Enhanced accumulation and turnover of esterified cholesterol in injured rabbit aorta

Transendothelial transport of lipoproteins

The accumulation of low-density lipoproteins (LDL) in the arterial wall plays a pivotal role in the initiation and pathogenesis of atherosclerosis. Conversely, the removal of cholesterol from the intima by cholesterol efflux to high density lipoproteins (HDL) and subsequent reverse cholesterol transport shall confer protection against atherosclerosis. To reach the subendothelial space, both LDL and HDL must cross the intact endothelium. Traditionally, this transit is explained by passive filtration. This dogma has been challenged by the identification of several rate-limiting factors namely scavenger receptor SR-BI, activin like kinase 1, and caveolin-1 for LDL as well as SR-BI, ATP binding cassette transporter G1, and endothelial lipase for HDL. In addition, estradiol, vascular endothelial growth factor, interleukins 6 and 17, purinergic signals, and sphingosine-1-phosphate were found to regulate transendothelial transport of either LDL or HDL. Thorough understanding of transendothelial lipoprotein transport is expected to elucidate new therapeutic targets for the treatment or prevention of atherosclerotic cardiovascular disease and the development of strategies for the local delivery of drugs or diagnostic tracers into diseased tissues including atherosclerotic lesions.

The mechanisms of coronary restenosis: insights from experimental models

Since its introduction into clinical practice, more than 20 years ago, percutaneous transluminal coronary angioplasty (PTCA) has proven to be an effective, minimally invasive alternative to coronary artery bypass grafting (CABG). During this time there have been great improvements in the design of balloon catheters, operative procedures and adjuvant drug therapy, and this has resulted in low rates of primary failure and short-term complications. However, the potential benefits of angioplasty are diminished by the high rate of recurrent disease. Up to 40% of patients undergoing angioplasty develop clinically significant restenosis within a year of the procedure. Although the deployment of endovascular stents at the time of angioplasty improves the short-term outcome, 'in-stent' stenosis remains an enduring problem. In order to gain an insight into the mechanisms of restenosis, several experimental models of angioplasty have been developed. These have been used together with the tools provided by recent advances in molecular biology and catheter design to investigate restenosis in detail. It is now possible to deliver highly specific molecular antagonists, such as antisense gene sequences, to the site of injury. The knowledge provided by these studies may ultimately lead to novel forms of intervention. The present review is a synopsis of our current understanding of the pathological mechanisms of restenosis.

Atherogenecity of lipoprotein(a) and oxidized low density lipoprotein: insight from in vivo studies of arterial wall influx, degradation and efflux

The accumulation of atherogenic lipoproteins in the arterial intima is pathognomonic of atherosclerosis. Modification of LDL by covalent linkage of apo(a) (resulting in the formation of Lp(a)) or oxidation probably enhances its atherogenecity. Although the metabolism of LDL in arterial intima has been rather extensively characterized, little has been known about the interaction of Lp(a) and oxidized LDL (ox-LDL) with the arterial wall. The present paper reviews a series of recent in vivo studies of the interaction of Lp(a) and ox-LDL with the arterial wall. The results have identified several factors that affect the accumulation of Lp(a) and ox-LDL in the arterial intima and have provided fresh insight into unique metabolic characteristics of Lp(a) and ox-LDL that may explain the large atherogenic potential of these modified LDL species.

Heart allograft vascular disease: an obliterative vascular disease in transplanted hearts

More than 30 years have passed since the first human heart transplantation was performed. Since then, short-term survival after heart transplantation has been markedly improved, but this development has not been paralleled with a similar improvement in long-term survival. One of the major reasons for this is the subsequent development of heart allograft vascular disease, an obliterative disease in the coronary arteries of the transplanted heart. The dubious effect of re-vascularization in this disease, the less favorable outcome after repeat heart transplantation, and the low donor supply have called for intensified research for new and efficient prophylactic therapies against heart allograft vascular disease. This research has lead to improved knowledge about diagnosis, etiology, pathogenesis, prophylaxis, and treatment possibilities. The most important among these seem to be: (i) the introduction of intravascular ultrasound for early detection of the disease; (ii) evidence to suggest that hyperlipidemia, insufficient immunosuppressive therapy, human leukocyte antigen (HLA)-mismatch, and infection with cytomegalovirus (CMV) all may promote allografts vascular disease; and (iii) the introduction of at least two promising prophylactic therapies in humans namely 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors and calcium entry blockers, and others potentially promising e.g. angiotensin-converting enzyme-inhibitors, angiopeptin, mycophenolate mofetil and rapamycin. This review summarizes present knowledge on the possibilities of inhibiting or treating heart allograft vascular disease incorporating evidence from both human and experimental studies.

