A novel calcium sensor stimulating inositol phosphate formation and [Ca2+](i) signaling expressed by GCT23 osteoclast-like cells

Osteoclast activity is inhibited by elevated [Ca2+](o); however. the: underlying molecular mechanism is unknown. We used the human osteoclast-like cells GCT23 to elucidate their cation-sensing properties. Cells responded to elevated [Ca2+](o) with rapid concentration-dependent [Ca2+](i) transients (EC50 = 7.8 mm, time to peak 44 +/- 4 sec) that were due to release from intracellular stores, followed by Ca2+ Influx across the plasma membrane. Ca2+ store depletion by thapsigargin, endothelin-l, or bradykinin activated calcium entry pathways. Cells responded similarly to Ni2+ and Cd2+ with albeit slower kinetics (EC50 <10 mu m and <100 mu m, times to peak 140 +/- 25 sec and 150 +/- 24 sec, respectively). The three cations stimulated inositol phosphate production (two-fold, p < .02) similar to bradykinin (25-fold, p < .002), which activates a phospholipase C (PLC)-coupled receptor in GCT23 cells. The cells did not respond to 0.1-1 mM Gd3+ or neomycin B, indicating that the parathyroid calcium receptor (PCaR) is not functionally expressed. In confirmation, PCaR could not be detected by reverse transcriptase polymerase chain reaction in GCT23 cells and in mouse osteoclasts, and the calcimimetic compound NPS R-568 failed to produce the left shift of the concentration-response curve characteristic for PCaR. Our data demonstrate for the first time that cation sensing by osteoclast-like GCT23 cells is mediated by a PLC-coupled receptor that is not identical to PCaR.