Peter A. Mayes

Modification of the fatty acid composition of dietary oils and fats on incorporation into chylomicrons and chylomicron remnants

Possible changes in the fatty acid composition of dietary fats and oils which might occur during digestion, absorption and formation of chylomicrons and chylomicron remnants were investigated. Chylomicrons were collected from the thoracic duct of rats tube-fed with olive, maize, palm or fish oil or butter fat, and their fatty acid composition was determined and compared with that of their parent lipids. In turn, these lipoproteins were converted to chylomicron remnants in functionally hepatectomized rats and their composition re-determined. The predominant fatty acids in each of the oils and fats also predominated in their respective chylomicrons, but their proportions were reduced during the processes leading to their formation. Endogenous contributions of linoleic, eicosapentaenoic, and docosahexaenoic acids were particularly noted when these fatty acids were not well-represented in the original oils and fats, suggesting that they may be obligatory constituents in the formation of chylomicrons. The conversion of chylomicrons to remnants further attenuated the extremes in fatty acid composition of the dietary oils and fats. These results indicate that following an acute intake of oil or fat, the resulting chylomicrons and chylomicron remnants presented to the tissues contain a more balanced distribution of saturated, mono- and polyunsaturated fatty acids than the oils and fats from which they were derived.

Acute effects of insulin on glycerol phosphate acyl transferase activity, ketogenesis and serum free fatty acid concentration in perfused rat liver

In the perfused rat liver insulin inhibits the oxidation of serum free fatty acids [1 ] and endogenous fatty acids [2], increases the secretion of very low density lipoprotein triacylglycerol [1-3] and also opposes the antilipogenic effect of long chain free fatty acids [3]. In isolated adipocytes insulin increases the synthesis of triacylglycerol. Although some of this insulin effect in adipose tissue may be attributed to enhancement of glucose transport and hence provision of triose phosphates for esterification, there is evidence for more direct insulin action upon glyceride synthesis [4]. In vitro adipocyte glycerol phosphate acyltransferase (GPAT) and long-chain fatty acyl CoA synthetase activities have been shown to increase as a result of insulin treatment [5 -7] . It was therefore of interest to examine the acute effects of insulin on activities of these enzymes in the perfused rat liver.

Comparison of the hepatic uptake and processing of cholesterol from chylomicrons of different fatty acid composition in the rat in vivo

The effect of the fatty acid composition of chylomicrons on the uptake and processing of the cholesterol they carry was investigated in the rat in vivo. Rats kept on a standard low fat pellet diet and tube fed a single dose of palm, olive, corn or fish oil (rich in saturated, n-9 monounsaturated, n-6 polyunsaturated and n-3 polyunsaturated fatty acids, respectively) were used to prepare [3H]cholesterol-labelled chylomicrons of different fatty acid composition. These were then injected intravenously into rats (kept on the standard diet), and the clearance of radioactivity from the blood, distribution in the plasma lipoprotein density fractions, uptake by the liver and appearance in the bile were studied. [3H]Cholesterol from fish and corn oil chylomicrons was cleared from the blood more rapidly than that from palm and olive oil chylomicrons. After 180 min the proportion of the radioactivity present in the plasma in high density lipoprotein (HDL) was less when the chylomicrons were derived from palm oil as compared to any of the other oils. Approx. 40% of the administered label was recovered in the liver after 180 min in all experiments. The percentage of the injected radioactivity secreted into bile during 180 min was significantly higher with corn and fish oil chylomicrons than with palm oil chylomicrons, with chylomicrons from olive oil in an intermediate position, and these differences were most pronounced between 60 and 120 min after administration of the label. These studies clearly demonstrate that the fatty acid composition of chylomicrons has important effects on the hepatic uptake and processing of the cholesterol they carry, with enrichment with polyunsaturated fatty acids leading to an increased rate of uptake and more rapid removal from the body via the bile as compared to enrichment with saturated or monounsaturated fatty acids.

Comparative effects of fructose and glucose on the lipid and carbohydrate metabolism of perfused rat liver.

