Liver uptake of amino acids and carbohydrates during a single circulatory passage

The uptake of 14C-labeled amino acids and carbohydrates by liver following rapid injection into the portal vein was measured relative to a simultaneously injected highly diffusible reference, tritium-labeled water, (3HOH). A 0.25-ml bolus of buffered Ringer solution containing 1-2 muCi of the 14C-labeled test substance and 3-6 muCi of 3HOH was administered by rabid portal injection in anesthetized rats. Circulation was terminated after a single passage of the bolus through the hepatic microvasculature and the tissue was immediately macerated, solubilized, and subjected to liquid scintillation counting. Liver uptake indices (LUI) were calculated from the ratio of 14C to 3H in liver tissue relative to the same ratio in the injection mixture. LUI's of five carbohydrates were measured: sucrose (24.3 percent), inulin (27.7 percent), D-mannitol (80.2 percent), D-glucose (96.8 percent) and L-glucose (26.7 percent). The LUI for cholic acid was 127.1 percent. Among 18 amino acids tested, the LUI's were the highest for the acidic ones (L-aspartic acid, 100.0 percent and L-glutamic acid, 86.4 percent) and lowest for the basic ones (L-arginine, 37.4 percent and L-lysine, 31.4 percent). Stereospecificity for glucose and alanine uptake, saturation kinetics for glutamic acid (Km equal to 4.8 mM) and aspartic acid (Km equal to 2.7 mM), and cross-inhibition among uptake of the acidic amino acids were observed. These findings confirmed the applicability of a technique which was originally developed for studies of amino acid uptake in brain to characterization of transport systems in liver.