Endotoxin Increases Hepatic Glutamine Transport Activity

Abstract

Glutamine uptake by the liver is accelerated during endotoxemia, but little is known regarding the influence of sepsis on the plasma membrane transport systems catalyzing hepatic glutamine uptake. We hypothesized that this augmented uptake was due to an increase in hepatocyte plasma membrane transport activity. We investigated the activities of the Na+-dependent transport System N (transports glutamine into the hepatocyte) and the Na+-independent System n (transports glutamine out of the cell) in hepatocyte plasma membrane vesicles (HPMVs) prepared from livers of rats treated with Escherichia coli endotoxin (LPS) in vivo. HPMVs were prepared by differential centrifugation and the transport of [3H]glutamine was assayed by a rapid mixing/filtration technique in the presence and absence of sodium. Vesicle integrity and functionality were confirmed by enzyme marker enrichments and classic "overshoots" in the presence of sodium. Carrier-mediated Na+-independent glutamine transport activity was not altered by LPS administration. In contrast, endotoxemia resulted in a time- and dose-dependent two- to threefold increase in Na+-dependent glutamine transport activity in HPMVs secondary to an increase in the transport Vmax, consistent with the appearance of increased numbers of corresponding transporter proteins in the hepatocyte plasma membrane. The Km (affinity for glutamine) of the System N transporter was not affected by LPS treatment. Maximal increases in transport were observed 4 hr after exposure to endotoxin. System N transport activity returned to basal levels by 12 hr. This increase in transport activity represents an important mechanism regulating the accelerated hepatic glutamine uptake that occurs during severe infection.