Preferential influx and decreased fractional loss of lipoprotein(a) in atherosclerotic compared with nonlesioned rabbit aorta

The aim was to investigate the atherogenic potential of lipoprotein(a) (Lp(a)) and to further our understanding of the atherogenic process by measuring rates of transfer into the intima-inner media (i.e., intimal clearance) and rates of loss from the intima-inner media (i.e., fractional loss) of Lp(a) and LDL using cholesterol-fed rabbits with nonlesioned (n = 13) or atherosclerotic aortas (n = 12). In each rabbit, 131I-Lp(a) (or 131I-LDL) was injected intravenously 26 h before and 125I-Lp(a) (or 125I-LDL) 3 h before the aorta was removed and divided into six consecutive segments of similar size. The intimal clearance of Lp(a) and LDL was similar and markedly increased in atherosclerotic compared with nonlesioned aortas (ANOVA, effect of atherosclerosis: P < 0.0001). Fractional losses of labeled Lp(a) and labeled LDL in atherosclerotic aorta were on average 25 and 43%, respectively, of that in nonlesioned aortas (ANOVA, effect of atherosclerosis: P < 0.0001). Fractional loss of Lp(a) was 73% of that of LDL (ANOVA, effect of type of lipoprotein: P = 0.07). These data suggest that the development of atherosclerosis is associated with increased influx as well as decreased fractional loss of Lp(a) and LDL from the intima. Accordingly, Lp(a) may share with LDL the potential for causing atherosclerosis.

Accumulation of lipoprotein fractions and subfractions in the arterial wall, determined in an in vitro perfusion system

A large proportion of a dense subfraction of LDL in plasma is coupled with an increased risk of coronary artery disease, CAD. This may reflect an increased inflow of such LDL subfractions into the intima, since the inflow of lipoproteins is supposed to be inversely related to the size of the particles. In order to evaluate this possibility we used an in vitro perfusion system for aortic intima-media from rabbits with experimental atherosclerosis. The uptake of human VLDL, LDL, HDL and subfractions of LDL (LDL1, 1.019-1.035 and LDL2, 1.035-1.063 g/ml) in lesions and non-involved areas was studied. Our results indicate that particle size is an important factor for the clearance of lipoproteins into the arterial tissue, both for plaques (VLDL 7.6, LDL 25, HDL 58 nl/mg wet wt./h) and in other areas (VLDL 3.8, LDL 4.1, HDL 12 nl/mg wet wt./h). Interestingly, the uptake of LDL2 was as much as 1.5-1.9 times higher than LDL1. This supports the view that an increased lipid load in the arterial wall may be one mechanism behind the association between denser LDL and CAD. Our data also suggest that the difference between LDL uptake in plaque (576 nl/mg wet wt.) and other areas (48 nl/mg wet wt.) not only reflects a rapid clearance but a large distribution volume of the intima (plaque > 60%, non-involved areas 5.7%).

Specific accumulation of lipoprotein(a) in balloon-injured rabbit aorta in vivo

To explore whether lipoprotein(a), Lp(a), may accumulate preferentially to LDL in the arterial wall at sites of injury, cholesterol-fed rabbits were injected intravenously with radiolabeled Lp(a) and/or LDL 3.1 +/- 0.1 days (mean +/- SEM, n = 30) after a balloon injury of the thoracic aorta. After 5 to 10 minutes' exposure to labeled lipoproteins, more labeled LDL than labeled Lp(a) was recovered in the intima-inner media of the balloon-injured segment (n = 9; paired t test, P < .0001); however, the amount of tightly bound labeled lipoprotein was similar for the two lipoprotein fractions. In the second set of experiments, 131I-Lp(a) (or 131I-LDL) was injected 26 hours before and 125I-Lp(a) (or 125I-LDL) 3 hours before the aorta was removed. Permeability and fractional loss of labeled Lp(a) (n = 8) versus LDL (n = 7) in the balloon-injured aortic intima-inner media were: permeability, 0.46 +/- 0.10 microL/cm2 per hour versus 1.41 +/- 0.32 microL/cm2 per hour (nonpaired t test, P < .0001); and fractional loss, 0.12 +/- 0.02 h-1 versus 0.44 +/- 0.05 h-1 (nonpaired t test, P = .0001), respectively. Finally, after 23 hours' exposure to labeled lipoproteins, the total accumulation and the amount of tightly bound labeled Lp(a) in the balloon-injured intima-inner media were, respectively, 174% (n = 6; ANOVA, P = .03) and 256% ANOVA, P = .005) of the values for labeled LDL. For labeled Lp(a) in the balloon-injured compared with the normal aortic intima-inner media, the recovery after 5 to 10 minutes, the permeability, and the accumulation after 23 hours were all increased, whereas the fractional loss was unchanged. These data suggest that the accumulation of Lp(a) is much larger in injured vessels than in normal vessels. Moreover, the data support the idea of a specific accumulation of Lp(a) compared with LDL in injured vessels.

