Amino acid-dependent regulation of the hepatic glutamine transport system N

System N is a sodium-dependent amino acid transport system in the plasma membrane of hepatocytes which mediates the uptake of glutamine, histidine, and asparagine. Addition of certain individual amino acids to the culture medium of freshly-isolated hepatocytes has been shown to stimulate the activity of the System N transporter 2- to 3-fold after 30 to 60 minutes of exposure. The induction of transport activity was demonstrated to be rapid, cycloheximide-insensitive, and due to neither trans-stimulation nor recruitment of additional carriers to the plasma membrane. Collectively, these data indicate that the number of functional transporters in the plasma membrane remains unchanged during the induction, and that amino acids activate the intrinsic activity of pre-existing transporters by a novel mechanism. Further investigations revealed that the induction was sodium-dependent, supporting the related observation that stimulatory amino acids were exclusively substrates of sodium-dependent transport systems. The data therefore suggested that the concentrative uptake of amino acids via sodium-dependent systems is necessary for the activation to occur.Treatment of hepatocytes with stimulatory amino acids such as asparagine caused cell swelling, which was observed prior to transporter activation. Both asparagine-induced cell swelling and System N stimulation were suppressed by hypertonic buffer. Additionally, swelling of rat hepatocytes with hypotonic buffer in the absence of extracellular amino acids mimicked the stimulatory effects of asparagine. Collectively, these data indicate that cell swelling is a prerequisite for the amino acid-dependent induction, but also sufficient alone for the activation of System N.Treatment of hepatocytes with asparagine or valinomycin increased the permeability of the plasma membrane to potassium and resulted in both net potassium loss and increased System N activity. The stimulatory effects of asparagine were blocked in a concentration-dependent manner by increasing extracellular potassium. Among the sodium-dependent transport systems in rat hepatocytes, System N was selectively stimulated by asparagine, hypotonic buffer, and valinomycin. The results demonstrate that amino acid-induced cell swelling and concomitant potassium movements serve to regulate hepatic System N transport activity.