Modulation of amino acid transport in rat liver slices by tissue preincubation.

In rat liver slices, a prolonged preincubation resulted in a stimulation of AIB and alanine uptake. The increase of transport of both amino acids can be ascribed to a reduction of Ki without an alteration of Vt. The enhancement of transport seems to be restricted to the A mediation as indicated by the findings that only the Na+-dependent, MeAIB-inhibitable portion of AIB transport was increased by the prolonged preincubation, and, furthermore, that the uptake of cysteine, a specific substrate for system ASC in liver cells, was not affected. The effect of an extended preincubation on AIB transport was abolished by the addition of high concentrations of AIB and alanine to the preincubation medium. Several other amino acids tested did not affect the increase of AIB transport. The observed stimulation of transport seems not to be due to the derepression of the synthesis of transport proteins, as cycloheximide and 5-azacytidine did not block the increase of AIB uptake. The enhancement of amino acid transport does not appear to be a result of a release from transinhibition since the uptake of alanine was markedly enhanced after 3 h of preincubation while the alanine concentrations in the incubated liver slices did not change significantly.