Timing of the initial muscle biopsy does not affect the measured muscle protein fractional synthesis rate during basal, postabsorptive conditions

Gordon I. Smith, Dennis T. Villareal, Charles P. Lambert, Dominic N. Reeds, B. Selma Mohammed, Bettina Mittendorfer

Research output: Contribution to journalArticlepeer-review

18 Scopus citations

Abstract

The muscle protein fractional synthesis rate (FSR) is determined by monitoring the incorporation of an amino acid tracer into muscle protein during a constant-rate intravenous tracer infusion. Commonly two sequential muscle biopsies are obtained some time after starting the tracer infusion. However, other protocols, including those with an initial biopsy before starting the tracer infusion to measure the background enrichment and those with only a single biopsy after several hours of tracer infusion have been used. To assess the validity of these approaches, we compared the muscle protein FSR obtained by calculating the difference in [ring 2H 5]phenylalanine and [5,5,5- 2H 3]leucine incorporation into muscle protein at ∼3.5 h after starting the tracer infusion and 1) at 60 min; 2) before starting the tracer infusion (background enrichment); 3) a population average muscle protein background enrichment; and 4) by measuring the tracer incorporation into muscle protein at ∼3.5 h assuming essentially no background enrichment. Irrespective of the tracer used, the muscle protein FSR calculated from the difference in the muscle protein labeling several hours after starting the tracer infusion and either the labeling at 60 min or the background enrichment were not different (e.g., 0.049 ± 0.007%/h vs. 0.049 ± 0.007%/h, respectively, with [ 2H 5] phenylalanine; P = 0.99). However, omitting the initial biopsy and assuming no background enrichment yielded average FSR values that were ∼15% (with [ 2H 5]phenylalanine) to 80% (with [ 2H 3]leucine) greater (P ≤ 0.059); using a population average background enrichment reduced the difference to ∼3% (P = 0.76) and 22% (P = 0.52) with 2H 5]phenylalanine and [ 2H 3]Ieucine, respectively. We conclude that during basal, postabsorptive conditions, valid muscle protein FSR values can be obtained irrespective of the timing of the initial biopsy so long as the protein labeling in two sequential biopsies is measured whereas the single biopsy approach should be avoided.

Original languageEnglish
Pages (from-to)363-368
Number of pages6
JournalJournal of Applied Physiology
Volume108
Issue number2
DOIs
StatePublished - Feb 2010

Keywords

  • Amino acid
  • Muscle protein turnover

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