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: under lying molecular mechanism is unknown. We used the human osteoclast-like cel ls GCT23 to elucidate their cation-sensing properties. Cells responded to e levated [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 int racellular stores, followed by Ca2+ Influx across the plasma membrane. Ca2 store depletion by thapsigargin, endothelin-l, or bradykinin activated cal cium 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 p hosphate production (two-fold, p < .02) similar to bradykinin (25-fold, p < .002), which activates a phospholipase C (PLC)-coupled receptor in GCT23 c ells. 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 polym erase chain reaction in GCT23 cells and in mouse osteoclasts, and the calci mimetic compound NPS R-568 failed to produce the left shift of the concentr ation-response curve characteristic for PCaR. Our data demonstrate for the first time that cation sensing by osteoclast-like GCT23 cells is mediated b y a PLC-coupled receptor that is not identical to PCaR.