Functional consequences of the interaction of Ni2+ with the osteoclast Ca2+ 'receptor'

Ni2+ was used as an extracellular activator of the Ca2+ 'receptor' in order to study the regulation of osteoclast function in vitro. Application of different micromolar concentrations of Ni2+ to osteoclasts bathed in 1.25 mM [Ca2+] and 0.8 mM [Mg2+] caused a concentration-dependent elevation of cytosolic [Ca2+] measured in single cells using fura-2 fluorescence. Cytosolic [Ca2+] responses to 5 mM [Ni2+] showed a rapidly developing and use-dependent inactivation, unlike those induced by the application of 10 mM [Ca2+]. Pre-treatment with 5 mM [Ni2+] reduced the magnitude of responses to a subsequent extracellular application of 10 mM [Ca2+] and vice versa. Ni2+ treatment elicited a number of functional effects. It produced an inhibition of osteoclastic bone resorption which was sustained over hours. This was associated with a pronounced cell retraction or R effect over the 40 min period following Ni2+ exposure as observed by time-lapse video image analysis. Both these effects varied with concentration. In contrast, granule movement, cell migration, and quantitative indicators of margin ruffling were all unchanged. These findings are consistent with the initiation of a causally related set of specific functional and morphometric events following activation of a specific membrane receptor sensitive to divalent cations.