The G-protein-coupled calcium receptors bind extracellular calcium and may be found on the surface on a wide variety of cells distributed in the brain, heart, skin, stomach, C cells, and other tissues. In the parathyroid gland, high concentrations of extracellular calcium result in activation of the Gq G-protein coupled cascade through the action of phospholipase C.  This hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to liberate intracellular messengers IP3 and diacylglycerol (DAG). Ultimately, these two messengers result in a release of calcium from intracellular stores into the cytoplasmic space. Hence a high extracellular calcium concentration leads to an increase in the cytoplasmic calcium concentration. In contrast to the mechanism that most secretory cells use, this high cytoplasmic calcium concentration inhibits the fusion of vesicles containing granules of preformed PTH with the membrane of the parathyroid cell, and thus inhibits release of PTH. In the parathyroids, magnesium serves this role in stimulus-secretion coupling. A mild decrease in serum magnesium levels stimulates the reabsorptive activity PTH has on the kidneys. Severe hypomagnesemia inhibits PTH secretion and also causes resistance to PTH, leading to a form of hypoparathyroidism that is reversible. Stimulators
Decreased serum [Ca2+]. Mild decreases in serum [Mg2+]. An increase in serum phosphate (increased phosphate causes it to complex with serum calcium,  forming calcium phosphate, which reduces stimulation of Ca-sensitive receptors (CaSr) that do not sense calcium phosphate, triggering an increase in PTH). Adrenaline
Histamine

Inhibitors
Increased serum [Ca2+]. Severe decreases in serum [Mg2+], which also produces symptoms of hypoparathyroidism (such as hypocalcemia). Calcitriol
Increase in serum phosphate.
Reticulocytosis is a condition where there is an increase in reticulocytes, immature red blood cells. It is commonly seen in anemia. They are seen on blood films when the bone marrow is highly active in an attempt to replace red blood cell loss such as in haemolytic anaemia, haemorrhage. == External links ==