The interaction of convection and diffusion in the transport of 131I-albumin within the media of the rabbit thoracic aorta

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.

Elastic fiber fragmentation increases transmural hydraulic conductance and solute transport in mouse arteries

Transmural advective transport of solute and fluid was investigated in mouse carotid arteries with either a genetic knockout of Fibulin-5 (Fbln5-/-) or treatment with elastase to determine the influence of a disrupted elastic fiber matrix on wall transport properties. Fibulin-5 is an important director of elastic fiber assembly. Arteries from Fbln5-/- mice have a loose, non-continuous elastic fiber network and were hypothesized to have reduced resistance to advective transport. Experiments were carried out ex vivo at physiological pressure and axial stretch. Hydraulic conductance (Lp ) was measured to be 4.99·10-6 ± 8.94·10-7, 3.18·-5 ± 1.13·10-5 (P < 0.01), and 3.57·10-5 ± 1.77·10-5 (P < 0.01) mm·s-1·mmHg-1 for wild-type, Fbln5-/-, and elastase-treated carotids, respectively. Solute fluxes of 4, 70, and 150 kDa FITC-dextran were statistically increased in Fbln5-/- compared to wild-type by a factor of 4, 22, and 3 respectively. 70 kDa FITC-dextran solute flux was similarly increased in elastase-treated carotids by a factor of 27. Solute uptake by Fbln5-/- carotids was decreased compared to wild-type for all investigated dextran sizes after 60 minutes of transmural transport. These changes in transport properties of elastic fiber compromised arteries have important implications for the kinetics of biomolecules and pharmaceuticals in arterial tissue following elastic fiber degradation due to aging or vascular disease.

Chronic hypertension increases aortic endothelial hydraulic conductivity by upregulating endothelial aquaporin-1 expression

Numerous studies have examined the role of aquaporins in osmotic water transport in various systems, but virtually none have focused on the role of aquaporin in hydrostatically driven water transport involving mammalian cells save for our laboratory's recent study of aortic endothelial cells. Here, we investigated aquaporin-1 expression and function in the aortic endothelium in two high-renin rat models of hypertension, the spontaneously hypertensive genetically altered Wistar-Kyoto rat variant and Sprague-Dawley rats made hypertensive by two-kidney, one-clip Goldblatt surgery. We measured aquaporin-1 expression in aortic endothelial cells from whole rat aortas by quantitative immunohistochemistry and function by measuring the pressure-driven hydraulic conductivities of excised rat aortas with both intact and denuded endothelia on the same vessel. We used them to calculate the effective intimal hydraulic conductivity, which is a combination of endothelial and subendothelial components. We observed well-correlated enhancements in aquaporin-1 expression and function in both hypertensive rat models as well as in aortas from normotensive rats whose expression was upregulated by 2 h of forskolin treatment. Upregulated aquaporin-1 expression and function may be a response to hypertension that critically determines conduit artery vessel wall viability and long-term susceptibility to atherosclerosis.NEW & NOTEWORTHY The aortic endothelia of two high-renin hypertensive rat models express greater than two times the aquaporin-1 and, at low pressures, have greater than two times the endothelial hydraulic conductivity of normotensive rats. Data are consistent with theory predicting that higher endothelial aquaporin-1 expression raises the critical pressure for subendothelial intima compression and for artery wall hydraulic conductivity to drop.

Intimal and medial contributions to the hydraulic resistance of the arterial wall at different pressures: a combined computational and experimental study

The hydraulic resistances of the intima and media determine water flux and the advection of macromolecules into and across the arterial wall. Despite several experimental and computational studies, these transport processes and their dependence on transmural pressure remain incompletely understood. Here, we use a combination of experimental and computational methods to ascertain how the hydraulic permeability of the rat abdominal aorta depends on these two layers and how it is affected by structural rearrangement of the media under pressure. Ex vivo experiments determined the conductance of the whole wall, the thickness of the media and the geometry of medial smooth muscle cells (SMCs) and extracellular matrix (ECM). Numerical methods were used to compute water flux through the media. Intimal values were obtained by subtraction. A mechanism was identified that modulates pressure-induced changes in medial transport properties: compaction of the ECM leading to spatial reorganization of SMCs. This is summarized in an empirical constitutive law for permeability and volumetric strain. It led to the physiologically interesting observation that, as a consequence of the changes in medial microstructure, the relative contributions of the intima and media to the hydraulic resistance of the wall depend on the applied pressure; medial resistance dominated at pressures above approximately 93 mmHg in this vessel.

The water channel AQP1 is expressed in human atherosclerotic vascular lesions and AQP1 deficiency augments angiotensin II-induced atherosclerosis in mice

AIM

The water channel aquaporin 1 (AQP1) promotes endothelial cell migration. It was hypothesized that AQP1 promotes neovascularization and growth of atherosclerotic plaques.

METHODS

AQP1 immunoreactivity and protein abundance was examined in human and murine atherosclerotic lesions and aortic aneurysms. Apolipoprotein E (ApoE) knockout (-/-) and AQP1-/-ApoE-/- mice were developed and fed Western diet (WD) for 8 and 16 weeks to accelerate the atherosclerosis process. In ApoE-/- and AQP1-/-ApoE-/- mice abdominal aortic aneurysms (AAA) were induced by angiotensin II (ANGII) infusion by osmotic minipumps for 4 weeks.

