Net albumin transport across the wall of the rabbit common carotid artery perfused in situ

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.

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.

The effects of slip velocity on a micropolar fluid through a porous channel with expanding or contracting walls

In this paper, a simple mathematical model depicting blood flow in the deforming porous channel is developed with an emphasis on the permeability property of the blood vessel and slip boundary based on Beavers and Joseph slip condition. In this study, the blood is represented by a micropolar fluid. With such an ideal model, the governing equations are reduced to ordinary ones by introducing suitable similar transformations. Homotopy analysis method is employed to obtain the expressions for velocity and microrotation fields. Graphs are sketched for some values of parameters such as slip coefficient and expansion ratio, and the associated dynamic characteristics are analysed in detail.

Effect of the shape and configuration of smooth muscle cells on the diffusion of ATP through the arterial wall

In this study, the shape and the configuration of smooth muscle cells (SMCs) within the arterial wall are altered to investigate their influence on molecular transport across the media layer of the thoracic aorta wall. In a 2D geometry of the media layer containing SMCs, the finite-element method has been employed to simulate the diffusion of solutes through the media layer. The media is modeled as a heterogeneous system composed of SMCs having elliptic or circular cross sections embedded in a homogeneous porous medium made of proteoglycan and collagen fibers with an interstitial fluid filling the void. The arrangement of SMCs is in either ordered or disordered fashion for different volume fractions of SMCs. The interstitial fluid enters the media through fenestral pores, which are assumed to be distributed uniformly over the internal elastic lamina (IEL). Results revealed that in an ordered arrangement of SMCs, the concentration of adenosine 5'-triphosphate (ATP) over the surface of SMCs with an elliptic cross section is 5-8% more than those of circular SMCs in volume fractions of 0.4-0.7. The ATP concentration at the SMC surface decreases with volume fraction in the ordered configuration of SMCs. In a disordered configuration, the local ATP concentration at the SMC surface and in the bulk are strongly dependent on the distance between neighboring SMCs, as well as the minimum distance between SMCs and fenestral pores. Moreover, the SMCs in farther distances from the IEL are as important as those just beneath the IEL in disordered configurations. The results of this study lead us to better understanding of the role of SMCs in controlling the diffusion of important species such as ATP within the arterial wall.

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).

Effect of time of day and rabbit strain on patterns of aortic wall permeability

Lipid deposits occur more frequently downstream of branch points than upstream in immature rabbit and human aortas but the opposite pattern is seen in mature vessels. These distributions correlate spatially with age-related patterns of aortic permeability, observed in rabbits, and may be determined by them. The mature but not the immature pattern of permeability is dependent on endogenous nitric oxide synthesis. Although the transport patterns have hitherto seemed robust, recent studies have given the upstream pattern in some mature rabbits but the downstream pattern in others. Here we show that transport in mature rabbits is significantly skewed to the downstream pattern in the afternoon compared with the morning (P < 0.05), and switches from a downstream to an upstream pattern at around 21 months in rabbits of the Murex strain, but at twice this age in Highgate rabbits (P < 0.001). The effect of time of day was not explained by changes in nitric oxide production, assessed from plasma levels of nitrate and nitrate, nor did it correlate with conduit artery tone, assessed from the shape of the peripheral pulse wave. The effect of strain could not be explained by variation in nitric oxide production nor by differences in wall structure. The effects of time of day and rabbit strain on permeability patterns explain recent discrepancies, provide a useful tool for investigating underlying mechanisms and may have implications for human disease.

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.

Effect of age on the pattern of short-term albumin uptake by the rabbit aortic wall near intercostal branch ostia

