Uptake of amino acids by liver following rapid portal injection

A tracer technique which was developed for studies on amino acid uptake in brain has been used to investigate the uptake of ¹⁴C labeled substances by liver. AAn 0.2 ml bolus of test solution containing 1-2 μCi of ¹⁴C amino acid and 3-6 μCi of ³HOH (as an internal reference standard) was administered by rapid portal injection in anesthetized rats. Circulation was terminated after a single passage of the bolus through the hepatic microvasculature and the tissue was immediately 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 the substances tested were arranged in a spectrum between inulin (22.5%), sucrose (24.3%), D mannitol (80.2%) and D glucose (138.0%). For the amino acids, LUI's were highest for the acidics (L Asp, 107.3%; L Glu, 84.05%) and lowest for the basics (L Arg, 37.4%; L lys, 31.4%). Stereospecificity (L Ala, 41.2%; D Ala, 22.7%), saturation kinetics (Km for L Glu, 4.8 mM), and competition demonstrated the applicability of this method for characterizing amino acid transport systems in liver.