Evidence for two Na+-independent neutral amino acid transport systems in primary cultures of rat hepatocytes. Time-dependent changes in activity.

Adult rat hepatocytes placed in primary culture contain at least two distinct Na+-independent transport systems for neutral amino acids. The characteristics of the two systems do not allow assignment to previously described Na+ independent agencies, so we have tentatively termed the two processes Systems L1 and L2. Uptake by System L1 is substantially inhibited by cysteine, valine, isoleucine, leucine, methionine, histidine, tryptophan, tyrosine, phenylalanine, and 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid. In contrast, System L2-mediated transport is completely inhibited by isoleucine, leucine, phenylalanine, and 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid. Amino acids transported by both systems show biphasic kinetics yielding Km values for the System L1 component in the micromolar range, whereas the corresponding values for System L2 are an order of magnitude higher. In freshly isolated hepatocytes, the activity of System L2 is relatively high and declines over the initial 24 to 48 h of culture. The Na+-dependent Systems N and ASC also show a significant decay in activity during this time period. In contrast to the decrease in uptake by System L2, transport by System L1 increases during culture following an initial lag period of 12 to 24 h. The increase in System L1 activity can be blocked by the addition of either cycloheximide or actinomycin D. System L1 appears to be present also in fetal hepatocytes, although, in the hepatoma cell line, HTC, the Na+-independent component appears to be homogeneous as though one of the two systems present in the normal adult hepatocyte is not expressed in these transformed cells.