Relative significance of endothelium and internal elastic lamina in regulating the entry of macromolecules into arteries in vivo

Blood Lipoproteins Shape the Phenotype and Lipid Content of Early Atherosclerotic Lesion Macrophages: A Dual-Structured Mathematical Model

Macrophages in atherosclerotic lesions exhibit a spectrum of behaviours or phenotypes. The phenotypic distribution of monocyte-derived macrophages (MDMs), its correlation with MDM lipid content, and relation to blood lipoprotein densities are not well understood. Of particular interest is the balance between low density lipoproteins (LDL) and high density lipoproteins (HDL), which carry bad and good cholesterol respectively. To address these issues, we have developed a mathematical model for early atherosclerosis in which the MDM population is structured by phenotype and lipid content. The model admits a simpler, closed subsystem whose analysis shows how lesion composition becomes more pathological as the blood density of LDL increases relative to the HDL capacity. We use asymptotic analysis to derive a power-law relationship between MDM phenotype and lipid content at steady-state. This relationship enables us to understand why, for example, lipid-laden MDMs have a more inflammatory phenotype than lipid-poor MDMs when blood LDL lipid density greatly exceeds HDL capacity. We show further that the MDM phenotype distribution always attains a local maximum, while the lipid content distribution may be unimodal, adopt a quasi-uniform profile or decrease monotonically. Pathological lesions exhibit a local maximum in both the phenotype and lipid content MDM distributions, with the maximum at an inflammatory phenotype and near the lipid content capacity respectively. These results illustrate how macrophage heterogeneity arises in early atherosclerosis and provide a framework for future model validation through comparison with single-cell RNA sequencing data.

A multiphasic model for determination of water and solute transport across the arterial wall: effects of elastic fiber defects

Transport of solute across the arterial wall is a process driven by both convection and diffusion. In disease, the elastic fibers in the arterial wall are disrupted and lead to altered fluid and mass transport kinetics. A computational mixture model was used to numerically match previously published data of fluid and solute permeation experiments in groups of mouse arteries with genetic (knockout of fibulin-5) or chemical (treatment with elastase) disruption of elastic fibers. A biphasic model of fluid permeation indicated the governing property to be the hydraulic permeability, which was estimated to be 1.52×10^-9, 1.01×10^-8, and 1.07×10^-8 mm⁴/μN.s for control, knockout, and elastase groups, respectively. A multiphasic model incorporating solute transport was used to estimate effective diffusivities that were dependent on molecular weight, consistent with expected transport behaviors in multiphasic biological tissues. The effective diffusivity for the 4 kDA FITC-dextran solute, but not the 70 or 150 kDa FITC-dextran solutes, was dependent on elastic fiber structure, with increasing values from control to knockout to elastase groups, suggesting that elastic fiber disruption affects transport of lower molecular weight solutes. The model used here sets the groundwork for future work investigating transport through the arterial wall.

Flexible endoscopic micro-optical coherence tomography for three-dimensional imaging of the arterial microstructure

Micro-optical coherence tomography (µOCT) is a novel imaging approach enabling visualization of the microstructures of biological tissues at a cellular or sub-cellular level. However, it has been challenging to develop a miniaturized flexible endoscopic µOCT probe allowing helical luminal scanning. In this study, we built a flexible endoscopic µOCT probe with an outer diameter of 1.2 mm, which acquires three-dimensional images of the arterial microstructures via helical scanning with an axial and lateral resolutions of 1.83 µm and 3.38 µm in air, respectively. Furthermore, the depth of focus of the µOCT imaging probe was extended two-fold using a binary phase spatial filter. We demonstrated that the present endoscopic µOCT could image cellular level features of a rabbit artery with high-risk atheroma and a bioresorbable scaffold-implanted swine coronary artery. This highly-translatable endoscopic µOCT will be a useful tool for investigating coronary artery disease and stent biology.

Vascular Mural Cells in Healing Canine Myocardial Infarcts

Angiogenesis is a critical process in healing of myocardial infarcts, leading to the formation of highly vascular granulation tissue. However, effective cardiac repair depends on mechanisms that inhibit the angiogenic process after a mature scar is formed, preventing inappropriate expansion of the fibrotic process. Using a canine model of reperfused myocardial infarction, we demonstrated that maturation of the infarct leads to the formation of neovessels, with a thick muscular coat, that demonstrate distinct morphological characteristics. Many of these “neoarterioles” lack a defined internal elastic lamina and demonstrate irregular deposits of extracellular matrix in the media. Vascular mural cells in healing infarcts undergo phenotypic changes, showing minimal expression of desmin during the proliferative phase (1 hr occlusion/7 days reperfusion) but in the mature scar (8 weeks reperfusion) acquire a phenotype similar to that of vascular smooth muscle cells in control areas. Non-muscle myosin heavy chains A and B are induced in infarct endothelial cells and myofibroblasts, respectively, but are not expressed in neovascular mural cells. Recruitment of a muscular coat and formation of neoarterioles in mature scars may inhibit endothelial cell proliferation and vascular sprouting, stabilizing the infarct vasculature.

