Drugs affecting myometrial tone and contractile activity
7 topics with concise reviews: definitions, key facts, and class questions.
This section covers the pharmacology of drugs that regulate uterine contractile activity. They are the primary pharmacological tool in obstetrics and gynecology, essential for both labor and delivery and maintaining pregnancy.
Here we will examine in detail myometrial stimulants (oxytocin and posterior pituitary preparations, prostaglandins, ergot alkaloids), as well as agents that affect cervical dilation. The counterpart group consists of uterine-relaxing drugs: tocolytics and hormonal agents for pregnancy maintenance. In addition, the section includes appetite-regulating agents (as a related topic within the systemic function regulation module.
During tests and exams, examiners are strictest about the differences in the mechanisms of action between oxytocin and ergot alkaloids (why the former are needed to induce labor, while the latter are used exclusively to stop bleeding). Pay close attention to the side effects of tocolytics and the effects of prostaglandins on vascular tone.
Why is oxytocin ineffective for early pregnancy termination?
The sensitivity of myometrial receptors to oxytocin directly depends on the hormonal background and gestational age. In early stages, the non-pregnant or slightly enlarged uterus is virtually insensitive to it.
What is the main difference between prostaglandins and oxytocin?
Myometrial sensitivity to prostaglandins does not depend on gestational age, which is why they effectively stimulate the uterus at any stage of pregnancy, whereas oxytocin works predominantly in late terms.
Why are ergot alkaloids contraindicated during labor?
They cause sustained tonic contraction of the myometrium rather than rhythmic labor activity. Such a spasm cuts off blood flow and leads to fetal asphyxia.
What is the difference between tocolytics and cervical-relaxing agents?
Tocolytics act on the myometrium of the uterine body, decreasing its tone to stop contractions. Drugs that relax the cervix are used for the opposite clinical purpose — to accelerate delivery or perform intrauterine interventions.
Due to what mechanism does uterine tone decrease when taking these hormonal agents?
Tone is reduced due to the pharmacological attenuation of the physiological effects of oxytocin. This directly leads to a drop in myometrial excitability and contractility.
Which drugs are primary for stimulating cervical dilation?
The drugs of choice in this case are prostaglandins, such as dinoprost and dinoprostone.
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