Abnormalities of the arterial wall and its metabolism in atherogenesis

Aggregation of dimyristoylphosphatidylglycerol liposomes by human plasma low density lipoprotein

Turbidity (absorbance at 470 nm) measurements revealed human serum low density lipoprotein (LDL) to cause, within a few minutes and at physiological pH and [NaCl], the aggregation of liquid crystalline large unilamellar liposomes (LUVs) of dimyristoylphosphatidylglycerol (DMPG). No evidence for concomitant lipid or aqueous contents mixing was obtained with fluorescent assays for these processes, in keeping with the lack of fusion of LUVs. Involvement of apoB is implicated by the finding that tryptic digestion of LDL abrogates its ability to cause aggregation. Aggregation is not caused by VLDL, HDL2, or HDL3. Interestingly, also oxidised LDL failed to aggregate DMPG vesicles. Aggregation of DMPG LUVs by LDL did depend on the ionic strength of the medium as well as on the phase state of the lipid. More specifically, below the main transition temperature Tm maximal aggregation was seen in the presence of 25-100 mM NaCl, whereas slightly higher (up to 150 mM) [NaCl] were required when T>Tm. Aggregation due to LDL was also observed for dimyristoylphosphatidylserine as well as for dipalmitoylphosphatidylglycerol LUVs, whereas liposomes composed of either unsaturated acidic phospholipids or different phosphatidylcholines were not aggregated. Involvement of electrostatic attraction between the acidic phosphate of DMPG and cationic residues in apoB is suggested by the finding that increasing the content of dimyristoylphosphatidylcholine (DMPC) in DMPG liposomes reduced their aggregation and at XDMPC=0.50 no response was evident. Notably, increasing the mole fraction of 1-palmitoyl-2-oleyl-PG (POPG) in DMPG LUVs progressively reduced their aggregation by LDL and at XPOPG=0.50 there was complete inhibition. The latter effect of POPG is likely to be due to augmented hydration of the unsaturated lipid constituting a barrier for the contact between apoB and the vesicle surface. In keeping with this view, the presence of the strongly hygroscopic polymer, poly(ethylene glycol) at 1% (by weight) enhanced the aggregation and could partly reverse the inhibition by POPG.

Non-ischemic hypoxia of the arterial wall is a primary cause of atherosclerosis

The response-to-injury hypothesis has been the dominant model of atherosclerosis for 20 years. However, it does not explain the experimental role of oxygen in atherogenesis, does not explain many of the clinical features of atherosclerosis, and has failed to provide useful countermeasures. I propose that arterial wall hypoxia results from risk factors for atherosclerosis. The primary mechanism is decreased oxygen delivery by a microcirculatory derangement resulting from impaired erythrocyte deformability. As in a healing wound, hypoxia causes growth factor release within the arterial media. Diffusion of these factors causes intimal proliferation and atheroma formation. This hypothesis implies that simple inexpensive oxygenation regimens might prevent the morbidity and mortality of atherosclerosis. Despite demonstrated effectiveness in experimental models, such treatments have not been extensively studied in clinical atherosclerosis because they conflict with the dominant model. This dogma needs to be re-examined.

Early change of vascular permeability in hypercholesterolemic rabbits

To study the effect of hyperlipidemia on vascular permeability, serial anterior chamber fluorophotometric examinations were carried out on 33 control rabbits (group 1) and 32 diet-induced hypercholesterolemic (group 2) rabbits. Changes in the blood-aqueous barrier function associated with total serum cholesterol (CHO) and triglyceride (TG) levels were studied at the beginning of the study and every 2 weeks thereafter for up to 16 weeks following 0.5% cholesterol-enriched diet feeding. Concurrently, a slit-lamp biomicroscope was used to examine the iris for evidence of atheromatous plaque. In group 1, the CHO level decreased slightly during the first 6 weeks and remained rather steady thereafter. The status of the blood-aqueous barrier correlated significantly with serum CHO and TG levels (r = .46, P < .001; r = .23, P = .01, respectively). In group 2, CHO and TG levels increased significantly after 2 and 8 weeks of cholesterol-enriched diet feeding, respectively. The blood-aqueous barrier also became more permeable than that in group 1 after 2 weeks' and increased above its baseline level after 6 weeks' feeding. Both CHO and TG levels correlated well with the degree of blood-aqueous barrier breakdown (r = .51, P < .001; r = .25, P < .001, respectively). The first evidence of iridic lipid-streak deposition was noted at 7.6 +/- 0.7 weeks, while definite iridic atheromatous plaque appeared 11.2 +/- 0.7 weeks after feeding. The change in the blood-aqueous barrier also correlated well with the semiquantitative score of iridic plaque (r = .58, P < .001) and usually preceded visual evidence of plaque formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Arterial intimal hyperplasia after occlusion of the adventitial vasa vasorum in the pig

