Effect of atorvastatin on chylomicron remnant metabolism in visceral obesity: a study employing a new stable isotope breath test

Elevated plasma concentration of chylomicron remnants may be causally related to atherosclerosis in obesity. We examined the effect of atorvastatin on chylomicron remnant metabolism in 25 obese men with dyslipidaemia. A remnant-like emulsion labeled with cholesteryl [(13)C]oleate was injected intravenously into patients; the fractional catabolic rate (FCR) of the remnant-like emulsion was determined by measurement of (13)CO(2) in the breath and analyzed using compartmental modelling. Compared with placebo, atorvastatin significantly decreased the plasma concentrations of total cholesterol, triglycerides, LDL cholesterol, apolipoprotein B (apoB), and lathosterol (P < 0.001). ApoB-48 and remnant-like particle-cholesterol (RLP-C) both decreased significantly by 23% (P = 0.002) and 33% (P = 0.045), respectively. The FCR of the remnant-like emulsion increased significantly from 0.054 +/- 0.008 to 0.090 +/- 0.010 pools/h (P = 0.002). The decrease in RLP-C was associated with the decrease in plasma triglycerides (r = 0.750, P = 0.003). Furthermore, the change in FCR of remnant-like emulsions was inversely associated with the change in LDL-C (r = -0.575, P = 0.040), suggesting removal of LDL and chylomicron remnants by similar hepatic receptor pathways. We conclude that in obese subjects, inhibition of cholesterol synthesis with atorvastatin decreases the plasma concentrations of both LDL-C and triglyceride-rich remnants and that this may be partially due to an enhancement in hepatic clearance of these lipoproteins.     

Obesity and post-prandial lipid metabolism. Feast or famine?

Both in Western countries and in third world countries there is an increasing incidence of obesity. Obesity per se or insulin resistance associated with obesity may increase cardiovascular risk factors including dyslipidemia, hypertension and Type 2 diabetes. Over the past decade the understanding has increased of specific mediators in the hypothalamus that are involved in regulating food intake and body weight. In obese humans fasting plasma lipids can be normal but postprandial lipid metabolism is abnormal with an accumulation of triglyceride-rich remnant lipoproteins. In viscerally obese men chylomicron remnant catabolism was markedly decreased when compared with lean individuals. The decreased clearance of chylomicron remnants in viscerally obese subjects may be explained by competition between chylomicron remnants and the increased hepatic production of VLDL for clearance by low density lipoprotein receptors. Increased food intake in rodent models of obesity was shown to be associated with a delay in the catabolism of remnant lipoprotein particles. Prevention of hyperphagia was found to correct the impairment in the metabolism of remnant lipoproteins. Under fasting and food restricted conditions the improvement of remnant metabolism was associated with an increased oxidation of remnant lipids as determined by a novel stable isotope breath test. Anti-obesity and lipid lowering drugs have been used for the treatment of obesity. Inhibitors of cholesterol synthesis inhibitors (statins) have been shown to be effective in treating dyslipidemia. Inhibition of cholesterol synthesis with Atorvastatin was shown to improve chylomicron metabolism by increasing chylomicron remnant catabolism in obese subjects as assessed by the newly developed stable isotope breath test.     

Competition between chylomicrons and their remnants for plasma removal: a study with artificial emulsion models of chylomicrons

In previous studies, protein-free emulsions of defined lipid composition were shown capable of simulating either the metabolism of chylomicrons (chylomicron-like emulsion) or their remnants (remnant-like emulsion), depending on the content of free, unesterified cholesterol. To validate further the assumption that remnant-like and chylomicron-like emulsion have metabolic pathways in common with their natural counterparts, studies of competition for plasma removal were undertaken: the remnant-like emulsion labeled with [3H]triolein was injected sequentially twice in the carotid arteries of rats to compare the clearance of remnant-like emulsion of the second injection with the first (control). Prior to the second injection, a large bolus of the chylomicron-like emulsion or rat lymph chylomicron was injected, to check the hypothesis that remnant generated from chylomicron-like emulsion or natural chylomicrons could compete with and displace remnant-like emulsion particles from their tissue receptor sites. Experiments were also performed in rats treated with Triton WR-1339, to block the generation of remnants. Results showed that remnants derived from either natural chylomicrons or chylomicron-like emulsion both strongly competed with the remnant-like emulsion. In contrast, when transformation of remnants was prevented by Triton, the undegraded particles of chylomicron-like emulsion or natural chylomicron were unable to compete with or displace remnant-like emulsion from its sites of removal from the plasma. In agreement with plasma clearance data, the hepatic uptake of the remnant-like emulsion was inhibited by the surplus dose of natural chylomicrons. In contrast, the spleen uptake was unaffected by it.     

