l-Carnitine effects on chemical composition of plasma lipoproteins of rabbits fed with normal and high cholesterol diets

l-Carnitine plays an important role in the mitochondrial uptake of long-chain fatty acids in mammals. It has recently been shown that this compound has a marked hypo-cholesterolemic effect when used in conjunction with lipid-rich diets. The aim of this study was to investigate the effects of l-carnitine on the fatty acid composition of plasma lipoproteins in rabbits fed with different diets. Four different groups were investigated: group I (standard diet), group II (standard diet supplemented with l-carnitine at 80 mg/kg), group III (standard diet supplemented with 0.5% cholesterol), and group IV (standard diet supplemented with 0.5% cholesterol plus l-carnitine at 80 mg/kg). The feeding period was 126 d. Total plasma cholesterol was indistinguishable in groups I and II, but increased nearly 40-fold in group III. This increment was reduced by 50% in group IV. Correspondingly, total cholesterol content in lipoprotein fractions very low density lipoprotein (VLDL), low density lipoprotein (LDL), high density lipoprotein (HDL) separated by agarose gel chromatography was the same for groups I and II, while for animals fed a cholesterol-rich diet (III) total cholesterol in VLDL+LDL increased nearly 100-fold when compared with groups I and II but, again, the increment was reduced by 50% in group IV. In contrast, total cholesterol in HDL increased only fivefold for both groups III and IV when compared with groups I and II, indicating no effects of l-carnitine on this parameter. The reduction of total cholesterol in VLDL+LDL particles in animals fed a cholesterol-rich diet plus l-carnitine was associated with a marked decrease in the ratio of cholesteryl ester to free cholesterol and a dramatic increase in their phospholipid content; opposite effects were observed for HDL. l-Carnitine induced a marked decrease in the saturated to unsaturated C₁₆+C₁₈ fatty acid ratio in cholesteryl esters associated with VLDL and LDL from animals fed with both normal and cholesterol-rich diets. The opposite effect (a large increase in the saturated to unsaturated fatty acid ratio) was observed for both cholesteryl esters and phospholipids associated with HDL in animals fed with both diets. The results suggested that the hypocholesterolemic effects of l-carnitine could be associated with increased systemic breakdown of cholesteryl esters, a probable increase in reverse cholesterol transport, and the stabilization of a phospholipid-based structure of VLDL+LDL particles.