Metabolic acidosis in sheep alters expression of renal and skeletal muscle amino acid enzymes and transporters

To determine the effect of metabolic acidosis on expression of l-Gln, l-Glu, and l-Asp metabolizing enzymes and transporters, the relative content of mRNA, protein, or mRNA and protein, of 6 enzymes and 5 transporters was determined by real-time reverse transcription-PCR and immunoblot analyses in homogenates of kidney, skeletal muscle, and liver of growing lambs fed a common diet supplemented with canola meal (control; n = 5) or HCl-treated canola meal (acidosis; n = 5). Acidotic sheep had a 790% greater (P = 0.050) expression of renal Na⁺-coupled neutral AA transporter 3 mRNA and a decreased expression of renal glutamine synthetase mRNA (47% reduction, P = 0.037) and protein (57% reduction, P = 0.015) than control sheep. No change in renal cytosolic phosphoenolpyruvate carboxykinase (protein and mRNA), glutaminase (mRNA), or l-Glu dehydrogenase (protein) was found. In skeletal muscle, acidotic sheep had 101% more (P = 0.026) aspartate transaminase protein than did control sheep, whereas no change in the content of 3 Na⁺-coupled neutral AA transporters (mRNA) or 2 high-affinity l-Glu transporter proteins was found. In liver, no change in the content of any assessed enzyme or transporter was found. Collectively, these findings suggest that tissue-level responses of sheep to metabolic acidosis are different than for nonruminants. More specifically, these results indicate the potential capacity for metabolism of l-Asp and l-Glu by skeletal muscle, and l-Gln absorption by kidneys, but no change in hepatic expression of l-Gln metabolism, elaborates previous metabolic studies by revealing molecular-level responses to metabolic acidosis in sheep. The reader is cautioned that the metabolic acidosis model employed in this study differs from the increased plasma lactateinduced metabolic acidosis commonly observed in ruminants fed a highly fermentable grain diet.