SUMMARY1. Nutrient transport in cultured human umbilical vein endothelial cells was characterized using a rapid dual-isotope dilution technique. Microcarrier beads with confluent endothelial cells were perfused in small columns, and uptake and efflux were assessed relative to D-mannitol (extracellular tracer) during a single transit through the column.2. At tracer concentrations significant unidirectional uptakes were measured for L-leucine (53 + 2 %), L-phenylalanine (73 + 2 %), L-serine (40 +4%), L-arginine (42 + 3 %) and L-ornithine (26 + 3 %). Uptake for L-proline, D-glucose, dopamine and serotonin was lower (6-10 %), whereas uptake for the system A analogue 2-methylaminoisobutyric acid (2-MeAIB) was negligible. Uptakes rapidly decreased with time due to tracer efflux. 6. Our results provide the first evidence that cultured human endothelial cells of venous origin express a saturable transport system for large neutral amino acids resembling system L described in brain microvascular endothelium. Detection of Na+-dependent and Na+-independent L-arginine uptake is of interest in view of recent reports that this cationic amino acid may be the physiological precursor for nitric oxide released by endothelium.

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