INTERMITTENT CYCLIC LOADING OF CARTILAGE EXPLANTS MODULATES FIBRONECTIN METABOLISM

Objective: The aim of this study was to evaluate systematically the ef fect of tissue load, its amplitude, time of intermittence and duration of loading on the biosynthesis and release of fibronectin by intermit tently loaded mature bovine articular cartilage explants. Methods: Cyc lic compressive pressure was introduced using a sinusoidal waveform of 0.5 Hz-frequency with a peak stress of 0.1, 0.5 or 1.0 MPa for a peri od of 10 s followed by an unloaded period lasting 10, 100 or 1000 s. F ibronectin and total proteins were radiolabeled with 10 mu Ci/ml [H-3] -phenylalanine during the final 18 h of the 1, 3 or 6 day experiments. The content of endogenous fibronectin was determined using enzyme-lin ked immunosorbant assay (ELISA), whereas the viability of explants was measured using sections of cartilage explants stained with fluorescei n diacetate and propidium iodide. The deformation of loaded explants w as determined using a load-displacement transducer system. Results: Th e mechanical factor time of intermittence significantly altered the sy nthesis and release of fibronectin by cartilage explants, whereas the tested range of load magnitudes, as well as the duration of loading, s eemed to be of subordinate importance. Loading affected the viability of the superficial zone in the cartilage, whereas the chondrocytes of the intermediate and deep zone remained viable. The compression of loa ded explants was dependent on the magnitude of stress, as well as on t he duration of unloading between each loading cycle. Synthesis of fibr onectin, the retention of newly synthesized fibronectin within the ext racellular matrix, and the portion of newly synthesized proteins that were fibronectin was significantly increased in cartilage explants whi ch were cyclically compressed with 0.5 MPa for 10 s followed by a peri od of unloading lasting 100 s. Conclusions: Previous studies reporting that cartilage explants of human and animal osteoarthritic joints syn thesize and retain elevated amounts of fibronectin imply that in our e xperiments mechanical stimuli can induce a fibronectin metabolism in v itro which mimics some of the osteoarthritic characteristics.