Activation of the Ca²⁺ “receptor” on the osteoclast by Ni²⁺ elicits cytosolic Ca²⁺ signals: Evidence for receptor activation and inactivation, intracellular Ca²⁺ redistribution, and divalent cation modulation

Earlier studies have demonstrated that a high (mM) extracellular Ca²⁺ concentration triggers intracellular [Ca²⁺] signals with a consequent inhibition of bone resorptive activity. We now report that micromolar concentrations of the divalent cation, Ni²⁺, elicited rapid and concentration‐dependent elevations of cytosolic [Ca²⁺]. The peak change in cytosolic [Ca²⁺] increased monotonically with the application of [Ni²⁺] in the 50–5,000 μM range in solutions containing 1.25 mM‐[Ca²⁺] and 0.8 mM‐[Mg²⁺]. The resulting concentration‐response function suggested Ni²⁺‐induced activation of a single class of binding site (Hill coefficient = 1). The triggering process also exhibited a concentration‐dependent inactivation in which conditioning Ni²⁺ applications in the range 5–1,500 μM‐[Ni²⁺] inhibited subsequent responses to a maximally effective [Ni²⁺] of 5,000 μM. Ni²⁺‐induced cytosolic [Ca²⁺] responses were not dependent on extracellular [Ca²⁺]. Thus, when 5,000 μM‐[Ni²⁺] was applied to osteoclasts in Ca²⁺‐free, ethylene glycol bis‐(aminoethyl ether) tetraacetic acid (EGTA)‐containing medium (≤5 nM‐[Ca²⁺] and 0.8 mM‐[Mg²⁺]), cytosolic [Ca²⁺] responses resembled those obtained in the presence of 1.25 mM‐[Ca²⁺]. Prior depletion of intracellular Ca²⁺ stores by ionomycin prevented Ni²⁺‐induced cytosolic [Ca²⁺] responses, suggesting a major role for intracellular Ca²⁺ redistribution in the response to Ni²⁺. The effects of Ni²⁺ were also modulated by the extracellular concentration of the divalent cations, Ca²⁺ and Mg²⁺. When these cations were not added to the culture medium (0 μM‐[Ca²⁺] and [Mg²⁺]), even low [Ni²⁺] ranging between 5 pM and 50 μM elicited progressively larger cytosolic [Ca²⁺] transients. However, the response magnitude decreased at higher, 250–5,000 μM‐[Ni²⁺], resulting in a “hooked” concentration‐response curve. Furthermore, increasing extracellular [Mg²⁺] or [Ca²⁺] (0–1 mM) diminished the response to 50 μM‐[Ni²⁺], a concentration on the rising phase of the “hook.” Similar increases (0–10 mM) in extracellular [Mg²⁺] or [Ca²⁺] increased the response to 5,000 μM‐[Ni²⁺], a concentration on the falling phase of the “hook”. These findings are consistent with the existence of a membrane receptor strongly sensitive to Ni²⁺ as well as the divalent cations, Ca²⁺ and Mg²⁺. Receptor occupancy apparently activates intracellular Ca²⁺ release followed by inactivation. Furthermore, repriming is independent of intracellular Ca²⁺ stores, suggesting that such inactivation operates at a transduction step between receptor occupancy and intracellular Ca²⁺ release. © 1993 Wiley‐Liss, Inc.