Mathematical methods for the simultaneous measurement of arterial influx of esterified and free cholesterol from two lipoprotein fractions and in vivo hydrolysis of arterial cholesterol ester

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.

Different efflux pathways for high and low density lipoproteins from porcine aortic intima

To study the efflux of high (HDL) and low (LDL) density lipoproteins from the arterial wall in vivo, a surgical model in pigs was used. An isolated segment of the lesion-free thoracic aorta was pulse labeled from the lumen of the artery with 3H-cholesteryl ester labeled HDL and 14C-cholesteryl ester labeled LDL. Subsequently, the labeled aortic segment was exposed to cold chase in vivo. The transfer of HDL cholesteryl ester from plasma into intima expressed as intimal clearance was three to seven times greater than that of LDL cholesteryl ester. At least 50%, but possibly as much as 95%, of the HDL cholesteryl ester that entered the arterial intima during a period of 4 hours penetrated the arterial wall beyond the internal elastic lamina. In contrast, less than 15% of the LDL cholesteryl ester that entered the arterial intima in the same period penetrated beyond the luminal layer. After 24 hours of cold chase in vivo, more than 80% of both labeled HDL esterified cholesterol and labeled LDL esterified cholesterol had disappeared from the arterial wall. Transmural profiles after 9 hours of cold chase showed that labeled HDL was present throughout the entire arterial wall, whereas labeled LDL in quantitative amounts was present only in the luminal layer. The results suggest that the most important efflux route for HDL esterified cholesterol is through the vasa vasorum and lymphatics in the outer media and adventitia, whereas LDL esterified cholesterol predominantly leaves intima via the lumen of the artery.

A method for quantifying lipoprotein flux rates between plasma and arterial intima in vivo

A new method is proposed for measuring in vivo the rates of transfer of plasma lipoprotein into and out of arterial intima in man. The technique requires the injection of a lipoprotein which can be labelled with two different tracers, followed by a single arterial sample during elective surgery. The feasibility of the method was tested by computer simulation and it was found that measurement uncertainties of the order of those likely in practice produced uncertainties of 7% SD in the calculated influx and 16% SD in the calculated efflux rates. The principal assumption, that lipoprotein in arterial intima can be modelled as a single compartment system, was tested in a preliminary study in an anaesthetised dog: reasonable evidence of monocompartmental behaviour was obtained. The new method was used in a preliminary study of three patients undergoing elective arterial surgery. Low density lipoprotein (LDL) labelled with 125I was injected about 24 h before surgery, followed by 131I-labelled LDL 4 to 6 h before surgery. The radioactivities of washed extracts of arterial intima were then measured in a gamma counter. LDL flux rates were higher in atherosclerotic specimens than in less seriously diseased or normal tissue, and were highest in the most hyperlipidaemic patient.

Effect of verapamil on accumulation of free and esterified cholesterol in the thoracic aorta of cholesterol-fed rabbits

In order to investigate the effect of the calcium antagonist verapamil on atherogenesis in cholesterol-fed rabbits, 3 groups of 11 animals were fed a 2% cholesterol-enriched diet for 10 weeks. One group received verapamil in a daily dose of 16 mg/kg orally plus 2 mg/kg subcutaneously. This dosage resulted in plasma concentrations of verapamil in the same range as the usual therapeutic levels in humans. Another group received verapamil in a daily dose of 8 mg/kg orally and 0.5 mg/kg subcutaneously. The third group received placebo capsules orally and isotonic saline subcutaneously. Total cholesterol concentrations in plasma over the 10 weeks were 37 +/- 4, 42 +/- 4 and 45 +/- 3 mM (mean +/- SE) in the high verapamil-, in the low verapamil- and in the placebo group, respectively. These values were not significantly different. The distribution of cholesterol between HDL, LDL and VLDL in plasma was similar in the 3 groups. The high verapamil group had a significantly (P less than 0.05) lower concentration of cholesterol in the thoracic aorta than the placebo group (29 +/- 5 vs 43 +/- 9 mumol/g wet weight). The low verapamil group and the placebo group had the same aortic cholesterol concentrations. Neither dosage of verapamil affected the permeability of the aortic endothelium to plasma lipoproteins and albumin, as measured by use of radioactive tracers at the end of the experiment.

