Christiane Lacombe

Identification of α2-adrenoceptors and of non-adrenergic idazoxan binding sites in pancreatic islets from young and adult hamsters

1. The current study was undertaken to investigate the characteristics of alpha 2-adrenoceptors and to search for the presence of NAIBS in hamster pancreatic islets. 2. Pancreatic islets were isolated from young (6-7 weeks) and adult (14-15 weeks) animals. 3. The identification of alpha 2-adrenoceptors using [3H]RX821002 indicated that adults exhibited higher number of alpha 2-adrenoceptors than the young animals (194 +/- 20 vs 105 +/- 16 fmol/mg protein) while the Kd value was unchanged. 4. Glucose-evoked insulin release was completely inhibited by the alpha 2-agonist clonidine (0.1 microM) whatever the age of the animals. Agonist inhibition curves showed the following rank order of potency: clonidine > UK14304 > adrenaline. 5. Blockade of UK14304-elicited inhibition by various antagonists indicated that yohimbine has a low affinity for the receptor supporting the conclusion that the receptor is of the alpha 2-D subtype. 6. Binding experiments with [3H]idazoxan under conditions allowing to discriminate between alpha 2-adrenoceptors and NAIBS showed that hamster pancreatic islets express a high number of NAIBS. The density of NAIBS was similar in young and adult hamsters (1550 +/- 245 and 1342 +/- 332 fmol/mg protein).

DIETARY RESTRICTION AMPLIFIES THE HYPERCHOLESTEROLEMIC EFFECT OF CHOLESTEROL FEEDING IN THE RAT

Abstract Dietary restriction (half of the control ration) was performed in rats given either standard or hypercholesterolemic diets. The plasma cholesterol was measured throughout the 2 month experiment after which time the lipoproteins were analysed. Underfed rats with or without cholesterol supplementation showed a rapid rise in plasma cholesterol. Moreover, dietary restriction greatly enhanced the effect of the hypercholesterolemic diet which contained cholesterol and cholic acid. The major changes occurred in the very low density lipoprotein (VLDL) cholesterol which was increased in cholesterol-fed rats. This was amplified more than 2-fold by dietary restriction. In addition cholesterol feeding associated with dietary restriction induced a significant rise in the intermediate density lipoprotein (IDL)+low density lipoprotein (LDL) cholesterol. The findings indicate that dietary restriction could be an additional risk factor of hyperlipoproteinemia, specially when associated with high cholesterol intake.

Effect of dietary restriction on the plasma apolipoprotein pattern in cholesterol-fed rabbits.

Dietary restriction (half of the control ration) was performed in rabbits given either standard or cholesterol-rich diets. The plasma apolipoproteins were studied on the total, d<1.21, lipoprotein fraction using polyacrylamide gel electrophoresis. A marked rise in the amount of both apo-B and apo-E appeared in cholesterol-fed rabbits and was enhanced by dietary restriction. These results reflect the aggravation of hypercholesterolemia when cholesterol feeding is carried out in underfed rabbits. In all groups only one molecular species of apo-B, identified as apo-B-100, was present after overnight fasting. Thus, lipoproteins which accumulate in the plasma following cholesterol feeding, associated with dietary restriction or not, probably are remnants of hepatogenous triglyceride-rich lipoproteins.

Fate of exogenous and newly synthesized cholesterol in intestinal cell lines

1. The current study was undertaken to test the existence of functionally distinct intracellular pools of cholesterol depending on the origin: neosynthesis or exogenous. 2. This was performed on two subpopulations, either differentiated or undifferentiated, of the HT29 cell line. 3. A parallel study was also carried out on Caco-2 cells. 4. First we checked the ability of differentiated HT29 cells to secrete lipids into the medium and found that lipid production was efficient but less so than in Caco-2 cells. 5. In contrast, undifferentiated HT29 cells were unable to secrete lipids into the medium. 6. Then we studied the fate of [14C]cholesterol incorporated into micellar preparations and of [14C]mevalonate in the different models. 7. The data obtained with labelled exogenous cholesterol show that it enters the membrane cholesterol pool as well as, for the differentiated models, the cholesteryl ester pool. 8. Similarly, labelled newly synthesized cholesterol could be used for membrane formation as well as for incorporation into cholesteryl esters. 9. Thus, in HT29 subpopulations as well as in Caco-2 cells, the results suggest the existence of a common pool of cholesterol whatever its origin.

