狭窄血管远心端低密度脂蛋白浓度极化促进动脉粥样硬化形成

低密度脂蛋白（low density lipoprotein,LDL）浓度极化可能是动脉粥样硬化局灶性的重要原因,本文以狭窄血管远心端为研究对象,探讨LDL浓度极化对动脉粥样硬化发生、发展的影响。用数值计算模拟狭窄血管远心端LDL的壁面浓度分布,用激光扫描共聚焦显微镜测定狭窄血管远心端LDL沿z轴的浓度分布;用外科手术方法建立颈总动脉局部狭窄的实验模型,从整体动物水平观察LDL浓度极化对动脉粥样硬化形成的影响。数值计算和激光扫描共聚焦显微镜测定的结果表明,狭窄血管远心端存在显著的LDL浓度极化现象,且LDL壁面浓度与入口液流速度和狭窄程度有关：在相同的速率下,LDL壁面浓度在狭窄度为40％的圆管内最大;在狭窄程度相同的情况下,雷诺数（Re）为250时测得的LDL壁面浓度高于Re为500时测得的壁面浓度。整体动物实验表明,在狭窄血管远心端LDL浓度极化显著的区域形成明显的动脉粥样硬化病变,并且有大量的脂质沉积。以上结果提示,LDL浓度极化可能是导致动脉粥样硬化局灶性的重要因素。     

Effect of the endothelial glycocalyx layer on arterial LDL transport under normal and high pressure

To quantitatively investigate the role of the endothelial glycocalyx layer (EGL) in protecting the artery from excessive infiltration of atherogenic lipids such as low density lipoproteins (LDLs), a multilayer model with the EGL of an arterial segment was developed to numerically simulate the flow and the transport of LDLs under normal and high pressure. The transport parameters of the layers of the model were obtained from the hydrodynamic theory, the stochastic theory, and from the literature. The results showed that the increase in the thickness of the EGL could lead to a sharp drop in LDL accumulation in the intima. A partial damage to the EGL could compromise its barrier function, hence leading to enhanced infiltration/accumulation of LDLs within the wall of the arterial model. Without the EGL, hypertension could lead to a significantly enhanced LDL transport into the wall of the model. However, the intact EGL could protect the arterial wall from hypertension so that the LDL concentration in the intima layer was almost the same as that under normal pressure conditions. The results also showed that LDL concentration within the arterial wall increased with Φ (the fraction of leaky junctions) on the intima layer. The increase in LDL concentration with Φ was much more dramatic for the model without the EGL. For instance, without the EGL, a Φ of 0.0005 could lead LDL concentration within the arterial wall to be even higher than that predicted for the EGL intact model with a Φ of 0.002. In conclusion, an intact EGL with a sufficient thickness may act as a barrier to LDL infiltration into the arterial wall and has the potential to suppress the hypertension-driven hike of LDL infiltration/accumulation in the arterial wall.     

The Effect of a Spatially Heterogeneous Transmural Water Flux on Concentration Polarization of Low Density Lipoprotein in Arteries

Uptake of low density lipoprotein (LDL) by the arterial wall is likely to play a key role in atherogenesis. A particular process that may cause vascular scale heterogeneity in the rate of transendothelial LDL transport is the formation of a flow-dependent LDL concentration polarization layer on the luminal surface of the arterial endothelium. In this study, the effect of a spatially heterogeneous transmural water flux (that traverses the endothelium only via interendothelial cell clefts) on such concentration polarization is investigated numerically. Unlike in previous investigations, realistic intercellular cleft dimensions are used here and several values of LDL diffusivity are considered. Particular attention is paid to the spatially averaged LDL concentration adjacent to different regions of the endothelial surface, as such measures may be relevant to the rate of transendothelial LDL transport. It is demonstrated in principle that a heterogeneous transmural water flux can act to enhance such measures, and cause them to develop a shear dependence (in addition to that caused by vascular scale flow features, affecting the overall degree of LDL concentration polarization). However, it is shown that this enhancement and additional shear dependence are likely to be negligible for a physiologically realistic transmural flux velocity of 0.0439 μm s⁻¹ and an LDL diffusivity (in blood plasma) of 28.67 μm² s⁻¹. Hence, the results imply that vascular scale studies of LDL concentration polarization are justified in ignoring the effect of a spatially heterogeneous transmural water flux.     