Influx of [3H,14C]cholesterol-labelled lipoprotein into re-endothelialized and de-endothelialized areas of ballooned aortas in normal-fed and cholesterol-fed rabbits

The entry of [3H]- and [3H,14C]cholesterol-labelled lipoprotein into de-endothelialized and re-endothelialized areas of balloon-injured rabbit aortas was studied in normal-fed and cholesterol-fed rabbits. Studies were carried out 11-15 weeks after the initial injury when endothelial regeneration involved approximately half of the aortic area. The entry into the aorta of 3H-labelled free and ester cholesterol in lipoprotein over a 72-h period was studied following the ingestion of a single dose of 3H-labelled cholesterol. The entry of double labelled [3H,14C]cholesterol-labelled lipoprotein was also studied over a 6-h period following the injection of plasma from donor rabbits. The accumulation of cholesterol and cholesterol ester in the aorta in both the normal- and cholesterol-fed rabbits was significantly greater for the re-endothelialized (white) areas than for the de-endothelialized (blue) areas or the sham-operated aortas. Where the rabbits were cholesterol-fed 4-10 times the amount of cholesterol accumulated in re-endothelialized intima compared to normal intima. Both entry (micrograms/day/100 mg wet weight aortic intima) and clearance (mu 1 plasma/day/cm2) of free and ester cholesterol were increased in the neointima compared with the normal intima for both normal-fed and cholesterol-fed rabbits. Hydrolysis of cholesterol ester occurred in the neointima and was greater than in the corresponding de-endothelialized area but less than for the sham-operated intima. Synthesis of cholesterol ester was minimal in all areas. Removal of labelled cholesterol and cholesterol ester from the intima during a 20-h efflux period following the initial 72-h loading period indicated that for aortas of both normal-fed and cholesterol-fed rabbits, there was greater removal for normal intima than for either re-endothelialized or de-endothelialized intima. However, no clear difference between the blue and white areas was observed. It is concluded that the accumulation of cholesterol in neointima after balloon injury is associated with a marked increase in permeability to lipoprotein of the neointima as well as to possible binding of lipoprotein to glycosaminoglycan in the artery.

No effect of nifedipine on atherogenesis in cholesterol-fed rabbits

It was recently reported that the calcium antagonist nifedipine suppresses aortic cholesterol accumulation in cholesterol-fed rabbits without reducing hypercholesterolemia. We extended this study on plasma lipoprotein levels and aortic influx of cholesteryl ester. We gave 40 mg per day of nifedipine orally to 17 rabbits fed a 2% cholesterol diet for 8 weeks. For the same period of time 15 control rabbits received placebo capsules and the same diet. During the study, total cholesterol, HDL, LDL, and VLDL cholesterol concentrations in plasma were not significantly different in the experimental and control animals. At the end of the study we found no difference in the two groups in accumulation of cholesterol in the intima media of the proximal thoracic aorta, the distal thoracic aorta, and the corresponding media layers. Furthermore, aortic influx of free cholesterol, cholesteryl ester, and albumin from plasma measured by radioactive tracers was not significantly affected by nifedipine.

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.