Ultrasound-mediated drug delivery for cardiovascular disease

INTRODUCTION

Ultrasound (US) has been developed as both a valuable diagnostic tool and a potent promoter of beneficial tissue bioeffects for the treatment of cardiovascular disease. These effects can be mediated by mechanical oscillations of circulating microbubbles, or US contrast agents, which may also encapsulate and shield a therapeutic agent in the bloodstream. Oscillating microbubbles can create stresses directly on nearby tissue or induce fluid effects that effect drug penetration into vascular tissue, lyse thrombi or direct drugs to optimal locations for delivery.

AREAS COVERED

The present review summarizes investigations that have provided evidence for US-mediated drug delivery as a potent method to deliver therapeutics to diseased tissue for cardiovascular treatment. In particular, the focus will be on investigations of specific aspects relating to US-mediated drug delivery, such as delivery vehicles, drug transport routes, biochemical mechanisms and molecular targeting strategies.

EXPERT OPINION

These investigations have spurred continued research into alternative therapeutic applications, such as bioactive gas delivery and new US technologies. Successful implementation of US-mediated drug delivery has the potential to change the way many drugs are administered systemically, resulting in more effective and economical therapeutics, and less-invasive treatments.

A novel model of accelerated intimal hyperplasia in the pig iliac artery

There is no good animal model of large artery injury-induced intimal hyperplasia (IH). Those available are reproducible, providing only a few layers of proliferating cells or have the disadvantage of the presence of a metallic stent that complicates histology evaluation. This study was designed to develop a new, simple model of accelerated IH based on balloon injury in conjunction with disruption of the Internal Elastic Lamina (IEL) in pig external iliac arteries. Iliac artery injury (n = 24) was performed in 12 Yorkshire pigs divided in two groups: Group I (n = 10), overdistention injury induced by an oversized non-compliant balloon; Group II (n = 14), arterial wall disruption by pulling back an isometric cutting balloon (CB) followed by stretching with a compliant Fogarty Balloon (FB). At two weeks, arteries were processed for morphometric analysis and immunohistochemistry (IHC) for smooth muscle cells (SMC) and proliferating cell nuclear antigen (PCNA). When comparing the two groups, at 2 weeks, arteries of group II had a higher incidence of IH (100%vs. 50%, P = 0.0059), increased intimal areas (2.54 ± 0.33 mm(2) vs. 0.93 ± 0.36 mm(2) , P = 0.004), increased intimal area/Media area ratios (0.95 ± 0.1 vs. 0.28 ± 0.05; P < 0.0001) and decreased lumen areas (6.24 ± 0.44 vs. 9.48 ± 1.56, P = 0.026). No thrombosis was noticed in Group II. Neointima was composed by proliferating SMC located with the highest concentration in the area of IEL disruption (IHC). Arterial injury by pulling back CB and FB induces significant IH in pig iliac arteries by two weeks without thrombosis. This model is superior to the classical overdistention non-compliant model and should be useful and cost-effective for preclinical testing of procedures designed to inhibit IH in large peripheral arteries.

Permeability change of arterial endothelium is an age-dependent function of lesion size in apolipoprotein E-null mice

The remodeling process of the arterial wall in atherosclerosis involves intimal thickening, which can be related to the barrier functions of the endothelial cell layer (ECL) and internal elastic lamina (IEL) using horseradish peroxidase (HRP) as a tracer. To evaluate the ECL and IEL permeabilities ( PECL and PIEL, respectively) and intimal transport parameters, e.g., apparent HRP velocity ( VI) and diffusivity, we compared simulations with a mathematical model to experimental data. In this study, we injected HRP into the vein of apolipoprotein E-null mice and measured HRP concentration profiles in lesioned areas of aortas. Lesion size was characterized by lower, middle, and upper ranges of the intimal/medial thickness (δI/δM): 0 < δI/δM ≤ 0.5, 0.5 < δI/δM ≤ 1.0, and δI/δM > 1.0. The PECL (in micrometers per minute) of 5-mo-old mice in the middle range (0.98 ± 0.14) was significantly greater than that in the lower range (0.21 ± 0.03) but not significantly different from mice in the upper range (0.99 ± 0.55). The PECL of 12-mo-old mice increased significantly with the relative intimal thickness: 0.27 ± 0.04 in the lower range, 1.12 ± 0.15 in the middle range, and 1.74 ± 0.24 in the upper range. In both age groups, VI (in micrometers per minute) increased significantly from lower to upper ranges of intimal thickness. However, PIEL did not change significantly with relative intimal thickness and age. In the upper range of intimal thickness, PECL and VI were significantly greater in 12-mo-old mice than in 5-mo-old mice. These data indicate an interaction between lesion growth and aging that leads to progressive loss in the integrity of the endothelial barrier function. Furthermore, the IEL is not a significant barrier between the intima and tunica media in the atherosclerotic process.