RESULTS

In human atherosclerotic lesions and AAA, AQP1 immunoreactive protein was associated with intralesional small vessels. In ApoE-/- mouse aorta, APQ1 mRNA levels were increased with time on WD (n = 7-9, P < 0.003). Both in murine lesions at the aortic root and in the abdominal aortic aneurysmal wall, AQP1 immunoreactivity was associated with microvascular structures. The atherosclerotic lesion burden was enhanced significantly in ANGII-infused AQP1-/-ApoE-/- mice compared with ApoE-/- mice, but neither incidence nor progression of AAA was different. The aortic lesion burden increased with time on WD but was not different between ApoE-/- and AQP1-/-ApoE-/- mice at either 8 or 16 weeks (n = 13-15). Baseline blood pressure and ANGII-induced hypertension were not different between genotypes.

CONCLUSION

AQP1 is expressed in atherosclerotic lesion neovasculature in human and mouse arteries and AQP1 deficiency augments lesion development in ANGII-promoted atherosclerosis in mice. Normal function of AQP1 affords cardiovascular protection.

Noradrenaline has opposing effects on the hydraulic conductance of arterial intima and media

The uptake of circulating macromolecules by the arterial intima is thought to be a key step in atherogenesis. Such transport is dominantly advective, so elucidating the mechanisms of water transport is important. The relation between vasoactive agents and water transport in the arterial wall is incompletely understood. Here we applied our recently-developed combination of computational and experimental methods to investigate the effects of noradrenaline (NA) on hydraulic conductance of the wall (Lp), medial extracellular matrix volume fraction (ϕECM) and medial permeability (K11) in the rat abdominal aorta. Experimentally, we found that physiological NA concentrations were sufficient to induce SMC contraction and produced significant decreases in Lp and increases in ϕECM. Simulation results based on 3D confocal images of the extracellular volume showed a corresponding increase in K11, attributed to the opening of the ECM. Conversion of permeabilities to layer-specific resistances revealed that although the total wall resistance increased, medial resistance decreased, suggesting an increase in intimal resistance upon application of NA.

Endothelial glycocalyx, apoptosis and inflammation in an atherosclerotic mouse model

BACKGROUND AND AIMS

Previous experiments suggest that both increased endothelial cell apoptosis and endothelial surface glycocalyx shedding could play a role in the endothelial dysfunction and inflammation of athero-prone regions of the vasculature. We sought to elucidate the possibly synergistic mechanisms by which endothelial cell apoptosis and glycocalyx shedding promote atherogenesis.

METHODS

4- to 6-week old male C57Bl/6 apolipoprotein E knockout (ApoE(-/-)) mice were fed a Western diet for 10 weeks and developed plaques in their brachiocephalic arteries.

RESULTS

Glycocalyx coverage and thickness were significantly reduced over the plaque region compared to the non-plaque region (coverage plaque: 71 ± 23%, non-plaque: 97 ± 3%, p = 0.02; thickness plaque: 0.85 ± 0.15 μm, non-plaque: 1.2 ± 0.21 μm, p = 0.006). Values in the non-plaque region were not different from those found in wild type mice fed a normal diet (coverage WT: 92 ± 3%, p = 0.7 vs. non-plaque ApoE(-/-), thickness WT: 1.1 ± 0.06 μm, p = 0.2 vs. non-plaque ApoE(-/-)). Endothelial cell apoptosis was significantly increased in ApoE(-/-) mice compared to wild type mice (ApoE(-/-):64.3 ± 33.0, WT: 1.1 ± 0.5 TUNEL-pos/cm, p = 2 × 10(-7)). The number of apoptotic endothelial cells per unit length was 2 times higher in the plaque region than in the non-plaque region of the same vessel (p = 3 × 10(-5)). Increased expression of matrix metalloproteinase 9 co-localized with glycocalyx shedding and plaque buildup.

CONCLUSIONS

Our results suggest that, in concert with endothelial apoptosis that increases lipid permeability, glycocalyx shedding initiated by inflammation facilitates monocyte adhesion and macrophage infiltration that promote lipid retention and the development of atherosclerotic plaques.

Aquaporin-1 facilitates pressure-driven water flow across the aortic endothelium

Aquaporin-1, a ubiquitous water channel membrane protein, is a major contributor to cell membrane osmotic water permeability. Arteries are the physiological system where hydrostatic dominates osmotic pressure differences. In the present study, we show that the walls of large conduit arteries constitute the first example where hydrostatic pressure drives aquaporin-1-mediated transcellular/transendothelial flow. We studied cultured aortic endothelial cell monolayers and excised whole aortas of male Sprague-Dawley rats with intact and inhibited aquaporin-1 activity and with normal and knocked down aquaporin-1 expression. We subjected these systems to transmural hydrostatic pressure differences at zero osmotic pressure differences. Impaired aquaporin-1 endothelia consistently showed reduced engineering flow metrics (transendothelial water flux and hydraulic conductivity). In vitro experiments with tracers that only cross the endothelium paracellularly showed that changes in junctional transport cannot explain these reductions. Percent reductions in whole aortic wall hydraulic conductivity with either chemical blocking or knockdown of aquaporin-1 differed at low and high transmural pressures. This observation highlights how aquaporin-1 expression likely directly influences aortic wall mechanics by changing the critical transmural pressure at which its sparse subendothelial intima compresses. Such compression increases transwall flow resistance. Our endothelial and historic erythrocyte membrane aquaporin density estimates were consistent. In conclusion, aquaporin-1 significantly contributes to hydrostatic pressure-driven water transport across aortic endothelial monolayers, both in culture and in whole rat aortas. This transport, and parallel junctional flow, can dilute solutes that entered the wall paracellularly or through endothelial monolayer disruptions. Lower atherogenic precursor solute concentrations may slow their intimal entrainment kinetics.