Oxygenation of the arterial wall is provided by diffusion of oxygen outward from the main vessel lumen and inward from the adventitial vasa vasorum. In a group of four Yucatan miniature pigs the oxygenation profiles across the superficial femoral arteries were recorded by polarographic oxygen microelectrodes. The profiles obtained suggested a relatively poorly oxygenated media (a trough value of approximately 25% that of the intimal oxygenation) with a progressive rise in oxygenation toward the intimal and adventitial surfaces. In four other survival experiments, occlusion of the adventitial vasa vasorum by flush ligation of the arterial branches that supply them resulted in the production of a focal, intimal hyperplastic lesion that was absent in control vessels (intimal to medial ratios [mean +/- SEM] of 0.053 +/- 0.008, n = 8, p < 0.001 and 0.013 +/- 0.001, n = 8, respectively). By electron microscopy this lesion was seen to be composed mainly of smooth muscle cells. This evidence would support the hypothesis that arterial wall hypoxia may be involved in the initiation of intimal hyperplasia. It is proposed that human atherosclerosis may be initiated by occlusion of the vasa vasorum and concomitant hypoxia.

Two variants of coronary atherosclerosis in six populations

Coronary arteries and aortas obtained at autopsy in six populations were examined microscopically. Two variants of atherosclerosis could be separately quantified by objective morphometry. These two variants, fibroplasia and atheronecrosis, were both found to increase with age and to correlate with each other from one autopsy to the next after age adjustment. Despite the correlation of these two variables among individuals, the measures of fibroplasia and atheronecrosis did not correlate with each other across population groupings. A positive correlation of this kind between some populations was canceled by an inverse correlation between other populations. For example, Manila was the population with the greatest intimal fibroplasia in the coronary arteries at each age, and Sao Paulo was among those with the least intimal fibroplasia. Nevertheless, Sao Paulo showed the greatest tendency toward atheronecrosis, while Manila was among the populations with the least tendency toward atheronecrosis. The possibility arises, therefore, that the subjects from Sao Paulo were exposed to a different assortment of etiologic agents than were the subjects from Manila, and that their arteries, therefore, manifested a different kind of atherosclerosis.

Adaptation to arterial wall hypoxia demonstrated in vivo with oxygen microcathodes

Iliofemoral arteries of 9 rabbits were balloon de-endothelialized resulting in subintimal thickening. Contrary to expectation, enzyme and lactate determinations did not indicate arterial wall hypoxia when compared with arteries of 10 control rabbits. The explanation came from in vivo measurement of oxygen tension profiles across the de-endothelialized and control femoral arteries and from the subsequent histological findings. They showed that the impaired oxygen supply of the de-endothelialized arteries with subintimal thickening was counteracted by a centripetal oxygenation of the arterial wall obviously induced by proliferation of newly formed nutrient vessels in the adventitia. Such adaptation is an important mechanism against hypoxia induced by arterial injury and may be an essential protective factor in atherogenesis.

