It is now established that calcium is a second messenger mediating the action of calcitonin on the osteoclast. We have demonstrated that an increase in the concentration of intracellular free calcium ([Ca ²⁺ ] _i ) is associated with (and possibly mediates) the functional effects of calcitonin, including an acute reduction of cell spread area (the R effect) and, in the longer term, a reduction in enzyme release. The present study addresses questions relating to mechanisms of calcitonin action on osteoclast [Ca ²⁺ ] _i . We have used asusuberic(1–7) eel and human calcitonin as agonists, and an indo-1-based dual-emission microspectrofluorimetric method for the measurement of [Ca ²⁺ ] _i in single osteoclasts. Whilst asusuberic(1–7) eel calcitonin caused a biphasic increase in [Ca ²⁺ ] _i , human calcitonin produced only a monophasic [Ca ²⁺ ] _i response of a much lower magnitude. Each biphasic response consisted of a rapid initial transient increase, occurring within seconds of exposure, followed by a sustained increase in [Ca ²⁺ ] _i . The magnitude of the latter response was more variable, but was consistently below the peak value of [Ca ²⁺ ] _i . The sustained phase of the calcitonin effect was abolished in extracellular Ca ²⁺ -free medium. This phase is therefore dependent on extracellular [Ca ²⁺ ] ([Ca ²⁺ ] _e ) whilst the rapid transient increase appeared to be dependent on Ca ²⁺ _i redistribution. The effects of calcitonin on [Ca ²⁺ ] _i were concentration-dependent, with neither latency nor oscillations. Repetitive 30-s exposures to calcitonin failed to produce subsequent responses. There was a marked concentration-dependent correlation between changes in osteoclast [Ca ²⁺ ] _i and the magnitude of the R effect. Thus the likely components of the biphasic [Ca ²⁺ ] _i response are a rapid redistribution followed by the transmembrane flux of Ca ²⁺ . We suggest that the increase in [Ca ²⁺ ] _i may mediate, in part, the inhibitory effect of calcitonin.

Journal of Endocrinology (1992) 132, 241–249