Arterial influx of esterified cholesterol from two plasma lipoprotein fractions and its hydrolysis in vivo in hypercholesterolemic rabbits

Transport of Apolipoprotein B-Containing Lipoproteins through Endothelial Cells Is Associated with Apolipoprotein E-Carrying HDL-Like Particle Formation

Passage of apolipoprotein B-containing lipoproteins (apoB-LPs), i.e., triglyceride-rich lipoproteins (TRLs), intermediate-density lipoproteins (IDLs), and low-density lipoproteins (LDLs), through the endothelial monolayer occurs in normal and atherosclerotic arteries. Among these lipoproteins, TRLs and IDLs are apoE-rich apoB-LPs (E/B-LPs). Recycling of TRL-associated apoE has been shown to form apoE-carrying high-density lipoprotein (HDL)-like (HDL_E) particles in many types of cells. The current report studied the formation of HDL_E particles by transcytosis of apoB-LPs through mouse aortic endothelial cells (MAECs). Our data indicated that passage of radiolabeled apoB-LPs, rich or poor in apoE, through the MAEC monolayer is inhibited by filipin and unlabeled competitor lipoproteins, suggesting that MAECs transport apoB-LPs via a caveolae-mediated pathway. The cholesterol and apoE in the cell-untreated E/B-LPs, TRLs, IDLs, and LDLs distributed primarily in the low-density (LD) fractions (d ≤ 1.063). A substantial portion of the cholesterol and apoE that passed through the MAEC monolayer was allotted into the high-density (HD) (d > 1.063) fractions. In contrast, apoB was detectable only in the LD fractions before or after apoB-LPs were incubated with the MAEC monolayer, suggesting that apoB-LPs pass through the MAEC monolayer in the forms of apoB-containing LD particles and apoE-containing HD particles.

Hormone replacement therapy enhances postprandial lipid metabolism in postmenopausal women

Postmenopausal estrogen therapy reduces cardiovascular morbidity and mortality, except in women with advanced coronary disease. This beneficial effect is partly attributed to a reduction of fasting plasma total and low-density lipoprotein cholesterol (LDL-C) and an elevation of plasma high-density lipoprotein cholesterol (HDL-C) concentrations. Since postprandial lipemia seems to play a role in the pathogenesis of coronary artery disease, we evaluated the effect of hormone replacement therapy (HRT) on postprandial lipoprotein metabolism in 14 normolipemic postmenopausal women. A vitamin A fat-loading test before and after three cycles of treatment with a sequential combination of conjugated equine estrogen (CEE) and medroxyprogesterone acetate (MPA) was used to label chylomicrons and chylomicron remnants with retinyl palmitate (RP), and RP clearance was assessed over an 8-hour period postprandially. Following 3 months of HRT, fasting total cholesterol and LDL-C levels were reduced 9.8% (P = .049) and 16.5% (P = .023), respectively. Fasting HDL-C levels increased 18.9% (P = .001). Fasting triglycerides (TGs) increased, but not significantly. Postprandial integrated plasma TGs did not change significantly. The integrated RP levels in whole plasma and chylomicron (Svedberg flotation units [Sf] > 1,000) and nonchylomicron (Sf < 1,000) fractions were reduced 58% (P = .043), 78% (P = .041), and 75% (P = .001), respectively, after hormonal treatment. Enhanced clearance of chylomicrons and chylomicron remnants by HRT may contribute to the protective effect of estrogens against cardiovascular disease in normolipemic postmenopausal women.

Thyroxine replacement therapy enhances clearance of chylomicron remnants in patients with hypothyroidism

To further confirm the benefit of replacement therapy in terms of risk for coronary artery disease, we evaluated the effect of T4 on postprandial lipoproteins in patients with hypothyroidism. Nine normolipidemic patients (aged 62.75+/-7.6 yr) with TSH of 32.2+/-13.2 mU/L and free T4 of 0.66+/-0.17 ng/mL were treated with T4 (50-100 microg/day) for at least 4 months. The behavior of postprandial lipoproteins was assessed before and during treatment by determining retinyl palmitate levels in the total plasma, chylomicrons (Sf >1000) and chylomicron remnants (Sf <1000) fractions for 8 h after a mixed meal plus vitamin A. During T4 treatment, serum levels of TSH and FT4 were 4.4+/-4.9 mU/L and 1.2+/-0.34 ng/mL (P = 0.001 and P = 0.002), respectively. Fasting low density lipoprotein cholesterol decreased from 166+/-35 to 135+/-23 mg/dL (P = 0.035). Retinyl palmitate (RP) levels in the chylomicron remnant fraction was reduced significantly during therapy from 6948+/-2790 to 5174+/-2401 microg/L x h (area under the curve +/-SD; P = 0.014). Total plasma RP and chylomicron RP remained unchanged. We conclude that T4 enhances the clearance of chylomicron remnants in normolipidemic patients with hypothyroidism.

Direct assessment of lipoprotein outflow from in vivo-labeled arterial tissue as determined in an in vitro perfusion system

The rate of cholesterol deposition during the atherosclerotic process is determined by the balance between the inflow and outflow of plasma lipoproteins in the arterial wall. Whereas the rate of inflow may be measured directly, the rate of outflow has most often been calculated indirectly from lipoprotein uptake by using the 2-compartment model. One objection against such calculations is that lipoprotein binding is not being considered. In the present study 2 different protocols were used to obtain a direct measure of the outflow of lipoproteins from atherosclerotic rabbit aortas. Thus, 3 rabbits with experimental atherosclerosis were given 125I-LDL intravenously and 3 were given [14C]cholesterol perorally. Twenty-four hours later the aortas were removed and the outflow of label was monitored during in vitro perfusion. Despite the different protocols, our results were consistent and indicated that fractional loss relative to whole tissue was approximately 0.01 pool/h, which is 1 order of magnitude lower than current estimates based on the 2-compartment model (0.04 to 0.4 pool/h). Furthermore, whereas as much as 2/3 to 3/4 of the tracer that had entered the arterial wall was effectively trapped, the remainder equilibrated at a faster rate (0.06 pool/h). In conclusion, it seems that tissue binding constitutes a prominent and possibly underrated mechanism of lipoprotein deposition, at least in the atherosclerotic rabbit aorta. Furthermore, this means that current estimates of lipoprotein exchange parameters based on the 2-compartment model (eg, fractional loss) may rest on invalid assumptions and should be regarded with caution.

