GTRAP3-18 Protein Negatively Modulates Canalicular Glutamate Transport and Glutamine Synthesis Capacity in the Liver of Finishing Vs. Growing Beef Steers.

In rodents, system X^-_AG Glu transporters, EAAC1 and GLT-1, are linked to glutamine synthetase (GS) activity and glutathione content, respectively. The goal of this study was to determine if the hepatic content and activities of these proteins, and GTRAP3-18, an inhibitor of EAAC1 and GLT-1, change as steers developed from predominately lean to lipid growth phases. Weaned Angus steers (BW = 209 ± 29.4 kg) were randomly assigned (n = 8) to develop through lean (GROW, final BW = 301 ± 7.06 kg) vs. lipid (FINISH, final BW = 576 ± 36.9 kg) growth phases and individually fed enough of a cotton seed hull-based diet to achieve a constant ADG (1.5 kg/d) throughout the trial. The effect of development on experimental parameters was assessed by 1-way ANOVA using the GLM procedure of SAS. As expected, final BW (576, 301 kg), 12^th rib adipose (1.73, 0.54 cm), marbling score (668, 296), and yield grade (3.65, 2.13) were greater (P < 0.01), respectively, for FINISH vs. GROW steers. Western blot analysis of hepatic homogenates (n = 8) found more GTRAP3-18 (63%, P = 0.01) in FINISH vs. GROW steers, whereas EAAC1 content was less (32%, P = 0.05), and GLT-1 did not differ (P ≥ 0.66). System X^-_AG activity (pmol ∙ 10 min^-1 ∙ μg^-1 protein) in canalicular membrane vesicles (cMV) was greater (P = 0.07) in GROW (2.78, n = 6) than in FINISH steers (0.0, n = 4), but did not differ (P = 0.55) between basolateral membrane vesicles (bMV) of GROW (1.85, n = 6) and FINISH (1.15, n = 4) steers. Western blot analysis of MV (n = 4) found that EAAC1 content in bMV was less (25%, P = 0.05), and tended to be less (47%, P = 0.09) in cMV, for FINISH vs. GROW steers, whereas GLT-1 content in bMV and cMV was not affected (P ≥ 0.50). GS activity in liver homogenates (n = 8) of FINISH steers (0.67 nmol ∙ min^-1 ∙ mg^-1 wet tissue) was less (32%, P = 0.01) than GROW steers, whereas GS content did not differ (P = 0.72). Hepatic glutathione content (mg/g wet tissue) did not differ (P = 0.96) between GROW (1.08) and FINISH (1.07) steers. We conclude that as steer compositional gain changes from lean to lipid, the change in Gln synthesis capacity is related to canalicular membrane-associated EAAC1 function, which is inversely proportional to GTRAP3-18 content.