Mechanical compression modulates matrix biosynthesis in chondrocyte/agarose culture

Abstract

This study focuses on the effect of static and dynamic mechanical compression on the biosynthetic activity of chondrocytes cultured within agarose gel. Chondrocyte/agarose disks (3 mm diameter) were placed between impermeable platens and subjected to uniaxial unconfined compression at various times in culture (2-43 days). [35S]sulfate and [3H]proline radiolabel incorporation were used as measures of proteoglycan and protein synthesis, respectively. Graded levels of static compression (up to 50%) produced little or no change in biosynthesis at very early times, but resulted in significant decreases in synthesis with increasing compression amplitude at later times in culture; the latter observation was qualitatively similar to that seen in intact cartilage explants. Dynamic compression of approximately 3% dynamic strain amplitude (approximately equal to 30 microns displacement amplitude) at 0.01-1.0 Hz, superimposed on a static offset compression, stimulated radiolabel incorporation by an amount that increased with time in culture prior to loading as more matrix was deposited around and near the cells. This stimulation was also similar to that observed in cartilage explants. The presence of greater matrix content at later times in culture also created differences in biosynthetic response at the center versus near the periphery of the 3 mm chondrocyte/agarose disks. The fact that chondrocyte response to static compression was significantly affected by the presence or absence of matrix, as were the physical properties of the disks, suggested that cell-matrix interactions (e.g. mechanical and/or receptor mediated) and extracellular physicochemical effects (increased [Na+], reduced pH) may be more important than matrix-independent cell deformation and transport limitations in determining the biosynthetic response to static compression. For dynamic compression, fluid flow, streaming potentials, and cell-matrix interactions appeared to be more significant as stimuli than the small increase in fluid pressure, altered molecular transport, and matrix-independent cell deformation. The qualitative similarity in the biosynthetic response to mechanical compression of chondrocytes cultured in agarose gel and chondrocytes in intact cartilage further indicates that gel culture preserves certain physiological features of chondrocyte behavior and can be used to investigate chondrocyte response to physical and chemical stimuli in a controlled manner.