Research Article: Hepatic glutamate transport and glutamine synthesis capacities are decreased in finished vs. growing beef steers, concomitant with increased GTRAP3-18 content

Date Published: February 1, 2018

Publisher: Springer Vienna

Author(s): J. Huang, Y. Jia, Q. Li, W. R. Burris, P. J. Bridges, J. C. Matthews.

http://doi.org/10.1007/s00726-018-2540-8

Abstract

Hepatic glutamate uptake and conversion to glutamine is critical for whole-body N metabolism, but how this process is regulated during growth is poorly described. The hepatic glutamate uptake activities, protein content of system documentclass[12pt]{minimal}
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begin{document}$${text{X}}^{ – }_{text{AG}}$$end{document}XAG- transporters (EAAC1, GLT-1) and regulatory proteins (GTRAP3-18, ARL6IP1), glutamine synthetase (GS) activity and content, and glutathione (GSH) content, were compared in liver tissue of weaned Angus steers randomly assigned (n = 8) to predominantly lean (growing) or predominantly lipid (finished) growth regimens. Steers were fed a cotton seed hull-based diet to achieve final body weights of 301 or 576 kg, respectively, at a constant rate of growth. Liver tissue was collected at slaughter and hepatic membranes fractionated. Total (75%), Na+-dependent (90%), system documentclass[12pt]{minimal}
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begin{document}$${text{X}}^{ – }_{text{AG}}$$end{document}XAG–dependent (abolished) glutamate uptake activity, and EAAC1 content (36%) in canalicular membrane-enriched vesicles decreased as steers developed from growing (n = 6) to finished (n = 4) stages, whereas Na+-independent uptake did not change. In basolateral membrane-enriched vesicles, total (60%), Na+-dependent (60%), and Na+-independent (56%) activities decreased, whereas neither system documentclass[12pt]{minimal}
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begin{document}$${text{X}}^{ – }_{text{AG}}$$end{document}XAG–dependent uptake nor protein content changed. EAAC1 protein content in liver homogenates (n = 8) decreased in finished vs. growing steers, whereas GTRAP3-18 and ARL6IP1 content increased and GLT-1 content did not change. Concomitantly, hepatic GS activity decreased (32%) as steers fattened, whereas GS and GSH contents did not differ. We conclude that hepatic glutamate uptake and GS synthesis capacities are reduced in livers of finished versus growing beef steers, and that hepatic system documentclass[12pt]{minimal}
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begin{document}$${text{X}}^{ – }_{text{AG}}$$end{document}XAG- transporter activity/EAAC1 content is inversely proportional to GTRAP3-18 content.

Partial Text

Improvement of feeding regimens for production animals has been hindered by a lack of fundamental knowledge about how the capacity to regulate nutrient absorption across cell membranes affects the function of nutrient metabolizing enzymes. Hepatic l-glutamate (Glu) transport and metabolism are critical to support whole-animal energy and N homeostasis (Burrin and Stoll 2009; Heitmann and Bergman 1981; Wu 1998). In the liver, Glu is a central substrate for ureagenesis, gluconeogenesis, glutathione production, de novo protein synthesis, and nitrogen shuttling via glutamine (Meijer et al. 1990; Brosnan 2000; Watford 2000). Although plasma membrane transport capacity is thought to limit Glu metabolism (Gegelashvili et al. 2003; Nissim 1999; Low et al. 1994), knowledge of how hepatic Glu transport capacity may be regulated to support changes in metabolic capacity associated with different phases of growth (e.g., predominately lean to predominately lipid tissue accretion) is limited.

A primary function of the mammalian liver is to coordinate whole-body energy and N metabolism. Hepatic transport and intermediary metabolism of Glu is critical to these processes as Glu is a central substrate for hepatic ureagenesis, gluconeogenesis, glutathione production, de novo protein synthesis, and nitrogen shuttling via l-glutamine (Gln) (Meijer et al. 1990; Watford 2000). Understanding how the expression and function of system documentclass[12pt]{minimal}
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begin{document}$${text{X}}^{ – }_{text{AG}}$$end{document}XAG- transporters, their regulatory proteins, and GS activity and GSH content are coordinated could yield important insight into the importance of Glu use and metabolism as production animals grow, especially as they transition from predominantly low to high lipid accretion phases. Accordingly, the objective of this study was to determine if hepatic activities and protein content of EAAC1, GLT-1, GTRAP3-18, and ARL6IP1, and GS activity and protein content, and GSH content, in liver changed as steers developed from growing through finished stages, using a commercially relevant development regimen. To obviate potential metabolic changes as a result of ruminants growing at different rates (Howell et al. 2003), diets for the growing and finished treatment groups were formulated to support the same average rate of growth. As planned, ADG did not differ in finished vs. growing steers and lipid accretion clearly differed among development stages (Table 1). That is, the transition from predominantly lean phenotypes of growing steers to predominantly lipid phenotype of finished steers was demonstrated by higher final BW, HCW, ribeye area, adjusted 12th rib adipose, marbling scores, and yield grade in finished vs. growing steers. Thus, the steers of each treatment group in this study were typical of growing and finished beef cattle phenotypes.

 

Source:

http://doi.org/10.1007/s00726-018-2540-8

 

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