Intestinal Cholesterol Uptake Research Articles

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.

Intestinal brush border membrane marker enzymes, lipid composition and villus morphology: Effect of fasting and diabetes mellitus in rats

1. 1. Diabetes mellitus is associated with enhanced passive intestinal uptake of cholesterol and fatty acids. 2. 2. In order to determine the basis for these changes in intestinal permeability, the jejunal morphology and the lipid content of purified brush border membranes (BBM) were measured in fasted and fed control (C) and streptozotocin diabetic (DM) rats. 3. 3. There was no difference between C and DM in BBM sucrase or alkaline phosphatase; fasting had no effect on BBM enzymes in C, but in DM fasting was associated with increased sucrase activity per length of jejunum. 4. 4. In C fasting was associated with higher levels of BBM total phospholipid, lecithin, choline and amine phospholipids, whereas fasting in DM was associated with higher BBM cholesterol and lower free fatty acids. 5. 5. In the fasting DM, there was a greater villus and mucosal surface area than in the fasting C. 6. 6. A previous study demonstrated that with fasting in DM versus C, cholesterol uptake was unchanged, but when animals were fed, cholesterol and fatty acid uptake were greater into the jejunum of fed DM as compared with fed C. 7. 7. In the BBM of fed DM as compared with C, there was a significant increase in total phospholipid, lecithin, phosphatidyl ethanolamine, choline and amine phospholipids, and phospholipid/cholesterol ratio. 8. 8. Thus, (1) fasting is associated with changes in intestinal morphology, BBM lipids; (2) the effect of fasting is different in DM and C; (3) the enhanced uptake of lipids into the jejunum of fed diabetic rats is not due to changes in villus morphology, but may be due to alterations in the BBM phospholipids.

Effect of ethanol on intestinal uptake of fatty acids, fatty alcohols, and cholesterol.

The effect of acute exposure of the rabbit jejunum to ethanol on the uptake of fatty acids, fatty alcohols, and cholesterol was examined using a previously validated in vitro technique. The effective resistance of the intestinal unstirred water layer was determined from the rate of uptake of a homologous series of fatty alcohols. The addition of ethanol to the incubation or preincubation solutions had no effect on the uptake of these probes when the bulk phase was stirred at 600 rpm, but uptake was higher in the ethanol-exposed samples when the bulk phase was unstirred. Increasing the concentration of ethanol in the bulk phase was associated with a progressive decline in the rate of uptake of acetic, lauric, myristic, and palmitic acids, whereas the uptake of hexanoic, octanoic, and decanoic acids was unaffected, and the uptake of cholesterol was increased. Acute exposure of the intestine to ethanol was associated with an increase in the electrical conductance of the tissue, with no associated change noted in the tight junctions on transmission electron microscopy or in the surface epithelium on scanning electron microscopy. The results suggest that acute exposure of the rabbit intestine to ethanol is associated with a selective decline in the passive permeability properties of the membrane towards only certain lipids and that the effective resistance of the unstirred layer is influenced by ethanol only when the bulk phase is unstirred and the resistance is high.

Some aspects of mechanism of inhibition of cholesterol absorption by β-sitosterol

(1) Mixed bile salt micelle solubilized either cholesterol or β-sitosterol to a comparable extent. When added simultaneously, β-sitosterol restricted the micellar solubility of cholesterol. (2) β-Sitosterol also reduced the cholesterol content in the aqueous (micellar) phase of the intestinal contents of rats, the extent of reduction being comparable with that observed in vitro. The intestinal uptake of cholesterol in vivo was equivalent to the micellar incorporation of cholesterol both in vitro and in vivo. (3) β-Sitosterol had no inhibitory effect on cholesterol absorption from the micellar solution in jejunal loops in situ, whereas the rate of β-sitosterol uptake was only about one-fifth that of cholesterol. (4) The intestinal uptake of β-sitosterol intubated into the stomach of rats was about one-fifth that of cholesterol. The intestinal brush-border membrane discriminated these sterols. These results suggest that the restriction of the micellar solubility of cholesterol, rather than the inhibition of uptake from brush-border membrane, is the major determinant for the interference of β-sitosterol with cholesterol absorption.

