Divalent cations mimic the inhibitory effect of extracellular ionised calcium on bone resorption by isolated rat osteoclasts: Further evidence for a “calcium receptor”

Abstract

Osteoclast activity is thought to be regulated by calcitonin, as well as by the level of ionised calcium generated locally as a result of bone resorption. The exposure of isolated osteoclasts to elevated ambient calcium levels has been shown to lower resorptive activity and to reduce rates of enzyme release. We have attempted to determine whether these effects are mediated by a divalent cation-sensitive "calcium receptor," as has been reported for the parathyroid chief cells. Thus, we compared the effect of alkaline earth metal cations on osteoclast function using a morphometric measure of bone resorption and a spectrophotometric method for measuring the activity of the released enzyme, acid phosphatase. The exposure of resorbing osteoclasts to between 5 and 20 mM extracellular ionised calcium ([Ca2+]e) inhibited bone resorption and enzyme release to an extent similar to that seen with 0.1 to 10 microM ionomycin. The effect of combining submaximal concentrations of [Ca2+]e (15 mM) and ionomycin (0.1 microM) resulted in additivity, suggesting that the influence of [Ca2+]e on bone resorption was mediated by elevated intracellular calcium levels ([Ca2+]i). The other cations studied (Mg2+, Ba2+) were effective and elicited similar effects, although some required higher concentrations. Thus, whilst Ca2+ and Mg2+ were effective at 10 to 15 mM levels, Ba2+ was effective only at high (20 mM) concentrations. These findings are consistent with an influence of [Ca2+]e on osteoclast activity through an action on a surface membrane "calcium receptor" that can also bind other divalent cations, rather than by passive changes of [Ca2+]i with [Ca2+]e elevation.

Selection and Structure of Ion-selective Ligands for Platelet Integrin αIIbβ3

“…For peptide A22, which was examined in Me 2 SO, the amide proton temperature coefficients were measured at 25, 30, 35, 40, and 45°C. Gradient heteronuclear multiple quantum experiments, 13 C HSQC, were performed at natural abundance to assign C ␣ and C 5 . NMR data were processed and analyzed using Felix2000 software (MSI, San Diego) on an SGI Indigo work station.…”

“…The adherent osteoclast resorbs mineralized bone, liberating free Ca 2ϩ , which can rise above 10 mM (11). This elevation in [Ca 2ϩ ] is then sensed by the osteoclast via a receptor (12,13), and this ultimately causes the osteoclast to detach from the bone surface. The identity of this osteoclast Ca 2ϩ receptor remains unclear, but it appears to have many of the properties of the allosteric I site on the ␣ v ␤ 3 integrin (4, 10).…”

Osteoblast calcium‐sensing receptor has characteristics of ANF/7TM receptors

“…In addition, osteoclasts and osteoblasts display a functional response to extracellular calcium and other cations [Kanatani et al, 1991;Quarles et al, 1991Quarles et al, , 1994Quarles et al, , 1997Chang et al, 1999;Pi et al, 2000;Shalhoub et al, 2003]. Ca 2+ released by osteoclastmediated bone resorption could potentially target calcium-sensing receptors located in osteoclasts and osteoblasts to, respectively, inhibit bone resorption and stimulate bone formation [Kellner, 1939;Quarles et al, 1988Quarles et al, , 1990Quarles et al, , 1991Quarles et al, , 1994Silver et al, 1988;Malgaroli et al, 1989;Zaidi et al, 1989Zaidi et al, , 1991Zaidi et al, , 1995Kanatani et al, 1991Kanatani et al, , 1999Brown et al, 1993;Ruat et al, 1995;Riccardi et al, 1996;Bowler et al, 1998]. …”

“…The identities of the skeletal extracelluar calcium-sensing mechanisms in osteoclasts and osteoblasts are uncertain [Quarles et al, 1988;Zaidi et al, 1989Zaidi et al, , 1991Zaidi et al, , 1995Pollak et al, 1993;Sugimoto et al, 1993;Kameda et al, 1998;Kanatani et al, 1999;Seuwen et al, 1999;Seeman and Delmas, 2001;Yamaguchi et al, 1998b,d]. In osteoclasts, the prototypic calciumsensing receptor, CASR, a channel, and a recently described cell surface Type II ryanodine receptor are proposed mechanisms for cation-sensing [Zaidi et al, 1989[Zaidi et al, , 1991[Zaidi et al, , 1995Pollak et al, 1993;Sugimoto et al, 1993;Kameda et al, 1998;Kanatani et al, 1999;Seuwen et al, 1999].…”

Calcium Signalling and Calcium Transport in Bone Disease

“…Nevertheless, one could then suggest an activation scheme in which Ca 2+ acts both as extracellular regulator and intracellular messenger. This suggestion was compatible with the action of even some membrane-impermeant divalent or trivalent ions that similarly triggered cytosolic [Ca 2+ ] changes: this would remain compatible with their interaction with a surface membrane CaR [Malgaroli et al, 1989;Zaidi et al, 1991;Shankar et al, 1992a,b]. Thus, applications of the divalent cation Ni 2+ as surrogate extracellular trigger elicited rapid, concentration-dependent, cytosolic [Ca 2+ ] elevations.…”