Plasma lipids and lipoproteins in the rat during fasting. Absence of regulation of high density lipoprotein catabolism

Abstract

The plasma lipid and lipoprotein composition of rats was followed over a fasting period of 4 days duration. Plasma cholesterol concentration remained stable over the 4 days of fast and was predominantly located in HDL particles, which were enriched in apoE. While plasma apoE concentration was more than doubled, apoAI was decreased by 38% and apoAIV by 76%. The Triglyceride Rich Fraction (TRF) and HDL levels were significantly decreased by 81% and 25% respectively after 4 days, whereas LDL level was not affected. The TRF and LDL particles were enriched with cholesterol, which accounted for the stability of the plasma cholesterol concentration despite the decrease of HDL plasma level. As HDL is the main lipoprotein fraction in the rat, we also examined the effect of fasting on apoAI and HDL catabolism. In vivo half-time decay of 125I-apoAI labeled rat HDL was similar in fed and fasted animals. In vitro specific binding of rat apoE-free 125I-HDL to both liver and kidney plasma membranes were also unaffected by fasting. Our results demonstrate that, in the fasting rat, apoAI-HDL catabolism is similar to that of control animals. However, both the concentration and the composition of HDL are affected by fasting, suggesting that fasting exerts its effect on HDL primarily at the level of apolipoprotein production rather than on degradation.