The effects of Triton WR-1339, protamine sulfate and heparin on the plasma removal of emulsion models of chylomicrons and remnants in rats

Protein-free lipid emulsions with compositions modelling chylomicrons (chylomicron-like emulsion) or chylomicron remnants (remnant-like emulsion) were injected intra-arterially into nonanesthetized rats. Compared with control untreated rats, treatment with Triton WR-1339, protamine sulfate or heparin strongly modified the plasma removal of triacylglycerols and cholesteryl ester moieties of chylomicron-like emulsions, but had little effect on removal rates of triacylglycerols or cholesteryl esters of remnant-like emulsions. The effects on chylomicron-like removal were similar to those on natural lymph chylomicrons. The relative lack of effects on remnant-like emulsion removal provides additional evidence that remnant-like emulsions are a metabolic model for natural chylomicron remnants.     

Relative roles of LDLr and LRP in the metabolism of chylomicron remnants in genetically manipulated mice

Remnant-like emulsions labeled with cholesteryl [(13)C]-oleate were prepared with lipid compositions similar to remnants derived from triacylglycerol-rich lipoproteins. When injected into the bloodstream of conscious mice, the remnant-like emulsions were metabolized in the liver leading to the appearance of (13)CO(2) in the breath. Previously, using this technique, we found that remnant metabolism was significantly impaired but not completely inhibited in mice lacking low density lipoprotein receptors (LDLr). We have now found in mice with non-functional low density lipoprotein receptor-related protein (LRP) that breath enrichment of (13)CO(2) was significantly decreased, indicating that the LRP also plays an important role in the metabolism of chylomicron remnants (CR). The enrichment of (13)CO(2) in the expired breath was negligible in mice lacking both LDLr and receptor-associated protein (-/-), essential for normal function of LRP. In mice pre-injected with gluthatione S-transferase-receptor-associated protein to block LRP binding, there was a marked inhibition of the appearance of (13)CO(2) in the expired breath of homozygous LDLr-deficient mice, supporting the role of LRP in vivo. Whether or not LDLr were present, in mouse and human fibroblast cells human apoE3 or E4 but not apoE2 were essential for binding of remnant-like emulsions, while lactoferrin and suramin completely inhibited binding. We conclude that in normal mice LDLr are important for the physiological metabolism of CR. When LDLr are absent the evidence supports a role for the LRP in the uptake of CR in liver cells and in fibroblasts, with binding characteristics for CR-associated apoE similar to LDLr.     

Kinetics of chylomicron remnant clearance in normal and in hyperlipoproteinemic subjects

The kinetics of chylomicron metabolism have been studied by measuring retinyl palmitate in chylomicrons and their remnants for 10-12 hr following oral administration of vitamin A and Lipomul in three groups of adult male subjects: A) normal plasma triglyceride levels (n = 7); B) endogenous hypertriglyceridemia (n = 12); C) apolipoprotein E (apoE) phenotype E2/2, with Type 3 hyperlipoproteinemia (n = 4) or normal plasma lipids (n = 1). A multicompartmental model was developed using SAAM 27 to characterize the appearance, intravascular metabolism, and clearance from the plasma of retinyl palmitate-labeled dietary lipoproteins. The half-times for retinyl palmitate clearance from the chylomicron remnant fraction (T1/2 REMNANT) were 14.1 +/- 9.7 min in Group A; they were prolonged in Group B (50.7 +/- 20.8 min) and were extremely prolonged for Type 3 subjects in Group C (611.9 +/- 419.9 min). One subject with the apoE 2/2 phenotype and normal plasma triglycerides had a T1/2 REMNANT of 66.8 min. T1/2 REMNANT was highly correlated with fasting plasma triglycerides in Group A and B (r = 0.77, slope = 0.15), and in Group C (r = 0.97, slope = 0.85). These results support the interpretation that delayed chylomicron remnant clearance in subjects with endogenous hypertriglyceridemia may be largely secondary to overproduction of VLDL particles, whose remnants compete with chylomicron remnants for removal by the liver via apoE receptor-mediated endocytosis. The subjects with apoE 2/2 have an additional defect in the removal of chylomicron remnants presumably due to the structural abnormality in their apoE.     