1. Livers from rats fed on a standard diet were perfused with whole blood, and infused continuously with glucose and fructose at equimolar rates. 2. Infusion of fructose increased both the secretion of very-low-density-lipoprotein (VLDL)-triglycerides and the incorporation of free fatty acids (FFA) from the perfusate into VLDL-lipids, but neither of these two processes was affected by infusion of glucose. 3. Infusion of fructose decreased the oxidation and increased the esterification of FFA, but glucose infusion had no effect on these processes. When fructose and glucose were infused together was a further decrease in oxidation. 4. When fructose was infused alone or together with glucose, blood concentrations rapidly became stabilized at those found in the hepatic portal vein in vivo, with similar rates of hepatic uptake to those found in the intact animal. Infusion of glucose alone resulted in continuously increasing perfusate glucose concentrations, and rates of uptake which were only 20% of those for fructose. Blood glucose concentrations were reduced, and lactate concentrations were increased by fructose infusion, and when glucose and fructose were infused together there was a greatly increased rate of glucose uptake. 5. Liver glycogen was not affected by the infusion of fructose or glucose alone; however, their combined addition significantly increased its concentration. 6. Uptake of perfusate FFA was not affected by either fructose or glucose infusions. 7. The results are discussed in terms of the differences in nutrition and metabolism between glucose and fructose, with particular reference to the development of hypertriglyceridaemia.

Investigations into the direct effects of insulin on hepatic ketogenesis, lipoprotein secretion and pyruvate dehydrogenase activity

In the rat liver perfused with whole rat blood containing either decreased or increased concentrations of non-esterified fatty acids, insulin decreased production of acetoacetate and 3-hydroxybutyrate and stimulated secretion of very-low-density-lipoprotein triacylglycerol. In these same livers, pyruvate dehydrogenase activity was not altered by insulin addition, although it was diminished by non-esterified fatty acids.

The immediate and long term effects of clofibrate on the metabolism of the perfused rat liver

Abstract A study was made of the immediate effects of CPIB (chlorophenoxy-isobutyrate) and of the effects of clofibrate (ethyl-CPIB) pretreatment on the metabolism of the perfused liver. Both treatments caused an increased hepatic uptake of lactate and free fatty acids. Pretreatment with clofibrate resulted in a decrease in perfusate glucose, an increase in ketogenesis and a decreased output of very low density lipoprotein triacylglycerol. A more oxidized redox state of both the cytosol and the mitochondria was indicated by decreased ratios of perfusate [lactate]/[pyruvate] and [3-hydroxybutyrate]/[acetoacetate] respectively. Increased hepatic O 2 consumption was associated with the increased liver weight of rats treated with the drug for 1 week. The fate of free fatty acids was followed by infusing [1— 14 Cloleate. The increased oxidation of oleate to both CO 2 and ketone bodies in livers from animals pretreated with clofibrate was accompanied by a corresponding decreased incorporation of 14 C into very low density lipoprotein triacylglycerol. Lipogenesis was depressed upon addition of CPIB to the perfusate, but was increased after pretreatment with clofibrate. No changes in cholesterol synthesis were detected. A hypothesis to account for the hypolipidaemic and other effects of clofibrate pretreatment is advanced. This is based on a primary enhancement of fatty acid oxidation accompanied by a reciprocal decrease in hepatic triacylglycerol secretion. It is suggested that increased peroxisomal oxidation of fatty acids may be a cause of the decreased redox potential. A consequent activation of pyruvate dehydrogenase would explain both the changes in carbohydrate metabolism and the increase in lipogenesis.

Comparison of short- and long-term effects of different dietary fats on the hepatic uptake and metabolism of chylomicron remnants in rats

The uptake and metabolism of [14C]oleate-labelled chylomicron remnants derived from olive oil, maize oil, palm oil, fish oil or butter fat was investigated using perfused livers from rats fed on the corresponding fat-supplemented diet (providing 40% of the dietary energy) or a low-fat diet for 21 d. The percentage of added [14C]oleate-labelled remnant removed from the perfusate was similar for livers from rats fed on the fat-supplemented diets irrespective of the type of fat fed, whereas livers from rats fed on the low-fat diet removed more labelled fish oil and butter fat remnants than olive, maize or palm oil remnants. Following hepatic uptake in the fat-supplemented groups, the oxidation of [14C]oleate-labelled remnant lipid from maize oil, fish oil, and butter fat remnants was greater than that of the lipids from olive and palm oil remnants, although only the oxidation of lipids from maize and palm oil remnants was increased by prior fat-supplementation of the diet. In addition, the livers from rats fed on the fish-oil-supplemented diet incorporated more [14C]oleate-labelled remnant lipid into phospholipid compared with the livers from rats fed on the other fat-supplemented diets or the low-fat diets. These investigations show that both prior fat feeding and the composition of the fat fed, as well as the fatty acid composition of the chylomicron remnant particles themselves, influence the uptake and metabolism of chylomicron remnants by the liver.

