Increasing aortic permeability by atherogenic diet

A high-throughput microtissue platform to probe endothelial function in vitro

A critical role of vascular endothelium is as a semi-permeable barrier, dynamically regulating the flux of solutes between blood and the surrounding tissue. Existing platforms that quantify endothelial function in vitro are either significantly throughput limited or overlook physiologically relevant extracellular matrix (ECM) interactions and thus do not recapitulate in vivo function. Leveraging droplet microfluidics, we developed a scalable platform to measure endothelial function in nanoliter-volume, ECM-based microtissues. In this study, we describe our high-throughput method for fabricating endothelial-coated collagen microtissues that incorporate physiologically relevant cell-ECM interactions. We showed that the endothelial cells had characteristic morphology, expressed tight junction proteins, and remodeled the ECM via compaction and deposition of basement membrane. We also measured macromolecular permeability using two optical modalities, and found the cell layers: (1) had permeability values comparable to in vivo measurements and (2) were responsive to physiologically-relevant modulators of endothelial permeability (TNF-α and TGF-β). This is the first demonstration, to the authors' knowledge, of high-throughput assessment (n > 150) of endothelial permeability on natural ECM. Additionally, this technology is compatible with standard cell culture equipment (e.g. multi-well plates) and could be scaled up further to be integrated with automated liquid handling systems and automated imaging platforms. Overall, this platform recapitulates the functions of traditional transwell inserts, but extends application to high-throughput studies and introduces new possibilities for interrogating cell-cell and cell-matrix interactions.

A definition of initial, fatty streak, and intermediate lesions of atherosclerosis. A report from the Committee on Vascular Lesions of the Council on Arteriosclerosis, American Heart Association

The compositions of lesion types that precede and that may initiate the development of advanced atherosclerotic lesions are described and the possible mechanisms of their development are reviewed. While advanced lesions involve disorganization of the intima and deformity of the artery, such changes are absent or minimal in their precursors. Advanced lesions are either overtly clinical or they predispose to the complications that cause ischemic episodes; precursors are silent and do not lead directly to complications. The precursors are arranged in a temporal sequence of three characteristic lesion types. Types I and II are generally the only lesion types found in children, although they may also occur in adults. Type I lesions represent the very initial changes and are recognized as an increase in the number of intimal macrophages and the appearance of macrophages filled with lipid droplets (foam cells). Type II lesions include the fatty streak lesion, the first grossly visible lesion, and are characterized by layers of macrophage foam cells and lipid droplets within intimal smooth muscle cells and minimal coarse-grained particles and heterogeneous droplets of extracellular lipid. Type III (intermediate) lesions are the morphological and chemical bridge between type II and advanced lesions. Type III lesions appear in some adaptive intimal thickenings (progression-prone locations) in young adults and are characterized by pools of extracellular lipid in addition to all the components of type II lesions.

A definition of initial, fatty streak, and intermediate lesions of atherosclerosis. A report from the Committee on Vascular Lesions of the Council on Arteriosclerosis, American Heart Association

The compositions of lesion types that precede and that may initiate the development of advanced atherosclerotic lesions are described and the possible mechanisms of their development are reviewed. While advanced lesions involve disorganization of the intima and deformity of the artery, such changes are absent or minimal in their precursors. Advanced lesions are either overtly clinical or they predispose to the complications that cause ischemic episodes; precursors are silent and do not lead directly to complications. The precursors are arranged in a temporal sequence of three characteristic lesion types. Types I and II are generally the only lesion types found in children, although they may also occur in adults. Type I lesions represent the very initial changes and are recognized as an increase in the number of intimal macrophages and the appearance of macrophages filled with lipid droplets (foam cells). Type II lesions include the fatty streak lesion, the first grossly visible lesion, and are characterized by layers of macrophage foam cells and lipid droplets within intimal smooth muscle cells and minimal coarse-grained particles and heterogeneous droplets of extracellular lipid. Type III (intermediate) lesions are the morphological and chemical bridge between type II and advanced lesions. Type III lesions appear in some adaptive intimal thickenings (progression-prone locations) in young adults and are characterized by pools of extracellular lipid in addition to all the components of type II lesions.