Effects of cholesterol content on the metabolism of protein-free emulsion models of lipoproteins

After intravenous injection, emulsions with compositions similar to chylomicrons behaved metabolically as described for chylomicrons, with faster removals of triacylglycerols than cholesteryl esters from the blood after injection into rats, and with greater uptakes of cholesteryl esters than triacylglycerols by the liver. In contrast, emulsions with a high content of free cholesterol showed equal removal rates from the blood of triacylglycerols and cholesteryl esters; and similar uptakes by the liver. This pattern of metabolism was that expected for a chylomicron core remnant particle. Emulsions poor in cholesteryl ester but rich in free cholesterol showed remnant-like behavior, whereas emulsions rich in cholesteryl ester but poor in free cholesterol were metabolized like nascent chylomicron particles. The amount of free cholesterol appeared to regulate metabolism by affecting the binding of apolipoproteins to the particle surface. Emulsions with a high content of free cholesterol bound less A-I, A-IV and C apolipoproteins, and the relative amount of apolipoprotein E was increased. All of these effects are consistent with the metabolic differences between chylomicrons and remnant particles, suggesting that the amount of free cholesterol plays a regulatory role in chylomicron metabolism.     

The diurnal rhythms of cholesterol metabolism and plasma clearance of model chylomicrons: comparison of normal and genetically hypercholesterolemic rats (RICO)

Previous studies showed a slower clearance of cholesterol-labeled lymph chylomicrons in genetically hypercholesterolemic rats (RICO) compared with normocholesterolemic rats. In this study, we compared rates of lipolysis and remnant clearance in RICO versus control normocholesterolemic rats of the same strain (RAIF) or with control Wistar rats, by injecting chylomicron-like lipid emulsions labeled with ¹⁴C-triolein to trace lipolysis, and ³H-cholesteryl ester to trace remnant clearance. Our findings showed slower clearance of chylomicron remnants in RICO compared with control RAIF or with control Wistar rats. During the light period, the clearance of lipids from chylomicron-like lipid emulsions injected intravenously was significantly slower in RICO rats compared with normocholesterolemic control rats of the same strain, RAIF. Within the RICO group, clearance of emulsion triolein (TO) was faster during the dark period compared with the light period. In contrast, however, the clearance of the emulsion remnants traced by cholesteryl oleate (CO) was slower during the dark period. This behaviour was not found within the Wistar group, where the clearances of TO and CO were similar in the light and dark period. Hepatic clearance of chylomicron remnants is mediated primarily by the low density lipoprotein (LDL) receptor, the expression of which shows diurnal variation. In both Wistar and RICO rats, the expression of LDL receptors was highest during the dark period. The LDL receptors in hepatic microsomal membranes from RICO rats migrated faster on SDS polyacrylamide gel electrophoresis when compared with normal Wistar and the RAIF. However in hepatic plasma membranes the LDL receptors from RICO and Wistar rats appeared identical after immunoblotting. Furthermore the LDL receptors from RICO and Wistar rats responded similarly to treatment with neuraminidase. An alteration in post-translational processing of the LDL receptor could possibly account for the slower clearance of chylomicron remnants in the RICO.     

Clearance of chylomicron remnants in normolipidaemic patients with coronary artery disease: case control study over three years.

