Human Biochemistry, Volume 2 - Murray R. 1993

Biochemistry of Intra- and Intercellular Communication
Pituitary and Hypothalamic Hormones
Posterior Pituitary Hormones - Oxytocin

Regulation of Secretion

The primary stimuli for oxytocin release are nerve impulses generated by The stimulation of the nipples. Distension of the Vagina and Uterus plays a secondary role. Many stimuli that trigger oxytocin secretion also induce the release of prolactin; it is hypothesized that an oxytocin fragment may act as a prolactin-releasing factor. Estrogens stimulate, whereas progesterone inhibits, The production of oxytocin and neurophysin I.

MECHANISM OF ACTION

The Mechanism of action of oxytocin remains unknown. It induces contractions of the uterine smooth Muscle and is therefore used in pharmacological doses to induce labor in women. Interestingly, pregnant animals with a damaged hypothalamo-hypophyseal system do not necessarily experience impaired labor. The most likely physiological function of oxytocin is to stimulate the contraction of myoepithelial Cells surrounding the mammary alveoli. This facilitates the movement of milk into The system of alveolar ducts and results in milk ejection. Membrane Receptors for oxytocin have been identified in uterine and mammary gland Tissues. Their density increases in response to estrogens and decreases under METABOLISM/18.html">The Influence of progesterone. The onset of Lactation prior to parturition can presumably be explained by the simultaneous rise in estrogen levels and the drop in progesterone levels immediately before birth. Progesterone derivatives are frequently used to suppress postpartum lactation in women. Oxytocin and neurophysin I appear to be synthesized in the Ovaries as well, where oxytocin may inhibit steroidogenesis.

Chemical groups essential for oxytocin activity include the primary amino group of the N-terminal Cysteine, the phenolic group of Tyrosine, the 3 carboxamide groups of asparagine, glutamine, and glycinamide, and the disulfide (S—S) bond. Numerous analogues of oxytocin have been produced by removing or substituting these groups. For example, the removal of the free primary amino group of the terminal hemicysteine residue (position 1) yields deaminooxytocin, an analogue whose antidiuretic activity is 4 to 5 times greater than that of native oxytocin.