Concentration polarization of atherogenic lipids in the arterial system

The transport of atherogenic lipids (LDL) in a straight segment of an artery with a semi-permeable wall was simulated numerically. The numerical analysis predicted that a mass transport phenomenon called ’concentration polarization’ of LDL might occur in the arterial system. Under normal physiological flow conditions, the luminal surface LDL concentration was 5%–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 across the arterial wall and inversely with wall shear rate. At low wall shear rate, the luminal surface LDL concentration was very sensitive to changes in flow conditions, decreasing sharply as wall shear rate increased. In order to verify the numerical analysis, the luminal surface concentration of bovine serum albumin (as a tracer macromolecule) in the canine carotid artery was measured in vitro by directly taking liquid samples from the luminal surface of the artery. The experimental result was in very good agreement with the numerical analysis. The authors believe that the mass transport phenomenon of ‘concentration polarization’ may indeed exist in the human circulation and play an important role in the localization of atherosclerosis.     

<Emphasis Type="Italic">Chlamydia pneumoniae</Emphasis> aggravates vein graft intimal hyperplasia in a rat model

Background  

Along with angioplasty, autologus vein grafts are commonly used for artery bypass grafting in patients with advanced arterial stenosis and drug-resistant angina pectoris. Although initially a successful procedure, long-term functionality is limited due to proliferation and migration of smooth muscle cells. Like in atherosclerosis, common chronic infections caused by viruses and bacteria may contribute to this process of vein graft failure. Here we investigated the possible role of Chlamydia pneumoniae (Cpn) in the pathogenesis of venous graft failure in an experimental animal model. In 2 groups (n = 10 rats/group), an epigastric vein-to-common femoral artery interposition graft was placed. Immediately thereafter, rats were infected with Cpn (5*10⁸ IFU) or injected with control solutions. Rats were sacrificed three weeks after surgery and the grafts were harvested for morphometrical and immunohistochemical analysis.     

Connexin43 Inhibition Prevents Human Vein Grafts Intimal Hyperplasia

Venous bypass grafts often fail following arterial implantation due to excessive smooth muscle cells (VSMC) proliferation and consequent intimal hyperplasia (IH). Intercellular communication mediated by Connexins (Cx) regulates differentiation, growth and proliferation in various cell types. Microarray analysis of vein grafts in a model of bilateral rabbit jugular vein graft revealed Cx43 as an early upregulated gene. Additional experiments conducted using an ex-vivo human saphenous veins perfusion system (EVPS) confirmed that Cx43 was rapidly increased in human veins subjected ex-vivo to arterial hemodynamics. Cx43 knock-down by RNA interference, or adenoviral-mediated overexpression, respectively inhibited or stimulated the proliferation of primary human VSMC in vitro. Furthermore, Cx blockade with carbenoxolone or the specific Cx43 inhibitory peptide ⁴³gap26 prevented the burst in myointimal proliferation and IH formation in human saphenous veins. Our data demonstrated that Cx43 controls proliferation and the formation of IH after arterial engraftment.     

Coupled computational analysis of arterial LDL transport -- effects of hypertension

Hypertension, a risk factor for atherosclerosis, increases the uptake of low density lipoproteins (LDL) by the arterial wall. Our objective in this work was to use computational modeling to identify physical factors that could be partially responsible for this effect. Fluid flow and mass transfer patterns in the lumen and wall of an arterial model were computed in a coupled manner, replicating as closely as possible previous experimental studies in which LDL uptake into the artery wall was measured in straight, excised arterial segments. Under conditions of both flow and no-flow, simulations predicted an increase in concentration polarization of LDL at the artery wall when arterial pressure was increased from 120 to 160 mmHg. However, this led to only a slight increase in mean LDL concentration within the arterial wall. However, if the permeability of the endothelium to LDL was allowed to vary with intra-arterial pressure, then the simulations predicted that the uptake of LDL would be enhanced 1.9-2.6 fold at higher pressure. The magnitude of this increase was consistent with experimental data. We conclude that the concentration polarization effects, enhanced by elevated intra-arterial pressure, cannot explain the increase in LDL uptake seen under hypertensive conditions. Instead, the data are most consistent with a pressure-linked increase in endothelial permeability to LDL.     

