The effects of the related cytokines interleukin-6 (IL-6), leukemia inhibitory factor (LIF) and oncostatin-M on bone resorption and cytosolic Ca(2+) signaling were compared in isolated rat osteoclasts. In the traditional disaggregated osteoclast (pit) assay, IL-6 and LIF, but not oncostatin-M, conserved the bone resorption otherwise inhibited by high extracellular [Ca(2+)] (15 mM). It produced a paradoxical, concentration-dependent stimulation of resorption by elevated extracellular Ca(2+). In the micro-isolated single osteoclast resorption assay, IL-6, high [Ca(2+)] or IL-6 plus high [Ca(2+)] all increased pit formation. In contrast, the IL-6 receptor (IL-6R)-specific agonist antibody MT-18 inhibited bone resorption in a concentration-dependent manner (1:500 to 1:500 000). MT-18 triggered cytosolic Ca(2+) signals in fura 2-loaded osteoclasts within approximately 10 min of application. Each cytosolic Ca(2+) transient began with a peak deflection that persisted in Ca(2+)-free, EGTA-containing extracellular medium, consistent with a release of intracellularly stored Ca(2+). This was followed by a sustained elevation of cytosolic [Ca(2+)] that was abolished in Ca(2+)-free medium, as expected from an entry of extracellular Ca(2+), and by the Ca(2+) channel antagonist Ni(2+). The inclusion of either IL-6 or soluble human (sh) IL-6R specifically reversed both the above effects of MT-18, confirming that both effects were specific for the IL-6R. The findings suggest that IL-6R activation by IL-6 stimulates osteoclastic bone resorption either by reversing the inhibitory effect of high extracellular Ca(2+) in stromal-containing systems or itself stimulating bone resorption along with Ca(2+) by micro-isolated osteoclasts. In contrast, activation of the IL-6R by an agonist antibody produces an inhibition of bone resorption and an associated triggering of the cytosolic Ca(2+) signals previously associated with regulation of bone resorptive function in other situations.