Experimental Atherosclerosis

GENETIC ASPECTS OF CARDIOVASCULAR DISEASES IN ANIMALS

When the incidences and types of cardiovascular disease present in various species, breeds, and strains, or families of animals are compared, certain differences are apparent. It is often difficult, however, to separate hereditary from environmental influences.

Studies of vascular disease in zoo animals have shown that changing environmental conditions can alter the incidence of certain types of lesions in various species. Species differences, however, in resistance to diet-induced atherosclerosis appear to be genetically determined. The prevalence and types of congenital cardiac malformations appear to differ from species to species, but further systematic study is required. Arterial blood pressure is higher in the giraffe and turkey than in other species, and normal variants in cardiac rhythm are characteristic of the dog, horse, and mole.

Relatively high incidences of specific cardiovascular diseases are found in certain breeds of animals. The White Carneaux, Autosexing King, and Silver King breeds of pigeons have a high incidence of atherosclerosis. Congenital heart disease appears to be more common in purebred than in mongrel dogs, and an unusual aggregation of cases of subaortic stenosis in the Boxer and German Shepherd breeds has been found. In a survey of heart disease in dogs, the prevalence of chronic congestive heart failure in the male Cocker Spaniel greatly exceeded that in the male and female of all other breeds. Arterial blood pressure is higher in Broad Breasted Bronze turkeys than in the Jersey Buff breed. This is associated in the former breed with a relatively high incidence of spontaneous aortic rupture.

The occurrence of cardiovascular disease is unusually high in certain families and strains of animals. Among swine, litter and strain differences in serum cholesterol levels and in susceptibility to atherogenic diets occur. The White Carneaux breed of pigeons is actually a highly inbred strain with a remarkable predisposition to the development of atherosclerosis. Strains of chickens and rats with high incidences of interventricular septal defects have been developed by selective breeding. The familial occurrence of congenital heart disease in dogs and swine has been observed, and an inherited vascular anomaly in cattle has been described. Through selective breeding of laboratory rodents, strains with various types of cardiomyopathies have been developed. Certain diseases thought to be similar to the heritable disorders of connective tissue in man have been identified in domestic species. The level of arterial blood pressure is a heritable characteristic, and strains of rabbits, rats, and chickens with relative hypertension have been produced by selective breeding.

Many of these observations indicate the importance of inheritance in determining susceptibility to various types of acquired cardiovascular disease. Genetic factors appear to operate in determining the occurrence of certain congenital malformations. Breeding experiments provide the most convincing evidence of genetic influence on the development of specific cardiovascular lesions. This experimental approach holds the greatest promise for furthering knowledge and understanding of the role of inheritance in the etiology of disease of the heart and blood vessels.

Atherosclerosis in rabbits identified as high and low responders to an atherogenic diet and the effect of treatment with a beta 1-blocker

In order to investigate whether initial plasma lipid concentrations could be used to distinguish between high and low responders to an atherogenic diet, rabbits were divided into 3 groups according to their plasma concentrations of cholesterol and phospholipids after 4 weeks on a standard rabbit diet. Plasma cholesterol and phospholipid levels were less than 0.5 mM, less than 1.1 mM, respectively, in group 1 (n = 17), greater than 0.5 mM, less than 1.1 mM, in group 2 (n = 13), and greater than 0.5 mM, greater than or equal to 1.1 mM, in group 3 (n = 14). After 7 weeks on a diet containing 0.25% cholesterol and 3% coconut oil, animals in groups 1 and 2 had a lower increase in their plasma lipid levels compared with group 3. Half of each group was then treated with the beta 1-adrenoceptor antagonist metoprolol during the next 14 weeks on the atherogenic diet. At the end of the study, the extent of atherosclerosis both in the aortas and in the coronary arteries of the control animals showed a positive correlation to plasma cholesterol and to plasma phospholipid concentrations integrated over time. The metoprolol-treated animals in groups 1 and 2 had a reduction of atherosclerosis compared with their respective controls. We conclude that subpopulations of rabbits that react differently on an atherogenic diet can be identified by their initial plasma lipid levels, and that metoprolol treatment of low responders to an atherogenic diet significantly reduces atherosclerotic lesions of the aorta.