Densitometric measurement of increased endothelial permeability in arteriosclerotic plaques and inhibition of permeability under the influence of two calcium antagonists

A densitometric technique was established to investigate quantitative changes in endothelial permeability for horseradish peroxidase (HRP), mol. wt. 40,000 daltons, in rabbit carotid artery. Repeated weak electrical stimulations of rabbit carotid arterial walls with implanted electrodes lead to fibromuscular plaques mainly beneath the anode. It could be demonstrated that there exists a typical growth curve of the plaques dependent on the number of days of electrostimulation, with a fast proliferation rate of smooth muscle cells in the first 2 weeks of electrostimulation, and an increasing retardation of proliferation during the next 4 weeks. Endothelial permeability for HRP increases in close relation to the plaque development. Intravenous applications of single doses of the calcium entry blockers flunarizine or nimodipine are able to inhibit the increased permeability of the endothelial lining covering arteriosclerotic plaques. The intensity of the inhibitory action of these calcium antagonists correlates with the size of the arteriosclerotic plaques in inverse proportion, but nevertheless in large plaques an inhibitory effect is seen.

The effect of pentoxifylline on endothelial permeability of rabbit carotid artery wall

Weak electrical stimulation of the rabbit carotid artery wall with direct current impulses over a period of 45 min increased the endothelial permeability for horseradish peroxidase (HRP), molecular weight 40,000 Da, beneath the anode region of the electrodes. The stimuli caused a massive accumulation of the reaction products of peroxidase in the subendothelial space. With a microdensitometric technique, it was possible to quantify the amount of the peroxidase reaction products in the subendothelium, and to compare the amounts in the non-stimulated regions with those in the stimulated regions. Intravenous administration of pentoxifylline before stimulation inhibited the transendothelial transport of peroxidase. Inhibition was pronounced in the stimulated regions, whereas the uptake of HRP in the non-stimulated regions was only slightly lower than that in non-treated animals. It was demonstrated that the extent of inhibition was dose-dependent (3, 10 and 30 mg pentoxifylline/kg). Electron microscopic examination showed that the main route of permeation of HRP was through the interendothelial clefts. However, electrical stimulation also caused an increase in the vesicular uptake of HRP into endothelial cells. Both pathways seem to be influenced by pentoxifylline.

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.

Effect of mast cell stimulators or inhibitors on plasma lipids and aortic content of lipids, hexosamine and hydroxyproline in rats

Twenty-one-day-old rats were treated with mast cell stimulators or inhibitors. Mast cell inhibition produced decrease of total cholesterol in plasma and aorta, while the stimulation led to an increase. Both stimulation and inhibition produced increased levels of plasma and aorta triglycerides, and decrease of aorta content of hexosamine, hydroxyproline and total protein; these same effects could be due to different release of specific mast cell mediators.

Mast cell: its mediators and effects on arterial wall metabolism

Some general aspects of mast cells and their functional mediators, mainly histamine and heparin are reviewed. The actions of mast cells, histamine and heparin are analyzed in the context of their possible role in the physiology and pathophysiology of vascular tissue metabolism.

