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 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 muM 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 muM-[Ni2+] inhibited subsequent responses to a maximally effective [Ni2+] of 5,000 muM. Ni2+-induced cytosolic [Ca2+] responses were not dependent on extracellular [Ca2+]. Thus, when 5,000 muM-[Ni2+] was applied to osteoclasts in Ca2+-free, ethylene glycol bis-(aminoethyl ether) tetraacetic acid (EGTA)-containing medium (less-than-or-equal-to 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 muM-[Ca2+] and [Mg2+]), even low [Ni2+] ranging between 5 pM and 50 muM elicited progressively larger cytosolic [Ca2+] transients. However, the response magnitude decreased at higher, 250-5,000 muM-[Ni2+], resulting in a ''hooked'' concentration-response curve. Furthermore, increasing extracellular [Mg2+] or [Ca2+] (0-1 MM) diminished the response to 50 muM-[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 muM-[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.