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Somatostatin refers to a number of polypeptides consisting of chains of 14 to 28 amino acids. The name was coined when its discoverers found that an extract of the hypothalamus strongly inhibited the release of growth hormone from the pituitary gland. Somatostatin is also a powerful inhibitor of pituitary TSH secretion. Somatostatin, like TRH, is widely distributed in the central nervous system and in other tissues. It serves an important paracrine function in the islets of Langerhans, by blocking the secretion of both insulin and glucagon from adjacent cells. Somatostatin has emerged not only as a powerful blocker of the secretion of GH, insulin, glucagon, and other hormones but also as a potent inhibitor of many functions of the gastrointestinal tract, including the secretion of stomach acid, the secretion of pancreatic enzymes, and the process of intestinal absorption. Despite these multiple, widespread actions, the term somatostatin has persisted because of its major role as a regulator of GH secretion, and impaired somatostatin secretion may cause some forms of hypersecretion of growth hormone.

No examples of somatostatin deficiency have been found, but a tumour called a somatostatinoma has been well characterized in a number of patients. Persons with a somatostatinoma have cramping abdominal pain, persistent diarrhea, a mild elevation of blood glucose levels, and sudden flushing of the skin.

Prolactin-inhibiting and releasing hormones

The hypothalamic regulation of prolactin secretion from the pituitary is different from the hypothalamic regulation of other pituitary hormones in two respects. First, the hypothalamus primarily inhibits rather than stimulates the release of prolactin from the pituitary (the hypothalamus stimulates the release of all other pituitary hormones). Thus, if pituitary cells are removed from the influence of the hypothalamus, few or none of the pituitary hormones are secreted, except for prolactin, which continues to be secreted by the prolactin-secreting cells (lactotrophs). Second, this major inhibiting factor is not a neuropeptide, but rather the neurotransmitter dopamine, a fact exploited in afflicted persons by physicians who are able to reduce abnormally high concentrations of prolactin by using drugs that mimic the prolactin-inhibiting effects of dopamine. Another prolactin-inhibiting factor (PRF) comes into play primarily during pregnancy and lactation. Prolactin-stimulating factors also exist, among them TRH.

Prolactin deficiency is known to occur, but only rarely. Excessive prolactin production (hyperprolactinemia) is a common endocrine abnormality, and the prolactinoma is the most frequently encountered pituitary tumour.
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