Lipids, clotting factors, and diabetes: endogenous risk factors for cardiovascular disease

Theories of intimal injury leading to plaque formation include platelet adhesion and production of growth factors, hypercholesterolemia, smooth muscle cell proliferation, macrophage activity, defective utilization of low-density lipoproteins via deficient receptors, and deficiency in cellular lysosomal enzymes. High levels of low-density lipoproteins and intermediate-density lipoproteins, as well as their apoproteins, are strong risk factors for cardiovascular disease. The lowering of the cholesterol level has been shown to produce significant regression of atherosclerotic lesions. Data also suggest an interaction between lipids and platelets, although the role of coagulation disorders as an independent risk factor for atherosclerosis is difficult to assess. Although much of the data are controversial, there is evidence that platelet survival time is a strong predictor of severe vessel damage. In addition, some studies have reported decreased activity of antithrombin III with coronary artery disease, and there appears to be a direct correlation between fibrinogen and cholesterol levels. Finally, diabetes mellitus (both types I and II) is a significant independent risk factor for atherosclerosis. The risk is not related to the severity or duration of diabetes, and it appears to be greater in women than in men.

Metabolite concentration gradients in the arterial wall of experimental atherosclerosis

Development of intimal necrosis is characteristic of advanced atherosclerotic lesions. In order to study metabolic alterations preceding cellular death, the concentration gradients of glucose, glycogen, ATP, and lactate were measured within the walls of rabbit carotid arteries which were transmurally stimulated by dc impulses. This experimental model allows the induction of lipid-free intimal myocyte proliferations usually void of necrosis and, under additional hypercholesterolemia, typical lipid-laden atheromas, which become necrotic after 5-6 weeks of stimulation. Frozen samples (1 X 3 mm2) obtained from normal arterial wall and from both types of plaques as well as from spontaneously occurring aortic lesions of the hypercholesterolemic rabbits were cut in consecutive slices (10 microns thick) parallel to the luminal surface from the intima toward the adventitia. Compared with normal media, the results show that in the neointima the concentrations of glucose and glycogen decreased whereas that of lactate increased, independent of whether or not hypercholesterolemia was induced. The ATP concentration in lipid-free plaques was comparable with that in normal media, but strongly decreased in lesions which developed in combination with hypercholesterolemia. These observations support the view that in the initial stages of plaque growth, the intimal cells reveal an increased energy turnover leading to a decreased glycogen content. The development of necrosis, however, seems to involve additional events associated with a decrease in ATP, obviously depending on the effects of lipids or lipoproteins in the cells of the arterial wall.

Physicochemical transfer of [3H]cholesterol from plasma lipoproteins to cultured human fibroblasts

The transfer of free cholesterol from [3H]cholesterol-labelled plasma lipoproteins to cultured human lung fibroblasts was studied in a serum-free medium. The uptake of [3H]cholesterol depended upon time of incubation, concentration of lipoprotein in the medium, and temperature. Modified (reduced and methylated) low-density lipoprotein (LDL), which did not enter the cells by the receptor pathway, gave a somewhat lower transfer rate than unmodified LDL, but if the transfer values for native LDL were corrected for the receptor-mediated uptake of cholesterol the difference was eliminated. The initial rates of transfer of [3H]cholesterol from LDL and high-density lipoprotein (HDL) were of the same order of magnitude (0.67 +/- 0.05 and 0.75 +/- 0.06 nmol of cholesterol/h per mg of cell protein, respectively) while that from very-low-density lipoprotein (VLDL) was much lower (0.23 +/- 0.02 nmol of cholesterol/h per mg) (means +/- S.D., n = 5). The activation energy for transfer of cholesterol from reduced, methylated LDL to fibroblasts was determined to be 57.5 kJ/mol. If albumin was added to the incubation medium the transfer of [3H]cholesterol was enhanced, while that of [14C]dipalmitoyl phosphatidylcholine was decreased compared with the protein-free system. The results demonstrate that, in spite of its low water solubility, free cholesterol can move from lipoproteins to cellular membranes, probably by aqueous diffusion. We propose that physicochemical transfer of free cholesterol may be a significant mechanism for net uptake of the sterol into the artery during atherogenesis.

Renal vascular disease: pathology of large blood vessel disease

Lesions of the extrarenal arteries which are associated with hypertension are described. Included are descriptions of the development, stages, and complications of atherosclerotic plaques; intimal fibroplasia and the medial fibromuscular dysplasias; Takayasu's aortitis; radiation injury; and a newly described arterial disease, medial agenesis. Also described is the development of atherosclerosis in saphenous vein bypass grafts and the effects of transluminal angioplasty.