Concentration polarization of atherogenic lipids in the arterial system

Nomenclature c, Normalized LDL concentration (C*/C0); C0, incoming (bulk) LDL concentration (gr/cm3); Cw, LDL concentration on the luminal surface (gr/cm3); ,wC time average value of LDL concentration on the luminal surface (gr/cm3); D, diffusion coef-ficient of LDL (cm2/s); Q, blood flow rate (mL/s); 0R, average internal radius of the artery (cm); Re, Reynolds number (002/Run); Sc, Schmidt number (/Dn); t, normalized time (00*/tuR); u, normalized axial velocity (0*/uu); 0u, time a…     

Low-density lipoprotein concentration in the normal left coronary artery tree

Background

The blood flow and transportation of molecules in the cardiovascular system plays a crucial role in the genesis and progression of atherosclerosis. This computational study elucidates the Low Density Lipoprotein (LDL) site concentration in the entire normal human 3D tree of the LCA.

Methods

A 3D geometry model of the normal human LCA tree is constructed. Angiographic data used for geometry construction correspond to end-diastole. The resulted model includes the LMCA, LAD, LCxA and their main branches. The numerical simulation couples the flow equations with the transport equation applying realistic boundary conditions at the wall.

Results

High concentration of LDL values appears at bifurcation opposite to the flow dividers in the proximal regions of the Left Coronary Artery (LCA) tree, where atherosclerosis frequently occurs. The area-averaged normalized luminal surface LDL concentrations over the entire LCA tree are, 1.0348, 1.054 and 1.23, for the low, median and high water infiltration velocities, respectively. For the high, median and low molecular diffusivities, the peak values of the normalized LDL luminal surface concentration at the LMCA bifurcation reach 1.065, 1.080 and 1.205, respectively. LCA tree walls are exposed to a cholesterolemic environment although the applied mass and flow conditions refer to normal human geometry and normal mass-flow conditions.

Conclusion

The relationship between WSS and luminal surface concentration of LDL indicates that LDL is elevated at locations where WSS is low. Concave sides of the LCA tree exhibit higher concentration of LDL than the convex sides. Decreased molecular diffusivity increases the LDL concentration. Increased water infiltration velocity increases the LDL concentration. The regional area of high luminal surface concentration is increased with increasing water infiltration velocity. Regions of high LDL luminal surface concentration do not necessarily co-locate to the sites of lowest WSS. The degree of elevation in luminal surface LDL concentration is mostly affected from the water infiltration velocity at the vessel wall. The paths of the velocities in proximity to the endothelium might be the most important factor for the elevated LDL concentration.     

Computational model of blood flow in the aorto-coronary bypass graft

Background

Coronary artery bypass grafting surgery is an effective treatment modality for patients with severe coronary artery disease. The conduits used during the surgery include both the arterial and venous conduits. Long- term graft patency rate for the internal mammary arterial graft is superior, but the same is not true for the saphenous vein grafts. At 10 years, more than 50% of the vein grafts would have occluded and many of them are diseased. Why do the saphenous vein grafts fail the test of time? Many causes have been proposed for saphenous graft failure. Some are non-modifiable and the rest are modifiable. Non-modifiable causes include different histological structure of the vein compared to artery, size disparity between coronary artery and saphenous vein. However, researches are more interested in the modifiable causes, such as graft flow dynamics and wall shear stress distribution at the anastomotic sites. Formation of intimal hyperplasia at the anastomotic junction has been implicated as the root cause of long- term graft failure.Many researchers have analyzed the complex flow patterns in the distal sapheno-coronary anastomotic region, using various simulated model in an attempt to explain the site of preferential intimal hyperplasia based on the flow disturbances and differential wall stress distribution. In this paper, the geometrical bypass models (aorto-left coronary bypass graft model and aorto-right coronary bypass graft model) are based on real-life situations. In our models, the dimensions of the aorta, saphenous vein and the coronary artery simulate the actual dimensions at surgery. Both the proximal and distal anastomoses are considered at the same time, and we also take into the consideration the cross-sectional shape change of the venous conduit from circular to elliptical. Contrary to previous works, we have carried out computational fluid dynamics (CFD) study in the entire aorta-graft-perfused artery domain. The results reported here focus on (i) the complex flow patterns both at the proximal and distal anastomotic sites, and (ii) the wall shear stress distribution, which is an important factor that contributes to graft patency.

