LINKAGE OF EXTRACELLULAR AND INTRACELLULAR CONTROL OF CYTOSOLIC CA2+ IN RAT OSTEOCLASTS IN THE PRESENCE OF THAPSIGARGIN

Cytosolic [Ca2+] was measured in single osteoclasts using fura-2 in ex periments investigating the effects of Ca2+ ''receptor'' activation us ing thapsigargin as a means of depleting intracellular Ca2+ stores. Ap plication of 4 muM thapsigargin to osteoclasts in Ca2+-free Solutions resulted in an elevation of CytoSoliC [Ca2+]. Under similar conditions , activation of the osteoclast Ca2+ receptor by the substitute divalen t cation agonist, Ni2+, resulted in a transient elevation of cytosolic [Ca2+]. In both instances, restoration of extracellular [Ca2+] to 1.2 5 mM resulted in an ''overshoot '' of cytosoliC [Ca2+].Prior depletion of intracellular Ca2+ stores by thapsigargin markedly reduced the mag nitude of the cytosolic [Ca2+] response to a subsequent application of 5 mM Ni2+. The application of 2 muM thapsigargin to intercept the fal ling phase of the Ni2+-induced cytosolic Ca2+ signal resulted in a sus tained elevation of cytosolic [Ca2+], which was terminated by a second application of the same Ni2+. Furthermore, the sustained elevation of cytosolic [Ca2+] induced by thapsigargin application alone was abolis hed by late application of Ni2+. We conclude that activation of the su rface membrane Ca2+ receptor on the osteoclast results in the cytosoli c release of Ca2+ from intracellular storage organelles; the refilling of such stores depends upon a thapsigargin-sensitive Ca2+-ATPase; sto re depletion induces capacitative Ca2+ influx; and the Ca2+ influx pat hway is sensitive to blockade by Ni2+.