Arterial calcification in diabetics

Calcification of the media of the arteries of the lower limbs, giving a linear type of calcification roentgenologically is typical for diabetic arteriosclerosis. Spotty calcification of the arteries of the lower extremity, which histologically is found in the intima, is also seen a little more often in diabetics with gangrene. There are no major differences between the other histological vascular structures of diabetics and arteriosclerotics. Maximal stenosis of the lumen is found more peripherally in diabetics than in persons with normal carbohydrate metabolism. The results favour the idea of predominant femoropopliteal occlusion in arteriosclerosis and popliteal-tibial occlusion in diabetics.

Aortic enzymes and lactate in high altitude-raised and cholesterol-fed rabbits

Fourteen male rabbits born at elevation 4000 ft (first experimental series) were transferred at age of 2 months to elevation 12470 ft and raised there for 18 weeks. Half of the animals remained on a commercial rabbit chow (group H) while the other half was on the same diet supplemented with cholesterol (group C). Eight male rabbits raised at sea level served as controls (group S). Intima-media homogenates from the thoracic aortas were assayed for lactate dehydrogenase (LDH), malate dehydrogenase (MDH), lipoamide dehydrogenase, pyruvate kinase (PK), phosphofructokinase (PFK) and the lysosomal hydrolases beta-glucuronidase and N-acetyl-beta-glucosaminidase (NAGA). Aortic lactate and glucose were also measured. Thirty-two male rabbits (second experimental series) were subdivided into 4 groups. Rabbits were fed a cholesterol-supplemented diet not only at high altitude (8 rabbits matching group C) but also 8 animals raised at sea level. The degree of atherosclerosis in the aortas of these 4 groups was assessed by measuring the aortic cholesterol contents. Plasma cholesterol was also determined. In the aortas of the rabbits of group H the activity of PK was significantly elevated, and the activity of the lysosomal hydrolases significantly decreased compared with aortas of group S rabbits. There was no difference in the other enzyme activities or in the aortic glucose and lactate content of these groups. Cholesterol feeding of the animals of group C resulted in a significantly increased activity of the lysosomal hydrolases as well as of LDH and PK. The lipid analyses (second experimental series) revealed a trend to a lower concentration of aortic cholesterol in the high altitude than in the sea level animals, both fed a cholesterol diet, in spite of the higher plasma cholesterol concentrations in the high altitude animals. The low aortic lysosomal hydrolase activities in the high altitude rabbits are in accord with their comparatively lower susceptibility to experimental atherosclerosis. This metabolic feature may be due to a lower degree of exposure of these aortas to injurious factors, such as infections or lower blood pressure. The elevated activity of PK without increased lactate content in group H animals seems to parallel the well-known general adaptation of the organism to high altitude hypoxia, and does not indicate a metabolic switch toward anaerobic glycolysis.

Effect of D-penicillamine on the mechanical properties of aorta, muscle tendon and skin in rats

The biomechanical properties of rat aorta, muscle tendon and skin were studied after daily D-penicillamine treatment (500 mg/kg) for 5, 10 and 42 days. D-Penicillamine treatment for 5 days resulted in increased aortic extensibility. After long-term treatment the aorta exhibited a shift towards decreased extensibility and increased stiffness at small 'stress' values. Simultaneously the dry weight and diameter of the aortic samples were increased after D-penicillamine treatment for 42 days. After correction of the mechanical parameters for the increased amount of tissue of the samples, the stiffness at small 'stress' values was still increased and the mechanical stability at high 'stress' values retained. This is in contrast to the marked reduction in the strength of muscle tendon and skin of the same animals after D-penicillamine treatment for 42 days. This study demonstrates that a primary increase in the extensibility of aorta may elicit a reactive formation of vascular connective tissue. It is proposed that aortic smooth muscle cells are activated by an increased pulsatile distension of the vessel wall secondary to the early effect of D-penicillamine on collagen and elastin. The resulting excess deposition of collagen and elastin leads to increased stiffness, which may in turn increase the susceptibility of the aortic wall to hemodynamic injury. Consequently D-penicillamine may not as proposed counteract the development of atherosclerosis.

