The effect of blood pressure on the passage of labeled plasma albumin into canine aortic wall

Lymphatic Vessel Network Structure and Physiology

The lymphatic system is comprised of a network of vessels interrelated with lymphoid tissue, which has the holistic function to maintain the local physiologic environment for every cell in all tissues of the body. The lymphatic system maintains extracellular fluid homeostasis favorable for optimal tissue function, removing substances that arise due to metabolism or cell death, and optimizing immunity against bacteria, viruses, parasites, and other antigens. This article provides a comprehensive review of important findings over the past century along with recent advances in the understanding of the anatomy and physiology of lymphatic vessels, including tissue/organ specificity, development, mechanisms of lymph formation and transport, lymphangiogenesis, and the roles of lymphatics in disease. © 2019 American Physiological Society. Compr Physiol 9:207-299, 2019.

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).

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.

LIPOPROTEIN MOVEMENT THROUGH CANINE AORTIC WALL

Low-density (1.019 to 1.063) lipoprotein labeled with radioiodine enters the inner layer of the canine aortic wall directly from the aortic lumen. Its rate of entry is greatest in the proximal aorta and decreases progressively down the length of the aorta. A similar gradient was observed previously for the accumulation of cholesterol early in experimental atherosclerosis.

Effects of pressure-induced stretch and convection on low-density lipoprotein and albumin uptake in the rabbit aortic wall

The effects of pressure-driven convection and vessel wall stretching in the pressure-related changes in low-density lipoprotein (LDL) and albumin transport across the arterial wall were studied in vitro in freshly excised rabbit thoracic aorta held at in vivo length and pressurized at 70, 120, or 160 mm Hg for 30 minutes. External rigid polyester sleeves of various diameters (4, 5, or 6 mm) were passed around half of the arterial segments in order to prevent vessel distension during pressurization. The intraluminal solution contained 131I-LDL and 125I-albumin. The transmural distribution of relative concentrations of LDL (CLDL) and albumin (Calb) across the wall was determined in wrapped and unwrapped segments using a serial frozen-sectioning technique. In the unwrapped segments, Calb increased uniformly between 70 and 120 mm Hg (P < .0001) but did not change significantly between 120 and 160 mm Hg (0.0063 +/- 0.0009 [n = 4], 0.0520 +/- 0.0055 [n = 9], and 0.0620 +/- 0.0071 [n = 12], respectively). In contrast, CLDL increased markedly both between 70 and 120 mm Hg (P < .001) and between 120 and 160 mm Hg (P < .05) (0.0025 +/- 0.0005 [n = 4], 0.0234 +/- 0.0029 [n = 9], and 0.0393 +/- 0.0056 [n = 12], respectively), with the increase being much more pronounced in the inner than in the outer media. In the segments wrapped with the 4-mm sleeves, both CLDL and Calb did not vary significantly between 70, 120, and 160 mm Hg. In the segments wrapped with the 5-mm sleeves, CLDL increased significantly between 120 and 160 mm Hg, whereas Calb did not vary significantly with increasing pressure. Our results demonstrate that (1) pressure-induced stretching of the arterial wall is a major determinant of arterial mass transport, and (2) pressure-driven convection accentuates LDL accumulation in the inner media, which may explain enhanced atherosclerosis in hypertension.

Increase by adrenaline or angiotensin II of the accumulation of low density lipoprotein and fibrinogen by aortic walls in unrestrained conscious rats

