Maintenance of extracellular calcium (Ca2+ o) homeostasis in mammals and other tetrapods requires the coordinated and tightly regulated interactions of Ca2+ o-regulating hormones [eg, parathyroid hormone (PTH), 1, 25-dihydroxyvitmain D3 [1, 25 (OH) 2D3] and, to a lesser extent, calcitonin (CT)], and the effector organs acted upon by these hormones (ie, kidney, intestine and bone)[1]. All three of the cell types that secrete Ca2+ o-regulating hormones are capable of recognizing and responding to (eg,``sensing'') small changes in the level of the extracellular ionized calcium concentration with alterations in their secretion of these hormones [1]. For the Ca2+ o-elevating hormones, PTH and