Intestinal uptake of fatty acids and cholesterol in four animals species and man: Role of unstirred water layer and bile salt micelle

1. 1. The uptake ( J d ) of fatty acids (FA), fatty alcohols (Alc) and cholesterol (C) into the jejunum of rats (R, Rattus norvegiens), rabbits (RAB, Oryctolagus cuniculus), guinea pigs (GB, Curia porcellus). and hamsters (H, Mesocricetus auratus) was assessed in vitro. 2. 2. Using jejunal discs the J d of Alc was H > R = GP > RAB. the J d of FA was H-RAB > R > GP, but the J d of C was R > H > RAB > GP. 3. 3. The J d of FA was quantitatively and qualitatively different when using jejunal biopsies; in man the J d of FA into biopsies was greater than in the other animal species, but there was no difference in J d of FA into normal human jejunal biopsies and those showing severe abnormalities in villus architecture. 4. 4. There are marked species differences in the passive permeability properties of the jejunum and in the effective resistance of the overlying unstirred water layer, but these differences do not explain the species variations in the uptake of cholesterol.

Mixed micelle properties and intestinal cholesterol uptake

The solubilizing powers of taurocholate, taurochenodeoxycholate and tauroursodeoxycholate for monoolein and cholesterol, and the size of the bile salt-monoolein-cholesterol micelles have been determined. For the three bile salt species, the micellar size depends on the saturation with monoolein. As a result, for a given bile salt to monoolein ratio, the taurochenodeoxycholate micelles are smaller than those of taurocholate and both are smaller than those of tauroursodeoxycholate. Intestinal cholesterol uptake has been studied in vitro as a function of the micellar size and the saturation degree with cholesterol. For a given bile salt to monoolein ration and 1) for low cholesterol concentrations, taurocholate leads to the greatest rates of uptake ; 2) for high cholesterol content, taurochenodeoxycholate induces the largest uptake. The specific micellar characteristics of the tauroursodeoxycholate micelles clearly demonstrate why this bile salt is of so little help in the intestinal uptake of cholesterol.

Intestinal cholesterol uptake from phospholipid vesicles and from simple and mixed micelles.

This study was undertaken in vitro to examine the rat jejunal uptake of cholesterol from phospholipid vesicles and from mixed bile salt micelles, under conditions of low effective resistance of the intestinal unstirred water layer. Cholesterol uptake, Jd, occurred from vesicles only when the cholesterol:phospholipid ratio was high. The addition of phospholipid (PL) to micelles comprising 20 mM taurodeoxycholic acid (TDC) extended the concentration of cholesterol, beyond which the relationship between cholesterol concentration and uptake remained linear. When the concentration of cholesterol in the bulk phase was held constant and the concentration of TDC or of PL added to the TDC was increased, there was a decline in cholesterol uptake; this effect was masked when the concentration of TDC was high, or when higher concentrations of PL were added to the mixed micelle. When increasing concentrations of palmitic acid were added to mixed micelles composed of cholesterol, TDC and PL, the uptake of cholesterol decreased; in contrast, cholesterol uptake progressively increased when palmitic acid was added to simple TDC micelles. The results suggest that the mechanism responsible for cholesterol uptake may vary, depending on the nature of the constituents of the micelle, and it is proposed that PL inhibits the intestinal uptake of cholesterol by altering the partitioning of cholesterol out of the micelle.

Intestinal cholesterol uptake from mixed micelles in vitro effects of taurocholate, taurochenodeoxycholate and tauroursodeoxycholate

Cholesterol uptake by everted rat jejunal sacs is lower from mixed micelles containing tauroursodeoxycholate than from those with taurocholate or taurochenodeoxycholate. This occurs in spite of a greater saturation with cholesterol of tauroursodeoxycholate micelles as measured by equilibrium solubility studies. The results suggest that cholesterol saturation of solutions containing tauroursodeoxycholate is overestimated when calculated with reference to solubility in micellar form.

