Effects of palm-kernel oil and sunflower-seed oil on serum lipids and atherogenesis in alloxan-diabetic rabbits

Hyperglycemia in normotriglyceridemic, hypercholesterolemic insulin-treated diabetic rabbits does not accelerate atherogenesis

Severely hyperlipidemic alloxan-diabetic cholesterol-fed rabbits were treated with different daily doses of insulin in order to study the effect of insulin on plasma lipids, lipoproteins and postheparin lipoprotein lipase activity. At plasma triglyceride levels of 15,000 mg/dl, untreated diabetic rabbits carried 73% (1950 mg/dl) of plasma total cholesterol in lipoproteins with a diameter larger than 75 nm (Sf greater than 400), 25% in smaller very low density lipoproteins (VLDL) and 1% in both low and high density lipoproteins (LDL, HDL). Insulin treatment greatly reduced plasma total cholesterol and triglyceride concentrations. The decrease of plasma total cholesterol concentration was paralleled by a decrease in the cholesterol of the largest lipoproteins (Sf greater than 400) and an increase in cholesterol of both smaller very low density lipoproteins and low density lipoproteins. At the same time, postheparin plasma lipoprotein lipase activity increased 2-8-fold. When plasma triglyceride levels were normalized by insulin treatment, the lipoprotein cholesterol distribution in diabetic cholesterol-fed rabbits was similar to that of normal cholesterol-fed rabbits. To study development of atherosclerosis, diabetic rabbits were cholesterol-fed and treated with insulin for eight weeks such that the triglyceride levels were normalized, but plasma glucose levels were still greatly elevated. Nondiabetic rabbits were cholesterol-fed simultaneously. Plasma cholesterol and triglyceride levels were similar in the two groups of rabbits, as well as cholesterol in Sf greater than 400 or smaller VLDL and cholesterol in HDL. However, LDL-cholesterol concentration in the insulin-treated diabetic rabbits was 1.5-2 times that in the nondiabetic rabbits. The two groups of rabbits developed similar degrees of atherosclerosis, as judged by aortic cholesterol content. Apparently, partially controlled diabetes in cholesterol-fed rabbits does not accelerate atherogenesis beyond that observed in nondiabetic cholesterol-fed rabbits.

Reduced atherogenesis in cholesterol-fed diabetic rabbits. Giant lipoproteins do not enter the arterial wall

In cholesterol-fed rabbits, alloxan-diabetes has an anti-atherogenic effect, which is associated with severe elevation of plasma triglyceride concentrations. To study this effect, we measured lipoprotein sizes and aortic permeability coefficients for cholesteryl ester and for albumin in hypertriglyceridemic diabetic cholesterol-fed rabbits and in normotriglyceridemic cholesterol-fed rabbits. With the same high cholesterol concentration in plasma, hypertriglyceridemic diabetic rabbits had 70% of plasma cholesterol in very large lipoproteins (diameter greater than 75 nm), whereas normotriglyceridemic rabbits had only about 10% of plasma cholesterol in these giant lipoproteins. The aortic permeability coefficients for cholesteryl ester in hypertriglyceridemic diabetic cholesterol-fed rabbits was only 10% to 50% of that in normotriglyceridemic cholesterol-fed rabbits. Aortic permeability coefficients for albumin did not differ significantly between the hypertriglyceridemic and normotriglyceridemic rabbits. The results suggest that the large size of a major fraction of plasma lipoproteins in the hypertriglyceridemic diabetic cholesterol-fed rabbits is responsible for the relatively low aortic permeability coefficient for cholesteryl ester from plasma and hence for reduced atherogenesis in these animals.

Evidence for an inverse relation between plasma triglyceride and aortic cholesterol in the coconut oil/cholesterol-fed rabbit

Rabbits fed a commercial chow diet containing 0.5% cholesterol and 14% coconut oil developed more severe hyperlipidemia and atherosclerosis than rabbits fed the same diet containing olive oil in place of coconut oil. Average plasma cholesterol was twice as high in the coconut oil/cholesterol-fed rabbits than in olive oil/cholesterol-fed rabbits. Final plasma triglycerides, although highly variable, were approx. 20-fold higher than basal plasma triglyceride in coconut oil/cholesterol-fed rabbits; plasma triglyceride in olive oil/cholesterol-fed rabbits remained unchanged throughout the study period. In coconut oil/cholesterol-fed rabbits, a direct relationship between plasma triglyceride and aortic cholesterol was not found. Plasma cholesterol and aortic cholesterol were also not correlated at a statistically significant level (r = 0.26, P greater than 0.25). However, when both plasma cholesterol and triglyceride were simultaneously introduced as predictors of aortic cholesterol, the correlation between these plasma lipids and aortic cholesterol became highly significant (r = 0.64, P less than 0.02). Aortic cholesterol increased in proportion to plasma cholesterol concentrations but appeared to be inversely related to plasma triglyceride levels.

Pathology of macrovascular disease

In this chapter we have presented information on the development of large vessel damage in diabetes mellitus. A series of changes occur independent of the presence of atherosclerosis. The abnormalities include accumulation of PAS-positive material, laminin, fibronectin, type IV collagen and connective tissue with lack of acid mucopolysaccharides, and deposition of calcium. It is of particular interest that accumulation of PAS-positive material and lack of acid mucopolysaccharides are recognized as the histological markers of diabetic microangiopathy. These changes are in agreement with the hypothesis of a non-atherosclerotic large vessel damage, that is, diabetic macroangiopathy. From this standpoint the working hypothesis of a specific diabetic macroangiopathy should generate new ways to study the mechanism of the large vessel disease of diabetes over and above the traditional concept of classical atherosclerosis.