Effects of injury on apoB kinetics and concentration in rabbit aorta

Endothelial injury or dysfunction and deposition of lipoproteins and cholesterol are key events during the development of atherosclerosis. We have studied the lipoprotein kinetics in arterial tissue in relation to endothelial injury and re-endothelialization. Endothelial injury was induced in rabbits by use of a balloon catheter. With a specific immunoradiometric assay, apoB levels in arterial tissue were measured at different time points for up to 10 weeks after injury. Forty-five minutes before being killed, the rabbits were injected with 125I-LDL, and influx of LDL was calculated from the accumulation of radioactivity in the arterial tissue. The concentration of apoB in the injured arterial tissue was four times higher than that in control arterial tissue (P < .0001). Within the lesion the concentration was as high in nonendothelialized as in re-endothelialized regions. The tissue pool of apoB was divided into a loosely bound fraction and a tightly bound fraction. The increase of apoB in the injured areas was primarily due to an increase in the tightly bound fraction. The influx of apoB was severalfold higher in nonendothelialized tissue than in re-endothelialized tissue or control areas (P < .005). When retention time was calculated, this was found to dramatically increase (by seven times) the tightly bound pool of apoB in the re-endothelialized areas. In addition to the large increase of a tightly bound apoB pool in injured areas, we found a prolonged retention time of apoB in the lesions, but only in the re-endothelialized areas.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective retention of VLDL, IDL, and LDL in the arterial intima of genetically hyperlipidemic rabbits in vivo. Molecular size as a determinant of fractional loss from the intima-inner media

To explore possible mechanisms whereby the triglyceride-rich lipoproteins IDL and VLDL may promote atherosclerosis, fractional loss of these lipoproteins from the intima-inner media was measured in vivo in genetically hyperlipidemic rabbits of the St Thomas's Hospital strain and compared with the fractional loss of LDL, HDL, and albumin. These rabbits exhibit elevated plasma levels of VLDL, IDL, and LDL. In each rabbit, two aliquots of the same macromolecule, one iodinated with 125I and the other with 131I, respectively, were injected intravenously on average 24 and 3 hours, respectively, before removal of the aortic intima-inner media. The fractional loss from the intima-inner media of newly entered macromolecules was then calculated. The average fractional losses for VLDL, IDL, LDL, HDL, and albumin in lesioned aortic arches were 0.1%/h (n = 4), -0.2%/h (n = 3), 1.8%/h (n = 4), 11.4%/h (n = 3), and 26.3%/h (n = 1), respectively; in nonlesioned aortic arches fractional losses for IDL, LDL, HDL, and albumin were 1.7%/h (n = 1), 0.6%/h (n = 2), 14.6%/h (n = 3), and 25.9%/h (n = 3). In both lesioned and nonlesioned aortic arches, the logarithms of these fractional loss values were inversely and linearly dependent on the diameter of the macromolecules (R2 = .57, P = .001 and R2 = .84, P < .001), as determined from electron photomicrographs of negatively stained lipoproteins. These results suggest that after uptake into the arterial intima, VLDL and IDL as well as LDL are selectively retained in comparison with HDL and albumin.