OBJECTIVE--To test the hypothesis that subjects who clear chylomicron remnants slowly from plasma may be at higher risk of coronary artery disease than indicated by their fasting plasma lipid concentrations. DESIGN--Case control study over three years. SETTING--An 800 bed general municipal hospital. SUBJECTS--85 normolipidaemic patients with coronary artery disease selected prospectively and matched with 85 normolipidaemic subjects with normal coronary arteries on angiography. INTERVENTIONS--All subjects were given a vitamin A fat loading test which specifically labels intestinal lipoproteins with retinyl palmitate. MAIN OUTCOME MEASURE--Postprandial lipoprotein metabolism. RESULTS--The area below the chylomicron remnant retinyl palmitate curve was significantly increased in the coronary artery disease group as compared with the controls (mean 23.4 (SD 15.0) v 15.3 (8.9) mumol/l.h; 95% confidence interval of difference 4.37 to 11.82). CONCLUSION--Normolipidaemic patients with coronary artery disease had significantly higher concentrations of chylomicron remnants in plasma than normolipidaemic subjects with normal coronary vessels. This may explain the mechanism underlying the susceptibility to atherosclerosis of coronary artery disease patients with normal fasting lipid values. As diet and drugs can ameliorate the accumulation of postprandial lipoproteins in plasma, the concentration of chylomicron remnants should be measured in patients at high risk of coronary artery disease.     

Contraceptive steroids increase hepatic uptake of chylomicron remnants in healthy young women

In an investigation of alterations in cholesterol metabolism during contraceptive steroid use, we studied plasma clearance of chylomicron remnants. Six healthy women were studied on and off contraceptive steroid therapy. Remnant clearance was measured from the disappearance of retinyl palmitate administered intravenously in plasma endogenously labeled with retinyl palmitate. We also measured cholesterol in HDL and its subfractions and postheparin lipoprotein lipase and hepatic triglyceride lipase activities. Plasma decay of retinyl palmitate was biexponential. The rapid component, reflecting chylomicron remnant removal, accounted for about 90% of the total clearance in all studies. During contraceptive steroid intake, both rapid and slow decay constants and the calculated plasma clearance rates were significantly increased (mean values: rapid decay constant, control 0.048 versus treated 0.101 min-1, P less than 0.05; slow decay constant, 0.004 versus 0.014 min-1, P less than 0.01; plasma clearance 74 versus 115 ml/min, P less than 0.025) indicating enhanced hepatic uptake of chylomicron remnants and probably an increased hepatic uptake of higher density lipoproteins (d greater than 1.006 g/ml). Total postheparin lipolytic activity and lipoprotein lipase activity were depressed in all six women (P less than 0.05) and hepatic triglyceride lipase activity was increased in four of five subjects. Contraceptive steroids also caused a decrease in the HDL2/HDL3 cholesterol ratio (P less than 0.05), implying impaired peripheral lipoprotein triglyceride hydrolysis and/or increased HDL2 clearance by hepatic triglyceride lipase. In conclusion, during intake of contraceptive steroids, the plasma clearance of chylomicron remnants and higher density lipoproteins was increased.(ABSTRACT TRUNCATED AT 250 WORDS)     

High cholesterol absorption efficiency and rapid biliary secretion of chylomicron remnant cholesterol enhance cholelithogenesis in gallstone-susceptible mice

The study of chylomicron pathway through which it exerts its metabolic effects on biliary cholesterol secretion is crucial for understanding how high dietary cholesterol influences cholelithogenesis. We explored a relationship between cholesterol absorption efficiency and gallstone prevalence in 15 strains of inbred male mice and the metabolic fate of chylomicron and chylomicron remnant cholesterol in gallstone-susceptible C57L and gallstone-resistant AKR mice. Our results show a positive and significant (P<0.0001, r=0.87) correlation between percent cholesterol absorption and gallstone prevalence rates. Compared with AKR mice, C57L mice displayed significantly greater absorption of cholesterol from the small intestine, more rapid plasma clearance of chylomicrons and chylomicron remnants, higher activities of lipoprotein lipase and hepatic lipase, greater hepatic uptake of chylomicron remnants, and faster secretion of chylomicron remnant cholesterol from plasma into bile. All of these increased susceptibility to cholesterol gallstone formation in C57L mice. We conclude that genetic variations in cholesterol absorption efficiency are associated with cholesterol gallstone formation in inbred mice and cholesterol absorbed from the intestine provides an important source for biliary hypersecretion. Differential metabolism of the chylomicron remnant cholesterol between C57L and AKR mice clearly plays a crucial role in the formation of lithogenic bile and gallstones.     

