An appraisal of cholesterol feeding in experimental atherogenesis

Koch's postulates and experimental atherosclerosis

Koch's postulates contributed substantially to the evolution of scientific knowledge of infectious diseases, but are inapplicable to chronic non-infectious, degenerative diseases such as atherosclerosis. In experimental atherogenesis compliance with appropriately modified postulates is essential to preclude spurious causes from consideration. The crux of such postulates is that the experimental procedure must reproduce the disease and its complications and their pathogenesis and experimental conditions must be analogous to those prevailing in man. Since atherosclerosis is not species specific to man and consists of multiple lesions, which develop independently of one another but can ultimately coalesce, reproduction of the disease in a localized segment of a blood vessel in susceptible animals under conditions similar to those prevailing in man would comply with the spirit of Koch's postulates.

Arterial lipid biochemistry in the spontaneously hyperlipidemic Zucker rat and its similarity to early atherogenesis

In the present studies, arterial lipid metabolism was evaluated in the spontaneously hyperlipidemic obese Zucker rat (fa/fa), the lean Zucker rat (Fa/-), and the Sprague-Dawley (SD) rat. Mean serum cholesterol levels in the obese Zucker, lean Zucker and SD rats were 216 +/- 18 mg/dl, 145 +/- 14 mg/dl and 84 +/- 5 mg/dl, respectively. Arterial cholesterol content was in the same rank order as plasma cholesterol and ranged from a mean of 2.23 +/- 0.10 mg/gm wet wt. in the obese rats to 1.36 +/- 0.04 mg/gm wet wt. in the SD rats. The increased arterial sterol in the obese rats was associated with increased lipid metabolism activity. The in vitro incorporation of [14C]oleate into arterial cholesteryl esters was increased 3-4-fold (P less than 0.01) and incorporation into phospholipids and triglycerides was also elevated (P less than 0.001 and P less than 0.01, respectively). The arterial sterol content and arterial lipid metabolism pattern observed in obese Zucker rat aortas are similar to those found in vessels of other species undergoing atherogenic change.

Atherosclerosis in the aorta of hypercholesterolemic rabbits and the influence of daltroban

These studies have examined aortic atherogenesis in cholesterol-fed rabbits and have correlated the effects of daltroban to the pathomechanism of the vessel wall lesions. After feeding a 0.5% cholesterol-enriched diet for 96 d atherosclerotic alterations were seen, which exhibited a proximo-distal pattern, to which the branching of the aorta contributed considerably. Depending on their localization and size a varying cellular constitution of the plaques was obvious. Large plaques, which were mainly seen in the aortic arch and the proximal descending thoracic aorta, consisted of numerous proliferating cells, masses of fibrillar ground substance, clusters of foam cells, and rarely contained cholesterol crystals and necroses. Emerging plaques mainly found in distal thoracic and abdominal aorta imposed as fatty streaks. Daltroban treatment, used in a clinically relevant doses of 10 mg/kg b. wt. per day, reduced extension and protrusional area of plaques to about 40%, which was evaluated using a newly developed computerized morphormetric method, in association with significant reductions in free cholesterol content within the aorta. The results suggest that daltroban inhibits the progression of atherosclerosis in cholesterol-fed rabbits. This effect may be related to its antagonistic interaction with the thromboxane A2 receptor and also to an inhibition of the cholesterol metabolism.

Imprecision of the clinical diagnosis of coronary heart disease in epidemiological studies and atherogenesis

Coronary heart disease (CHD) is an imprecise diagnosis associated with a substantial diagnostic error and provides an erroneous assessment of the morbidity and mortality of myocardial ischemia and of severe coronary atherosclerosis. Due to the inability to measure the severity of atherosclerosis during life, epidemiologists frequently use the incidence of CHD as a surrogate monitor of severe atherosclerosis. It is therefore important in clinical epidemiological studies to appreciate the degree of inaccuracy of CHD statistics, which detracts from the value of such studies and their reliability.

Human equivalent doses of platelet inhibitors and experimental atherosclerosis

The effects of aspirin, dipyridamole and sulfinpyrazone on the development of atherosclerotic lesions in the aorta were studied in cholesterol-fed rabbits. Aspirin and sulfinpyrazone prevented the development of atherosclerosis, whereas dipyridamole-treated animals developed advanced atherosclerotic lesions. As all the three drugs inhibit platelet function, some non-platelet effects are probably responsible for the differences in results. It is felt that dipyridamole should be avoided in hypercholesterolemic individuals.

