synthetic nine residue cyclic peptide. The hormone is prepared synthetically to keep away from possible infection with vasopressin (ADH) and different small polypeptides with biologic interest.
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.
Used for labor induction, augmentation of labor, postpartum abbreviation of third stage of labor, postpartum control of uterine bleeding, termination of pregnancy and for the evaluation of fetal respiratory capability. Oxytocin cannot be used for elective induction of labor, there must be a clear medical requirement.
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.