Activation of the Ca 2+ “receptor” on the osteoclast by Ni 2+ elicits cytosolic Ca 2+ signals: Evidence for receptor activation and inactivation, intracellular Ca 2+ redistribution, and divalent cation modulation

Earlier studies have demonstrated that a high (mM) extracellular Ca 2+ concentration triggers intracellular [Ca 2+ ] signals with a consequent inhibition of bone resorptive activity. We now report that micromolar concentrations of the divalent cation, Ni 2+ , elicited rapid and concentration‐dependent elevations of cytosolic [Ca 2+ ]. The peak change in cytosolic [Ca 2+ ] increased monotonically with the application of [Ni 2+ ] in the 50–5,000 μM range in solutions containing 1.25 mM‐[Ca 2+ ] and 0.8 mM‐[Mg 2+ ]. The resulting concentration‐response function suggested Ni 2+ ‐induced activation of a single class of binding site (Hill coefficient = 1). The triggering process also exhibited a concentration‐dependent inactivation in which conditioning Ni 2+ applications in the range 5–1,500 μM‐[Ni 2+ ] inhibited subsequent responses to a maximally effective [Ni 2+ ] of 5,000 μM. Ni 2+ ‐induced cytosolic [Ca 2+ ] responses were not dependent on extracellular [Ca 2+ ]. Thus, when 5,000 μM‐[Ni 2+ ] was applied to osteoclasts in Ca 2+ ‐free, ethylene glycol bis ‐(aminoethyl ether) tetraacetic acid (EGTA)‐containing medium (≤5 nM‐[Ca 2+ ] and 0.8 mM‐[Mg 2+ ]), cytosolic [Ca 2+ ] responses resembled those obtained in the presence of 1.25 mM‐[Ca 2+ ]. Prior depletion of intracellular Ca 2+ stores by ionomycin prevented Ni 2+ ‐induced cytosolic [Ca 2+ ] responses, suggesting a major role for intracellular Ca 2+ redistribution in the response to Ni 2+ . The effects of Ni 2+ were also modulated by the extracellular concentration of the divalent cations, Ca 2+ and Mg 2+ . When these cations were not added to the culture medium (0 μM‐[Ca 2+ ] and [Mg 2+ ]), even low [Ni 2+ ] ranging between 5 pM and 50 μM elicited progressively larger cytosolic [Ca 2+ ] transients. However, the response magnitude decreased at higher, 250–5,000 μM‐[Ni 2+ ], resulting in a “hooked” concentration‐response curve. Furthermore, increasing extracellular [Mg 2+ ] or [Ca 2+ ] (0–1 mM) diminished the response to 50 μM‐[Ni 2+ ], a concentration on the rising phase of the “hook.” Similar increases (0–10 mM) in extracellular [Mg 2+ ] or [Ca 2+ ] increased the response to 5,000 μM‐[Ni 2+ ], a concentration on the falling phase of the “hook”. These findings are consistent with the existence of a membrane receptor strongly sensitive to Ni 2+ as well as the divalent cations, Ca 2+ and Mg 2+ . Receptor occupancy apparently activates intracellular Ca 2+ release followed by inactivation. Furthermore, repriming is independent of intracellular Ca 2+ stores, suggesting that such inactivation operates at a transduction step between receptor occupancy and intracellular Ca 2+ release. © 1993 Wiley‐Liss, Inc.