Somatostatin

Overview
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Somatostatin (also known as growth hormone inhibiting hormone (GHIH) or somatotropin release-inhibiting hormone (SRIF)) is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferation via interaction with G-protein-coupled somatostatin receptors and inhibition of the release of numerous secondary hormones.

Somatostatin has two active forms produced by alternative cleavage of a single preproprotein: one of 14 amino acids, the other of 28 amino acids.

Digestive system
Somatostatin is secreted in several locations in the digestive system:
 * stomach
 * intestine
 * delta cells of the pancreas

Brain
Somatostatin is produced by neuroendocrine neurons of the periventricular nucleus of the hypothalamus. These neurons project to the median eminence, where somatostatin is released from neurosecretory nerve endings into the hypothalamo-hypophysial portal circulation. These blood vessels carry somatostatin to the anterior pituitary gland, where somatostatin inhibits the secretion of growth hormone from somatotrope cells. The somatostatin neurons in the periventricular nucleus mediate negative feedback effects of growth hormone on its own release; the somatostatin neurons respond to high circulating concentrations of growth hormone and somatomedins by increasing the release of somatostatin, so reducing the rate of secretion of growth hormone.

Somatostatin is also produced by several other populations that project centrally - i.e. to other areas of the brain, and somatostatin receptors are expressed at many different sites in the brain. In particular, there are populations of somatostatin neurons in the arcuate nucleus, the hippocampus and the brainstem nucleus of the solitary tract.

Actions


Somatostatin is classified as an inhibitory hormone, whose actions are spread to different parts of the body:

Anterior pituitary
In the anterior pituitary gland, the effects of somatostatin are:
 * Inhibit the release of growth hormone (GH) (thus opposing the effects of Growth Hormone-Releasing Hormone (GHRH))
 * Inhibit the release of thyroid-stimulating hormone (TSH)

Gastrointestinal system

 * Suppress the release of gastrointestinal hormones
 * Gastrin
 * Cholecystokinin (CCK)
 * Secretin
 * Motilin
 * Vasoactive intestinal peptide (VIP)
 * Gastric inhibitory polypeptide (GIP)
 * Enteroglucagon
 * Lowers the rate of gastric emptying, and reduces smooth muscle contractions and blood flow within the intestine
 * Suppress the release of pancreatic hormones
 * Inhibit the release of insulin
 * Inhibit the release of glucagon
 * Suppress the exocrine secretory action of pancreas.

Synthetic substitutes
Octreotide (brand name Sandostatin, Novartis Pharmaceuticals) is an octopeptide that mimics natural somatostatin pharmacologically, though is a more potent inhibitor of growth hormone, glucagon, and insulin than the natural hormone.