Regulation of amino acid transport in the liver. Emergence of a high affinity transport system in isolated hepatocytes from fasting rats.

The increased capability of the liver to extract glucogenic substrates during fasting was investigated at. the cellular level by measuring the transport of neutral amino acids in hepatocytes freshly isolated from fed and 4%h-fasted rats. The uptake of 2-a-amino[l-14C]isobutyric acid, a nonmetabolizable analog of alanine, was markedly increased in hepatocytes from fasting rats when measured at 0.1 mu. At the plateau a distribution ratio of 30 to 40 was achieved, compared to 10 in hepatocytes from fed rats. This marked increase was not observed when uptake was measured at 50 mM. Analysis of the relationship between influx and substrate concentration revealed that two independent saturable components contributed to entry of a-aminoisobutyric acid in hepatocytes from fasting rats: 1) a low affinity component (Km E 50 to 70 mM) similar to that observed in cells from fed animals; 2) a high affinity component (K,,, E 0.6 to 0.8 mM) not observed in cells from fed rats. The transport of a-aminoisobutyric acid occurring through the high affinity component was dependent on Na+, and could be completely inhibited by a-(methylamino)isobutyric acid (Ki z 0.6 mM), the specific substrate for the “A” (alanine preferring) transport system. Ouabain, valinomycin, and gramicidin D partly inhibited cy-aminoisobutyric acid transport occurring through this high affinity component. Puromycin and cycloheximide administered in vivo prevented the emergence of the high affinity transport component induced by fasting. We conclude that the increase in amino acid active transport in hepatocytes from fasting rats results from the emergence of a high affinity transport component which has the properties of a pure “A” system. This system endows the hepatocyte with a high power for concentrating amino acids at low ambient levels, and thus may play an important role in the regulation of gluconeogenesis.