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Human soleus and vastus lateralis muscle protein metabolism with an amino acid infusion

¹Nutrition, Metabolism, and Exercise Laboratory, DWR Department of Geriatrics, ²Department of Physiology and Biophysics, University of Arkansas for Medical Sciences; and ³Central Arkansas Veterans HealthCare System, Little Rock, Arkansas

Submitted 25 August 2004 ; accepted in final form 19 October 2004

The calf muscles, compared with the thigh, are less responsive to resistance exercise in ambulatory and bed-rested individuals, apparently due to muscle-specific differences in protein metabolism. We chose to evaluate the efficacy of using amino acids to elevate protein synthesis in the soleus, because amino acids have been shown to have a potent anabolic effect in the vastus lateralis. Mixed muscle protein synthesis in the soleus and vastus lateralis was measured before and after infusion of mixed amino acids in 10 individuals (28 ± 1 yr). Phosphorylation of ribosomal protein p70 S6 kinase (p70^S6K; Thr³⁸⁹) and eukaryotic initiation factor 4E-binding protein-1 (4E-BP1; Thr^37/46) was also evaluated at rest and after 3 h of amino acid infusion. Basal protein synthesis was similar (P = 0.126), and amino acids stimulated protein synthesis to a similar extent (P = 0.004) in the vastus lateralis (0.043 ± 0.011%/h) and soleus (0.032 ± 0.017%/h). Phosphorylation of p70^S6K (P = 0.443) and 4E-BP1 (P = 0.192) was not increased in either muscle; however, the soleus contained more total (P = 0.002) and phosphorylated (P = 0.013) 4E-BP1 than the vastus lateralis. These data support the need for further study of amino acid supplementation as a means to compensate for the reduced effectiveness of calf resistance exercise in ambulatory individuals and those exposed to extended periods of unloading. The greater 4E-BP1 in the soleus suggests that there is a muscle-specific distribution of general translational initiation machinery in human skeletal muscle.

protein synthesis; ribosomal protein p70 S6 kinase; eukaryotic initiation factor 4E-binding protein-1; unloading; calf muscles

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