A combined numerical and experimental framework for determining permeability properties of the arterial media

The medial layer of the arterial wall may play an important role in the regulation of water and solute transport across the wall. In particular, a high medial resistance to transport could cause accumulation of lipid-carrying molecules in the inner wall. In this study, the water transport properties of medial tissue were characterised in a numerical model, utilising experimentally obtained data for the medial microstructure and the relative permeability of different constituents. For the model, a new solver for flow in porous materials, based on a high-order splitting scheme, was implemented in the spectral/hp element library nektar++ and validated. The data were obtained by immersing excised aortic bifurcations in a solution of fluorescent protein tracer and subsequently imaging them with a confocal microscope. Cuboidal regions of interest were selected in which the microstructure and relative permeability of different structures were transformed to a computational mesh. Impermeable objects were treated fictitiously in the numerical scheme. On this cube, a pressure drop was applied in the three coordinate directions and the principal components of the permeability tensor were determined. The reconstructed images demonstrated the arrangement of elastic lamellae and interspersed smooth muscle cells in rat aortic media; the distribution and alignment of the smooth muscle cells varied spatially within the extracellular matrix. The numerical simulations highlighted that the heterogeneity of the medial structure is important in determining local water transport properties of the tissue, resulting in regional and directional variation of the permeability tensor. A major factor in this variation is the alignment and density of smooth muscle cells in the media, particularly adjacent to the adventitial layer.

Kinetic modeling of low density lipoprotein oxidation in arterial wall and its application in atherosclerotic lesions prediction

Oxidation of low-density lipoprotein (LDL) is one of the major factors in atherogenic process. Trapped oxidized LDL (Ox-LDL) in the subendothelial matrix is taken up by macrophage and leads to foam cell generation creating the first step in atherosclerosis development. Many researchers have studied LDL oxidation using in vitro cell-induced LDL oxidation model. The present study provides a kinetic model for LDL oxidation in intima layer that can be used in modeling of atherosclerotic lesions development. This is accomplished by considering lipid peroxidation kinetic in LDL through a system of elementary reactions. In comparison, characteristics of our proposed kinetic model are consistent with the results of previous experimental models from other researches. Furthermore, our proposed LDL oxidation model is added to the mass transfer equation in order to predict the LDL concentration distribution in intima layer which is usually difficult to measure experimentally. According to the results, LDL oxidation kinetic constant is an important parameter that affects LDL concentration in intima layer so that existence of antioxidants that is responsible for the reduction of initiating rates and prevention of radical formations, have increased the concentration of LDL in intima by reducing the LDL oxidation rate.

A new correlation for inclusion of leaky junctions in macroscopic modeling of atherosclerotic lesion initiation

Vascular endothelium cells are the main barriers between vessel wall and blood flow; they play an essential role in the progression of atherosclerosis. Various experimental and computational studies have been carried out to identify the pathways and mechanisms by which Low Density Lipoprotein (LDL) transfers through the endothelium cells. The most conventional hypothesis in LDL transfer is the presence of leaky junctions. Leaky junctions are large pores in endothelium cells associated with cell mitosis or apoptosis. Although some studies have microscopically modeled leaky junctions, none however have evaluated their effects in a macroscopic level modeling. In this study, a new approach is proposed to consider the presence of the leaky junction as the main pathway in LDL transport from the lumen into the arterial wall. LDL transport in macroscopic scale is simulated in a simplified axisymmetric model and Staverman filtration coefficient (SFC) is used as a measurement criterion for estimating the amount of leaky junctions. According to the results, decreasing SFC corresponds to decreasing the resistance of endothelium cells. In other words, an increase in the number of leaky junctions causes an increase in the LDL concentration inside the arterial wall. Additionally, a new correlation is presented for evaluating the fraction of leaky junctions in the endothelial cells by comparing the results of macroscopic and microscopic models. This correlation accredits each SFC to a specified fraction of leaky junction in the endothelial cells. Therefore, it can be used for the inclusion of leaky junctions in the macroscopic modeling without incorporating any of the complications that are raised by the microscopic modeling studies. This correlation has important implications in the modeling of the atherosclerosis lesions propagation.

A theory for water and macromolecular transport in the pulmonary artery wall with a detailed comparison to the aorta

The pulmonary artery (PA) wall, which has much higher hydraulic conductivity and albumin void space and approximately one-sixth the normal transmural pressure of systemic arteries (e.g, aorta, carotid arteries), is rarely atherosclerotic, except under pulmonary hypertension. This study constructs a detailed, two-dimensional, wall-structure-based filtration and macromolecular transport model for the PA to investigate differences in prelesion transport processes between the disease-susceptible aorta and the relatively resistant PA. The PA and aorta models are similar in wall structure, but very different in parameter values, many of which have been measured (and therefore modified) since the original aorta model of Huang et al. (23). Both PA and aortic model simulations fit experimental data on transwall LDL concentration profiles and on the growth of isolated endothelial (horseradish peroxidase) tracer spots with circulation time very well. They reveal that lipid entering the aorta attains a much higher intima than media concentration but distributes better between these regions in the PA than aorta and that tracer in both regions contributes to observed tracer spots. Solutions show why both the overall transmural water flow and spot growth rates are similar in these vessels despite very different material transport parameters. Since early lipid accumulation occurs in the subendothelial intima and since (matrix binding) reaction kinetics depend on reactant concentrations, the lower intima lipid concentrations in the PA vs. aorta likely lead to slower accumulation of bound lipid in the PA. These findings may be relevant to understanding the different atherosusceptibilities of these vessels.