Lipid deposition occurs more frequently downstream than upstream of branches in immature human aorta but the opposite pattern is seen in mature vessels. These distributions may reflect variation in the uptake of plasma macromolecules by the aortic wall. We have recently shown that the quasi-steady state uptake of albumin is greater downstream than upstream of branches in immature rabbit aortas and that the opposite pattern occurs in mature animals. Additionally, there is a sharp drop in the mean uptake shortly after weaning. In the present study, the mechanisms underlying these phenomena were investigated by examining the short-term uptake of albumin and its distribution across the wall. Albumin was labeled with a fluorescent dye and introduced into the circulation of conscious New Zealand White rabbits. Thoracic aortas were fixed in situ 10 minutes later and were sectioned through the center of intercostal ostia. Fluorescence from sections was measured by using digital imaging fluorescence microscopy and was converted to tracer concentrations after appropriate autofluorescence levels had been subtracted. In animals aged 45 days, more tracer was detected in the wall downstream than upstream of branches; the difference between regions was > 100% of the mean value. This percentage halved and the mean uptake decreased almost threefold by 75 days. In mature animals, the mean value remained at the 75-day level but the converse distribution was seen; 22% more tracer was detected upstream than downstream. These trends were insensitive to the depth of the intimal-medial layer examined. In each region, the maximum tracer concentration occurred close to the luminal surface but not always within the first 2.9-microns-thick layer of the wall. Maxima were similar in magnitude to those observed at quasi-steady state, but the fall with increasing distance into the wall was much sharper. In many cases concentrations remained constant over most of the media, and rises toward the adventitial boundary were rarely seen. Uptake after 10 minutes predominantly reflects the rate at which tracer enters the wall. The concentration profiles were consistent with most of the tracer having entered from the luminal surface and with the involvement of convective transport. The trends observed with age closely paralleled those occurring at quasi-steady state. Consequently, the latter are also likely to be determined by changes in the resistance of the wall to macromolecule influx.

Hemodynamic stress and experimental aortoiliac atherosclerosis

PURPOSE

Human aortic atherosclerosis is predominantly localized to the infrarenal aorta where flow is bidirectional and wall shear stress oscillates. Similar flow patterns have been related to carotid atherosclerosis. The thoracic aorta is usually spared, where flow and shear stress are unidirectional. We hypothesized that because heart rate and systemic blood pressure modulate flow velocity and shear stress oscillation, both these hemodynamic forces may enhance aortoiliac atherogenesis.

METHODS

Eighteen male cynomolgus monkeys were fed an atherogenic diet for 6 months (mean serum cholesterol = 535 +/- 35 mg/dl). Heart rate was determined with 24-hour electrocardiographic telemetry at monthly intervals and blood pressure was measured by direct arterial cannulation. The product of mean heart rate and mean blood pressure was used to define hemodynamic stress for each animal. Atherosclerotic lesion formation at three standard thoracic aortic sites was quantitatively compared with lesion formation at five standard infrarenal aortoiliac locations with computer-assisted morphometry.

RESULTS

There was significantly more plaque in the aortoiliac segment than in the thoracic aorta (12.4% +/- 9.0% vs. 6.4% +/- 4.5% area stenosis, p = 0.02). No correlation was found between the degree of serum lipid elevations and lesion formation in either aortic location. Mean heart rate was 113 +/- 18 beats/min (87 to 163 beats/min) and mean blood pressure was 85 +/- 19 mm/Hg (62 to 130 mm Hg). Heart rate and blood pressure alone were not significantly related to lesion formation. A significant correlation was, however, found between hemodynamic stress and maximum lesion thickness (r = 0.47, p < 0.05) in the aortoiliac region but not in the thoracic aorta (r = 0.19, p > 0.10).

CONCLUSIONS

This study demonstrates that heart rate and blood pressure exert a mutually potentiating effect on aortoiliac atherosclerosis but not on thoracic aortic atherosclerosis. Regional differences in aortic atherosclerosis may therefore be attributable to the interaction between these hemodynamic forces and the local flow patterns specific to each aortic location. Additional investigation of these hemodynamic factors in relation to human aortic atherosclerosis is warranted.

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.

Measurement of absolute tracer concentrations in tissue sections by using digital imaging fluorescence microscopy. Application to the study of plasma protein uptake by the arterial wall

