Atherosclerosis--its pathogenesis in perspective

What is the cardioprotective role of hormone replacement therapy?

A very large body of literature has yielded strong biologic and mechanistic plausibility for the consistent observational findings that estrogen is cardioprotective. Recently completed randomized, controlled trials have been interpreted as challenging the doctrine that hormone replacement is cardioprotective for postmenopausal women. However, other than the Estrogen in the Prevention of Atherosclerosis Trial, none of the currently completed (and no ongoing) randomized, controlled trials have appropriately tested the hypothesis generated from observational data that estrogen replacement is cardioprotective. This mainly results from the fact that randomized, controlled trials have not tested the same pattern and type of hormone use in the same population of women observed in the epidemiologic studies. On the other hand, recently completed randomized, controlled trials provide important but limited information concerning the clinical use of a specific regimen of hormone replacement for the prevention of cardiovascular disease in a particular population of postmenopausal women. Observations made from epidemiologic studies will have to be appropriately tested in randomized, controlled trials before any real conclusions can be drawn as to whether hormone replacement is cardioprotective.

Antiatherosclerosis interventions in women

Although important in reducing atherosclerosis progression and cardiovascular events in women, lowering of low-density lipoprotein cholesterol levels is not sufficient for optimum prevention. Additional potential targets for intervention include reducing levels of triglyceride-rich lipoproteins, increasing high-density lipoprotein cholesterol, and replacing certain hormones that decrease during menopause. Hormone replacement therapy (HRT) for the reduction of atherosclerosis progression and cardiovascular events has become complicated because of conflicting outcomes from recent surrogate end point studies and secondary prevention randomized clinical trials. The key to HRT may be early intervention, when women first enter menopause and the atherosclerotic process appears to be relatively quiescent. Although many questions remain to be answered in this important area of women's health, there is a fact that is certain: administration of HRT for the reduction of cardiovascular events is not a straightforward proposition. Powerful imaging tools for monitoring atherosclerosis progression exist that can be used to help address many questions relating to type, dose, regimen, and timing of HRT.

Atherosclerosis imaging methods: assessing cardiovascular disease and evaluating the role of estrogen in the prevention of atherosclerosis

Various interventions for cardiovascular disease (CVD) slow or reverse the progression of atherosclerosis and reduce the risk of clinical coronary events. Although the cardiovascular benefits of hormone replacement therapy have been demonstrated in observational studies in predominantly healthy women, no benefit has been found in a randomized clinical trial conducted in older women with established CVD. It is possible that the benefit of hormone therapy occurs when it is used relatively early in the progression of atherosclerosis. Techniques are now available to monitor the various stages of atherosclerosis. Quantitative coronary angiography, a technique used to evaluate relatively late-stage atherosclerosis, has been shown to predict the risk of subsequent clinical coronary events. B-mode ultrasonography of the intima-media wall thickness (IMT) of the carotid artery can assess the earlier stages of atherosclerosis and correlates with atherosclerosis risk factors, as well as with clinical cardiovascular and cerebrovascular outcome. This technique offers a relatively rapid and cost-effective method to test therapies for CVD and to screen for individuals who are at high risk for cardiovascular events. As an example of the use of atherosclerosis imaging to evaluate possible therapeutic interventions, measurements of IMT were performed in a randomized, controlled trial comparing oral 17beta-estradiol with placebo. The results demonstrated that 17beta-estradiol significantly reduces the progression of subclinical atherosclerosis in healthy, postmenopausal women when compared with placebo.

Postmenopausal hormone replacement therapy as antiatherosclerotic therapy

Cardiovascular disease remains the number one killer of women. Although important for the reduction of cardiovascular events, lipid alteration does not appear to be sufficient to obtain optimum reduction in cardiovascular risk. Women have a potential opportunity for further reduction in cardiovascular risk through postmenopausal hormone replacement therapy. More than 50 observational studies indicate that postmenopausal use of hormone replacement therapy reduces atherosclerosis and cardiovascular events. However, recently reported, randomized, controlled clinical trials have yielded mixed results as to whether hormone replacement therapy reduces cardiovascular events relative to placebo. These, as well as other randomized controlled trials of hormone replacement therapy and cardiovascular disease in postmenopausal women, are reviewed. Although conclusions concerning the specific hormones used in the specific populations studied can be offered from the completed trials, more broad conclusions concerning the use of hormone replacement therapy in the prevention of cardiovascular disease will have to await conduction and completion of other trials.

