• skeletal development • cellular calcium ion homeostasis • G-protein coupled receptor protein signaling pathway • cell-cell signaling • lactation • induction of apoptosis by hormones • bone resorption • cAMP metabolic process
Parathyroid hormone (PTH), or parathormone, is secreted by the parathyroid glands as a polypeptide containing 84 amino acids. It acts to increase the concentration of calcium (Ca2+) in the blood, whereas calcitonin (a hormone produced by the parafollicular cells (C cells) of the thyroid gland) acts to decrease calcium concentration.
PTH acts to increase the concentration of calcium in the blood by acting upon parathyroid hormone receptor in three parts of the body:
It enhances the release of calcium from the large reservoir contained in the bones. Bone resorption is the normal destruction of bone by osteoclasts, which are indirectly stimulated by PTH. Stimulation is indirect since osteoclasts do not have a receptor for PTH; rather, PTH binds to osteoblasts, the cells responsible for creating bone. Binding stimulates osteoblasts to increase their expression of RANKL, which can bind to osteoclast precursors containing RANK, a receptor for RANKL. The binding of RANKL to RANK stimulates these precursors to fuse, forming new osteoclasts which ultimately enhances the resorption of bone.
It enhances the absorption of calcium in the intestine by increasing the production of activated vitamin D. Vitamin D activation occurs in the kidney. PTH up-regulates the enzyme responsible for 1-alpha hydroxylation of 25-hydroxy vitamin D, converting vitamin D to its active form (1,25-dihydroxy vitamin D). This actived form of vitamin D affects the absorption of calcium (as Ca2+ ions) by the intestine via calbindin.
Effects on serum phosphate (decrease, with compensation)
However, PTH also enhances the uptake of phosphate from the intestine and bones into the blood. Slightly more calcium than phosphate is released from the breakdown of bone, and the intestinal absorption of phosphate (mediated by an increase in activated vitamin D) is not as dependent on vitamin D as is that of calcium. The end result is a small net drop in the serum concentration of phosphate.
Increased calcium concentration in the blood acts (via feedback inhibition) to decrease PTH secretion by the parathyroid glands.
This is achieved by the activation of calcium-sensing receptors located on parathyroid cells.
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