The transport of LDL across the deformable arterial wall: the effect of endothelial cell turnover and intimal deformation under hypertension

A multilayered model of the aortic wall is introduced to investigate the transport of low-density lipoprotein (LDL) under hypertension, taking into account the influences of increased endothelial cell turnover and deformation of the intima at higher pressure. Meanwhile, the thickness and properties of the endothelium, intima, internal elastic lamina (IEL), and media are affected by the transmural pressure. The LDL macromolecules enter the intima through leaky junctions over the endothelium, which are created by dying or dividing cells. Water molecules enter the intima via the paracellular pathway through breaks in tight junctions after passing the glycocalyx as well as through leaky junctions. The glycocalyx is modeled as a Brinkman porous medium to describe the fluid filtration associated with its structure. Combined Navier-Stokes and Brinkman equations are solved for the transmural flow, and the convective-diffusion equation is employed for LDL transport. The permeation of LDL over the surface of smooth muscle cells is modeled through a uniform reaction evenly distributed in the macroscopically homogeneous media layer. Simulations are performed in an axisymmetric plane centered at a leaky cell. The overriding issue addressed is that LDL fluxes across the leaky junction, the intima, fenestral pores in the IEL, and the media layer are highly affected by the transmural pressure, which affects the endothelial cell turnover rate and the compaction of intima. The present model, for the first time and with no adjustable parameters, is capable of making many realistic predictions including the proper magnitudes for the permeability of endothelium and intimal layers and the hydraulic conductivity of all layers as well as their trends with pressure. Results for the volume flux through the wall and the hydraulic conductivity of the entire arterial wall, the endothelium, and subendothelial layers at 70 and 180 mmHg are in good agreement with previous experimental studies.

A perfusable 3D cell-matrix tissue culture chamber for in situ evaluation of nanoparticle vehicle penetration and transport

A key factor in gene or drug therapy is the development of carriers that can efficiently reach targeted cells from a distal administration. In many gene/drug delivery studies, results obtained in 2D cultures fail to translate to similar results in vivo. In this work, we developed a perfusable 3D chamber for studying nanoparticle penetration and transport in cell-gel soft tissue cultures. The compartmented chamber is made of a polydimethylsiloxane (PDMS) top layer with the chamber features, created using micromachined lithography, bonded to a bottom glass coverslip. A solution of cells embedded in a hydrogel is loaded in the chamber between PDMS posts that serve as anchors to the cell-matrix at the gel-media interface. The chamber offers the following unique features: (i) rapid fabrication and simplicity in assembly, (ii) direct in situ cell imaging in a plane normal to the direction of flow or action, (iii) an easily configurable and controllable environment conducive cell culture under static or interstitial flow conditions, and (iv) facile recovery of live cells from chambers for post-experimental analysis. To assess the chamber, we delivered fluorescently labeled nanoparticles of three distinct sizes to cells-embedded Matrigels in the 3D chamber under flow and static conditions. Penetration of nanoparticles were enhanced under interstitial flow while live cell imaging and flow cytometry of recovered cells revealed particle size restrictions to efficient delivery. Although designed for delivery studies, the chamber is versatile and can be easily modified. Thus it may have broad applications for biological, tissue engineering, and therapeutic studies.

Transient diffusion of albumin in aortic walls: effects of binding to medial elastin layers

The goal of this study was to measure diffusive transport of albumin through artery walls experimentally and to analyze the results theoretically, taking into account the binding of albumin to elastic lamellae. Segments of rabbit aorta were placed in solutions of fluorescently labeled albumin for periods of 30, 60, 90, and 120 min, and the distributions of fluorescence intensity through the arterial media were observed. On average, intensity increased almost linearly with time. Bands of high intensity were observed corresponding to elastin layers within the media. The temporal and spatial variations of intensity were compared with predictions of theoretical models, including effects of albumin binding and hindered diffusion resulting from the complex wall structure. Based on these analyses, it was concluded that the spatial distribution of free albumin within the media equilibrated relatively rapidly, and that the observed linear increase in intensity reflected gradual accumulation of albumin bound to medial elastin layers. The results imply that previous theoretical analyses, in which binding was neglected, substantially underestimated albumin diffusivity in the aortic wall. With respect to stent-associated delivery of inhibitors of vascular cell proliferation, the results suggest that albumin might serve as an “affinity vehicle” for drug delivery to the aorta, by attaching the drug to an abundant component of the artery wall.

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.

Interactive effect of chondroitin sulphate C and hyaluronan on fluid movement across rabbit synovium

