Activation of chondrocytes calcium signalling by dynamic compression is independent of number of cycles

Mechanical loading is necessary for the development and maintenance of healthy articular cartilage through the control of extracellular matrix synthesis and catabolism. However, the underlying process of chondrocyte mechanotransduction remains unclear. This study examined the influence of cyclic compression on intracellular calcium (Ca2+) signalling within isolated articular chondrocytes cultured in agarose constructs. A validated experimental system was developed for applying controlled cyclic cell deformation. Cell-agarose constructs were subjected to 1 Hz cyclic compression between 0 and 10% gross strain for 1, 10, 100 or 300 cycles. The cells were subsequently visualised for 300 s in the unstrained state using confocal microscopy and the Ca2+ indicator, Fluo-4 AM. Within unloaded control constructs, a sub-population of approximately 50% of chondrocytes exhibited characteristic spontaneous Ca2+ transients each lasting approximately 40-60 s. Cyclic compression, for only 1 cycle, significantly up-regulated the percentage of cells exhibiting Ca2+ transients in the subsequent 5 min period (p < 0.05). Increasing the number of cycles to 10 or 100 had no additional effect. The up-regulated Ca2+ signalling was maintained for up to 5 min before returning to basal levels. By contrast, 300 cycles were followed by Ca2+ signalling that was not significantly different from that in unloaded controls. However, this response was shown to be due to the increased time following the start of compression. In conclusion, this study indicates that chondrocyte Ca2+ signalling is stimulated by dynamic compression, probably mediated by cyclic cell deformation. The overall response appears to be independent of the number of cycles or duration of cyclic compression. The sustained up-regulation of Ca2+ signalling after 1, 10 or 100 cycles suggests the involvement of an autocrine-paracrine signalling mechanism. Furthermore, the reduced response following 300 cycles indicates a possible receptor desensitisation mechanism. Therefore, Ca2+ signalling may be part of a mechanotransduction pathway through which chondrocyte populations can modulate their metabolic activity in response to changing mechanical stimuli.