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Drugs Affecting the Myometrium

Q: What are the clinical indications for oxytocin?

Indications for oxytocin include: Stimulation of labor — used for uterine inertia; oxytocin does not relax the cervix and is used when cervical dilation is complete or combined with cervical-relaxing drugs. In multiple gestation, oxytocin may be used if contractions are absent or weak.Postpartum period — used for the prevention and treatment of hypotonic uterine hemorrhage and to accelerate uterine involution.Lactation support — used when milk ejection is impaired in the early postpartum period.

Uterotonica

For medical students2 min readUpdated 2026-10-10

Drugs affecting the myometrium are pharmacological agents capable of enhancing or diminishing uterine function. Their primary sites of action are rhythmic contractile activity and the baseline tone of smooth muscle, allowing clinicians to manage labor and maintain pregnancy.

Sites of ActionDrugs target two parameters: rhythmic contractile activity and tone.

UterotonicsMyometrial stimulants divided into those increasing tone and those enhancing rhythmic contractions.

Pregnancy HormoneProgesterone relaxes the uterine body and contracts the cervix to ensure successful gestation.

Oxytocin StructureA nonapeptide (nine amino acids) with a disulfide bridge between cysteine molecules.

Receptor and Humoral Regulation

The functional state of the uterus depends directly on the activity of the autonomic nervous system and the hormonal balance. Specific receptors are located on the membranes of smooth muscle cells, and their stimulation produces opposing effects:

Stimulatory receptors: $M$-cholinergic receptors and $\alpha$-adrenergic receptors. Their activation leads to increased tone and contractility of the myometrium.
Inhibitory receptors: Extrasynaptic $\beta_2$-adrenergic receptors. They bind circulating blood catecholamines or exogenous adrenergic agonists, inhibiting muscular activity.

Humoral regulation is mediated by biologically active substances. Stimulants include oxytocin, estrogens, and prostaglandins (groups $E_2$ and $F_{2\alpha}$). An important feature of prostaglandins is their dual action: they cause the uterine body to contract while simultaneously relaxing and dilating the cervix. The primary inhibitory factor is progesterone.

Uterine Dynamics During Pregnancy and Labor

Depending on the gestational age, the hormone ratio shifts to allow the female body to fulfill specific biological tasks.

Gestation Period: Progesterone dominates. Under its influence, the myometrium of the uterine body is relaxed, and the cervix is tightly closed. The main goal of this stage is the reliable maintenance of pregnancy.
Pre-labor Period (Late Gestation): Estrogen levels begin to rise. They sensitize the uterus by increasing the number of oxytocin receptors and enhancing the production of stimulatory prostaglandins.
Labor and Postpartum Period: The combined action of oxytocin and prostaglandins initiates labor, providing strong rhythmic contractions (labor pains). After delivery, high myometrial tone is critical for uterine involution (reduction in size) and the cessation of postpartum hemorrhage.

Principles of Pharmacological Regulation

Three main groups of substances are used to pharmacologically manage contractile activity:

Endogenous compound drugs (analogs of natural regulators).
Agents affecting uterine adrenergic receptors.
Substances modulating the hormonal background.

Medicines that enhance uterine function (uterotonics) are classified by their predominant mode of action. The first group comprises agents that increase rhythmic contractions. They mimic natural labor by causing alternating phases of tension and relaxation with a simultaneous increase in tone. The second group consists of agents that predominantly increase tone, causing prolonged tonic tension of the muscle fibers.

Agents Enhancing Rhythmic Contractions

This group includes prostaglandins and oxytocin preparations. Oxytocin is a natural hormone of the posterior pituitary gland (neurohypophysis) and serves as a physiological labor stimulant.

Several dosage forms are used in pharmacological practice:

Native hormone: Oxytocin (synonym — Syntocinon).
Modified analogs: Methyloxytocin and demoxytocin (synonyms — Sandopart, Desaminooxytocin).
Posterior pituitary extracts: Pituitrin (Pituitrinum) and hyphotocin, containing a complex of hormones.

