UP-REGULATION OF SYSTEM-A ACTIVITY IN THE REGENERATING RAT-LIVER

System A activity for neutral amino acid transport, measured as the MeAIB-sensitive Na+-dependent L-alanine uptake, is induced 6 h after partial hepatectomy in plasma membrane vesicles from rat livers. Other Na+-dependent transporters, like system ASC (MeAIB-insensitive Na+-dependent L-alanine transport) and the nucleoside carrier show similar inductions. Up-regulation of system A is not explained by changes in the dissipation rate of the Na+ transmembrane gradient, as deduced from uptake measurements performed in the presence of monensin. To determine whether induced system A shared any similarity with the activity found in hepatoma cell lines, we analyzed the N-ethylmaleimide (NEM) sensitivity of system A in both regenerating and control rat liver plasma membrane vesicles, NEM treatment was equally effective in inhibiting system A in both experimental groups. Thus, during the prereplicative phase of liver growth, a transport activity similar to basal system A is up-regulated in liver parenchymal cells, by a stable mechanism that does not involve changes in the Na+ transmembrane gradient.