1. Earlier experiments of ours with anaesthetized rabbits showed that infusions of catecholamines into the carotoid blood stream significantly increased the uptake of radioiodinated low density lipoprotein (LDL) by the artery wall after as little as 2 h. This observation has now been extended to much longer time periods, i.e., 6 days, and another species, viz. conscious and unrestrained rats; also to another pressor agent, angiotensin II, as well as to another plasma protein, fibrinogen. 2. Groups of rats were infused from subcutaneously implanted osmotic minipumps for 6 days. The infusions were either into a carotid artery or into the surrounding tissues, with essentially the same effects. Control animals were infused with saline, and test animals with either adrenaline or angiotensin II. The minipump concentration of adrenaline of 4-5 microM, which gave blood concentrations of 25-41 nM, increased the blood pressure significantly after 3 days. The minipump concentration of angiotensin II of 9.9 mg ml-1 was chosen to produce similar increases in blood pressure. 3. Five days after starting the infusion, rats were injected i.v. with either homologous or human LDL labelled with [125I]-tyramine cellobiose (TC), or with [131I]-TC labelled human fibrinogen. Twenty-four hours later, the animals were killed and the radioactivities determined in the whole aorta. The labelled TC radioactivities represent primarily metabolised protein (because TC is trapped intracellularly), but also include the fraction of intact, i.e., non-metabolized protein in transit through the vessel wall. To determine the contribution of the latter, in some experiments we injected double-labelled [131I]-[125TC]-LDL only. These experiments showed that the [131I]-LDL counts representing protein in transit accounted for approx. 20% of the total 125TC counts, and that this percentage was not significantly affected by adrenaline or angiotensin II. Therefore, the bulk of the experiments was carried out with single labelled proteins, using 125I to label TC-LDL and 131I to label TC-fibrinogen. In these experiments, the radioactivity of the arterial wall thus provides a cumulative measure of the uptake and degradation of proteins. 4. Aortic wall radioactivities from rat and human LDL and from human fibrinogen were significantly increased by both agents. Adrenaline at 25-41 nM increased the radioactivities by 52 and 47% for rat and human LDL respectively, and by 31% for human fibrinogen; these differences were highly significant (P < 0.01). Angiotensin II at ca. 10 nM also increased the radioactivities significantly, by 21% for human LDL and by 109% for human fibrinogen (P < 0.05). 5. The results suggest that the accumulation of LDL and of fibrinogen by rat aorta is increased by adrenaline or by angiotensin II at concentrations which raise the blood pressure progressively and significantly after 3 or 5 days respectively.

Effect of angiotensin II and enalapril on transfer of low-density lipoprotein into aortic intima in rabbits

To assess the mechanism behind a possible atherosclerosis-promoting effect of angiotensin II, the influence of angiotensin II, noradrenaline, and enalapril on transfer of low-density lipoprotein (LDL) into the arterial wall was investigated in conscious rabbits. Intravascular infusion of angiotensin II (1.4 micrograms/kg per minute) initially increased the mean blood pressure from 70 to 80 mm Hg to 125 to 150 mm Hg; this effect was transient, and the blood pressure returned to baseline values within 2 hours, despite continuous infusion of angiotensin II. The normalized influx of LDL into the aortic intima, determined after in vivo exposure to 125I-LDL for 1 hour, was 88 +/- 17 (n = 6), 12 +/- 12 (n = 5), and 28 +/- 6 (n = 5) nL/cm2 per hour (mean +/- SEM) during angiotensin II infusion at high blood pressure, during angiotensin II infusion after the blood pressure had been normalized, and during continuous saline infusions, respectively (P < .05 for high blood pressure versus low blood pressure and saline). When noradrenaline was used to increase blood pressure to a level similar to that induced by angiotensin II, the normalized influx of LDL in noradrenaline-treated rabbits was also increased markedly. Production of endogenous angiotensin II was inhibited with enalapril (2.9 mg/kg per day). Compared with placebo rabbits, enalapril-treated rabbits had a 92% lower plasma angiotensin-converting enzyme activity and a 23% lower blood pressure. The normalized influx of LDL, however, was similar in the two groups at 18 +/- 2 (n = 10) and 20 +/- 3 (n = 10) nL/cm2 per hour, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Influx, efflux, and accumulation of LDL in normal arterial areas and atherosclerotic lesions of white Carneau pigeons with naturally occurring and cholesterol-aggravated aortic atherosclerosis

