Adrenergic Agonists Overview

Adrenergic Agonists

• Adrenergic agonists mimic the actions of the sympathetic nervous system
• Direct-acting agonists bind directly to adrenergic receptors
  • Catecholamines are naturally occurring compounds
  • Non-catecholamines are synthetic
• Indirect-acting agonists enhance the effects of norepinephrine by increasing its release or inhibiting its reuptake
  • They do not directly bind to post-synaptic receptors

Direct-Acting Catecholamines

• Epinephrine
  • Alpha (all types) and beta (all types) agonist
  • Used to treat anaphylaxis and with local anesthetics
  • Side effects: hyperglycemia, arrhythmias, hypertension
• Norepinephrine
  • Alpha (all types) and beta1 agonist
  • Used to treat neurogenic shock
  • Last resort therapy for shock patients
• Dopamine
  • Stimulates D1, beta1, and alpha1 receptors
• Dobutamine
  • Beta1 agonist

Direct-Acting Non-Catecholamines

• Alpha 1 agonist: Phenylephrine
• Alpha 2 agonist: Clonidine
• Beta 1 agonist: No specific drugs (only catecholamines)
• Beta 2 agonists: Salbutamol & Ritodrine

Indirect-Acting Adrenergic Agonists

• Amphetamine, Ephedrine, and Pseudoephedrine
• They enhance the effects of norepinephrine, but do not directly affect post-synaptic receptors

Adrenergic Antagonists (Sympatholytics)

• Agents blocking adrenergic receptors
  • Labetalol and carvedilol block both alpha and beta receptors
  • Phenoxybenzamine is a non-selective alpha blocking agent
  • Prazosin, terazosin, doxazosin, and tamsulosin are selective alpha1 blockers
  • Propranolol, timolol, and nadolol are non-selective beta antagonists
  • Acebutolol, atenolol, and metoprolol are selective beta1 antagonists

Cholinergic Agonists

• Mimic the actions of the parasympathetic nervous system
• Direct-acting agonists bind directly to cholinergic receptors
  • Acetylcholine
  • Bethanechol
  • Carbachol
  • Pilocarpine
• Indirect-acting agonists inhibit the breakdown of acetylcholine
  • Reversible: Neostigmine, Pyridostigmine, Physostigmine, Donepezil
  • Irreversible: Not used clinically due to permanent inactivation of cholinesterase

Cholinergic Antagonists

• Block the actions of acetylcholine
• Atropine
  • Uses: Mydriasis and cycloplegia, antispasmodic, treatment of heart block, drying of secretions, antidote for cholinergic agonists
  • Side effects: CNS stimulation, increased body temperature, dry eyes/mouth, blurred vision, tachycardia, urinary retention, constipation
• Scopolamine (Hyoscine)
  • Uses: Antispasmodic, motion sickness, amnesic drug
  • Side effects: Sedation, euphoria, abuse potential
• Ipratropium
  • Uses: Bronchodilator in asthmatic patients
  • Local action in lungs, minimizing systemic effects