Computational modeling of coupled blood-wall mass transport of LDL: effects of local wall shear stress

The work herein represents a novel approach for the modeling of low-density lipoprotein (LDL) transport from the artery lumen into the arterial wall, taking into account the effects of local wall shear stress (WSS) on the endothelial cell layer and its pathways of volume and solute flux. We have simulated LDL transport in an axisymmetric representation of a stenosed coronary artery, where the endothelium is represented by a three-pore model that takes into account the contributions of the vesicular pathway, normal junctions, and leaky junctions also employing the local WSS to yield the overall volume and solute flux. The fraction of leaky junctions is calculated as a function of the local WSS based on published experimental data and is used in conjunction with the pore theory to determine the transport properties of this pathway. We have found elevated levels of solute flux at low shear stress regions because of the presence of a larger number of leaky junctions compared with high shear stress regions. Accordingly, we were able to observe high LDL concentrations in the arterial wall in these low shear stress regions despite increased filtration velocity, indicating that the increase in filtration velocity is not sufficient for the convective removal of LDL.

Macromolecular Transport in the Arterial Wall: Alternative Models for Estimating Barriers

Early atherosclerosis, or atherogenesis, is characterized by the abnormal accumulation of plasma-borne macromolecules (e.g., LDL) in the arterial intima. The change of barrier characteristics of tissue in the arterial wall requires evaluation of macromolecular transport across the endothelial cell layer (ECL) and internal elastic lamina (IEL), the luminal and abluminal boundaries of the arterial intima, respectively. In this study, alternative mathematical models are derived from dynamic mass balances to describe macromolecular transport across the arterial wall. One model considers each medial layer as a spatially lumped compartment, whereas another model consists of a spatially lumped intima and spatially distributed media. Model simulations of a tracer concentration distribution in the arterial wall are compared with concentration distributions of horseradish peroxidase (HRP) after i.v. injection in mice. For each model, optimal parameter values are obtained that yield model outputs matching the data well for two different HRP circulation times. The model parameter estimates show that the ECL is the major barrier for macromolecular transport across the normal arterial wall. Sensitivity analysis indicates that the parameter estimates of the transport coefficients of the ECL and IEL are well determined. Optimal circulation times are determined and expected to yield improved precision of parameter estimates in future experiments to reflect disease progression.

Effect of hypercholesterolemia on transendothelial EBD-albumin permeability and lipid accumulation in porcine iliac arteries

Hypercholesterolemia is associated with increased cardiovascular mortality and is known to promote the advancement of atherosclerotic lesions in experimental animal models. Juvenile swine were fed a normal or high-cholesterol diet, and the transendothelial macromolecular permeability of the external iliac arteries of these animals was assessed by measuring the uptake rate of circulating Evans blue dye (EBD). The extent and patterns of lipid-containing lesions were also determined using en face staining with Oil Red O (ORO). Sites of ORO staining often excluded EBD, possibly via the fragmentation of the internal elastic lamina, to which EBD binds. By spatially averaging the EBD uptake in arterial segments relatively free of ORO-positive lesions, it was found that endothelial permeability to albumin was greater in hypercholesterolemic pigs than in those on a normal diet (p=0.056).

Internal elastic lamina affects the distribution of macromolecules in the arterial wall: a computational study

