Epithelia form barriers to separate cells from the internal and external environment, allowing specific exchange of nutrients and other substances across epithelial cells. Among the many electrolytes, Ca2++ is an important intracellular messenger and a major component of the mineral phase of bones and teeth. Thus, at the cellular level, the cytosolic Ca2++ concentration is kept low (around 100 nM at rest), whereas its concentration in the extracellular fluids (ECF), such as blood, is maintained at a much higher level and within a relatively narrow limit (around 1 mM) . The bone, which contains roughly 99% of the Ca2++ in the human body, is subject to constant Ca2++ exchange with the ECF. Any reduction in the extracellular Ca2++ concentration ([Ca2++]o) in the ECF is being monitored by the G-protein-coupled Ca2++-sensing receptor (CaSR) in the parathyroid gland, triggering secretion of the parathyroid hormone (PTH) . The PTH increases bone resorption and renal reabsorption of Ca2++ and stimulates the hydroxylation of 25-hydroxyvitamin D3 [25(OH)2D3] in the proximal tubule of the kidney to form 1,25(OH)2D3 . An important role of this activated form of vitamin D is the stimulation of intestinal Ca2++ absorption via the nuclear vitamin D receptor (VDR) . Several organs and hormones work in concert to maintain stable levels of Ca2++ in the ECF. The intestinal and renal handling of Ca2++ is important as the intestine determines how much Ca2++ enters the body and the kidney determines how much Ca2++ is removed from the body.