domingo, 16 de septiembre de 2012

Somatostatin


Somatostatin (also known as growth hormone-inhibiting hormone (GHIH) or somatotropin release-inhibiting factor (SRIF)) or somatotropin release-inhibiting hormone[citation needed] is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferationvia 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.[1]
In all vertebrates, there exists six different somatostatin genes that have been named SS1, SS2, SS3, SS4, SS5, and SS6. [2] The six different genes along with the five different somatostatin receptors allows somatostatin to possess a large range of functions.[3] Humans have only one somatostatin gene, SST.[4][5][6]
Production
Digestive system
Somatostatin is secreted in several locations in the digestive system:
§       stomach
§       intestine
§       delta cells of the pancreas[7]
[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 system through neuron axons. Somatostatin is then carried to the anterior pituitary gland, where it 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[citation needed], thehippocampus[citation needed], and the brainstem nucleus of the solitary tract[citation needed].
Actions
Somatostatin is classified as an inhibitory hormone,[1] 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)[8] (thus opposing the effects of Growth Hormone-Releasing Hormone (GHRH))
§       Inhibit the release of thyroid-stimulating hormone (TSH)[9]
§       It is induced by low pH.
§       Inhibit adenylyl cyclase in parietal cells.
Gastrointestinal system
§       Somatostatin is homologous with cortistatin (see somatostatin family) and suppresses the release of gastrointestinal hormones
§       Gastrin
§       Cholecystokinin (CCK)
§       Secretin
§       Motilin
§       Vasoactive intestinal peptide (VIP)
§       Gastric inhibitory polypeptide (GIP)
§       Enteroglucagon
§       Decrease rate of gastric emptying, and reduces smooth muscle contractions and blood flow within the intestine[8]
§       Suppresses the release of pancreatic hormones
§       Inhibits insulin release when somatostatin is released from delta cells of pancreas[10]
§       Inhibits the release of glucagon[10]
§       Suppresses the exocrine secretory action of pancreas.
Octreotide (brand name Sandostatin, Novartis Pharmaceuticals) is an octapeptide that mimics natural somatostatin pharmacologically, though is a more potent inhibitor of growth hormone, glucagon, and insulin than the natural hormone and has a much longer half-life (approximately 90 minutes, compared to 2–3 minutes for somatostatin). Since it is absorbed poorly from the gut, it is administered parenterally (subcutaneously, intramuscularly, or intravenously). It is indicated for symptomatic treatment of carcinoid syndrome, acute variceal bleeding, and acromegaly. It is also finding increased use in polycystic diseases of the liver and kidney.
Lanreotide (INN) is a medication used in the management of acromegaly and symptoms caused by neuroendocrine tumors, most notably carcinoid syndrome. It is a long-acting analogue of somatostatin, like octreotide.

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