The internal elastic lamina (IEL), which separates the arterial intima from the media, affects macromolecular transport across the medial layer. In the present study, we have developed a two-dimensional numerical simulation model to resolve the influence of the IEL on convective-diffusive transport of macromolecules in the media. The model considers interstitial flow in the medial layer that has a complex entrance condition because of the presence of leaky fenestral pores in the IEL. The IEL was modeled as an impermeable barrier to both water and solute except for the fenestral pores that were assumed to be uniformly distributed over the IEL. The media were modeled as a heterogeneous medium composed of an array of smooth muscle cells (SMCs) embedded in a continuous porous medium representing the interstitial proteoglycan and collagen fiber matrix. Results for ATP and low-density lipoprotein (LDL) demonstrate a range of interesting features of molecular transport and uptake in the media that are determined by considering the balance among convection, diffusion, and SMC surface reaction. The ATP concentration distribution depends strongly on the IEL pore structure because ATP fluid-phase transport is dominated by diffusion emanating from the fenestral pores. On the other hand, LDL fluid-phase transport is only weakly dependent on the IEL pore structure because convection spreads LDL laterally over very short distances in the media. In addition, we observe that transport of LDL to SMC surfaces is likely to be limited by the fluid phase (surface concentration less than bulk concentration), whereas ATP transport is limited by reaction on the SMC surface (surface concentration equals bulk concentration).

Effects of altering infusion parameters on intimal hyperplasia following local catheter-based delivery into the rabbit iliac artery

BACKGROUND

Efficient local gene or drug therapy requires optimized application modalities to avoid vessel damage, which might lead to increased neointimal hyperplasia. Aim of the study was to evaluate different application parameters for local delivery using the channeled balloon catheter in order to minimize vessel trauma induced by local application.

METHODS AND RESULTS

Sixty cholesterol fed rabbits were randomly enrolled into twelve groups of different local application parameters: group I, application pressure 2atm/application volume 1ml physiologic saline; group II, 2atm/2ml; group III, 2atm/5ml; group IV, 4atm/1ml; group V, 4atm/2ml; group VI, 4atm/5ml. The other six groups received Ringer's solution instead of saline. Administration of the solution was randomly performed in one iliac artery using the channeled balloon catheter with simultaneous balloon angioplasty (8atm). The contralateral iliac artery served as a control and was treated with balloon angioplasty only. Four weeks after local therapy, calibrated angiography was performed; the animals were sacrificed, vessel segments were excised and quantitative morphometric measurements were obtained. In none of the animals acute complications, e.g. dissection, thrombosis or perforation of the vessel, was noted. Up to an application pressure of 4atm and an application volume of 5ml, no significant neointima formation was seen compared to arteries which underwent angioplasty only. Additionally, no significant differences between saline and Ringer's solution were detected. In a multivariate analysis, neither application pressure nor volume were found to have a statistically significant influence on the amount of neointimal hyperplasia.

CONCLUSIONS

Local application of "drugs" using the channeled balloon catheter is safe and feasible without significant induction of neointimal hyperplasia compared to angioplasty, if an application volume of 5ml and a pressure of 4atm is not exceeded.

Role of monocytes in atherogenesis

This review focuses on the role of monocytes in the early phase of atherogenesis, before foam cell formation. An emerging consensus underscores the importance of the cellular inflammatory system in atherogenesis. Initiation of the process apparently hinges on accumulating low-density lipoproteins (LDL) undergoing oxidation and glycation, providing stimuli for the release of monocyte attracting chemokines and for the upregulation of endothelial adhesive molecules. These conditions favor monocyte transmigration to the intima, where chemically modified, aggregated, or proteoglycan- or antibody-complexed LDL may be endocytotically internalized via scavenger receptors present on the emergent macrophage surface. The differentiating monocytes in concert with T lymphocytes exert a modulating effect on lipoproteins. These events propagate a series of reactions entailing generation of lipid peroxides and expression of chemokines, adhesion molecules, cytokines, and growth factors, thereby sustaining an ongoing inflammatory process leading ultimately to lesion formation. New data emerging from studies using transgenic animals, notably mice, have provided novel insights into many of the cellular interactions and signaling mechanisms involving monocytes/macrophages in the atherogenic processes. A number of these studies, focusing on mechanisms for monocyte activation and the roles of adhesive molecules, chemokines, cytokines and growth factors, are addressed in this review.

Impact of transport and drug properties on the local pharmacology of drug-eluting stents

Drugs released from stents are driven by physiological transport forces, principally solvent-driven flow (convection) and random molecular agitation (diffusion). The relative strength of these two forces determines drug penetration and distribution in the arterial wall. Drug physicochemical factors can induce critical modulations to the primary distribution, both transiently and at steady state. Hydrophobic interactions and nonspecific binding, for example, can both result in tissue drug concentrations severalfold above administered concentration. Drug interaction with native proteins may also interfere with drug transfer at the stent-artery interface. These transport forces and tissue interactions can induce local drug concentrations even at steady state to vary by one or more orders of magnitude over the span of a few cells. To account for significant local variations in drug concentrations following stent-based delivery, rational design of vascular delivery systems requires consideration of drug distribution and tissue interactions on a local, continuum basis. Continuum analysis adapts traditional pharmacokinetics to the local environment by supplementing discrete global parameters of drug content with continuous local values of concentration, transport and binding. The interplay of these parameters with local flux conditions and drug and tissue properties defines the local drug distribution in space and over time. This type of analysis may well become increasingly relevant given the trend toward stent-based drug therapy in cardiovascular care.

