Muscular work accelerates many metabolic processes. Because amino acids play a central role in these processes, their metabolism is of added importance during exercise. In addition to being constituents of protein, amino acids regulate the balance of protein synthesis and breakdown. Moreover, they are essential to interorgan nitrogen flux, are substrates for gluconeogenesis and oxidation, and are involved in acid-base regulation. Despite the importance of amino acids, major gaps in the understanding of their role and regulation exist. Amino acid metabolism is difficult to study technically and as a result there is not a clear consensus on even fundamental aspects of the exercise response. By combining arteriovenous difference and stable isotopic methods in the dog our preliminary data show that the gut is a major site of protein catabolism during exercise. The gut response results in a massive amino acid inflow to the liver. Interestingly, only the branched chain amino acids, in a net sense, escape the liver. Studies in humans have shown that the splanchnic bed is also a major site of metabolism for amino acids and NH3 released by the working muscle. We have extended these findings by showing that both the gut and liver participate in this process. In the experiments outlined, we will study amino acid metabolism using stable isotopes (5-15N-glutamine, 1-13C-leucine) in dogs chronically catheterized for blood sampling from the vessels which perfuse and drain the gut and liver. There are three major aims of the proposed studies. The first is to determine the mechanism of the increase in gut protein catabolism (increased proteolysis or decreased protein synthesis) and the means of regulation by hormones (glucagon, insulin), and substrates (glutamine, NH3). Second, the role of glutamine in nitrogen transfer to the gut and liver will be studied. Again, we will assess putative regulatory factors.
The third aim i s to assess the functional significance of the increase in splanchnic amino acid metabolism. This will be assessed by examining protein synthesis using alpha-ketoisocaproic acid and hepatic protein enrichments. In addition, the fate of the glutamine amide nitrogen will be assessed by measuring the secondary labeling of urea, alanine, and NH3. Two general hypotheses are proposed which transcend the aims outlined above. First, we hypothesize that the increased glutamine uptake causes a reduction in glutamine availability which is part of the stimulus for gut protein catabolism. Second, we propose that the gut and liver function as a syncytium in the regulation of amino acids so that the gut response reflects the demands of the liver. These studies will further our knowledge of the mechanisms involved in the regulation of amino acid metabolism during exercise and lead to a better understanding of the means by which amino acids are involved in the adaptations that occur.
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