Catabolism of low density lipoproteins by perfused rabbit livers: Cholestyramine promotes receptor-dependent hepatic catabolism of low density lipoproteins

Rabbits fed a wheat starch/casein diet develop a marked hypercholesterolemia accompanied by a decrease in the number of EDTA-sensitive binding sites on plasma membrane fractions of the liver for low density lipoproteins (LDL) and β-migrating very low density lipoproteins [Chao, Y.-S., Yamin, T.-T. & Alberts, A. W. (1982) J. Biol. Chem., in press]. Inclusion of 1% cholestyramine resin in this diet prevents the increase in plasma cholesterol, increases the removal of LDL from plasma, and increases the number of hepatic plasma membrane LDL-binding sites. To determine the functional role of hepatic LDL-binding sites in the catabolism of LDL, we studied the catabolism of ¹²⁵I-labeled LDL (¹²⁵I-LDL) by in situ perfused rabbit livers in a recirculating system. The rate of catabolism was measured from the increment of nonprotein-bound radioiodine in the perfusate. The receptor-dependent catabolism of LDL by the liver was calculated from the difference of hepatic catabolism of ¹²⁵I-LDL and catabolism of ¹²⁵I-labeled cyclohexanedione-modified LDL, which does not bind to LDL receptors. The data show that about 74% of LDL catabolized by perfused livers from chow-fed rabbits is through the receptor-dependent pathway and 26% is through the receptor-independent pathway. In rabbits fed a cholesterol diet, the hepatic catabolism of ¹²⁵I-LDL is reduced, and the receptor-dependent catabolism of ¹²⁵I-LDL is abolished. In rabbits fed the wheat starch/casein diet, the receptor-dependent catabolism of ¹²⁵I-LDL is reduced by 40% when compared with hepatic catabolism in chow-fed rabbits. Perfused livers from rabbits fed the wheat starch/casein diet supplemented with 1% cholestyramine show a 5,4-fold increase of receptor-dependent catabolism of ¹²⁵I-LDL when compared with that of livers from rabbits fed the wheat starch/casein diet alone. Thus, these studies demonstrate that the change in the number of rabbit hepatic membrane LDL receptors induced by dietary manipulation and drugs is correlated to the functional rate of removal of LDL by the liver.