Effect of experimental renal hypertension on experimental thiouracil-cholesterol atherosclerosis in dogs

Coronary artery disease and arterial hypertension: clinical, angiographic and follow-up data

OBJECTIVES

To evaluate how the presence of arterial hypertension affects coronary atherosclerosis and prognosis in patients with, or at high risk of, ischaemic heart disease.

DESIGN

Retrospective analysis of clinical records and follow-up data.

SETTINGS

Single referral centre for ischaemic heart disease.

SUBJECTS

All consecutive patients (n = 1700, 38% with hypertension) undergoing coronary angiography for the evaluation of ischaemic heart disease during 1983-92.

RESULTS

On angiography, the likelihood of having three-vessel disease was higher amongst hypertensives (odds ratio = 1.41; 95% confidence interval [CI] = 1.08-1.85) after adjustment for age, sex, and angina symptoms. The sum of all visible stenoses (an index of overall atherosclerotic involvement) was 19% higher in hypertensives (262 +/- 204 vs. 220 +/- 194 units, P < 0.005). By multivariate analysis, the presence of hypertension made a modest (+ 28 units), albeit statistically significant, independent contribution to the total atherosclerosis score. On follow-up (median = 96 months), cardiovascular mortality was slightly higher in the hypertensive patients than in the normotensive group (P < 0.05 in a Kaplan-Meier analysis), but a proportional hazard analysis adjusting for age and gender showed no significant independent contribution of hypertension. Hypertensive patients, however, remained at higher risk of non-fatal myocardial infarction following discharge (adjusted odds ratio = 1.21, 95% CI = 1.03-1.46; P < 0.05).

CONCLUSIONS

In this referral population, hypertension is a risk factor for presence of three-vessel disease. Distribution, severity and extension of coronary stenosis are similar to those of normotensive patients, and prognosis is only marginally affected.

Frequency of systemic hypertension in various cardiovascular diseases

The clinical frequencies of systemic hypertension and necropsy evidence of cardiomegaly in various cardiovascular conditions are summarized. Systemic hypertension is present in greater than 50% of patients with various coronary events, in greater than 75% of patients with various cerebrovascular events and in greater than 90% of patients with aortic dissection. Hypertension is the sole underlying factor in most patients with nontraumatic cerebral arterial or aortic (dissection = partial rupture) rupture. In association with hypercholesterolemia (serum total cholesterol levels greater than 150 mg/dl), hypertension clearly accelerates atherosclerosis and its devastating consequences.

Effects of two forms of hypertension on atherosclerosis in the hyperlipidemic baboon

We examined the relationship of hypertension and plasma renin activity to atherogenesis in 48 moderately hyperlipidemic (total serum cholesterol was about 200 mg/dl) baboons (Papio sp.). We used renal artery stenosis (two-kidney, one clip model) to produce hypertension associated with elevated plasma renin activity, and used cellophane wrapping of both kidneys (bilateral perinephritis model) to produce hypertension with normal renin activity. Renal artery stenosis and bilateral perinephritis increased both systolic and diastolic blood pressure by about 30 mm Hg. Renal artery stenosis approximately doubled, but bilateral perinephritis did not change plasma renin activity. Both hypertensive groups, to about the same degree, had significantly more extensive atherosclerosis than the control group in the abdominal aorta and brachial, iliac-femoral, and carotid arteries. The effect of hypertension was greatest in the carotid arteries where the extent of atherosclerosis was nearly tripled. Hypertension did not influence lesions in the thoracic aorta. By multiple regression analysis, very low plus low density lipoprotein cholesterol, high density lipoprotein cholesterol, and systolic blood pressure were consistently strong predictive variables for the extent of atherosclerotic lesions. Most of the effects of renal hypertension on atherosclerotic lesions appeared to be accounted for by the increase in blood pressure. In the carotid arteries, however, there was a suggestion of an effect above that due to increased blood pressure. Additional analyses indicated that these treatment effects were associated with serum potassium concentration, plasma renin activity, or other closely related variables.