Methods

The three-dimensional coronary bypass models of the aorto-right coronary bypass and the aorto-left coronary bypass systems are constructed using computational fluid-dynamics software (Fluent 6.0.1). To have a better understanding of the flow dynamics at specific time instants of the cardiac cycle, quasi-steady flow simulations are performed, using a finite-volume approach. The data input to the models are the physiological measurements of flow-rates at (i) the aortic entrance, (ii) the ascending aorta, (iii) the left coronary artery, and (iv) the right coronary artery.

Results

The flow field and the wall shear stress are calculated throughout the cycle, but reported in this paper at two different instants of the cardiac cycle, one at the onset of ejection and the other during mid-diastole for both the right and left aorto-coronary bypass graft models. Plots of velocity-vector and the wall shear stress distributions are displayed in the aorto-graft-coronary arterial flow-field domain. We have shown (i) how the blocked coronary artery is being perfused in systole and diastole, (ii) the flow patterns at the two anastomotic junctions, proximal and distal anastomotic sites, and (iii) the shear stress distributions and their associations with arterial disease.

Conclusion

The computed results have revealed that (i) maximum perfusion of the occluded artery occurs during mid-diastole, and (ii) the maximum wall shear-stress variation is observed around the distal anastomotic region. These results can enable the clinicians to have a better understanding of vein graft disease, and hopefully we can offer a solution to alleviate or delay the occurrence of vein graft disease.

     

Flow-dependent concentration polarization of plasma proteins at the luminal surface of a cultured endothelial cell monolayer

Flow-dependent concentration or depletion of atherogenic low density lipoproteins which has been theoretically predicted to occur at a blood/endothelium boundary may play an important role in the genesis, progression, and regression of atherosclerosis in man and intimal hyperplasia in vascular grafts implanted in the arterial system in man and experimental animals. Hence to explore such a possibility, we have studied the effect of a steady shear flow on concentration polarization of plasma proteins and lipoproteins at the luminal surface of a cultured bovine aortic endothelial cell (BAEC) monolayer which served as a model of the vessel wall of an artery or an implanted vascular graft. The study was carried out by circulating a cell culture medium containing fetal calf serum or bovine plasma lipoproteins in steady flow through a parallel-plate flow cell in which a cultured BAEC monolayer was installed, over the physiologic ranges of wall shear rate and water filtration velocity at the BAEC monolayer. The water (cell culture medium) filtration velocity at the BAEC monolayer was determined to provide a measure of the change in concentration of plasma protein particles at the luminal surface of the BAEC monolayer. It was found that for perfusates containing plasma proteins and/or lipoproteins, water filtration velocity varied as a function of flow rate, being lowest in the absence of flow. Water filtration velocity increased or decreased as flow rate increased or decreased from an arbitrarily set non-zero value, indicating that surface concentration of protein particles varied as a direct function of flow rate, and the process was reversible. It was also found that at particle concentrations equivalent to those found in a culture medium containing serum at 20% by volume, plasma lipoproteins which were much smaller in number and lower in concentration but larger in size than albumin, showed almost the same effect as observed with serum which contained both lipoproteins and albumin, indicating that the substance responsible for this phenomenon is not albumin but lipoprotein whose diffusivity is much smaller than that of albumin. The results strongly support our hypothesis that flow-dependent concentration polarization of lipoproteins occurs at a blood endothelium boundary, and this in turn promote the localization of various vascular diseases which develop in our arterial system.     

