Up-regulation of system A activity in the regenerating rat liver

Abstract

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.