The influence of mechanical loading on isolated chondrocytes seeded in agarose constructs

Articular cartilage is subjected to dynamic compressive loading during normal activity which influences chondrocyte metabolism through various mechanotransduction pathways. A well characterised and reproducible model system, involving chondrocytes embedded in agarose gel, has been used to investigate the effects of mechanical compression on chondrocytes, isolated from full depth cartilage or separately from the superficial and deep zone tissue. The role of nitric oxide as a mediator of mechanical-induced effects has also been studied. Chondrocytes were isolated, separately, from full depth, superficial and deep zone cartilage and seeded in 3% agarose constructs. Dynamic compressive strain was applied to the constructs using a range of frequencies (0.3, 1 and 3 Hz). Glycosaminoglycan synthesis, cell proliferation and nitrite production were assessed. In further experiments, constructs were compressed in the presence of 1 mM L-NAME or 10 microM dexamethasone. Glycosaminoglycan synthesis by full depth chondrocytes was affected by compressive strain in a frequency dependent manner. Dynamic strain at all frequencies induced an increase in [3H]-thymidine incorporation. Glycosaminoglycan synthesis by deep zone cells was affected by the strain regimes in a similar fashion to full depth cells, while superficial cells exhibited a similar proliferative response to full depth cells. Dynamic compression inhibited nitrite production, the effect being reversed by L-NAME. Compression induced stimulation of [3H]-TdR incorporation was reversed by L-NAME. These studies demonstrate that glycosaminoglycan synthesis and proliferation are influenced by the dynamic strain regimes in a distinct manner. Indeed the data suggest that these processes occur in different chondrocyte sub-populations. It may be speculated that nitric oxide acts as a mediator of mechanotransduction processes affecting proliferation primarily in the superficial cell sub-population.