This study investigated the hypothesis that increased influx of low-density lipoprotein (LDL) accounts for the natural development of atherosclerosis in a characteristic (susceptible) site in the distal thoracic aorta of White Carneau (WC) pigeons and the exacerbation of atherosclerosis by cholesterol feeding. The influence of dietary cholesterol-induced changes in LDL composition on LDL influx into the artery was also investigated. As a measure of the influx of LDL into the artery, we determined the arterial accumulation of radiolabeled LDL after 1 hour. Nine 50-month-old WC pigeons with naturally occurring atherosclerosis and seven 14-month-old WC pigeons with atherosclerosis accelerated by 10 months of cholesterol feeding were studied. In the absence of atherosclerotic lesions, we found no evidence for increased accumulation of LDL at the susceptible site. In fact, more LDL accumulated in less susceptible normal arterial areas near the heart (approximately 90 nl/h per square centimeter) than in the susceptible distal thoracic aorta (approximately 35 nl/h per square centimeter). In the absence of atherosclerotic lesions, LDL accumulation (nanoliters per hour per square centimeter) was not influenced by hypercholesterolemia, although mass transport of LDL cholesterol into the artery was increased. Naturally occurring atherosclerotic lesions accumulated five times as much LDL as the adjacent normal arterial area (P < .001), whereas cholesterol-aggravated atherosclerotic lesions in different arterial sites accumulated four to 26 times as much LDL as the adjacent normal artery (P < .05). Cholesterol-aggravated atherosclerotic lesions at the most susceptible site accumulated five times as much LDL as naturally occurring atherosclerotic lesions in the corresponding arterial site (823 +/- 241 vs 175 +/- 45 nl/h per square centimeter, mean +/- SEM; P < .005). Arterial accumulation of LDL was influenced very little by changes in LDL composition induced by cholesterol feeding. In another study with young WC pigeons free of atherosclerosis and other WC pigeons with cholesterol-aggravated atherosclerosis, we injected differently labeled LDL 0.5 and 1 hour before sacrifice to investigate whether efflux of LDL from the artery was significant during a 1-hour period of LDL uptake. Although efflux of LDL from all arterial sites occurred during 1 hour, differential efflux could not account for regional differences in 1-hour arterial LDL accumulation. This study suggests that the characteristic susceptibility of the distal thoracic aorta of WC pigeons to atherosclerosis and the exacerbation of atherosclerosis by cholesterol feeding cannot be explained by differences in influx or efflux of LDL.(ABSTRACT TRUNCATED AT 400 WORDS)

Albumin transport characteristics of rat aorta in early phase of hypertension

The effects of early-stage hypertension on the macromolecular transport characteristics of the aorta have been investigated in rats 1 week after the ligature of the abdominal aorta between the two renal arteries. The animals were left untreated or treated for 1 week with an angiotensin converting enzyme inhibitor (enalapril, 6 mg/kg per day). Blood pressure of a subgroup of hypertensive rats was acutely lowered to a normal level by injection of enalaprilat (1.5 mg/kg) at the time of the experiment. 131I-Albumin and 125I-albumin were injected 90 minutes and 5 minutes, respectively, before the rats were killed. The transmural distribution of the relative tissue concentrations across the wall was obtained using a serial frozen-section technique. Short-term albumin uptake permitted calculation of apparent endothelial permeability coefficients, and 90-minute uptake was used to estimate the steady-state albumin distribution within the media. The effect of early-stage hypertension on the characteristics of the arterial macromolecular transport depended on the aortic site; the ascending aortic arch appeared not to be affected. In the thoracic and abdominal aorta, the endothelial permeability coefficients increased significantly in hypertensive rats. This increase was not a direct effect of the arterial pressure, since the values were not significantly different when the pressure was acutely normalized. The 90-minute albumin concentration in the media was enhanced in hypertensive rats and returned to the normal value by acutely lowering the blood pressure, indicating that the increase observed in hypertensive rats resulted from a direct effect of pressure, possibly increased pressure-driven convection and/or pressure-induced stretching of the wall. Treatment by angiotensin converting enzyme inhibitor prevented hypertension and protected against its effects in hypertensive animals.

Use of the perforated balloon catheter to infuse marker substances into diseased coronary artery walls after experimental postmortem angioplasty

A perforated balloon catheter was used in human coronary arteries after postmortem angioplasty had been performed. The catheter used has a standard angioplasty balloon with a pattern of laser-produced holes, 25 microns in size, which generate streams of fluid under pressure. Studies of the routes by which marker substances enter diseased arterial tissue when infused by the perforated balloon after experimental angioplasty are described. A colored marker dye entered the new crevices and dissection planes created by the angioplasty, but did not extend greater than 2 cm either proximal or distal to the perfused segment. Horseradish peroxidase entered tissue not only from the lumen and adventitia as occurs with its infusion into normal tissue with the perforated balloon, but also extended from new crevices and dissection planes created by the angioplasty. Platelet aggregation, coagulation and cell proliferation, the likely causes of restenosis after angioplasty, originate in the sites of greatest tissue disruption and blood stasis. These postmortem studies suggest that active drugs are delivered to the arterial wall in a manner likely to be effective in preventing these events.

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.