The polysaccharide hyaluronan (HA) conserves synovial fluid by keeping outflow low and almost constant over a wide pressure range ('buffering'), but only at concentrations associated with polymer domain overlap. We therefore tested whether polymer interactions can cause buffering, using HA-chondroitin sulphate C (CSC) mixtures. Also, since it has been found that capillary filtration is insensitive to the Starling force interstitial osmotic pressure in frog mesenteries, this was assessed in synovium. Hyaluronan at non-buffering concentrations (0.50-0.75 mg ml(-1)) and/or 25 mg ml(-1) CSC (osmotic pressure 68 cmH(2)O) was infused into knees of anaesthetised rabbits in vivo. Viscometry and chromatography confirmed that HA interacts with CSC. Pressure (P(j)) versus trans-synovial flow (;Q(s)) relations were measured.;Q(s) was outwards for HA alone (1.2 +/- 0.9 microl min(-1) at 3 cmH(2)O, mean +/- S.E.M.; n = 6). CSC diffused into synovium and changed;Q(s) to filtration at low P(j) (-4.1 microl min(-1), 3 cmH(2)O, n = 5, P < 0.02, t test). Filtration ceased upon circulatory arrest (n = 3). At higher P(j), 0.75 mg ml(-1) HA plus CSC buffered;Q(s) to approximately 3 microl min(-1) over a wide range of P(j), with an outflow increase of only 0.04 +/- 0.02 microl min(-1) cmH(2)O(-1) (n = 4). With HA or CSC alone, buffering was absent (slopes 0.57 +/- 0.04 microl min(-1) cmH(2)O(-1) (n = 4) and 0.86 +/- 0.05 microl min(-1) cmH(2)O(-1) (n = 5), respectively). Therefore, polymer interactions can cause outflow buffering in joints. Also, interstitial osmotic pressure promoted filtration in fenestrated synovial capillaries, so the results for frog mesentery capillaries cannot be generalised. The difference is attributed to differences in pore ultrastructure.

Local gene delivery to the vessel wall

This review will provide an overview of delivery strategies that are being evaluated for vascular gene therapy. We will limit our discussion to those studies that have been demonstrated, utilizing in vivo model systems, to limit post-interventional restenosis. We also discuss the efficacy of the vectors and methods currently being used to transfer genetic material to the vessel wall. The efficiency of these techniques is a critical issue for the successful application of gene therapy.

A biphasic, anisotropic model of the aortic wall

A biphasic, anisotropic elastic model of the aortict wall is developed and compared to literature values of experimental measurements of vessel wall radii, thickness, and hvdraulic conductivity as a function of intraluminal pressure. The model gives good predictions using a constant wall modulus for pressures less than 60 mmHg, but requires a strain-dependent modulus for pressures greater than this. In both bovine and rabbit aorta, the tangential modulus is found to be approximately 20 times greater than the radial modulus. These moduli lead to predictions that, when perfused in a cylindrical geometry, the aortic volume and its specific hydraulic coonductivity are relatively independent of perfusion pressure, in agreement with experimental measurements. M, the parameter that relates specific hydraulic conductivy, to tissue dilation, is found to be a positive quantity correcting a previous error in the literature.

Simulated lipoprotein transport in the wall of branched arteries

Study of arterial blood flow dynamics improves our understanding of the development of cardiovascular diseases such as atherosclerosis. The transport and accumulation of macromolecules in the arterial wall can be influenced by local fluid mechanics. We used numeric simulations to investigate such transport in a T-junction model. Presumably an in vitro experiment would consist of gel segments inserted in the walls of a mechanical flow T-junction model near branch points where separation and recirculation zones are expected. The transport of low density lipoprotein (LDL) was investigated theoretically at these sites in a two dimensional numeric T-branch model. In the numeric model, the hydraulic conductivity of the porous gel wall segments was varied for a fixed species diffusivity to provide simulations with wall transmural Peclet numbers ranging from 0.3 to 30. Steady state flow patterns in the lumen of the two dimensional T-branch were simulated at Reynolds numbers of 250 and 500, using the software package FIDAP 7.61 to implement the finite element method. The simulations demonstrated that wall Peclet numbers greater than 1.0 were needed to achieve species concentration gradients within the wall that varied in the axial direction, thereby reflecting the influence of disturbed flow and pressure patterns in the lumen. As expected, the transmural concentration gradients were steeper when convection predominated. Blood flow in the lumen can influence the distribution of macromolecules in the arterial wall and needs to be investigated for the relevance to atherosclerosis.

Alternative blood conduits: assessment of whether the porosity of synthetic prostheses is the key to long-term biofunctionality

The paper examines the effects of water permeability on solid particle (platelet) adhesion and lipid transport through the wall of a blood conduit. Also tested is the capacity of external supports to reduce lipid infiltration into venous grafts. The results indicate that water permeability not only facilitates particle adhesion, but also affects the spatial distribution of the adhesion. The presence of filtration flow leads to a concentration polarisation of atherogenic lipids at the blood/wall interface, with increased lipid concentration from the bulk value towards the interface, thus enhancing the drive potential for lipid infiltration into the vessel wall. An external support to a venous graft guards against excessive distention and significantly reduces lipid infiltration into the venous wall. These results strongly suggest that too high a water permeability or porosity can lead to the late failure of arterial grafting by affecting blood cell interaction with the graft and lipid infiltration into the wall. Therefore the pore structure of an arterial prosthesis is crucial to its long-term biofunctionality. Ideally, a synthetic prosthesis should display pores of adequate size and a structural network that promotes tissue ingrowth, while maintaining water porosity at a physiological level.

Analysis of adenoviral transport mechanisms in the vessel wall and optimization of gene transfer using local delivery catheters

Local delivery devices have been used for adenovirus-mediated gene transfer to the arterial wall for the potential treatment of vascular proliferative diseases. However, low levels of adenoviral gene expression in vascular smooth muscle cells may pose a serious limitation to the success of these procedures in the clinic. In this study, we examined the mechanisms controlling adenoviral transport to the vessel wall, using both hydrogel-coated and infusion-based local delivery catheters, with the goal of enhancing in vivo gene transfer under clinically relevant delivery conditions. The following delivery parameters were tested in vivo: applied transmural pressure, viral solution volume and concentration, and delivery time. We found that viral particles are transported into the vessel wall in a manner consistent with diffusion rather than pressure-driven convection. Consistent with diffusion, viral concentration was shown to be the key variable for viral transport in the vessel wall and thus gene expression in vascular smooth muscle cells. A transduction level of 17.8+/-3.2% was achieved by delivering a low volume of concentrated adenoviral beta-galactosidase solution through an infusion balloon catheter at low pressure without an adverse effect on medial cellularity. Under these conditions, effective gene transfer was accomplished within a clinically relevant time frame of 2 min, indicating that longer delivery times may not be necessary to achieve efficient gene transfer.