Measurement of the flow in coronary artery bypass grafts

The aim of our study was to measure the flow in coronary artery bypass grafts and to compare the flow between two groups of patients. In group A the arterial revascularization was performed with both internal thoracic arteries using as a Y graft and in group B conventional revascularization using left internal thoracic artery (ITA) attached to the left anterior descending artery (LAD) and venous grafts to the other branches of the left coronary artery was performed. The flow in all grafts was measured at six time points during the operation. The cumulative flow at the end of the operation in the group A (arterial Y graft) was 51.8 +/- 24.5 ml/min and in group B (conventional technique) it was 96.8 +/- 41.1 ml/min (p < 0.05). The flow in left ITA to LAD was similar in both groups (27.3 +/- 15.9 ml/min and 26.3 +/- 16.1 ml/min in group A and B). The flow in right ITA (25.2 +/- 18.4 ml/min) was significantly lower than in venous grafts (72.5 +/- 45.5 ml/min). The calculated flow reserve was 2.2 in group A and 2.1 in group B. We found that the cumulative flow in arterial Y graft was lower in comparison with conventional revascularization. This is due to the lower flow in the right ITA branch of the Y graft compared to venous grafts. However based on clinical results, we can postulate that the flow in the Y graft is sufficient to meet the demand of the myocardium originally supplied by the left coronary artery.     

Effect of non-Newtonian and pulsatile blood flow on mass transport in the human aorta

To investigate the effects of both non-Newtonian behavior and the pulsation of blood flow on the distributions of luminal surface LDL concentration and oxygen flux along the wall of the human aorta, we numerically compared a non-Newtonian model with the Newtonian one under both steady flow and in vivo pulsatile flow conditions using a human aorta model constructed from MRI images. The results showed that under steady flow conditions, although the shear thinning non-Newtonian nature of blood could elevate wall shear stress (WSS) in most regions of the aorta, especially areas with low WSS, it had little effect on luminal surface LDL concentration (c(w)) in most regions of the aorta. Nevertheless, it could significantly enhance c(w) in areas with high luminal surface LDL concentration through the shear dependent diffusivity of LDLs. For oxygen transport, the shear thinning non-Newtonian nature of blood could slightly reduce oxygen flux in most regions of the aorta, but this effect became much more apparent in areas with already low oxygen flux. The pulsation of blood flow could significantly reduce c(w) and enhance oxygen flux in these disturbed places. In most other regions of the aorta, the oxygen flux was also significantly higher than that for the steady flow simulation. In conclusion, the shear shining non-Newtonian nature of blood has little effect on LDL and oxygen transport in most regions of the aorta, but in the atherogenic-prone areas where luminal surface LDL concentration is high and oxygen flux is low, its effect is apparent. Similar is for the effect of pulsatile flow on the transport of LDLs. But, the pulsation of blood flow can apparently affect oxygen flux in the aorta, especially in areas with low oxygen flux.     

Changes in plasma atrial natriuretic peptide--immunoreactivity in patients undergoing coronary artery bypass graft placements

Plasma atrial natriuretic peptide immunoreactivity (IrANP) was measured before, during, and after cardiopulmonary bypass for coronary artery bypass graft placement. Eight subjects scheduled for elective operation had in the premedicated preoperative state slightly elevated IrANP compared to controls. Neither induction of anesthesia with a high dose narcotic/non-depolarising relaxant/diazepam technique nor cardiopulmonary bypass changed IrANP significantly. Mixed venous and arterial IrANP increased immediately after discontinuing bypass, and remained elevated 1 h later. Because ANPs affect peripheral resistance as well as urinary sodium loss, the post-bypass elevations in these peptides may contribute to cardiovascular and diuretic effects after cardiopulmonary bypass.     

CD68 expression in aortocoronary saphenous vein bypass grafts

Atherosclerosis commonly affects the arteries harvested from patients 70 years of age or older. Saphenous vein grafts appear to maintain a higher patency rate after coronary artery bypass grafting in these subjects. The infiltration of macrophages is an early step in saphenous vein graft atherosclerosis; however, little is known regarding the underlying mechanisms of infiltration. The objective of the present report is to evaluate the presence of CD68-positive cells in the saphenous vein wall and correlate initial CD68-positive infiltration to specific clinical and biochemical parameters and the graft patency rate as estimated in patients undergoing coronary artery bypass grafting. A total of 309 patients were allocated into two groups: A1 patients, who were between 50 and 70 years of age, and A2 patients, who were 70 years or older at the time of vein harvesting. CD68 expression was evaluated by immunohistochemistry. There were no significant differences between A1 and A2 patients regarding macrophage expression within any of the analyzed vascular regions. Saphenous vein macrophages were never present in the tunica intima unless they were also expressed in the media or the adventitia. The patients with CD68-positive cells in the tunica intima had a significantly higher number of bypass stenoses when compared with the subjects who did not have CD68-positive cells in this layer. These findings suggest that the CD68-positive cells (those that have not yet developed into foam cells) present in the intima of saphenous vein grafts might serve as a very early marker of graft occlusion.     