Correlations of aortic histology with gross aortic atherosclerosis and metabolic measurements in diabetic and nondiabetic Macaca nigra

We studied the aortic histology of 28 Macaca nigra males and females, from 6 to more than 20 years old, normal and manifesting various degrees of spontaneous diabetes. Correlations of several metabolic and hormonal indicators of diabetes severity with gross and microscopic findings in the aortas demonstrated direct associations with the severity of atherosclerosis. Mild to relatively severe aortic lesions were present. These monkeys showed many changes similar to those observed in medium and large arteries of diabetic humans. Intimal proliferation, prominent extracellular fibers as part of the intimal thickening, and lipid deposition--mostly in extracellular locations--were particularly evident. Significant relationships were observed when glucose clearance, insulin secretion, and fasting glucose levels were correlated with all aortic microscopic findings. Cholesterol concentrations had no correlation with the histological state of the aortas, and triglyceride levels correlated only with aortic lipid content and intimal thickness. Aortic pathologic changes increased with age; diabetics had significantly greater changes than nondiabetics. Macaca nigra can be useful in the study of how diabetes affects the development of atherosclerosis without the influence of an atherogenic diet.

The complementary interaction of epidemiological and experimental animal studies: a key foundation of the preventive effort

This overview will examine the contributions made by animal models of atherosclerosis to our understanding of its pathogenesis and will describe the principles for prevention of this disease process. In addition, some of the recent biochemical and cell biological discoveries, many utilizing animal cells and animal lipoproteins, will be discussed. Further, the increasing knowledge brought to worldwide efforts for the prevention and regression of atherosclerosis by the aforementioned research approaches will be reviewed. The results of some of the most promising animal studies of advanced atherosclerotic plaque regression will be briefly summarized, emphasizing the quantitation of changes. The prospects for verifying some of these findings in quantitative studies of human atherosclerosis will be presented along with some of the obstacles to be overcome.

Ultrastructure of the umbilical artery from a newborn delivered at term by a mother who smoked 80 cigarettes per day

The ultrastructure was studied of an umbilical artery from a newborn child delivered by a healthy mother who smoked as many as 80 cigarettes per day. Focal areas in the luminal part of the media appeared acellular consisting of collagen fibers only. In the deeper parts of the media the smooth muscle cells were hypertrophic and showed sign of proliferation. These smooth muscle cells had accumulated huge amounts of glycogen in the perinuclear region. The glycogen accumulations were always found in close connection with the mitochondria and often associated with lipid droplets. Occasionally phagocytosis and degradation of collagen by the smooth muscle cells were visualized.

Pathogenesis of rheumatoid arthritis: a vascular hypothesis

Past research into the pathogenesis of RA has generally concerned itself with established inflammation. The present review summarizes alterations in microvascular anatomy and function which occur during the hypoxic state, in various experimental and disease conditions. It further shows that tissue hypoxia is a common finding in RA and that the microvascular alterations of RA are similar to those produced by experimental hypoxia. The available data suggest that microcirculatory compromise, concomitant with an increase in metabolic needs of synovial tissue, may initiate tissue injury via anoxia and acidosis, resulting in hydrolytic enzyme release, increased vascular permeability and acceleration of inflammatory processes. It is further believed that the microcirculatory abnormality may be generalized, accounting for the systemic manifestations often seen in RA. Factors effecting arteriolar blood flow obstruction are reviewed to identify areas for future investigation in RA and other disorders involving microvasculopathy. The multitude of longknown and newly recognized factors predisposing to vasospasm and vasodilatation have been outlined as a guide to possible mechanisms which may be operative in RA. An attempt has been made to gather and synthesize the available data in the hope that it may stimulate other investigators to pursue more definitive research into specific areas which may show early microvascular abnormalities in the pathophysiology of RA. Identification of factors operative early in the pathogenesis of RA, before it becomes self-perpetuating, may well be a step in the direction of preventing the ravages of this disease, or providing insight to more effective control.

