Standardized ileal digestible tryptophan to lysine ratios affect performance and regulate intestinal mRNA expression of amino acid transporters in weaning pigs fed a low crude protein diet

The objective of this study was to estimate the effect of standardized ileal digestible (SID) tryptophan (Trp) to lysine (Lys) ratios on growth performance and jejunal expression of amino acid transporters in weaning pigs. A total of 240 hybrid [(Duroc, male) x (Large White x Landrace, female)] piglets (6.52 ? 0.42 kg) were used in a 28-day growth trial and randomly allotted to one of four diets providing SID Trp to Lys ratios of 0.15, 0.18, 0.21, or 0.24 and were allocated in 6 pens per treatment with 10 pigs per pen. The average daily gain (ADG), average daily feed intake (ADFI), and serum levels of Trp, serine, isoleucine and leucine in pigs significantly increased when the SID Trp to Lys ratios increased from 0.15 to 0.18, 0.21, or 0.24 (P < 0.05). The pigs fed the diet with the 0.24 ratio had a reduced serum urea nitrogen (SUN) concentration compared with the pigs fed the 0.15 ratio diet (P < 0.05). The villus heights of the duodenum and jejunum in pigs fed the diet with a 0.21 ratio were greater than those of pigs fed the 0.15 ratio diet (P < 0.05). The increase in the SID Trp to Lys ratio from 0.15 to 0.18 decreased the intestinal ASCT2 mRNA level in weaning pigs (P < 0.05). The increase in the SID Trp to Lys ratio from 0.15 to 0.21 or 0.24 increased the mRNA abundance of intestinal B?AT1 (P < 0.05). The increase in the SID Trp to Lys ratio from 0.15 to 0.18 increased the mRNA abundances of intestinal CAT-1 and rBAT (P < 0.05). The intestinal b0,+AT mRNA level in the pigs fed the 0.21 ratio diet was higher than that in pigs in other three treatments (P < 0.05). The mRNA abundances of y+LAT1 and 4F2hc in the pigs fed the 0.15 ratio diet were lower than those in pigs in other three treatments (P < 0.05). The intestinal PepT-1 mRNA level in the pigs fed the 0.15 or 0.24 ratio diet was lower than that of the pigs fed the 0.21 ratio diet (P < 0.05). In conclusion, the optimal SID Trp to Lys ratio was determined to be 0.18 for weaning pigs that were fed a low crude protein diet using ADG and jejunal amino acid transporter mRNA expression as the response criteria.