This study examined whether the calcium-sensing receptor (CaR) is expressed in normal adult human osteoblastic and osteoclastic cells in culture, and whether the calcimimetic, cinacalcet HCl (AMG 073), potentiates the effects of calcium (via CaR, or some other receptor/mechanism). When mouse or human osteoblastic cells were treated with higher concentrations of calcium (6.6 or 8.6 mM in alpha-MEM/10% FBS) than present in control cultures (1.6 mM), the previously well-documented increase in cell number was demonstrated. Cinacalcet HCl affected cell proliferation of CHO cells transfected with CaR, dose dependently, but had no effect on human or mouse osteoblastic cell proliferation in calcium-containing medium (1.6 or 8.6 mM). To test cinacalcet HCl and calcium on osteoclastic cells, peripheral blood mononuclear cells were cultured in medium containing RANK ligand and M-CSF, supplemented with calcium, and/or cinacalcet HCl. Tartrate-resistant acid phosphatase-positive multinucleated osteoclastic cells on plastic or bone were then counted at 11 and 21 days, respectively. Calcium (greater than 6.0 mM) inhibited osteoclast formation, but cinacalcet HCl (30-1000 nM) had no effect on osteoclastic formation or resorption in the presence of calcium (1.6 or 6.1 mM). RT-PCR did not detect CaR in human, rat, or mouse primary osteoblastic cells and cell lines or osteoclastic cells. In conclusion, these studies indicate that the calcium-induced increase in osteoblastic cell number, and the decrease in formation/function of osteoclastic cells, involves a mechanism or receptor other than CaR. In addition, the calcimimetic agent did not potentiate the effects of calcium on normal adult human bone cells in vitro.

Osteoprotegerin (OPG) acts by neutralizing the receptor activator of nuclear factor-kappaB ligand (RANKL), the primary mediator of osteoclast differentiation, function, and survival. We examined whether OPG could affect the bone loss associated with chronic kidney disease (CKD) in a rodent model of CKD and secondary hyperparathyroidism (SHPT). SHPT was induced in rats by 5/6 nephrectomy (5/6 Nx) and a 1.2% P/0.6% Ca(2+) diet. Starting 1 week after 5/6 Nx, rats were treated with vehicle (veh) or OPG-Fc (3 mg/kg, intravenously) every 2 weeks for 9 weeks. At baseline, 3, 6, and 9 weeks, blood was taken and bone mineral density (BMD) and bone mineral content (BMC) were assessed by dual-energy X-ray absorptiometry. Serum parathyroid hormone (sPTH) levels reached 912 pg/ml in 5/6 Nx rats vs. 97 pg/ml in shams at 9 weeks. OPG-Fc had no effect on sPTH or Ca(2+) levels throughout the 9-week study, indicating that SHPT was a renal effect independent of bone changes. At 3 weeks, 5/6 Nx-veh rats had osteopenia compared with sham-veh rats and 5/6 Nx-OPG-Fc rats had significantly higher percent changes in whole-body BMC, leg BMD, and lumbar BMD versus 5/6 Nx-veh rats. By 6-9 weeks, elevated sPTH was associated with reversal of bone loss and osteitis fibrosa in the proximal tibial metaphysis. OPG-Fc decreased this sPTH-driven high bone turnover, resulting in augmented thickness of proximal tibial trabeculae in 5/6 Nx rats. Thus, RANKL inhibition with OPG-Fc can block the deleterious effects of continuously elevated sPTH on bone, suggesting that RANKL may be an important therapeutic target for protecting bone in patients with CKD and SHPT.