Cyclosporin suppresses transplant arteriosclerosis in the aorta-allografted, cholesterol-clamped rabbit. Suppression preceded by decrease in arterial lipoprotein permeability

The immunosuppressant cyclosporin has been suggested to aggravate as well as retard the development of transplant arteriosclerosis, the major long-term problem for patients with heart transplants. We examined the effect of human therapeutic levels of blood cyclosporin on the development of experimental transplant arteriosclerosis. The thoracic aorta from one rabbit was transplanted as an end-to-side bypass on the abdominal aorta of another rabbit, and plasma cholesterol was clamped at 5 to 7 mmol/L. Cyclosporin markedly suppressed the severity of transplant arteriosclerosis, judged both biochemically and histologically: cholesterol content in aortic transplants was reduced by 70% and 80% after 10 days and 20 days of cholesterol feeding, respectively (both comparisons, P < .01), and after 20 days of cholesterol feeding myointimal proliferation was totally inhibited in grafts from cyclosporin-treated animals, judged from maximal intimal thickness and intimal area on cross sections of grafts (both comparisons, P < .05). In another group of non-cholesterol-fed, aorta-transplanted rabbits, cyclosporin reduced by 90% (P < .01) an otherwise markedly increased permeability to low-density lipoprotein in transplanted aortas. These results suggest that cyclosporin causes a substantial decrease in the severity of transplant arteriosclerosis and that this effect is mediated at least partly via a large decrease in aortic lipoprotein permeability.

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)

Hypertension in rats does not potentiate hypercholesterolemia and aortic cholesterol deposition induced by a hypercholesterolemic diet

The effect of a hypercholesterolemic diet (HCD) on hyperlipemia and atherogenesis was investigated using normotensive Wistar/Kyoto rats (WKY), spontaneously hypertensive rats (SHR) and stroke-prone SHR (SHRSP), with systolic blood pressures increasing in that order. Feeding an HCD diet containing cholesterol, cholate and suet induced hypercholesterolemia in all the strains examined as compared with a normal diet. The plasma cholesterol levels were significantly higher in WKY than in SHR and SHRSP fed the HCD diet. The HCD diet also induced hepatic fat deposition, particularly deposition of cholesteryl esters, a slight increase in aortic cholesterol deposition, and elevation of both monoenoic/saturated fatty acid ratios and linoleate/arachidonate ratios in tissue lipids. The changes induced in the three strains by the HCD diet were not positively correlated with blood pressures. The HCD diet affected hepatic acyl-CoA:cholesterol acyltransferase and plasma lecithin:cholesterol acyltransferase activities differently in WKY and SHR which, in addition to the induction of delta 9 desaturase, may partly account for the difference in the diet-induced changes in the fatty acid compositions of plasma cholesteryl esters. The results indicate that hypertension per se does not stimulate the development of hypercholesterolemia and arterial cholesterol deposition induced by an HCD diet, suggesting that other factors are involved.

Aortic permeability to LDL as a predictor of aortic cholesterol accumulation in cholesterol-fed rabbits

The aim of this study was to investigate the possibility that the permeability characteristics of the arterial wall are related to the development of atherosclerosis. The in vivo regional variation of aortic permeability to iodinated human low density lipoprotein (LDL) in normal rabbits was compared with the regional variation in aortic cholesterol accumulation in cholesterol-fed rabbits. Aortas were divided into the aortic arch, thoracic aorta, and abdominal aorta, and each of these three parts was further subdivided into four segments of similar size. The permeability to LDL was 40 +/- 7 nl.cm-2.hr-1 (mean +/- SEM, n = 11) in the most proximal segment of the aortic arch and decreased throughout the length of the aorta to 3 +/- 1 nl.cm-2.hr-1 in the most caudal segment of the abdominal aorta. In such normal rabbits the aortic cholesterol content was similar in all 12 arterial segments at 0.08 +/- 0.005 mumol/cm2 (mean +/- SEM, n = 3 x 12). Aortic cholesterol accumulation was determined in other rabbits with an average plasma cholesterol level of 32 +/- 1 mmol/l for 96 days; the cholesterol content in the most proximal segment of the aortic arch was 2.7 +/- 0.5 mumol/cm2 (mean +/- SEM, n = 11) and decreased with increasing distance from the heart to 0.17 +/- 0.03 mumol/cm2 in the most caudal segment of the abdominal aorta. Linear regression analysis showed a close positive association between the permeability to LDL of a given aortic segment and the cholesterol accumulation in that same aortic segment after cholesterol feeding (r2 = 0.96, p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Accelerated cholesterol accumulation in homologous arterial transplants in cholesterol-fed rabbits. A surgical model to study transplantation atherosclerosis

