F. Chevallier

Effects of the thyroid state on cholesterol metabolism in the rat.

An isotopic equilibrium method which permits the in vivo measurements of cholesterol turnover processes was applied to different groups of rats: (1) radiothyroidectomized, (2) low-iodine fed, and (3) L-thyroxin fed. Plasma cholesterol concentrations were enhanced after thyroidectomy and reduced by large dose of L-thyroxin. A low-iodine diet decreased plasma thyroxin level but did not affect plasma cholesterol. Thyroid levels in plasma modified the coefficient of intestinal absorption of cholesterol. After thyroidectomy or under conditions of reduced thyroxin formation this absorption coefficient was enhanced. The absorption coefficient of cholesterol was decreased in rats receiving 31 or 61.5 mug/day of L-thyroxin but was not changed in rats fed 110 mug/day L-thyroxin. The proportion of de novo biosynthetisized cholesterol eliminated into the feces (external secretion) was reduced while thyroxin levels were low. The fecal excretion of cholesterol and the in vitro exchange of cholesterol between erythrocytes and plasma were increased by L-thyroxin ingestion. The rate of cholesterol biosynthesis was decreased after thyroidectomy and enhanced by L-thyroxin feeding. In fact, changes in thyroid state modified indirectly the biosynthesis of cholesterol by its effects on metabolism and on the coefficient of intestinal absorption of cholesterol.

Time course of biosynthetic cholesterol in the adult rat brain

Abstract In a first experiment, the time course of biosynthetic cholesterol was studied in the whole brain of adult rats, 1 h to 15 months after an intraventricular injection of 5-[ 14 C]mevalonic acid. Radioactivities of nonsaponifiable compounds, sterols and cholesterol purified by gas-liquid chromatography were measured in the brain and those of precursor sterols and squalene fraction were calculated. In the other experiments, only cholesterol radioactivities were measured from 3 days to 1–3 months after injection (1) in the whole brain and in a piece of skull, (2) in the cerebellum and cerebral hemispheres and (3) in a fraction of whole brain homogenate and in cerebral myelin. The most striking results are the following: 1. Brain cholesterol specific radio-activity reaches two maxima, one at 3 days, and the other, slightly lower, at 1 month after injection. 2. After these two peaks, brain cholesterol specific radioactivity decreases as an exponential function of time, with a slope of 0.004 day −1 . 3. Cholesterol specific radioactivity of myelin increases up to 40 days after injection whereas that of the other subcellular fractions decreases up to 6 days and increases from 6 to 40 days after injection. Two complementary explanations of these results are discussed. The first involves the existence of two groups of subcellular structures, those of group 1 exchanging their cholesterol with that of plasma, those of group 2 exchanging their cholesterol between themselves and with cholesterol of the first group. The second interpretation involves two chains of biosynthesis for brain cholesterol, working at different rates.

Vitesse des processus de renouvellement du cholestérol contenu dans son espace de transfert, chez le rat: III. Modifications et étude critique de la méthode d'équilibre isotopique

Abstract Since the isotopic equilibrium method as applied to rat cholesterol was described, some practical modifications have been made which are reported here. Sometimes the use of a subcutaneous implantation of capsules containing tritiated cholesterol leads to an over-estimate of the rate of fecal cholesterol excretion. For the questionable cases, it is necessary to use the initial method of daily subcutaneous injections of radioactive cholesterol. In another connection, 17 experiments using the isotopic equilibrium method have shown great variability of the weekly radioactive balance. The radioactivity of cholesterol and its transformation products released from the organism of the rat and that of absorbed cholesterol are rarely equal as would be expected. To define the origins of these losses an experiment using the isotopic equilibrium method was carried out on rats which were killed after 4, 8 or 12 weeks. Bacterial degradation cannot explain the deficit in the weekly radioactive balances, for the total radioactive balance which includes the radioactive cholesterol of the rat is always nil. In fact, these deficits can be explained on the basis that isotopic equilibrium is not really obtained or that the organism is not in a steady state but in an expansion state. In both cases the application of suitable relations permits the calculation of all the rates involved in the dynamics of cholesterol with certainty. But for an organism in an expansion state it is necessary to know the rate of cholesterol increase. After these corrections have been made, the percentage of error on each rate is defined. In the particular case of this critical experiment, animals were fed with a basal diet to which, 0.2% taurocholate was added. Although this results in an expansion state, the rates of synthesis, excretion and absorption are constant. The increase of mobile cholesterol in the organism is associated with a decrease of its transformation to bile acids.

