Regulation of extracellular calcium sensing in rat osteoclasts by femtomolar calcitonin concentrations

Abstract

Certain eukaryotic cells can sense changes in their extracellular Ca2+ concentration through molecular structures termed Ca(2+)-sensing receptors (CaRs). We have shown recently that in the bone-resorbing osteoclast, a unique cell surface-expressed ryanodine receptor (RyR), functions as the CaR. The present study demonstrates that the sensitivity of this receptor is modulated by physiological femtomolar concentrations of the bone-conserving hormone, calcitonin. Calcitonin was found to inhibit cytosolic Ca2+ responses to both Ca2+ and Ni2+. The latter inhibition was mimicked by amylin (10(-12) M), calcitonin gene-related peptide (10(-12) M), cholera toxin (5 micrograms/l) and dibutyryl adenosine 3',5'-cyclic monophosphate (cAMP) (2.5 x 10(-4) or 5 x 10(-4) M) and was reversed by the protein kinase A phosphorylation inhibitor, IP-20. Finally, using a quench flow module, we showed that cellular cAMP levels rise to a peak within 25 ms of calcitonin application; this is consistent with the peptide's rapid effect on CaR activation. We conclude, therefore, that cAMP plays a critical role in the control of CaR function by calcitonin.

Calcium sensing and cell signaling processes in the local regulation of osteoclastic bone resorption

“…It was pointed out above (Section IV) that all these agents would be expected to influence the osteoclast cAMP signaling system. Furthermore, quench flow studies demonstrated increases in cellular cAMP levels within 25 ms of CT application, consistent with a rapid effect of the peptide on CaR activation (Zaidi et al, 1996).…”

“…Thus, CT inhibited cytosolic Ca 2+ responses to both Ca 2+ and Ni 2+ . The latter inhibition was mimicked by amylin (10 x12 mol l x1 ), calcitonin generelated peptide (CGRP : 10 x12 mol l x1 ), cholera toxin (5 mg l x1 ) and dibutyryl-cAMP (2.5r10 x4 or 5r 10 x4 mol l x1 ) and was reversed by the protein kinase A phosphorylation inhibitor, IP-20 (Zaidi et al, 1996). It was pointed out above (Section IV) that all these agents would be expected to influence the osteoclast cAMP signaling system.…”

“…Finally, the cytosolic [Ca 2+ ] response to caffeine (5r10 x4 mol l x1 ), another RyR modulator, was also strongly attenuated by pretreatment with 5r10 x9 mol l x1 Ruthenium Red. In the presence of an intact nonleaky cell membrane, these results would favour a plasma membrane locus of action for the two modulators that in turn interact with the process of sensing the levels of ambient divalent cations (Adebanjo et al, 1996).…”

Biochemical Variables in Pre- and Postmenopausal Women: Reconciling the Calcium and Estrogen Hypotheses

“…This could be explained in terms of increased sensitivity of bone to PTH in the absence of estrogen [35], which has been supported by in vitro studies [36] and by PTH infusions [37,38], but this concept needs testing at the cellular and subcellular levels. Perhaps estrogens modify the osteoclast setpoint for calcium (the external calcium concentration at which osteoclast activity is suppressed [39,40]) in the way that calcitonin does [41]. However, although one needs to invoke an effect of estrogen deficiency on bone itself to fully explain menopausal bone loss, there is an important difference between postulating that estrogen directly regulates bone resorption and postulating that it regulates the bone's response to calcium demand.…”

Osteobiology of Aging

“…Zaidi et al have shown that osteoclast sensitivity to this peptide exists in vitro and serves a mechanism of skeletal conservation [136]. At the cellular level, calcitonin inhibits extracellular Ca 2+ sensing which serves as an antiresorptive signal [137].…”