Digital imaging fluorescence microscopy (DIFM) of tissue sections was used to quantify uptake of labelled plasma proteins by the arterial wall. Several aspects of the measuring system were investigated so that absolute tracer concentrations and their local variation could be derived from digitized images. These investigations may be relevant to other studies employing DIFM. Nonlinearities were found to arise from offsets in the video digitizers, from background fluorescence and stray light within the microscope and from the transfer characteristics of the intensified CCD camera. Camera gain controls showed complex behaviour. Camera output fell substantially for several hours after switching on and was affected by room temperature. Large spatial variations in response were caused by the geometry of the microscope optics and by the image intensifier. However, the ratios between areas were not affected by light intensity or camera gain settings. Measured intensities were independent of the depth-wise location of fluorophores within tissue sections but they were affected by the emission from objects outside the measuring area. Photobleaching of tracer varied significantly over the range of excitation intensities and durations used but was not concentration dependent. Methods used to correct these effects and obtain a uniform, linear and constant relationship between concentration and grey level are described. Using the system and appropriate corrections, in vivo uptake of sulphorhodamine-B-labelled serum albumin by the rabbit aortic wall was investigated. Results obtained for the mean uptake of tracer and its local variation were quantitatively similar to those previously obtained with nonmicroscopic methods.

Measurement of endothelial permeability to 125I-low density lipoproteins in rabbit arteries by use of en face preparations

A procedure of en face quantitative autoradiography of the endothelium (Hautchen preparations) was developed to examine regional variations in 125I-low density lipoprotein (125I-LDL) permeability in the arterial wall in vivo. Endothelial preparations from fixed arterial tissue and calibration standards consisting of known concentrations of 125I-albumin were dipped in nuclear emulsion, exposed for 1-3 months, developed, and stained with hematoxylin. Digital image analysis was used to analyze dark-field images of autoradiographs. Background grain densities on cold endothelial preparations were 30-100% higher than on glass, but the variability in grain densities on the two different surfaces was similar. Regression slopes of grain density versus concentration for calibration standards were the same for sections placed on cold tissue or glass. For 1-5-microns-thick calibration standards of the same concentration, the grain density was proportional to the total amount of radioactivity per unit area. The results indicated that errors arising from nonuniformities in preparation thickness were minimal, and permeabilities and intimal concentrations could be determined. Rabbits were killed 10 minutes after injection of 125I-LDL, and endothelial preparations were made. For regions of uniformly low grain density in the rabbit aorta, the 125I-LDL permeability was 1.9 +/- 0.8 x 10(-8) cm/sec, and the effective diffusion coefficient was 5.4 +/- 3.1 x 10(-10) cm2/sec. Errors in the estimated permeability arising from nonuniformities in tissue thickness were the same as the reported experimental variability. Analysis of elevated regions of permeability suggested that 125I-LDL was binding to the extracellular matrix. Approximately 25% of the sites of elevated grain density were associated with mitotic endothelial cells, and such regions had higher permeabilities than sites associated with nonmitotic cells. Around intercostal arteries, sites of highest permeability were distal and lateral to the vessels and occurred where lesions first develop in hypercholesterolemic rabbits.

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.

Albumin and Cr-EDTA uptake by systemic arteries, veins, and pulmonary artery of rabbit

Experiments have been performed both in vivo and in vitro to measure the steady-state uptake of labeled albumin and Cr-ethylenediaminetetraacetate by various blood vessels of the rabbit: the ascending and descending portions of the thoracic aorta, the carotid artery, the pulmonary artery, and the inferior vena cava. The in vitro experiments indicated that the wall tissues of the pulmonary artery and the vena cava have much greater distribution volumes for albumin than do the systemic arteries. This may in part explain the differences in wall tissue concentrations in vivo and, in turn, the differences between vessels in their susceptibility to atherosclerosis.

Effect of transmural pressure on low density lipoprotein and albumin transport and distribution across the intact arterial wall