Uptake of hematoporphyrin derivative by valvular vegetations in experimental infective endocarditis

Drugs that localize in valvular vegetations may be useful in the diagnosis and treatment of infective endocarditis. We therefore tested the hypothesis that parenterally injected hematoporphyrin derivative (HPD), which is concentrated in tumors and atherosclerotic plaques, localizes in the vegetations of experimental infective endocarditis. In 14 rabbits, various bacteria were given intra-arterially immediately after injury to the aortic valve. In 12 additional rabbits, sterile vegetations on the aortic valve were produced by the trauma caused by an indwelling catheter that had been in place over a long period. HPD, 2.5 mg/kg, was injected intravenously 1 to 2 days before the animals were killed in six rabbits with sterile vegetations and in seven rabbits with infected vegetations. In all rabbits, multiple vegetations on the aortic valve leaflets were identified. On exposure to ultraviolet light, strong porphyrin fluorescence of all vegetations, whether sterile or infected, was observed only in rabbits given HPD. In two rabbits given HPD 10 weeks after catheter implantation across the aortic valve, however, only mild fluorescence could be detected in healing endocardial vegetations. In frozen sections of HPD-laden lesions, a patchy distribution of fluorescence was observed that was similar to the pattern of HPD localization in atheromatous plaques. Since vegetations in experimental infective endocarditis selectively concentrate HPD, porphyrins could be useful in the diagnosis and treatment of infective endocarditis.

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.

Selective resolution of plaques and treatment of atherosclerosis by biophysical alteration of "cellular" and "intracellular" properties

Presenting a totally new approach to the effective treatment of Atherosclerosis through the alteration of biophysical properties both "intracellularly" and "extracellularly." The experimental data is preliminary but the results obtained indicate that by allowing the atherosclerotic lesions to take up the magnetically excitable submicron particles and then applying an external alternating electromagnetic field, the atherosclerotic lesions may be selectively resolved without damaging normal blood vessels. This concept suggests many areas of investigation since there are a variety of means of biophysically altering the atherosclerotic plaques, and many substances may be utilized to enhance the process. This new technology and this initial experimentation introduces a "new era" in the effective treatment of Atherosclerosis.

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.

Permeability in atherosclerosis: fluorescence test in green light with trypan blue

A new microscopic fluorescence method for trypan blue at 570 nm has been used to follow the entry of albumin into the atheromatous rabbit aorta. Permeability into the inner aortic wall increases before the onset of gross lesions and seems just to precede intraendothelial deposition of lipid. Thereafter, permeability of the inner wall progressively increases until streaks or small plaques develop. These raised lesions stain and fluoresce variably, some intensely so while others are almost unreactive. This variability might reflect the difference between progressive and quiescent lesions. However, a zone of increased permeability surrounds many raised lesions, suggesting that the edge is a major site of growth and progression.

Aortic endothelial cell morphology observed in situ by scanning electron microscopy during atherogenesis in the rabbit

The morphology of endothelial cells during the induction of atherosclerosis in the descending aortic arch of the hypercholesterol rabbit was studied in situ by scanning electron microscopy (SEM) following silver staining, fixation at physiological pressure, and air-drying of specimens- The earliest deviations from normal endothelial morphology were observed 3 weeks after starting to feed a semi-synthetic diet containing 20% beef fat and 0.2% cholesterol. These were (1) the occurrence of brightly silver stained (argyrophilic) cells, (2) areas of irregularly shaped cells which were often larger and more weakly stained than normal cells and (3) increased incidence of stigmata and stomata associated with the irregular cells. After 6 weeks of hypercholesterolaemia, similar changes were present in the endothelium, but were often also associated with sub-endothelial swelling. These represented the first atherosclerotic lesions. Following 12, 20 and 24 weeks of hypercholesterolaemia, larger raised macroscopic lesions were observed which were always endothelialized. Endothelial morphology and lesion topography suggested that early fatty streaks were composed of numerous focal swellings. In addition to the abnormal endothelial morphology noted at 6 weeks, endothelial cells overlying more advanced lesions became rounded in outline.

