J Babraj

Coordinated collagen and muscle protein synthesis in human patella tendon and quadriceps muscle after exercise

We hypothesized that an acute bout of strenuous, non‐damaging exercise would increase rates of protein synthesis of collagen in tendon and skeletal muscle but these would be less than those of muscle myofibrillar and sarcoplasmic proteins. Two groups (n= 8 and 6) of healthy young men were studied over 72 h after 1 h of one‐legged kicking exercise at 67% of maximum workload (Wmax). To label tissue proteins in muscle and tendon primed, constant infusions of [1‐13C]leucine or [1‐13C]valine and flooding doses of [15N] or [13C]proline were given intravenously, with estimation of labelling in target proteins by gas chromatography–mass spectrometry. Patellar tendon and quadriceps biopsies were taken in exercised and rested legs at 6, 24, 42 or 48 and 72 h after exercise. The fractional synthetic rates of all proteins were elevated at 6 h and rose rapidly to peak at 24 h post exercise (tendon collagen (0.077% h−1), muscle collagen (0.054% h−1), myofibrillar protein (0.121% h−1), and sarcoplasmic protein (0.134% h−1)). The rates decreased toward basal values by 72 h although rates of tendon collagen and myofibrillar protein synthesis remained elevated. There was no tissue damage of muscle visible on histological evaluation. Neither tissue microdialysate nor serum concentrations of IGF‐I and IGF binding proteins (IGFBP‐3 and IGFBP‐4) or procollagen type I N‐terminal propeptide changed from resting values. Thus, there is a rapid increase in collagen synthesis after strenuous exercise in human tendon and muscle. The similar time course of changes of protein synthetic rates in different cell types supports the idea of coordinated musculotendinous adaptation.

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 (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.

Human bone collagen synthesis is a rapid, nutritionally modulated process

We developed a direct assay of human bone collagen synthesis using [13C] or [15N] proline and applied it to determine the effects of feeding in young healthy men. Surprisingly, postabsorptive bone collagen synthesis is not sluggish, being ∼0.07%/h more rapid than that of muscle protein, and capable of being stimulated within 4 h of intravenous feeding by 66 ± 13%.