Regional differences in cholesterol content of aorta in response to experimental coarctation in spontaneously atherosclerosis-susceptible pigeons

The effect of experimental coarctation of the aorta at a site above the coeliac bifurcation (site of occurance of spontaneous atherosclerotic lesions) in White Carneau pigeons was examined. Cholesterol content of the aorta in the site where spontaneous atherosclerosis usually occurs (coeliac bifurcation = lesion site) was decreased in pigeons with aortic coarctation. On the other hand, in the region proximal to the site of coarctation which is usually free of atherosclerotic events cholesterol accumulation was increased. A decrease in percent composition of oleic acid (the major fatty acid which increases during atherosclerosis) was noted in the lesion site. These studies have shown regional differences in response of aorta to experimental coarctation in spontaneously atherosclerotic-susceptible pigeons and suggest that lowering of blood pressure at lesion site might decrease cholesterol accumulation and perhaps might retard subsequent atherogenic process.

Hypertension-induced cerebral atherosclerosis in the cholesterol-fed rabbit

Foam cell lesions were found in cholesterol-fed rabbits with induced hypertension, particularly in intimal cushions at branching sites, where permeability to horseradish peroxidase was enhanced. Permeability to horseradish peroxidase was enhanced at the edge of intimal cushions without foam cell accumulation. This finding suggests that permeability is increased before foam cell infiltration. No foam cell lesions were observed in the intima of cerebral arteries distant from branching sites, but insudation of plasma constituents here caused endothelial cells to separate from the subendothelial matrices. Foam cell lesions were absent from the cerebral arteries in normotensive cholesterol-fed rabbits.

Effect of hypertension on the in vitro incorporation of [1-14C]-oleate into phospholipid and cholesterol ester in the aortae of normal-fed and cholesterol-fed rabbits

The effect of prior hypertension on the in vitro incorporation of [1-14C]-oleate into phospholipid and cholesterol ester in aortae from cholesterol-fed and normal fed rabbits was studied. Incorporation of [1-14C]-oleate into phospholipid was not increased in aortae from either hypertensive normal-fed or hypertensive cholesterol-fed rabbits when compared to the appropriate normotensive controls. In the normal-fed rabbits, incorporation of [1-14C]-oleate into cholesterol ester was increased by hypertension in all aortic regions. In cholesterol-fed rabbits cholesterol esterification was found to be proportional to the intimal cholesterol concentration, irrespective of the prior blood pressure or the particular aortic region studied. It is concluded that the increased lipid synthesis in atherosclerotic vessels from hypertensive rabbits is a consequence of the increased lipid accumulation produced by hypertension and not the result of hypertension directly stimulating arterial wall metabolism.

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.

Incorporation in vitro of 14C-labelled acetate and 32P-labelled phosphate into lipid in thoracic aortae from hypertensive and nomotensive rabbits

The effect of prior hypertension on lipid synthesis in the thoracic aortae of normal-fed and cholesterol-fed rabbits was studied in vitro using[1(-14)C] acetate and [32P] phosphate as lipid precursors. In normally fed rabbits, prior hypertension did not increase the incorporation of the labelled precursors into either phospholipid or neutral lipid. In cholesterol-fed rabbits, hypertension increased the incorporation of [32P] phosphate into phosphatidyl-choline and of [1(-14)C-acetate into cholesterol ester. The increased incorporation of [1(-14)C] acetate into cholesterol ester was accompanied by an increase in intimal total cholesterol concentration. For both normotensive and hypertensive cholesterol-fed rabbits there was a close correlation between cholesterol esterification and total cholesterol concentration of the thoracic intima. It is concluded that the increase in aortic lipid synthesis in hypertensive cholesterol-fed rabbits is secondary to the increased cholesterol accumulation induced by hypertension rather than to a direct stimulation of arterial wall lipid synthesis by hypertension per se.

