We wished to study adaptations in human musculoskeletal connective tissue in relation to physical activity. We previously developed a method to measure human bone collagen synthesis (Babraj et al. 2002) and have now applied it to the study of collagen turnover in human tendon, ligament and muscle. These studies had the subjects’ informed consent and approval from ethics committees in Tayside and Copenhagen. We studied eight overnight fasted subjects (24 ± 8 years; means ± S.D. throughout); four were undergoing surgical repair of the anterior cruciate ligament. In all subjects, a flooding dose of [1-¹³C]proline (3.75 g of 20 atoms percent) was administered I.V. and blood samples taken for up to 120 min. Four other subjects also received a primed constant infusion of [1-¹³C]leucine (1 mg kg^-1 h^-1), started 2 h before the proline bolus. Surgical biopsies were taken from tendon and ligament immediately after induction of general anaesthesia. Muscle biopsies were taken by the conchotome technique under local anaesthesia (lignocaine 1 %) in the non-surgical subjects. Collagen was isolated from connective tissue and muscle, and myofibrillar and sarcoplasmic proteins were isolated from muscle using standard methods; fractional synthesis rates were calculated from the incorporation of proline into collagen hydroxyproline and incorporation of both leucine and proline into muscle proteins, determined by gas-chromatography-combustion-isotope ratio mass spectrometry. Using leucine or proline, there were no differences between the myofibrillar and sarcoplasmic fractional synthetic rates (grand means: 0.043 ± 0.013 and 0.077 ± 0.008 % h^-1, respectively) obtained, confirming that the flooding dose of proline does not stimulate protein synthesis, an assumption underlying the method. Human muscle collagen had a markedly lower synthesis rate (0.016 ± 0.002 % h^-1) than myofibrillar or sarcoplasmic protein. We also demonstrate for the first time the feasibility of direct measurement of collagen synthesis in human tendon (0.052 ± 0.014 % h^-1) and ligament (0.042 ± 0.004 % h^-1). The synthetic rates were higher than in muscle collagen but similar to those in mature bone collagen (0.04-0.06 % h^-1), indicating substantial collagen metabolic activity. These data suggest that the technique will be applicable to the study of human musculoskeletal connective tissue adaptation.

This work was supported by the UK MRC, The Wellcome Trust and the Danish MRC (22-01-0154).