DIFFERENT CALCIUM SENSITIVITY IN OSTEOCLASTS ON GLASS AND ON BONE ANDMAINTENANCE OF CYTOSKELETAL STRUCTURES ON BONE IN THE PRESENCE OF HIGH EXTRACELLULAR CALCIUM

The sensitivity of rat osteoclasts to increased extracellular calcium concentrations ([Ca2+](e)) was investigated by single cell measurement s of free cytosolic calcium concentrations ([Ca2+](i)), by changes in microfilament organization of resorbing osteoclasts, and by in vitro b one resorption assays. Osteoclasts cultured on glass and on bone showe d clear differences in their responses, as in 44% and 52% of osteoclas ts on glass but in only 21% and 25% of osteoclasts on bone [Ca2+](i) i ncreased when [Ca2+](e) was increased from 2 mM to 6 or 10 mM via perf usion, respectively. Bone resorption was inhibited without changes in the osteoclast numbers only by 10 mM [Ca2+](e) in 2 day cultures. Furt hermore, there were no changes in the organization of microfilament st ructures in resorbing osteoclasts after increased [Ca2+](e) (up to 20 mM [Ca2+](e), 30 min incubation). These results suggest that the sensi tivity of osteoclasts to increased [Ca2+](e) is dependent on their act ivation phase (resting/migrating vs. resorbing) and that resorbing ost eoclasts are not sensitive to increased [Ca2+](e) or that the sensing system cannot be reached in polarized resorbing osteoclasts. In contra st, increasing [Ca2+](i) through the use of calcium ionophores dispers ed specific microfilament structures at the sealing zone transiently i n a few minutes. This shows that [Ca2+](i) is used as a signaling mech anism to inactivate osteoclasts, with a similar end result on microfil ament structures at the sealing zone as caused by increased concentrat ion of cAMP and activation of protein kinase C. (C) 1996 Wiley-Liss, I nc.