Receptors and their effects
| Receptor | Main effects | Drug examples |
|---|---|---|
| α₁ | Vasoconstriction, raised blood pressure, mydriasis | Phenylephrine, epinephrine, norepinephrine, levonordefrin |
| α₂ | Presynaptic inhibition of norepinephrine release; central sedation and lower blood pressure | Clonidine, dexmedetomidine |
| β₁ | Raised heart rate and contractility; renin release | Epinephrine; blocked by metoprolol and atenolol |
| β₂ | Bronchodilation; vasodilation in skeletal muscle; glycogenolysis | Albuterol, epinephrine; blocked by nonselective β-blockers (propranolol) |
| Muscarinic (M) | Salivation, bronchoconstriction, bradycardia, miosis, GI and bladder activity | Agonists pilocarpine and cevimeline; antagonists atropine, glycopyrrolate, scopolamine |
| Nicotinic (Nn ganglia, Nm muscle) | Ganglionic transmission; skeletal muscle contraction | Succinylcholine and rocuronium act at Nm |
- Preganglionic fibers of both divisions release acetylcholine; postganglionic sympathetic fibers release norepinephrine (except sweat glands, which use acetylcholine).
Dental applications
- Xerostomia from Sjögren syndrome or head and neck radiation: pilocarpine or cevimeline (muscarinic agonists) if some gland function remains. Side effects: sweating, flushing, GI upset; caution in asthma and narrow-angle glaucoma.
- Drying the field: atropine or glycopyrrolate reduce saliva; avoid in narrow-angle glaucoma, prostatic hypertrophy and tachyarrhythmias.
- Saliva: parasympathetic stimulation gives copious watery saliva; sympathetic stimulation gives small amounts of thick, protein-rich saliva.
- Many common drugs have anticholinergic side effects that cause dry mouth: tricyclic antidepressants, first-generation antihistamines, antipsychotics, bladder antispasmodics.
Epinephrine and β-blockers
- With a NONSELECTIVE β-blocker (propranolol, nadolol), epinephrine's β₂ vasodilation is blocked, leaving unopposed α₁ vasoconstriction → sharp blood pressure rise and reflex bradycardia. Use minimal epinephrine and monitor.
- Cardioselective β₁ blockers (metoprolol, atenolol) carry much less of this risk.
- Anaphylaxis in a patient on β-blockers may respond poorly to epinephrine (glucagon is the hospital option).
⚠️ Common traps
- Calling pilocarpine an antimuscarinic: it is a muscarinic agonist that increases saliva.
- Using atropine in a patient with narrow-angle glaucoma.
- Expecting the epinephrine–β-blocker reaction with metoprolol as much as with propranolol.
- Saying all postganglionic sympathetic fibers release norepinephrine: sweat glands are cholinergic.
Mini-quiz
- A patient with Sjögren syndrome and some remaining salivary function asks about dry mouth medication. Which drug fits?
- A patient taking propranolol receives several cartridges with epinephrine and develops a sharp rise in blood pressure with a slow pulse. Why?
- Which receptor mediates bronchodilation?
- Which condition is a contraindication to atropine?
- Sympathetic stimulation of the salivary glands mainly produces:
Answer key: 1 D · 2 C · 3 A · 4 C · 5 A