Amino acid-dependent increase in hepatic system N activity is linked to cell swelling.
Treatment of cultured rat hepatocytes with certain amino acids stimulates the activity of the System N transporter. The present report investigates the mechanism by which the stimulatory amino acids elicit their effect. Activation of System N-mediated transport by amino acids is rapid, cycloheximide-insensitive, and involves neither trans-stimulation nor recruitment of additional carriers to the plasma membrane. In addition, the activation is Na(+)-dependent, supporting the related observation that the most effective stimulatory amino acids are substrates of Na(+)-dependent transport Systems A, ASC, and N whereas substrates of Na(+)-independent System L and non-amino acid metabolites are ineffective. The data suggest that active accumulation of amino acids via Na(+)-dependent carriers is necessary for the activation to occur. The amino acid-dependent stimulation is blocked in a concentration-dependent manner by increasing extracellular K+. Treatment of hepatocytes with an amino acid such as asparagine causes cell swelling and stimulation of System N activity; both of these effects are reduced by hypertonic media. Furthermore, swelling of rat hepatocytes with hypotonic media mimics the System N-stimulatory effects of asparagine. Among the Na(+)-dependent amino acid transport systems present in rat hepatocytes, System N is stimulated preferentially by amino acid-containing or hypotonic media. Collectively, these results demonstrate that cell swelling is a prerequisite for the amino acid-dependent activation of the hepatic System N transporter.
Bode
B P
BP
Department of Biochemistry and Molecular Biology, University of Florida School of Medicine, Gainesville 32610.
Kilberg
M S
MS
D000596:Amino Acids / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D002478:Cells, Cultured; D003470:Culture Media; D003513:Cycloheximide / Q000494:pharmacology; D008099:Liver / Q000166:cytology / Q000378:metabolism*; D008297:Male; D009994:Osmolar Concentration; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism*
D000596:Amino Acids; D003470:Culture Media; D003513:Cycloheximide; D012964:Sodium