To study the effect of a continuous cyclic mechanical load on the release of newly synthesized proteoglycans (PGs) from mature bovine articular cartilage.
Viable cartilage explants were continuously loaded with 1 MPa cyclic stress at 1 Hz frequency for 24 h, and the release of labeled (35SO4) PGs measured before, during and after application of the compressive load. To separate the effect of active chondrocyte catabolism from that of passive PG release, PG release in live explants, with and without protease inhibitors to inhibit PG breakdown, was compared to PG release in explants whose chondrocytes were killed prior to loading.
In live explants, a continuous cyclic load significantly reduced PG release by as much as 50% compared to unloaded explants. In killed explants which were unloaded, the PG release increased five to 10 times, while a cyclic load reduced PG release to that found in viable, loaded explants. Twenty-four hours after load removal PG release in all loaded explants returned (increased) to that of the unloaded explants.
These results indicate that PG release from the cartilage matrix is inhibited by continuous cyclic mechanical loading, independent of cellular metabolism, and suggest that a primary mechanism for reducing PG release is by decreasing the interstitial porosity through which the PGs can escape.