To investigate the effect of hyperpressure on the transport of low density lipoprotein (LDL) and albumin in the arterial wall, we measured in vitro the uptake of both iodine-131-labeled LDL and iodine-125-labeled albumin in intact rabbit thoracic aorta, held at in vivo length and pressurized to 70 or 160 mm Hg. Arteries were incubated for 2 hours (n = 8) at 70 mm Hg, and for 5 minutes (n = 4), 30 minutes (n = 4), 1 hour (n = 5), and 2 hours (n = 5) at 160 mm Hg. The transmural distribution of the relative concentrations of LDL (CLDL) and albumin (Calb) across the wall was determined by using a serial frozen sectioning technique. At 70 mm Hg, the mean medial CLDL and Calb values were 0.0018 +/- 0.0007 and 0.0039 +/- 0.0013, respectively. At 160 mm Hg, CLDL and Calb were markedly increased. The distribution of labeled albumin was almost uniform across the media and reached a steady state after 30 minutes, whereas labeled LDL accumulated in the first inner layers, a steady state being achieved after 1 hour. The 1-hour values of CLDL in the first and second luminal sections (0.24 +/- 0.03 and 0.13 +/- 0.05, respectively) were much higher than those of Calb, the CLDL/Calb ratios being 4.12 +/- 0.94 and 2.34 +/- 0.42 (p less than 0.01), respectively. In the subsequent sections, the CLDL decreased markedly and became much lower than the Calb, the CLDL/Calb ratio averaging 0.2 in the two-thirds outer media. To investigate whether LDL was trapped at high pressure in the inner layers, vessels were exposed to a tracer-free intraluminal solution for 30 minutes, after a 30-minute incubation with tracers. After washout, albumin was almost totally removed from the wall, while the CLDL were practically unchanged. Compaction of the media induced by high distending stresses applied to the vessel might have hindered the efflux of LDL, whereas albumin moved freely through the wall. Synergy between increased endothelial permeability and compaction of the media together with enhanced pressure-driven convection might account for the marked increase in LDL concentration observed in the inner wall at high pressure.

Distribution of endogenous albumin across the rat aortic wall as revealed by quantitative immunocytochemistry

Endogenous albumin was revealed over thin sections of rat aortic wall, with high resolution and specificity, by applying the protein A-gold immunocytochemical technique. Gold particles, revealing albumin antigenic sites, were observed over plasmalemmal vesicles in endothelial cells and over the interstitial space throughout the thickness of the aortic wall. The distribution of the labeling in the interstitial space varied from region to region and was associated with the collagen fibers, following the orientation of the bundles. The morphometric evaluation of this labeling demonstrated a first peak in labeling intensity in the intima followed by a steep decrease with low levels in the media, and an increasing gradient towards the adventitia. In the subendothelium, a moderate labeling was observed at the base of the endothelial cells of both aortic and capillary endothelia, followed by a decreasing gradient. Ratios between the labeling density in the intima as well as in the adventitia and that in the capillary lumen (plasma albumin) revealed different concentrations of albumin in these compartments. Endogenous albumin, under steady-state conditions, is thus unevenly distributed over the interstitial spaces across the rat aortic wall, and appears associated along the collagen fibers.

Inhibition of platelet and red blood cell accumulation on damaged arterial surfaces with albumin pretreatment

Damaged arterial surfaces initiate platelet accumulation which leads to thrombus formation. On artificial surfaces, albumin pretreatment inhibits platelet deposition. To investigate albumin pretreatment on damaged vessel surfaces, 16 carotid arteries were obtained from 8 anesthetized dogs (11-15 kg). Each artery was divided into 2 segments. Each segment was mounted in a perfusion system, distended to 100 mmHg, and the middle section damaged. One segment was perfused with Tyrodes solution plus bovine albumin (5 g/100 ml), while the other segment was perfused only with Tyrodes solution. After 120 mins, both segments were perfused with whole citrated blood containing Indium-111 labeled platelets. Without albumin pretreatment, the proximal section contained 11 +/- 8 (X +/- SEM) percent of total blood radioactivity, while the damaged section contained 53 +/- 11 percent (p less than 0.01). Albumin pretreatment significantly reduced platelet deposition in the damaged section (53 +/- 11 versus 9 +/- 6 percent, p less than 0.01). Further, with albumin pretreatment the radioactive counts in the damaged section were not significantly different from the nondamaged proximal section (9 +/- 6 vs 8 +/- 7 percent, p greater than .8). Quantitative examination of the scanning electron micrographs demonstrated significantly more platelet and red blood cell coverage of the damaged segments (60 +/- 12.8 percent) than of the albumin treated segments (12.9 +/- 5 percent). In four additional experiments, we pretreated arterial segments with albumin for varying time intervals. After 15, 30 and 60 mins of albumin pretreatment, each artery was perfused with radiolabeled platelets and whole blood for 5 mins at 100 mmHg perfusion pressure. Radioactive evidence of platelet deposition on arterial segments treated with albumin for 15, 30, and 60 minutes was also significantly less than control (p less than .05). Beneficial effects of albumin were apparent up to 30 mins of blood flow at 100 mm Hg. Our results suggest that albumin may inhibit platelet and red blood cell deposition on damaged arteries. This could be an adjunct therapy for vessel preservation during artery bypass procedures.

