Receptor and humoral regulation
The functional state of the uterus directly depends on the activity of the autonomic nervous system and the hormone balance. Specific receptors are located on the membranes of smooth muscle cells, the stimulation of which produces multidirectional effects:
- Stimulating receptors: M-cholinergic receptors and $\alpha$-adrenergic receptors. Their activation leads to an increase in myometrial tone and contractility.
- Inhibitory receptors: Extrasynaptic $\beta_2$-adrenergic receptors. They bind to circulating blood catecholamines or exogenous adrenomimetics, suppressing muscular activity.
Humoral regulation is provided 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 body of the uterus to contract while relaxing and dilating its cervix. The main inhibitory factor is progesterone.
Dynamics of the uterine state during pregnancy and labor
Depending on the gestational age, the hormone ratio changes so that the female body can perform specific biological tasks.
- Gestation period: Progesterone dominates. Under its influence, the myometrium of the uterine body is relaxed, and the cervix is tightly contracted. The main goal of this stage is the reliable maintenance of pregnancy.
- Pre-labor period (late terms): Estrogen levels begin to rise. They ensure the sensitization of the uterus: they increase the number of oxytocin receptors and enhance the production of stimulating prostaglandins.
- Labor and postpartum period: The combined action of oxytocin and prostaglandins initiates labor, ensuring strong rhythmic contractions (bouts). After delivery, high myometrial tone is critically important for uterine involution (reduction in size) and the cessation of postpartum hemorrhage.
Principles of pharmacological regulation
Three main groups of substances are used for the pharmacological management of contractile activity:
- Drugs of endogenous compounds (analogues of natural regulators).
- Agents acting on the adrenergic receptors of the uterus.
- Substances that modulate the hormonal background.
Medicines that enhance uterine function (uterotonics) are classified according to their predominant character of action. The first group consists of agents that increase rhythmic contractions. They mimic natural labor, causing an alternation of tension and relaxation phases with a simultaneous increase in tone. The second group consists of agents that primarily increase tone, causing prolonged tonic tension of muscle fibers.
Agents increasing rhythmic contractions
This group includes prostaglandins and oxytocin preparations. Oxytocin is a natural hormone of the posterior pituitary gland (neurohypophysis) and is a physiological stimulant of labor.
Several release forms are used in pharmacological practice:
- Native hormone: Oxytocin (synonym — Syntocinon).
- Modified analogues: Methylergometrine and demoxytocin (synonyms — Sandopart, Desaminooxytocin).
- Posterior pituitary extracts: Pituitrin (Pituitrinum) and hyphotocin, containing a complex of hormones.
The pharmacodynamics of oxytocin strictly depends on the dose. In physiological concentrations, it stimulates normal labor activity. However, with an increased dose, the effect shifts: the baseline myometrial tone sharply increases, creating a high risk of dangerous spasm.
Briefly: answers to topical questions
Which drugs belong to the group of agents that predominantly increase myometrial tone?
The group of agents that predominantly increase myometrial tone includes drugs that cause persistent tonic contraction of the uterine muscle rather than rhythmic labor activity. Two main subgroups are distinguished in clinical practice:
- Ergot alkaloids — include ergometrine (ergometrine maleate) and ergotamine (cornutamin, rigetamin).
- Synthetic agents — represented by cotarnine chloride (stypiticin).
Which drugs are used as tocolytics (agents that weaken uterine contractions)?
Drugs from several pharmacological groups are used as tocolytics (agents for myometrial relaxation):
- $\beta_2$-adrenomimetics — hexoprenaline (Gynipral), salbutamol, terbutaline, fenoterol, ritodrine.
- Progestogens — oxyprogesterone, hydroxyprogesterone caproate, as well as a 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 indications for the use of oxytocin?
Indications for the use of oxytocin:
- Stimulation of labor activity — used for uterine inertia; meanwhile, oxytocin does not relax the cervix and is used at full cervical dilation or in combination with cervical-relaxing drugs. In multiple pregnancy, oxytocin may be used in the absence of contractions or their weakening.
- Postpartum period — used for the prevention and cessation of hypotonic uterine hemorrhage, as well as to accelerate uterine involution.
- Lactation support — used for difficulty in milk ejection in the early postpartum period.
What complications are caused by oxytocin overdose?
With increased concentration or prolonged use of oxytocin, the following complications are possible:
- Excessive increase in baseline myometrial tone (uterine spasm) — with increased concentration, the effect of oxytocin shifts towards increasing myometrial tone.
- Water retention in the body — possible with prolonged infusion due to the weak antidiuretic (vasopressin-like) action of the drug.
- Cardiovascular disorders — tachycardia and arterial hypotension are possible.
Which specific prostaglandin drugs are used for myometrial stimulation?
Drugs that are synthetic analogues of endogenous prostaglandins are used for myometrial stimulation.
- Dinoprostone — a prostaglandin $E_2$ preparation (trade names: Prostenon, Prostin $E_2$).
- Dinoprost — a prostaglandin $F_{2\alpha}$ preparation (trade name: Enzaprost-F).
These agents exhibit a pronounced stimulating effect, cause rhythmic contractions, and increase uterine tone. Their key difference is that they are effective at any stage of pregnancy and additionally promote relaxation and dilation of the cervix.
How to remember the influence of the autonomic nervous system on the uterus: M and Alpha — Uterus is Active (stimulation). Beta — Blocks activity (inhibition).
Class and exam questions
How do prostaglandins affect different parts of the uterus?
They possess a dual effect: they stimulate the contraction of the smooth muscle of the uterine body, while also promoting the relaxation and dilation of the cervix.
Why is an increase in estrogen levels necessary before labor?
Estrogens cause sensitization of the uterus — they increase the number of oxytocin receptors and raise the concentration of stimulating prostaglandins (E2 and F2α).
What are the risks of oxytocin overdose?
In physiological doses, the drug causes normal rhythmic contractions, but when the concentration is exceeded, the baseline myometrial tone sharply increases, which can lead to spasm.
What else to study on this topic
- Mechanism of action of drugs that predominantly increase uterine tone.
- Pharmacodynamics and clinical use of E2 and F2α group prostaglandins.
- Agents that decrease myometrial tone and their effect on β2-adrenoceptors.
- Features of the use of posterior pituitary extracts (pituitrin, hypophytocin).
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