The uptake of glutamine and release of arginine, citrulline and proline by the small intestine of developing pigs.
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Arteriovenous (A-V) differences in the plasma concentrations of amino acids across the jejunum were studied in preweaning (14- to 21-d-old) and post-weaning (29- to 58-d-old) pigs in the postabsorptive state. Glutamine was the only amino acid that was extracted by the small intestine in both pre- and post-weaning pigs. The production of citrulline by the jejunum was low in preweaning pigs, but was threefold greater in the post-weaning pigs than in the preweaning pigs. The output of proline by the intestine was observed in the post-weaning pigs but not in the preweaning pigs. Arginine and alanine were the predominant amino acids released by the jejunum of the pre- and post-weaning pigs, respectively. Whereas glutamate was the major amino acid formed from glutamine in pig enterocytes in vitro, the jejunum of the post-weaning pigs released comparable amounts of citrulline, proline and glutamate in vivo. There were no significant A-V differences in the concentrations of urea, ornithine, taurine, tyrosine, serine, glycine and nutritionally essential amino acids in either the preweaning or the post-weaning pigs. Thus, we demonstrated for the first time the uptake in vivo of glutamine and the release of arginine, alanine, citrulline, glutamate and proline by the small intestine of developing pigs. Our findings on the release in vivo of citrulline (the precursor of arginine) and proline by the small intestine of the pre- and post-weaning pigs provide a basis for explaining why arginine and proline are nutritionally essential amino acids for young suckling piglets but not for adult pigs.