Protein synthesis rates in human muscles: neither anatomical location nor fibre-type composition are major determinants.

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 (tricepsversussoleus 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. [ABSTRACT FROM AUTHOR]