Effects of fibric acid derivatives and metformin on postprandial lipemia

It was suggested that postprandial lipoproteins (PPLp) may play an important role in atherogenesis. We studied PPLp metabolism and its response to drugs in seven hypertriglyceridemic subjects, 23 men with isolated low HDL-C levels, and nine non-diabetic glucose intolerant subjects. Results were compared with those found in a group of 19 healthy normolipidemic individuals. We used the vitamin A-fat loading test which specifically labels PPLp with retinyl palmitate (RP). In the hypertriglyceridemics the areas under RP curves of the chylomicrons were 6.3-fold and those of non-chylomicrons 2.9-fold higher than in normals (P < 0.01). Gemfibrozil 1200 mg/day caused a dramatic decrease in chylomicrons 73% and nonchylomicrons 31%. In subjects with isolated low HDL-C, RP chylomicron curves were significantly higher than in normals (17.733+/-6.821 vs 13939+/-6217 microg/l per h, P < 0.005). Bezafibrate 400 mg/day reduced RP chylomicrons and nonchylomicron levels by 35% (P < 0.0001) in 15 responders with an increase in fasting HDL-C 35+/-3 to 40+/-22 mg/dl (P < 0.0001). No response was found in eight subjects. In the nine glucose intolerant subjects, metformin reduced postprandial insulin area under the curve from 389 to 245 mU/ml (P <0.01) chylomicron and nonchylomicron RP areas were 3.6- and 3-fold higher than in normals and were reduced by 56 and 32%, respectively. In conclusion gemfibrozil, bezafibrate and metformin were shown to be beneficial in the clearance of PPLp in hypertriglyceridemic patients, subjects with isolated low HDL-C levels and nondiabetic glucose intolerant subjects, respectively.

Concentration polarization of low density lipoproteins (LDL) in the arterial system

The atherogenic lipid concentration at the luminal surface of a blood vessel may vary according to its location in the arterial tree because of regional differences in wall shear rate, blood pressure, and vascular permeability. We therefore hypothesized that these local variations in the luminal surface lipid concentration may contribute to the localization of atherosclerosis. To verify this hypothesis, the transport of low-density lipoproteins from flowing blood to the arterial wall was studied numerically under both steady-state and pulsatile flow conditions. Numerical analysis predicted that "concentration polarization" of LDL may occur in the arterial system under these conditions. In contrast to steady-state flow conditions, the luminal surface LDL concentration varied with time in a cardiac cycle. However, its time-average value was slightly higher than the corresponding value under steady-state flow conditions. The time-average value of the luminal surface LDL concentration was 5 to 14% greater than the bulk concentration in a straight segment of an artery. The luminal surface LDL concentration at the arterial wall was flow-dependent, varying linearly with the filtration rate through the vessel wall and inversely with wall shear rate. This may therefore have some significant implications for the pathogenesis and localization of vascular disorders.

Fluid filtration across the arterial wall under flow conditions: is wall shear rate another factor affecting filtration rate?

The effect of flow on fluid filtration across an arterial wall was investigated in the canine common carotid artery. The arteries were cannulated in situ to maintain their in vivo length and endothelium intact. The excised vessels were pressurized at 120 mmHg through an overflow head-tank system that provided a constant flow rate to the perfused vessels. Filtration rates across the walls of the carotid arteries tested were measured under 4 different experimental conditions: 1) albumin-free Krebs solution under absent flow condition; 2) albumin-free Krebs solution with flow (148 +/- 8 ml/min); 3) Krebs solution containing 1.0 g/dl bovine serum albumin under absent flow condition; 4) Krebs solution containing 1.0 g/dl bovine serum albumin with flow (148 +/- 8 ml/min). Under absent flow conditions, the addition of albumin to the Krebs solution (1.0 g/dl) led to an approximate 25% drop in filtration rate (p < 0.001). It was found that fluid flow affected the filtration rate of the albumin solution, yet failed to affect the filtration rate of albumin-free solution across the arterial wall. The present study suggests that the change in filtration rate of the albumin solution under flow condition may indicate the change in the luminal surface concentration of albumin due to flow.

Efficiency of an external support to reduce lipid infiltration into venous grafts: in vitro evaluation

Excessive distension of venous grafts due to arterial pressure enhances the convective water transport (filtration flow) through the vessel wall, and thus might affect the infiltration of macromolecules such as lipoproteins. In this paired experimental study, filtration velocities were measured at 100 mm Hg for canine jugular veins with or without external supports of expanded polytetrafluoroethylene (ePTFE) arterial prostheses. In addition, to assess the effect of filtration velocity on lipid infiltration or uptake, canine jugular veins were wrapped over half of their lengths with ePTFE arterial prostheses and perfused with dog serum containing 3H-cholesterol at a pressure of 100 mm Hg. At 100 mm Hg, the average filtration velocity of the wrapped jugular veins was 7.9 +/- 1.3 x 10(-6) cm/s whereas the average filtration velocity of the unwrapped veins was 27.3 +/- 2.7 x 10(-6) cm/s (p < 0.005). Moreover, the unwrapped veins had a significantly higher uptake rate of labeled cholesterol than the wrapped veins (10.9 +/- 7.3 x 10(-4) cm/h and 5.0 +/- 1.6 x 10(-4) cm/h, respectively, p < 0.005). In conclusion, under arterial pressure, veins experience excessive distention, which leads to significant increases in both filtration flow and cholesterol uptake. An external wrap or support of ePTFE material protects veins from excessive distension and thus may prevent atherosclerosis in venous grafts by reducing cholesterol uptake.

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.

Effect of glipizide treatment on postprandial lipaemia in patients with NIDDM

The primary goal of the present study was to examine the effects of improved glycaemic control associated with glipizide treatment on postprandial lipaemia in non-insulin-dependent diabetic patients. The metabolism of triglyceride-rich lipoproteins of intestinal origin was assessed by measuring the retinyl palmitate content in plasma and the Svedberg flotation index (Sf) > 400 and Sf 20-400 lipoprotein fractions. Fasting plasma glucose concentrations (14.5 +/- 0.5 vs 9.0 +/- 0.5 mmol/l), glycated haemoglobin levels (13.1 +/- 0.6 vs. 9.7 +/- 0.6%), and daylong plasma glucose concentrations were all significantly lower after glipizide treatment (p < 0.001). The improvement in glycaemic control was associated with increases in insulin-mediated glucose uptake (p < 0.001) and plasma post-heparin lipoprotein and hepatic lipolytic activities (p < 0.02). Both fasting plasma triglyceride (3.09 +/- 0.51 vs 2.37 +/- 0.34 mmol/l), and postprandial triglyceride concentrations (p < 0.05-0.001) were lower following glipizide treatment, associated with a significant fall in retinyl palmitate content in all three lipoprotein fractions (p < 0.02-0.001), with the most substantial decrease seen in the Sf20-400 fraction. These data indicate that glipizide-induced improvement in glycaemic control was associated with changes in the metabolism of triglyceride-rich lipoproteins of intestinal origin that would be anticipated to reduce risk of coronary heart disease in non-insulin-dependent diabetic patients.

