Drugs Acting on the Uterus
1. Introduction
The uterus is a smooth-muscle organ that can contract and relax under the influence of hormones. Drugs that act on the uterus are used to either stimulate contractions or relax the uterus. They are important in obstetrics to start or speed up labor, control bleeding after delivery, induce abortion, or stop premature labor. There are two main groups: (1) Uterine stimulants (Oxytocics) – make the uterus contract; (2) Uterine relaxants (Tocolytics) – stop or reduce contractions.
2. Physiology of Uterine Contraction
Uterine contraction depends on calcium ions (Ca²⁺). Oxytocin and prostaglandins increase calcium and cause contraction, while progesterone keeps the uterus relaxed. Near delivery, estrogen makes the uterus more sensitive to oxytocin.
3. Classification of Drugs Acting on the Uterus
A. Uterine Stimulants (Oxytocics): Oxytocin, Ergometrine, Methylergometrine, Prostaglandins (Dinoprostone, Carboprost, Misoprostol), Mifepristone.
B. Uterine Relaxants (Tocolytics): β₂-agonists (Salbutamol, Terbutaline, Ritodrine), Calcium channel blockers (Nifedipine), Magnesium sulfate, Oxytocin antagonist (Atosiban), Indomethacin.
4. Uterine Stimulants (Oxytocics)
a) Oxytocin: Natural hormone that increases uterine contractions by releasing calcium in uterine muscles. Used to induce or speed labor, prevent postpartum hemorrhage, and help milk ejection. Side effects include uterine rupture and water intoxication.
b) Ergometrine: Causes strong, continuous contractions by acting on alpha and serotonin receptors. Used to control postpartum bleeding. Side effects include hypertension and nausea.
c) Prostaglandins: Cause uterine contraction and soften the cervix. Used to induce labor, induce abortion, and control postpartum bleeding. Side effects include diarrhea, fever, and cramps.
d) Mifepristone: Blocks progesterone and increases uterine sensitivity to prostaglandins. Used for medical abortion with misoprostol.
5. Uterine Relaxants (Tocolytics)
a) β₂-Agonists (Salbutamol, Terbutaline): Relax uterine muscles by increasing cAMP. Used to stop premature labor. Side effects include tremor and tachycardia.
b) Nifedipine: Calcium channel blocker that reduces calcium entry, causing relaxation. Used to delay preterm labor. Side effects include dizziness and hypotension.
c) Magnesium Sulfate: Competes with calcium to relax uterine muscles. Used in eclampsia and preterm contractions. Antidote: Calcium gluconate.
d) Atosiban: Blocks oxytocin receptors. Used for short-term treatment of preterm labor.
e) Indomethacin: Reduces prostaglandin synthesis. Used in early preterm labor.
6. Clinical Uses
| Condition | Common Drug | Purpose |
|------------|--------------|----------|
| Induce labor | Oxytocin, Dinoprostone | Start or increase contractions |
| Prevent PPH | Oxytocin, Ergometrine, Misoprostol | Stop bleeding |
| Abortion | Mifepristone + Misoprostol | Expel uterine contents |
| Delay preterm labor | Nifedipine, MgSO₄, Atosiban | Relax uterus |
| Eclampsia | MgSO₄ | Prevent seizures |
7. Side Effects and Antidotes
| Drug | Side Effect | Antidote |
|------|--------------|-----------|
| Oxytocin | Uterine rupture | Stop infusion, β₂-agonist |
| Ergometrine | Hypertension | Nitroglycerin |
| MgSO₄ | Respiratory depression | Calcium gluconate |
| β₂-agonists | Tachycardia | Stop drug |
8. Summary Table
| Group | Example | Mechanism | Use | Side Effects |
|--------|----------|------------|------|--------------|
| Oxytocic | Oxytocin | Increases Ca²⁺ | Induce labor | Uterine rupture |
| Ergot | Ergometrine | α-agonist | Control PPH | Hypertension |
| Prostaglandin | Misoprostol | PGE₁ analogue | Abortion | Fever, diarrhea |
| Tocolytic | Nifedipine | Blocks Ca²⁺ | Delay labor | Hypotension |
| Tocolytic | MgSO₄ | Competes Ca²⁺ | Eclampsia | Depression |
9. Multiple Choice Questions
1. Oxytocin is used to: (B) Induce labor.
2. Drug contraindicated in hypertension: (B) Ergometrine.
3. Drug to delay labor: (A) Nifedipine.
4. Antidote for magnesium toxicity: (A) Calcium gluconate.
5. Misoprostol is a: (A) Prostaglandin analogue.
10. Essay Questions
1. Classify uterine drugs and describe uses.
2. Explain mechanism and uses of oxytocin.
3. Describe management of postpartum hemorrhage.
4. Write notes on uterine relaxants.
5. Compare oxytocin and ergometrine.
11. References
Tripathi KD (2023), Katzung BG (2021), Rang HP (2020), WHO (2023).