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.

The contributions of glycosaminoglycans, collagen and other interstitial components to the hydraulic resistivity of porcine aortic wall

A pressure-driven flux of water occurs across the arterial wall in vivo. We have investigated the role of several interstitial components in determining the resistance of the wall to this flow. Pieces of porcine thoracic aorta were modified by thermal denaturation, enzymatic digestion or disruptive chemical treatments. The effect of these procedures on the wall content of glycosaminoglycans, collagen and elastin was determined by biochemical assay of uronic acid and hydroxyproline. Effects on hydraulic conductivity were measured by using a flow cell in which tissue was free to deform under applied pressure. Untreated tissue showed considerable variation in uronic acid content but conductivities were substantially less variable and averaged 0.75 x 10(-12) cm4/dyne.s. In tissue autoclaved for < 1 h, resistivity increased, possibly because interstitial components had been denatured but not removed from the wall. After longer periods, resistivity decreased by a factor of one hundred. More specific treatments showed that resistivity decreased by up to a factor of ten when glycosaminoglycans were removed and by a similar factor when collagen was removed. Tissue in which both were removed showed a hundred-fold decrease in resistivity. As with tissue subjected to prolonged autoclaving, the resistivity was still an order of magnitude higher than that of alkali- or acid-extracted elastin despite an apparently similar composition, suggesting the existence of a non-assayed component with important properties. The resistivity of the samples was decreased further by treatment with chymotrypsin, consistent with this component being microfibrillar protein.

Effect of intra-articular hyaluronan on pressure-flow relation across synovium in anaesthetized rabbits

1. Hyaluronan is the major polysaccharide of synovial fluid, responsible for its high viscosity. The effect of hyaluronan on fluid transport across the synovial lining of the joint was investigated. Rate of fluid absorption from the joint cavity (Qs) was measured at intra-articular pressures (Pj) of up to 24 cmH2O in knees of anaesthetized rabbits, in the presence or absence of hyaluronan in intra-articular infusates. 2. Viscometry studies in vitro showed that the commercial hyaluronan used had a molecular weight of 549,000-774,000, a radius of gyration of 48-99 nm and a critical concentration for molecular overlap of 1.3 g l-1. 3. With intra-articular Krebs solution (control) or subnormal, subcritical concentrations of hyaluronan (0.5 g l-1), flow increased with pressure. Hyaluronan reduced the fluid escape rate by reducing slope dQs/dPj by 32-64% relative to Krebs solution. 4. At normal to high hyaluronan concentrations (3-6 g l-1) and low pressures, hyaluronan again reduced slope dQs/dPj, by 39-64%. The reduction in slope was slight, however, when compared with the reduction in bulk fluidity (1/relative viscosity). Fluidity at high shear rates was reduced to 6% of control values by 6 g l-1 hyaluronan. The effect on slope did not correlate significantly with the effect on fluidity. 5. At pressures above approximately 12 cmH2O, 3-6 g l-1 hyaluronan altered the shape of the pressure-flow relation: a flow plateau developed. In some joints raising pressure even reduced trans-synovial flow slightly. The pressure required to drive unit trans-synovial flow (an index of outflow resistance) increased 2.5-fold between 5 and 25 cmH2O in the presence of hyaluronan. By contrast, in the absence of hyaluronan the outflow resistance fell as pressure was raised. 6. It is suggested that the increasing resistance to flow in the presence of hyaluronan may be caused by partial molecular sieving of hyaluronan by the small porosities of the synovial interstitial matrix, leading to accumulation of a resistive filter cake of hyaluronan chains at the tissue-cavity interface. Since hyaluronan impedes fluid escape when pressure is raised, it may serve to preserve synovial fluid volume in vivo, e.g. during sustained joint flexion.

Age-related variations in transport properties of the rabbit arterial wall near branches

Lipid accumulation in the human aorta occurs predominantly downstream of branches in foetuses, neonates and infants but upstream at later ages. The lipid in these deposits may derive from plasma lipoproteins. We have examined uptake of plasma proteins by the rabbit aortic wall near branches as a function of age. Albumin was labelled with a fluorescent dye and introduced into the circulation of animals fed a normal diet. The aorta was fixed in situ 3 h later and the distribution of tracer in sections through the wall was measured by using digital imaging fluorescence microscopy. Net uptake by the intima-media was higher downstream of intercostal ostia than upstream in young animals but this difference decreased and then reversed with age. Furthermore, the average of uptake by both regions was higher shortly after weaning than at later ages. These age-related variations in transport properties may explain discrepancies between previous studies of uptake, resolve apparent inconsistencies between the properties of rabbit and human arteries and, if applicable to man, might account for the non-uniform and changing pattern of lipid accumulation around arterial branches.

Extravascular circulation of lipoproteins: their role in reverse transport of cholesterol

The transfer of lipoproteins out of plasma into peripheral tissues, their flow through the interstitium and their return to blood has been reviewed. In this context special emphasis has been given to reports dealing with the movement of lipoproteins into and out of the wall of arteries.

