High-affinity glutamate transporter and glutamine synthetase content in longissimus dorsi and adipose tissues of growing Angus steers differs among suckling, weanling, backgrounding, and finishing production stages1

Skeletal muscle and adipose tissues play important roles in maintaining whole-body Glu and N homeostasis by the uptake of Glu and release of Gln. To test the hypothesis that expression of highaffinity Glu transporters (GLAST1, EAAT4, EAAC1, GLT-1) and glutamine synthetase (GS) would increase in longissimus dorsi and adipose tissue of newborn Angus steers randomly assigned (n = 6) to develop through suckling (S; 32 d) and/or weanling (W; 184 d), backgrounding (B; 248 d), and finishing (F; 423 d) production stages. Carcass quality was determined at slaughter to verify shifts in adipose and lean deposition with development. Expression of mRNA (RT-PCR/Southern) and relative protein abundance (Western analysis) were determined in tissue homogenates isolated from longissimus dorsi, and kidney and subcutaneous adipose. The effect of production stage or tissue type on carcass and protein abundance was assessed by 1-way ANOVA using the GLM procedure of SAS, and Fisher’s protected LSD procedure was used to separate data means. Neither GLAST1 nor EAAT4 mRNA or protein was detected. EAAC1, GLT-1, and GS mRNA were identified in all tissues, but GLT-1 and GS protein were not detected in kidney or subcutaneous adipose, and GS protein was not detected in longissimus dorsi. The EAAC1 content of subcutaneous (P = 0.06) and kidney (P = 0.02) adipose was 2 times greater in B and F than W steers, whereas GS was 5 times greater (P < 0.07) in B than F steers (B = W > F). For longissimus dorsi, EAAC1 (P < 0.01) and GLT-1 (P < 0.04) content decreased with development (S > W > B = F, S = W > B = F, respectively). Within F steers, EAAC1 and GLT-1 mRNA was expressed by subcutaneous, kidney, omental, mesenchymal, and intramuscular adipose tissues, whereas GS mRNA was expressed by all except for intramuscular. Only EAAC1 protein was detected in any adipose tissue, with EAAC1 content being 104% and 112% greater (P < 0.01) in intramuscular than in kidney or subcutaneous adipose, respectively, and not differing (P > 0.45) from omental or mesenchymal adipose. These data demonstrate (1) longissimus dorsi and adipose tissues of steers developing through typical production stages have different capacities for Glu uptake and Gln synthesis, (2) the importance of EAAC1 and GS in adipose metabolism, and (3) the differential metabolic fate of Glu by adipose tissues as steers developed, as evidenced by the marked decrease of GS content in subcutaneous and kidney adipose of F steers.