The effect of the linoleic acid content of the diet on the fatty acid composition of the cholesterol esters isolated from the plasma of rabbits

Dietary linoleic acid and the plasma phospholipids of the rabbit

1. Groups of rabbits were given diets containing different proportions of butterfat and maize oil. After the animals had been given the experimental diets for 40 weeks the plasma phospho- lipids were fractionated and the fatty acid composition of each fraction was determined.

2. Phosphatidyl choline and lysophosphatidyl choline accounted for about 75 and 12% respectively of the total plasma phospholipids: phosphatidyl ethanolamine, sphingomyelin and phosphatidyl serine accounted for only about 5.3, 5.0 and 2.6% respectively. Changes in the linoleic acid content of the diet had little effect on the relative proportions of the individual plasma phospholipids, but there was an over-all decrease in the concentration of total phospholipids in the plasma as the linoleic acid content of the diet was increased from 0.25 to 10.6%.

3. When the diet contained 0.25 % linoleic acid, the linoleic acid:oleic acid ratio in the phosphatidyl choline (1.3) was similar to that in the phosphatidyl ethanolamine (1.2), but as the linoleic acid content of the diet was increased to 10.6% the linoleic acid:oleic acid ratio in the phosphatidyl choline increased to 48, whereas that in the phosphatidyl ethanolamine increased only to 2.2. Increases in the linoleic acid content of the diet resulted also in increases in the linoleic acid:oleic acid ratios in the phosphatidyl serine, lysophosphatidyl choline and sp hingomyelin.

4. When the linoleic acid content of the diet was increased, the stearic acid:palmitic acid ratio in the phosphatidyl choline increased, whereas it decreased in the phosphatidyl ethanol- amine and remained relatively unaltered in the phosphatidyl serine. The stearic acid contents of the lysophosphatidyl choline and sphingomyelin were unaltered by dietary treatments, but the palmitic acid content of these two phospholipids decreased as the linoleic acid content of the diet increased.

5. The results are discussed in terms of the metabolic relationships that exist between the various phospholipids.

The effect of diets containing different proportions of butterfat and maize oil on plasma lipids and aortic atherosis in rabbits

1. Seven groups of male rabbits (thirteen to fifteen per group) were givenad lib. a diet consisting of 80 parts of a low-fat basal ration to which were added 20 parts of fat in the form of butterfat, maize oil or various mixtures of the two.

2. After 40 weeks, samples of blood were taken and the animals were killed. The concentrations of total cholesterol and phospholipid in the plasma and the fatty acid compositions of the plasma cholesteryl esters and phospholipids were determined. The degree of atheromatous degeneration was determined in the aortas.

3. Little aortic atherosis was observed in the rabbits given the diets containing 0, 10 and 12% butterfat, but as the butterfat content of the diet was increased above 12%, pronounced increases occurred in the extent of atheromatous degeneration of the aorta. The concentration of cholesterol in the plasma increased progressively as the butterfat content of the diet was increased and the maize oil content was decreased. There was an inverse curvilinear relationship between the degree of aortic atherosis and the phospholipid: cholesterol ratio in the plasma.

4. The concentration of linoleic acid in the plasma cholesteryl esters was inversely related to the concentration of cholesterol in the plasma, and the concentration of linoleic acid in the plasma phospholipids was inversely related to the concentration of phospholipids in the plasma. There was a positive linear relationship between the stearic: palmitic acid ratio and the linoleic: oleic acid ratio in the plasma phospholipids.

Comparison of the lipid compositions of the aortic lesions in rabbits given different atherogenic diets

1. The compositions of the lipids in the atheromatous lesions of rabbits have been studied in an experiment in which two groups of rabbits (twelve/group) were given different atherogenic diets for a period of 38 weeks. One group of rabbits was given a diet in which 35% of the total calories was derived from butterfat and the other group was given a diet in which 48% of the total calories was derived from starch.

2. At the end of the feeding period the rabbits were killed and the atheromatous plaques were dissected out from the aortic intima of each rabbit. The lipid compositions of the aortic lesions were determined and the results compared with the compositions of the plasma lipids.

3. In the lipids of the aortic lesions in the rabbits given the high-butterfat diet, the proportions of cholesterol and cholesterol esters were higher and the proportion of triglycerides was lower than in the lipids of the aortic lesions in the rabbits given the high-starch diet. Irrespective of dietary treatment, free cholesterol constituted a higher proportion of the lesion lipids than it did of the plasma lipids.

4. In the rabbits on either dietary treatment, the lesion cholesterol esters contained higher concentrations of oleic acid and lower concentrations of linoleic acid than did the plasma cholesterol esters. In the lesion phospholipids there were higher concentrations of palmitic, palmitoleic, stearic and oleic acids and lower concentrations of linoleic and arachidonic acids than in the plasma phospholipids.

