Chemical Nature and Structure
Natural oxytocin is a hormone of the posterior pituitary gland (neurohypophysis). Synthetic analogs are used in clinical practice (synonyms of the native hormone include Syntocinon). Posterior pituitary extracts containing a complex of hormones (Pituitrin, Hyphotocin) also exist.
Chemically, it is a 9-amino acid polypeptide (octapeptide). Key structural elements include:
- Cyclic domain: 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 within the ring: Tyrosine residue (with a phenolic ring) and isoleucine.
- C-terminal tail: A linear chain of three amino acids (proline, leucine, glycine) terminating in 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 has low sensitivity to oxytocin.
In late gestation (the pre-labor period), estrogen levels rise. Estrogens sensitize the uterus by increasing the number of oxytocin receptors and raising prostaglandin levels ($E_2$ and $F_{2\alpha}$). Maximum receptor sensitivity is observed immediately before labor, during labor, and immediately after.
Pharmacological Effects
- Effect on the Uterus (Uterotonic Effect): In physiological concentrations, the drug stimulates normal labor activity (rhythmic contractions primarily of the uterine body). At higher doses, the effect shifts toward increasing baseline myometrial tone, carrying 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 surrounding 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 agents.
- Management of Postpartum Hemorrhage: Prescribed for uterine atony and subinvolution. Rare administration routes include direct injection into the cervical and uterine tissue.
- Stimulation of Lactation: To facilitate milk ejection in the early postpartum period.
Side Effects and Contraindications
Side Effects:
- Cardiovascular system: Tachycardia, arterial hypotension.
- Metabolic disturbances: Water retention (due to the antidiuretic effect) — risk increases with prolonged intravenous infusion.
- Myometrium: Uterine spasm in case of overdosage.
Contraindications:
- Preterm labor.
- Cephalopelvic disproportion.
- Elevated blood pressure. Note: In arterial hypertension, the modified analog demoxytocin (Demoxytocinum) is permitted, as it lacks the vasopressin-like effect.
Administration 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, intravenous infusion is used: 5 IU of the drug is diluted in 500 mL of 5% glucose solution. Infusion rate is strictly controlled at 10 drops per minute.
Sample Prescription: Rp.: Sol. Oxytocini 5 ED - 1 ml D.t.d. N. 5 in amp. S. Intravenous infusion. Dilute the contents of 1 ampoule in 500 mL of 5% glucose solution, infuse at a rate of 10 drops per minute.