Basic precepts and the lipid hypothesis of atherogenesis

In approaching circumstantial evidence provided by epidemiology in support of the lipid hypothesis of atherogenesis, basic precepts include accuracy in terminology and in defining the end point to be measured. Cause must be used as the sole prerequisite without which the disease does not occur. Risk factors are not causes unless proven by other means and coronary heart disease (CHD) is an inappropriate monitor of the severity of atherosclerosis. Misuse of cause, risk factor and CHD had lead to the multifactorial concept of the etiology of atherosclerosis.

The epidemiological relationship of hypercholesterolemia, hypertension, diabetes mellitus and obesity to coronary heart disease and atherogenesis

Coronary heart disease (CHD) is an imprecise, inappropriate monitor of atherosclerosis severity and by inapplicable extrapolation CHD risk factors are incorrectly assumed to be causes of atherosclerosis. Taking into account (1) the misuse and substantial diagnostic error of CHD, (2) errors in determining the prevalence of risk factors, (3) the use of a young non-representative minority of sufferers of CHD, (4) bias posed by inclusion of familial hypercholesterolemia (FH) in clinical studies and (5) mutual inter-relationships, genetic influence and age dependence of hypercholesterolemia, hypertension, diabetes mellitus and body mass or obesity, it is unlikely that multivariate statistical analyses can adequately differentiate between their effects. These factors are age dependent and so are CHD and atherosclerosis. The importance of hypercholesterolemia in atherogenesis is suspect particularly since the vascular pathology of familial hypercholesterolemia and of cholesterol-fed animals has been misrepresented and does not provide support for the role of hypercholesterolemia in atherogenesis.

Decreased renal preglomerular microvascular PGI2 and PGE2 biosynthesis in cholesterol-fed rabbits

Although atherosclerosis is a common disease afflicting the kidney, few studies have examined the biochemical disturbances associated with this process directly in the renal microvasculature. Using a technique developed in this laboratory to isolate renal preglomerular microvessels, we have examined whether these vascular segments have a reduced capacity to synthesize vasodilator prostanoids (i.e., PGI2 and PGE2), an observation which has been made in atherosclerotic blood vessels from other vascular beds. The synthesis of these two prostanoids was assessed by radioimmunoassay in a group of white New Zealand rabbits fed a 2% cholesterol diet for 30 days and the results compared to that obtained in a similar group of rabbits fed a normal diet. The results of these studies showed the development of hypercholesterolemia in the cholesterol-fed rabbits which was associated with a 35% decrease in the biosynthesis of PGI2 and a 51% decrease in PGE2 in freshly isolated renal preglomerular microvessels. These findings are similar to those observed in non-renal major blood vessels and suggest that common biochemical mechanisms may operate in the development of atherosclerosis in many organ systems.

Effect of lanthanum chloride on established atherosclerosis in the cholesterol-fed rabbit. Mitral valve as a site for assessment of treatment effects

The ability of lanthanum chloride (LaCl3) to retard the progression of established atherosclerosis was investigated in cholesterol-fed rabbits. Rabbits were initially maintained on a high-fat plus cholesterol-supplemented diet for 10 weeks to induce lesions and were then changed to a low-fat diet or a low-fat diet supplemented with LaCl3 for an additional 24 weeks to permit their serum cholesterol levels to normalize. LaCl3 did not affect the rate at which serum cholesterol levels returned to normal. The dose of LaCl3 was approximately 30 mg/kg body weight/day. In comparison with controls, LaCl3-treated rabbits exhibited histologically less severe coronary artery and mitral valve atherosclerosis. Lesion severity in the carotid arteries was unaffected by LaCl3 treatment. Although statistically significant, the salutary effects of LaCl3 were relatively small. The data support the hypothesis that calcium antagonists can retard the progression of established atherosclerotic lesions. The data also illustrate the value of the mitral valve as a site to assess treatment effects on monocyte/macrophages in vivo.

