oxytocin

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oxytocin

(ŏksĭtō`sĭn), hormonehormone,
secretory substance carried from one gland or organ of the body via the bloodstream to more or less specific tissues, where it exerts some influence upon the metabolism of the target tissue.
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 released from the posterior lobe of the pituitary glandpituitary gland,
small oval endocrine gland that lies at the base of the brain. It is sometimes called the master gland of the body because all the other endocrine glands depend on its secretions for stimulation (see endocrine system).
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 that facilitates uterine contractions and the milk-ejection reflex. The structure of oxytocin, a cyclic peptidepeptide,
organic compound composed of amino acids linked together chemically by peptide bonds. The peptide bond always involves a single covalent link between the α-carboxyl (oxygen-bearing carbon) of one amino acid and the amino nitrogen of a second amino acid.
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 consisting of nine amino acids, was determined in 1953 and in the same year it was synthesized in the laboratory. Both oxytocin and antidiuretic hormoneantidiuretic hormone
, polypeptide hormone secreted by the posterior pituitary gland. Its principal action is to regulate the amount of water excreted by the kidneys. Antidiuretic hormone (ADH), known also as vasopressin, causes the kidneys to resorb water directly from the
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 are biosynthesized in the hypothalamus of the brain and travel down neuronal axons to the posterior pituitary, where they accumulate prior to release. Stimuli that elicit the release of oxytocin include childbirth, suckling, and coitus; the uterine contractions that result may facilitate either childbirth or the ascent of spermatozoa through the fallopian tubes. Oxytocin may also play a role in the initiation of labor. The milk-ejection response occurs only in females immediately after childbirth. The role of oxytocin in males is unknown.

Oxytocin

 

(also pitocin), a hormone that is produced by the neurosecretory cells of the anterior nuclei of the hypothalamus. Oxytocin is transported over nerve fibers from the hypothalamus to the posterior lobe of the hypophysis, where it accumulates and is then released into the blood. It elicits contractions of the smooth muscles of the uterus and, to a lesser degree, the muscles of the urinary bladder and intestine. It also stimulates the flow of milk from the mammary glands. Oxytocin is secreted in response to stimulation of the nipple during lactation and distention of the uterus in the late stages of pregnancy. Adrenaline suppresses the secretion of oxytocin, and progesterone counteracts oxytocin’s effect on the uterine muscles.

Oxytocin is an octapeptide Whose four amino-acid groups are bound in a ring by cystine, which is also linked to a tripeptide:

Oxytocin—which is structurally similar to the other hypophyseal hormone, vasopressin—was isolated in pure form from biological substances (1950) and produced synthetically (1954) by V. du Vigneaud and co-workers; this was the first synthesis of a biologically active protein. A less expensive, synthetic oxytocin is used in medicine.

V. M. SAMSONOVA

oxytocin

[¦äk·sē¦tō·sən]
(biochemistry)
C43H66O12N12S2 A polypeptide hormone secreted by the neurohypophysis that stimulates contraction of the uterine muscles.
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