Calcium mass in human aortas from autopsy

We used a single-energy x-ray method and an image processing system to measure the amount of calcium salts deposited in 60 human aortas (41 male and 19 female) of ages 15-88 y. The aortas were removed at autopsy, slit lengthwise and preserved in formalin. The aortas were x-rayed while flat, together with a calibration block containing known amounts of CaHPO4. We determined the amount of calcium salt, referenced to CaHPO4, from the x-ray film with a personal computer using a frame-grabbing board. There was less calcium in the thoracic than the abdominal aorta both in males and females and those of ages less than 50 y. The coefficient of correlation for abdominal aortic calcium mass versus age in 19 females was 0.557 and in 41 males was 0.321. The higher correlation of mass with age for females may be related to differences in calcium metabolism between sexes.

Composition of atherosclerotic plaques in coronary arteries in women less than 40 years of age with fatal coronary artery disease and implications for plaque reversibility

This study analyzes the composition of atherosclerotic plaques in the 4 major epicardial coronary arteries in 8 women less than 40 years of age (mean 34) with fatal coronary artery disease (CAD) and compares these data to previous studies of 37 adults greater than 45 years of age (mean 59) with fatal CAD. Histologic sections were taken at 5-mm intervals from the entire lengths of the right, left main, left anterior descending and left circumflex coronary arteries. With the use of a computerized morphometry system, analysis of the 4 major epicardial coronary arteries showed the major component of plaque to be a combination of cellular (mean percent total plaque area = 65%, standard error = 6%) and dense (19%, standard error = 6%) fibrous tissue. Arterial segments narrowed greater than 75% in cross-sectional area from these young women were compared with similarly narrowed arteries from 37 older patients (32 men [86%]) with fatal CAD previously reported by this laboratory, and showed significantly more cellular fibrous tissue and lipid-rich foam cells, and lesser amounts of dense fibrous and heavily calcified tissue. The large amount of lipid-containing foam cells and relative lack of acellular scar tissue in coronary plaques in these young women suggests a greater potential for reversibility of these plaques in this subset of patients with CAD.

Sequestration hypothesis of atherosclerosis

Atherosclerosis of the human aorta has been studied by morphometric, chemical, and histochemical methods. Results of these separate approaches are converging upon a theory of pathogenesis. This theory begins with the standard view of a two stage process, intimal fibroplasia followed by atheronecrosis in the most thickened and aged places. The first stage, fibroplasia, can be described in terms of a stochastic process wherein smooth muscle cells, scattered in accordance with a Poison distribution, elaborate matrix materials over time, causing the realms of the cells to expand and to aggregate. The fusion of the expanded smooth muscle cell realms seems to mark the advent of necrosis. The second stage, atheronecrosis, can be described such that the probability of a necrotic core emerging at a site in a vessel is governed by the amount and the age of interstitial matrix materials at the site. Further evidence shows that the matrix materials tend to sequester lipids in greater than proportionate amounts as the intimal bulk increases. The sequestered perifibrous lipid is histochemically different from the lipids of the necrotic core, in that only the latter can be fixed with chromic acid. These results suggest that lipids undergo a qualitative change as well as a quantitative increase at the stage of impending necrosis. This qualitative change is governed by age, which raises the possibility that necrotizing toxicity accumulates in the sequestered lipid as it ages.

Replication of smooth muscle cells in vascular disease

Smooth muscle proliferation has been recognized as central to the pathology of both major forms of vascular disease: atherosclerosis and hypertension. Recent advances in our knowledge of mechanisms of control of proliferation suggest that events occurring in adult animals may recapitulate portions of the developmental biology of the smooth muscle cell. This review attempts to consider the current state of knowledge of the mechanisms controlling smooth muscle proliferation in these two diseases, to put that knowledge into the context of what is known about smooth muscle biology, and to offer two hypotheses on the possible roles of smooth muscle developmental biology in manifestations of atherosclerosis and hypertension in adult humans.

Chelation therapy of atherosclerosis

Chelation therapy with intravenous injections of edetate disodium is being promoted to the public as a nonsurgical means to treat coronary or other arterial atherosclerosis. The rationale for use, clinical efficacy, and safety are reviewed. Acceptable evidence supporting chelation therapy for atherosclerotic vascular disease is lacking.

Endothelial cell coat modifications in rat thoracic aorta. Effect of ovariectomy and cigarette smoke

The effects of acute cigarette smoking and bilateral ovariectomy on the thickness of rat aortic cell coat (Con A) were investigated. Ovariectomized rats showed a significant increase in the thickness of the cell coat. When cigarette smoking was combined with ovariectomy the thickness of the reaction product was similar to controls. Cigarette smoke without ovariectomy resulted in a decreased thickness, but these changes were not significant.