Cytotoxicity of cholesterol oxidase to cells of hypercholesterolemic guinea pigs

1. A high cholesterol diet caused guinea pig erythrocytes to become sensitive to lysis by cholesterol oxidase (CO), a protein not hemolytic to normal cells. 2. Lysis was associated with conversion of membrane cholesterol to its oxidation product (delta-4-cholesten-3-one). 3. Intravenous injection of CO to hypercholesterolemic guinea pigs produced a reduction in serum cholesterol, but was not lethal as it was in rabbits. 4. Homogenized spleen, liver and kidney from the hyperlipidemic animals were sensitive to in vitro cholesterol oxidation while tissues from non-lipemic animals were resistant to modification.

Functional and histological changes in mesenteric arteries and aortas from monkeys fed a high cholesterol diet

Treatment of Japanese monkeys for 8 months with a high fat, high cholesterol diet produced atherosclerotic lesions in the aorta and mesenteric arteries, such as fatty dots, streaks and plaques, intimal thickening with accumulation of spindle-shaped cells and macrophages and endothelial cell flattening. Contractile responses of mesenteric arteries from control and atherosclerotic monkeys to electrical stimulation of adrenergic nerves, norepinephrine and angiotensin II did not differ, whereas contractions caused by serotonin in the atherosclerotic monkey arteries were significantly greater. Ketanserin and cinanserin suppressed the serotonin-induced contraction. Relaxations caused by adenosine and K+ (5 mM) were moderately attenuated in atherosclerotic monkey mesenteric arteries, and those by acetylcholine were reduced only slightly or not affected in the arteries or aortas. Relaxations of control and atherosclerotic arteries in response to nitroglycerin, isoproterenol and prostaglandin I2 did not differ. The relaxant response to K+ was reversed to a contraction by ouabain. Acetylcholine-induced relaxations were dependent on the endothelium and suppressed by atropine. Diet-induced atherosclerosis appears to potentiate contractions mediated via serotonergic 5-HT2 receptors and to attenuate relaxations possibly caused by activation of the electrogenic Na+ pump in the smooth muscle cell membrane. Endothelium-dependent relaxations via muscarinic receptors would not evidently be affected in mesenteric arteries and aortas from atherosclerotic Japanese monkeys.

Effect of metoprolol on diet-induced atherosclerosis in rabbits

The effect of metoprolol, a beta 1-blocker, on atherogenesis was evaluated in rabbits fed a diet supplemented with 0.25% cholesterol and 3% coconut oil for 21 weeks. After 7 weeks on the diet, the rabbits were randomly divided into treated (n = 22) and untreated (n = 22) groups. Treated animals received metoprolol subcutaneously by an osmotic pump for 14 weeks, resulting in a plasma level of 774 +/- 69 nM during the investigation. Plasma concentrations of cholesterol, triglycerides, and phospholipids did not differ between the two groups. Nor were there any significant differences between the two groups in plasma concentrations of apolipoprotein A-I, apolipoprotein B, apolipoprotein C-III, and apolipoprotein E measured by electroimmunoassay. At the end of the study, the aortas were cut into three portions and the extent of atherosclerosis was determined by morphometry. The group that had received metoprolol had significantly (p less than 0.015) less atherosclerosis in the aorta (ascending plus arch 37.8 +/- 6.8%, thoracic 32.9 +/- 6.1%, abdominal 19.8 +/- 6.1% of total intimal area; mean +/- SEM) than the controls (ascending plus arch 54.9 +/- 7.1%, thoracic 48.0 +/- 6.2%, abdominal 25.9 +/- 5.5%).

Toxicity of enzymically-oxidized low-density lipoprotein

Intravenous injection of cholesterol oxidase into hyperlipidemic rabbits in which aortic atheromatous lesions have been induced by dietary means is lethal within hours, whereas injection of the same enzyme into normal rabbits has no visible adverse effect. The lethal effect of the enzyme is explicable by the finding that injection of cholesterol-oxidase treated low-density lipoprotein kills normal rabbits, in contrast to untreated low-density lipoprotein which does not. Enzymically oxidized low-density lipoprotein was also found to be cytotoxic for two human cell lines and for cultured bovine aortic endothelial cells. We suggest that in vivo enzymic conversion of low-density lipoprotein cholesterol to low-density lipoprotein cholestenone may possibly play a role in the initiation of atheromatous lesions in humans.