Ammonia production from intraluminal amino acids in canine jejunum

Abstract

Dietary protein increases the blood ammonia concentration when hepatic metabolic function is impaired, but the site of ammonia production and its specific precursors have not been clearly defined. The purpose of this study is to determine if individual luminal amino acids are metabolized to ammonia by the jejunum during the process of absorption. In anesthetized, fasted dogs, a cannula was inserted into the mesenteric vein draining a segment of the jejunum weighing approximately 18 g, and the ends of the segment were ligated to isolate its blood flow. Ammonia and amino acids were determined in luminal fluid as well as arterial and mesenteric venous blood. One of six amino acids (10 mM) was luminally perfused for a 15-min equilibration period and two 15-min collection periods, and the results were compared with control periods that preceded and followed the amino acid perfusion. Alanine, leucine, and glutamine significantly (P less than 0.01) increased ammonia release into mesenteric venous blood by 37, 42, and 106%, respectively, whereas threonine, serine, and glycine had no effect. Net jejunal uptake of glutamine from arterial blood, which accounts for ammonia release by the jejunum in the basal state, was not altered by perfusions other than with glutamine. Luminal glycine perfusion also caused an increased release of serine into mesenteric venous blood and alanine perfusion increased the release of glutamate. Glutamine perfusion caused increased release of glutamate, alanine, proline, and citrulline. These results indicate that some, but not all, luminal amino acids are partially metabolized to ammonia during the process of absorption in the small intestine.