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1Washington University, School of Medicine, St Louis, MO, USA 2Copenhagen Muscle Research Center, Copenhagen, Denmark 3University of Dundee, School of Life Sciences, Dundee, UK 4University of Nottingham, Medical School, Derby, UK
In many animals the rate of protein synthesis is higher in slow-twitch, oxidative than fast-twitch, glycolytic muscles. To discover if muscles in the human body also show such differences, we measured [13C]leucine incorporation into proteins of anatomically distinct muscles of markedly different fibre-type composition (vastus lateralis, triceps, soleus) after an overnight fast and during infusion of a mixed amino acid solution (75 mg amino acids kg–1 h–1) in nine healthy, young men. Type-1 fibres contributed 83 ± 4% (mean ±
S.E.M.) of total fibres in soleus, 59 ± 3% in vastus lateralis and 22 ± 2% in triceps. The basal myofibrillar and sarcoplasmic protein fractional synthetic rates (FSR, % h–1) were 0.034 ± 0.001 and 0.064 ± 0.001 (soleus), 0.031 ± 0.001 and 0.060 ± 0.001 (vastus), and 0.027 ± 0.001 and 0.055 ± 0.001 (triceps). During amino acid infusion, myofibrillar protein FSR increased to 3-fold, and sarcoplasmic to 2-fold basal values (P < 0.001). The differences between muscles, although significant statistically (triceps versus soleus and vastus lateralis, P < 0.05), were within 
15%, biologically probably insignificant. The rates of collagen synthesis were not affected by amino acid infusion and varied by < 5% between muscles and experimental conditions.
(Received 12 October 2004;
accepted after revision 8 December 2004;
first published online 20 December 2004)
Corresponding author B. Mittendorfer: Washington University School of Medicine, Division of Geriatrics & Nutritional Sciences, 660 South Euclid Avenue; Campus Box 8031, St Louis, MO 63110, USA. Email: mittendb{at}wustl.edu
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