Influx, efflux, and accumulation of LDL in normal arterial areas and atherosclerotic lesions of white Carneau pigeons with naturally occurring and cholesterol-aggravated aortic atherosclerosis

This study investigated the hypothesis that increased influx of low-density lipoprotein (LDL) accounts for the natural development of atherosclerosis in a characteristic (susceptible) site in the distal thoracic aorta of White Carneau (WC) pigeons and the exacerbation of atherosclerosis by cholesterol feeding. The influence of dietary cholesterol-induced changes in LDL composition on LDL influx into the artery was also investigated. As a measure of the influx of LDL into the artery, we determined the arterial accumulation of radiolabeled LDL after 1 hour. Nine 50-month-old WC pigeons with naturally occurring atherosclerosis and seven 14-month-old WC pigeons with atherosclerosis accelerated by 10 months of cholesterol feeding were studied. In the absence of atherosclerotic lesions, we found no evidence for increased accumulation of LDL at the susceptible site. In fact, more LDL accumulated in less susceptible normal arterial areas near the heart (approximately 90 nl/h per square centimeter) than in the susceptible distal thoracic aorta (approximately 35 nl/h per square centimeter). In the absence of atherosclerotic lesions, LDL accumulation (nanoliters per hour per square centimeter) was not influenced by hypercholesterolemia, although mass transport of LDL cholesterol into the artery was increased. Naturally occurring atherosclerotic lesions accumulated five times as much LDL as the adjacent normal arterial area (P < .001), whereas cholesterol-aggravated atherosclerotic lesions in different arterial sites accumulated four to 26 times as much LDL as the adjacent normal artery (P < .05). Cholesterol-aggravated atherosclerotic lesions at the most susceptible site accumulated five times as much LDL as naturally occurring atherosclerotic lesions in the corresponding arterial site (823 +/- 241 vs 175 +/- 45 nl/h per square centimeter, mean +/- SEM; P < .005). Arterial accumulation of LDL was influenced very little by changes in LDL composition induced by cholesterol feeding. In another study with young WC pigeons free of atherosclerosis and other WC pigeons with cholesterol-aggravated atherosclerosis, we injected differently labeled LDL 0.5 and 1 hour before sacrifice to investigate whether efflux of LDL from the artery was significant during a 1-hour period of LDL uptake. Although efflux of LDL from all arterial sites occurred during 1 hour, differential efflux could not account for regional differences in 1-hour arterial LDL accumulation. This study suggests that the characteristic susceptibility of the distal thoracic aorta of WC pigeons to atherosclerosis and the exacerbation of atherosclerosis by cholesterol feeding cannot be explained by differences in influx or efflux of LDL.(ABSTRACT TRUNCATED AT 400 WORDS)

Local intimal-medial uptakes of 125I-albumin, 125I-LDL, and parenteral Evans blue dye protein complex along the aortas of normocholesterolemic minipigs as predictors of subsequent hypercholesterolemic atherogenesis

This report describes the normalized intimal-medial uptakes [uptake (M, mg.cm-2) divided serum concentration (c0, mg.cm-3)] of 125I-albumin, 125I-low-density lipoprotein (LDL), and in vivo Evans blue dye (EBD)-albumin complex as functions of pressure (P), time (t), molecular species (i), and location (z) along a ventral longitudinal z axis of the normal, intact, aortic endothelial surface in adult normocholesterolemic Sinclair Research Farm (SRF) minipigs and compares these uptake (M/c0) measurements with atherogenesis in hypercholesterolemic cohorts. Uptakes of porcine serum 125I-albumin (n = 21) and 125I-LDL (n = 10) were measured in freshly excised, metabolically supported aortas using a recently developed organ-support system. In vivo intimal-medial EBD uptake vs z data were measured photometrically on opened descending aortas from another group (n = 6) of normocholesterolemic, adult, SRF minipigs 18 hours after the intravenous administration of EBD. For comparison purposes, the corresponding incidence of atherosclerotic lesions along the aortic z axis was calculated using topographic data from hypercholesterolemic minipig cohorts (n = 39). The results showed that uptakes varied greatly with t, z, and macromolecule (i) but not with P. More specifically, the value of M/c0 at any location (z) rose with t, was insensitive to P, decreased with macromolecular (i) size, and varied with z in a pattern that "peaked" in the upstream region, decreased to a nadir in the downstream region, and then rose again as it approached the abdominal celiac orifice. The spatially z-averaged uptake rates for the three different labeled serum proteins were 0.31 x 10(-3) cm.h-1 for 125I-albumin, 0.42 x 10(-3) cm.h-1 for EBD-albumin, and 0.04 x 10(-3) cm.h-1 for 125I-LDL. Nondimensionalized analysis of the individual sets of uptake data indicated that the overall uptake relationship [M(t,P,z,i)/c(io), cm] could be characterized empirically by the simple product of two separate functions: one, a "scaling function" [m(z,i)], that described the uptake magnitude for a given i and z and appeared to be independent of t or P; the other, a "shape function" [s(t,P)], that described the shapes of the uptake vs t and P relationships and appeared to be independent of z or i. The "scaling function" [m(z,i)] vs z contour appeared to correlate well with the corresponding atherosclerotic lesion incidence vs z contour from the group of hypercholesterolemic minipig cohorts. Assuming passive transport, it was shown ("Appendix") that m(z,i) can be interpreted physically in terms of an endothelial diffusive permeability coefficient (P,cm.s-1).(ABSTRACT TRUNCATED AT 400 WORDS)

Regional differences in arterial low density lipoprotein metabolism in surgically postmenopausal cynomolgus monkeys. Effects of estrogen and progesterone replacement therapy

To determine if arterial lipoprotein metabolism may be involved in mediating well-known anatomic regional differences in susceptibility to atherosclerosis, arterial low density lipoprotein (LDL) metabolism and extent of atherosclerosis were studied in 17 ovariectomized female cynomolgus monkeys. The animals were fed an atherogenic diet for 18 weeks, during which time one group received 17 beta-estradiol and cyclic progesterone treatment (n = 9) and the controls received no hormone replacement therapy (n = 8). As reported previously, hormone replacement markedly reduced the accumulation of LDL in coronary arteries without affecting plasma lipoprotein patterns. We report here that LDL metabolism differed among arterial sites. LDL accumulation, LDL degradation rate, and the concentration of undegraded LDL were greatest in the coronary arteries and carotid bifurcations compared with the aorta, iliac arteries, and cerebral arteries. Although hormone replacement decreased indexes of LDL metabolism, there was no effect on intimal thickness or indexes of endothelial injury, such as leukocyte adhesion and endothelial cell turnover rate. There were, however, regional differences in these morphological parameters. The intima was thickest in the aorta, and leukocyte adhesion and endothelial cell turnover rates were greatest in the carotid bifurcation and thoracic aorta. The decreased accumulation and metabolism of LDL caused by hormone replacement therapy was specific to the arterial system and did not occur in the liver or other peripheral tissues.

