Cell swelling as an intermediate signal leading to activation of a Cl- channel in extracellular calcium-sensing of murine osteoclasts.

Osteoclasts possess extracellular Ca2+-sensing machinery to regulate bone-resorbing activity. In murine osteoclasts, an elevation of the extracellular Ca2+ concentration activated a volume-sensitive outwardly rectifying Cl- (OR(Cl)) channel. Exposure to 40 mM Ca2+ activated the OR(Cl) current in association with an increase in the planar cell area. An actin-destabilizer (cytochalasin D), removal of a major extracellular osmolyte (Na+), and a blocker for the Na+/Ca2+ exchanger (2'4'-dichlorobenzamil hydrochloride) inhibited both swelling and activation of the OR(Cl) channel by extracellular Ca2+. There was a positive correlation between the current density and increment in the cell area. The extracellular Ca2+-induced swelling was confirmed in intact (unclamped) cells by three-dimensional analysis using confocal scanning microscopy with a fluorescent dye (BCECF) in the extracellular medium. These results suggest that swelling is an intermediate signal for extracellular Ca2+ sensing of osteoclasts, which leads to activation of the OR(Cl) channel. Cell volume may be a second-message in the regulation of osteoclast function.