Disclaimer:
This page is intended solely for educational purposes as part of the Pharm.D curriculum. It discusses pharmacological agents, including oxytocin and related compounds, for academic study only. No content on this page is meant for non-medical or non-educational interpretation.
Introduction
Uterine stimulants and relaxants are medicines that control the contraction and relaxation of the uterus. These drugs are important in obstetrics for inducing labour, controlling postpartum bleeding and managing preterm labour. Oxytocin is the most commonly used uterine stimulant, while β2-agonists, calcium channel blockers and other agents are used as uterine relaxants.
Uterine Stimulants (Oxytocics)
Uterine stimulants increase the strength and frequency of uterine contractions. They include:
- Oxytocin
- Ergot derivatives: Ergometrine, Methylergometrine
- Prostaglandins: PGE2, PGF2α, Misoprostol
Oxytocin
Oxytocin is a natural hormone produced in the hypothalamus and stored in the posterior pituitary. It is responsible for labour contractions and milk ejection during breastfeeding. In medicine, synthetic oxytocin is used for controlled induction or enhancement of labour.
Pharmacological Actions
- Uterus: Increases force and frequency of contractions. Sensitivity rises near term. Low doses allow relaxation between contractions; high doses may cause continuous contractions.
- Breast: Contracts myoepithelial cells → milk ejection.
- Kidney: High doses show ADH-like action → water retention.
- Cardiovascular system: High doses may cause vasodilation, hypotension and reflex tachycardia.
Uses
- Induction of labour: Preferred drug; administered as IV infusion with monitoring of mother and fetus.
- Postpartum hemorrhage (PPH): Given IM or IV to contract the uterus and stop bleeding.
- Uterine inertia: Used when contractions are weak during labour.
- Breast engorgement: Intranasal oxytocin may help milk let-down.
Adverse Effects
- Uterine hyperstimulation → fetal asphyxia or uterine rupture
- Water intoxication (headache, vomiting, seizures)
- Hypotension and tachycardia with high doses
Carbetocin
Carbetocin is a long-acting oxytocin analogue used especially after caesarean section to prevent uterine atony and PPH.
Ergot Derivatives
Ergometrine and methylergometrine strongly contract the uterus and increase basal tone. Unlike oxytocin, they also contract the lower uterine segment.
Pharmacological Actions
- Powerful, sustained uterine contractions (uterine tetany at high doses)
- Increased GI motility
- Mild vasoconstriction and rise in blood pressure
Uses
- Prevention and treatment of PPH (commonly after delivery of the anterior shoulder)
- Control of bleeding after caesarean section
- To promote uterine involution
Adverse Effects & Contraindications
- Nausea and vomiting
- Hypertension
- Reduced prolactin → may affect breastfeeding
Contraindicated in: Hypertension, peripheral vascular disease, sepsis, preeclampsia and eclampsia.
Prostaglandins
Prostaglandins like PGE1 (misoprostol) and PGE2 (dinoprostone) soften and dilate the cervix and stimulate uterine contractions.
Uses
- Induction of labour (cervical ripening)
- Prevention and treatment of PPH
- Misoprostol: oral/sublingual; Carboprost: intramuscular
Uterine Relaxants (Tocolytics)
Tocolytics relax the uterus and are used to delay preterm labour or manage conditions where reducing contractions is necessary.
β2-Adrenergic Agonists
Examples: Isoxsuprine, Salbutamol, Terbutaline, Ritodrine
- Act by relaxing uterine smooth muscle
- Side effects: Tachycardia, palpitations, arrhythmias, hyperglycemia, pulmonary edema
- Avoid in heart disease and diabetes
Calcium Channel Blockers
Nifedipine inhibits calcium entry into myometrial cells → smooth muscle relaxation.
Fewer side effects compared to β2-agonists.
Oxytocin-Receptor Antagonist
Atosiban blocks oxytocin receptors and relaxes the uterus. It is given through IV infusion and has fewer cardiovascular side effects.
Prostaglandin Synthesis Inhibitors
NSAIDs such as indomethacin produce tocolysis by reducing prostaglandin synthesis. Not preferred because of risk of premature closure of ductus arteriosus in the fetus.
Magnesium Sulphate
Given IV; depresses uterine smooth muscle, CNS and myocardium. Used in eclampsia and when β2-agonists are contraindicated.
Progesterone
Helps maintain pregnancy by relaxing the uterus. Used in threatened abortion.
Nitric Oxide Donors
Nitroglycerin has tocolytic action but can cause significant maternal hypotension.
Other Agents
Halothane, an inhalational anesthetic, also has potent uterine relaxant properties.
Uses of Tocolytics
- Delay of preterm labour
- Treatment of threatened abortion
- Relief of dysmenorrhea
Detailed Notes:
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