Characterization of the sinusoidal transport process responsible for uptake of chylomicrons by the liver

Cholesterol synthesis in many tissues is regulated by uptake of specific lipoproteins. These studies were undertaken to characterize the transport process in the liver whereby cholesterol of intestinal origin carried in the chylomicron is taken up by the liver and suppresses the rate of hepatic cholesterol synthesis. Chylomicrons from donor rats fed radiolabeled cholesterol and/or fatty acid were obtained by cannulation of the intestinal lymphatic duct. In some experiments remnants of chylomicrons were prepared by the intravenous administration of the intestinal lipoproteins into functionally hepatectomized, eviscerated animals and then reisolating the particles 3 h later. After fractionation of the chylomicrons and remnants by preparative centrifugation, the lipoproteins of various sizes were suspended in Krebs-Ringer bicarbonate buffer and perfused through isolated rat livers at physiological flow rates using a nonrecirculating system. Initial experiments demonstrated that under these perfusion conditions the liver maintained normal metabolic activity with respect to rates of gluconeogenesis, CO₂ production, and ketone and cholesterol synthesis. The hepatic uptake of both the cholesterol and fatty acid moieties in the various lipoprotein particles was a linear function of the time of perfusion from 1 to 15 min. Furthermore, relative uptake rates of these two components were directly proportional to their relative concentrations in the lipoprotein particles. Unmetabolized chylomicrons were taken up at very low rates regardless of whether they were small [S(f)< 400] or large [S(f)> 400]. In contrast, remnants of chylomicrons were taken up by the perfused liver at much higher velocities, and this uptake process manifested saturation kinetics with an apparent K(m) of 1.9 mg/dl^-1 and a V(max) value of 0.035 mg/g^-1/min^-1 (expressed as cholesterol concentration in the perfusate and rate of cholesterol uptake per g of liver, respectively). Reduction in the temperature of the perfused liver by 10° reduced the rate of remnant uptake 4.5-fold, indicating an activation energy for the transport process of 27 kcal/mol^-1. Taken together, these findings indicate that the sinusoidal membrane of the liver possesses a finite number of transport sites capable of the selective and rapid uptake of the intact chylomicron remnant by a process presumably requiring considerable membrane deformation.