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

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

Role of LDL receptors in the in vitro uptake and degradation of LDL in the media of rabbit thoracic aorta

The possible role of plasma low-density protein (LDL) receptors in the uptake and degradation of LDL in the whole arterial wall was investigated by comparison of the in vitro uptake of 125I-native LDL (nLDL) and 131I-methylated LDL (mLDL) by the media of deendothelialized rabbit thoracic aorta excised at in vivo length and pressurized to 70 mm Hg, taking the advantage that mLDL is not recognized by the LDL receptor. The distribution of the relative concentrations of nLDL (Cn) and mLDL (Cm) across the wall was obtained using a serial frozen sectioning technique. The aorta was incubated under three different conditions for varying periods of incubation in order to analyze separately the processes of binding, binding-internalization, and degradation. At 39 degrees C, in which binding-internalization and degradation occurred, Cn was significantly higher than Cm at each position across the media. The mean medial Cn/Cm ratio was 1.36 +/- 0.15 (n = 5) after 1 hour of incubation, and decreased to 1.23 +/- 0.22 (n = 7) after 2 hours of incubation and to 1.13 +/- 0.11 (n = 5) after 4 hours of incubation. At 4 degrees C, in which internalization and degradation were blocked, the Cn/Cm ratio reflected the surface nLDL binding alone; the Cn/Cm ratio was 1.47 +/- 0.20 (n = 5) after 4 hours of incubation, higher than the value obtained at 39 degrees C. To investigate whether degradation of nLDL occurred after receptor binding, the interstitial LDL was washed out by an LDL-free solution after 2-hour incubation at 39 degrees C. After 30 minutes of washout, the Cn/Cm ratio decreased to 1.06 +/- 0.20 (n = 5) in the inner media and was unchanged in the outer media. After 1 hour of washout, the ratio declined to 0.57 +/- 0.18 (n = 7) in the inner part of the media and increased progressively to 1 at the media-adventitia boundary. The Cn/Cm ratio, at 0.67 +/- 0.12 (n = 5), was practically constant throughout the media after 2 hours of washout. The nLDL degradation rate across the media was obtained from the comparison of nLDL and mLDL before and after the washout. A steep decreasing gradient in nLDL degradation rate was observed from the luminal to the external surface. The mean medial nLDL degradation rate value was 9.6 +/- 4.5 microliters/cm3 wet tissue/hr. We concluded that functional LDL receptors participate in the uptake and degradation of LDL in the whole aorta.

Aortic 125I-albumin transport in patients with Marfan's syndrome and annuloaortic ectasia

The morphologic, biochemical, and mechanical abnormalities of connective tissue fibrous proteins in Marfan's syndrome have been well studied, and their role in cardiovascular complications is well accepted. Less is known, however, about the state of the amorphous components of the aortic connective tissue. In the course of a study of transmural transport in blood vessels, we have had the opportunity to study dystrophic aorta from two young men who survived elective surgery; both with aortic insufficiency (AI) histologically compatible with Marfan's syndrome. One had recurrent chronic dissecting aneurysm (RCDA) as well. The aorta of the first (but not the second) was histologically compatible with Marfan's syndrome. Fresh specimens of intact ascending aorta were incubated in Krebs solution, pH 7.4, containing 125I-labelled bovine serum albumin for 2 h at 37 degrees C. The samples were then frozen, and serially sectioned in the plane of the lumenal surface. The radioactivity of the 20-micron thick sections was then determined, and expressed as a tissue/labelled solution concentration ratio. Transmural profiles of these ratios revealed no difference between the aorta of the RCDA patient with non-specific aortic dystrophy, and that of a 70-year-old man undergoing aortocoronary bypass. However, in the patient with aortic histology compatible with Marfan's syndrome, the average media concentration ratio was 5-fold less (4% vs. 20%).

