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
✍ Scribed by Vijai S. Shankar; Christopher M. R. Bax; Bridget E. Bax; A. S. M. Towhidul Alam; Baljit S. Moonga; Bruce Simon; Michael Pazianas; Christopher L.-H. Huang; Mone Zaidi
- Publisher
- John Wiley and Sons
- Year
- 1993
- Tongue
- English
- Weight
- 1011 KB
- Volume
- 155
- Category
- Article
- ISSN
- 0021-9541
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✦ Synopsis
Earlier studies have demonstrated that a high (mM) extracellular CaL+ concentration triggers intracellular [Ca"] signals with a consequent inhibition of bone resorptive activity. We now report that micromolar concentrations of the divalent cation, Ni'+, elicited rapid and concentration-dependent elevations of cytosolic [Ca"]. The peak change in cytosolic [Ca2+] increased monotonically with the application of [Ni"] in the 50-5,000 pM range in solutions containing 1.25 mM-[Ca'+] 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 FM-LNi' ' I inhibited subsequent responses to a maximally effective [Ni2+l of 5,000 pM. Ni2 ' -induced cytosolic [Ca'+] responses were not dependent on extracellular [Ca"]. Thus, when 5,000 pM-[Ni'+] was applied to osteoclasts in Ca'+-free, ethylene glycol bis-(aminoethyl ether) tetraacetic acid (EGTA)-containing medium ( a 5 nM-[Ca'+] and 0.8 mM-[Mg2']), cytosolic [Ca"] responses resembled those obtained in the presence of 1.25 mM-ICa'+]. Prior depletion of intracellular Ca2+
stores by ionomycin prevented Ni'+-induced cytosolic [Ca'+] responses, suggesting a major role for intracellular Ca'+ redistribution in the response to Ni'+. The effects of Ni'+ were also modulated by the extracellular concentration of the divalent cations, CaL+ and Mg'+. When these cations were not added to the culture medium (0 pM-[CaZf] and [MgZ+l), even low [NiL+l ranging between 5 pM and 50 pM elicited progressively larger cytosolic [Ca2+] transients. However, the response magnitude decreased at higher, 250-5,000 pM-[Ni2+], resulting in a "hooked" concentration-response curve. Furthermore, increasing extracellular [Mg2*1 or [Ca' ' 1 (0-1 mM) diminished the response to 50 pM-[NiZ+], a concentration on the rising phase of the "hook." Similar increases (0-1 0 mM) in extracellular lMg2'] or [Ca"] increased the response to 5,000 pM-INi'+l, a concentration on the falling phase of the "hook. These findings are consistent with the existence of a membrane receptor strongly sensitive to Ni' ' as wet1 as the divalent cations, Ca2+ and MgL+. Receptor occupancy apparently activates intracellular Ca" 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.