Gut-liver interaction in glutamine homeostasis: portal ammonia role in uptake and metabolism.

The role of ammonia released by the gut on hepatic glutamine handling and metabolism was studied in postabsorptive anesthesized male Sprague-Dawley rats at spontaneous and elevated arterial glutamine concentrations. Glutamine handling and metabolite release across both organ beds were studied using arteriovenous concentration differences and simultaneously measured portal and hepatic venous plasma flows. At the spontaneous arterial glutamine load, fractional glutamine extraction, FE-Gln, by the gut and the liver was 24 and 10%, respectively. At the elevated glutamine load, gut glutamine uptake doubled, while FE-Gln remained at 24%; however, portal ammonia and alanine increased and decreased, respectively. In response, hepatic FE-Gln increased to 28% with a large release of glutamate and urea. The role of portal ammonia in modulating hepatic glutamine uptake was studied by infusing NH4HCO3 directly into the portal vein. Increasing the portal load promptly stimulated hepatic glutamine uptake and glutamate and urea release. Mitochondria isolated from these livers produced more glutamate from glutamine, suggesting ammonia activation of hepatic glutaminase flux; in addition, citrulline formation increased, suggesting a coupling of glutaminase flux to urea synthesis. The results are consistent with portal ammonia release acting as a key informational molecule in interorgan glutamine flow.