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 Ca2+ ([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, 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,. We first observed that low pH, stimulated Ca2+ efflux and cell acidification. We then demonstrated that the amplitude of the [Ca2+](o)-dependent [Ca2+](i) ''spikes'' is downregulated by low pH(e), with similar to 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.