Oxytocin is a nanopeptide formed in the supra-optic and paraventricular nuclei of the hypothalamus, and transported then stored by the posterior lobe of the pituitary gland which releases it into the blood circulation. Its main effect is to induce uterine contractions.
Oxytocin secretion is increased by stimulation of cervix, vagina, breast, and is decreased by ethanol intake, which explains why alcohol was used formerly as a tocolytic for preventing preterm delivery.
Its half-life in the plasma is five to ten minutes. It is eliminated by the kidney and is degraded by an aminopeptidase called ocytocinase.
Oxytocin acts on several organs, mainly on the uterus. The effects of oxytocin on the uterus and mammary gland result from stimulation of membrane receptors coupled to G proteins leading to increase of intracellular calcium concentration via activation of phospholipase C.
- On the uterus: oxytocin, by raising the intracellular concentration of Ca2+, increases the strength and the frequency of contractions. Its efficacy increases during gestation because the uterus becomes increasingly sensitive to its presence. The number of oxytocin receptors of a uterus at the end of gestation is about 200 times higher than that of a non-pregnant uterus. Smooth muscle cells of a non pregnant uterus have low-strength and asynchronous contractions. In late pregnancy uterine contractions are synchronous because, under the influence of estrogens, progesterone and prostaglandins, functional junctions via connexines have been established.
- On mammary gland: oxytocin constricts milk channels, which induces milk ejection.
- On cardiovascular system: in large doses, it has a vasodilator action but it has also a weak antidiuretic action.
- On the nervous system: in certain animal species, oxytocin could improve maternal behaviour
Oxytocin (Syntocinon*) is given by intravenous infusion in the following therapeutic uses:
- Induction and reinforcement of uterine contractions for labor, in absence of mechanical obstacle to vaginal delivery, with dilated cervix to three or four centimetres. The quantity of oxytocin necessary to elicit effective contractions is very variable from one person to another.
- For the treatment and prevention of postpartum haemorrhage .
From a chemical point of view, oxytocin antagonists can be divided in polypeptide and non-polypeptide molecules. Atosiban (Tractocile*) is a polypeptide antagonist whose structure is closely related to that of oxytocin. It inhibits oxytocin receptors and certain V1 vasopressin receptors. It decreases uterine contractions elicited by oxytocin and is indicated to prevent preterm deliveries. It is a tocolytic agent.
Methylergometrine, by its alpha-1-adrenergic effect, raises the intracellular calcium concentration and reinforces uterine contractions. It is used in the treatment of postpartum uterine hemorrhages.
Beta-2 mimetics reduce uterine contractions and are used for prevention of preterm delivery. They act by increasing the concentration of intracellular cyclic AMP.
Relaxine secreted by the corpus luteum decreases uterine contractions.