Time-dependent effects of intermittent hydrostatic pressure on articular chondrocyte type II collagen and aggrecan mRNA expression

The normal loading of joints during daily activities causes the articular c artilage to be exposed to high levels of intermittent hydrostatic pressure. This study quantified effects of intermittent hydrostatic pressure on expr ession of mRNA for important extracellular matrix constituents. Normal adul t bovine articular chondrocytes were isolated and tested in primary culture , either as high-density monolayers or formed aggregates. Loaded cells were exposed to 10 MPa of intermittent hydrostatic pressure at a frequency of 1 Hz for periods of 2, 4, 8, 12, and 24 hrs. Other cells were intermittently loaded for a period of 4 hrs per day for 4 days. Semiquantitative reverse transcription polymerase chain reaction assays were used to assess mRNA sig nal levels for collagen types II and I and aggrecan. The results showed tha t type II collagen mRNA signal levels exhibited a biphasic pattern, with an initial increase of approximately five-fold at 4 and 8 hrs that subsequent ly decreased by 24 hrs. In contrast, aggrecan mRNA signal increased progres sively up to three-fold throughout the loading period. Changing the loading profile to 4 hrs per day for 4 days increased the mRNA signal levels for t ype II collagen nine-fold and for aggrecan twenty-fold when compared to unl oaded cultures. These data suggest that specific mechanical loading protoco ls may be required to optimally promote repair and regeneration of diseased joints.