Cyclo-oxygenase blockade abrogates the endotoxin-induced increase in Na(+)-dependent hepatic amino acid transport.

Abstract

Endotoxemia is characterized by a marked increase in the uptake of amino acids by the liver, but the regulation of this response has not been fully elucidated. In the current study, we investigated the potential role of prostaglandins as mediators of this response. We examined the in vivo effects of the anti-inflammatory agent ketorolac, a cyclo-oxygenase inhibitor that blocks prostaglandin synthesis, on hepatic amino acid transport activity in endotoxin-treated rats. We assayed the activities of the Na(+)-dependent transport systems A and N in hepatic plasma membrane vesicles prepared from endotoxemic rats that were pretreated with ketorolac or vehicle. Hepatic plasma membrane vesicles were prepared by differential centrifugation, and the transport of [3H]glutamine (system N) and [3H]2-methylamino-isobutyric acid (system A) was assayed. Hepatic plasma membrane vesicles were also prepared from normal rats that received prostaglandin E2, and glutamine and MeAIB transport were measured. Endotoxin treatment resulted in a twofold to threefold increase in Na(+)-dependent amino acid transport activity in hepatic plasma membrane vesicles secondary to an increase in the transport Vmax, which was consistent with the appearance of increased numbers of corresponding transporter proteins in the hepatocyte plasma membrane. Pretreatment with ketorolac almost completely abrogated the endotoxin-induced increase in hepatic amino acid transport. Administration of prostaglandin E2 to normal rats resulted in a statistically significant increase in glutamine and alanine transport by hepatic plasma membrane vesicles prepared from these animals. Prostaglandins play a key role in mediating the accelerated hepatic amino acid transport that occurs during endotoxemia.