Internal elastic membrane in the internal mammary and left anterior descending coronary arteries and its relationship to intimal thickening

The internal mammary artery (IMA) is less prone to intimal thickening than coronary arteries and is routinely used in coronary bypass surgery. To resolve whether morphologic differences can explain why IMA develops less intimal thickening, morphometric measurement of the left anterior descending coronary artery (LAD) and IMA was carried out in 62 autopsies from individuals of various age groups. The amount of intimal thickening, the degree of folding of the internal elastic membrane, and media thickness were estimated by appropriate indices. Intimal thickening of LAD was more marked, occurred earlier and could be demonstrated even below one year of age, but was not found before 21 years of age in the IMA. A fold-index of the internal elastic membrane was significantly higher in the IMA than in the LAD and correlated negatively with an intimal thickening index in both arteries (P less than 0.0001 for both). The fold-index decreased with age, most markedly in LAD. Negative correlation was also found between a media thickness index and the intimal thickening index in LAD (P less than 0.02). The fold-index may indicate the magnitude of tangential intimal tension during life and may be of significance for development of intimal thickening and atherosclerosis. Lower fold-index may in part explain the propensity of atherosclerosis in LAD compared with IMA. Also, reduced fold-index with age accompanied the occurrence of atherosclerosis with age.

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.

In vivo hypertensive arterial wall uptake of radiolabeled liposomes

Using five sham-operated and seven aortic coarctation-induced hypertensive New Zealand White rabbits intravenously injected with neutral small unilamellar vesicles loaded with [111In]nitrilotriacetic acid, we demonstrated in vivo that the normal aortic arterial wall participates in liposome uptake and that this uptake is increased in the hypertensive aortic wall by approximately threefold (p less than or equal to 0.0001). Among the three regions examined, aortic arch, thoracic aorta, and lower abdominal aorta, the difference in uptake between the normotensive and hypertensive arterial walls was significantly different, p less than or equal to 0.05, p less than or equal to 0.0001, and p less than 0.05, respectively. The uptake by the different regions of the hypertensive arterial wall is consistent with the pathological changes present in these areas. Furthermore, the extent of liposome uptake by the aortic wall is strongly correlated with the height of the blood pressure (r = 0.85, p = 0.001, n = 11). We conclude that neutral small unilamellar liposomes can be used to carry agents into the arterial wall in vivo in the study of hypertensive vascular disease and could be especially useful for the delivery of pharmacologically or biologically active agents that would otherwise be inactivated within the circulation or are impermeable to the arterial wall.

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.

Initiation of atherosclerotic lesions in cholesterol-fed rabbits. II. Selective retention of LDL vs. selective increases in LDL permeability in susceptible sites of arteries

We asked if the arterial sites most prone to early lesions in cholesterol-fed rabbits have higher permeabilities to low density lipoprotein (LDL) in normolipidemic rabbits or if these sites become more permeable shortly after the onset of cholesterol feeding. We also considered whether the focal increases in the concentration of LDL within the arterial wall in lesion-susceptible sites before fatty streak formation can be explained by increased arterial permeability to LDL or by other mechanisms such as decreased rates of LDL efflux or degradation. 125I-tyramine cellobiose-labeled LDL was injected 1 hour before death to determine the initial rate of LDL entry into lesion-prone and lesion-resistant sites of aorta as a measure of permeability. This was studied in normal rabbits and in rabbits fed cholesterol for 4, 8, or 16 days. Combining this permeability data with the tracer data described in the accompanying article, we fit a kinetic model to calculate the mass and mean residence time of intact LDL within the artery and the fractional rates of LDL degradation and efflux from the artery. In normal rabbits, the permeability of lesion-susceptible branch sites of the abdominal aorta was about four times that of the lesion-resistant, nonbranched areas. However, the permeability of the aortic arch, a susceptible site, was similar to that of the lesion-resistant descending thoracic aorta. Permeability to LDL did not increase in any aortic site during the 16 days of cholesterol feeding, even in sites with the largest increases in arterial LDL concentrations. Plasma LDL cholesterol concentration increased substantially and total LDL cholesterol delivery into the artery increased many fold. Since there was no differential change in permeability between susceptible and resistant sites, the increased entry of LDL did not explain the selective increases in arterial LDL concentration in susceptible sites. Kinetic analysis indicated that the fractional rate of degradation of the arterial LDL pool was lower in lesion-prone sites than in lesion-resistant sites in all animals. Fractional rates of efflux of arterial LDL decreased in lesion-susceptible branch sites of the abdominal aorta and were low in the lesion-susceptible aortic arch. These results suggest that the focal increases in LDL concentration observed in all lesion-susceptible sites of cholesterol-fed rabbits before fatty streak formation are due to localized differences in LDL retention and diminished fractional rates of LDL degradation, not to selectively increased permeability.(ABSTRACT TRUNCATED AT 400 WORDS)