Effect of phosphatidylcholine on fatty acid and cholesterol absorption from mixed micellar solutions

Using the experimental model of the everted sac prepared from rat jejuna, kinetic studies on [ 14C]oleic acid uptake from bile salt micelles were conducted in the presence and absence of phosphatidylcholine. The concentration of oleic acid was varied between 0.625 and 5 mM. At every level of fatty acid concentration studied the addition of 2 mM phosphatidylcholine produced a significant inhibition of fatty acid uptake. It was further noted that the intact phospholipid molecule was required for this effect as lysophosphatidylcholine produced little, if any, inhibition of [ 14C]oleic acid uptake. The effect of varying the concentration of phosphatidylcholine on fatty acid uptake was also studied. The degree of inhibition was noted to be correlated grossly with media concentrations of this phospholipid although the decrease of fatty acid uptake was not strictly proportional to concentration of this material in the medium. Studies were also performed analyzing in vitro absorption of [ 14C]oleic acid and [ 3H]cholesterol simultaneously from mixed micelles composed of sodium taurocholate, oleic acid, monoolein and cholesterol. Control medium contained no phospholipid while experimental medium contained either diester or diether phosphatidylcholine, 2 mM. Both types of phosphatidylcholine caused significant inhibition of fatty acid and cholesterol uptake. In vivo absorption studies were also performed using the isolated jejunal segment technique. A mixed micellar solution containing [ 3H]cholesterol and [ 14C]oleic acid was used as the test dose. Phospholipid in the test dose for controls was supplied as lysophosphatidylcholine and for experimentais it was in the form of diether phosphatidylcholine. Significantly less radioactively labeled cholesterol and tatty acid was absorbed by experimentals as compared to controls over a 10-min period. It is concluded that the intact molecule of phosphatidylcholine inhibits intestinal uptake of cholesterol and fatty acid from mixed micellar solutions under both in vitro and in vivo conditions.

The effect of lecithin on intestinal cholesterol uptake by rat intestine in vitro.

1. Sacs 20 cm long were obtained from the upper half of the small intestine of bile fistula rats (bile duct cannulated 48 hours previously). The sacs were everted, filled with oxygenated phosphate buffer and incubated 1 hr at 37 degrees C in 25 ml. of a buffered micellar solution of oleic acid (0.6 mM), mono-olein (0.3 mM), sodium taurocholate (4.8 mM) and (3)H-labelled cholesterol (0.15 mM) plus glucose (28 mM).2. After incubation the amount of [(3)H]cholesterol taken up by the mucosal tissue was measured. It averaged 200 n-mole/hr.g tissue wet wt. +/- 6 (S.E.).3. Adding 3 ml. whole rate bile with other factors unchanged caused cholesterol uptake to decrease by 50% in confirmation of previous studies.4. Adding purified lecithin obtained from rat liver tissue, and from egg yolks, similarly decreased cholesterol uptake. A significant response was obtained with 2.5 mg liver lecithin (concentration 0.13 mM) and a near maximum response with 15 mg (concentration 0.80 mM). 10 mg lecithin decreased uptake by an amount equivalent to that obtained with 3 ml. whole bile.5. Lecithin is an active component of whole bile causing reduced intestinal cholesterol uptake from micelles.6. The decreased uptake of cholesterol in the presence of lecithin may have been the result of expansion of the cholesterol-containing micelles with consequent reduction in cholesterol permeability.

Intestinal absorption and lymphatic transport of cholesterol in the rat: influence of the fatty acid chain length of the carrier triglyceride

This paper deals with the effect of the fatty acid chain length of dietary triglyceride on the intestinal uptake and lymphatic transport of exogenous and endogenous cholesterol in the rat. This question seemed of interest as the chain length of the monoglyceride and fatty acids formed in the intestinal lumen from the triglyceride fed could be expected to affect the concentration of cholesterol in the micellar or isotropic phase of intestinal content. Feeding rats medium- or short-chain triglycerides (C(12) to C(2)) did not affect the lymphatic transport of endogenous cholesterol from the intestine compared to the fasting state. The extent of lymphatic transport of cholesterol added to these fats increased proportionally with chain length (C(6)-C(18)) of the component fatty acids. The uptake of exogenous cholesterol into the intestinal wall was similarly related to the chain length of the carrier triglyceride, with the exception of triacetin, which gave a much higher intestinal uptake than lymphatic transport. When cholesterol was fed in octadecane, negligible amounts only were transported to the thoracic duct lymph. This again indicates the importance of the polar split products of dietary fat for cholesterol absorption.