Somatostatin
Somatostatin is a hormone comprising two peptides, one built of 14 amino acids, the other of 28 amino acids.
Somatostatin is secreted not only by cells of the hypothalamus but also by delta cells of stomach, intestine, and pancreas. It binds to somatostatin receptors.
Actions
Somatostatin is classified as an inhibitory hormone, whose main actions are to:
Inhibit the release of growth hormone (GH)
Inhibit the release of thyroid-stimulating hormone (TSH)
Suppress the release of gastrointestinal hormones
Gastrin
Cholecystokinin (CCK)
Secretin
Motilin
Vasoactive intestinal peptide (VIP)
Gastric inhibitory polypeptide (GIP)
Enteroglucagon (GIP)
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.
Somatostatin opposes the effects of Growth Hormone-Releasing Hormone (GHRH)
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. The Food and Drug Administration (FDA) has approved the usage of a salt form of this peptide, octreotide acetate, as an injectable depot formulation for the treatment of acromegaly, the treatment of diarrhoea and flushing episodes associated with carcinoid syndrome, and treatment of diarrhoea in patients with vasoactive intestinal peptide-secreting tumours (VIPomas). Octreotide has also been used off-label for the treatment of severe, refractory diarrhoea from other causes. It is used in Toxicology for the treatment of prolonged recurrent hypoglycaemia after sulfonylurea overdose.
Somatostatin in the 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.
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