Adrenergic Agonists

Adrenergic Receptors & Classification - Receptor Rhapsody

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Catecholamines - Adrenaline's A-Team

• IV admin (Epi IM/SC too). Rapid COMT/MAO metabolism. ADRs: Arrhythmias, HTN. ⚠️ Extravasation necrosis.

• Epinephrine (Adrenaline): α1,α2,β1,β2 agonist.

  • Uses: Anaphylaxis (0.3-0.5mg IM/SC 1:1000; IV 1:10000 shock), cardiac arrest, septic shock.

  ⭐ Epinephrine Reversal of Dale: With α-blocker, Epi causes ↓BP (unopposed β2 vasodilation).

• Norepinephrine (Noradrenaline): α1>β1 agonist (little β2).

  • Uses: Septic/cardiogenic shock (↑SVR, ↑MAP).

• Isoproterenol (Isoprenaline): β1,β2 agonist (non-selective).

  • Uses: Bradycardia/heart block (↑HR, ↓SVR).

• Dopamine: Dose-dependent:

  • Low: D1 (renal VD)
  • Med: β1 (↑inotropy, ↑HR)
  • High: α1 (VC)
  • 📌 D1→β1→α1 (↑dose).
  • Uses: Shock, bradycardia.

• Dobutamine: Mainly β1 agonist (mild β2/α1).

  • Uses: Acute HF/cardiogenic shock (↑inotropy).

Alpha-Selective Agonists - Alpha Force

• Alpha-1 ($\alpha_1$) Selective Agonists:
  • Drugs: Phenylephrine, Midodrine.
  • Uses:
    • Phenylephrine: Mydriasis (non-cycloplegic), nasal decongestant, ↑BP (pressor).
    • Midodrine: Orthostatic hypotension.
• Alpha-2 ($\alpha_2$) Selective Agonists:
  • Drugs: Clonidine, Methyldopa (prodrug), Apraclonidine, Brimonidine.
  • Uses:
    • Clonidine: Hypertension, ADHD, opioid/nicotine withdrawal.
    • Methyldopa: Hypertension in pregnancy.
    • Apraclonidine, Brimonidine: Glaucoma (↓aqueous humor production).
• Key Adverse Effects (AEs):
  • Clonidine: Sedation, dry mouth, ⚠️ rebound hypertension (abrupt withdrawal).
  • Methyldopa: Sedation, drug-induced lupus, Coombs-positive hemolytic anemia.

  ⭐ Methyldopa is a notable cause of drug-induced Coombs-positive hemolytic anemia.

Beta-Selective & Other Agonists - Beta Boosters & Mixers

• β1-Selective Agonist
  • Dobutamine: Primarily β1 (↑inotropy, chronotropy); cross-ref Catecholamines.
• β2-Selective Agonists
  • SABA (Short-Acting): Salbutamol (Albuterol), Terbutaline.
    • Uses: Acute asthma, COPD, hyperkalemia (Salbutamol), tocolysis (Terbutaline).
  • LABA (Long-Acting): Salmeterol, Formoterol.
    • Uses: Chronic asthma/COPD (with corticosteroids).
  • Ritodrine: Tocolytic (largely replaced).
  • AEs (common to β2 agonists): Tremor, tachycardia, palpitations, hypokalemia.
• Indirect-Acting Sympathomimetics
  • Amphetamine, Tyramine: MOA - Release stored NE.
  • Cocaine: MOA - Inhibits NE reuptake.
• Mixed-Acting Sympathomimetics
  • Ephedrine, Pseudoephedrine: MOA - Direct receptor agonism + release stored NE.
    • Uses: Nasal decongestion, pressor (Ephedrine). Tachyphylaxis.

⭐ Long-Acting Beta-Agonists (LABAs) like Salmeterol and Formoterol should not be used as monotherapy for asthma due to increased risk of asthma-related death; they must be used with an inhaled corticosteroid.

High-Yield Points - ⚡ Biggest Takeaways

• Dobutamine (β1 agonist): For cardiogenic shock, acute HF; primarily ↑cardiac contractility.
• Phenylephrine (α1 agonist): Causes vasoconstriction, mydriasis; used as nasal decongestant, raises BP.
• Clonidine (central α2 agonist): For hypertension, opioid withdrawal. (α-methyldopa: HTN in pregnancy).
• Salbutamol/Terbutaline (β2 agonists): For acute asthma/COPD; main side effects: tremor, tachycardia.
• Dopamine: Dose-dependent effects: low (D1 renal), medium (β1 cardiac), high (α1 vasoconstriction).
• Epinephrine (α & β agonist): Drug of choice for anaphylactic shock, cardiac arrest.