© 2017 by Taylor & Francis Group, LLC. 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.