AIM Step 10
Student Memory Support
Sympathomimetic Drugs: Adrenergic Pharmacology and Clinical Applications
3rd Year MBBS • High-yield memory reinforcement and last-minute KMU revision
1. High-Yield Flashcards
Tap each question to reveal the answer.
How are sympathomimetic drugs classified by mode of action?
Directly acting, indirectly acting and mixed acting.
Which endogenous sympathomimetics are important catecholamines?
Adrenaline, noradrenaline and dopamine.
Which enzymes rapidly metabolize catecholamines?
Catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO).
What is the major effect of α₁-adrenoceptor stimulation?
Vascular smooth-muscle contraction causing vasoconstriction and increased peripheral resistance.
What are the main cardiac effects of β₁ stimulation?
Increased heart rate, conduction and myocardial contractility.
Which receptor mediates sympathomimetic bronchodilation?
β₂-adrenoceptors.
Why can noradrenaline reduce heart rate despite β₁ stimulation?
Its strong α-mediated rise in blood pressure triggers baroreceptor-mediated reflex bradycardia.
Which receptors are predominantly stimulated by isoprenaline?
β₁ and β₂ receptors.
Why is adrenaline effective in anaphylaxis?
α₁ raises vascular tone, β₁ supports the heart and β₂ produces bronchodilation.
What is the receptor sequence as dopamine exposure increases?
D₁ → β₁ → α₁.
How does brimonidine lower intraocular pressure?
α₂ stimulation reduces aqueous humour production and can improve outflow.
How does acetazolamide lower intraocular pressure?
Carbonic anhydrase inhibition reduces aqueous humour formation.
How does mannitol rapidly lower intraocular pressure?
It raises plasma osmolarity and draws water from ocular tissues into the circulation.
What causes the hypertensive cheese reaction?
Tyramine during MAO inhibition causes excessive noradrenaline release and severe hypertension.
Which antagonist can oppose severe α-mediated hypertension after adrenaline overdose?
Phentolamine, an α-adrenoceptor antagonist.
2. Mnemonics
Mnemonic Title: Dopamine Dose Sequence
D → B → A
Meaning: Dopamine receptors first (D₁) → Beta₁ next → Alpha₁ at higher exposure.
Mnemonic Title: Adrenaline in Anaphylaxis
V-H-B
Meaning: Vessels via α₁ → Heart via β₁ → Bronchi via β₂.
Mnemonic Title: Catecholamine Metabolism
C-M
Meaning: Catecholamines are rapidly metabolized by COMT and MAO.
3. Memory Tables
Cardiovascular Sympathomimetic Comparison
| Drug | Main receptors | Diastolic BP | Heart rate | Memory clue |
|---|---|---|---|---|
| Adrenaline | α + β | May fall with β₂ effect | Increases | Anaphylaxis |
| Noradrenaline | α + β₁ | Increases | Reflex decrease | Strong vasoconstriction |
| Phenylephrine | α₁ | Increases | Reflex decrease | Pure α-like response |
| Isoprenaline | β₁ + β₂ | Decreases | Marked increase | β-only pattern |
Glaucoma Drug Mechanisms
| Drug | Mechanism | Effect |
|---|---|---|
| Brimonidine | α₂ stimulation | Reduced aqueous formation |
| Acetazolamide | Carbonic anhydrase inhibition | Reduced aqueous formation |
| Mannitol | Increased plasma osmolarity | Water leaves ocular tissues |
4. Rapid Revision Points — Last-Minute Revision
Must Remember:
- Direct sympathomimetics activate adrenoceptors; indirect agents increase endogenous catecholamine activity.
- Catecholamines are rapidly metabolized by COMT and MAO and generally have short actions.
- α₁ stimulation causes vasoconstriction; β₁ stimulates the heart; β₂ relaxes bronchial smooth muscle.
- Noradrenaline can produce reflex bradycardia despite direct β₁ stimulation.
- Isoprenaline raises heart rate while β₂-mediated vasodilation lowers diastolic pressure.
- Adrenaline combines α₁, β₁ and β₂ effects and is the key pharmacological treatment in anaphylaxis.
- Dopamine receptor predominance progresses from D₁ to β₁ to α₁ as exposure increases.
- Brimonidine and acetazolamide reduce aqueous humour production by different mechanisms.
- Mannitol lowers intraocular pressure by creating an osmotic gradient.
- Aged and fermented foods can contain tyramine and may precipitate severe hypertension during MAO inhibition.
- Adrenaline overdose with severe α-mediated hypertension may require α-adrenoceptor antagonism.
KMU Exam Trap: A fall in heart rate after noradrenaline does not mean β₁ blockade. The rise in blood pressure activates the baroreceptor reflex and produces vagal bradycardia.
5. Clinical Memory Hooks
Anaphylaxis with hypotension and wheeze
→
adrenaline
→
α₁ vascular support + β₂ bronchodilation.
→
adrenaline
→
α₁ vascular support + β₂ bronchodilation.
Marked rise in blood pressure with slowing pulse
→
strong α-mediated vasoconstriction
→
reflex bradycardia.
→
strong α-mediated vasoconstriction
→
reflex bradycardia.
Glaucoma needing rapid osmotic pressure reduction
→
mannitol
→
water moves from ocular tissues to plasma.
→
mannitol
→
water moves from ocular tissues to plasma.
MAO inhibitor + aged cheese + severe headache and hypertension
→
tyramine
→
excessive noradrenaline release.
→
tyramine
→
excessive noradrenaline release.
Increasing dopamine infusion with progression to peripheral vasoconstriction
→
increasing α₁ receptor predominance.
→
increasing α₁ receptor predominance.
6. Starred High-Yield Exam Points
- ⭐ Adrenaline is the key sympathomimetic in anaphylaxis because α₁, β₁ and β₂ effects address multiple life-threatening abnormalities simultaneously.
- ⭐ Noradrenaline raises systolic and diastolic pressure but may reduce heart rate through reflex bradycardia.
- ⭐ Dopamine shows the dose-related receptor sequence D₁ → β₁ → α₁.
- ⭐ Isoprenaline is a β₁/β₂ agonist: heart rate rises while peripheral resistance and diastolic pressure fall.
- ⭐ Tyramine during MAO inhibition can cause excessive noradrenaline release and a hypertensive cheese reaction.
- ⭐ Acetazolamide reduces aqueous humour formation by carbonic anhydrase inhibition, whereas mannitol works osmotically.
- ⭐ Severe α-mediated hypertension after adrenaline overdose can be opposed by an α-adrenoceptor antagonist such as phentolamine.