Effect of dietary restriction on cholesterol biosynthesis in the liver and the intestine of cholesterol-fed rabbits

The effect of dietary restriction (half of the control ration) on cholesterol biosynthesis was investigated in rabbits fed either standard or cholesterol-rich diets. Accompanying the amplification of hypercholesterolemia, additional disturbances of cholesterol metabolism were observed when cholesterol feeding was associated with dietary restriction. In the intestine, underfed rabbits showed a more marked inhibition of duodenal cholesterol biosynthesis from [14C]acetate following cholesterol feeding than rabbits on normal caloric ration. In contrast liver cholesterogenesis was equally suppressed in both groups receiving cholesterol-rich diets. Cholesterol biosynthesis from [14C]mevalonate was also inhibited by cholesterol feeding particularly in the duodenum of underfed rabbits. In addition cholesterol feeding induced a marked increase of the labeled esterified: free cholesterol ratio in the liver, demonstrating intensive esterification, this was enhanced by dietary restriction. The additional cholesterol which accumulates in the plasma and in various tissues in underfed rabbits is of dietary origin since the feedback control of cholesterogenesis by exogenous cholesterol was shown to be very effective in these animals.

Dietary restriction amplifies the metabolic disturbances of very-low-density lipoproteins in cholesterol-fed rabbits

The effect of dietary restriction (half of the control ration) on VLDL turnover was investigated in cholesterol-fed rabbits. Rabbits on standard, cholesterol and restricted cholesterol diets were injected with homologous 125I-labelled VLDL. Accompanying the amplification of hypercholesterolemia, additional disturbances of VLDL turnover were observed when cholesterol feeding was associated with dietary restriction. Cholesterol-fed rabbits with normal caloric ration exhibited delayed clearance of 125I-labelled apolipoprotein B component of VLDL compared to control rabbits. This was markedly accentuated in underfed rabbits, indicating further down-regulation of apolipoprotein B,E receptors in these animals. Furthermore, a reduced proportion of radiolabelled apolipoprotein B was converted from VLDL to intermediate-density lipoprotein (IDL) and LDL in both groups receiving cholesterol-rich diets. Thus, the combination of further impairment in plasma clearance of VLDL and the poor conversion into IDL and LDL could account for the massive increase of beta-VLDL in underfed animals on cholesterol-rich diets.

Toxicocinetique d'un pesticide du groupe des thioquinoxalines: L'oxythioquinox

Abstract Toxicokinetics of a thioquinoxaline component: the oxythioquinox The absorption and distribution of Morestan 35 S in the organism of the rat were studied after administration by stomach tube. It was shown that Morestan, in vivo and in vitro, forms a complex with plasma proteins and particularly with albumins. Most of the plasma Morestan is distributed like this. After intravenous administration, the constant of elimination, K 13 , and the biological half-life determination show that plasma Morestan 35 S is eliminated very slowly. The K 12 and K 21 constants indicate that there is an equilibrium between the plasma compartment and a peripheral compartment. After intragastric administration, the increase of radioactivity in epididymal adipose tissue, indicate that the peripheral compartment should include the adipose tissue.

Causal relationship between removal efficiency of low density lipoproteins and their composition following cholesterol feeding and cholestyramine therapy

Abstract The origin of cholesterol-rich low density lipoprotein (LDL) in diet-induced hypercholesterolemia was investigated by simultaneous examination of LDL composition and 125 I-LDL plasma clearance in control and in cholesterol-fed rabbits treated or not with cholestyramine. The results show a straight relationship between LDL plasma clearance and the cholesterol ester/triglyceride ratio among the three experimental groups. Cholesterol-fed rabbits present a marked increase in the cholesterol ester/triglyceride ratio of LDL associated with an impairment in the catabolism of LDL, isolated from normal rabbits, as indicated by their fractional catabolic rate (FCR). Both parameters, cholesterol ester content and FCR of the LDL remain intermediate in cholesterol-fed rabbits treated with cholestyramine. The intravascular transformation of labeled LDL from normal rabbits was evaluated by the study of changes in their density profile. The increase in modal density of LDL after 7 hours of exposure in the blood stream is accentuated in cholesterol fed-rabbits. Therefore the intravascular transformation of LDL depends on the removal efficiency of the recipient animals. Taken together the findings support the hypothesis that abnormal LDL mainly results from supplementary intravascular degradation as a consequence of increased lifetime. Additional data concerning the composition of the very low density lipoproteins (VLDL) and intermediate density lipoproteins (LDL) are given. Cholestyramine has clear preventive effects on the alteration of these lipoproteins following cholesterol feeding suggesting that the mechanism proposed for LDL alteration is involved in the appearance of the other apo-B- containing lipoproteins.