Hormonal regulation of amino acid transport in isolated rat hepatocytes: properties of a high affinity transport component induced by glucagon and cyclic AMP.

In isolated rat hepatocytes, glucagon and cyclic AMP induce in vitro the emergence of a high affinity component of amino acid transport similar to that induced by fasting or partial hepatectomy, situations characterized by high plasma glucagon levels (10,11). This high affinity component has the properties of a pure A system, is operative for both AIB entry and AIB exodus, is dependent on de novo protein synthesis and has a rapid rate of degradation. The high affinity of this transport system, as well as its strong specificity for amino acids sharing the A mediation, represent a mechanism whereby the liver can adjust to increased needs for amino acids for gluconeogenesis, or for macromolecular synthesis. It appears from our studies that glucagon can rapidly stimulate amino acid transport through the synthesis of high affinity carriers which, in addition, are characterized by a rapid rate of degradation. This rapid turnover may be essential for enabling the hepatocyte to adjust its amino acid transport activity to varying environmental conditions.