CHARACTERIZATION OF AMINO-ACID-TRANSPORT IN HUMAN ENDOTHELIAL-CELLS

The transport of amino acids has been studied in human umbilical vein endothelial cells. Neutral amino acids enter human umbilical vein endo thelial cells through three distinct agencies endowed with the charact eristics of systems A, ASC, and L. Each system has been studied by eva luating the influx of preferential substrates. The influx of L-proline and 2-methylaminoisobutyric acid occurs through an Na+-dependent adap tively regulated trans-inhibited agency identifiable with system A. L- Threonine influx occurs mainly through a distinct Na+-dependent trans- stimulated pathway corresponding to system ASC. System L accounts for Na+-independent influx of L-leucine. These systems cooperate for the t ransport of L-glutamine, which is due mainly to system ASC, whereas th e component due to the operation of system A increases upon amino acid starvation. No clear evidence was found for a glutamine-specific syst em (''system N''). Two systems, one Na+ dependent (system X(AG)-) and the other Na+ independent (system x(C)-), transport anionic amino acid s. L-Arginine influx exhibits a poor dependence on extracellular Na+, whereas it is sensitive to conditions known to change membrane potenti al and to trans-stimulation by intracellular amino acids. These featur es are consistent with a process mediated by system y+ and may be of s ignificance for the regulation of the intracellular concentration Of L -arginine.