Adaptive regulation of neutral amino acid transport system A in rat H4 hepatoma cells

Substrate regulation of System A transport activity in rat H4 hepatoma cells is described. The uptake of several amino acids was tested in the presence of system‐specific inhibitors. System A activity was increased in a RNA‐ and protein synthesis‐dependent manner by amino acid deprivation of the cells (adaptive regulation), whereas transport by Systems ASC, N, y + , and L was unaffected. Unlike human fibroblasts, the H4 cells did not require serum to exhibit the depression of System A. At cell densities between 88 × 10 3 and 180 × 10 3 cells/cm 2 , the degree of adaptive regulation was inversely related to cell density. Both transport of AIB and adaptive regulation of System A were nearly abolished if either K + or Li + was substituted for Na + in the medium. The presence of cycloheximide or tunicamycin blocked further increases in starvation‐induced activity within 1 hr of addition, suggesting the involvement of a plasma membrane glycoprotein. In contrast, if the medium was supplemented with actinomycin after the stimulation of System A had begun, the activity continued to increase for an additional 2 hr before being slowed by the inhibitor. The contributions of trans‐inhibition and repression to the amino acid‐induced decay of System A activity were estimated for several representative amino acids. In general, the System A activity in normal rat hepatocytes was much less sensitive to trans‐inhibition than the corresponding activity in H4 hepatoma cells. The half‐life values for the amino acid‐dependent decay of System A ranged from 0.5 to 2.0 hr.