HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text Full Text (PDF) All Versions of this Article: 293/3/E666    most recent 00185.2007v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (2) Citing Articles via Google Scholar Google Scholar Articles by Smith, G. I. Articles by Mittendorfer, B. Search for Related Content PubMed PubMed Citation Articles by Smith, G. I. Articles by Mittendorfer, B.

Measurement of human mixed muscle protein fractional synthesis rate depends on the choice of amino acid tracer

Washington University School of Medicine, St. Louis, Missouri

Submitted 23 March 2007 ; accepted in final form 29 May 2007

The goal of this study was to discover whether using different tracers affects the measured rate of muscle protein synthesis in human muscle. We therefore measured the mixed muscle protein fractional synthesis rate (FSR) in the quadriceps of older adults during basal, postabsorptive conditions and mixed meal feeding (70 mg protein·kg fat-free mass^–1·h^–1 x 2.5 h) by simultaneous intravenous infusions of [5,5,5-²H₃]leucine and either [ring-¹³C₆]phenylalanine or [ring-²H₅]phenylalanine and analysis of muscle tissue samples by gas chromatography-mass spectrometry. Both the basal FSR and the FSR during feeding were 20% greater (P < 0.001) when calculated from the leucine labeling in muscle tissue fluid and proteins (fasted: 0.063 ± 0.005%/h; fed: 0.080 ± 0.007%/h) than when calculated from the phenylalanine enrichment data (0.051 ± 0.004 and 0.066 ± 0.005%/h, respectively). The feeding-induced increase in the FSR (20%; P = 0.011) was not different with leucine and phenylalanine tracers (P = 0.69). Furthermore, the difference between the leucine- and phenylalanine-derived FSRs was independent of the phenylalanine isotopomer used (P = 0.92). We conclude that when using stable isotope-labeled tracers and the classic precursor product model to measure the rate of muscle protein synthesis, absolute rates of muscle protein FSR differ significantly depending on the tracer amino acid used; however, the anabolic response to feeding is independent of the tracer used. Thus different precursor amino acid tracers cannot be used interchangeably for the evaluation of muscle protein synthesis, and data from studies using different tracer amino acids can be compared qualitatively but not quantitatively.

muscle protein turnover

This article has been cited by other articles:

				G. I. Smith, P. Atherton, D. N. Reeds, B. S. Mohammed, H. Jaffery, D. Rankin, M. J. Rennie, and B. Mittendorfer No major sex differences in muscle protein synthesis rates in the postabsorptive state and during hyperinsulinemia-hyperaminoacidemia in middle-aged adults J Appl Physiol, October 1, 2009; 107(4): 1308 - 1315. [Abstract] [Full Text] [PDF]

				J. E. Tang, J. G. Perco, D. R. Moore, S. B. Wilkinson, and S. M. Phillips Resistance training alters the response of fed state mixed muscle protein synthesis in young men Am J Physiol Regulatory Integrative Comp Physiol, January 1, 2008; 294(1): R172 - R178. [Abstract] [Full Text] [PDF]