Serum low density lipoproteins with mitogenic effect on cultured aortic smooth muscle cells

Low density lipoprotein (LDL) subspecies of different size and lipid mass were isolated by density gradient ultracentrifugation from the serum of male rhesus monkeys (Macaca mulatta) fed both a low fat, low cholesterol commercial primate ration, and cholesterol-supplemented high-fat diets, as well as from the serum of human donors. The mitogenic effect of these lipoproteins was examined using primary cultures of rhesus aortic smooth muscle cells. It was observed that the smaller LDL (molecular weight 2.7 X 10(6) from normolipidemic monkeys and a small LDL (molecular weight 2.6 X 10(6) occurring in some normal human subjects exhibited no mitogenic action. In turn, the larger LDL subspecies (molecular weight greater than 3.0 X 10(6), and buoyant density less than 1.030 g/ml), whether from normolipidemic or hyperlipidemic monkeys, or from some normal human subjects, had a marked proliferative action. The results indicate that both hyperlipidemic and normal sera (both human and rhesus) contain mitogenic LDL species although in different amounts. LDL-III, the rhesus equivalent of human Lp(a) was not mitogenic despite its similarity on size and lipid composition to the stimulating particles. However, on the removal of most of its large sialic acid moiety, a clear mitogenic action was observed. The mechanisms responsible for the proliferative effect are unclear and may involve LDL mass, lipid composition, and surface charge although other speculations cannot at present be ruled out. Furthermore, since the small LDL subspecies of either rhesus or human origin were nonmitogenic and similar in mass to the LDL found in calf serum, the mitogenic response of the smooth muscle cells to large LDLs may depend on their early conditioning with the LDL of calf serum.

Endothelial morphology and plasma total and high density lipoprotein cholesterol changes in hypothalamically stimulated squirrel monkeys fed a modified atherogenic diet

Experimental animals fed atherogenic diets show endothelial damage, impairment of endothelial regeneration and plasma lipid changes characterized by elevation of LDL and decrease of HDL cholesterol concentrations. Previous studies in this laboratory disclosed that chronic electrical stimulation of the lateral hypothalamus was associated with electron-microscopic evidence of endothelial injury in rats and squirrel monkeys maintained on basal (low fat/cholesterol-free) diets. In the present investigation squirrel monkeys fed similar diets supplemented with "modest" amounts of caloric fat and cholesterol were subjected to chronic lateral hypothalamic stimulation for periods as long as 20 months with the expectation that endothelial injury would be greater than in the absence of the supplements. The expectations were not substantiated. Endothelium was found to be surprisingly intact by electron microscopy and similar to that of implanted nonstimulated controls. A further observation of interest was the cholesterolemic response, notably in the HDL fraction, observed in both groups, but more striking in experimental animals. The data suggest that an interaction between a modified lipid/cholesterol diet and hypothalamic stimulation may lead to elevation of plasma HDL cholesterol concentration and preservation of endothelial integrity. Further investigation is required to determine whether these two events are causally related.

Effect of hypothalamic stimulation on the endothelial morphology of the aorta in the conscious squirrel monkey

The role of neurogenic factors in the development of atherosclerosis has not previously been studied in detail. In recent years evidence has accumulated to implicate endothelial injury as a primary stimulus for the proliferation of myo-intimal cells resulting in the formation of the early morphologic lesion. In the present investigation, the effect on aortic endothelial morphology of repetitive electrical stimulation of the lateral hypothalamus in the conscious, unrestrained squirrel monkey, maintained on a cholesterol-free low-fat diet, has been studied. Stimulation was performed with a self-powered, miniaturized electronic stimulator connected to indwelling electrodes. Implanted nonstimulated animals served as controls. Endothelial injury in the form of cell degeneration, denudation, with plasma insudation and partial junctional separation were observed electron-microscopically in stimulated animals compared with controls. These alterations were found to be independent of hypercholesterolemia and/or hypertension. Possible pathways for the induction of injury in this neurogenic model are: (1) direct, through neural circuits from the brain to the vessel wall, and (2) indirect, by elaboration of angiopathic substances inside or outside of the CNS, released into the circulation and transported to the vessel wall where they exert their effects. Reversibility of the endothelial injury progression to established lesions and mechanisms involved remain to be determined in further investigations.

