ENTERAL GLUTAMATE IS ALMOST COMPLETELY METABOLIZED IN FIRST PASS BY THE GASTROINTESTINAL-TRACT OF INFANT PIGS

We studied the absorption of enteral glutamate and phenylalanine using isotopic tracer and arteriovenous difference techniques. Six piglets, implanted with portal, carotid, and gastric catheters and an ultrason ic portal flow probe received a 6-h intragastric infusion of [U-C-13]g lutamate ana [H-2]phenylalanine, with a high-protein diet offered one time each hour. Amino acid concentrations and the isotopic enrichments of all mass isotopomers of glutamate, glutamine, and phenylalanine we re measured in portal and arterial blood over the last hour. There was significant (P < 0.025) net absorption of the indispensable amino aci ds as well as arginine, proline, serine, and alanine. There was no por tal uptake of glutamate, aspartate, and glycine, and arterial glutamin e was removed by the portal drained viscera (P < 0.05). At isotopic st eady state, 72% of the [H-2]phenylalanine but only 5% of the [U-C-13]g lutamate tracer appeared in the portal blood. We conclude that, in fed infant pigs, the gut metabolizes virtually all of the enteral glutama te during absorption. Therefore, glutamate and glutamine in the body a s a whole must derive almost entirely from synthesis de novo.