Accelerated coronary artery disease has become a major complication to heart transplantation in humans. Therefore, we have developed a surgical model in the rabbit, with transplantation of the thoracic aorta as a bypass graft onto the abdominal aorta of another rabbit. The model permits the study of cholesterol metabolism in transplanted arteries. The graft did not accumulate cholesterol for as long as 298 days, provided that the rabbits were normocholesterolemic, i.e., with plasma cholesterol levels of 0.3-0.7 mmol/l. However, after a few weeks of cholesterol feeding resulting in plasma cholesterol levels of 2-5 mmol/l, the homologous graft accumulated cholesterol compared with intact aortic tissue in the rabbits and also compared with autologous aortic grafts. The intimal clearance of plasma cholesteryl ester, mainly high density lipoprotein cholesteryl ester, in the luminal layer of the aortic graft was 60-150 nl x cm-2 x hr-1 1-2 hours after transplantation. The intimal clearance in the corresponding intact thoracic aorta of the recipient animal was 5-20 nl x cm-2 x hr-1. The values were 1,500-3,000 nl x cm-2 x hr-1 51-298 days after transplantation, while the intimal clearance of the rabbit's own aorta remained unchanged. A pronounced increase in plasma lipoprotein permeability is thus an early event in transplanted arteries. It results in a higher cholesteryl ester influx that leads to cholesterol accumulation in the artery, but only if the rabbits are fed a cholesterol-enriched diet. This rabbit model may be useful in the search for interventional measures to prevent or diminish the accelerated coronary artery disease in transplanted hearts in humans.

Arterial response to mechanical injury: balloon catheter de-endothelialization

Coronary angioplasty has been used clinically for over a decade. Its initial promise as an alternative to coronary bypass surgery has only partially been fulfilled because of the high rate of post-operative restenosis. A number of animal models have been devised to study this phenomenon and although none is entirely satisfactory, they have, together with recent advances in molecular biology provided an insight into the cellular mechanisms that may contribute to this complication. This knowledge may ultimately lead to a means of therapeutic intervention. This review summarises our present understanding of the pathology of post-angioplasty re-stenosis as revealed by studies using the balloon catheter de-endothelialization model, and discusses some of the intervention strategies that have been attempted.

Quantitative studies of transfer in vivo of low density, Sf 12-60, and Sf 60-400 lipoproteins between plasma and arterial intima in humans

To assess the potential of various plasma lipoprotein classes to contribute to the lipid content of the arterial intima, influx and efflux of these plasma lipoprotein fractions into and from the intima of human carotid arteries were measured in vivo. While low density lipoprotein (LDL) is known to transfer from plasma into the arterial wall, there is less information on the atherogenic potential of lipoproteins of intermediate density (Sf 12-60) or of very low density (Sf 60-400). Aliquots of the same lipoprotein (LDL, Sf 12-60 lipoprotein particles, or Sf 60-400 lipoprotein particles) iodinated with iodine-125 and iodine-131 were injected intravenously 18-29 hours and 3-6 hours, respectively, before elective surgical removal of atheromatous arterial tissue, and the intimal clearance of lipoproteins, lipoprotein influx, and fractional loss of newly entered lipoproteins were calculated. Intimal clearance of Sf 60-400 particles was not detectable (less than 0.3 microliter x hr-1 x cm-2), whereas the average value for both LDL and Sf 12-60 lipoprotein particles was 0.9 microliter x hr-1 x cm-2. Since the fractional loss of newly entered LDL and Sf 12-60 lipoprotein particles was also similar, the results suggest similar modes of entry and exit for these two particles. However, due to lower plasma concentrations of Sf 12-60 lipoproteins as compared with LDL, the mass influx of cholesterol in the Sf 12-60 particles was on the order of one 10th of that in LDL, and that of apolipoprotein B was about one 20th.(ABSTRACT TRUNCATED AT 250 WORDS)

Low density lipoprotein uptake by an endothelial-smooth muscle cell bilayer

To study the interaction of endothelial and smooth muscle cells, and the means by which such interaction may affect lipid permeability of the arterial wall, cell bilayers were established by use of a transwell culture system. After confluent growth of both cell types had been achieved, iodine 125 bound to low-density lipoprotein (10 ng protein/ml) was added to the media of the upper well. After a 3-hour incubation period, the iodine 125-bound low-density lipoprotein content of the upper and lower media demonstrated an impedance to lipoprotein movement across the endothelial cell monolayer as compared to the bare porous polycarbonate filter of the transwell (p less than 10(-6)). The presence of smooth muscle cells in the bottom well significantly enhanced the permeability of the endothelial cell layer (p less than 10(-60). This effect remained unchanged over a 9-day time course. Membrane binding and cellular uptake of low-density lipoprotein by endothelial cells was not altered by smooth muscle cells, indicating that this change in permeability could not be easily attributed to changes in receptor-mediated transport or transcytosis. Membrane binding (p less than 0.02) and cellular uptake (p less than 10(-6)) of low-density lipoprotein by smooth muscle cells in the bilayer, when adjusted for counts available in the smooth muscle cell media, were both reduced in the early incubation period as compared to isolated smooth muscle cells. The disproportionate reduction in uptake as compared to binding would suggest that this was not entirely a receptor-dependent process.(ABSTRACT TRUNCATED AT 250 WORDS)