Pathopysiological aspects of immune complex diseases. Part II. Phagocytosis, exocytosis, and pathogenic depositions

Elimination of IC by the phagocytic system occurs mainly by macrophages and contrarotates to the pathogenic effect. Decisive to prevent systemic IC disease is the capacity of the phagocytic system. In the case of its saturation, the danger of the occurrence of IC disease is greatly enhanced. Conclusive evidence seems to exist that IC of extremely small or extremely high lattice structure (precipitates) are less pathogenic than soluble IC of medium network. Small IC in extreme antigen and antibody excess or precipitates exhibit a reduced complement activating potency. Small IC in extreme antigen or antibody excess hardly interact in vitro with membrane receptors and do not induce IC disease when injected or formed in vivo. Highly lattices IC, especially precipitates, are eliminated extremely quickly from the circulation, mainly by macrophages and there deposition in the kidney is significantly reduced. Thus, lack of quality of the antibody to precipitate the antigen and a reduced capacity and effectivity of the phagocytic system to eliminate the IC may be extremely important in the generation of IC diseases. Facing the overwhelming and partly even inconsistant data of this topic, one may doubt whether IC diseases may be regarded to be a defined and coherent disease. Too many variables and questions exist concerning the nature of the antigen, especially in tumor and autoimmune diseases, concerning the quality of the antibody and the characteristics of the pathogenic IC and concerning localization and the elimination process. Nevertheless, common pathophysiological pathways of IC diseases may be recognized.

Selective phospholipid adsorption and atherosclerosis

Disaturated (fully saturated) lecithins adsorb onto solid surfaces more readily than lecithins in which one or both fatty acids are unsaturated. If saturated lecithins adsorb to arterial walls as they do to glass and polystyrene surfaces, there may be increased probability of atherosclerosis when the disaturated lecithin content of plasma is elevated. Analyses of lecithins in plasma samples from patients with myocardial infarction, and from patients with premature atherosclerosis but with low concentrations of plasma cholesterol and triglycerides, are consistent with the hypothesis that a high concentration of disaturated lecithin in plasma may be a significant risk factor for atherosclerosis, independent of triglyceride and cholesterol concentrations.

Lipoproteins versus structural glycoproteins: atherosclerosis as an analogue and competitor of normal connective tissue interactions

There is a striking resemblance, in amino-acid composition and properties, between one class of structural glycoproteins from connective tissue and the apo low density lipoproteins (LDL) of blood plasma. It is suggested that there is a structurally specific analogy between the early atherogenic accumulation of LDL (and more particularly Lp(a), in arterial connective tissue and the normal interactions of structural glycoproteins with other connective tissue components, particularly proteoglycans and collagen.

Hyperreactive arterial endothelial cells: a clue for the treatment of atherosclerosis

Arterial endothelial cells, which are capable of phagocytizing carbon particles of the same size as beta- and pre-beta-lipoprotein, were found only in endothelial cells of arterial segments susceptible to atheromatous changes in susceptible animal species, and the distribution closely corresponded to the susceptibility. The distribution of such endothelial cells is dense in large arteries, in the openings to their branches, especially in downstream portions, of rabbits, hens, and cocks; however, the distribution is relatively scanty in arteries of rhesus monkeys and is very scanty in dogs. Carbon particles were also rare in the suckling rabbit and tended to increase with age. They were not found in Wistar rats but were found in spontaneously hypertensive rats, which showed a characteristically diffuse distribution, even in relatively small arteries. The carbon particles, phagocytized, were released to the subendothelial space but were difficult to pass through the internal elastic lamina and tended to stagnate there for more than one month. The authors therefore call these cells hyperreactive endothelial cells. Various vasoactive substances, such as angiotensin II, histamine, and serotonin, significantly enhanced the phagocytic activities of arotic endothelial cells in rabbits; epinephrine and norepinephrine also slightly enhanced these activities. Various smooth muscle relaxants, such as ATP, pyridinol carbamate (ATP synthesis-enhancing substance), cycli-AMP, dibutyryl cycli-AMP, phthalazinol (cyclic-AMP phosphodiesterase inhibitor), iproveratril (calcium entry-inhibiting substance), colchicine, and vinblastine, with their different modes of action, commonly inhibited phagocytic activities, a finding that suggests a significant role for contractile protein in the permeability problem of atherogenesis. The atheromatous lesions of cholesterol-fed rabbits exhibited a striking increase in hyperreactive endothelial cells, accompanied by a marked rise in the activity of low-Km cyclic-AMP phosphodiesterase activity in atheromatous lesions and adjacent muscular layers, especially in rabbits with rapidly progressing atheroma.