Strontium potently inhibits mineralisation in bone-forming primary rat osteoblast cultures and reduces numbers of osteoclasts in mouse marrow cultures

The basic mechanisms by which strontium ranelate acts on bone are still unclear. We show that an important action of strontium salts is to block calcification in cultures of osteoblasts, the bone-forming cells. These results suggest that strontium treatment could have previously overlooked effects on bone.

The basic mechanisms of action of strontium ranelate (SrR) on bone have remained unclear. We studied the direct actions of Sr²⁺ salts in functional cultures of osteoblasts and osteoclasts.

Cultures of primary osteoblasts from rat calvariae and osteoclast-forming mouse marrow cells were treated continuously with either SrR or strontium chloride (SrCl₂).

Abundant, discretely mineralised ‘trabecular’ bone structures formed in control osteoblast cultures after 14 days. SrR at 0.01, 0.1 and 1 mM inhibited mineralisation to 59, 98 and 100 % (all p < 0.001) of control values, respectively. SrCl₂ at the same concentrations caused similar inhibitions. Osteoblast cell numbers and alkaline phosphatase activity were unaltered. SrR dose-dependently reduced the formation of multinucleated osteoclasts from marrow mononuclear cells cultured on dentine for 8 days in the presence of macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor kappa B ligand (RANKL), with a 50 % inhibition occurring at 1 mM; SrCl₂ was slightly less effective, eliciting a maximal 30 % inhibition. Corresponding decreases in total resorption pit formation were observed, suggesting Sr²⁺ salts affect osteoclast formation rather than resorptive activity.

Our findings are consistent with the documented physicochemical inhibitory action of Sr²⁺ on mineralisation but contrast with reports that Sr²⁺ increases osteoblast activity and number in vitro. Our results suggest that rather than acting as an agent that ‘uncouples’ bone formation and resorption, Sr²⁺ acts as a global inhibitor of bone cell function, with particularly marked effects on mineralisation. The potential effects of long-term Sr²⁺ on secondary mineralisation in bone deserve investigation.