Hepatic Na(+)-independent amino acid transport in endotoxemic rats: evidence for selective stimulation of arginine transport

Abstract

The effects of endotoxin on the activities of the major Na(+)-independent amino acid transporters in rat liver (Systems n, asc, L, bo,+, and y+) were studied using using hepatic plasma membrane vesicles (HPMVs). Rats were treated with a single dose of Escherichia coli endotoxin (E. coli lipopolysaccharide 0127:B8 (LPS), 7.5, 15, or 30 mg/kg BW) and HPMVs were prepared by Percoll density gradient centrifugation at various timepoints after LPS administration. Vesicle purity and integrity was established by assay of enzyme markers and identical equilibrium uptakes. The activities of the Na(+)-independent amino acid transport systems y+ and bo,+ (arginine), asc (alanine and cysteine), L (leucine), and n (glutamine) were evaluated by measuring the uptake of radiolabeled amino acids using a rapid mixing/filtration technique. Amino acid uptake by HPMVs consisted of saturable and nonsaturable components. Prior treatment with endotoxin did not alter the activities of Systems n, asc, or L but resulted in a time- and dose-dependent stimulation 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 LPS treatment. Kinetic analysis of arginine transport demonstrated the presence of both a high affinity and a low affinity carrier. Treatment with LPS resulted in a 73% increase in the Vmax of the high affinity carrier (System y+) and a 25% increase in the Vmax of the low affinity transporter (System bo,+). The data indicate selective stimulation of Na(+)-independent arginine transport in the liver during endotoxemia which may serve to support important arginine-dependent pathways during sepsis.