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 surgical model to study in vivo efflux of cholesterol from porcine aorta. Evidence for cholesteryl ester transfer through the aortic wall

We describe a surgical procedure in pigs which makes it possible to follow the influx into, the penetration through and the efflux from the arterial wall of labeled lipoproteins. After 4 h exposure of the luminal side of the arterial wall to labeled lipoproteins, labeled esterified cholesterol was found in all layers of the aortic wall, whereas labeled free cholesterol gained access only to the most luminal layer. The data suggest that at least 40%, if not 80-90%, of the cholesteryl ester that enters the aortic wall from the luminal side, passes through the entire wall and leaves the aortic wall through vasa vasorum and lymphatics. They also suggest that free cholesterol in the lipoproteins exchanges extensively with cellular free cholesterol while the lipoproteins penetrate through the most luminal layer. More than 90% of the labeled cholesteryl ester disappeared from the arterial wall during a cold chase period of 4 days. By the simultaneous use of [3H]cholesterol and [14C]cholesterol it was shown that only 10-20% of this disappearance can be explained by cholesteryl ester hydrolysis in the arterial wall.

The arterial barrier to lipoprotein influx in the hypercholesterolemic rabbit. 2. Long-term studies in deendothelialized and reendothelialized aortas

These studies consider whether a mild aortic injury that does not increase cholesteryl ester influx during the first few days promotes atheromatosis in the hypercholesterolemic rabbit. The cholesteryl ester influx in uninjured, deendothelialized, and reendothelialized aorta was also measured in order to account for the different cholesteryl ester contents in these areas. By 32-33 days after localized aortic injury which was made after 5-7 days of cholesterol feeding, uninjured (control) areas of the thoracic aortas had accumulated 48 micrograms/cm2 each of esterified and nonesterified cholesterol due to continued cholesterol feeding. However, the previously injured deendothelialized and reendothelialized areas of the aortas had accumulated 6 and 10 times as much esterified cholesterol, and 2 and 5 times as much nonesterified cholesterol, respectively, as the adjacent uninjured areas. Esterified cholesterol influx was low during the second day after injury but increased with time so that 30-31 days later the esterified cholesterol influx in deendothelialized and reendothelialized aorta was respectively 44 and 7 times as great as the 0.1 microgram/h/cm2 in the adjacent uninjured aorta. However, expressed per mg aortic cholesteryl ester, cholesteryl ester influx in reendothelialized aorta was no greater than in noninjured aorta. These studies suggest that an injury that does not initially increase cholesteryl ester influx can promote atheromatosis and that the higher rate of cholesteryl ester influx that develops with time in reendothelialized areas is closely related to its increased cholesteryl ester content.

The arterial barrier to lipoprotein influx in the hypercholesterolemic rabbit. 1. Studies during the first two days after mild aortic injury

These studies examine the hypothesis that removal of aortic endothelium eliminates a barrier to lipoprotein cholesterol influx. The aortas of rabbits fed a cholesterol-rich diet from 7 to 16 days before balloon injury were studied 1 or 2 days after deendothelialization of specific areas of the aorta. By this design the aortic sterol content was near normal on the day of injury, but areas of injured and noninjured aorta were exposed to identical levels of elevated plasma cholesterol. Measuring the arterial [3H]- and [14C] cholesterol fractions accumulated during 2 different intervals after dosage in the same animals permitted calculation of total influx and fractional loss of aortic cholesterol. During the first 2 days after deendothelialization, total (unidirectional) cholesteryl ester influx in deendothelialized aorta was similar to that in adjacent uninjured aorta, but total influx of nonesterified cholesterol was increased. The calculated increase in influx of nonesterified cholesterol was not a result of an increase in hydrolysis of entered cholesteryl ester but probably represents increased exchange of labeled cholesterol between artery and plasma. These results suggest that subendothelial layers of the aorta of short-term cholesterol-fed rabbits function as significant barriers to lipoprotein influx and that processes other than increased permeation by lipoproteins initiate injury-induced lesions.