The degradation of very low density lipoprotein by the extrahepatic tissues of the rat

Very low density lipoprotein metabolism after insulin over-treatment and during a euglycaemic clamp

The disturbance of very low density lipoprotein (VLDL) metabolism that occurs as a result of intensive insulin treatment and during a euglycaemic clamp have been investigated in a rat model. Normal rats were maintained with fed blood glucose levels below 5 mmol l-1 for 8 weeks by subcutaneous insulin injections (normal fed levels 5.8 +/- 0.4 (SD) mmol l-1). Glucose requirement to maintain a glucose clamp was significantly reduced (116 +/- 3 mumol min-1 kg-1 (SE) vs. 173 +/- 5 mumol min-1 kg-1, P less than 0.001), compared with weight-matched normal control rats. In the fasting state (blood glucose 3.5 +/- 0.2 mmol l-1 vs. 3.9 +/- 0.1 mmol l-1, NS) plasma non-esterified fatty acid levels were reduced. Fasting VLDL-triglyceride turnover, measured by bolus injection of 14C-VLDL, was also lower (3.17 +/- 0.12 mumol min-1 kg-1 vs. 3.50 +/- 0.07 mumol min-1 kg-1, P less than 0.05). Despite decreased turnover, insulin over-treated rats had normal plasma triglyceride concentrations indicating a removal defect. At the end of a 3-h euglycaemic clamp, plasma triglyceride concentrations and VLDL-triglyceride turnover were decreased in both normal control and insulin over-treated animals, and turnover remained significantly lower in the insulin over-treated rats (2.59 +/- 0.13 mumol min-1 kg-1 vs. 3.08 +/- 0.10 mumol min-1 kg-1, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Retention of apolipoprotein B and cholesterol by perfused heart during lipolysis of very-low-density lipoprotein

The fate and mechanism of removal of apolipoproteins and lipids of human very-low-density lipoproteins were determined in the perfused rat heart. Approx. 50% of the VLDL triacylglycerol was hydrolyzed during a 2 h perfusion. Phospholipid phosphorus, apolipoproteins C-II, C-III and E were quantitatively recovered in the medium. However, there was a loss of unesterified (17 +/- 6%) and esterified (19 +/- 8%) cholesterol from the perfusion medium. Apolipoprotein B was retained by the heart, as determined by the loss of immunoassayable apolipoprotein B (30 +/- 5%) or the uptake of 125I-labelled apolipoprotein of VLDL (9 +/- 2%) from the perfusion medium. The discrepancy in the two methods for estimating apolipoprotein removal was shown to be due to the modification of apolipoprotein B-containing lipoproteins, which was such that they were no longer precipitated with antibodies to apolipoprotein B. The labelled apolipoprotein B, retained by the heart, could be partially released by perfusion of the heart with buffer containing heparin (14 +/- 2%) or trypsin (50 +/- 2%). Labelled apolipoprotein uptake by the heart was reduced by 90% when lipoprotein lipase was first released by heparin or when VLDL was treated with 1,2-cyclohexanedione to modify arginine residues of apolipoproteins. Very little extensive degradation of the apoprotein to low molecular weight material occurred during the 2 h perfusion, since 95% of the tissue label was precipitated by trichloroacetic acid. It is concluded that there is retention of apolipoprotein B, cholesteryl ester and cholesterol by the perfused heart during catabolism of VLDL. The data are consistent with the concept that the retention of apolipoprotein B requires membrane-bound lipoprotein lipase or an interaction with the cell surfaces that is modified by heparin. The overall process also involves arginine residues of apolipoproteins. At least 50% of the labelled apolipoprotein retained in the tissue is associated with lipoprotein lipase and other cell surface sites, while the remainder may be taken up by the cells.

