Influence of tracer selection on protein synthesis rates at rest and postexercise in multiple human muscles

Journal Title (Medline/Pubmed accepted abbreviation): Metabolism- Clinical and Experimental
Year: 2011
Volume: 60
Page numbers:689-697
doi (if applicable): 10.1016/j.metabol.2010.07.003

Summary of background and research design

Background: In order to measure rates of protein synthesis, isotopically-labeled amino acids can be injected and then muscle protein can be analyzed for the incorporation of the isotopes. An isotope is simply a way to label a specific atom so that its fate can be monitored; all isotopes have nearly identical chemical properties to their specific element. Different labs have chosen different amino acids to label for rate of protein synthesis experiments, for example phenylalanine or leucine, and it is unknown whether the results from these labs are comparable to one another.

Research goals: 1) To compare the usage of isotopically-labeled phenylalanine or leucine for measurement of protein synthesis, 2) to analyze the effect of aerobic exercise (45 min at 75% VO2max on a treadmill) on muscle protein synthesis.

Subjects: Eight aerobically trained male subjects, age 26 ± 2 yrs. old

Experimental design: Repeated measures

Protocol: Subjects were evaluated for their VO2maxand then reported to the laboratory 3 times:  1) Measurement of resting rate of muscle protein synthesis, 2) Exercise by running on the treadmill at 75% VO2max for 45 min, 3) Measurement of post-exercise rate of muscle protein synthesis (24 hrs later).
           All trials were performed in the morning after an overnight fast.  To measure the rate of resting protein synthesis, the subjects were infused with deuterated phenylalanine and leucine.  The infusion was primed to achieve equilibrium and then the rate of infusion was 0.05 µmol/min/kg body weight for 2H-Phe (following a prime of 2 µmol/kg) and 0.12 µmol/min/kg body weight for 2H-Leu (following a prime of 4.8 µmol/kg).  Blood was drawn at baseline and every hour for 6 hrs (except hour 1).  At 2 and 6 hrs, 8 total muscle biopsies were obtained from the vastus lateralis (outer quad) and the soleus (part of the calf muscle).  The post-exercise rate of muscle synthesis was performed similarly, but unlabeled amino acids were infused at 2 hrs (1.35 mL/hr/kg after a priming dose of 0.45 mL/kg).  The infusion rate of the labeled amino acids was also doubled and an additional prime was injected to maintain the isotope concentration. 

Summary of research findings
  • Both labeled Phe and Leu yielded the same results for rate of resting muscle protein synthesis: 0.080 ± 0.007%/hr for Phe and 0.085 ± 0.004%/hr for Leu in the vastus lateralis and 0.086 ± 0.008%/hr for Phe and 0.094 ± 0.008%/hr for Leu in the soleus.
  • Protein synthesis occurred at a faster rate after exercise in both muscles: 0.110 ± 0.010%/hr for Phe and 0.109 ± 0.005%/hr for Leu in the vastus lateralis and 0.080 ± 0.007%/hr for Phe and 0.123 ± 0.008%/hr for Leu in the vastus lateralis.  These values are statistically significant when comparing resting to post-exercise but not when comparing between the tracers or muscles.

Interpretation of findings/Key practice applications

  • Different laboratories use different protocols to study rate of muscle synthesis.  It appears that deuterated Phe or Leu report similar rates and, therefore, rates reported by these two tracers, assuming other aspects of the protocol are the same, could be comparable. 
  • Resistance exercise has clearly shown to increase the rate of muscle synthesis 24 hrs after exercise; this study shows that endurance exercise also enhances protein synthesis the next day.


It was not explicitly stated why the authors infused unlabeled amino acids during the post-exercise protein synthesis assessment but not the resting assessment. The higher concentrations of amino acids in the blood could possibly, independently, lead to an increase in muscle protein synthesis. In addition, depending on the specific amino acid profile of the amino acid solution, it is possible that a bias could be introduced regarding the protein synthesis rates that were determined with each tracer (e.g., the amino acid solution might have relatively more leucine than phenylalanine, or vice versa).
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