Accumulation of 125I-tyramine cellobiose-labeled low density lipoprotein is greater in the atherosclerosis-susceptible region of White Carneau pigeon aorta and further enhanced once atherosclerotic lesions develop

Previous studies have suggested that greater arterial concentrations of undegraded low density lipoprotein (LDL) and/or greater arterial rates of LDL degradation may play role(s) in determining regional differences in arterial susceptibility to atherosclerosis in rabbits (Schwenke and Carew, Arteriosclerosis 1989;9:895-918). The White Carneau (WC) pigeon is also useful for investigating potential mechanism(s) that might account for regional variation in arterial susceptibility to atherosclerosis because atherosclerosis develops predictably in the aorta at the level of the celiac bifurcation (atherosclerosis-susceptible celiac site). In this study we sought to determine whether the 125I-tyramine cellobiose (125I-TC) content 1 day after injecting 125I-TC-LDL ("125I-TC-LDL accumulation") would be greater in the celiac site in arteries of WC pigeons and whether 125I-TC-LDL accumulation would be exaggerated by cholesterol feeding. Because 125I-TC remains trapped in cells after cellular degradation, arterial sites that either degrade LDL at higher rates or contain higher concentrations of undegraded LDL or both will demonstrate greater 125I-TC-LDL accumulation. Young WC pigeons were studied while consuming a cholesterol-free diet and after consuming a cholesterol-containing diet for 1, 2, 4, and 8 weeks. In pigeons fed a cholesterol-free diet, 125I-TC-LDL accumulation in the celiac site was equivalent to 0.24 +/- 0.02 micrograms LDL cholesterol/cm2 aortic surface/day compared with only 0.14 +/- 0.02 micrograms LDL cholesterol/cm2/day for the adjacent aorta, which is resistant to atherosclerosis (atherosclerosis-resistant site) (p less than 0.025). In atherosclerotic lesions excised from the celiac site, 125I-TC-LDL accumulation was equivalent to 21 +/- 10 micrograms LDL cholesterol/cm2 aortic surface/day compared with 0.66 +/- 0.17 micrograms LDL cholesterol/cm2/day for the adjacent atherosclerosis-resistant site (p less than 0.001). During cholesterol feeding, 125I-TC-LDL cholesterol accumulation in the celiac site as a whole increased 30-fold compared with a fivefold increase in plasma LDL cholesterol. In comparison, 125I-TC-LDL cholesterol accumulation in the adjacent atherosclerosis-resistant arterial site increased at the same rate as the plasma LDL cholesterol, while 125I-TC-LDL cholesterol accumulation in two other relatively atherosclerosis-resistant arterial sites that we studied increased relatively little during cholesterol feeding. The results of this study suggest that differences in arterial 125I-TC-LDL accumulation, both those present in normal animals and those induced by cholesterol feeding, may contribute to the characteristic regional variation in arterial susceptibility to atherosclerosis in WC pigeons.(ABSTRACT TRUNCATED AT 400 WORDS)

Influx in vivo of low density, intermediate density, and very low density lipoproteins into aortic intimas of genetically hyperlipidemic rabbits. Roles of plasma concentrations, extent of aortic lesion, and lipoprotein particle size as determinants

To compare the atherogenic potential of low density lipoprotein (LDL), intermediate density lipoprotein (IDL), and very low density lipoprotein (VLDL) under conditions where plasma levels of these lipoproteins are elevated, the influx of cholesterol in these lipoproteins into the aortic intima was measured in vivo in genetically hyperlipidemic rabbits from the St. Thomas's Hospital strain, an animal model that shares many of the features of the human disorder familial combined hyperlipidemia. Univariate linear regression showed that the arterial influx of LDL cholesterol (n = 25), IDL cholesterol (n = 14), and VLDL cholesterol (n = 10) was positively and linearly associated with plasma concentrations of LDL cholesterol in the range 0.2-6.4 mmol/l, of IDL cholesterol in the range 0.1-7.0 mmol/l, and of VLDL cholesterol in the range 0.7-8.5 mmol/l, respectively, and also with the extent of lesions in the arterial intima in the range 0-100% of the surface area. Multiple linear regression suggested that the arterial influx of LDL, IDL, and VLDL cholesterol was linearly dependent on plasma concentration, independent of lesion size. Furthermore, it appeared that the arterial influx of the three lipoproteins was linearly dependent on the extent of the lesions, independent of lipoprotein concentration. When influx was normalized for plasma concentration (intimal clearance) and for lesion size (compared within the same aorta), the intimal clearance of the larger IDL and VLDL particles was 15-35% less than that of the smaller LDL particles. These findings suggest that the quantitatively most important mechanism for transfer of plasma lipoproteins into the arterial intima involves nonspecific molecular sieving and that at elevated plasma levels, IDL and VLDL share with LDL the potential for causing atherosclerosis.

Hemodynamics and low density lipoprotein metabolism. Rates of low density lipoprotein incorporation and degradation along medial and lateral walls of the rabbit aorto-iliac bifurcation

We have investigated whether arterial wall low density lipoprotein (LDL) metabolism in areas of disturbed flow differs from the metabolism in adjacent regions of undisturbed flow. Using the rabbit aorto-iliac bifurcation as a model, we examined the rates of LDL incorporation and catabolism in vivo and correlated them to the arterial flow patterns in these regions. The trapped ligand method was used to quantitate the rates of LDL incorporation and degradation over a 20-hour period in three hemodynamic zones of the daughter iliac branch: 1) a region of flow separation where the shearing forces are elevated along the medial wall and reduced along the lateral wall, 2) a transition region where the flow patterns begin to approach the fully established situation, and 3) a unidirectional flow region with symmetric fluid shearing forces along the medial and lateral walls. Our results indicate an elevated rate of LDL incorporation into the lateral versus the medial wall in the proximal zone of flow separation (5.2 +/- 0.8 nl/mg/hr vs. 3.7 +/- 0.5 nl/mg/hr, p less than 0.01). A similar elevation in the degradation rate of the lateral over the medial wall of this most proximal zone was also observed (2.1 +/- 0.4 vs. 1.4 +/- 0.2, p less than 0.05). No such differences were observed regarding LDL incorporation and degradation in the transitional or unidirectional hemodynamic zones. These results suggest that modifications in arterial wall LDL incorporation and catabolism are induced by hemodynamic forces. The implications of these findings for the formation of the atherosclerotic lesion are discussed.

