Calcitonin

Encyclopedia : C : CA : CAL : Calcitonin

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.

Contents

1 Synthesis
2 Physiology
3 Pharmacology
4 History
5 See also
6 Reference

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 indicates that calcitonin is of very little physiological importance to humans. Rather, calcium and phosphate homeostasis is primarily under the control of parathyroid hormone (PTH).

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 G_s 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.

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