Interrelationship of hypertension, plasma lipids and atherosclerosis

The relationship between hypertension and atherosclerosis has been illustrated by epidemiological, clinical and experimental observations. Typical atherosclerotic lesions develop in arterial wall when hypercholesterolaemia is present. Hypertension aggravates these lesions by causing vascular structural changes. In clinical studies, however, the correction of high blood pressure does not decrease the incidence of coronary heart disease. Several hypotheses have been formulated to account for this observation: one is that reversibility of the structural vascular changes induced by hypertension is not complete when the blood pressure is lowered; another is that antihypertensive drugs have a deleterious effect on the vascular wall.

Influence of pressure on permeability of normal and diseased muscular arteries to horseradish peroxidase. A new catheter approach

The effects of different distending pressures on permeability of dog and human arteries to horseradish peroxidase (HRP) were studied. A new catheter was employed to achieve the distention of defined vessel segments to the desired pressure. Normal dog brachial arteries were studied both post mortem and in vivo. Mildly to moderately diseased human coronary arteries were studied post mortem. A predictable linear relationship between pressure and penetration of HRP into the dog arterial media was found, using pressures of 0, 150, 300 and 500 mm Hg. Postmortem vessels were consistently less permeable than those studied in vivo. Full penetration of the media by HRP was achieved by application of 300 mm Hg pressure for 45 sec with the new catheter. When human coronary lesions were examined under these same conditions, plaques were readily demonstrated to be permeable to HRP, even to a depth of many hundreds of micrometer. Thus, penetration of arterial wall thickness by HRP (Mr 40,000 dalton) is related to the distending pressure applied. Human coronary plaques also show ready penetrance by HRP. The new catheter described allows the application of these pressures to defined segments of the arterial tree.

Role of the altered transmural permeability in the pathomechanism of arteriosclerosis. History of arteriosclerosis theories. Role of the altered permeability in experimental arteriosclerosis models

After presenting an overview on classification and history of arteriosclerosis theories, the physiological factors involved in the transmural permeability of the arteries are discussed in detail. The development and characteristic features of the altered transmural permeability were studied in various experimental models such as in rat's hypercholesterolemia, local aortic hypoxia, lymphedema of the vascular wall and in hypertension. Results appear to show that alterations in permeability invariably developed in all of the pathological conditions examined, they were transient in nature and preceded the onset of intimal proliferation(s). The disturbance of transmural permeability might be the common pathologic clue which initiates uniform vascular responses to injuries produced by a variety of noxious stimuli. The possible role of the altered transmural permeability in the induction of smooth muscle cell proliferation is also discussed and evidence is provided that after withdrawal of stimulus for vascular injury intimal proliferation will not develop despite the manifest disorders in permeability.

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.

The extra-cellular-matrix in hypertension: the link between renal function, autoregulation and sodium metabolism, a review and hypothesis

In this paper the nature of the extra-cellular-matrix (ECM) is outlined briefly. Changes in the ECM in hypertension have been reviewed as has their possible involvement in the pathophysiological process of hypertension. It appears that significant and widespread changes occur in the ECM in hypertensive states. Such changes may alter the characteristics of (1) the renal perfusion pressure/sodium excretion curve (2) autoregulation and (3) sodium metabolism, all of which have been implicated in the pathogenesis of hypertension. The ECM may well be the link between these three. The ECM changes will also contribute to many of the differences, e.g. altered fluid distribution, altered handling of a sodium load, altered electrolyte and water content of vascular tissue, and altered passive and active mechanical properties of vascular tissue seen in hypertensive states. The ECM changes are unlikely to be the root cause of hypertension but are a more fundamental difference than altered renal perfusion/excretion characteristics, altered fluid distribution etc. Indeed the perfusion/excretion characteristics, fluid distribution etc. appear to be dependent upon the properties of the ECM. What now requires to be done is to determine what controls the properties of the ECM and to determine what fault in the control may lead to the changes which could ultimately manifest themselves as hypertension.

