Divalent cations mimic the inhibitory effect of extracellular ionised calcium on bone resorption by isolated rat osteoclasts: further evidence for a "calcium receptor".

Abstract

Osteoclast activity is thought to be regulated by calcitonin, as well as by the level of ionised calcium generated locally as a result of bone resorption. The exposure of isolated osteoclasts to elevated ambient calcium levels has been shown to lower resorptive activity and to reduce rates of enzyme release. We have attempted to determine whether these effects are mediated by a divalent cation-sensitive "calcium receptor," as has been reported for the parathyroid chief cells. Thus, we compared the effect of alkaline earth metal cations on osteoclast function using a morphometric measure of bone resorption and a spectrophotometric method for measuring the activity of the released enzyme, acid phosphatase. The exposure of resorbing osteoclasts to between 5 and 20 mM extracellular ionised calcium ([Ca2+]e) inhibited bone resorption and enzyme release to an extent similar to that seen with 0.1 to 10 microM ionomycin. The effect of combining submaximal concentrations of [Ca2+]e (15 mM) and ionomycin (0.1 microM) resulted in additivity, suggesting that the influence of [Ca2+]e on bone resorption was mediated by elevated intracellular calcium levels ([Ca2+]i). The other cations studied (Mg2+, Ba2+) were effective and elicited similar effects, although some required higher concentrations. Thus, whilst Ca2+ and Mg2+ were effective at 10 to 15 mM levels, Ba2+ was effective only at high (20 mM) concentrations. These findings are consistent with an influence of [Ca2+]e on osteoclast activity through an action on a surface membrane "calcium receptor" that can also bind other divalent cations, rather than by passive changes of [Ca2+]i with [Ca2+]e elevation.