Linkage of extracellular and intracellular control of cytosolic Ca 2+ in rat osteoclasts in the presence of thapsigargin

Cytosolic [Ca 2+ ] was measured in single osteoclasts using fura‐2 in experiments investigating the effects of Ca 2+ “receptor” activation using thapsigargin as a means of depleting intracellular Ca 2+ stores. Application of 4 μM thapsigargin to osteoclasts in Ca 2+ ‐free solutions resulted in an elevation of cytosolic [Ca 2+ ]. Under similar conditions, activation of the osteoclast Ca 2+ receptor by the substitute divalent cation agonist, Ni 2+ , resulted in a transient elevation of cytosolic [Ca 2+ ]. In both instances, restoration of extracellular [Ca 2+ ] to 1.25 mM resulted in an “overshoot” of cytosolic [Ca 2+ ]. Prior depletion of intracellular Ca 2+ stores by thapsigargin markedly reduced the magnitude of the cytosolic [Ca 2+ ] response to a subsequent application of 5 mM Ni 2+ . The application of 2 μM thapsigargin to intercept the falling phase of the Ni 2+ ‐induced cytosolic Ca 2+ signal resulted in a sustained elevation of cytosolic [Ca 2+ ], which was terminated by a second application of the same Ni 2+ . Furthermore, the sustained elevation of cytosolic [Ca 2+ ] induced by thapsigargin application alone was abolished by late application of Ni 2+ . We conclude that activation of the surface membrane Ca 2+ receptor on the osteoclast results in the cytosolic release of Ca 2+ from intracellular storage organelles; the refilling of such stores depends upon a thapsigargin‐sensitive Ca 2+ ‐ATPase; store depletion induces capacitative Ca 2+ influx; and the Ca 2+ influx pathway is sensitive to blockade by Ni 2+ .