Effect of serotonin on albumin and low density lipoprotein uptake in perfused rabbit femoral arteries

Uptake by the arterial wall of plasma constituents has considerable clinical implication; thus, uptake of low density lipoprotein (LDL) plays an important role in the development of atherosclerosis. Serotonin and other vasoactive material may result in changes in the arterial wall leading to increased uptake of albumin or low density lipoprotein. This study was conducted to determine the effect of serotonin (10(-5)M 5-hydroxytryptamine) and serotonin-induced arterial constriction on albumin and low density lipoprotein uptake rates in perfused rabbit femoral arteries. The results show that the presence of serotonin inhibits the rate of uptake of both albumin and LDL. The effect on albumin uptake does not have a direct dose response dependence and is linearly dependent on transmural pressure. In contrast, LDL uptake rates are only slightly affected by pressure. Thus, albumin and LDL uptake processes appear to be due to separate mechanisms.

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.

Effects of particle size and perfusate composition on the uptake of colloidal gold by the rabbit thoracic aorta perfused in situ

The influence has been investigated of particle size on the uptake of radioactive gold colloid by the rabbit thoracic aorta perfused in situ. Particles ranging in diameter from 14 nm to 40 nm were suspended in 0.9% NaCl and infused either at a pressure of 15 mm Hg for times of between 2 1/2 and 60 min or at pressure of between 15 and 160 mm Hg for 5 min. Uptake by the whole intima-media increased with perfusion time and hydrostatic pressure but did not depend on particle size. Radioactive assay of serial sections across the aortic wall also showed that particle size did not influence the distribution of tracer. An effect of perfusate composition on uptake was demonstrated in further experiments in which particles either 14 or 40 nm in diameter were suspended in pooled rabbit serum and infused at pressures of between 15 and 140 mm Hg for 5 min. Uptake and transmural distribution were again independent of particle size, but uptake was 4-5-fold less than when the particles were perfused in saline. Under all perfusion conditions radioactivity fell steeply across the intima and then rose gradually across the media and adventitia. Radioactivity in the outer media and adventitia increased with perfusion time but little change could be detected in intimal activity. In transmission electron micrographs, particles in the intima were not seen to penetrate the internal elastic lamella and in the outer media particles remained extracellular and did not enter collagen bundles. Autoradiographs showed that particles in the intima were uniformly distributed around the circumference of the vessel but in the outer media and adventitia particles usually clustered close to the vasa vasorum.

The permeability of large arteries to labeled protein

Using labeled albumin, the permeability index was determined for the aorta and other arteries of the rabbit after different circulation times. The results were reproducible. Different indices of permeability were obtained for different arteries. When the circulation time of the tracer was extended, the increase in the permeability index was greater for the coronary and femoral arteries than for the aorta. 5-hydroxytryptamine, bradykinin and prostaglandin E2 tended to increase the permeability index. There was no relation to the level of blood pressure. The different indices of permeability obtained for the different arteries were not leveled by the mediators. The permeability index is seen to become greater postmortally.

Arterial connective tissue changes and distribution of 125I-labelled low density lipoprotein in hypertensive pigs

Young pigs with hypertension of 10 weeks duration, resulting from cellophane perinephritis, were injected with 125I-labelled low-density lipoprotein ([125I]LDL) before being killed 24 h or 48 h later. Intimal thickening and increased acid mucopolysaccharide were demonstrated in the aortas and major arteries of the hypertensive animals. Increased accumulation of [125I]LDL was observed in the inner media beneath areas of intimal thickening. It is suggested that the primary effect of hypertension in atherosclerosis is to produce structural changes in arterial connective tissue which allow increased accumulation of LDL by altering the permeability and binding properties of the arterial wall.

Relationship between inhibition of aortic histamine formation, aortic albumin permeability and atherosclerosis

Effects of partial inhibition of aortic histamine formation on aortic albumin uptake and lipid deposition were examined in male, New Zealand white rabbits maintained on Purina Rabbit Chow containing 0.5% cholesterol for a 2-week period. Aortic histamine synthesis was inhibited by partial inhibition of aortic histidine decarboxylase (HD) through administration of alpha-hydrazinohistidine (alpha-HH, MK785, Regis Chemical Co., 25 mg/kg, i.p. at 12-h intervals). Additional rabbits were maintained on either the cholesterol diet or on Purina Rabbit Chow without cholesterol. Results indicate that administration of alpha-HH for the 2-week period produced a 31% reduction (P less than 0.05) in aortic HD activity in those rabbits maintained on the cholesterol diet, and that concurrently there was a 51% reduction in aorta albumin uptake (P less than 0.025) and a 63% reduction in the extent of oil red O staining. By regression analysis a significant correlation coefficient (r = 0.71, P less than 0.005) was obtained between the aortic albumin uptake and the aortic histamine forming capacity (HFC) in rabbits maintained on this cholesterol diet. These findings indicate that the aortic HD system may be an important enzymatic coupler involved in vascular permeability alterations occurring early in the atherogenic sequence.