Differential effects of chylomicron remnants derived from corn oil or palm oil on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes

The effects of chylomicron remnants derived from corn oil (rich in n-6 polyunsaturated fatty acids) and palm oil (rich in long chain saturated fatty acids) on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes were studied. Incubation of the cells with corn oil remnants led to increased bile acid production, while the secretion of lipid in very low density lipoprotein remained unchanged. In contrast, addition of palm oil remnants to the medium did not affect bile acid synthesis, but resulted in the secretion of cholesterol-rich very low density lipoprotein. These findings show that chylomicron remnants of different fatty acid composition have differential effects on cholesterol metabolism in liver cells, and provide part of the explanation for the hyper- and hypocholesterolaemic effects of saturated and polyunsaturated fatty acids.

Hepatic uptake and processing of cholesterol and cholesteryl ester from chylomicron remnants: An in vivo study in the rat

The metabolic fate of cholesterol in chylomicron remnants was studied after intravenous infusion into biliary drained rats. The remnants were radiolabelled with [3H]cholesterol in vivo, so that radioactivity was incorporated into both the unesterified (27% of label) and esterified (73% of label) cholesterol fractions, or with 14C-labelled unesterified cholesterol after exchange in vitro. Blood and bile samples were collected at intervals for 180 min, after which the animals were killed for analysis. The total amount of radioactivity found in the liver (46%) and bile (4.5%) after infusion of [3H] remnants was higher than that found when the label was 14C (33 and 2.7%, respectively). Radioactivity from 14C-labelled unesterified cholesterol was cleared more rapidly from the blood, but the distribution of the label between the lipoprotein fractions VLDL + LDL, HDL2 and HDL3 at the end of the experiment was similar to that found when total [3H]cholesterol was used. In experiments with both types of label, approximately 94% of the total radioactivity secreted into bile was associated with the bile acid, with only about 6% in biliary unesterified cholesterol, and these proportions did not change during 180 min. When the chylomicron remnants were labelled with total [3H]cholesterol the specific radioactivity of the bile acid, taurochenodeoxycholic acid, in the bile was approximately twice that observed when the label was unesterified [14C]cholesterol. The specific radioactivity of unesterified biliary cholesterol was very low in the latter case, but higher and more comparable to that of taurochenodeoxycholic acid in the former. Thus, the metabolic fate of chylomicron remnant cholesterol differs, depending on whether it is in the esterified or unesterified form, suggesting that hepatic cholesterol originating from the two fractions may mix to a different extent with the various intracellular pools. In addition, the experiments with 14C indicate that the behaviour of chylomicron remnant unesterified cholesterol resembles that exhibited by cholesterol in HDL more than that carried in VLDL or LDL.

Evaluation in vivo of the differential uptake and processing of high-density lipoprotein unesterified cholesterol and cholesteryl ester in the rat.

The uptake and processing of high-density lipoprotein (HDL) unesterified and esterified cholesterol were compared in vivo in the rat. HDL labelled with 3H in either unesterified cholesterol or cholesteryl ester was administered intravenously, and the clearance of radioactivity from the blood, its distribution in plasma lipoprotein density fractions, uptake by tissues, and appearance in bile were studied at intervals up to 180 min. 3H in HDL unesterified cholesterol was cleared more rapidly from the blood than that in HDL cholesteryl ester, and this difference was mainly due to rapid sequestration of [3H]unesterified cholesterol by the liver, with 58.2% of the administered dose found in this tissue after 10 min, compared to 6.8% of the [3H]cholesteryl ester dose. Non-hepatic tissues took up only a small proportion of the administered label from both HDL unesterified and esterified cholesterol, but on a per gram wet weight basis, the specific uptake of HDL cholesteryl ester in the adrenal glands and the spleen was higher than in the liver, particularly in the first 60 min. The distribution of radioactivity in the plasma lipoprotein density fractions remained constant between 10 and 180 min when [3H]unesterified cholesterol was used, but the proportion of plasma radioactivity from HDL labelled in esterified cholesterol in the very-low-density lipoprotein (VLDL) fraction increased from 0% to 26%, while in HDL there was a shift in the distribution of radioactivity from the most (d 1.125-1.250 g/ml) to the least (d 1.050-1.085 g/ml) dense sub-fractions. A greater percentage of the administered label from HDL unesterified cholesterol (8.8%) than from HDL cholesteryl ester (3.3%) was secreted into bile during 180 min, but the proportions secreted in bile acids and unesterified cholesterol were similar with both labels. These findings indicate that there are significant differences in the uptake and processing of HDL unesterified as compared to esterified cholesterol in the rat in vivo.