Effects of exogenous apo E-3 and of cholesterol-enriched meals on the cellular metabolism of human chylomicrons and their remnants

The effects of exogenous apo E-3 and of cholesterol-enriched meals on the binding, cell association and proteolytic degradation of human chylomicrons and their remnants were determined in cultured human skin fibroblasts. Chylomicrons were prepared from plasma of normolipemic humans 4 h after a fat meal with normal or high cholesterol content. Remnants were obtained after incubation of chylomicrons with lipoprotein lipase in vitro. Cellular metabolism of chylomicrons was minimal, less than 10% that of LDL. Exogenous apo E-2 enhanced chylomicron metabolism by 3-4-fold. The cellular metabolism of remnants was 2.5-3.5-fold higher as compared to intact chylomicrons but their response to exogenous apo E-3 was considerably lower. The cellular metabolism of chylomicrons and chylomicron remnants obtained from subjects eating cholesterol-enriched fat meal was the highest either without or with added exogenous apo E-3. Yet, even in the preparation that exhibits the highest metabolic activity (apo E-3 enriched remnants from cholesterol-enriched meals) the absolute proteolytic degradation was about two-thirds that of LDL. We conclude that although LDL-receptors take up and degrade chylomicron remnants, the rate of catabolism of remnants by this route can not explain the rapid and complete remnant removal process as observed in vivo.     

Scavenger receptor BI plays a role in facilitating chylomicron metabolism

The function of scavenger receptor class B type I (SR-BI) in mediating the selective uptake of high density lipoprotein (HDL) cholesterol esters is well established. However, the potential role of SR-BI in chylomicron and chylomicron remnant metabolism is largely unknown. In the present investigation, we report that the cell association of 160 nm-sized triglyceride-rich chylomicron-like emulsion particles to freshly isolated hepatocytes from SR-BI-deficient mice is greatly reduced (>70%), as compared with wild-type littermate mice. Competition experiments show that the association of emulsion particles with isolated hepatocytes is efficiently competed for (>70%) by the well established SR-BI ligands, HDL and oxidized low density lipoprotein (LDL), whereas LDL is ineffective. Upon injection into SR-BI-deficient mice the hepatic association of emulsion particles is markedly decreased ( approximately 80%) as compared with wild-type mice. The relevance of these findings for in vivo chylomicron (remnant) metabolism was further evaluated by studying the effect of SR-BI deficiency on the intragastric fat load-induced postprandial triglyceride response. The postprandial triglyceride response is 2-fold higher in SR-BI-deficient mice as compared with wild-type littermates (area-under-the-curve 39.6 +/- 1.2 versus 21.1 +/- 3.6; p < 0.005), with a 4-fold increased accumulation of chylomicron (remnant)-associated triglycerides in plasma at 6 h after intragastric fat load. We conclude that SR-BI is important in facilitating chylomicron (remnant) metabolism and might function as an initial recognition site for chylomicron remnants whereby the subsequent internalization can be exerted by additional receptor systems like the LDL receptor and LDL receptor-related protein.     

Defective plasma clearance of chylomicron-like lipid emulsions in Watanabe heritable hyperlipidemic rabbits

