Stimulation of hepatocyte System y(+)-mediated L-arginine transport by an inflammatory agent

Arginine participates in several distinct metabolic pathways, including polyamine and nitric oxide biosynthesis. Normally, arginine is effectively sequestered from the hepatocyte intracellular space by the low basal activity of membrane transport system y+. This has implications for the subsequent metabolism of arginine and for hepatic arginine requirements during a septic insult. We investigated the influence of tumor necrosis factor (TNF) on the activity of System y(+)-mediated hepatocyte arginine transport employing hepatic plasma membrane vesicles (HPMVs).

Rats were treated with a single intraperitoneal injection of TNF (50 or 150 micrograms/kg body weight) for 2, 4, or 24 hours, and HPMVs were prepared by Percoll density gradient centrifugation. Vesicle purity was established by assay of enzyme markers. Vesicle arginine transport activity was evaluated by measurement of tritiated arginine uptake employing a rapid mixing-filtration technique.

Arginine transport by HPMVs was entirely independent of sodium and consisted of saturable and nonsaturable components. Prior treatment with TNF resulted in a time- and dose-dependent stimulation of saturable transport within 2 hours and a return to basal levels after 24 hours. Nonsaturable uptake was unchanged. Inhibition analysis indicated that the TNF-induced increase in saturable arginine transport activity was mediated by an increase in System y+. Kinetic analysis revealed that accelerated transport was caused by a 78% increase in the maximal velocity of transport without alteration in transport affinity.

In vivo treatment with TNF results in a rapid stimulation of saturable, System y(+)-mediated arginine transport in the liver. This TNF-induced stimulation of hepatic arginine transport may serve to increase the normally restricted availability of extrahepatic arginine to the hepatocyte intracellular space during a septic insult to support important arginine-dependent pathways in the liver.