Localization of acid and alkaline phosphatase activities in the blood vessels of normotensive and hypertensive rats

Samples of aortae and caudal arteries from normotensive and hypertensive rats were studied for cytochemical and biochemical determinations of acid and alkaline phosphatase activities. Cytochemical examination revealed an increased amount of acid phosphatase reaction product in hypertensive samples, with extensive localization to the extracellular matrix. Alkaline phosphatase activity was localized to the plasma membrane of fibroblasts and vascular smooth muscle cells and to the extracellular matrix. Biochemical assays of enzyme activities supported the cytochemical findings, showing increased activity in aortae from hypertensive rats.

Detection of atherosclerotic plaques in carotid arteries by the use of 123I-fibrinogen

A non-invasive method for detecting atherosclerotic plaques by the administration of radioactively labelled fibrinogen has been used in a pilot study in one non-atherosclerotic patient and in four patients with unilateral or mainly unilateral atherosclerotic changes. Immediately after 1 mCi 123I-labelled fibrinogen had been given intravenously, the cervical region was examined with a gamma camera and again after 4 h and 20 h. In the control case there was little difference in activity between the left and right carotid regions. The other cases showed a significant increase of activity over the region of the plaque after 4 h. This increased uptake had disappeared after 20 h. Studies to clarify the nature of this uptake and the usefulness of the method are in progress.

Time dependence of [131I]albumin uptake following removal of an experimental aortic stenosis

The uptake of trypan blue and [131I]human serum albumin (HSA) has been studied in the dog's abdominal aorta between 1 and 42 days after removal of an experimental stenosis (approximately 90%) applied 1 week previously. Previous work has shown that when the stenosis was present during circulation of these markers, their uptake was increased immediately proximal to the stenosis decreasing to normal by the renal artery level. Distal to the stenosis uptake was reduced apart from small areas of high uptake probably due to turbulent jet impacts. Within the stenosed section the uptake was normal. In this present study it was found that, after removal of the stenosis, proximal uptake initially remained elevated, returning to normal after approximately 15 days whilst the distal uptake returned to normal after approximately 10 days. In the previously stenosed section uptake was increased markedly following the release of the stenosis but returned to normal within approximately 20 days. The relationship of these findings to alterations in the local haemodynamic state and to possible changes in endothelial morphology are discussed.

Effect of serum and liver extracts from hypercholesterolemic rats on the synthesis of collagen by isolated aortas and cultured aortic smooth muscle cells

Rats were made hypercholesterolemic by feeding them a high-cholesterol, olive oil diet for one week. The effect of sera and 35,000 X g supernatants of liver homogenates on collagen synthesis was studied in isolated aortas, cultured arterial smooth muscle cells and the same cells in suspension. Compared to the preparations from normal rats, the liver preparations from hyperlipidemic rats stimulated collagen synthesis in both isolated aortas and cultured smooth muscle cells by about 25%. In these test systems hyperlipidemic serum was without effect but when added to smooth muscle cells incubated in suspension, produced a significant increase in the amount of collagen secreted. Hyperlipidemic serum caused an increase of about 50% in the incorporation of [3H]-thymidine by cultured smooth muscle cells.