The pharmacodynamics of oxytocin strongly depend on the dose. At physiological concentrations, it stimulates normal labor activity. However, when the dose is increased, the effect shifts: the baseline myometrial tone rises sharply, creating a high risk of dangerous spasm.

Mnemonic

How to remember the autonomic nervous system's effect on the uterus: M and Alpha = Muterus Active (stimulation). Beta = Blocks activity (inhibition).

Frequently asked questions

Which drugs belong to the group of agents that predominantly increase myometrial tone?

Drugs that predominantly increase myometrial tone include agents that cause sustained tonic contraction of the uterine muscle rather than rhythmic labor activity. Clinical practice distinguishes two main subgroups:

Ergot alkaloids — include ergometrine (ergometrine maleate) and ergotamine (cornutamine, rigetamine).
Synthetic agents — represented by cotarnine chloride (stypticin).

Which drugs are used as tocolytics (agents that reduce uterine contractions)?

Tocolytics (agents for myometrial relaxation) include drugs from several pharmacological groups:

$\beta_2$-adrenergic agonists — hexoprenaline (Gynipral), salbutamol, terbutaline, fenoterol, ritodrine.
Progestins — oxyprogesterone, hydroxyprogesterone caproate, as well as the combination of ethinyl estradiol and acetomepregenol.
Oxytocin receptor antagonists — atosiban.
Myotropic antispasmodics — magnesium sulfate.
Central nervous system depressants — general anesthetics (enflurane) and anxiolytics (diazepam).

What are the clinical indications for oxytocin?

Indications for oxytocin include:

Stimulation of labor — used for uterine inertia; oxytocin does not relax the cervix and is used when cervical dilation is complete or combined with cervical-relaxing drugs. In multiple gestation, oxytocin may be used if contractions are absent or weak.
Postpartum period — used for the prevention and treatment of hypotonic uterine hemorrhage and to accelerate uterine involution.
Lactation support — used when milk ejection is impaired in the early postpartum period.

What complications does oxytocin overdose cause?

With increased concentration or prolonged use of oxytocin, the following complications may occur:

Excessive increase in baseline myometrial tone (uterine spasm) — at higher concentrations, oxytocin's effect shifts toward increasing myometrial tone.
Water retention — possible during prolonged infusion due to the weak antidiuretic (vasopressin-like) action of the drug.
Cardiovascular disturbances — tachycardia and arterial hypotension are possible.

Which specific prostaglandin preparations are used to stimulate the myometrium?

Synthetic analogs of endogenous prostaglandins are used to stimulate the myometrium.

Dinoprostone — a prostaglandin $E_2$ preparation (trade names: Prostin $E_2$, Prostenon).
Dinoprost — a prostaglandin $F_{2\alpha}$ preparation (trade name: Enzaprost-F).

These agents exert a marked stimulatory effect, causing rhythmic contractions and increasing uterine tone. Their key feature is that they are effective at any stage of gestation and additionally promote relaxation and dilation of the cervix.

How do prostaglandins affect different parts of the uterus?

They have a dual effect: they stimulate smooth muscle contraction in the uterine body while promoting relaxation and dilation of the uterine cervix.

Why is an increase in estrogen levels necessary before labor?

Estrogens sensitize the uterus by increasing the number of oxytocin receptors and raising the concentration of stimulatory prostaglandins ($E_2$ and $F_{2\alpha}$).

What is the danger of oxytocin overdose?

In physiological doses, the drug causes normal rhythmic contractions, but at excessive concentrations, the baseline myometrial tone increases sharply, which can lead to spasm.

Go deeper

Mechanism of action of drugs that predominantly increase uterine tone.
Pharmacodynamics and clinical use of $E_2$ and $F_{2\alpha}$ prostaglandins.
Agents that decrease myometrial tone and their effect on $\beta_2$-adrenergic receptors.
Clinical features of pituitary extracts (pituitrin, hyphotocin).