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.

Comparative Trial of Local Pharmacotherapy withl-Arginine, r-Hirudin, and Molsidomine to Reduce Restenosis after Balloon Angioplasty of Stenotic Rabbit Iliac Arteries

PURPOSE

To determine if local application of L-arginine, r-hirudin, or molsidomine significantly reduces restenosis after balloon angioplasty in stenotic rabbit iliac arteries.

MATERIALS AND METHODS

Thirty-one male cholesterol-fed New Zealand white rabbits underwent balloon dilation of both common iliac arteries to induce arterial stenosis. Four weeks later, one stenotic iliac artery was simultaneously dilated and received local application of L-arginine (210 mg/mL, n = 7), r-hirudin (0.5 mg/mL, n = 8), or molsidomine (0.2 mg/mL, n = 8) with a channeled balloon catheter. On the contralateral side, 0.9% saline was injected as a control. In eight sham animals, saline was applied to one iliac artery and balloon dilation to only the contralateral artery. Six weeks after local treatment, vessels were harvested, and computerized morphometric and immunohistologic analyses were performed.

RESULTS

Application of drugs resulted in a significant reduction of neointimal area as follows: 53% with L-arginine (1.01 mm(2) vs. 2.17 mm(2), P <.05), 43% with molsidomine (1.04 mm(2) vs. 1.89 mm(2), P <.05), and 20% with r-hirudin (1.79 mm(2) vs. 2.24 mm(2), P <.05). Infusion of saline led to a significant increase (50%, 1.21 mm(2) vs. 1.93 mm(2), P <.05) in neointimal area compared with balloon dilation alone. Immunohistologic findings showed a significant reduction of macrophages (5.0% vs. 10.2%, P <.05) and proliferating cells (6.2% vs. 10.6%, P <.05) in the neointima after local application of L-arginine.

CONCLUSION

Reduction of neointimal area was significant for L-arginine and molsidomine but not for r-hirudin. Saline infusion caused significant arterial trauma, resulting in additional neointimal proliferation.

Effect of endothelial injury and increased blood pressure on albumin accumulation in the arterial wall: a numerical study

The present study investigates the influence of endothelial damage and of blood pressure on albumin accumulation in the arterial wall. For this purpose a numerical model for the coupled mass transport processes in the arterial lumen and in the various layers of the arterial wall is developed. The model considers the transport in the endothelium, intima, internal elastic lamina (IEL) and media of a straight axisymmetric arterial segment. In the arterial lumen fully developed stationary blood flow is assumed, the filtration velocity in the wall layers is calculated applying Darcy's law. The description of the luminal mass transport uses the stationary convection-diffusion equation, the transport in the porous intima and media is modelled applying the volume-averaged stationary convection-diffusion-reaction equation. The transport processes in the lumen, intima and media are coupled by the flux across the endothelium and IEL, which is mathematically described using the Kedem-Katchalsky equations. The numerical solution of the transport equations applies the finite element method. The results demonstrate a high resistance of the healthy endothelium to macromolecule exchange between blood and the artery wall. The reduced resistance of an injured endothelium causes an increased mass flux into the wall which results in higher concentration levels within the wall. The effect of the blood pressure on the wall concentration level is different for a helathy and an injured endothelium. In the case of a healthy endothelium a blood pressure increase causes a decrease of the intimal concentration and an increase of the medial concentration, whereas in the case of an injured endothelium an increased blood pressure results in higher concentration levels within the intima and media.

The effect of caloric restriction on the aortic tissue of aging rats

Connective tissue shows peculiar and complex age-related modifications, which can be, at least in part, responsible for altered functions and increased susceptibility to diseases. Food restriction has long been known to prolong life in rodents, having antiaging effects on a variety of physiologic and pathologic processes. Therefore, the aorta has been investigated in rats fed normal or hypocaloric diet, from weaning to senescence. Compared with controls, caloric-restricted animals showed less pronounced age-dependent alterations such as elastic fiber degradation, collagen accumulation and cellular modifications. Immunocytochemical analyses revealed that elastic fibers were positively labelled for biglycan, decorin, ApoB100 (LDL), ApoA1 (HDL) and elastase and that the intensity of the reactions was time- and diet-dependent. With age, the major changes affecting aortic elastic fibers were increased positivity for decorin, LDL and elastase. Compared with age-matched normal fed rats, caloric restricted animals revealed lower content of LDL, decorin and elastase and higher positivity for HDL. These data suggest that a caloric restricted diet might influence the aging process of the arterial wall in rats, delaying the appearance of age-related degenerative features, such as structural alterations of cells and matrix and modified interactions of elastin with cells and with other extracellular matrix molecules.