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

Arterial influx of esterified cholesterol from 2 plasma lipoprotein fractions, d less than 1.019 and d greater than 1.019, and influx of plasma free cholesterol were determined in each of 15 hypercholesterolemic rabbits with approximately the same plasma cholesterol concentrations but with different extents of arterial lesions. The procedure consisted of injecting intravenously into recipient rabbits [14C]- or [3H]cholesterol-labeled lipoproteins prepared from donor rabbits. The esterified cholesterol of one lipoprotein fraction was labeled primarily with one isotope and that of the other lipoprotein fraction was labeled with the other isotope. Thoracic aortas were removed 4-6 h after lipoprotein injections. The arterial influx of esterified cholesterol was up to 50 times higher in rabbits with maximal lesions than in those with minimal cholesterol deposits. the arterial influx of cholesteryl ester derived from d less than 1.019 lipoproteins was about equal to that derived from the d greater than 1.019 fraction. The amount of cholesteryl ester in plasma d less than 1.019 was approximately 3 times that in d greater than 1.019. Thus, per unit of cholesteryl ester concentration the d less than 1.019 lipoproteins delivered about 1/3 as much cholesteryl ester to the artery as the lipoproteins in the higher density fractions. some 5-40% of plasma esterified cholesterol which had entered the artery was hydrolyzed in the artery during the experimental period. The influx of free cholesterol that could not be accounted for by the influx of intact plasma lipoproteins was 5-80% of the free cholesterol influx. This excess probably represents free cholesterol influx by an exchange between the plasma lipoproteins and the intimal surface of the artery.

In vivo influx, tissue esterification and hydrolysis of free and esterified plasma cholesterol in the cholesterol-fed rabbit

The influx of free and esterified cholesterol into various tissues of cholesterol-fed rabbits is calculated from the tissue [3H] cholesterol and [14C] cholesterol content - corrected for radioactivity in contaminating plasma - after a 3--6 h exposure to in vivo-labeled plasma. The plasma free cholesterol was labeled primarily with 3H and the esterified cholesterol with 14C or vice versa. The influx calculation is based on total 3H and 14C in tissues and two linear equations that take into account esterification and hydrolysis of sterol fractions by the tissues. The influx of esterified cholesterol into tissue samples from aorta, heart, small intestine and lung was 10--80 nmol-g--1-h--1, whereas the influx into adrenal, spleen and liver was from 400--2500 nmol--g-1--h-1. The influx of free cholesterol was considerably higher than expected if free and esterified cholesterol had entered the tissues together as part of plasma lipoproteins. This excess of free cholesterol influx can be ascribed to cholesterol exchange between plasma lipoproteins and tissues, which in several tissues amounted to more than 80% of the total free cholesterol influx. From tissue free and esterified cholesterol radioactivity, one can calculate that 20--70% of the newly entered esterified cholesterol was hydrolyzed by various tissues and that most tissues esterified less than 10% of newly entered cholesterol during the experimental period. However, esterification of plasma cholesterol by adrenals averaged 50% of that taken up during a 3-6 h period.

Transfer of plasma lipoprotein components and of plasma proteins into aortas of cholesterol-fed rabbits. Molecular size as a determinant of plasma lipoprotein influx

The arterial influx of esterified and free cholesterol from low density lipoproteins and very low density lipoproteins in 20 hypercholesterolemic rabbits was measured simultaneously by the use of lipoproteins labeled in vivo with [3H]- and [14C]-cholesterol. The simultaneous arterial influx of either [3H]-leucine-labeled very low density lipoproteins, low density lipoproteins, high density lipoproteins, or plasma proteins was also measured in each rabbit. The arterial influx was calculated as intimal clearance, i.e., the influx of a given fraction divided by its plasma concentration. The intimal clearance of low density lipoprotein esterified cholesterol was equal to that for the apolipoproteins of that fraction, which is compatible with an arterial influx of intact low density lipoprotein molecules. The intimal clearance of very low density apolipoprotein or cholesteryl ester was less than that for low density lipoprotein, whereas high density lipoprotein and albumin clearances exceeded low density lipoprotein clearance by 1.5- to 3-fold. The intimal clearances of plasma proteins, high density, low density, and very low density lipoproteins decreased linearly with the logarithm of the macromolecular diameter. This indicates that the arterial influx of three plasma lipoprotein fractions and of plasma proteins proceeds by similar mechanisms. Apparently the relative intimal clearances of lipoproteins are more dependent on their size relative to pores or vesicular diameters at the plasma-artery interface than on specific interactions between lipoproteins and the arterial intimal surface.