Metabolism of Vascular and Luminal Glutamine by Intestinal Mucosa in Vivo

Abstract

Studies over the last 15 years have revealed that the small intestine is a major site for the metabolism of circulating glutamine in most animal species examined, including man. Because of the quantitative importance of glutamine as a vehicle for transporting nitrogen among tissues, the small intestine is now seen to play a significant role in processing the waste nitrogen from other organs. Meanwhile, oxidation of the glutamine carbon provides an important source of energy for the epithelial cells of the intestinal mucosa, where glutamine utilization is largely localized. Glutamine has emerged as a quantitatively more important respira¬tory fuel than glucose in this tissue. Also, it is now apparent that the small intestine is more than a site for digestion and absorption of dietary ingredients; it also serves some vital metabolic functions for the whole organism. Neptune’s work in 1965 [23], showing that glutamine was vigorously oxidized to C0₂ by incubated ileal tissue from several laboratory animal species, appears to be the first reported indication that glutamine may be a preferred substrate for intestine. These studies seem to have attracted little attention, however, perhaps because the conditions employed in vitro were not easily related to conditions in vivo. More direct clues emerged from arteriovenous difference measurements across the nonhepatic splanchnic organs of dogs [1, 7, 14], sheep [49], rats [2], and man [9], studies that revealed an uptake of circulating glutamine by the combined organs drained by the hepatic portal vein — the stomach, small and large intestine, cecum, spleen, and pancreas.