Initiation of atherosclerotic lesions in cholesterol-fed rabbits. I. Focal increases in arterial LDL concentration precede development of fatty streak lesions

We have reported that arterial low density lipoprotein (LDL) concentrations and degradation rates in normal rabbits are elevated in those aortic sites most susceptible to early atheromatous lesions resulting from cholesterol feeding. Here we asked whether the focal differences in LDL metabolism observed in normolipidemic rabbits are accentuated during the first 16 days of feeding 2% cholesterol and whether such changes occur before accumulation of significant numbers of macrophage foam cells. No lesions were grossly visible after 16 days of cholesterol feeding. Histology indicated that macrophage foam cells were sparse during this interval and were found only in lesion-prone sites at longer feeding times. We used LDL labeled both with 131I (to trace undegraded LDL), and with the intracellularly trapped ligand 125I-tyramine cellobiose (to trace degraded plus undegraded LDL). The most profound change was a marked and focal increase in the concentration of intact LDL within the arterial wall of lesion-prone sites. After 16 days of cholesterol feeding when the plasma LDL cholesterol concentration had increased 7.6-fold, the concentration of intact LDL in lesion-prone branch sites of the abdominal aorta was increased by 22-fold (from 2.5 to 54 micrograms LDL cholesterol/g). These concentrations were two and 5.7 times, respectively, as great as in the lesion-resistant, nonbranch areas of the abdominal aorta of the same animals. Similar, but less striking, results were found when the lesion-prone aortic arch and intercostal orifices of the thoracic aorta were compared with adjacent lesion-resistant sites. Arterial LDL degradation rates expressed in terms of LDL mass also increased with time of cholesterol feeding, and were greater in the aortic arch and in branch sites of the abdominal aorta than in adjacent lesion-resistant sites. However, fractional rates of degradation of arterial LDL were decreased in all sites by cholesterol feeding, and were lower in susceptible than in resistant sites. This was probably due in large part to saturation and down-regulation of LDL receptors. The timing, focal nature, and site-specificity of these changes suggest that an increased concentration of LDL in the arterial wall may be an early indication of lesion formation and, in fact, may be a necessary first step in the pathogenesis of the fatty streak lesion in cholesterol-fed rabbits.

Passage of low density lipoproteins through monolayers of human arterial endothelial cells. Effects of vasoactive substances in an in vitro model

The endothelium controls the influx of lipoproteins into the arterial wall, a process that may be disturbed in arteriosclerotic blood vessels. We have used an in vitro model to investigate the characteristics of the passage of low density lipoproteins (LDL) through monolayers of human arterial endothelial cells. Umbilical artery, aorta, or carotid artery endothelial cells were cultured on polycarbonate filters and formed a tight monolayer in which the cells were connected by tight junctions. Passage of 125I-LDL through these monolayers proceeded linearly over a 24-hour period. It was threefold lower through monolayers of aorta or carotid artery cells than through monolayers of umbilical artery cells. The LDL passage process did not show saturation with LDL concentrations up to 800 micrograms/ml LDL-protein (i.e., 1.6 nmol/ml apolipoprotein B) between 2 and 4 hours after addition. However, during the first 30 to 60 minutes after addition of high concentrations of LDL, a reduction of the passage rate of both LDL and peroxidase, resulting in an apparent saturation of the passage process, was observed. The passage rate of the negatively charged acetylated LDL was twofold lower than that of native LDL. Addition of histamine to the endothelial monolayer resulted in a large, but transient, increase in permeability paralleled by a decrease in electrical resistance. The effects of histamine were mediated via an H1 receptor. Thrombin and Ca++ ionophore also induced an increase in permeability of the monolayer, while bradykinin did not. The effects of histamine and thrombin were paralleled by a rapid and marked increase in cytoplasmatic Ca++ concentration of the endothelial cells, while bradykinin induced only a small increase. Although the cyclic adenosine 5'-monophosphate-elevating agent, forskolin, markedly decreased the basal rate of LDL passage through the endothelial cell monolayers, it did not change the relative increase in permeability induced by histamine. Thus, histamine induces small, but significant, increases in the permeability of tight endothelial cell monolayers.

Dietary polyunsaturated fats of the W-6 and W-3 series reduce postprandial lipoprotein levels. Chronic and acute effects of fat saturation on postprandial lipoprotein metabolism

The chronic and acute effects of different types of dietary fat on postprandial lipoprotein metabolism were studied in eight normolipidemic subjects. Each person was placed for 25 d on each of three isocaloric diets: a saturated fat (SFA), a w-6 polyunsaturated fat (w-6 PUFA) and a w-3 polyunsaturated fat (w-3 PUFA) diet. Two vitamin A-fat loading tests were done on each diet. The concentrations in total plasma and chylomicron (Sf greater than 1,000) and nonchylomicron (Sf less than 1,000) fractions of retinyl palmitate (RP) were measured for 12 h postprandially. Compared with the SFA diet, the w-6 PUFA diet reduced chylomicron and nonchylomicron RP levels 56 and 38%, respectively, and the w-3 PUFA diet reduced these levels 67 and 53%, respectively. On further analysis, the main determinant of postprandial lipoprotein levels was the type of fat that was chronically fed, which appeared to mediate its effect by changing the concentration of the endogenous competitor for the system that catabolizes triglyeride-rich lipoproteins. However, there was a significant effect of the acute dietary fat load, which appeared to be due to a differential susceptibility to lipolysis of chylomicrons produced by SFA as opposed to PUFA fat loads. The levels of postprandial lipoproteins are determined by the interaction of these chronic and acute effects.

Lipid composition of aorta of Watanabe heritable hyperlipemic and comparably hypercholesterolemic fat-fed rabbits. Plasma lipid composition determines aortic lipid composition of hypercholesterolemic rabbits

Aortic and plasma lipid compositions were compared during a 12-month period in Watanabe heritable hyperlipemic (WHHL), comparably hypercholesterolemic fat-fed, and age-matched control normolipidemic rabbits to determine whether exposure to equivalent degrees of endogenous or exogenous hypercholesterolemia led to differences in the composition and concentration of lipids deposited in the artery wall. Although there were marked differences in the distribution of cholesterol among the lipoproteins in the WHHL versus the fat-fed rabbits, the contents of both free and esterified cholesterol were elevated to an equivalent degree in the aorta and plasma. In contrast, the triglyceride content of both the plasma and aorta were elevated only in the WHHL rabbits. However, aortic total phospholipids were increased in both the WHHL and fat-fed animals as compared to control rabbits. In the control rabbits, there was an age-dependent enrichment in aortic relative to plasma cholesteryl-oleate consistent with low density lipoprotein (LDL) receptor-directed intracellular cholesteryl ester processing. In contrast, enrichment in cholesteryl-oleate in aortae relative to plasma was not detected in either WHHL or fat-fed groups, suggesting that the plasma cholesteryl ester composition was the primary determinant of the aortic cholesterol composition. Thus, during chronic hypercholesterolemia, deposition of lipids in the artery wall appears to be determined by plasma lipoprotein levels and composition if the LDL receptor is either absent on a genetic basis or suppressed due to a high-fat, high-cholesterol diet.

