Effect of compressive loading and unloading on the synthesis of total protein, proteoglycan, and fibronectin by canine cartilage explants.

Full-thickness canine articular cartilage explants were subjected to compressive loads equivalent to a uniaxial stress of 0.025-1.2 MPa. A single cycle (18 h) of unconfined compression resulted in inhibition of total protein, proteoglycan, and fibronectin synthesis. The inhibition of fibronectin synthesis followed that of total protein synthesis. The magnitude of inhibition increased nonlinearly with increasing load levels. The signal that depressed synthesis remained effective for several hours after removal of load, but by 24 h proteoglycan synthesis had partially recovered and fibronectin and protein synthesis had fully recovered and sometimes exceeded the rate of synthesis in free-swelling controls. Forty-eight hours after five cycles of intermittent unconfined compression with similar loads, proteoglycan content and synthesis did not differ in loaded disks and in disks that were never loaded in vitro. Interestingly, the percentage of water in disks that had never been loaded in vitro increased significantly after 10 days in culture, relative to the percentage of water in free-swelling disks on the day of harvest. Intermittent compressive loading in the range of 0.5-1.2 MPa partially prevented this increase. Our results confirmed the previously reported inhibition of biosynthesis with static loading but also suggested that exposure to intermittent compressive loading may help to maintain the normal ratio of dry to wet weight in the explant.