Origins of Neutral Sterols in Human Feces Studied by Stable Isotope Labeling (D and 13C)

An experimental procedure using stable isotope-labeled cholesterol (13C and D) was carried out on 15 healthy subjects to distinguish the different origins of neutral fecal sterols in man: nonabsorption of dietary cholesterol, fecal excretion by transfer of plasmatic cholesterol and external secretion of cholesterol biosynthetized in digestive tract and directly eliminated. For a mean daily mass of 652 mg of fecal cholesterol, unabsorbed dietary cholesterol is 20% (133 mg), excreted cholesterol 67% (434 mg) and cholesterol from external secretion 13% (85 mg). A short treatment (4 days) with cholestyramine or different bile acids was then administered to each subject to study the possible variations in their fecal elimination of cholesterol. The more evident effect was the large stimulation of external secretion of cholesterol (234 mg/day) observed after chenodeoxycholic acid feeding (1 g/day). This treatment tends also to decrease dietary cholesterol absorption and to enhance excretion of cholesterol.

Analyse des sterols des contenus digestifs et des feces du rat

Resume The sterols of the digestive tract contents (stomach, intestine, caecum and colon) and faeces were studied with control rats and with others, washed daily, fed a sterol-free diet and having a tails cup to prevent coprophagy (“treated rats”). All rats were killed at 9 a.m. In the control rats, the synthetic “precursors” of cholesterol are more abundant in the digestive contents and faeces than in the alimentary tract wall: lathosterol (12–15%), methostenol (5–8%), desmosterol, dihydrolanosterol and lanosterol (0–4%). In treated rats the ranges of lathosterol and methostenol are 14–37% and 2–13%, respectively. The other “precursors” of cholesterol were not detected by our gasliquid chromatographic analysis. It seems that their disappearance is due to the treatments given to the rats. In treated rats, analysis of the sterol content of the stomach demonstrates the existence of sterol secretion by the higher portion of the alimentary tract: cholesterol (55%). lathosterol (39%) and methostenol (7%). In the intestine contents, there are stomach and endogenous sterols. The variations in their relative proportions demonstrates an important dilution of cholesterol. Also, in the control rats, the dilution factor is 5.3. This result is only valid for the time of sacrifice (9 a.m.). The transformation cholesterol-coprosterol is only achieved in the caecum but continues in the colon and faeces. No sterol absorption, secretion or excretion occurs in the colon. For the two groups of animals, the esterified sterol percentage, high in the stomach contents, is lowered in the intestine and increases slowly in the caecum and colon. The finding of esterified sterols in the stomach contents suggests that secreted sterols are partially esterified. The decrease in the esterified cholesterol level in the intestinal contents is explained by the dilution of free cholesterol. For all contents, the percentage of esterified sterols is about 3 times higher in the treated rats than in the controls. In the faeces of treated rats, the esterified cholesterol is hydrolysed. This process is not apparent in control rats.

Origin and Fate of Cholesterol in Rat Plasma Lipoproteins in vivo

After a single ingestion of a diet containing 14C-cholesterol, cholesterol radioactivity in the stomachal and intestinal contents, in the different organs and in the very low density lipopr

Diet and sterol biohydrogenation in the rat: Occurrence of epicoprostanol

The fecal sterols from rats fed several types of semipurified or commercial diets were analyzed by a combination of thin layer and gas liquid chromatography. In rats fed semipurified diets with lard, sucrose, and casein, increasing proportions of lard (0, 8, 20, 65%) enhanced the fecal coprostanol/coprostanol + cholesterol ratio (from 0.50 to 0.85). This ratio was reduced by replacing lard with triolein or a mixture of calcium oleate and linoleate (1∶1) and did not change when trierucin was substituted. No coprostanol formation was observed in rats fed a diet with tripalmitin or tristearin. The addition of sodium hyodeoxycholate (0.5%) or cholestyramine (2%) to the basal diet was without effect on the coprostanol/coprostanol + cholesterol ratio in the feces. The addition of sodium taurocholate (0.2, 0.75, and 4%) strongly reduced coprostanol formation, while a chronic bile duct ligation led to an enhancement. Cholesterol feeding (0.05, 0.2, and 0.5% in the diet) slightly increased (from 51 to 66%) coprostanol formation. Trace amounts of epicoprostanol were generally found in the feces. However, in some cases a very high proportion (up to 60%) of this sterol was observed. Possible relationships between the presence of epicoprostanol and the nature of the diet are discussed.

