Figure 1.

Calcium homeostasis. Calcium homeostasis is finely regulated by a complex hormonal system including several hormones and its receptors (PTH/PTHrP and its receptor PTH1R; 1,25(OH)₂D) and the vitamin D receptor; ionized calcium itself and CaSR) and 3 main target-organs (kidney, gut and bone). A decrease in serum calcium inactivates the CaSR in the parathyroid glands and in the kidney. It causes an increase in PTH secretion which in turn, stimulates renal and bone reabsorption. PTH promotes the synthesis of 1,25(OH)₂D in the kidney leading to the increase of intestinal calcium absorption and to a feedback loop inhibition of PTH secretion. Inactivation of CaSR in the kidney leads to additional calcium reabsorption and enhancement of the renal PTH action of PTH. In contrast, an increase in serum calcium level increases calcium excretion and bone storage. PTH, parathyroid hormone; PTHrP, PTH-related Protein; PTH1R, PTH-1 receptor; CaSR, calcium-sensing receptor; 1,25(OH)₂D, 1,25-dihydroxyvitamin D; VDR, vitamin D receptor.