Intraarterial protein delivery via intimally-adherent bilayer hydrogels

Arterial structure plays an important role in drug delivery from intraarterial depots. The internal elastic lamina forms a major diffusive resistance to the transport of macromolecular drugs from intimally-adherent hydrogel depots to the arterial media. The objectives of this study were to develop an approach by which to form a bilayer hydrogel depot with a higher permeability intimally-adherent layer, containing the drug, and a lower permeability luminal layer, and to evaluate ex vivo whether this luminal layer could enhance the delivery of a protein to the arterial media. Sequential interfacial photopolymerization of polyethyleneglycol diacrylate precursors (molecular weight 4000 for the luminal layer, 10,000 for the intimal layer) with eosin Y and triethanolamine as an initiation system was employed to form these bilayer hydrogels. Horseradish peroxidase was used as a model protein, and delivery to the arterial media was measured in rat carotid arteries ex vivo. The lower permeability luminal layer served to enhance delivery of the model protein into the arterial media for delivery periods at least up to 72 h. Thus, it was possible to compensate for the diffusional resistance of the internal elastic lamina on the one side of the hydrogel depot with a second diffusional resistance on the other side of the hydrogel.

Local administration of ramiprilat is less effective than oral ramipril in preventing restenosis after balloon angioplasty in an animal model

PURPOSE

To test the hypothesis that local administration of angiotensin converting enzyme (ACE) inhibitor via a microporous balloon catheter would be more effective than oral administration of ACE inhibitor in preventing neointima formation after balloon angioplasty.

MATERIALS AND METHODS

Neointima formation was induced by balloon denudation followed by 0.5% cholesterol diet in 29 New Zealand White rabbits. Directly after denudation, local administration of 1.8 mg of ramiprilat (n = 7) or saline (n = 7) with a microporous balloon catheter at a pressure of 3 atm was performed. Both groups additionally received ramipril orally (1 mg/d). Seven animals were treated exclusively with oral ramipril. The control group was fed a 0.5% cholesterol diet and given no medication (n = 8). Six weeks after intervention, the animals were killed and morphometric and immunohistologic analyses were performed.

RESULTS

Oral administration of ramipril resulted in a significant reduction of placque area (-66%, P < .05). Oral and local administration of the ACE inhibitor was followed by a nonsignificant reduction of the neointimal area (-17%). Local administration of saline combined with oral ramipril failed to prevent neointimal formation (reduction of 6%, NS).

CONCLUSION

Oral administration of ramipril resulted in a significant reduction of neointimal proliferation in New Zealand White rabbits. The possible benefit of an additional administration of local ramiprilat was diminished by an excessive neointimal hyperplasia, which was most likely caused by the inherent vessel trauma with use of the microporous balloon catheter.

Measurement of drug distribution in vascular tissue using quantitative fluorescence microscopy

Quantitative tools to assess vascular macromolecular distributions have been limited by low signal-to-noise ratios, reduced spatial resolution, postexperimental motion artifact, and the inability to provide multidimensional drug distribution profiles. Fluorescence microscopy offers the potential of identifying exogenous compounds within intact tissue by reducing autofluorescence, the process by which endogenous compounds emit energy at the same wavelength as fluorescent labels. A new technique combining fluorescence microscopy with digital postprocessing has been developed to address these limitations and is now described in detail. As a demonstration, histologic cross-sections of calf carotid arteries that had been loaded endovascularly with FITC-Dextran (20 kD) ex vivo were imaged at two different locations of the electromagnetic spectrum, one exciting only autofluorescent structures and the other exciting both autofluorescent elements and exogenous fluorescent labels. The former image was used to estimate the autofluorescence in the latter. Subtraction of the estimated autofluorescence resulted in an autofluorescence-corrected image. A standard curve, constructed from arteries that were incubated until equilibrium in different bulk phase concentrations of FITC-Dextran, was used to convert fluorescent intensities to tissue concentrations. This resulted in a concentration map with spatial resolution superior to many of the previous methods used to quantify macromolecular distributions. The transvascular concentration profiles measured by quantitative fluorescence microscopy compared favorably with those generated from the proven en face serial sectioning technique, validating the former. In addition, the fluorescence method demonstrated markedly increased spatial resolution. This new technique may well prove to be a valuable tool for elucidating the mechanisms of macromolecular transport, and for the rational design of drug delivery systems.

