A reappraisal of the effect of extracellular calcium on osteoclastic bone resorption

During bone resorption osteoclasts are exposed to high levels of extracellular calcium solubilized from bone mineral and it has been suggested that this may act as a physiological negative feedback to control the resorptive process. We have confirmed that calcium (1.5-20 mM) dose-dependently inhibits osteoclastic bone resorption when added at the start (t = 0 hr) of the 24 hr bone slice assay. When 20 mM calcium was added at t = 0, 1, 3 or 6 hr after osteoclast attachment to bone slices, resorption was inhibited by 100%, 100%, -10% and 6% respectively. In contrast, human calcitonin (1 ng/ml) inhibited bone resorption by 100%, 100%, 91% and 52% when added at t = 0, 1, 3 and 6 hr respectively. Osteoclasts were not seen on bone slices after 24 hr incubation when 20 mM calcium was added at t = 0 or 1 hr, but when calcium was added at t = 3 or 6 hr osteoclast numbers were similar to controls, indicating that 20 mM calcium is not toxic to osteoclasts. Human calcitonin did not significantly affect osteoclast numbers regardless of time of addition to the bone slice assay. The absence of osteoclasts on bone slices exposed to 20 mM calcium at early time points indicates that high levels of extracellular calcium prevent osteoclast adhesion to bone slices, and later addition of high Ca(e) to the assay does not inhibit ongoing osteoclastic bone resorption.