High extracellular calcium concentrations directly stimulate osteoclast apoptosis

Although the inhibitory effects of high extracellular calcium concentrations (Ca](e)) 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](e) 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](e) 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](e) generated during bone demineralization, osteoclasts are subjected to negative-feedback regulation due, at least in part, to the induction of apoptosis.