Chemical nature and structure
Endogenous oxytocin is a hormone secreted by the posterior pituitary (neurohypophysis). Its synthetic analogue is used in medical practice (a synonym for the native hormone is Syntocinon). There are also posterior pituitary extracts containing a complex of hormones (Pituitrin, Hyphotocin).
Chemically, it is a 9-amino-acid polypeptide (octapeptide). Key structural elements:
- Cyclic part: formed by a disulfide bond ($-S-S-$) between two cysteine residues at positions 1 and 6 (pentapeptide ring). The N-terminus contains a free amino group.
- Side chains in the ring: tyrosine residue (with a phenol ring) and isoleucine.
- C-terminal part (tail): a linear chain of three amino acids (proline, leucine, glycine) ending with an amide group ($NH_2$).
Mechanism of action
Oxytocin exerts a selective effect on the uterine myometrium by stimulating specific oxytocin receptors.
Uterine sensitivity to the hormone depends on the gestational age and humoral regulation. During pregnancy, progesterone dominates, relaxing the myometrium of the uterine body and contracting the cervix to maintain pregnancy. The non-pregnant uterus is poorly sensitive to oxytocin.
In late gestation (the pre-labor period), estrogen levels increase. They sensitize the uterus by increasing the number of oxytocin receptors and raising the levels of prostaglandins ($E_2$ and $F_{2\alpha}$). Maximum receptor sensitivity is observed immediately before labor, during labor, and right after it.
Pharmacological effects
- Effect on the uterus (uterotonic effect): At physiological concentrations, the drug stimulates normal labor activity (rhythmic contractions primarily of the uterine body). When the dose is increased, the effect shifts toward raising the baseline myometrial tone, creating a risk of spasm. Crucial rule: oxytocin does not relax the cervix (unlike prostaglandins, which stimulate the body while dilating the cervix).
- Effect on lactation (lactogenic effect):
- Central action: stimulates the release of prolactin from the anterior pituitary, increasing milk secretion.
- Peripheral action: contracts myoepithelial cells around the alveoli and ducts of the mammary glands, facilitating milk ejection (let-down).
- Effect on the kidneys: Exhibits a mild antidiuretic (vasopressin-like) effect.
Indications for use
- Induction of labor: Used strictly upon full cervical dilation or in combination with cervical-relaxing drugs.
- Cessation of postpartum hemorrhage: Indicated for uterine atony and subinvolution. Rare routes of administration include direct injection into the tissues of the cervix and uterine body.
- Stimulation of lactation: To facilitate milk ejection in the early postpartum period.
Side effects and contraindications
Adverse effects:
- Cardiovascular system: tachycardia, arterial hypotension.
- Metabolic disorders: water retention (due to the antidiuretic effect) — poses a risk during prolonged intravenous infusion.
- Myometrium: uterine spasm upon overdose.
Contraindications:
- Premature labor.
- Cephalopelvic disproportion (mismatch between the fetal head and the mother's pelvis).
- Elevated blood pressure (BP). Note: in arterial hypertension, a modified analogue, demoxytocin (Demoxytocinum), is permitted for use as it lacks the vasopressin-like effect.
Administration guidelines and prescription
The drug is administered intramuscularly or intravenously. The half-life ($t_{1/2}$) is very short at 3–5 minutes.
For labor induction, continuous intravenous infusion is used: 5 IU of the drug is diluted in 500 ml of a 5% glucose solution. The infusion rate is strictly controlled at 10 drops per minute.
Prescription example: Rp.: Sol. Oxytocini 5 ED - 1 ml D.t.d. N. 5 in amp. S. Intravenously by drip infusion. Dilute the contents of 1 ampoule in 500 ml of 5% glucose solution, administer at a rate of 10 drops per minute.
Briefly: answers to topical questions
What are the contraindications to oxytocin administration?
Contraindications to oxytocin administration include conditions that threaten the health of the mother and fetus during uterine stimulation. These include:
- Premature labor — stimulation is inappropriate.
- Size disproportion — between the fetal head and the maternal pelvis.
- Arterial hypertension — elevated blood pressure.
Additionally, the use of the drug poses a serious hazard and may lead to mechanical uterine rupture in multiparous women with a large fetus in the presence of pathological changes in the uterine wall.
What drugs are used for cervical ripening and relaxation prior to oxytocin administration?
To ripen, relax, and dilate the cervix prior to oxytocin administration, drugs that eliminate cervical rigidity are used. These include:
- Prostaglandins (dinoprost, dinoprostone) — drugs of choice that promote cervical relaxation and dilation.
- M-anticholinergics (atropine) — used less frequently as alternative agents.
Oxytocin itself does not relax the cervix; therefore, in the absence of full dilation, it is used only in combination with the aforementioned relaxant drugs.
Which amino acids are part of the cyclic portion of the oxytocin molecule?
The cyclic portion of the oxytocin molecule is formed by a disulfide bond between two cysteine residues at positions 1 and 6.
Sources also explicitly mention the following for the cyclic portion:
- tyrosine — contains a phenol ring with an OH group;
- isoleucine — has a branched aliphatic chain.
The C-terminal part is a linear chain of three amino acids: proline, leucine, and glycine, ending with an amide group.
What oxytocin receptor antagonists exist?
Atosiban is used in clinical practice as an oxytocin receptor antagonist (blocker).
- Atosiban — a highly selective oxytocin receptor blocker.
Its mechanism of action involves blocking receptors, leading to decreased myometrial tone and contractility. The drug is used as a tocolytic to prolong pregnancy in threatened or early preterm labor (at 24–34 weeks of gestation). It is administered intravenously when other tocolytic agents are contraindicated.
Oxytocin CONTRACTS the uterus, but DOES NOT RELAX the cervix. No dilation — no oxytocin!
Class and exam questions
Why is oxytocin contraindicated when the cervix is immature (unripe)?
Oxytocin stimulates powerful contractions of the uterine body but does not relax the cervix at all. If the cervix is closed, stimulation will lead to spasm, risk of uterine rupture, and acute fetal hypoxia.
What is the pharmacological advantage of demoxytocin over oxytocin?
Demoxytocin (an analogue with deaminated cysteine) is resistant to salivary peptidases and can therefore be administered buccally. Furthermore, it lacks the antidiuretic (vasopressin-like) effect and is permitted in hypertension.
How do estrogens and progesterone affect oxytocin efficacy?
Progesterone suppresses uterine contractility during pregnancy. Estrogens, whose levels rise before labor, sensitize the myometrium by increasing the number of oxytocin receptors, making the uterus maximally sensitive to the drug.
What else to study on this topic
- Prostaglandins (dinoprostone) and their dual effect on the uterus and cervix
- Demoxytocin: pharmacokinetic features and buccal administration
- Humoral regulation of myometrial contractility at different gestations
- Uterine bleeding cessation drugs (methylergometrine)
- Tocolytics: myometrium-relaxing agents (oxytocin antagonists)
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