Investigation by immunofluorescence of arterial lesions in rabbits on two different lipid supplements and treated with pyridinol carbamate

Rabbits maintained on a pellet diet supplemented with cholesterol, or on a semi-synthetic diet containing beef fat but no added cholesterol, have been studied in relation to their development of hyperlipidaemia and of lipid-filled arterial lesions. The influence of pyridinol carbamate on animals on both diets was also examined but found to produce no significant effect. Animals on both diets developed a hyperlipoproteinaemia. In cholesterol-fed animals this developed quickly, became gross, and was characterized by the presence of an anomalous lipoprotein of very low density, large molecular size and abnormally high cholesterol content. Beef fat fed animals showed a more moderate hyperlipidaemia which developed more slowly and the lipoproteins qualitatively resembled those in normal rabbits. Differences in the rate and severity of development of aortic lesions between the two different dietary supplements were found to reflect differences in the duration and intensity of hyperlipoproteinaemia between the groups. Arterial lesions in cholesterol-fed animals were more extensive and contained larger numbers of fat-filled cells than those in beef fat-fed animals. Comparisons were made (in many cases on the identical section) between lesions treated with a fluorescein labelled antiserum to total rabbit serum low density lipoproteins (TLDL) and with a conventional lipid stain. Precise agreement was found between the distribution of lipid reacting with Oil red 0 and specific fluorescence for TLDL in endothelial cells, in extracellular deposits in the intimal ground-substance and in medial smooth muscle cells. But fat-filled cells in the intima and in reticulo-endothelial tissue showed variable immunofluorescent reactivity. The reason for this discrepancy is discussed. Agreement between the distribution of conventional lipid staining and specific immunofluorescence for TLDL was also found in extracellularly distributed material in arterioles and smaller vessels at certain sites. It is suggested that these results establish that rabbit TLDL serve as the vehicles transporting lipid into the experimental lesions, just as the homologous human lipoproteins do in human atherosclerosis.

The effect of hypercholesterolemia on aortic endothelium studied en face

The aortic endothelium of rabbits fed a stock diet containing 1 or 2% cholesterol for intervals of 1 day to 21 weeks was compared with that of normal rabbits on a stock diet without cholesterol. The endothelium was examined en face using the Häutchen technique. The aortic endothelium of the experimental animals revealed an increase in stigmata and stomata, multinucleated giant cells and leukocytes particularly about sites of branching. These changes were also observed in the endothelium overlying lipid deposits but in addition, there was an increase in mitoses, an alteration in the orientation of endothelial cells and their nuclei, and a change in their argyrophilic staining properties. Closely related to the endothelium was an increasing number of foam cells which appeared to be derived from monocytic leukocytes.

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.

Effects of angiotensin II and some analogues on vascular permeability in the rabbit

Recent investigations in the rat have shown that endothelial cell contraction should be included among the multiple actions of angiotensin II (AII) in large arteries. In the current study, after intravenous injections (60 mg/kg) of the azo dye Evans blue, a segment of the rabbit abdominal aorta was isolated between temporary ligatures and injected with 1 x 10 -10 g of AII diluted in Ringer's solution. Diffuse increased permeability of the aortic endothelium occurred within 60 seconds only in areas exposed to AII. Although injections of Ringer's solution alone produced no blueing, surface staining and scanning electron microscopy of the aortic endothelium after AII administration showed endothelial cell contraction, widening of the interendothelial junctions, and surface sudanophilia. Similar studies were carried out to determine changes in dermal vascular permeability. Of all the substances tested, AII, prostaglandin E 1 (1 x 10 -10 -1 x 10 -8 g), and serum triglycerides (1 x 10 -6 g) produced the most marked vascular response characterized by the appearance, within 15 minutes, of a bluish wheal or papule. Injections of angiotensin I or norepinephrine at concentrations of 10-100 x 10 -9 g or of Ringer's solution alone did not induce skin changes. Synthetic peptides, competitive antagonists to the myotropic and vasopressor activities of AII, blocked extravasation of Evans blue dye injected simultaneously with the octapeptide, but antihistaminics did not inhibit its effects. We concluded that the multiple actions of AII in blood vessels are related not only to the control of blood pressure but also to the regulation of the permeability and the function of the vascular wall as a whole.