Oxytocin and posterior pituitary preparations

Pharmacodynamics: mechanism of action and effects

The primary action of oxytocin targets smooth muscle. Primarily, it stimulates the contractile activity of the myometrium, with the most pronounced effect exerted on the uterine body. Notably, the response of muscle cells is variable: it is closely linked to the woman's current hormonal background and gestational age. While the non-pregnant uterus barely responds to the hormone, as the fetus develops, the number of receptors increases, reaching an absolute maximum just by the onset of labor.

In addition to influencing the reproductive system, the drug plays a key role in lactation by acting at two levels:

• Central effect: acts on the anterior pituitary gland, triggering the release of prolactin, which leads to increased breast milk secretion.
• Peripheral effect: causes contraction of myoepithelial cells surrounding the ducts of the mammary glands, thereby facilitating milk ejection (release).

Pharmacokinetics and route of administration dependence

Upon entering the systemic circulation, drug molecules do not bind to plasma proteins. The active substance has an extremely short half-life ($t_{1/2}$), ranging from only one to four minutes.

The onset speed of the clinical effect and its duration directly depend on the chosen administration route:

• With intravenous administration (often prescribed as a drip, diluted in a glucose solution), the action unfolds rapidly—within 30–60 seconds.
• With intramuscular injections, the effect takes 3 to 7 minutes to appear, but its duration increases and can range from half an hour to three hours.

Clinical use in obstetrics

Drugs of this group are dosed in special international units (IU) and are used in strictly defined situations:

1. Labor induction and augmentation. The main indication is diagnosed uterine inertia.
2. Postpartum management. Uterotonics are essential for the prevention and emergency cessation of hypotonic uterine hemorrhage. Increasing myometrial tone promotes faster uterine involution.
3. Breastfeeding support. For difficulties with milk ejection, intranasal forms (spray) are prescribed, applied 2–3 minutes before breastfeeding.

Safety profile and limitations

The use of uterine contraction stimulants is associated with certain risks. Maternal and fetal cardiovascular systems may experience blood pressure surges, rhythm disturbances (arrhythmias), bradycardia, and in rare cases, hypotension and shock. Severe obstetric complications include uterine hypertonus (tetany) and premature placental abruption. Allergic reactions, bronchospasm, nausea, and vomiting may also occur. If any such symptoms are recorded, drug administration must be stopped immediately.

Absolute contraindications:

• Mechanical barriers to spontaneous labor (clinically narrow pelvis, face presentation, oblique or transverse fetal lie).
• Presence of uterine scars (consequences of cesarean section or cervical surgeries).
• Severe maternal somatic pathology (renal failure, arterial hypertension).

Analogues and animal-derived preparations

In addition to classic oxytocin, physicians have other agents with a similar mechanism of action:

• Demoxytocin (deaminooxytocin). A synthetic analogue exceeding the original in pharmacological activity. Its main advantage is resistance to the destructive action of salivary enzymes. It is available as buccal tablets, which are well absorbed through the oral mucosa. Used for uterine inertia, to accelerate uterine involution, and to stimulate lactation.
• Pituitrin. An aqueous extract of the posterior pituitary gland obtained from slaughtered livestock. It contains not only oxytocin but also vasopressin (antidiuretic hormone). Due to vasopressin, the drug has a pronounced side effect—the ability to raise blood pressure.
• Gyphotocin. Another animal-derived extract, but unlike pituitrin, it contains predominantly oxytocin, making its action more selective.

Class and exam questions

What else to study on this topic

• Regimens for intravenous drip infusion of oxytocin in glucose solution for uterine inertia.
• Mechanisms of fetal bradycardia development against the background of uterotonic use.
• Features of using oxytocin intranasal spray to support lactation.
• Risks of using posterior pituitary preparations in arterial hypertension and renal failure.
• Pathogenesis of hypotonic uterine hemorrhage and the role of uterotonics in their management.

Other topics in the section "Drugs affecting myometrial tone and contractile activity"

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