intravenously with [1-C-13]acetate (0.6 mmol/h per kg) from 12:00 to 24:00 h, and fractional and absolute cholesterol-synthesis rates were determined by mass isotopomer distribution analysis (MIDA). As bile diversion leads to changes in cholesterol metabolism, rats were equipped with permanent catheters in the bile duct and duodenum, allowing sampling of small amounts of bile from an intact enterohepatic circulation. For comparison, rats with chronic bile diversion were also studied. Fractional synthesis of plasma cholesterol was 10.8 +/- 1.7% (mean +/- S.D.) after 12 h in rats with intact circulation. Fractional synthesis of biliary cholesterol was significantly higher than that of plasma cholesterol, i.e. 16.5 +/- 2.0% (P < 0.05) after 12 h. In contrast, no differences between fractional synthesis of cholesterol in plasma and bile were found in bile-diverted animals (31.8 +/- 2.1 and 33.1 +/- 3.3% respectively after 12 h). The calculated absolute rate of cholesterol biosynthesis increased from 53 +/- 10 to 221 +/- 19 mu mol/day per kg after bile diversion. A comparison of MIDA results with those obtained from balance studies indicated that MIDA does not assess total body synthesis in rats, presumably because of incomplete equilibration of newly synthesized molecules with cholesterol in the plasma compartment. These studies demonstrate that the contribution of newly synthesized cholesterol to biliary cholesterol is higher than to plasma cholesterol under physiological conditions, probably reflecting bile-salt-induced secretion of newly formed cholesterol by the periportal hepatocytes.