Human patients with familial hypercholesterolemia (FH) and Watanabe heritable hyperlipidemic rabbits (WHHL), while lacking normal receptors recognizing low-density lipoproteins (LDL), are said to have normal clearance of chylomicrons. In the present study, emulsions with a similar lipid composition to chylomicrons were injected intravenously in homozygous WHHL rabbits and normal control rabbits fed diet with low or high cholesterol. Radioactive labels tracing emulsion triolein and cholesteryl oleate were both removed rapidly from the bloodstream, with the removal rate of triolein always faster than that of cholesteryl oleate. This pattern was similar to the clearance of normal chylomicrons in rabbits or rats, and was consistent with the formation of remnant lipoproteins after hydrolysis of emulsion triolein by lipoprotein lipase, followed by hepatic uptake of the remnants. The removal of cholesteryl oleate was significantly slower in WHHL rabbits than in normal controls, suggesting that the absence of LDL receptor function led to impaired remnant clearance. Measured in post-heparin plasma, the activity of lipoprotein lipase was decreased in WHHL rabbits, but this was not associated with clear evidence of defective lipolysis of emulsion triolein. Apolipoprotein E did not appear to be deficient in WHHL rabbits. Plasma devoid of lipoproteins less than 1.006 g/ml from WHHL and normal control rabbits transferred similar amounts of apolipoprotein E to chylomicron-like emulsions after incubation. Impaired clearance of chylomicron remnants possibly contributes to the hypertriglyceridemia of WHHL rabbits and to accelerated atherogenesis when the function of LDL receptors is defective.     

Effects of cholesterol in chylomicron remnant models of lipid emulsions on apoE-mediated uptake and cytotoxicity of macrophages

Chylomicron remnants have been suggested to be involved in the development of atherosclerosis. To investigate the mechanisms of chylomicron remnant-induced atherosclerosis, we prepared cholesterol (Chol)-containing emulsion particles as models for chylomicron remnants. Chol markedly increased the apolipoprotein E (apoE) binding maximum of emulsions without changing the binding affinity and thereby promoted emulsion uptake by J774 macrophages. Fluorescence measurements showed that Chol increased acyl chain order and head group hydration of the surface phospholipid (PL) layer of emulsions. The binding maximum of apoE was closely correlated with the hydration and the increase in the PL head group separation at the emulsion surface. From experiments using inhibitors for lipoprotein receptors, heparan sulfate proteoglycans and low density lipoprotein receptor-related protein were found to be the major contributors to the uptake of Chol-containing emulsions. Trypan blue dye exclusion revealed that the uptake of Chol-containing emulsions induced cytotoxicity to J774 macrophages. This study proposes a mechanism of atherosclerosis induced by chylomicron remnants.     

Characterization of chylomicron remnant clearance by retinyl palmitate label in normal humans.

To characterize chylomicron remnant clearance by the liver, plasma elimination of retinyl palmitate-labeled chylomicron remnants was studied in 18 healthy subjects, ages 21-42 years. Autologous plasma containing retinyl palmitate-labeled chylomicrons and their remnants was injected intravenously, and retinyl palmitate disappearance was measured in serial plasma samples in all subjects and in lipoprotein fractions in 11 subjects. The injected doses (n = 18) ranged from 0.34 to 7.11 mumol retinyl palmitate in d less than or equal to 1.006 g/ml particles with an average molar ratio of 330/1 of retinyl palmitate/apoB-48 (n = 8). The label distributed in the intravascular space and exhibited apparent first order elimination, monoexponential in 6 and biexponential in 12 subjects. The first rapid component k1 (t1/2 18.8 +/- 11.4 min, n = 18) was shown to represent retinyl palmitate in particles of d less than or equal to 1.006 g/ml, i.e., chylomicron remnants, and the second slow component k2 (t1/2 123 +/- 62 min, n = 12) small amounts of retinyl palmitate (11 +/- 7%) injected in d greater than 1.006 g/ml particles (therefore excluded from analysis). Assuming a single-compartment model, initial rates of elimination (= dose x k1) of labeled chylomicron remnants obeyed (P = 0.06) Michaelis-Menten saturation kinetics: Km was 921 +/- 305 nmol retinyl palmitate label and Vmax 124 +/- 14 nmol/min corresponding to 0.88 nM apoB-48 for Km and 0.25 x 10(-3) nmol apoB-48.min-1.g-1 liver for Vmax. Their elimination was limited neither by the injected triglyceride dose nor theoretically by the liver blood flow. After the intake of 70 g of fat (cream) containing retinyl palmitate, the plasma retinyl palmitate concentration exceeded the estimated saturation concentration for 7 h. In conclusion, physiological chylomicron remnant catabolism by the liver appears to be saturable by ordinary lipid intake in healthy humans.     