Role of hypertension in atherosclerosis and cardiovascular disease

Clinical, experimental and pathologic studies strongly indicate that hypertension is a major factor in coronary heart disease, sudden death, stroke congestive heart failure and renal insufficiency. The deleterious effect of the elevated blood pressure on the cardiovascular system appears to be due mainly to the mechanical stress placed on the heart and blood vessels. Humoral factors and vasoactive hormones such as angiotensin, catecholamines and prostaglandins may play a role in the pathogenesis of hypertensive cardiovascular disease but this role has not yet been defined and is probably secondary. Hypertension and the resulting increase in tangential tension on the myocardial and arterial walls, leads to the development of hypertensive heart disease and congestive heart failure as well as hypertensive vascular disease that affects not only the kidneys but also the heart and brain. Hypertensive vascular disease involves both large and small arteries as well as arterioles and is characterized by fibromuscular thickening of the intima and media with luminal narrowing of the small arteries and arterioles. The physical stress of hypertension on the arterial wall also results in the aggravation and acceleration of atherosclerosis, particularly of the coronary and cerebral vessels. Moreover, hypertension appears to increase the susceptibility of the small and large arteries to atherosclerosis. Thus the patient with hypertension is a candidate for both hypertensive and atherosclerotic vascular disease of the coronary and cerebral vessels leading to occlusive disease of both the large and small arteries and resulting in myocardial infarction and stroke. Other major complications of hypertensive vascular disease include rupture and thrombotic occlusion of blood vessels, especially in the brain. Disease of the arterial media, which begins in childhood with the deposition of calcium in the vessels, may be an important cause of arterial hypertension. This form of hypertension may manifest itself in adults as arteriosclerotic hypertension and lead to cardiovascular complications very similar to those of essential hypertension. The relation of arteriosclerotic hypertension to nutritional factors, including dietary salt intake, deserves study.

Pathogenetic mechanisms in atherosclerosis

Four theories of atherogenesis are briefly reviewed and criticized: the degenerative, the thrombogenic, the platelet aggregation and the insudative theory. Evidence is presented in detail to suggest that a modified form of the insudative theory (1) accounts more satisfactorily than the other theories for the known association of risk factors with atherosclerosis and (2) allows one to understand how some of the more important risk factors operate at the level of the arterial wall. It is proposed that atherosclerotic plaques, and also certain extravascular lesions broadly associated with atherosclerosis (corneal arcus, xanthomas), arise because altered endothelial permeability allows certain reactive macromolecular plasma proteins (the plasma low density and very low density lipoproteins and fibrinogen, which are normally largely confined to the circulation) to permeate endothelium and interact with charged components of the connective tissue gel of the arterial wall or other tissues. The effect of hyperlipidemia, hypertension, arterial disease or injury upon this process, and the manner in which these factors interact, is examined in relation to experimental findings and clinical observations.

Pathogenesis of hypertensive retinopathy. An experimental study in the monkey

Retinal changes in accelerated hypertension were studied in seventeen monkeys with experimental hypertension by means of ophthalmoscopy and colour and flourescence photography during life, and by injection and digest preparations and light and electron microscopy after the animals had been killed. Cotton-wool spots developed in all but three monkeys. The arteries became tortuous and dilated and the light reflex decreased in those animals that became hypertensive. The earliest abnormality was a development of many points of fluorescein leakage on terminal arterioles or small arteries. Such leaking points were always present in relation to cotton-wool spots but were not confined to such areas. Focal narrowing of arteries was not observed but arteriolar occlusion and retrograde filling of the distal segment was present in three animals. Superficial linear haemorrhages were noted in five animals. Light microscopy revealed cotton-wool spots which were identical to those observed in man with a collection of swollen axons containing densely staining pseudonuclei. Study of the arterioles by electron microscopy showed findings ranging from normality to extensive necrosis. Many precapillary arteries were constricted and some were virtually occluded. Degenerative changes were present in smooth muscle cells in the wall of many of the constricted arterioles. Many arteries also showed insudation into their wall of plasma which had seeped into the muscular coat displacing and sometimes entirely replacing the smooth muscle cells. Except for arterioles with advanced necrosis, there was no indication of how plasma insudation occurred. Two arterioles with extensive necrosis showed a break within the endothelial cell cytoplasm through which penetration of plasma proteins had probably occurred. The extravascular tissues showed collections of amorphous material, sone of it with the typical banded configuration of fibrin. The sequence of events proposed to explain these features is as follows: (1) The arterioles constrict as the pressure rises, most likely as a result of vascular autoregulation. This may head to occlusion of the precapillary arterioles and is associated with necrosis of vascular smooth muscle. (2) Dilatation then occurs with insudation of plasma into the unsupported wall through a damaged endothelium. This stage probably corresponds to the autoregulatory break-point and is evidenced clinically by focal leakage of fluorescein. (3) Progressive plasma insudation into the vessel wall with further muscle necrosis results in secondary occlusion and the typical picture of advanced fibrinoid necrosis.