Dietary levels of protein and free amino acids affect pancreatic proteases activities, amino acids transporters expression and serum amino acid concentrations in starter pigs

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The supplementation of low protein diets with free AA for pigs improves the AA profile and reduces N excretion (Carter et al., 1996). Five free essential AA, feed‐grade, are currently available to be included in practical diets and, when added properly, can substantially reduce their protein content. Low‐protein AA‐supplemented diets may contain up to 60% free Lys and as low as 40% protein‐bound (PB) Lys. The free AA are available for absorption as they reach the proximal small intestine but the PB AA need to be released during protein digestion before their absorption takes place. Thus, the content of PB and free AA in the diet may affect the absorption site of AA in the small intestine of pigs.
The protein concentration in the small intestine digesta, which reflects the dietary protein content, seems to affect the secretion of pancreatic proteases in animals. Rats fed low protein diets secrete less pancreatic trypsin and chymotrypsin than those fed high‐protein diets (Green et al., 1986; Fushiki and Iwai, 1989). Moreover, the availability of free AA appears to affect the abundance of AA transporters in the small intestine of pigs (Morales et al., 2013). Lysine absorption is critical because it is the first limiting AA in most feed ingredients for pigs. Lysine is absorbed via the systems b0,+AT (Majumder et al., 2009) and y+LAT1 (Bröer, 2008) in exchange for neutral AA (Leu). System B0AT1 is the major Leu transporter (Bröer, 2008). Our hypothesis was that the activities of pancreatic proteases decrease in pigs fed a low‐protein diet compared with pigs fed a high‐protein diet, and that the supplementation with free AA stimulates the abundance (expression) of mRNA coding for AA transporters along the whole small intestine. In turn, the absorption and serum concentration of AA would be affected as well. However, there is no available data regarding these assumptions in young pigs.
The objective of the present study was to compare the effect of feeding pigs with either a low‐protein diet supplemented with free AA or a high‐protein diet with no supplemental free AA on: (i) the activity of trypsin and chymotrypsin in duodenal and jejunal digesta, (ii) the expression of two cationic AA and one neutral AA transporters in duodenum, jejunum and ileum, (iii) the concentrations of free AA in serum. The enzyme activities and the expression of AA transporters among the intestinal segments were also compared.
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