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

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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.

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