Sympathomimetic Drugs

Sympathomimetic drugs stimulate adrenergic receptors, increasing heart rate, blood pressure, and bronchodilation. (Also called Adrenergic Agonists)

Definition of Sympathomimetic Drugs:

• These drugs mimic the effects of the sympathetic nervous system by activating adrenergic receptors (alpha and beta receptors).

Classification with MOA:

• Direct-acting: Act directly on adrenergic receptors (e.g., epinephrine, dobutamine)
• Indirect-acting: Increase release or prevent reuptake of norepinephrine (e.g., amphetamine, cocaine)
• Mixed-acting: Both direct and indirect (e.g., ephedrine)

Examples of Sympathomimetic Drugs:

1. Direct-acting:

   • Epinephrine – acts on α and β receptors
   • Phenylephrine – selective α1 agonist
   • Salbutamol (Albuterol) – selective β2 agonist

2. Indirect-acting:

   • Amphetamine
   • Tyramine
   • Cocaine (blocks reuptake of NE)

3. Mixed-acting:

   • Ephedrine

Pharmacological Effects:

1. Eye

   • Mydriasis (α1 receptor): Contraction of radial muscle.
   • Decreased intraocular pressure: Via α2 activation → reduced aqueous humor production.

2. Cardiovascular System

   • β1 stimulation:
     • Increased heart rate (positive chronotropy)
     • Increased force of contraction (positive inotropy)
     • Increased conduction velocity (positive dromotropy)
   • α1 stimulation: Vasoconstriction → increased BP (used in shock, hypotension).
   • β2 stimulation: Vasodilation in skeletal muscles and coronary arteries → decreased diastolic BP.

3. Respiratory System

   • Bronchodilation (β2 receptors): Relaxation of bronchial smooth muscle (used in asthma, COPD).
   • Decreased mucosal edema (α1 vasoconstriction): Nasal decongestant action.

4. Gastrointestinal Tract

   • Decreased motility and tone (β2 & α2): Relaxation of GI smooth muscle → constipation.
   • Contracted sphincters (α1): Delays passage.

5. Urinary Tract

   • α1 stimulation: Contracts bladder neck and prostate → urinary retention.
   • β3 stimulation: Relaxes detrusor muscle (used in overactive bladder).

6. Metabolic Effects

   • β2 stimulation: Glycogenolysis in liver, gluconeogenesis → increased blood glucose.
   • β3 stimulation: Lipolysis in adipose tissue.
   • Insulin secretion: α2 inhibits; β2 stimulates → net effect depends on balance.

Clinical Uses:

• Anaphylactic shock (Epinephrine)
• Cardiac arrest
• Nasal congestion (Phenylephrine)
• Asthma and COPD (Salbutamol, Formoterol)
• Hypotension
• ADHD (Amphetamines)

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