Quantification in vivo of increased LDL content and rate of LDL degradation in normal rabbit aorta occurring at sites susceptible to early atherosclerotic lesions

While the exact mechanisms that initiate atherosclerotic lesions are unknown, considerable evidence supports a role for low density lipoprotein (LDL). We investigated whether in the normal rabbit, LDL metabolism in areas of aorta that are destined to become lesioned during cholesterol feeding differed from the metabolism in adjacent lesion-resistant aorta. These studies took advantage of the predictable pattern of early atherosclerotic lesions in the cholesterol-fed rabbit. Early lesions occur diffusely in the aortic arch and ascending aorta and distal to branch orifices in the abdominal aorta and the descending thoracic aorta. Arterial rates of irreversible degradation of LDL and concentrations of intact LDL were measured in susceptible and resistant sites with homologous doubly labeled LDL. LDL was labeled directly with 131I and with 125I-tyramine cellobiose. The latter label provides a highly sensitive means to determine the sites and rates of lipoprotein degradation in vivo. The arterial concentration of intact LDL in the lesion-prone aortic arch was 3.12 +/- 0.45 micrograms LDL cholesterol/g (n = 14), 3.6 +/- 0.69 times that in the relatively lesion-resistant descending thoracic aorta (p less than 0.001). The rate of LDL degradation in the aortic arch was 3.14 +/- 0.41 micrograms LDL cholesterol/g/day, 2.14 +/- 0.24 times that in the descending thoracic aorta (p less than 0.001). In the abdominal aorta, the LDL (per gram wet weight) concentration and degradation rate (per square centimeter surface area) at branch sites exceeded that at nonbranch sites by 88 +/- 11% (p less than 0.001) and by 61 +/- 8% (p less than 0.001), respectively. These data provide evidence that in the normal rabbit, which does not develop atherosclerotic lesions, focal elevations of arterial LDL degradation rate and concentrations of intact LDL occur at sites that first develop atherosclerotic lesions in the hypercholesterolemic animal. These differences in LDL metabolism may be linked causally to the propensity to develop atherosclerotic lesions at these sites.

Influx and cellular degradation of low density lipoproteins in rabbit aorta determined in an in vitro perfusion system

The accumulation of 125I-low density lipoprotein (LDL) into normal and atherosclerotic arterial tissue and cellular uptake in arterial cells were studied in an in vitro perfusion system for rabbit aorta. The accumulation of 125I-LDL in normal tissue could be fitted to an inverse exponential function with an initial influx rate of 1.39 nl/mg wet weight/hour and an equilibration volume of about 2% of the tissue volume. The influx rate into atherosclerotic plaques was about 10 times faster and the equilibration volume, 50 times higher. In atherosclerotic tissue there was a steep concentration gradient between the plaque and the underlying media. The accumulation of 125I-LDL in the media under plaque and in normal tissue adjacent to plaques was similar to that seen in normal tissue. For studies of cellular uptake of LDL a trace label, 125I-tyramine-cellobiose (TC), was used. Normal or atherosclerotic rabbit aorta was perfused in vitro with medium containing 125I-TC-LDL. After perfusion the tissue was digested and the cells were isolated by density gradient centrifugation. Two main cell fractions with characteristics of smooth muscle cells and foam cells, respectively, were obtained. A 70-fold higher uptake was seen in the foam cells. In conclusion, these studies suggest a higher influx rate into atherosclerotic plaques, as well as a high LDL concentration in the plaque, compared with normal tissue or underlying media. We suggest that most of the cellular uptake of LDL in the arterial wall is caused by the foam cells.

Different patterns of postprandial lipoprotein metabolism in normal, type IIa, type III, and type IV hyperlipoproteinemic individuals. Effects of treatment with cholestyramine and gemfibrozil

To study exogenous fat metabolism, we used the vitamin A-fat loading test, which specifically labels intestinally derived lipoproteins with retinyl palmitate (RP). Postprandial RP concentrations were followed in total plasma, and chylomicron (Sf greater than 1,000) and nonchylomicron (Sf less than 1,000) fractions. In normal subjects postprandial lipoproteins were present for more than 14 h, and chylomicron levels correlated inversely with lipoprotein lipase activity and fasting high density lipoprotein (HDL) cholesterol levels and nonchylomicron levels correlated inversely with hepatic triglyceride lipase activity. The main abnormality in type IV patients was a 5.6-fold increase in the chylomicron fraction, whereas in type III patients it was a 6.4-fold increase in nonchylomicrons. Type IIa patients had abnormally low chylomicron fractions. In type IV patients gemfibrozil decreased, whereas in type IIa patients cholestyramine increased the chylomicron fraction 66 and 88%, respectively. This study demonstrates an unexpectedly large magnitude and long duration of postprandial lipemia in normal subjects and patients. These particles are potentially atherogenic, and their role in human atherosclerosis warrants further study.

Regulation of lipoprotein receptors on rat hepatomas in vivo

It has been shown previously that the rat hepatoma no. 7288C grown in vivo or in vitro expresses fewer receptors which recognize chylomicron remnants than does normal rat liver, and it was suggested that this may contribute to the deletion of dietary cholesterol-induced regulation of cholesterol synthesis in hepatomas (Barnard, G., Erickson, S. and Cooper, A. (1984) J. Clin. Invest. 74, 173-184). To investigate this further, Buffalo rats bearing hepatomas (HTC no. 7288C) were made hypercholesterolemic by feeding an atherogenic diet and hypocholesterolemic by ethinyl estradiol injections. Under all circumstances, tumor membranes had fewer receptors than liver membranes as measured by specific binding of [125I]chylomicron remnants. Ethinyl estradiol treatment increased the number of lipoprotein receptors 1.7-fold in liver membranes and 1.2-1.6-fold in tumor membranes, but hypercholesterolemia did not produce any significant changes in remnant binding to either liver or hepatoma membranes. Feeding an atherogenic diet induced a 2.4-fold increase in total cholesterol content in the liver, primarily as cholesterol ester; however, there was no change in total, free or ester cholesterol in the hepatomas. Acyl coenzyme A:cholesterol acyltransferase activity was low in this hepatoma line and neither treatment significantly affected its activity. One explanation for the lack of effect of the atherogenic diet on hepatoma cholesterol metabolism in addition to the decreased number of lipoprotein receptors might be the failure of access of lipoproteins to the tumor cell. To assess this, radioiodinated apo E-rich lipoproteins of various sizes were injected intravenously into rats with hepatomas. Their disappearance from the circulation was followed, and the uptake of each lipoprotein into a variety of tissues was determined. Chylomicron remnants were the most avidly removed particles. VLDLH, IDLH and HDLC were removed more slowly and less completely. None of the lipoproteins accumulated substantially in the tumors suggesting a limited access to the hepatoma tissue. Thus, in addition to the observed reduction in lipoprotein receptor number, limited lipoprotein access to the hepatoma tissue may be a significant factor in contributing to the apparent lack of feedback regulation of cholesterol synthesis by hepatoma tissue in vivo.