Anatomic barriers influence the distribution of in vivo gene transfer into the arterial wall. Modeling with microscopic tracer particles and verification with a recombinant adenoviral vector

We evaluated the extent to which anatomic barriers to vector penetration might influence the distribution of successful in vivo gene transfer into the normal arterial wall. A double-balloon catheter technique with infusion pressures of 100 to 400 mm Hg was used to infuse microscopic tracer particles of the size range of liposomes and viral vectors into normal elastic arteries of sheep. Localization of the tracer particles in tissue sections by light, fluorescence, and electron microscopy suggested that vector-sized particles were delivered to the intima by direct infusion and to the adventitia via the arterial vasa vasorum. Particles were virtually absent from the arterial media. To test the predictions made from the particle studies, we repeated the infusion protocol with high-titer adenoviral vectors. Gene transfer occurred at high levels in the intima and along the adventitial vasa vasorum but again was virtually absent within the media. The ability of medial smooth muscle cells to be transduced was established in separate experiments with a high-pressure (5 atm) porous balloon infusion catheter. We conclude that (1) the anatomy of the normal elastic arterial wall imposes significant limitations on the penetration of particles in the size range of most gene-transfer vectors and (2) the distribution of in vivo gene transfer with adenoviral vectors is correctly predicted by the distribution of inert tracer particles. These findings have important implications for the design of arterial gene-transfer and gene-therapy protocols.

Effect of extravascular plasma protein on pressure-flow relations across synovium in anaesthetized rabbits

1. The effect of extravascular plasma protein on fluid flux through interstitial matrix was investigated in vivo by studying the pressure-flow relation across synovium during intra-articular infusions of protein solutions (usually bovine serum albumin). Synovium is a sheet of non-epithelial cells separated by interstitium-filled gaps, beneath which are fenestrated capillaries: synovium regulates synovial fluid volume and composition. 2. Albumin solutions (10-150 g l-1) of measured oncotic pressure and viscosity were infused at known pressure into the synovial cavity of knees of anaesthetized rabbits. Flow across the synovial lining in the steady state (absorption rate Qs) was recorded at a series of joint pressures (Pj) to define the pressure-flow relation. Krebs solution was infused into the opposite knee as a control (26 animals). 3. Infusion of a low albumin concentration (10 g l-1, bovine or rabbit) or diluted rabbit serum revealed no specific effect of plasma protein on interstitial matrix permeability (cf. specific protein effect on capillary glycocalyx permeability). Physiological (22.5 g l-1) and higher concentrations reduced trans-synovial absorption rate. The slope of the pressure-flow relation was reduced and the pressure intercept displaced to the right (i.e. Pj at zero flow was raised). 4. Slope dQs/dPj correlated negatively with intra-articular viscosity (P = 0.001-0.04), in keeping with viscous interstitial flow. The reduction in normalized slope, however, did not equal the reduction in fluidity (1/viscosity) quantitatively. It is proposed that apparent fluidity within the interstitial matrix is higher than in the bulk phase due to steric exclusion of albumin (radius 3.55 nm) by the interstitial glycosaminoglycans. The latter form spaces of estimated mean hydraulic radius 14-18 nm in synovium. 5. The joint-pressure intercept at zero net trans-synovial flow was displaced 0.015 cmH2O per cmH2O intra-articular oncotic pressure (pi j; S.E.M. +/- 0.006). Thus large trans-synovial osmotic gradients were not maintained at physiological flow velocities. The 1.5% displacement of the Pj intercept by pi j was attributed principally to interstitial albumin exerting pericapillary oncotic pressure and enhancing net Starling filtration pressure. Indeed, net trans-synovial flow at low joint pressure sometimes reversed from absorption to filtration into the joint cavity at high intra-articular oncotic pressures. 6. The displacement of the trans-synovial flow intercept per unit change in intra-articular oncotic pressure, (dQs/d pi j)Pj = 0, was 18 +/- 3 nl min-1 cmH2O-1.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of pressure on aortic hydraulic conductance

This study was performed to determine whether the transmural hydraulic conductance (Lp) of the rabbit aortic wall depends on its distension. In 19 rabbits, the aorta was cannulated in situ and perfused at a given pressure with a physiologically buffered solution containing 4% bovine serum albumin. The output cannula was then occluded to limit fluid flow to that traversing the artery wall. External diameter and transmural fluid flow were measured at three pressures (eight rabbits, group 1) or at four pressures (12 rabbits, group 2) in each vessel. Transmural fluid flow was determined by monitoring the velocity of an air bubble within a buffer-filled tube leading to the input cannula. From group 1 measurements, Lp values (mean +/- SD) at 50, 100, and 150 mm Hg were calculated to be 3.8 +/- 2.8, 3.5 +/- 1.3, and 4.1 +/- 1.2 x 10(-8) cm/sec/mm Hg, respectively. Group 2 measurements gave values of 4.2 +/- 1.6, 3.8 +/- 1.1, 3.8 +/- 1.1, and 4.2 +/- 1.1 x 10(-8) cm/sec/mm Hg at 75, 100, 125, and 150 mm Hg, respectively. Paired Student's t tests indicated no significant change in Lp with pressure. However, linear regression analysis demonstrated a weak correlation between Lp values obtained at 50 and 100 mm Hg (r2 = 0.30) and at 75 and 100 mm Hg (r2 = 0.36). Values of Lp at 100 and 150 mm Hg and at 125 and 150 mm Hg were closely correlated in each case. These results suggest that between 50 and 100 mm Hg the structural properties of the aortic wall change so as to alter Lp but not in the same way in each vessel. Lp may increase or decrease depending on which structural change predominates in a particular vessel.