The effect of varying albumin concentration of the hydraulic conductivity of the rabbit common carotid artery

Rabbit common carotid arteries were cannulated in situ, after ligation of their branches, and transferred to a perfusion apparatus in such a way that they were maintained at their physiological dimensions and the endothelium remained intact. The vessels were pressurized to 150 cm H2O with Krebs solution and the wall smooth muscle was relaxed with 10(-4) M NaNO2. The rate of inflow of perfusate into the vessels was measured by following the movement of a bubble in a calibrated capillary which, when steady, was taken to indicate the transmural filtration rate. The filtration rate was 1.48 +/- 0.26 X 10(-6) cm sec-1 (11) (mean, SD, n) with 1 g/dl bovine serum albumin in Krebs solution. The values with 0, 4, 7, and 10 g/dl, normalized by the 1 g/dl value were 1.38 +/- 0.16 (7), 0.80 +/- 0.05 (9), 0.65 +/- 0.03 (8), and 0.47 +/- 0.06 (9), respectively. The hydraulic conductivity of the wall was also found to depend on perfusate albumin concentration. The 1 g/dl value was 0.92 +/- 0.17 X 10(-8) cm sec-1 (cm H2O)-1 (11) and the values with 0, 4, 7 and 10 g/dl normalized by the 1 g/dl value were 1.35 +/- 0.16 (7), 0.87 +/- 0.06 (9), 0.81 +/- 0.03 (8), and 0.72 +/- 0.06 (9), respectively. The findings were analyzed in relation to models involving interaction of albumin with the endothelial glycocalyx, concentration polarization at the blood/wall interface, dependence of flux on solvent viscosity and dependence of the porosity of the wall interstitium on solvent flux. Both concentration polarization and variation of the porosity of the wall interstitium provide reasonable quantitative explanations for the findings.

Inhibition of cholesterol-induced increases in arterial wall permeability by propranolol

Increased arterial permeability to [125I]albumin is one of the earliest abnormalities found in the cholesterol-fed rabbit model for atherosclerosis and precedes morphologic changes in the arterial wall. Since propranolol is reported to retard atherogenesis in this model, the current study was undertaken to study the effect of propranolol on arterial permeability to albumin in cholesterol-fed rabbits. Carotid permeability to [125I]albumin was measured after 1 week of (a) a control diet containing only trace cholesterol (N = 9), (b) a 1.5% cholesterol diet (N = 11), or (c) a 1.5% cholesterol diet plus intraperitoneal propranolol 5 mg/kg/day (N = 19). The carotid artery was cannulated proximally and distally and peristaltically perfused with oxygenated Modified Eagle's Medium to which [125I]albumin had been added. The permeability of the carotid artery to albumin was estimated by measuring 125I activity in peripheral venous samples obtained after 0, 15, 30, 60, 120, 180, and 240 min of carotid perfusion. 125I-radioactivity of the perfused carotid artery was also counted at the end of the experiment to measure 125I retained in the arterial wall. The transfer of [125I]albumin across the arterial wall to the venous circulation was greater in the untreated cholesterol-fed group than in controls (P less than 0.05), but cholesterol-fed animals treated with propranolol did not differ significantly from controls. 125I-albumin in the arterial wall after 240 min of perfusion was decreased in both untreated cholesterol-fed animals (1444 dpm/cm +/- 350) and in propranolol-treated animals (1629 +/- 309) when these were compared to controls (3909 +/- 693, P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of cigarette smoking on the pattern of arterial blood flow: possible insight into mechanisms underlying the development of arteriosclerosis

By means of non-invasive multichannel doppler ultrasound measurements, cigarette smoking in healthy subjects was shown to increase arterial wall stiffness and to alter the pattern of arterial blood flow, decreasing the pulsatility index. The pattern of blood flow seems to be a contributory factor in the development of arteriosclerosis, and these findings may provide insight into the underlying mechanisms.

Effect of variations in pressure and flow on the geometry of isolated canine carotid arteries

A study is described in which the effects of hemodynamics on arterial geometry are investigated in vitro. A novel perfusion apparatus is employed to deliver pulsatile flow through excised canine carotid arteries under carefully controlled conditions. Data of perfused vessel diameter and arterial wall thickness are derived from the radial displacement of the pulsating vessel as measured using a scanning laser micrometer whose accuracy is determined to be 0.0125 mm (0.0005 in). The results of 30 perfusion experiments suggest that the hemodynamic variables of transmural pressure, pulse pressure and flow rate influence vessel size and radial strain. The physiologic implications of these findings are discussed.

