Calcium signaling as a novel method to optimize the biosynthetic response of chondrocytes to dynamic mechanical loading

Chondrocyte sensitization and desensitization to mechanical stimuli are complex phenomena that have not been fully described. In this study, we investigated the temporal response of chondrocytes to dynamic mechanical loading and whether changes in calcium signaling could be used a predictor of the biosynthetic response. Cell-seeded agarose gels pre-incubated with an intracellular Ca2+ dye (Fluo-4) were subjected to dynamic compressive loading under varying conditions (amplitude and duration). Induced changes in Ca2+ signaling were determined by confocal imaging and matrix biosynthesis by radioisotope incorporation. It was observed that chondrocytes required a minimum amount of stimulation in order to elicit an anabolic response and they quickly became insensitive to the imposed stimulus. The response appeared to be amplitude dependent and could be predicted by measuring resultant changes in Ca2+ signaling. A positive correlation between Ca2+ signaling and matrix synthesis was achieved when changes in Ca2+ signaling was expressed as a relative number of cells experiencing multiple transients. In addition, these changes in Ca2+ signaling were effective at determining optimal recovery period between successive applications of intermittent mechanical loading, in which full mechanosensitivity was achieved when Ca2+ signaling was allowed to return to baseline (control) levels. The use of Ca2+ signaling to predict the effectiveness of a particular mechanical stimulus as well as to determine optimal refractory periods appears to be advantageous over empirical-based approaches. Future work will investigate the process of Ca2+ ion sequestration into intracellular stores to elucidate potential desensitization mechanisms to dynamic mechanical loading.