Plasma lipoproteins after triglyceride clearance in cholesterol-fed rats.

The clearance of particulate triglyceride from the plasma of cholesterol-fed rats with appreciable stores of hepatic cholesterol ester produces a substantial increment in plasma cholesterol. Most of this plasma cholesterol increment arises from existing tissue sources. The increment begins from 4 to 6 h after clearance and is due to the appearance of larger cholesterol-rich, triglyceride-poor, beta migrating lipoproteins, which are isolated in the d < 1.063 fraction with an apoprotein (Apo) content consisting primarily of Apo E and smaller amounts of Apo B. A concurrent decrease in alpha lipoproteins occurs with the beta lipoprotein increment. Within 1 d of clearance the beta lipoproteins fall and alpha lipoproteins increase. The increase in total plasma Apo E and Apo B initially parallels that of the cholesterol, but it persists even when cholesterol falls. A modest decrease in plasma Apo A1 was observed during the time alpha lipoproteins declined. A significant increase in plasma lecithin cholesterol acyl transferase preceded the increase in beta lipoprotein cholesterol. This enzyme increment was absent in rats with little lipoprotein response despite increased hepatic cholesterol. In vivo inhibition of this enzyme with dithionitrobenzoic acid virtually eliminated the postclearance hypercholesterolemia. Plasma particulate triglyceride clearance induces an increase in beta lipoproteins. Coupling of this clearance and hepatic lipoprotein secretion occurs by an unknown mechanism modulated by lecithin cholesterol acyl transferase.