Cholesterol synthesis in cultured human peripheral lymphocytes. Influence of LDL, HDL, cholesterol/phosphatidylcholine liposomes and complete serum

Lipoprotein-deficient milieu, freshly isolated human peripheral blood lymphocytes lose about 50% of their membrane cholesterol into the medium within 8 h. The cholesterol loss is counter-regulated by de novo synthesis commencing after a lag phase of 8-12 h, and reaching a steady state within 24 h at a diminished membrane cholesterol level. About 50 micrograms free cholesterol/ml, offered in the form of low-density lipoproteins (LDL) and cholesterol/phosphatidylcholine liposomes, suppressed cholesterol synthesis to about 20% of that controls (lipoprotein-deficient culture). By contrast, pure phosphatidylcholine liposomes enhanced cholesterol synthesis to about 150% of control values. High-density lipoproteins (HDL) exerted a slightly suppressive effect on cholesterol synthesis only at high concentrations (greater than 100 micrograms HDL cholesterol/ml). HDL added to cultures containing fixed concentrations of LDL led to a dose-dependent neutralization of LDL suppression of cholesterol synthesis. Culture medium containing complete serum caused a suppression of cholesterol synthesis to about 50% of the control. The lesser reduction in cholesterol synthesis caused by complete serum compared with LDL or cholesterol/phosphatidylcholine liposomes can be explained by the presence of HDL in the former. Our results support the view that the cholesterol requirement of blood lymphocytes in their lipid-rich milieu is met by cholesterol neosynthesis as well as an exchange mechanism with surrounding lipoproteins. In our system, the cholesterol neosynthesis appears to be controlled by the ratio of LDL to HDL in the surrounding medium.

Radioiodinated Cholesteryl lopanoate as a Potential Probe for the in Vivo Visualization of Atherosclerotic Lesions in Animals

Radioiodinated cholesteryl iopanoate, a nonhydrolyzable cholesteryl ester probe, showed increased uptake into atherosclerotic aortas of cholesterol-fed rabbits in comparison with normal rabbits. Auto-radiography of the aortas showed the radioactivity to be concentrated in areas of visible atherosclerotic involvement. Lipid extraction and thin-layer chromatography of this tissue as well as liver, adrenal, and plasma confirmed the resistance of this probe to hydrolysis. These findings suggest that (125)I-cholesteryl iopanoate may prove useful for noninvasively monitoring atherosclerosis in intact laboratory animals.

Influx, efflux, and hydrolysis of cholesteryl ester in atheromatous lesions of cholesterol-fed rabbits

Total plasma lipoproteins were labeled with radioactive cholesteryl ester or cholesteryl ether by transfer of these lipids from phosphatidylcholine vesicles in the presence of plasma lipid transfer activity. Intravenous injection of these preparations into hypercholesterolemic rabbits showed disappearance curves identical to those of in vivo labeled lipoproteins. Disappearance of cholesteryl ester and ether were similar during the first 24 hours, but they diverged at later time intervals, indicating recirculation of labeled cholesteryl ester. Lipoproteins labeled with cholesteryl ether were injected at 25 days, 7 days and 1 day before sacrifice of the rabbits. The maximal loss of labeled ether from the aortas during a 24-hour period ranged from 1.6% to 8.9% of the labeled ether taken up from plasma. Hydrolysis of cholesteryl ester by the artery during 24 hours averaged 35% of the calculated cholesteryl ester influx. After hydrolysis, cholesteryl ester fatty acid appeared to be esterified more rapidly than the cholesterol moiety of the cholesteryl ester.

Cholesteryl ester efflux from extracellular and cellular elements of the arterial wall. Model systems in culture with cholesteryl linoleyl ether

Cholesteryl ester (CE) accretion in human atheroma is a slow process during which lipoproteins infiltrate the arterial extracellular space and then gain entry into the cellular components. The present aim was to simulate this process in model systems in culture to learn whether cholesteryl ester transfer protein (CETP) may promote CE efflux at different stages of atheroma formation. To simulate CE efflux from arterial interstitium, cationized LDL labeled with 3H-cholesteryl linoleyl ether (3H-CLE) was added to fixed aortic smooth muscle cells (SMC) or to extracellular matrix. To study efflux of 3H-CLE taken up by cells via receptor-mediated endocytosis of LDL, the SMC cultures were fixed and permeabilized prior to the determination of CE efflux. The cellular model included macrophages, which had ingested acetylated LDL labeled with 3H-CLE. Efflux of 3H-CLE and 14C-CE was studied during postincubation of the labeled cultures with human lipoprotein deficient serum (LPDS) or partially purified CETP. As controls, we used SMC cultures incubated with albumin. In all systems, a 3- to 12-fold increase of 3H-CLE or 14C-CE efflux was found in the postincubation medium containing human LPDS or partially purified CETP when compared to controls. Permeabilization of the cells with saponin enhanced cellular 3H-CLE and 14C-CE efflux in the presence of human LPDS. The findings indicate that CETP may promote CE efflux from aortic interstitium or disintegrating cells. We propose that CETP may play an important role in aortic CE homeostasis under physiological and pathological conditions.

Role of the low density lipoprotein receptor in penetration of low density lipoprotein into rabbit aortic wall

The present study was designed to determine whether binding of low density lipoprotein (LDL) to endothelial LDL receptors contributes significantly to the penetration of LDL into the normal rabbit aorta. Initial flux rate was used as a measure of uptake of LDL. Reductive methylation of LDL is known to block its recognition by the LDL receptor. Therefore, the difference in flux rates of native LDL and reductively methylated LDL (methyl-LDL) was assumed to represent the receptor-dependent uptake. Native LDL and methyl-LDL were labeled with different isotopes (125I or 131I) and both were injected simultaneously into the same rabbit. After 30 to 60 minutes, trichloroacetic acid-precipitable counts were determined in aortic specimens. The initial flux rates, expressed as plasma clearance (nl/g/hr), were 1787 for native LDL and 1924 for methyl-LDL. The difference was not significant, which suggests that the flux of LDL into the aorta is not significantly dependent upon, or regulated by, endothelial LDL receptors, but is mediated by other mechanisms.

