Content of ileal EAAC1 and hepatic GLT-1 high-affinity glutamate transporters is increased in growing vs. nongrowing lambs, paralleling increased tissue d- and l-glutamate, plasma glutamine, and alanine concentrations

• Published in: Journal of animal science 2003-04, Vol.81 (4), p.1030-1039
• Main Authors: Howell, J.A, Matthews, A.D, Welbourne, T.C, Matthews, J.C
• Format: Article
• Language: English
• Subjects: alanine; Alanine - blood; amino acid composition; Amino Acid Transport System X-AG - metabolism; Animal Nutritional Physiological Phenomena; Animal productions; Animals; Biological and medical sciences; epithelium; Excitatory Amino Acid Transporter 2 - metabolism; Fundamental and applied biological sciences. Psychology; glucose; Glutamate Plasma Membrane Transport Proteins; glutamic acid; Glutamic Acid - metabolism; glutamine; Glutamine - blood; ileum; Ileum - metabolism; Immunoblotting; Kidney - metabolism; kidneys; lambs; liver; Liver - metabolism; Male; messenger RNA; metabolism; protein content; Random Allocation; RNA, Messenger - metabolism; Sheep - blood; Sheep - growth & development; Sheep - metabolism; Symporters - metabolism; Terrestrial animal productions; transporters; Up-Regulation; Vertebrates; wethers
• ISSN: 0021-8812; 1525-3163
• DOI: 10.2527/2003.8141030x

Glutamate is a central metabolite for whole-animal energy and N metabolism. This study tested the hypothesis that ileal epithelium, liver, and kidney content of system X−AG glutamate transporters EAAC1 and GLT-1 would be up-regulated to support growth of wethers (30 ± 1.2 kg) fed a forage-based diet for at least 14 d to gain (2.0 × NEm; n = 9) vs. maintain (1.2 × NEm; n = 9) BW. We have previously demonstrated that two high-affinity glutamate transporters (EAAC1, GLT-1) are expressed by these extensive glutamate metabolizing epithelial tissues. Wethers fed at 2.0 × NEm gained (P < 0.001; 0.26 kg/d) BW, whereas those fed 1.2 × NEm did not. Although plasma concentrations (µM) of glucose and l- or d-glutamate did not differ, plasma glutamine (precursor of glutamate) and alanine concentrations (transamination product of glutamate) were 28% (P < 0.007) and 22% (P < 0.072) greater for growing lambs than nongrowing lambs. In tissues, the concentration of l-glutamate in ileum epithelia and d-glutamate of liver was 49% (P < 0.015) and 181% (P < 0.042) greater, respectively, in growing vs. nongrowing animals, whereas concentrations of glutamate isoforms did not differ in kidney. Paralleling these increased amino acid concentrations, ileal epithelium contained 313% more (P < 0.038) EAAC1 protein and liver contained 240% more (P < 0.001) GLT-1 protein, whereas kidney transporter content did not differ between growing and nongrowing wethers. In contrast to increased EAAC1 and GLT-1 protein content in ileal and liver tissue of growing lambs, messenger RNA levels did not differ. These results indicate that the increased capacity for high-affinity glutamate uptake in growing vs. nongrowing lambs is achieved through increased expression of EAAC1 by ileal epithelium and GLT1 by liver, which parallel increased tissue concentrations of glutamate and plasma concentrations of two major interorgan N carriers, glutamine and alanine.