Module 7: Figure PTH secretion
Control of parathyroid gland secretion and proliferation.
The primary function of the parathyroid gland is to synthesize and release parathyroid hormone (PTH). This PTH release has an inverse relationship to the plasma level of Ca2+ (see inset). The bar represents the normal range of plasma Ca2+ concentration. A Ca2+-sensing receptor (CaR) responds to changes in the plasma level by activating phospholipase C (PLC) to increase the level of inositol 1,4,5-trisphosphate (InsP3) and Ca2+. It can also inhibit adenylyl cyclase (AC) to reduce the level of cyclic AMP. Since an increase in these messengers is associated with a decrease in secretion, it is assumed that they somehow inhibit exocytosis. The vitamin D-derived hormone 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] acts on the vitamin D receptor (VDR) to inhibit both secretion and proliferation.