Ruminal and abomasal starch hydrolysate infusions selectively decrease the expression of cationic amino acid transporter mRNA by small intestinal epithelia of forage-fed beef steers

Although cat ionic amino acids (CAA) are considered essential to maximize optimal growth of cattle, transporters responsible for CAA absorption by bovine small intestinal epithelia have not been described. This study was conducted to test 2 hypotheses: 1) the duodenal, jejunal, and ileal epithelia of beef cattle differentially express 7 mRNA associated with 4 mammalian amino acid (AA) transport activities: y⁺ (CAT1), B^0,+ (ATB^0,+), b^0,+ (b^0,+AT and rBAT), and y⁺L (y ⁺LAT1, y⁺LAT2, and 4F2hc), and 2) the expression of these mRNA is responsive to small intestinal luminal supply of AA substrates (derived from ruminal microbes) or glucose-derived energy (from starch hydrolysate, SH), or both. Eighteen ruminally and abomasally catheterized Angus steers (body weight = 260 ± 17 kg) fed an alfalfa cube-based diet at 1.33 × net energy for maintenance requirement were assigned to 3 treatments (n = 6): ruminal and abomasal water infusion (control) ; ruminal SH and abomasal water infusion; and ruminal water and abomasal SH infusion. The dosage of SH infusion amounted to 20% of metabolizable energy intake. After 14 or 16 d of infusion, steers were slaughtered, duodenal, jejunal, and ileal epithelia were harvested, and total RNA was extracted. The relative amounts of mRNA expressed by epithelia were quantified using real-time reverse transcription-PCR. All 7 mRNA species were expressed by the epithelium from each region, but their abundance differed among the regions. Specifically, duodenal expression of CAT1 and ATB ^0,+ mRNA was greater than jejunal or ileal expression; ileal expression of b^0,+AT, rBAAT, and y⁺LAT1 mRNA was greater than jejunal or duodenal expression, whereas the expression of y⁺LAT2 and 4F2hc mRNA did not differ among the 3 epithelia. With regard to SH infusion effect, ruminai infusion down-regulated or tended to down-regulate the jejunal expression of CAT1, rBAT, y⁺LAT2, and 4F2hc mRNA. Abomasal infusion down-regulated the jejunal expression of y⁺LAT2 mRNA and tended to down-regulate the jejunal expression of 4F2hc mRNA. This study characterized the pattern of CAA transporter mRNA expressed by growing beef cattle fed an alfalfa-based diet. Moreover, this study demonstrated that increasing the luminal supply of microbe-derived AA (by ruminai supplementation of SH) results in a reduced capacity of apical and basolateral membrane to transport of CAA, whereas increasing luminal glucose supply (by abomasal supplementation of SH) reduces only the basolateral transport capacity, assuming that CAA transporter mRNA content represents functional capacity.