Measurement in vivo of irreversible degradation of low density lipoprotein in the rabbit aorta. Predominance of intimal degradation

The development of a highly sensitive method for assessing, tissue by tissue, the rates of irreversible protein degradation in vivo has allowed us to quantify low density lipoprotein degradation in the normal rabbit aorta and to localize it. The method depends upon the fact that tyramine-cellobiose, like sucrose used in previous studies, can be covalently attached to proteins, enter cells with them, and then remains trapped within the cell after the remainder of the protein molecule has been degraded. Rabbit LDL (d = 1.02 to d = 1.06 g/ml) was labeled with 125I-tyramine-cellobiose and injected into rabbits. Aortic 125I content 24 hours later served as a cumulative measure of degraded LDL (after appropriate corrections for any intact, nondegraded LDL present). Calculated aortic degradation of LDL averaged 9.4 X 10(-3) percent of the plasma pool per g aortic wet weight per day (n = 6). Intimal cells, obtained by gentle swabbing, accounted for fully 40% of total aortic degradation even though the intima represented less than 5% of the aortic mass. Autoradiography confirmed the high concentration of label in the intima. Degradation of unmodified and reductively methylated LDL were compared. The fractional rate of degradation of methylated LDL by the intima was 50% to 60% of that for native LDL, indicating that 40% to 50% of LDL degradation in the intima, predominantly endothelial cells, is mediated by LDL receptors.

Effect of chronic renal failure and methylprednisolone treatment on the uptake of labelled plasma cholesterol into the aorta of normocholesterolemic rabbits

The effects of chronic renal failure (CRF) and corticosteroid treatment on the aortic uptake of labelled free and esterified cholesterol (FC and EC) were investigated in normocholesterolemic rabbits. Methylprednisolone, 0.4 mg/day, or placebo was administered for 14 weeks to rabbits with normal renal function and with CRF. Then [3H]- and [14C]cholesterol were administered intravenously and orally, respectively. The radioactivity levels of FC and EC in plasma were measured at regular intervals. After 48 h the accumulation of 3H and 14C radioactivity of FC and EC in the intima-media of the thoracic aorta was determined. An aortic uptake coefficient was calculated by dividing the tissue radioactivity (dpm/cm2/h) by the mean plasma radioactivity (dpm/ml). The mean uptake coefficient of EC in normal rabbits was 6 nl/cm2/h, the value for FC being 180 nl/cm2/h. In normal rabbits treated with methylprednisolone the uptake coefficients of both FC and EC were significantly decreased to about 50% of the values in normal rabbits receiving placebo. A similar significant decrease in the uptake coefficients was found in the CRF rabbits receiving placebo. No further decrease was observed in the CRF rabbits treated with methylprednisolone. The cholesterol content of the aortic intima-media was significantly decreased only in CRF rabbits on methylprednisolone treatment. The results do not indicate an acceleration of uremic arterial disease by steroid treatment in the rabbit.

Comparison of cholesteryl ester transfer from chylomicrons and other plasma lipoproteins to aortic intima media of cholesterol-fed rabbits

Lymph chylomicrons labeled with 14C-cholesterol and plasma very low and intermediate density lipoproteins (d less than 1.019) labeled with 3H-cholesterol were injected simultaneously into recipient cholesterol-fed rabbits. The cholesteryl ester of chylomicron origin (14C) found in the tissues 1 to 4 hours after the injection is the sum of both direct and indirect uptake of chylomicron cholesteryl ester. The direct uptake of chylomicron cholesteryl ester is defined as occurring in situ during the interaction between chylomicrons and lipoprotein lipase in endothelial cells. The indirect uptake of chylomicron cholesteryl ester results from the uptake of chylomicron remnants and of other plasma lipoproteins which become labeled during the degradation of the injected chylomicrons by net lipid transfer or by exchange. The indirect contribution of chylomicron cholesteryl ester to tissues was calculated from tissue 3H-cholesteryl ester derived from injected plasma d less than 1.019 lipoproteins. In rabbits fed 0.5 g cholesterol daily for 40-60 days, with plasma cholesterol concentrations between 20 and 30 mM (800-1200 mg/dl), less than 1% of the cholesteryl ester influx into aortic intima media and less than 10% in liver was derived from the direct uptake of chylomicron cholesteryl ester.

Use of 3H-cholesteryl linoleyl ether for the quantitation of plasma cholesteryl ester influx into the aortic wall in hypercholesterolemic rabbits

In this study use was made of 3H-cholesteryl linoleyl ether CLE), a nondegradable analogue of cholesteryl ester (CE) to measure plasma lipoprotein CE influx into rabbit aorta. Autologous serum labeled with 3H-CLE was injected into seven hypercholesterolemic rabbits, and more than 90% of the label was recovered in the plasma compartment 10 minutes after injection. Between 4 hours and 3 days the label was cleared from the circulation with a t1/2 of about 24 hours. Between 4 and 24 hours the lipoproteins isolated at d less than 1.006, d less than 1.019, and d less than 1.063 approached similar specific activity, assuming that 3H-CLE had mixed with the lipoprotein CE pool. The rabbits were killed 7 to 14 days after injection when plasma radioactivity decreased to less than 0.03% of injected dose/ml. Total recovery of the CLE ranged from 70% to 95% and 48% to 72% were found in the liver. The minimum influx of plasma CE into the aortic intima was determined by dividing the label found in the artery by the mean specific activity of the labeled compound in the plasma. The minimum influx into regions with atheromatous involvement ranged from 0.8 to 3.4 micrograms CE/cm2/hr. The rate of influx was highly correlated with the amount of CE mass in the intima and media indicating that the bulk of aortic CE is derived from plasma lipoprotein CE. The method described might be useful in distinguishing between possible effects of "antiatherogenic" drugs on plasma CE influx into the aortic wall from an effect on intracellular CE hydrolysis and subsequent efflux of free cholesterol from the artery.

Uptake of [3H]cholesterol from low density lipoprotein by cultured human fibroblasts

The uptake of [3H]cholesterol from low density lipoprotein (LDL) was studied in LDL receptor-positive and receptor-negative human fibroblasts. In both cell lines the uptake depended upon temperature, time of incubation and the concentration of LDL in the medium. Although the incorporation of 125I-labeled LDL was minimal after 2 h of incubation in the receptor-negative (homozygous familial hypercholesterolemia, FH) cells, the uptake of [3H]cholesterol was only slightly less than that of the receptor-positive (WI-38) cells. With longer periods of incubation, a larger difference in labeled cholesterol incorporation was observed; this appeared to be due to a continued accumulation of the steroid in the WI-38 cells. After 8 and 24 h of incubation, some of the [3H]cholesterol was present as the ester in the WI-38 cells, but not the FH cells. Modified (reduced and methylated) LDL did not enter WI-38 cells by the receptor-mediated pathway during 2 h of incubation, as indicated by 125I uptake. [3H]Cholesterol uptake, however, was not significantly different from modified and unmodified LDL. While experiments indicated that significant amounts of cholesterol moved rapidly from LDL to cultured cells with a dependence on time and LDL concentration, no increase in total cell cholesterol was detected in either cell line. FH cells contained less total cholesterol and had a higher 3H specific activity than the WI-38 cells. These data suggest that there may be important mechanisms in addition to the LDL pathway for the movement of lipids into cells.