Differential toxicities of air (mO-LDL) or copper-oxidized LDLs (Cu-LDL) toward endothelial cells.

In vivo low density protein (LDL) oxidation is a progressive phenomenon leading to the presence of minimally and highly oxidized LDLs in the subendothelial arterial space. Oxidized LDLs have been reported to be cytotoxic against endothelial cells. The goal of this study was to determine which of the minimally and highly oxidized LDLs were the most cytotoxic against bovine aortic endothelial cells (BAEC). Both the morphological aspect of the cells themselves, and LDH or MTT tests revealed that mO- or Cu-LDLs had similar cytotoxicity with up to 8 hours of oxidation, showing no relation with the level of LDL oxidation; for longer oxidation times, Cu-LDL cytotoxicity decreased. This phenomenon is linked to their different oxidation kinetics. Moreover, in the initial hours following BAEC incubation with mO- or Cu-LDLs, total cell glutathione dropped, whereas after 16 hours of incubation, highly oxidized Cu-LDL increased the glutathione level in the cell. The biphasic evolution of glutathione concentration corresponds to an autoprotective mechanism of cells against oxidized LDL cytotoxicity. This study suggests that the specific chemical characteristics of the different types of oxidized LDLs should always be precisely described in future assays devoted to studying the biological effects of what are known under the generic term as "oxidized LDLs". This precaution should prevent any confusion in interpreting different studies.     

Aldehyde adducts generated during lipid peroxidation modification of proteins

Abstract

Various lines of evidence indicate that an important part in the pathogenesis of atherosclerosis is the modification of the plasma low-density lipoproteins (LDLs). A large number of pro-inflammatory and pro-atherogenic properties have been ascribed to the oxidatively modified LDLs and their components. There is considerable evidence to support the role of lipid peroxidation products, reactive aldehydes in particular, originating from the oxidized LDL as important signaling molecules in the context of the atherosclerotic lesion. These aldehydes generated during the peroxidation of LDL exhibit a facile reactivity with proteins, generating a variety of intra- and intermolecular covalent adducts on the apolipoprotein B-100 particle in LDL. Characterization of the aldehyde adducts generated in the protein is therefore critical in understanding the nature of the oxidized LDL. However, the majority of adducts generated during the oxidative modification of LDL have not yet been chemically characterized. In this review, the current status of aldehyde adducts quantitatively analyzed in the Cu²⁺-oxidized LDL is reviewed.     

LDL particle core enrichment in cholesteryl oleate increases proteoglycan binding and promotes atherosclerosis

Several studies in humans and animals suggest that LDL particle core enrichment in cholesteryl oleate (CO) is associated with increased atherosclerosis. Diet enrichment with MUFAs enhances LDL CO content. Steroyl O-acyltransferase 2 (SOAT2) is the enzyme that catalyzes the synthesis of much of the CO found in LDL, and gene deletion of SOAT2 minimizes CO in LDL and protects against atherosclerosis. The purpose of this study was to test the hypothesis that the increased atherosclerosis associated with LDL core enrichment in CO results from an increased affinity of the LDL particle for arterial proteoglycans. ApoB-100-only Ldlr^−/− mice with and without Soat2 gene deletions were fed diets enriched in either cis-MUFA or n-3 PUFA, and LDL particles were isolated. LDL:proteogylcan binding was measured using surface plasmon resonance. Particles with higher CO content consistently bound with higher affinity to human biglycan and the amount of binding was shown to be proportional to the extent of atherosclerosis of the LDL donor mice. The data strongly support the thesis that atherosclerosis was induced through enhanced proteoglycan binding of LDL resulting from LDL core CO enrichment.