HEPATIC NA-INDEPENDENT AMINO-ACID-TRANSPORT IN ENDOTOXEMIC RATS - EVIDENCE FOR SELECTIVE STIMULATION OF ARGININE TRANSPORT()

The effects of endotoxin on the activities of the major Na+-independen t amino acid transporters in rat liver (Systems n, asc, L, b(o,+) and y(+)) were studied using using hepatic plasma membrane vesicles (HPMVs ). Rats were treated with a single dose of Escherichia coil endotoxin (E. coli lipopolysaccharide 0127:B8 (LPS), 7.5, 15, or 30 mg/kg BW) an d HPMVs were prepared by Percoll density gradient centrifugation at va rious timepoints after LPS administration. Vesicle purity and integrit y was established by assay of enzyme markers and identical equilibrium uptakes. The activities of the Na+-independent amino acid transport s ystems y(+) and b(o,+) (arginine), asc (alanine and cysteine), L (leuc ine), and n (glutamine) were evaluated by measuring the uptake of radi olabeled amino acids using a rapid mixing/filtration technique. Amino acid uptake by HPMVs consisted of saturable and nonsaturable component s. Prior treatment with endotoxin did not alter the activities of Syst ems n, asc, or L but resulted in a time- and dose-dependent stimulatio n of saturable arginine transport. Arginine transport increased within 2 h of LPS administration and exhibited a return towards basal levels by 24 h. Nonsaturable uptake (diffusion) in HPMVs was unaltered by LP S treatment. Kinetic analysis of arginine transport demonstrated the p resence of both a high affinity and a low affinity carrier. Treatment with LPS resulted in a 73% increase in the V-max of the high affinity carrier (System y(+)) and a 25% increase in the V-max of the low affin ity transporter (System b(o,+)). The data indicate selective stimulati on of Na+-independent arginine transport in the liver during endotoxem ia which may serve to support important arginine-dependent pathways du ring sepsis.