Functional alteration of endothelium by short-term cholesterol feeding

The influence of varying periods of cholesterol feeding and of superimposed endothelial denudation on the transmural transport of albumin has been studied in rabbit carotid artery perfused in vivo with Dulbecco's modified Eagle's medium. The concentration of labeled albumin in jugular venous blood was measured serially over 4 hr and was used as the index of arterial transport. Three groups of rabbits were fed a diet containing 1.5% cholesterol for either 1, 3-7, or 12-28 weeks. A significant increase in albumin transport across the carotid artery was noted in all groups when compared to control animals who were fed a standard rabbit chow. The transport abnormalities observed after 1 week occurred in the absence of any grossly visible atherosclerotic lesions or of any evidence of endothelial cell loss by scanning electron microscopy. In an additional group of rabbits who were fed the cholesterol diet for 16-28 weeks and in which endothelial denudation was performed just prior to carotid perfusion, removal of carotid artery endothelium did not cause a further enhancement of albumin transport above that observed by cholesterol feeding alone. The findings suggest that cholesterol feeding of even very brief duration can alter endothelial function and enhance arterial permeability to albumin.

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

The interaction of convection and diffusion in the transport of 131I-labeled albumin within the wall of rabbit thoracic aorta was studied in vessels excised at in vivo length. They were pressurized with a solution containing no tracer and immersed in a solution containing labeled albumin. The label then entered the wall tissue via the adventitia and had to diffuse against the convective flux which occurred from the lumen to the adventitia. Experiments were performed on intact and deendothelialized vessels pressurized to 70 and 180 mm Hg. At the end of each experiment the vessels were subjected to sequential frozen sectioning parallel to the lumenal surface. The radioactivity of the 20-micron-thick sections was determined and expressed as a tissue:labeled solution concentration ratio. Transmural profiles of these ratios were thus obtained. The steady state was found to be achieved by about 90 minutes. When the convection was enhanced by removal of the endothelium, the average ratios were lower than when the endothelium was intact, and the profile was much flatter. The results suggest that convection influenced macromolecular transport within the arterial wall, even in vessels with intact endothelium.

Transport of 125I-albumin across normal and deendothelialized rabbit thoracic aorta in vivo

Transmural concentration profiles of 125I-albumin in vivo were measured across the normal and balloon catheter-deendothelialized rabbit descending thoracic aorta as a function of time following intravenous injection. A tracer was injected 5 or 60 minutes after deendothelialization, and the animals were sacrificed after circulation times of 10, 30 or 60 minutes. The aorta was immediately excised and frozen flat between glass slides. Samples were serially sectioned parallel to the intimal surface in a refrigerated microtome, washed with trichloroacetic acid (TCA), and counted. Relative tissue concentration profiles of TCA-precipitable radioactivity from the media of control animals showed entry from both luminal and adventitial sides, as previously found with conscious normal rabbits, but spatial gradients at both luminal and medial-adventitial borders were less steep. Relative concentration levels in ballooned animals were 10- to 40-fold higher than in controls, and the profiles were flatter. Uptake rates at equivalent circulation times were greater in experiments initiated 60 minutes, as compared with 5 minutes, after deendothelialization, suggesting that progressive medial edema may have occurred following balloon injury. These results show that the intact endothelium is the dominant mass transfer resistance for 125I-albumin transport across the aortic wall. The data also suggest that the incomplete monolayer of platelets adherent to the subendothelium after balloon deendothelialization is not a substantial resistance to transport, as compared to that of the media, and that convection plays a more important role than diffusion for 125I-albumin transport across the deendothelialized aortic wall.

