ABSTRACT
Bone turnover helps accomplish long-term correction of the extracellular calcium (Ca2+ o) homeostasis by the actions of osteoblasts and osteoclasts. These processes are highly regulated by the actions of hormones, most prominently parathyroid hormone (PTH), the release of which is a function of the Ca2+ o, and is regulated by the action of the Ca2+ -sensing receptor (CaR) in the parathyroid gland. Various mutations of the CaR gene give rise to gain or loss of functions leading respectively to hypo- or hypercalcaemic conditions. CaR could conceivably be a target for local changes in the Ca2+ o in the bone microenvironment thereby acting as a 'growth factor' in various cells residing in the bone marrow. This review discusses about the roles of the CaR in bone. In osteoblasts, CaR promotes its proliferation, differentiation and mineralization. In osteoclasts, CaR mediates high Ca2+ o-stimulated osteoclast differentiation as well as osteoclast apoptosis. CaR regulates localization of haematopoietic stem cells from the foetal liver to endosteal niche, the socalled homing. Although the CaR plays a key role in the defense against hypercalcaemia, its function can be aberrant in humoral hypercalcaemia of malignancy in which CaR activation stimulates secretion of parathyroid hormone-related peptide (PTHrP) secretion. Increased levels of PTHrP cause a vicious hypercalcaemic state resulting from its increased bone-resorptive and positive renal calcium reabsorbing effects give rise to hypercalcaemia. CaR mediates a variety of functions of Ca2+ o in the bone microenvironment under both normal and pathological conditions.