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secretory substance carried from one gland or organ of the body via the bloodstream to more or less specific tissues, where it exerts some influence upon the metabolism of the target tissue.
..... Click the link for more information. secreted by the parathyroid glandsparathyroid glands
, four small endocrine bodies, located behind the thyroid gland, that govern calcium and phosphorus metabolism. These four masses of tissue (each about the size of a pea) are difficult to distinguish from the thyroid and are often embedded in it.
..... Click the link for more information. that regulates the metabolismmetabolism,
sum of all biochemical processes involved in life. Two subcategories of metabolism are anabolism, the building up of complex organic molecules from simpler precursors, and catabolism, the breakdown of complex substances into simpler molecules, often accompanied by
..... Click the link for more information. of calcium and phosphate in the body. It has been purified extensively and appears to be a protein containing 84 amino-acid residues, a sequence of which about 33 to 35 are necessary for biological activity. Parathyroid hormone acts to raise the extracellular calcium concentration, that is, the concentration of calcium ions in the spaces between the cells of the body and in the blood plasma; it promotes the absorption of calcium by the intestine, mobilizes calcium salts from the bones, and increases the tendency of the kidney to recover calcium from the urine. The hormone also enhances both the excretion of phosphate by the kidneys and its uptake by the cells. This removes phosphate, which tends to form a relatively insoluble salt with calcium, from the extracellular spaces, allowing more calcium to remain in solution. Calcium is intimately involved not only in the formation of bone, but also in the functioning of the nervous system; thus hypoparathyroidism, the disease associated with a deficiency in parathyroid hormone secretion, is characterized by muscle spasms leading eventually to generalized convulsions and various psychiatric symptoms. This condition is sometimes successfully treated by the administration of the hormone. Hyperparathyroidism, the result of oversecretion of the hormone, often leads to the resorption of bone and can only be treated by the surgical removal of all of the parathyroid glands, which can be found in unusual locations.
The secretory product of the parathyroid glands. Parathyroid hormone (PTH) is a single-chain polypeptide composed of 84 amino acids. The sequences of human, bovine, and porcine parathyroid hormone are known, and the gene for human parathyroid hormone has been cloned and sequenced.
The major regulator of parathyroid hormone secretion is the serum concentration of calcium ions, to which the parathyroid cells are exquisitely sensitive. Only a limited amount of parathyroid hormone is stored in secretory granules, so that a hypocalcemic stimulus must ultimately influence biosynthesis as well as secretion of the hormone. Parathyroid secretory protein is a large, acidic glycoprotein which is stored and cosecreted with parathyroid hormone in roughly equimolar amounts; the biological function of parathyroid secretory protein is unknown.
Parathyroid hormone is responsible for the fine regulation of serum calcium concentration on a minute-to-minute basis. This is achieved by the acute effects of the hormone on calcium resorption in bone and calcium reabsorption in the kidney. The phosphate mobilized from bone is excreted into the urine by means of the hormone's influence on renal phosphate handling. Parathyroid hormone also stimulates calcium absorption in the intestine, this being mediated indirectly by 1,25-dihydroxyvitamin D. Thus, a hypocalcemic stimulus of parathyroid hormone secretion results in an increased influx of calcium from three sources (bone, kidney, and intestine), resulting in a normalization of the serum calcium concentration without change in the serum phosphate concentration. See Parathyroid gland, Thyrocalcitonin
(also parathormone), a hormone secreted by the parathyroid glands. It regulates the content of calcium and phosphorus in blood, interstitial fluid, and bone. It was isolated from parathyroid glands of bulls in 1925 by the Canadian biochemist J. Collip and obtained in pure form in 1961 by L. Craig and H. Rasmussen. Chemically it is a polypeptide that consists of 80 amino-acid residues and has a molecular weight of 9,500.
Parathyroid hormone acts to reduce blood phosphate levels and increase blood calcium levels and promotes the mobilization of calcium from bone into the body’s fluids. It also promotes excretion of calcium and phosphates in urine. An excess of parathyroid hormone in animals that receive it by injection or in humans suffering from hyperthyroidism results in resorption of bone, whereas a deficiency of the hormone lowers blood calcium levels with various consequences, including the intensification of neuromuscular excitability (tetany in acute cases) and the development of alkalosis. The amount of parathyroid hormone secreted varies inversely with blood calcium levels; that is, the amount secreted increases as the calcium level decreases.
The mechanism of action of parathyroid hormone has not been completely elucidated. Apparently the hormone acts on both renal and bone tissue and stimulates the absorption of calcium from the intestine and its reabsorption from the glomerular filtrate into the blood. An aqueous extract of parathyroid glands of slaughtered cattle is administered, subcutaneously or intramuscularly, in various forms of tetany, spasmophilia, and certain other diseases.
V. M. SAMSONOVA