Many drugs work by copying or blocking the messengers of the autonomic system. Knowing which receptor a drug acts on predicts both its effect and its side effects.
Adrenergic receptors
Alpha-1 receptors tighten blood vessels, beta-1 receptors speed and strengthen the heart, and beta-2 receptors relax the airways. Epinephrine stimulates all three, which is why it treats anaphylaxis.
Blocking the sympathetic side
Beta blockers such as metoprolol slow the heart and lower blood pressure. Nonselective ones such as propranolol also block beta-2 receptors and can trigger bronchospasm in asthma.
Cholinergic drugs
Atropine blocks muscarinic receptors, so it speeds a slow heart and dries secretions. Drugs that block the breakdown of acetylcholine, and organophosphate poisons, flood the body with parasympathetic effects.
Side effects to expect
Anticholinergic drugs cause dry mouth, urinary retention, constipation, a fast heart and confusion, especially in older adults. Sympathomimetic drugs can cause tachycardia, high blood pressure and tremor.
| Drug | Acts on | Main use |
|---|---|---|
| Epinephrine | Alpha and beta agonist | Anaphylaxis, cardiac arrest |
| Salbutamol (albuterol) | Beta-2 agonist | Asthma, COPD |
| Metoprolol | Beta-1 blocker | High blood pressure, heart rate control |
| Atropine | Muscarinic blocker | Symptomatic bradycardia |
At the bedside
Before giving a beta blocker, check heart rate and blood pressure and ask about asthma. After epinephrine or atropine, watch for tachycardia and recheck the vital signs.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).