Quantitative effects of bile salt and fatty acid on cholesterol absorption in the rat

Enteral administration of soybean phosphatidylcholine enhances the lymphatic absorption of lycopene, but reduces that of alpha-tocopherol in rats

Dietary phosphatidylcholine (PC) increases the lymphatic absorption of triglyceride (TG). This result suggests that dietary PC might also enhance the absorption of other fat-soluble nutrients. We examined the effects of PC on lycopene and alpha-tocopherol absorption in male rats fitted with a thoracic lymph cannula. The lymphatic output was collected after administration of 1 mL of emulsified test oils containing lycopene and/or alpha-tocopherol in 3 separate experiments. The sodium taurocholate-emulsified test oils contained soybean oil (SO; 113 micromol triglyceride), SO containing soybean PC (SPC; 82.5 micromol SO plus 30.5 micromol purified soybean PC) or SO containing egg PC (EPC; 82.5 micromol SO plus 30.5 micromol purified egg PC) with both lycopene and alpha-tocopherol (Expt. 1) or SO, SPC, or EPC with lycopene (Expt. 2) or alpha-tocopherol alone (Expt. 3). In rats administered SPC or EPC, the lymphatic outputs of TG and lycopene were higher, and that of alpha-tocopherol was lower compared with rats administered SO (Expt. 1). The absorption rate for lycopene increased from 0.59% (SO group) to 2.16 and 1.28% in the SPC and EPC groups (P < 0.05), respectively, whereas the corresponding rates for tocopherol were 21.5% for the SO, 14.8% for the SPC, and 12.9% for the EPC groups. The increase in lycopene, but not in triglyceride absorption, was higher in the SPC than in the EPC groups. The promotive effects of SPC and EPC were decreased when lycopene alone was added to the test lipids (Expt. 2), and the inhibitory effects of PC were reduced when alpha-tocopherol alone was added to the test lipids (Expt. 3). Dietary PC increased the lymphatic output of lycopene and TG and decreased that of alpha-tocopherol, suggesting that differences exist between lycopene and alpha-tocopherol in the absorptive mechanisms. The present results also show that the promotive effects of PC on lycopene absorption are influenced by the type of fatty acids in PC.

Effect of bile acid composition and manipulation of enterohepatic circulation on leptin gene regulation

In the rat, the ob gene product, leptin, putatively regulates energy balance via appetite control and energy expenditure. Bile acids in the intestinal lumen are necessary for efficient absorption of dietary lipids and may trigger the release of regulatory peptides. To investigate whether bile acids play a role in leptin gene expression, we altered the bile acid pool and then measured leptin mRNA levels in adipose tissue. Rats fed cholic acid (1% of chow wt/wt) for 2 weeks did not gain weight as rapidly as pair-fed control animals. Despite the lower weight, normalized leptin mRNA levels were 24% greater in cholic acid-fed rats compared with controls. Conversely, cholestyramine, a bile acid sequestrant, in chow (5% wt/wt) resulted in a 26% decline in leptin mRNA. Ligation of the common bile duct or chronic biliary diversion, experimental manipulations that decreased the intestinal concentration of bile salts, decreased leptin gene expression by 30% and 50%, respectively. A fluid and electrolyte (F/E) solution with and without taurocholate (36 micromol/h x 100 g rat[-1]) was then infused for 12 hours into the duodenum in animals with chronic biliary diversion. Taurocholate infusion resulted in a fourfold increase in steady-state adipocyte leptin mRNA levels compared with F/E infusion. Intravenous infusion of taurocholate or incubation of cultured adipocytes with taurocholate had no effect on leptin mRNA levels. We conclude that bile acids upregulate leptin gene expression indirectly, probably via effects on the absorption of dietary lipids or the release of neurohumoral mediators.

Failure of intravenous infusion of taurocholate to down-regulate cholesterol 7 alpha-hydroxylase in rats with biliary fistulas

BACKGROUND/AIMS

The decrease in cholesterol 7 alpha-hydroxylase induced by intraduodenal infusion of taurocholate in bile fistula rats may be indirect, i.e., mediated through release or absorption of an intestinal factor in response to the presence of bile salts in the intestine. The aim of this study was to determine if negative feedback regulation of cholesterol 7 alpha-hydroxylase can be shown when equimolar concentrations of taurocholate are administered intravenously, thus bypassing the intestine.

METHODS

After 96 hours of biliary diversion, taurocholate (36 mumol.h-1.100 g, rat-1) was infused into the rats either intravenously or intraduodenally for the final 24 hours. Livers were then harvested for analysis of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase specific activity, cholesterol 7 alpha-hydroxylase specific activity, messenger RNA levels, and transcriptional activity.

RESULTS

Intraduodenally administered taurocholate significantly decreased HMG-CoA reductase and cholesterol 7 alpha-hydroxylase specific activity by more than 50% and cholesterol 7 alpha-hydroxylase steady-state messenger RNA levels and transcriptional activity by 50%-75%. In contrast, intravenous administration of taurocholate failed to down-regulate either cholesterol 7 alpha-hydroxylase or HMG-CoA reductase.

CONCLUSIONS

Passage of taurocholate through the intestine strongly potentiates negative feedback regulation of cholesterol 7 alpha-hydroxylase. A putative intestinal factor, released or absorbed in the presence of bile acids in the intestinal lumen, may play a role in the regulation of bile acid synthesis.

Intestinal mucins and cholesterol uptake in vitro

A mucus material, secreted by intestinal segments, with a high affinity for cholesterol, has been isolated and chemically characterized. The mucin contained 11% carbohydrate, largely as glucosamine, galactose and N-acetylneuraminic acid, and 19% lipid, of which 86% was unesterified fatty acid. The isolated material readily bound cholesterol in a stoichiometric manner. Conditions known to enhance cholesterol absorption in vivo also decreased mucin complexing to cholesterol in vitro. This association of cholesterol and intestinal surface mucin also occurred during incubations of intestinal segments with dispersed cholesterol, resulting in a high level of intestinal adsorption, with little or no cellular absorption of the sterol. However, when cholesterol was solubilized in simple or complex micelles containing bile salts, surface adsorption of cholesterol was reduced and net absorption was increased. The results suggest that surface mucin binding of cholesterol may represent at least one major diffusion limitation to cholesterol absorption in the intestine.

THE ABSORPTION OF OLEIC ACID IN THE BILE FISTULA RAT

A technique is described for collecting thoracic duct lymph in a portable glass saddle from an unrestrained rat. Uniformly labelled oleic acid in various physical states was given to rats with thoracic duct and bile fistulae to study the influence of bile salts on the amount absorbed and on the route of transport and esterification after absorption. It is suggested that in addition to their emulsifying action bile salts may stimulate the esterification of absorbed oleic acid by the intestinal mucosa.