Reduction of transmural 125I-albumin concentration in rat aortic media by chronic hypertension

Relative 125I-albumin concentration was measured in vivo in the aortic media of sham-operated (n = 10) and hypertensive (two-kidney, one clip) rats, untreated (n = 8) or treated (n = 10) by an angiotensin converting enzyme inhibitor (CEI, Trandolapril). Blood pressure was acutely lowered to a normal level at the time of the experiment in hypertensive rats (n = 7) to separate the direct effect of increased pressure from the effect of pressure-induced structural changes. Relative tissue concentration profiles of labeled albumin across the media were obtained using a serial frozen-sectioning technique. In hypertensive rats, the mean medial albumin concentration decreased by 35% in the ascending arch and 32% in the descending arch (p less than 0.01). When blood pressure was acutely lowered in hypertensive animals, this value decreased further by 56% in the ascending arch, 48% in the descending arch (p less than 0.01), and 22% in the thoracic aorta (p less than 0.05) as compared with controls. The medial thickness in hypertensive rats was significantly increased (more in the ascending arch than in the rest of the aorta). Four-week CEI treatment reversed hypertension and medial thickening, but the mean medial albumin concentration remained significantly lower in the arch (by 36% in the ascending part and 40% in the descending part, p less than 0.01). The collagen content in the thoracic aorta was significantly increased in hypertensive rats (by 40%, p less than 0.01) and remained increased (by 29%, p less than 0.01) after CEI treatment. These results suggested that the hypertension-induced structural changes might reduce the medial distribution volume for albumin, whereas elevated blood pressure per se tended to enhance albumin concentration within the media. However, the net result of chronic hypertension was a reduction of the mean medial albumin concentration. The aortic arch appeared to be more affected than the rest of the aorta. Fiber content, more than medial thickness, might be responsible for the observed differences in albumin concentration. Lowering of blood pressure seemed to be insufficient to restore normal albumin concentration profiles and perhaps those of other macromolecules. This finding may be relevant in evaluating some of the complications associated with hypertension.

Experimental determination and mathematical model of the transient incorporation of cholesterol in the arterial wall

Experimental data of the radial incorporation of labeled cholesterol [14C-4] into the artery wall is regressed against a mathematical model that predicts macromolecular transport in this biological system. Data is obtained using excised canine carotid arteries that are perfused in vitro under pulsatile hemodynamic conditions for 2 hr. Vessels are exposed to either normotensive hemodynamics, hypertensive hemodynamics, or simulations in which the rate of flow or vessel compliance is deliberately altered. Several arteries are studied under normotensive conditions following balloon catheter deendothelialization. Transmural concentration profiles of [14C-4] activity are determined by microcryotomy of longitudinal sections of perfused vessels. Nonlinear Marquardt regression on 12 experimental cases yields parameter estimates of effective diffusivity, D and solute filtration velocity, V. Results of this experimental investigation support our hypothesis that hemodynamics and the endothelial lining influence wall flux in intact vessels. Exposure to altered (vs normotensive) hemodynamics is associated with increased incorporation of labeled cholesterol. A similar observation is made for deendothelialized vessels (e.g. a greater accumulation of label and a rise in convective flux). Based upon our companion measurements of vessel wall forces and endothelial cellular morphology accompanying hemodynamic simulations, we suggest that hemodynamically induced alterations to endothelial structures lead to the increased permeability, convection and incorporation that we observe in this work.

On the time-dependent diffusion of macromolecules through transient open junctions and their subendothelial spread. I. Short-time model for cleft exit region

In this two-part study we shall quantitatively study, using time-dependent models, the hypothesis that transient open junctions associated with widely scattered endothelial cells undergoing mitosis are the structural equivalent for the large pore pathway via which macromolecules the size of albumin or larger cross the vascular endothelium. In an earlier steady-state model [Am. J. Physiol. 248, H945-960 (1985)], the authors demonstrated that such an open-junction pathway could quantitatively account for the regional differences in macromolecular permeability observed in various mammalian arteries in regions of enhanced cell turnover as indicated by 3H-thymidine although these cells were less than 1% of the population and the open junctions occupied less than 10(-5) of the endothelial surface. The time-dependent models described herein have been used to identify a time window and size of probe molecule wherein this hypothesis could be tested experimentally in the larger blood vessels. The first stages of these experiments have now been completed and provide convincing evidence that the junctions of virtually all endothelial cells in the M phase of the cell cycle are leaky to macromolecules (Lin et al., 1988). The statistical frequency of such leakage sites has also been determined. The time-dependent models developed herein contain two important refinements that were not contained in the earlier steady state model. First the finite resistance of the open cleft as a function of molecular size is accounted for by introducing a diffusion coefficient ratio Dj/Dz describing the relative resistance of the open cleft compared to the subendothelial tissue in the direction normal to the endothelial surface. Second the non-isotropy of the vessel wall due to the elastic lamina is considered by introducing a second diffusion coefficient ratio Dx/Dz describing the relative resistance in the lateral as compared to the normal direction. This second ratio can be as large as 100 for the arterial intima, but is of order unity for capillaries. In Part I a short time model is presented to describe the initial labeling of the open cleft and the subendothelial space in the vicinity of the cleft exit following the introduction of a tracer macromolecule. This model is valid for both larger vessels and capillaries since wall thickness and curvature and the interaction between leakage sites does not enter into the model description. In Part II (Wen et al., 1988) a long-time model is developed for larger vessels only which is valid for greater times including steady-state labeling.