Evaluation of the components of the chylomicron remnant removal mechanism by use of the isolated perfused mouse liver.

The isolated perfused mouse liver was utilized to evaluate the relative contribution of various molecules believed to participate in the removal of chylomicron remnants by the liver. Sixty percent of asialofetuin was removed from the perfusate per pass; bovine serum albumin was not removed. Normal mouse livers removed chylomicron remnants more efficiently (40-50%/pass) than nascent chylomicrons (10-20%/pass). The fractional removal rate of remnants decreased as their concentration in the perfusate increased demonstrating saturability. Remnant removal by livers of low density lipoprotein receptor-deficient (LDLRD) mice paralleled that of normal mice at low remnant concentrations (0.05, 0.2 microg protein/ml); as concentration increased (4-16 microg protein/ml), removal by LDLRD livers was reduced. About 50% of the capacity to remove remnants was due to the LDL receptor. The role of the LDLR-related protein (LRP) was estimated using the receptor-associated protein (RAP). Four microg/ml of RAP inhibited only LRP; it reduced the removal of remnants by 30-40% in normal livers. When RAP was included in the perfusate of LDLRD livers, remnant removal persisted but was diminished, particularly late in the perfusion; the capacity was approximately 30% of controls. The present study has established that there is more than one mechanism operating for the removal of chylomicron remnants by the liver, provides estimates of the concentration of each to the removal of remnants, and indicates a method for further studies.It is concluded that in normal livers, the LDL receptor has the greatest capacity for removing chylomicron remnants. The LRP contributes to the process as well and a third component, perhaps "sequestration," accounts for up to 30% of the capacity for the initial removal of chylomicron remnants.     

Hepatic vitamin A fat-storage cells and the metabolism of chylomicron cholesterol.

These experiments were designed to test the hypothesis that the vitamin A fat-storage cell removes chylomicron remnant cholesterol from hepatic portal venous blood; A modified Ficoll density gradient ultracentrifugation procedure was used to isolate from rat liver cellular fractions that were enriched in vitamin A. In rats fed a normal diet and in rats fed excess vitamin A isolated hepatocytes were fractionated 15 min after the intravenous injection of chylomicrons labelled in vivo with radioactive cholesterol. The results showed that cholesterol radioactivity was not concentrated in the vitamin A enriched cellular fractions, so it was concluded that the vitamin A fat-storage cell is not implicated in clearance of chylomicron remnants by the liver.     

Plasma clearance of chylomicrons labeled with retinyl palmitate in healthy human subjects

To estimate hepatic uptake of chylomicron remnants in humans, chylomicrons and intestinal very low density lipoproteins (VLDL) were endogenously labeled with retinyl esters, harvested by plasmapheresis, and pulse-injected into the donor 44 hr after plasmapheresis. Plasma decay of retinyl palmitate was measured in eight healthy volunteers. Retinyl palmitate plasma disappearance obeyed an apparent first order function in seven studies and, in one study, a biexponential function with the second, slow exponential accounting for only 13% of the retinyl palmitate plasma decay. The mean fractional removal of rate was 0.037 +/- 0.037 min-1 (mean +/- SD) in a one-compartment model. The apparent volume of distribution, Vd, was 109 +/- 25% of the estimated plasma volume. Plasma clearance of retinyl palmitate was 130 +/- 97 ml/min calculated as Vd x Ke. Mean T 1/2 was 29 +/- 16 min. Both in vitro and in vivo the retinyl palmitate remained largely within chylomicrons and intestinal VLDL. Only 4.3% was transferred from chylomicrons to other lipoprotein classes during in vitro incubation for 5 hr. After plasma was stored for 42 hr, 5% was transferred to higher density lipoproteins. During 12 hr after a test meal containing retinyl palmitate, only 6.4 +/- 1.5% of the retinyl palmitate absorbed was found in the LDL fraction and 3.1 +/- 3.8% in the d 1.063 g/ml lipoproteins. We conclude that retinyl palmitate is a useful marker for chylomicrons and their remnants in humans and that the plasma clearance of retinyl palmitate-labeled chylomicrons is probably an estimate of chylomicron remnant plasma clearance in man.