Mouvements des sterols dans le tube digestif du rat. Absorption du cholesterol de synthese

Rats of approximately synchronous alimentary behaviour and known coprophagous cycle were used. During one day they were fed a diet containing [4-14C]-cholesterol. Sterol composition and radioactivity of the content of each digestive organ (stomach, intestine divided into four fractions, caecum, colon) and of faeces were determined eight times during this 24-h period. Daily fluxes of cholesterol, coprosterol and precursor sterols (mainly lathosterol and methostenol) passing through each digestive organ were calculated. Following coprophagy, the daily influx into the stomach accounted for 2.1 mg precursor sterols, 2.8 mg coprosterol and 1.2 mg cholesterol. The dietary cholesterol influx was 2.6 mg. In addition to exogenous sterols, the stomach content was rich in endogenous sterols. Their daily influx reached 3.7 mg for precursor sterols and 3.1 mg for cholesterol. The fluxes of endogenous and exogenous sterols were simultaneous. The mean quantities of sterols and cholesterol in the contents of the intestinal fractions increased approximately as an exponential function of the length of the intestine. Besides the cholesterol from the stomach (exogenous for intestine), their content was rich in endogenous cholesterol. The ratio of endogenous to exogenous cholesterol varied all along the intestine. At the pyloric extremity there was 14 times as much endogenous as exogenous cholesterol. If we consider the intestinal content as a homogeneous mixture of exogenous and endogenous cholesterol, or as divided into two separate compartments, the “axial” and “intermediary”, respectively, quantitative deductions are unacceptable. There was in fact, a transfer (11 mg/day) of endogenous cholesterol from the “intermediary” to the “axial” compartment. Consequently, endogenous cholesterol (5 mg/day) was absorbed to a similar extent as exogenous cholesterol (5.5 mg/day). For the stomach and intestine contents we also determined the origin of endogenous cholesterol accurately. Approximatively half of it came from plasma while the rest resulted from synthesis in the digestive tract. It could be demonstrated that 4.2 mg synthetic cholesterol passed into the plasma per day by means of absorption. Coprosterol absorption conformed with classical data. In addition, the intestine contained endogenous precursor sterols besides those from the stomach. The precursor sterols were partially absorbed. The caecum and the colon were transit organs, in which only transformation of cholesterol to coprosterol took place.

Effect of portacaval or mesentericocaval anastomosis on cholesterol metabolism in rats

Portacaval anastomosis (PCA) lowered by 50% the cholesterol concentration in the plasma of rats. The free and esterified cholesterol contents in the lipoproteins were decreased with the very low density lipoproteins most affected (-85%). Cholesterol concentration as total content in the liver was reduced. The major change in the cholesterol metabolism, as studied with an isotopic equilibrium method, was the decrease in the intestinal absorption coefficient of dietary cholesterol (56.0 +/- 2.7% instead of 73.3 +/- 1.9% in controls). The rate of cholesterol transformation into bile acids was decreased (10.5 +/- 0.3 vs 14.5 +/- 0.5 mg/day/rat in controls). The rate of internal secretion of cholesterol was slightly reduced while the rate of fecal external secretion was increased, suggesting that the synthesis of cholesterol by extra-digestive tissues (including liver) was reduced after PCA. The effects of PCA on cholesterol metabolism were similar to those described for glucagon administration. Since this shunt results in hyperglucagonemia, it is suggested that this hormonal perturbation was the main factor involved in the modifications of cholesterol metabolism after PCA. Moreover, mesentericocaval anastomosis, which shunts only the intestinal blood and allows the pancreatic hormones a normal transport through the liver, did not significantly modify cholesterol metabolism. Only cholesterolemia (-28%) and the absorption coefficient of dietary cholesterol (66.0 +/- 2.3%) were slightly reduced.

In vivo study of free and esterified cholesterol turnover in various tissues of the rat

Rats were infused for 3.5 to 10 hrs with either red cells or plasma previously labelled in vivo by [3H]-cholesterol. Cholesterol specific radioactivities were measured in plasma, HDL, LDL and VLDL, and various tissues. Red cell infusions led to a higher labelling of free than of esterified cholesterol in the plasma of infused rats. The opposite situation was observed following plasma infusion. Comparison of free and esterified cholesterol specific radioactivities in each tissue showed that esterified cholesterol was transferred from plasma to all the tissues, except the adrenals. Study of the ratios of cholesterol specific radioactivities from one experimental group to the other in each tissue, made it possible to demonstrate clearly the occurence of hydrolysis within all the studied tissues except 5 of them where its existence remains uncertain (lung, heart, kidney, tendon, muscle) and of esterification in 3 tissues (adrenal, liver lung). In addition, ratios of cholesterol radioactivities (free/ester) were found to be identical in plasma and in 4 tissues, where neither hydrolysis nor esterification were detected (heart, muscle, kidney, tendon). This finding is an argument in favor of a simultaneous transport of free and esterified cholesterol from plasma into these 4 tissues and suggests that the entire lipoprotein particles can penetrate these tissues, with no specificity of one special class. In adrenal, unlike all other tissues: 1) the turnover of esterified cholesterol was achieved mostly by hydrolysis and esterification in situ; 2) a preferential lipoprotein class (LDL) was responsible for the transport of free cholesterol from the plasma.