Effects of endothelial denudation and cholesterol feeding on in vivo transport of albumin, glucose, and water across rabbit carotid artery

An in vivo system for studying arterial transport was developed which utilized the rabbit carotid artery perfused in vivo with Dulbecco's modified Eagle's medium containing 125I-labeled albumin, [3H]-3-methyl-d-glucose, or tritiated water. The appearance of labeled materials in jugular venous blood was measured serially over 4 hours. Vascular integrity was assessed by scanning electron and transmission microscopy. Maintenance of endothelial integrity appeared dependent on perfusion with nutrient tissue culture medium, use of papaverine to inhibit arterial spasm, and circulation of the medium under pressure. Acute endothelial denudation with a balloon catheter induced an approximate 10-fold increase in plasma concentration of labeled albumin and a 3-fold rise in plasma [3H]-3-methyl-d-glucose activity, compared with results in animals with intact endothelium. Increased appearance of tritiated water in venous blood was also observed in the rabbits with denuded endothelium, although the relative rise was less than that with albumin or glucose. Feeding rabbits a diet containing 1.5% cholesterol for periods of 16-28 weeks produced approximately 10-fold increases in plasma concentration of 125I-labeled albumin after arterial perfusion to levels comparable to those present in chow-fed rabbits with experimental endothelial denudation. The increases in albumin transport with cholesterol feeding occurred even though a relatively small fraction of the intimal surface was involved with lesions. The results suggest that the arterial endothelium provides a relative barrier to albumin and, to a lesser extent, glucose and water. The findings also suggest that cholesterol feeding markedly increases arterial permeability to albumin, to a degree that is disproportionately greater than the extent of atherosclerotic involvement of the intimal surface.

The effect of 5-hydroxytryptamine and arterial blood withdrawal on cerebral microcirculation in the cat, arterial permeability in the rabbit

Studies dealing with the effect of 5-HT on cerebral cortical microcirculation of cats and on permeability of femoral arteries to RISA of rabbits are presented. The effect of 5-HT on cerebral cortical microcirculation was compared to that of arterial blood withdrawal and blood reinfusion. The effect of topical administration of 5-HT was also studied. Cortical microcirculation was observed by transillumination using a microtransilluminator. Motion pictures were taken at a speed of 400 frames/sec. and a magnification of 3000X. Permeability was investigated using arterial RISA uptake in vessels perfused in vitro, with continuous recording of perfusion pressure. Microcirculatory studies revealed that arterial blood withdrawal and injection of 5-HT diminished red cell velocity, although to a different degree. With blood withdrawal and reinjection, good correlation existed between blood pressure and red cell velocity. In contrast, no correlation between blood pressure and red cell velocity was found after intracarotid injection of 5-HT. Reactive hyperemia was noted during reinfusion of blood. Both arterial blood withdrawal and 5-HT injection resulted in disappearance of red cells in individual vessels (unperfused channels). Good correlation of blood pressure with capillary red cell velocity during arterial blood withdrawal suggests absence of autoregulation in this portion of the microcirculation. Topical administration of 5-HT caused general vasoconstriction. Permeability to 5-HT to RISA followed a parabolic curve. With slight arterial vasoconstriction, permeability declined, while it rose with severe vasoconstriction.

Water transport in the arterial wall--a theoretical study

Water transport in the arterial wall is studied using a mathematical model based on the theory for the consolidation of water saturated soils (Biot, 1941; Kenyon, 1976a). The intimal pressure is considered to be harmonic in time. Analytical results are obtained for both large and small consolidation times since both the situations are of physiological relevance. For large consolidation times, the filtration is confined to a thin boundary layer. Large pressure gradients exist within the boundary layer while the pressure gradient is negligible in the intermediate layer. Thus, the pulsatile flow is found to be confined to the boundary layer while a smaller mean flow exists throughout the wall.

Binding properties of circulating Evans blue in rabbits as determined by disc electrophoresis

Following injections of Evans blue (1-200 mg/kg) into rabbits, polyacrylamide gel disc electrophoresis showed that Evans blue binds to two protein fractions. The greater part was bound to albumin and the remainder to a plasma protein in the postalbumin fraction. Unbound Evans blue was present in each plasma sample analyzed. Attempts to liberate the dye from the coloured areas of the aorta and common carotid arteries by tissue electrophoresis failed unless very high concentrations of Evans blue were used. This indicates that at the concentrations used by many investigators areas dyed by Evans blue may not be equated with the presence of diffusible protein-dye complexes.