High Extracellular Ca2+ and Ca2+-Sensing Receptor Agonists Activate Nonselective Cation Conductance in Freshly Isolated Rat Osteoclasts

The effects of an increase of extracellular and intracellular Ca2+ on the membrane properties were examined in freshly isolated rat osteoclasts using the perforated patch-clamp method. Spread-type osteoclasts plated on a cover glass predominantly displayed an inwardly rectifying K+ current in a normal saline solution. Application of an extracellular high-Ca2+ solution transiently increased the membrane conductance in 15 (71%) of 21 osteoclasts. The external high Ca2+-induced current reversed at the membrane potential of −4.8 ± 2.4 mV (n = 8). The change of intracellular Cl− concentration did not affect the reversal potential, suggesting that the response was due to a nonselective cation conductance. Application of a calcium ionophore, ionomycin (3 μmol/L), continuously increased the membrane conductance, and the reversal potential was −12.5 ± 5.0 mV (n = 5). Extracellularly applied neomycin (100 μmol/L) and Gd3+ (100 μmol/L), which are agonists of Ca2+-sensing receptor (CaR), also increased the membrane conductance. These results suggest that rat osteoclasts detect high extracellular Ca2+ by an extracellular Ca2+-sensing mechanism functionally similar to the CaR in the cell surface, release Ca2+ from the internal stores, and display the activation of Ca2+-dependent cation channels in the cell membrane.