The determination of rates of muscle protein synthesis is important for a number of clinical studies including those on the loss of body protein following injury or infection. Currently amino acids labeled with stable (non-radioactive) labels are used to assess rates of muscle protein synthesis. Two different methods for the delivery of labeled amino acid tracer have been developed. One is the constant infusion technique in which label is given over several hours and the rate of muscle protein synthesis is determined from the increase in label in muscle protein determined from two muscle biopsies. Because there is some uncertainty with this method about the correct value of the enrichment of the amino acid at the site of protein synthesis, we have developed an alternative method for giving the labeled tracer. This method is the flooding technique where the labeled amino acid is given as a bolus injection. The application of these two techniques have given discrepant values for the rate of muscle protein synthesis. The hypothesis of this study is that the discrepancy between these two techniques can be resolved by using the enrichment of intracellular amino acid as a measure of the precursor enrichment. The study involved measurement of rates of muscle protein synthesis in healthy volunteers with either the infusion or flooding method, both in the post-absorptive state and after a day of eating. The study should resolve questions of methodology that will allow clinical investigations to proceed.
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