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John A.H. Wass

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date: 19 May 2022

Somatostatin was isolated in 1973 by Paul Brazeau in Roger Guillemin’s laboratory. It was found to have a widespread distribution, not only in the hypothalamus and brain but also in the gastrointestinal tract. Sixty-five per cent of the body’s somatostatin is in the gut, mostly in the D cells of the gastric and intestinal epithelium. It is also present in the myometric and submucosal plexuses. The highest concentration is in the antrum of the stomach and there is a gradual decrease of concentrations down the gastrointestinal tract. Five per cent of the body’s somatostatin is in the pancreas.

Infused somatostatin, which has a short half-life of 3 min, has a large number of actions on the pituitary gland, the endocrine and exocrine pancreas, gastrointestinal tract, other hormones, and on the nervous system (Box 6.8.1). Among its various actions of importance in the gastrointestinal tract is the inhibition of gastrin and cholecystokinin (CCK). In the pancreas, insulin and glucagon are inhibited. Nonendocrine actions include inhibition of gastric acid secretion, pancreatic exocrine function, gall bladder contraction, and intestinal motility. Intestinal absorption of nutrients, including glucose, triglycerides, and amino acids, is also inhibited (1).

Somatostatin exists in two main forms, as a 14-amino acid peptide (somatostatin 14) present mainly in the pancreas and the stomach, and as a 28-amino acid peptide present mainly in the intestine. Somatostatin 14 is the peptide present in enteric neurons.

Somatostatin receptors are present on many cell types, including the parietal cells of the stomach, G cells, D cells themselves, and cells of the exocrine and endocrine pancreas. A large number of tumours also have somatostatin receptors and these include pituitary adenomas, endocrine pancreatic tumours, carcinoid tumours, paragangliomas, phaeochromocytomas, small cell lung carcinomas, lymphomas, and meningiomas. Five different somatostatin receptors (SSTRs) have been cloned (SSTR1–SSTR5) and all are on different chromosomes. These have a varying affinity for somatostatin 14 and somatostatin 28 and a varying tissue distribution with SSTR2 and 5 being predominant in the pituitary (2).

Somatostatin can act either as an endocrine hormone or in a paracrine or autocrine way. It probably also has luminal effects in the gastrointestinal tract. Lastly, it can act as a neurotransmitter (3).

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