5. In both groups of rabbits the phospholipids of the aortic lesions contained higher proportions of cephalin and sphingomyelin and lower proportions of lecithin than did the plasma phospholipids.

6. The results of this investigation are discussed in relation to the possible origins of the lipids in the aortic lesions of the experimental rabbits.

The effect of an atherogenic diet on plasma lipid composition and aortic atherosis in two strains of New Zealand White rabbit

1. Susceptibility to dietary induction of hypercholesterolaemia and aortic atherosis was compared in two groups of male New Zealand White rabbits. 2. Twelve rabbits were purchased from one breeding establishment (group I) and twelve from another (group 2). On arrival at the laboratory six animals from each group were killed and the aortas were removed. Blood samples were taken from the remaining twelve animals and then they were givenad lib.for a period of 40 weeks an atherogenic diet containing 20% butterfat. During this period food intake and body-weight were recorded. At the end of the period blood samples were taken from the animals and immediately afterwards they were killed and the aortas were removed. 3. No atheromatous lesions were found in the aortas of the rabbits in either group killed at the beginning of the experiment. There were no differences between the two groups of animals with respect to body-weight or concentration of cholesterol in the plasma at the beginning of the experiment, food intake during the experiment or body-weight at the end of the experiment. At the end of the experiment, the degree of aortic atherosis in the rabbits of group I was considerably greater than that in the rabbits of group 2. 4. At the end of the experiment the concentrations of total lipids, free cholesterol, esterified cholesterol and phospholipids in the plasma of the rabbits in group 1 were significantly higher than the corresponding concentrations of these lipid components in the plasma of the rabbits in group 2. The concentrations of palmitic, stearic and linoleic acids in the cholesterol esters and the concentration of palmitic acid in the unesterified fatty acids in the plasma of the rabbits in group 2 were significantly higher than the corresponding concentrations of these fatty acids in the plasma cholesterol esters and unesterified fatty acids in the rabbits of group 1. 5. It is concluded that these differences in response to the atherogenic diet were reflections of the differences in the susceptibilities to the dietary induction of hypercholesterolaemia and atherosis of the two differents strain of rabbit that had been established by the two commercial breeders. Such differences in susceptibility could readily explain certain discrepancies in the results of various research workers engaged in this field of investigation.

The effect of diet on the compositions of the triglycerides and unesterified fatty acids isolated from the plasma, liver and adipose tissues of rabbits

1. Groups of male rabbits (ten to thirteen per group survived) were givenad lib. a diet consisting of 80 parts of a low-fat basal diet to which were added: for group I, 20 parts maize oil; for group 2, 20 parts butterfat; for group 3, 10parts maize oil and and 10parts butter- fat; for group 4, 0.47 parts maize oil and 43.1 parts wheat starch; and for group 5, 10.2parts maize oil and 21.6 parts wheat starch. The animals in group 6 were givenan ordinary com- mercial rabbit diet. 2. The rabbits were given the various diets for 38 weeks, after which a sample of blood was taken. The rabbits were then killed and the liver and a sample of perine- phric adipose tissue were removed from each animal. The plasma, liver and adipose tissue lipids werefractionated on columns of Florisil and the fatty acid compositions of the tri- glyceride and unesterified fatty acid fractions were determined by gas-liquid chromatography.

3. The effects of the different diets on the composition of the unesterifiedfatty acids in the plasma were very similar to the effects of the diets on the fatty acid composition of the plasma triglycerides, but in the plasma unesterified fatty acids the concentration of stearic acid was consistently higher and the concentration of linoleic acid was consistently lower than in the plasma triglycerides. 4. There appeared to be a positive rectilinear relationship between the concentration of stearic acid in the plasma triglycerides and the concentration of triglycerides in the plasma. 5. In the triglycerides of the plasma, the concentrations of palmitic and stearic acids were consistently greater and the concentration of linoleic acid was consistently less than the corresponding concentrations of these fatty acids in the triglycerides of the adipose tissues. In the two groups of rabbits given low-fat diets the fatty acid composition of the liver trigly- cerides was almost identical with that of the plasma triglycerides. 6. In each of the six groups of rabbits the composition of the unesterified fatty acids in the plasma was identical with that of the unesterified fatty acids in the adipose tissues. Inthe unesterified fatty acids of the liver the concentrations of linoleic andarachidonic acids were consistently greater and the concentrations of myristic, palmitic and palmitoleic acids were consistently less than the corresponding concentrations of these acids in the unesterified fatty acids of the plasma and adipose tissues. 7. The results of this investigation are discussedin the light of recent con-cepts on the metabolic relationships between the unesterified fatty acids and triglycerides of the liver, plasma and adipose tissues.