Extracellular Ca²⁺ sensing is modulated by pH in human osteoclast-like cells in vitro

Osteoclasts are polarized cells with a basolateral and an apical membrane exposed to different extracellular Ca^2- ([Ca²⁺](o)) and H⁺ (pH(e)) concentrations. Osteoclast bone resorption is inhibited in vitro by increases of [Ca²⁺](o) slightly above physiological levels, detected by a [Ca²⁺](o) sensing causing elevations of the intracellular signal, [Ca²⁺](i). Nevertheless, during bone resorption the apical membrane is exposed to [Ca²⁺](o) severalfold higher than physiological without apparent inhibition of osteoclast functions. Because pH(e) facing the apical membrane is acidic, in this single cell [Ca²⁺](i) and intracellular pH study we addressed the question of whether the responses of human osteoclast-like cells from a giant cell tumor of bone to elevated [Ca²⁺](o) are altered by reducing pH(e). We first observed that low pH(e) stimulated Ca²⁺ efflux and cell acidification. We then demonstrated that the amplitude of the [Ca^2-](o)- dependent [Ca²⁺](i) 'spikes' is downregulated by low pH(e), with ~70-fold higher [Ca²⁺](o) required to induce significant responses at pH(e) 6.0 compared with pH(e) 7.4. Similar downregulation was observed in authentic freshly isolated rat osteoclasts. Finally, we observed that occupancy of the [Ca²⁺](o) sensing by Ca²⁺ prompted rapid and transient cell acidification partially counteracted by a Na⁺-dependent amiloride derivative-sensitive H⁺ transport. These results demonstrate that the cascade of events triggered by activation of the [Ca²⁺](o) sensing is affected by environmental pH and in turn influences cellular H⁺ transport. Such pH related features of the [Ca²⁺](o)-sensing mechanism might be relevant for the regulation of osteoclast-like cell function.