Parathyroid hormone (Parathyrin) (PTH)
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Contents |
[edit] Description
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:<ref>Template:GeorgiaPhysiology</ref>
[edit] Functions
[edit] Effects on serum calcium (raising)
| Region | Effect |
| bones | It enhances the release of calcium from the large reservoir contained in the bones.<ref>Poole K, Reeve J (2005). "Parathyroid hormone - a bone anabolic and catabolic agent". Curr Opin Pharmacol 5 (6): 612–7. doi:. PMID 16181808.</ref> 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. |
| kidney | It enhances active reabsorption of calcium and magnesium from distal tubules and the thick ascending limb. As bone is degraded both calcium and phosphate is released. It also greatly increases the excretion of Phosphate, with a net loss in plasma Phosphate concentration. By increasing the calcium:phosphate ratio more calcium is therefore free in the circulation. <ref name="mcgill">http://sprojects.mmi.mcgill.ca/nephrology/presentation/presentation5.htm</ref> |
| intestine via kidney | 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 25-Hydroxyvitamin D3 1-alpha-hydroxylase, 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 activated form of vitamin D affects the absorption of calcium (as Ca2+ ions) by the intestine via calbindin. |
PTH was one of the first hormones to be shown to use the G-protein, adenylyl cyclase second messenger system.
Normal total plasma calcium level ranges from 8.5 to 10.2 mg/dL (2.12 mmol/L to 2.55 mmol/L).<ref> Template:Cite web </ref>
[edit] Effects on serum phosphate (decrease, with compensation)
PTH reduces the reabsorption of phosphate from the proximal tubule of the kidney<ref name="mcgill" /> which means more phosphate is excreted through the urine.
However, PTH enhances the uptake of phosphate from the intestine and bones into the blood. In the bone, slightly more calcium than phosphate is released from the breakdown of bone. In the intestines, which is mediated by an increase in activated vitamin D, the absorption of phosphate 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.
[edit] Activation of vitamin D indirectly
PTH increases the activity of 1-α-hydroxylase enzyme, which converts 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol, the active form of vitamin D.
[edit] Secretion of Parathyroid Hormone
Secretion of parathyroid hormone is chiefly controlled by serum [Ca2+] through negative feedback, which is achieved by the activation of calcium-sensing receptors located on parathyroid cells.<ref>Template:GeorgiaPhysiology</ref> The second messenger of parathyroid chief cells responsible for PTH secretion is cAMP.<ref name=brs />
[edit] Stimulators of parathyroid hormone release
- Decreased serum [Ca2+].
- Mild decreases in serum [Mg2+].
[edit] Inhibitors of parathyroid hormone release
- Increased serum [Ca2+].
- Severe decreases in serum [Mg2+], which also produces symptoms of hypoparathyroidism (such as hypocalcemia).<ref name=brs>Template:Cite book</ref>
[edit] Syndromes
- A high level of PTH in the blood is known as hyperparathyroidism.
- If the cause is in the parathyroid gland it is called primary hyperparathyroidism. The causes are parathyroid adenoma, parathyroid hyperplasia and parathyroid cancer.
- If the cause is outside the gland, it is known as secondary hyperparathyroidism. This can occur in chronic renal failure.
- A low level of PTH in the blood is known as hypoparathyroidism. Causes include surgical misadventure (eg inadvertent removal during routine thyroid surgery), autoimmune disorder, and inborn errors of metabolism.
[edit] Measurements
PTH can be measured in the blood in several different forms: intact PTH; N-terminal PTH; mid-molecule PTH, and C-terminal PTH, and different tests are used in different clinical situations.
The average PTH level is 10-60 pg/ml.
[edit] Source Organism
Equus caballus (Horse).
[edit] Taxomomy
Eukaryota; Metazoa; Chordata; Craniata; Vertebrata; Euteleostomi;Mammalia; Eutheria; Laurasiatheria; Perissodactyla; Equidae; Equus.
[edit] Subcellular Localization
Secreted protein (By similarity).
[edit] Developmental Stage
[edit] Similarity
Belongs to the parathyroid hormone family.
[edit] Post translational Modification
[edit] Function
PTH elevates calcium level by dissolving the salts in bone and preventing their renal excretion
[edit] GeneID
100034104
