Bile acid synthesis from newly synthesized vs. preformed cholesterol precursor pools in the rat

Abstract

The present study defines the origin of cholesterol subserving bile acid synthesis in male rats with an extracorporal bile duct by labeling newly formed cholesterol with tritiated water. Within 6 hr after interruption of the enterohepatic circulation, the bile acid pool was depleted. At this early time point the proportion from de novo cholesterol was 8% and 12% for biliary cholesterol and cholate, but 18% and 19% for muricholate and chenodeoxycholate, respectively. This proportion gradually rose to 40%, 34%, 51% and 51%, respectively, at 15 to 30 hr. At 78 hr after bile diversion, 64% of cholate was labeled, compared with 84% to 88% of the other biliary lipids and 71% of plasma cholesterol. Total and labeled bile acid secretion exhibited the same diurnal rhythm. To allow differentiation between direct hepatocytic de novo synthesis of bile acids from acetate and recycling of labeled plasma cholesterol, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase (pravastatin) was infused from 54 to 78 hr. It suppressed total synthesis of primary bile acids by 60% to 80% but decreased the tritium label of bile acids only from a range of 74% to 92% (54 hr) to a range of 54% to 63% (78 hr), which was in the range of plasma cholesterol (58%). We conclude that bile acids and biliary cholesterol are synthesized mostly from preformed (i.e., plasma) cholesterol, both immediately after depletion of the pool in enterohepatic circulation and after derepression. Moreover, the hepatic cholesterol pools subserving the synthesis of different bile acids and biliary cholesterol secretion are not identical.

Complex feedback regulation of bile acid synthesis in the hamster: The role of newly synthesized cholesterol

“…Under physiological conditions (6-9 hours), biliary secretion of de novo cholesterol amounted to no more than 18% in rats 45,47,55,56 and 5% in hamsters and humans. 44,57 Bile acid infusions did not significantly decrease biliary secretion of de novo cholesterol, suggesting that inhibition of cholesterol synthesis by bile acids may not be relevant in hamsters under these circumstances.…”

“…Using a further methanolpetroleum ether-extraction, both constituents were separated. The methanol phase served for high-pressure liquid chromatography (HPLC)-online liquid scintillation analysis of unconjugated bile acids 45 after they had been deconjugated enzymatically by 3␣-steroid dehydrogenase. Preceding an alkaline hydrolysis, biliary cholesterol was quantified from petroleum ether extract by a second HPLC-online liquid scintillation system.…”

“…This technique was used as described in detail previously. 45,[47][48][49] The specific [ 3 H]activity of water in bile served as a basis for these calculations. Biliary cholesterol and synthesis of bile acids from preformed (unlabeled) cholesterol was defined as the difference between synthesis from total and from de novo cholesterol.…”

The pravastatin-induced decrease of biliary cholesterol secretion is not directly related to an inhibition of cholesterol synthesis in humans

“…In contrast to studies by Hellerstein and Neese 39 and our own previous work, 9 [1][2][3][4][5][6][7][8][9][10][11][12][13] C]acetate was infused intraduodenally and not intravenously. Because of the preferential hepatic clearance of acetate from portal blood, 35,40 the dose could be reduced to one third of the amount given intravenously, and both experimental procedures resulted in comparable cytosolic [ 13 C]acetate enrichments.…”

“…In rats and hamsters the amount of newly synthesized cholesterol secreted into bile amounts to 8% to 18% and 2% to 5%, respectively. [3][4][5][6] In humans, most of the indirect evidence using standard radioisotope kinetics supported that 20% of biliary cholesterol were derived from de novo synthesis, 7 the majority being preformed cholesterol supplied by high-density lipoprotein cholesterol. 8 We have recently shown that the mass isotopomer distribution analysis (MIDA) technique allows for the direct measurement of the 4% to 5% of newly synthesized cholesterol formed from [ 13 C]acetate in bile and also that a significantly higher proportion of de novo cholesterol is secreted into bile as opposed to plasma in healthy individuals' circulation.…”