Schematic view of the absorption of amino acids (l-glutamate being taken as an example) from the lumen of the small intestine to the portal bloodstream. l-glutamate can be transported inside enterocytes through the Na+-dependent high affinity XAG − system and/or the low affinity B° system present in the brush border membranes (BBM). Oligopeptides (containing, for instance, glutamate) has to be degraded to dipeptides and tripeptides by brush border membranes before being transported inside enterocytes through PepT1 transporters. Then, l-glutamate is released from di and tripeptides by cytosolic peptidases. Intracellular glutamate which is not used for metabolism is released into the portal bloodstream through baso-lateral membranes using transport systems
Amino acid absorption by pig small intestine can be modified by different factors. For instance, the oral administration of insulin-like growth factor-I (IGF-I) to piglets is able to stimulate the Na+-dependent glutamine absorption by the jejunum without modifying the jejunal mucosal mass (Alexander and Carey 2002). Conversely, phytate, in addition to interfering with protein digestion, may inhibit amino acid absorption (Selle et al. 2012).
Supplementing the diet with alpha-ketoglutarate has been suggested as a mean to spare dispensable amino acids in pigs (Kristensen et al. 2002). Although transporters for di- and tricarboxylic acids have been detected in the pig small intestine, only a small percentage of alpha-ketoglutarate administered lumenally to pigs appears in the portal circulation, a result that is attributed to mucosal metabolism, and possibly to limited absorption by the small intestine (Lambert et al. 2002). Results of experiments involving pigs have suggested that small intestine absorption is probably also affected by low basolateral transfer from the enterocytes to the bloodstream as proposed by Buddington et al. (2004).
4.3 Amino Acid Absorption by the Piglet Colon
There is some evidence which suggest that the colonic epithelial cells of the piglets possess the capacity for the transport of some amino acid from the lumen. For instance, Smith and James (1980) have shown that it is possible to measure unidirectional fluxes of different amino acids across the newborn pig colon. Long-chain amino acids appeared to enter the proximal colon of newborn pigs more readily than did short- chain amino acids (Sepulveda and Smith 1979). The proximal colon of newborn pigs is able to transport methionine from the luminal side, but this capacity markedly decreases in 4-day-old piglets and remains undetectable in 10-day-old piglets (James and Smith 1976). The evolution of the capacity of colonocytes for amino acid transport in the piglets after birth may be related to marked ultrastructural changes of absorptive cells like the reduction of the height of the microvilli within few days following birth (Wooding et al. 1978). However, the nutritionally relevance of amino acid transport by the piglet colon has been questioned (Darragh et al. 1994).
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