Uterine motility depends on the formation of the contractile protein actomyosin under the influence of the Ca2+-dependent phosphorylating enzyme myosin light-chain kinase. Oxytocin promotes contractions by increasing the intracellular Ca2+, which in turn activates myosin's light chain kinase.. Oxytocin has specific receptors in the muscle lining of the uterus and the receptor concentration increases greatly during pregnancy, reaching a maximum in early labor at term.
Indirectly stimulates contraction of uterine smooth muscle by increasing the sodium permeability of uterine myofibrils. Increases contraction amplitude and frequency, which tends to decrease cervical activity, produce dilation and effacement of the cervix, and transiently impede uterine blood flow; contractions produced by oxytocin at term are similar to those occurring during spontaneous labor. High estrogen concentrations lower the threshold for uterine response to oxytocin. Uterine response increases with the duration of pregnancy and is greater in labor than when not in labor; only very large doses elicit contractions in early pregnancy. Contracts myoepithelial cells surrounding the alveoli of the breasts, forcing milk from the alveoli into the larger ducts and facilitating milk ejection. Minimal antidiuretic activity relative to vasopressin; water intoxication possible at high doses and/or excessive electrolyte-free fluid.
Oxytocin is rapidly metabolized in the liver and also in the plasma by oxytocinases. It is also metabolized to a smaller degree by the mammary glands.
Oxytocin can produce a severe water intoxication due to its antidiuretic effect. Prolonged IV infusions of more 40milliunits/min are usually the cause. Severe water intoxication with convulsions, coma, and death can occur. Some neonatal seizures have also been reported. Consider potential for water intoxication, particularly when administered by IV infusion and patient is receiving oral fluids. Uterine effects: High doses or hypersensitivity to oxytocin may cause uterine hypertonicity, spasm, tetanic contraction, or rupture of the uterus. There have also been reported allergic and anaphylactic reactions to oxytocin but they were rarely fatal.