Coconut oil (CO) induces a triacylglycerol infiltration in the hepatocytes of preruminant calves when given as the sole source of fat in the milk diet over a long-term period. Metabolic pathways potentially involved in this hepatic triacylglycerol accumulation were studied by in vitro methods on liver slices from preruminant Holstein × Friesian male calves fed a conventional milk diet containing CO (n 5) or beef tallow (BT, n 5) for 19 d. Liver slices were incubated for 12 h in the presence of 0·8 mM-[14C] oleate or -[14C] laurate added to the medium. Fatty acid oxidation was determined by measuring the production of CO2 (total oxidation) and acid-soluble products (partial oxidation). Production of CO2 was 1·7–3·6-fold lower (P 0·0490) and production of acid-soluble products tended to be lower (P = 0·0625) in liver slices of CO- than BT-fed calves. Fatty acid esterification as neutral lipids was 2·6– to 3·1–fold higher (P = 0·0088) in liver slices prepared from calves fed the CO diet compared with calves fed the BT diet. By contrast with what occurs in the liver of rats fed CO, the increase in neutral lipid production did not stimulate VLDL secretion by the hepatocytes of calves fed with CO, leading to a triacylglycerol accumulation in the cytosol. It could be explained by the reduction of fatty acid oxidation favouring esterification in the form of triacylglycerols, in association with a limited availability of triacylglycerols and/or apolipoprotein B for VLDL packaging and subsequent secretion.
Secretion of triglycerides by the liver in ruminants as components of very low density lipoproteins particles is low as compared with that in primates or rodents. The rate-limiting steps for the hepatic export of very low density lipoproteins have been studied in liver slices to determine the origin of the low lipotropic capacity of calf liver compared to that of rat liver. The rates of production of apolipoprotein B (apo B) and albumin as well as the rate of secretion of VLDL-apolipoproteins were measured during 12-h incubation of liver slices in organo-culture using [35S]methionine-cysteine labeling. Hepatic apo B production was similar in the two animal species but the VLDL-apolipoprotein secretion rate for calf liver slices amounted to only 20% of that observed for rat liver slices. Although calf and rat liver slices synthesized similar amounts of total protein, the hepatic production of albumin, measured in cells and media, was much higher in calf than rat liver slices (around 2.7-fold), whereas the rate secretion of albumin was similar in the two species. Our results showed that the slow rate of secretion of VLDL by calf liver cells was not consecutive to a low rate of synthesis of apo B but rather to a defect in VLDL assembly and/or secretion.
The health value for man of lipids in bovine muscles can be improved by the addition of PUFA to the animals' diets, but such treatments can modify fluidity of plasma lipoproteins and therefore their metabolic functions. The aim of the present study was to analyse whether changes in chemical composition of lipoproteins in steers fed sunflower oil-rich diets altered lipoprotein fluidity, measured by fluorescence polarization and electron spin resonance. LDL, light HDL and heavy HDL fractions were isolated by ultracentrifugation from plasma of eighteen crossbred Charolais £ Salers steers. For a period of 70 d, animals were given a control diet (C, n 6) consisting of hay (540 g/kg) and concentrate mixture (460 g/kg) or the same basal diet supplemented with sunflower oil rich in n-6 PUFA (40 g/kg diet DM), given either as crushed seeds (S, n 6) or as a free oil infused directly into the duodenum (O, n 6), thus avoiding ruminal hydrogenation of PUFA. We have shown that in bovine animals: (1) fluidity measurements by fluorescence polarization must be made at the bovine physiological temperature (38·58C); (2) heavy HDL always appear as the less fluid lipoparticles; (3) electron spin resonance, which does not depend on lipoparticle size, is more appropriate to compare the fluidity of LDL with that of light HDL. The values for lipoprotein fluidity measured by both methods indicated that linoleate-rich diets did not have any effect when compared with diet C; however, chemical variables support a fluidification of lipoparticles, since in steers given the diet O, n-6 PUFA concentrations increased in polar ( £ 1·8) and neutral ( £ 1·6) lipids in lipoparticles (P¼ 0·0001). The phospholipid:protein ratio increased in light (þ20 %, P¼0·019) and heavy (þ23 %, P¼ 0·06) HDL and especially in LDL (þ 46 %, P¼0·0001); the total cholesterol:phospholipid ratio decreased in the three lipoprotein classes (2 15 to 2 30 %, NS). Diet S led to similar but less pronounced effects. We concluded that linoleate-rich diets modified the chemical composition of plasma lipoproteins in steers, but did not alter their fluidity; this probably occurred as a result of 'homeoviscous adaptation', which ensured their functional capacity.Bovine: Fluidity of lipoproteins: Dietary n-6 polyunsaturated fatty acids
Incorporation of coconut oil (CO) rich in lauric acid into the milk diet induces a lipid infiltration of the liver (steatosis) in 1-month-old calves. Among possible steps involved in diet-induced liver steatosis, the ability of the calf liver to synthesize apolipoprotein (Apo) B and to secrete it as part of VLDL particles was investigated. Liver samples were taken from calves fed for 17 d on a conventional milk replacer containing CO (n 5) and beef tallow (BT, n 4) as reference. 35 S]ApoB in liver cells were 2-fold lower P 0´08Y 0´0004 and 0´03 respectively) in COthan in BT-fed calves. Although the total amount of proteins secreted (including albumin) was similar in both groups of calves, the amount of VLDL-[35 S]Apo secreted was 2-fold lower P 0´004 in CO-than in BT-fed calves. These results suggest that a CO-enriched milk diet induces in preruminant calves a lipid infiltration of the liver by decreasing ApoB synthesis, leading to a reduction in secretion of VLDL particles.
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