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+ (pHe) 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 pHe 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 pHe. We first observed that low pHe stimulated Ca2+ efflux and cell acidification. We then demonstrated that the amplitude of the [Ca2+]o-dependent [Ca2+]i "spikes" is downregulated by low pHe, with approximately 70-fold higher [Ca2+]o required to induce significant responses at pHe 6.0 compared with pHe 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 function.
Grano
M
M
Institute of Human Anatomy, School of Pharmacy, University of Bari, Italy.
Faccio
R
R
Colucci
S
S
Paniccia
R
R
Baldini
N
N
Zallone
A Z
AZ
Teti
A
A
D002118:Calcium / Q000378:metabolism*; D002628:Chemoreceptor Cells / Q000502:physiology*; D005110:Extracellular Space / Q000378:metabolism*; D016257:Fura-2; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007425:Intracellular Membranes / Q000378:metabolism; D009994:Osmolar Concentration; D010010:Osteoclasts / Q000378:metabolism*; D014407:Tumor Cells, Cultured
D002118:Calcium; D016257:Fura-2