High extracellular calcium concentrations directly stimulate osteoclast apoptosis.

Although the inhibitory effects of high extracellular calcium concentrations (Ca) on osteoclastic bone resorption have been known for several years, the exact mechanism remains poorly understood. The present study was performed to investigate the possible effect of Ca on osteoclast apoptosis. Using highly purified rabbit osteoclasts, we have shown that calcium directly promotes apoptosis in a dose-dependent manner which correlates with the dose range of calcium for the inhibition of bone resorption. A time-course experiment of apoptotic changes of osteoclasts cultured in presence of 1.8 or 20 mM calcium showed a significant difference after as early as 8 h of culture. After 72 h of culture, we observed that 80% of the cells cultured in the presence of 20 mM calcium displayed the typical features of apoptosis compared to only 20% in the medium containing 1.8 mM calcium. Calcium channel blockers and ryanodine abrogated the effects of Ca on apoptosis while neomycin, a calcium-sensing receptor agonist, did not alter cell viability. Taken together, these results suggest that calcium influx is involved in calcium-induced osteoclast apoptosis. Our results are consistent with the concept that in the presence of high Ca generated during bone demineralization, osteoclasts are subjected to negative-feedback regulation due, at least in part, to the induction of apoptosis.