Cause and course of acute myocardial infarction

A complete discussion of factors involved in causing acute myocardial infarcts is contained in ths review, and a detailed discussion of various complications of acute myocardial infarction is also provided. Interventions that are capable of reducing infarct size in animal models with experimentally produced acute myocardial infarcts are discussed, and factors that might alter infarct size in patients are reviewed. Prognostic factors that allow one to identify high risk patients with acute myocardial infarction following their hospital admission and in the follow-up period after hospital discharge are identified. The need to develop more accurate measurements of the extent of myocardial infarction and myocardial ischemia, and to develop additional prognostic markers that allow one to separate patients most at risk for sudden death and new myocardial infarction from those with ongoing chronic ischemic heart disease and those without additional complications from their ischemic heart disease is also emphasized in this review.

Significance of luminal plasma layer resistance in arterial wall oxygen supply

Previous analyses of the arterial wall oxygen supply system have assumed that a cell-free layer of plasma next to the endothelium is the major transport barrier in the lumen. Using a computer simulation, we have quantitatively tested this assumption. Our results show that oxygen diffusion gradients extend significantly into the flowing blood well beyond any plasma layer and that the major luminal transport resistance lies in the flowing blood and not in the plasma layer. The simulation was also employed to compute the effect of a reported 50% drop in plasma oxygen diffusivity. This rather large reduction did significantly lower oxygen levels within the arterial wall tissue. Whether such large reductions in diffusivity ever actually occur in human plasma is a subject of current controversy.

Carbon monoxide-induced arterial wall hypoxia and atherosclerosis

The elevated carbon monoxide level found in tobacco smokers has been suggested as one etiologic factor linking it with atherosclerosis. Unquestionably carbon monoxide does induce some arterial wall hypoxia, which has been established as an atherogenic factor, but without knowing the extent and location of this hypoxia the importance of this mechanism could not previously be assessed. Carbon monoxide acts both by inducing hypoxemia and shifting the oxyhemoglobin equilibrium curve, with these effects acting on the oxygen transport system from both the luminal blood and the vasa vasorum. We have studied this system using a computer simulation of the human arterial wall and found significant, mid-medial hypoxia with blood carbon monoxide levels routinely found in smokers. Because these levels fluctuate, the hypoxia they induce would be expected to be uncompensated by increased vascularization and therefore potentially represent a much more significant factor in atherogenesis than chronic hypoxia alone.

A scanning electron microscopic study of arterial endothelial cells using vascular casts

Vascular casts were made of rabbit aortas by infusing Batson's No. 17 anatomical corrosion compound into the artery at physiological pressure. The arterial tissue was then digested with sodium hydroxide and the cast viewed by scanning electron microscopy (SEM). Outlines of the endothelial cells and their silver stained boundaries were clearly visible. Cell nuclei and fine surface detail were also discernible. In EDTA damaged arteries, injured endothelial cells and platelets could also be observed in the vascular casts.

Spontaneous atherosclerosis: an ultrastructural study in the white Carneau pigeon

The atheroscleroit lesions, associated with the celiac intimal smooth muscle cushions, of four and five year old White Carneau pigeons were studied with the light and electron microscopes. Light microscopic examination of the spontaneous lesions demonstrated large intimal cushions composed of smooth muscle abundant collagen, clusters of foam cells and cholesterol crystal clefts. Ultrastructural examination of the intimal atheroma revealed dilatations between apposing endothelial cells which contained a flocculent material, similar to that seen in the subendothelial space. The subendothelial compartment contained abundant collagen, extracellular lipid, vesiculated material and cell processes which contained a flocculent matrix and tubular-like elements. In addition, fibroblast-like interlaminar cells were often observed. Numerous intimal smooth muscle cells were seen which displayed varied morphology. Abundant foam cells were also present within the intimal atheromas. The presence of atherosclerotic lesions in preexisting intimal smooth muscle cushions suggests that hemodynamic factors may be important in the progression of these spontaneous lesions. Endothelial cell dilatations may provide an important route of transport for circulating elements which may accumulate withing the subendothelial space. Morphologically, it appears that the smooth muscle cells undergo modification and may represent the precursors of foam cells in this species.