High extracellular calcium concentrations directly stimulate osteoclast apoptosis

Although the inhibitory effects of high extracellular calcium concentration s ([Ca](e)) on osteoclastic bone resorption have been known for several yea rs, the exact mechanism remains poorly understood. The present study was pe rformed to investigate the possible effect of [Ca](e), on osteoclast apopto sis. Using highly purified rabbit osteoclasts, we have shown that calcium d irectly promotes apoptosis in a dose-dependent manner which correlates with the dose range of calcium for the inhibition of bone resorption. A time-co urse 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 cultu red in the presence of 20 mM calcium displayed the typical features of apop tosis compared to only 20% in the medium containing 1.8 mM calcium. Calcium channel blockers and ryanodine abrogated the effects of [Ca](e) on apoptos is while neomycin, a calcium-sensing receptor agonist, did not alter cell v iability. Taken together, these results suggest that calcium influx is invo lved in calcium-induced osteoclast apoptosis. Our results are consistent wi th the concept that in the presence of high [Ca](e) generated during bone d emineralization, osteoclasts are subjected to negative-feedback regulation due, at least in part, to the induction of apoptosis. (C) 2000 Academic Pre ss.