Calcitonin

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Calcitonin is a 32 amino acid polypeptide hormone that is produced in humans primarily by the C cells of the thyroid, and in many other animals in the ultimobranchial body.

Synthesis

It is formed by proteolytic cleavage of a larger prepropeptide which is the product of the CALC1 gene, which itself is part of a superfamily of related protein hormone precusors including Islet Amyloid Precursor Protein, Calcitonin Gene-Related Peptide and the precursor of Adrenomedullin.

Physiology

The hormone participates in calcium and phosphorus metabolism and it was found in fish, reptiles, birds and mammals. Most evidence points to that Calcitonin is not of physiological importance to humans, except for it's pharmacological use (see below).

Specifically, calcitonin reduces blood calcium levels in three ways:

• Decreasing calcium absorption by the intestines
• Decreasing osteoclast activity in bones
• Decreasing calcium and phosphate reabsorption by the kidney tubules

Its actions, broadly, are:

• Bone mineral metabolism
  • Prevent postprandial hypercalcemia resulting from absorption of Ca++ from foods during a meal
  • Promote mineralization of skeletal bone
  • Protect against Ca++ loss from skeleton during periods of Ca++ stress such as pregnancy and lactation
• Vitamin D regulation
• A satiety hormone
  • Inhibit food intake in rats and monkeys
  • May have CNS action involving the regulation of feeding and appetite

Like the PTH receptor, the receptor of calcitonin is a serpentine G protein-coupled receptor with seven membrane spanning regions which is coupled by Gs to adenylyl cyclase and thereby to the generation of cAMP in target cells. Indeed, the PTH and calcitonin receptors are family members which are related in amino acid sequence, though their ligands are not.

Pharmacology

Salmon calcitonin is used for the treatment of:

• Postmenopausal osteoporosis
• Hypercalcaemia
• Paget's disease
• Bone metastases

History

Calcitonin was purified in 1962 by Copp and Cheney. While it was initially considered a secretion of the parathyroid glands, it was later identified as the secretion of the C-cells (parafollicular cells) of the thyroid.

See also

• Procalcitonin

Reference

• Copp DH, Cheney B. Calcitonin-a hormone from the parathyroid which lowers the calcium-level of the blood. Nature 1962;193:381-2. PMID 13881213

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Target-derived NGF, BDNF, NT-3

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