10. ADRENERGIC AND ANTI-ADRENERGIC DRUGS

Introduction

The sympathetic nervous system controls the body’s “fight or flight” responses. It uses adrenaline and noradrenaline (catecholamines) to activate different organs. Drugs that act on this system are called adrenergic (stimulating) or anti-adrenergic (blocking) drugs. These medicines are widely used in asthma, hypertension, shock, cardiac arrest and nasal congestion.

Adrenergic System Overview

Adrenergic nerves release noradrenaline as the main neurotransmitter. Sympathetic stimulation increases heart rate, blood pressure, blood glucose, bronchodilation and decreases digestive activity.

Main Receptors

  • Alpha-1: vasoconstriction, pupil dilation
  • Alpha-2: reduces noradrenaline release
  • Beta-1: increases heart rate and force
  • Beta-2: bronchodilation and relaxation of smooth muscles
  • Beta-3: lipolysis

Adrenergic Drugs (Sympathomimetics)

These drugs mimic the actions of sympathetic stimulation. They may act directly on receptors or indirectly by releasing stored noradrenaline.

1. Direct-Acting Adrenergic Drugs

a) Non-selective Adrenergic Agonists

  • Adrenaline: stimulates α and β receptors; used in cardiac arrest, anaphylaxis, acute asthma.
  • Noradrenaline: mainly α agonist; used to raise blood pressure in shock.
  • Dopamine: low dose → renal dilation; high dose → increases BP; used in shock.

b) Selective Alpha Agonists

  • Alpha-1 agonists: phenylephrine (nasal decongestion), midodrine (hypotension).
  • Alpha-2 agonists: clonidine, methyldopa (used in hypertension).

c) Selective Beta Agonists

  • Beta-1 agonist: dobutamine (acute heart failure).
  • Beta-2 agonists: salbutamol, terbutaline (asthma, COPD).

2. Indirect-Acting Adrenergic Drugs

Increase release of noradrenaline from nerve endings.

  • Amphetamine: CNS stimulant.
  • Tyramine: found in cheese; dangerous in patients taking MAO inhibitors.

3. Mixed-Acting Adrenergic Drugs

  • Ephedrine: bronchodilator, nasal decongestant.

Therapeutic Uses of Adrenergic Drugs

  • Asthma: salbutamol, terbutaline
  • Anaphylaxis: adrenaline
  • Shock / hypotension: dopamine, noradrenaline
  • Nasal congestion: phenylephrine
  • Cardiac arrest: adrenaline
  • Preterm labour: beta-2 agonists

Anti-Adrenergic Drugs (Sympatholytics)

These drugs block the effects of sympathetic stimulation. They work by blocking α or β receptors, reducing blood pressure, slowing heart rate or relaxing blood vessels.

1. Alpha Blockers

a) Alpha-1 Blockers

  • Prazosin, Terazosin, Doxazosin: used in hypertension, benign prostatic hyperplasia (BPH).

b) Alpha-2 Blockers

  • Yohimbine: rarely used; increases sympathetic outflow.

2. Beta Blockers

a) Non-selective Beta Blockers (β1 + β2)

  • Propranolol: hypertension, angina, migraine prophylaxis.

b) Selective Beta-1 Blockers

  • Atenolol, Metoprolol, Bisoprolol: preferred in hypertension, heart disease, fewer lung side effects.

c) Beta Blockers with Additional Actions

  • Carvedilol: blocks α1 + β receptors; used in heart failure.
  • Labetalol: used in pregnancy-induced hypertension.

Mechanisms of Action of Anti-Adrenergic Drugs

Alpha Blockers

  • Cause vasodilation → ↓ blood pressure
  • Relax bladder neck & prostate → improve urine flow

Beta Blockers

  • Reduce heart rate and force → ↓ BP
  • Reduce oxygen demand → used in angina
  • Reduce renin release → control BP

Side Effects

Adrenergic Drugs

  • Tachycardia
  • Tremors
  • Hypertension
  • Arrhythmias

Anti-Adrenergic Drugs

  • Bradycardia
  • Hypotension
  • Bronchoconstriction (non-selective beta blockers)
  • Fatigue, dizziness

Contraindications

Adrenergic Drugs

  • Severe hypertension
  • Cardiac arrhythmias

Beta Blockers

  • Asthma (avoid propranolol)
  • Severe bradycardia

Detailed Notes:

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