OXYTOCIN

hormone used clinically to help begin or to continue labour, to control bleeding following delivery, and to stimulate the secretion of breast milk. Oxytocin was first synthesized (along with the related antidiuretic hormone ) by Vincent du Vigneaud in 1953, and he received the Nobel Prize for Chemistry in 1955 for this work. Synthetic oxytocin has since become widely used in obstetric practice. Natural oxytocin is secreted by the posterior pituitary gland, which holds and secretes oxytocin produced by the hypothalamus. Oxytocin causes milk to be ejected from the breasts during lactation; the amount of oxytocin produced naturally, however, has little effect on uterine contractions and does not stimulate labour. When synthetic oxytocin is infused in larger amounts, however, it causes smooth muscle in the wall of the uterus to contract and initiate the process of labour. Smooth-muscle cells in the uterus contain proteins that bind specifically to oxytocin; the number of these oxytocin receptors increases during late pregnancy. Oxytocin's effect on uterine smooth muscle is dependent on the presence of estrogen, and for that reason oxytocin has little effect on the uterus during the early stages of pregnancy; near term, however, it is very effective and successfully produces uterine contractions in 8090 percent of the women to whom it has been administered.