Bile acid secretion by cultured rat hepatocytes. Regulation by cholesterol availability.

Abstract

A sensitive gas-liquid chromatographic technique employing electron capture detection of capillary column separated fluoro-substituted bile acid derivatives allowed us to examine regulation of bile acid secretion by cultured rat hepatocytes.In serum-free medium, the rate of secretion of cholic and 8-muricholic acids was constant for 96 h.Since the amount of cell-associated bile acids were similar at 0 and 72 h of the experiment, bile acid secretion was due to de novo synthesis.Under basal conditions (no additions), the secretion of cholic and 8-muricholic acids was nearly equal.The amount of chenodeoxycholic acid secreted was negligible.The net secretion rate of total bile acids was 8 nmol/g of liver/h, which is 20-50% of in vivo estimates.Within 18 h of adding up to 100 PM taurochenodeoxycholic acid, essentially all was converted to 8-muricholic (-80%) and cu-muricholic acids.Thus, the capacity of cultured hepatocytes to 6-hydroxylate is sufficient to account for the absence of chenodeoxycholic acid and the presence of 8-muricholic acid in the culture medium.To examine if cholesterol availability plays a role in regulating bile acid secretion, three different methods were used to alter its availability: 1) changing the rate of cholesterol biosynthesis, Le. addition of mevalonic acid (10 111~) increase, mevinolin (50 pg/ml) decrease, 2) feeding a cholesterol-rich diet to donor rats, and 3) addition of serum lipoproteins.Mevalonic acid, dietary cholesterol, and d < 1.02 g/ml lipoproteins obtained from cholesterol-fed rats decreased de novo cholesterol synthesis from ["Clacetate (65-95%), increased ["C] cholesterol esterification (5-10-fold), increased hepatocyte cholesterol ester concentrations (2-24-fold), and increased bile acid secretion (2-10-fold).In contrast, mevinolin inhibited cholesterol biosynthesis (-60%), inhibited ['4C]cholesterol esterification (-50%), slightly decreased hepatocyte cholesterol ester concentrations (-25%), and inhibited bile acid secretion (-40%).These results show that hepatocyte cholesterol concentrations and rates of bile acid secretion vary in parallel to these aforementioned effectors.Moreover, the results show for the first time a direct effect between hepatocyte uptake of lipoproteins and bile acid secretion.The liver is the major organ site of both the synthesis and degradation of plasma cholesterol.These metabolic processes,