Effect of Amino Acid Infusion on Human Postoperative Colon Protein Synthesis in Situ

Abstract

ABSTRACT. Background: Amino acids are an integral part of parenteral nutrition because of their anabolic action helping to conserve body protein after surgical stress. At the gastrointestinal tract, an adequate supply of amino acids may be particularly important because of the gut's high rate of protein turnover, cell division, and proliferation. However, no information is available about the effects of amino acids on human intestinal protein metabolism after surgery. Methods: Studies were performed in postabsorptive patients 8 -10 days after major abdominal surgery. Mass spectrometry techniques (capillary gas chromatography/ combustion isotope ratio mass spectrometry) were used to directly determine the incorporation rate of 1-[13 C]-leucine into colon mucosal protein. All subjects had a colostomy, which allowed easy access to the colon mucosa, and consecutive sampling from the same tissue was performed during continuous isotope infusion (0.16 mol/kg min). Isotopic enrichments were determined at baseline and after a 4-hour infusion of amino acids or after infusion of saline (control group). Results: Compared with baseline, infusion of amino acids reduced fractional colon protein synthesis significantly by Ϫ29.2 Ϯ 8.3%. This decrease was also significantly different from the corresponding (insignificant) change during saline infusion (ϩ19.4 Ϯ 26.9%, p Ͻ .05 vs amino acid group). Conclusions: After surgery, an amino acid infusion acutely reduces postoperative colon protein synthesis. This effect possibly may be attributed to interactions of specific amino acids (glutamine) with an altered intestinal immune system and enterocyte activity. Amino acid supply has gained its firm place during parenteral nutrition in a wide range of patients who cannot tolerate enteral feeding. Amino acids are usually considered necessary to reduce protein loss after surgical stress. An optimal conservation of body protein will be obtained with daily administration of 1.2-1.5 g amino acids per kg body weight.1 Potential anabolic actions of amino acids are known to occur in muscle and in splanchnic tissues.2 At the intestinal tract, amino acid metabolism and protein turnover are particularly important because an efficient formation of new protein is essential for maintaining a high rate of cell division and proliferation and for the production of cellular structure compounds and secreted enzymes.3,4 Unfortunately, in humans effects of specific substrates have almost exclusively been studied in the whole splanchnic bed, not allowing a differentiation between hepatic and intestinal changes.5-7 Virtually nothing is known on the effects of amino acids on human intestinal protein metabolism in situ.We sought in the present study to examine the effects of a standard amino acid infusion on colon protein synthesis using stable isotopes (1-[ 13 C]-leucine) and advanced mass spectrometry techniques. Studies were performed in patients after major abdominal surgery whose mucosal function may be altered and who are frequent candidates for parenteral...

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