CALCIUM INFLUX AND RELEASE IN ISOLATED RAT OSTEOCLASTS

Intracellular and extracellular sources of cytosolic [Ca2+] elevation in isolated rat osteoclasts were explored by a comparison of fura-2 si gnals in response to application of the Ca2+ ionophore, ionomycin, in Ca2+-containing and in Ca2+-free bathing solutions. Cytosolic [Ca2+] t ransients persisted in osteoclasts bathed in Ca2+-free, EGTA-containin g solutions. They consisted of a peak cytosolic [Ca2+] elevation follo wed by a full decay to baseline and were refractory to manipulations o f surface membrane potential through changes in extracellular [K+]. Th ey disappeared upon intracellular Ca2+ store depletion through repeate d ionophore applications. They were therefore attributable solely to i ntracellularly stored Ca2+. In contrast, the fura-2 peaks in osteoclas ts exposed to Ca2+-containing solutions decayed to sustained levels. C ytosolic [Ca2+] responses then persisted with repeated ionomycin appli cation. These latter phenomena are accordingly attributable to extrace llular Ca2+ entry. Finally, restoration of extracellular [Ca2+] to 1.2 5 mM following the depletion of intracellular Ca(2+)stores by treatmen t with ionomycin elicited a cytosolic [Ca2+] 'overshoot' consistent wi th capacitative Ca2+ entry via a cytosolic route. These results demons trate a refillable intracellular source of cytosolic Ca2+ that could f unction in osteoclastic regulation.