Arterial heparin deposition: role of diffusion, convection, and extravascular space

Transvascular transport has been studied with atherogenic, tracer, and inert compounds such as low-density lipoprotein, horseradish peroxidase, and albumin, respectively. Few studies used vasoactive compounds, and virtually all studies examined entry from the lumen and not from the perivascular space. We compared several mechanisms that govern arterial heparin deposition after administration to the perivascular and endovascular aspects of the calf carotid artery in vitro and the rabbit iliac artery in vivo. In the absence of transmural hydrostatic pressure gradients, heparin deposition following endovascular administration was unaffected by deendothelialization and was indistinguishable from perivascular delivery. Deposition in the former was enhanced by the addition of a pressure gradient and to a greater extent in denuded arteries, indicating that convection influences transport but is dampened by the endothelium. Neither the endothelium nor the adventitia pose significant resistances to heparin. Deposition in vivo was greater following endovascular hydrogel release than perivascular application from similar devices to native or denuded arteries. The loss of drug to extra-arterial microvessels exceeded the loss of drug to the lumen flow. These findings are essential for describing vascular pharmacokinetics and for implementing local pharmacotherapies.

Experimental hypercholesterolemia induces ultrastructural changes in the internal elastic lamina of porcine coronary arteries

The internal elastic lamina (IEL) serves as a barrier for cells and macromolecules migration between the intima and the media in the vascular wall. Several investigators have reported internal elastic lamina ultrastructural changes in elastic arteries with atherosclerosis. However, no quantitative and qualitative assessment of the internal elastic lamina architecture in muscular arteries such as the coronary circulation during early atherosclerosis have been performed yet. In this study, we therefore evaluated the ultrastructural morphological changes of the IEL in the coronary circulation of pigs fed with high cholesterol diet. Animals were sacrificed after being fed either a high cholesterol diet for 10-12 weeks (n = 5, 12 coronary segments) or a control diet (n = 4, 15 coronary segments). Coronary arteries were analyzed by transmission and scanning electron microscopy. In addition, computerized digital analysis of the images obtained by confocal scanning microscopy was performed for the quantitation of the morphologic changes in the internal elastic lamina. Confocal microscopy and scanning electron microscopy revealed an altered pattern characterized by large oval fenestration formation in the internal elastic lamina of hypercholesterolemic animals. Computerized morphometric analysis of confocal microscopy images demonstrated that compared to controls, the IEL of cholesterol-fed animals was characterized by an increase in the minor diameter of the fenestrae (2.16 +/- 0.04 microm versus 3.32 +/- 0.06 microm, P = 0.003) and a decrease in the fenestrae density (22333 +/- 1334/mm2 versus 17552 +/- 931/mm2, P = 0.015) of the internal elastic lamina. The percentage of the IEL area covered by the fenestrae correlated with the intimal thickness (r = 0.79, P = 0.004). This study demonstrates that experimental hypercholesterolemia is characterized by ultrastructural changes of the internal elastic lamina in the coronary circulation. This study suggests that the IEL may play an important role in the development of structural changes which characterize the early phase of coronary atherosclerosis.

Effect of atherosclerosis on transmural convection an arterial ultrastructure. Implications for local intravascular drug delivery

Local infusion of agents through perforated catheters may reduce neointimal formation following vascular angioplasty. Such treatment will succeed only if the drug is retained within the arterial intima long enough to promote repair. Drugs will be dispersed throughout the wall predominantly by transmural convection instead of diffusion if the Peclet number, Pe = J (1-delta f)/P, is greater than unity, where J is the transmural fluid flow per unit surface area and delta(f) and P are the reflection and permeability coefficients to the drug, respectively. Although the targets of local drug delivery will be atherosclerotic vessels, little is known about the transport properties of these vessels. Accordingly, we evaluated the effects of hypercholesterolemia and atherosclerosis on J per unit pressure (hydraulic conductance, Lp) and on ultrastructure in femoral arteries. Measurements were made at 30, 60, and 90 mm Hg in anesthetized New Zealand white rabbits fed a normal diet (n = 6) and after 3 weeks of lipid feeding (n = 19). Atherosclerosis was induced in six lipid-fed animals by air desiccation of a femoral artery. Hydraulic conductance was significantly greater in vessels from hypercholesterolemic than from normal animals and decreased with pressure only in hypercholesterolemic arteries. Atherosclerosis did not augment hydraulic conductance compared with hypercholesterolemia alone. Electron microscopic examination demonstrated damaged endothelium in hypercholesterolemic arteries and both altered endothelium and less tightly packed medial tissue, compared with controls, in atherosclerotic vessels, at least at lower pressures. Peclet numbers for macromolecules exceeded unity for all three groups of arteries and reached 0.3 to 0.4 for molecules as small as heparin. Thus, convection plays a dominant role in the distribution of macromolecular agents following local delivery and may result in their rapid transport to the adventitia in the femoral artery.