Quantification of plasma lipids and apolipoproteins in British Halflop rabbits. A comparison between normocholesterolemic rabbits, hypercholesterolemic rabbits (modified WHHL rabbits) and rabbits fed an atherogenic diet

We have established isolation methods and developed electroimmunoassays for rabbit apolipoprotein A-I (apo A-I), apo B, apo C-III and apo E. The assays were used to characterize a hyperlipidemic strain of the British Halflop rabbits (BHL rabbits), obtained after cross-breeding with WHHL rabbits and referred to as modified WHHL rabbits, and to investigate the changes in the apolipoprotein levels induced by feeding normal BHL rabbits an atherogenic diet (0.25% cholesterol and 3% coconut oil). The modified WHHL rabbits were characterized by increased levels of apo B, apo C-III and apo E as well as cholesterol, phospholipids and triacylglycerol as compared to chow-fed BHL rabbits, while the apo A-I levels were only half of those found in the chow-fed animals. The modified WHHL rabbits had virtually no low density lipoprotein (LDL) receptor activity and a low fractional catabolic rate (FCR) of LDL. These results indicate that the modified WHHL rabbit has the homozygous form of the LDL receptor deficiency. The BHL rabbits fed the atherogenic diet showed increased levels of cholesterol, triacylglycerol, apo B, apo C-III and apo E, as compared to those of the chow-fed BHL rabbits. The apo E and apo C-III reached levels in the range of or even higher than those of the modified WHHL rabbits. The apo A-I levels on the other hand did not differ from those of the chow-fed rabbits. Feeding an atherogenic diet led to a decrease in the FCR of LDL to a level similar to that found in the modified WHHL rabbits.

Effect of dietary fats and triiodothyronine administration on the lipid components of serum and tissues in the rat

Twelve groups of rats were included in this study. Six groups (3 males and 3 females) received high fat diets containing 20% of either peanut oil, maize oil or lard. The other six groups received the same diets plus triiodothyronine. Triiodothyronine T3 added to diets containing peanut oil caused slight increase in total serum lipids in female rats, while in male rats serum total lipids were increased about 20-fold. T3 administration caused almost no change in aortic total and all lipid fractions in male rats kept on diets containing any of the fats used. Female rats kept on lard plus T3 showed a rise in cholesterol free and esterified and phospholipid contents of aorta lipids.

Stimulation of lipid absorption in young rats by cholesterol: early time changes and effects on pentobarbital sleeping time

Four groups of young male and female rats were fed a chow diet (O), chow plus 10% corn oil (F), chow plus 1% cholesterol (C), or chow plus 1% cholesterol plus 10% corn oil (CF) for 1, 2, 4 and 8 days. After 2 dats, male F, C and CF rats exhibited a shorter anesthesia period (-20 to -30%) when given pentobarbital. By 4 days, male F and C rats had pentobarbital sleeping times (PB-ST) 20% less than O rats. These effects were additive and CF rats had 40% shorter PB-ST. Reduction of PB-ST by cholesterol and corn oil was similar but slightly less in female rats. Liver lipid content doubled in 4 days in CF rats, and liver cholesterol was 4 times than of O rats. These changes and the increases in metabolism of barbiturate suggested changes in liver microsomal enzyme activities. Serum glutamic oxaloacetic and glutamic pyruvic transaminase, two enzymes reflective of liver damage, did not increase after 8 days on C, F or CF diets. Our results suggest that consumption of an animal sterol and a high lipid diet by laboratory rats, normally consuming a diet low in fat (3-4%), increase the ability of the animal to detoxify a barbiturate. Storage of absorbed dietary cholesterol in the liver may represent a major mechanism for maintaining extra hepatic cholesterol homeostasis.

[Drug treatment of primary hyperlipoproteinemia (author's transl)]

Cardiovascular disease has become the major cause of death in the Western countries. There is strong evidence that elevations of serum lipids contribute to the pathogenesis of premature atherosclerosis. The classification of the hyperlipoproteinemias has been most beneficial as a guide to development of dietary and pharmacological regimens for lowering serum lipid concentrations. The results of dietary and drug prevention trials are discussed. Insight into the mechanisms involved in lipoprotein metabolism as well as the mode of action and of side-effects of hypolipidemic drugs is reviewed. Using present knowledge of heart disease research, it is reasonable to suggest dietary and drug treatments for the high risk patient.