Background Healthy articular cartilage is important for proper joint function. The impact of unloading due to disease or injury on healthy and diseased articular cartilage is not well understood and bed rest (BR) studies offer a unique and well-standardized model to study the effects of immobilization. Cartilage oligomeric matrix protein (COMP) is a structural protein found in cartilage and is a marker of cartilage turnover . Increased serum concentrations (sCOMP) have been associated with cartilage degradation in osteoarthritis  and rheumatoid arthritis  but also physiological loading [4, 5]. sCOMP decreases with with immobilization .
Objectives To investigate the impact of duration of immobilization (CON) and effectiveness of exercise (INT ex) and nutrition interventions (INTnut) on sCOMP concentration in healthy male subjects during medium-term BR-studies.
Methods Four BR-studies were conducted, each in cross-over design: study 1 (INTex; N=8) had a 14-days of BR, Study 2 (INTnut; N=7), Study 3 (INTnut; N=9) and Study 4 (INTex+nut; N=8) had 21 days of BR. Blood samples were taken after overnight fast. sCOMP concentration was analyzed using commercial enzyme immunoassays (COMP® ELISA, AnaMar Medical AB, Lund, Sweden). Statistical analysis (IBM SPSS Statistics, 19.01) was performed using repeated measures ANOVA (p<0.05).
Results BR lead to a reduction in sCOMP after 24h for all interventions and the control condition (Fig.1). sCOMP recovered to baseline after the end of BR for all conditions (study 1: INTex: +26%, CON: +28.6%; study 2: INTnut: +25%, CON: +25%; study 3: INTnut: +28.9%, CON: + 28.6%; study 4: INTex: +35.0%, INTex+nut: +33.2%, CON: +32.8%). INTnut and INTex did not affect the results. COMP data of 24 individuals from studies 1–3 were re-analyzed for the adaptation of sCOMP during BR. Baseline values of sCOMP decreased by -16% after 24h of BR and further by -5% until BRday 14.
Conclusions sCOMP concentration is sensitive to immobilization. After the distinct initial decrease in sCOMP during the first 24hrs of BR, sCOMP is significantly lowered at BR day 14 compared to the first 24h of BR. These results suggest that beside a reduced diffusion of molecules from the joint, cartilage turnover is decreased during prolonged immobilization. This presumably also negatively affects tissue properties. Thus the combined effects of disease and immobilization on cartilage health in arthritis patients need to be considered and require further investigation. High standardization of blood sampling with regards to mechanical loading is essential for data quality when evaluating sCOMP concentration.
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Disclosure of Interest None declared
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