The effect of renal hypertension on the regional deposition of cholesterol and phospholipid in the aorta of normally- and cholesterol-fed rabbits

The effect of renal hypertension on dry defatted tissue mass and lipid accumulation in different segments of the aortic intima was studied in both normally-fed and cholesterol-fed rabbits. In normally-fed rabbits hypertension caused an increase in intimal dry weight in the aorta. The increase was greatest in the lower thoracic intimal segment but was not significant in the aortic arch. The increase in tissue mass was not influenced by the addition of cholesterol to the diet and no regression of the increased tissue mass occurred when a 4-week period of hypertension was followed by a 4-week period of normotension. Hypertension did not increase the intimal cholesterol or phospholipid concentrations in normally-fed rabbits, suggesting that an observed increase in lipid content represented the cellular component of the intimal hypertrophy. Hypertension in cholesterol-fed animals caused preferential lipid accumulation in the lower thoracic segment, an effect that was independent of the total intimal cholesterol level. Intimal cholesterol, cholesterol ester and phospholipid were all increased. When a 4-week period of normotension and cholesterol feeding was preceded by a 4-week period of hypertension with normal feeding the amount of cholesterol deposited did not exceed that of the normotensive control, suggesting either that hypertension increased intimal permeability to lipid only in the presence of hypercholesterolaemia, or that healing of damaged intima had occurred before hypercholesterolaemia was fully established.

The hypertensive diseases. Evidence that systemic hypertension is a greater risk factor to the development of other cardiovascular diseases than previously suspected

This report summarizes the clinical frequencies of systemic hypertension and necropsy evidence of cardiomegaly in various cardiovascular conditions (Table I), termed "the hypertensive diseases" because of their frequent association with systemic hypertension. Although long recognized as a major risk factor, systemic hypertension appears to be an even greater risk factor to the development of various cardiovascular conditions than previously appreciated. Hypertension by itself appears to be the sole underlying factor in most cases of nontraumatic cerebral arterial or aortic (dissection = partial rupture) rupture. In association with hyperlipidemia, hypertension clearly accelerates atherosclerosis and its devastating consequences.

George Lyman Duff Memorial Lecture. Atherosclerosis complicating chronic hypertension

The accelerating effect of hypertension on atherogenesis in the aorta and iliofemoral and coronary arteries is presented here as a graded function of elevated intraarterial pressure. Predilection of the abdominal aorta and iliofemoral arteries for atherosclerosis can be explained on the basis of pressure augmentation by reflected pressure pulse waves and the increase in hydrostatic pressure that is produced by standing. Wave reflection is intensified by vasoconstriction, which is a characteristic not only of hypertension but also occurs with upright posture. Standing also raises the hydrostatic pressure of blood in vessels below heart level. Coronary atherosclerosis seems likely to relate to the fact that the coronary vessels are located in an area of high pressure in the arterial system and that their intramyocardial branches are normally completely occluded during systole — a condition which could be accompanied by subtle differences in the pressure-volume characteristics of the epicardial vessels.

Evidence is presented from actuarial statistics, epidemiologic studies, and laboratory experiments to indicate that the level of arterial pressure has a measurable effect on the metabolism of arterial smooth muscle cells. Thus arterial pressure is viewed as having a metabolic determinant role in addition to its vital role in the circulation of blood.

BLOOD PRESSURE, CHOLESTEROL CONTENT OF SERUM AND TISSUES, AND ATHEROGENESIS IN THE RAT

Rats on a stock diet with added cholesterol, cholic acid, and thiouracil developed increased concentrations of cholesterol, total lipide, and beta lipoprotein in the serum, and an increased content of cholesterol in the liver and carcass, despite the fact that the diet produced a cessation of endogenous cholesterol synthesis. Rats with high serum lipide concentrations developed intimal lesions similar to those of human atherosclerosis. The induction of hypertension by desoxycorticosterone and salt accelerated the development of hypercholesterolemia, hyperlipemia, increase in tissue cholesterol content, and atherosclerotic changes in the intima. Hypertension induced by renal artery constriction also intensified the hypercholesterolemia and hyperlipemia. On the other hand, rats receiving desoxycorticosterone acetate without salt or salt without desoxycorticosterone acetate did not show any intensification of hypercholesterolemia or hyperlipemia. The extent of the atherosclerotic lesions was correlated with the concentration of cholesterol in the serum. There was also a positive correlation between blood pressure and the degree of hypercholesterolemia. It remained uncertain whether the increase in atherosclerosis in the hypertensive animals was dependent on the increased lipide content of serum and tissues or on a local effect of the elevated blood pressure.