Metabolic fate of VLDL apolipoproteins B and E in hepatectomized rats

The metabolic fate of VLDL apolipoproteins B and E was examined in functionally hepatectomized rats. 1 h after hepatectomy, there was almost complete absence of ultracentrifugally isolated VLDL lipid and protein, including apolipoproteins B and E. Analysis of apolipoprotein concentrations by electroimmunoassay showed hepatectomy did not affect the total serum concentrations of apolipoproteins B and E; thus, hepatectomy caused a redistribution of these apolipoproteins from VLDL to higher density lipoproteins. In the LDL (d = 1.03--1.063 g/ml) fraction, hepatectomy, increased the concentrations of free cholesterol (40%), esterified cholesterol (57%) and protein (18--67%), due to an increase in apolipoproteins B (22--48%) and E (250--300%). After hepatectomy, the HDL fraction accumulated the greatest total amount of apolipoprotein E. Since the majority of apolipoprotein E was isolated in the d greater than 1.21 g/ml fraction after sequential ultracentrifugation, the redistribution of apolipoproteins B and E was further defined by fractionation of serum on 5 M agarose columns. Electroimmunoassay of the column fractions showed that the apolipoprotein B peak eluted before the apolipoprotein E peak. Although a considerable portion of apolipoprotein E eluted with A-I, the peak of apolipoprotein E eluted before the A-I peak in both groups. These data suggest that a portion of apolipoprotein E is associated with particles which are similar than LDL but are larger than A-I-rich HDL. Hepatectomy caused an accumulation of apolipoprotein B in LDL, and apolipoprotein E and cholesterol in particles which were smaller than LDL and may represent LDL1. It is likely that under normal physiological conditions the liver plays a role in the removal of these apolipoprotein E-rich particles which are derived, at least in part, from the metabolism of VLDL.

The removal of partially metabolized very-low-density lipoproteins by the perfused rat liver

1. Donor perfused rat livers were used to prepare VLD (very-low-density) lipoproteins, labelled in their triacylglycerol and protein components with [1-14C]oleic acid and L-[4,5-3H]leucine respectively. Partially metabolized VLD lipoproteins, similarly labelled, were obtained from supradiaphragmatic rats injected with the parent VLD lipoproteins. 2. The triacylglycerol and protein components of the partially metabolized VLD lipoproteins were removed by recipient perfused rat livers at rates much higher than those of the parent VLD lipoproteins. No degradation of the partially metabolized VLD lipoproteins to LD (low-density) lipoproteins occurred during the perfusions. 3. Removal of hepatic lipase from the livers did not significantly affect the rate of removal of the partially metabolized VLD lipoproteins.

[Effect of diurnal distribution of food intake on 24-h profiles of plasma lipoproteins (author's transl)]

The lipid infiltration theory of atherogenesis accepted, 24 h lipoprotein profiles may be more relevant than preprandial morning samples. Such profiles were performed in 12 metabolically healthy volunteers during two dietetic regimes identical in total food content but differing in the distribution over the day: form A meant an evening meal of 15% of total caloric intake, form B of 40%. After one week of each form, 24 h lipoprotein profiles differed significantly in the time course of triglyceride rich lipoproteins and in the mean values over 24 h in VLDL and LDL phospholipids and HDL cholesterol. These findings are cautiously interpreted as possible signs of differences in the catabolism of triglyceride rich lipoproteins, remnants and intermediate lipoproteins. The difference in HDL cholesterol which was higher in form A is discussed in the context of recent epidemiologic evidence.

Atherogenesis: a postprandial phenomenon

The hypothesis that plasma chylomicrons in persons who ingest a cholesterol-rich diet are atherogenic is evaluated. Evidence is presented that in humans, and experimental animals, chylomicron remnants as well as low-density lipoproteins are taken up by arterial cells. In persons who do not have familial hyperlipoproteinemia, atherogenesis may occur during the postprandial period. Research directions that may contribute to the evaluation of chylomicron remnants as a risk factor for atherogenesis are discussed. Lipoprotein studies after administration of a test meal containing fat and cholesterol are urgently needed.