Pregnancy oxytocin

Oxytocin (Figure 7.11) stimulates the pregnant uterus, causing contractions, and also brings about ejection of milk from the breasts. It thus plays a major role in the normal onset of labour at the end of pregnancy. Oxytocin for drug use is produced by synthesis, and is employed to induce or augment labour, as well as to minimize subsequent blood loss. 

Oxytocin alters transmembrane ionic currents in myometrial smooth muscle cells to produce sustained uterine contraction. The sensitivity of the uterus to oxytocin increases during pregnancy. Oxytocin-induced myometrial contractions can be inhibited by 13-adrenoceptor agonists, magnesium sulfate, or inhalation anesthetics. Oxytocin also causes contraction of myoepithelial cells surrounding mammary alveoli, which leads to milk ejection. Without oxytocin-induced contraction, normal lactation cannot occur. Oxytocin has weak antidiuretic and pressor activity. 

Oxytocin is contraindicated in patients with known hypersensitivity to the drug, cephalopelvic disproportion, unfavorable fetal position or presentation, in obstetric emergencies, situations of fetal distress when delivery is not imminent, severe toxemia (preeclampsia, eclampsia), hypertonic uterus, during pregnancy (intranasal administration), when there is total placenta previa, or to induce labor when vaginal delivery is contraindicated. Oxytocin is not expected to be a risk to the fetus when administered as indicated. When oxytocin is administered with vasopressors, severe hypertension may occur. 

Luteolysis The structural and functional degradation of the corpus luteum that occurs at the end of the luteal phase in the absence of pregnancy caused by the hormones prostaglandin-2(X and oxytocin. Communication between the corpus luteum and uterine endometrium is necessary for luteolysis. 

M. Naruki, S. Mizutani, K. Goto, M. Tsujimoto, H. Nakazato, A. Itakura, M. Mizuno, O. Kurauchi, F. Kikkawa, Y. Tomoda, Oxytocin Is Hydrolyzed by an Enzyme in Human Placenta That Is Identical to the Oxytocinase of Pregnancy Serum , Peptides 1996, 17, 257-261. 

Oxytocin also stimulates contraction of uterine smooth muscle in late phases of pregnancy. See Chapter 62 for a full discussion of the use of oxytocin in labor and delivery. 

Oxytocin is a peptide hormone secreted by the posterior pituitary that participates in labor and delivery and elicits milk ejection in lactating women. During the second half of pregnancy, uterine smooth muscle shows an increase in the expression of oxytocin receptors and becomes increasingly sensitive to the stimulant action of endogenous oxytocin. Pharmacologic concentrations of oxytocin powerfully stimulate uterine contraction. 

Reproductive smooth muscles Prostaglandins Ej, E2, and F cause contraction of the pregnant and nonpregnant human uterus, and produce laborlike contractions. In contrast to oxytocin, this effect is possible in all stages of pregnancy. 

Estrogens influence the breast tissue at puberty, during each ovulatory cycle, and during pregnancy they participate in duct formation, and, in conjugation with progesterone, oxytocin, and other hormones, prepare the breasts for lactation. 

Q8 During pregnancy, contractions of the smooth muscle of the uterus are suppressed because of actions of progesterone. But at the end of pregnancy the concentration of progesterone declines and the high oestrogen concentration increases the density of oxytocin receptors in the myometrium. 

Q9 Myometrial contractile activity is modified by both endocrine and autonomic factors. The increase in oestrogens during pregnancy gradually increases both the excitability of uterine smooth muscle and its sensitivity to agonists, particularly oxytocin. The uterus receives sympathetic innervation, which exerts excitatory effects via alpha-1-receptors (a i-receptors). Uterine smooth muscle also possesses beta-2-receptors (/d2-recept°rs)> which mediate relaxation. 

Bleeding due to incomplete abortion can be controlled with ergometrine and oxytocin (Syntometrine) given intramuscularly. Their combination is more effective in early pregnancy than either drug alone. 

In 56 women (18-20 weeks gestation), treated at the Marie Stopes Chnic in Jodhpur, India, who underwent termination of pregnancy with 0.1% ethacridine lactate 150 ml injected into the intrauterine extra-amniotic space and in whom intravenous oxytocin was used to expedite the dehvery of the abortus, ethacridine lactate induced successful abortion in 52 cases (1). Abortion failure occurred in the other four cases because of transverse lie of fetus (n = 2), cervical dystocia (n = 1), and uterine inertia (n = 1). In 41 women the abortion occurred at 12-24 hours after induction (mean 20 hours) which was shorter than that of previous reports (29.5-38 hours). There were complications in six cases three women had cervical tears and three had incomplete expulsion. There was one case each of severe bleeding and vaginal laceration. There were no cases of sepsis. The authors concluded that ethacridine lactate performed better than other instillation abortion methods. 

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