Phenotypic modulation of smooth muscle cells after arterial injury is associated with changes in the distribution of laminin and fibronectin

Earlier in vitro studies suggest opposing roles of laminin and fibronectin in regulation of differentiated properties of vascular smooth muscle cells. To find out if this may also be the case in vivo, we used immunoelectron microscopy to study the distribution of these proteins during formation of intimal thickening after arterial injury. In parallel, cell structure and content of smooth muscle alpha-actin was analyzed. The results indicate that the cells in the normal media are in a contractile phenotype with abundant alpha-actin filaments and an incomplete basement membrane. Within 1 week after endothelial denudation, most cells in the innermost layer of the media convert into a synthetic phenotype, as judged by loss of actin filaments, construction of a large secretory apparatus, and destruction of the basement membrane. Some of these cells migrate through fenestrae in the internal elastic lamina and invade a fibronectin-rich network deposited on its luminal surface. Within another few weeks a thick neointima forms, newly produced matrix components replace the stands of fibronectin, and a basement membrane reappears. Simultaneously, the cells resume a contractile phenotype, recognized by disappearance of secretory organelles and restoration of alpha-actin filaments. These findings support the notion that laminin and other basement membrane components promote the expression of a differentiated smooth muscle phenotype, whereas fibronectin stimulates the cells to adopt a proliferative and secretory phenotype.

Transfer of low density lipoprotein into the arterial wall and risk of atherosclerosis

The aim of the review is to summarize the present knowledge on determinants of transfer of low density lipoprotein (LDL) into the arterial wall, particularly in relation to the risk of development of atherosclerosis. The flux of LDL into the arterial wall (in moles of LDL per surface area per unit of time) has two major determinants, i.e. the LDL concentration in plasma and the arterial wall permeability. LDL enters the arterial wall as intact particles by vesicular ferrying through endothelial cells and/or by passive sieving through pores in or between endothelial cells. Estimates in vivo of the LDL permeability of a normal arterial wall vary between 5 and 100 nl/cm2/h. In laboratory animals, the regional variation in the arterial wall permeability predicts the pattern of subsequent dietary induced atherosclerosis. Moreover, mechanical or immunological injury of the arterial wall increases the LDL permeability and is accompanied by accelerated development of experimental atherosclerosis. This supports the idea that an increased permeability to LDL, like an increased plasma LDL concentration, increases the risk of atherosclerosis. Hypertension, smoking, genetic predisposition, atherosclerosis, and a small size of LDL may all increase the arterial wall permeability to LDL and in this way increase the risk of accelerated development of atherosclerosis. The hypothesis that atherosclerosis risk can be reduced by improving the barrier function of the arterial wall towards the entry of LDL remains to be investigated; agents which directly modulate the LDL permeability of the arterial wall in vivo await identification.

Kinetics of smooth muscle cell proliferation and intimal thickening in a pig carotid model of balloon injury

Restenosis as a result of neointimal smooth muscle cell accumulation is an important limitation to the effectiveness of balloon angioplasty as a treatment for end-stage atherosclerosis. Quantitative animal models allow the definition of pathophysiological mechanisms and the evaluation of new therapeutic strategies. In this study we quantified the time course of neointima formation by morphometry, and smooth muscle cell (SMC) proliferation by immunocytochemistry for proliferating cell nuclear antigen (PCNA), in the pig carotid artery 0-28 days following balloon injury. This led to two distinct kinds of injury observed also in clinical studies, namely medial dilatation or deep medial tearing with rupture of the internal elastic lamina. Dilatation injury alone led to medial enlargement and neointima formation by 7 days, which did not increase further up to 28 days. Medial enlargement was similar following rupture of the internal elastic lamina; however the sum of neointima formation plus the area of medial repair ('neomedia') increased progressively up to 21 days after balloon injury. Balloon injury increased the PCNA index of smooth muscle cells in the media underlying an intact internal elastic lamina maximally after 3 days. The PCNA index in the neointima and especially in the neomedia was greater and maximal after 7 days. Endothelial regrowth occurred by 21 days in the presence or absence of medial tears. Our results establish a quantitative pig model of balloon injury which will allow the assessment of new therapeutic strategies directed at two clinically relevant types of injury. Medial tearing is associated with an enhanced and localized proliferative response and may therefore be especially important in human restenosis.