Sympathomimetic Agents - Direct, Indirect & Mixed

Sympathomimetic Agents the actions of endogenous catecholamines.
They are broadly classified into three groups based on their mechanism of action:

Classification of Sympathomimetic Agents

Sympathomimetic agents are drugs that mimic the actions of the sympathetic (adrenergic) nervous system.
They are classified based on their mechanism of action in activating adrenergic receptors.

Mechanism:
- These agents directly bind to and stimulate adrenergic receptors (α and/or β), reproducing the effects of endogenous catecholamines.
Key Points:
- Their action is receptor-mediated.
- They can have selectivity for α-receptors, β-receptors, or both.
- Some of these drugs also have central effects.
Examples:
1. Non-Selective Adrenergic Agonists (α and β receptors):
  - Norepinephrine
  - Epinephrine
  - Dopamine
2. Selective α-Agonists:
  - Phenylephrine (α₁-selective)
  - Methyldopa (α₂-selective, centrally acting)
  - Clonidine (α₂-selective, centrally acting)
  - Naphazoline (α₁-selective, used as a decongestant)
  - Oxymetazoline (α₁ and partial α₂-selective, used as a decongestant)
  - Xylometazoline (α₁ and partial α₂-selective, used as a decongestant)
3. Selective β-Agonists:
  - Dobutamine (β₁-selective, used in heart failure)
  - Isoproterenol (non-selective β₁ and β₂ agonist)
  - Terbutaline (β₂-selective, used in asthma & preterm labor)
  - Salbutamol (Albuterol) (β₂-selective, used in asthma)
  - Bitolterol (β₂-selective, used in asthma)

Mechanism:
- These drugs do not directly activate adrenergic receptors but instead:
- Promote the release of endogenous catecholamines (norepinephrine, dopamine, epinephrine).
- Inhibit the reuptake of catecholamines, thereby enhancing the sympathetic response.
Key Points:
- Their effect depends on the presence of stored catecholamines.
- They indirectly increase the stimulation of adrenergic receptors.
Examples:
- Hydroxyamphetamine (induces norepinephrine release, used in pupil dilation)
- Pseudoephedrine (induces norepinephrine release, used as a decongestant)
- Propylhexedrine (causes norepinephrine release, used as a nasal decongestant)

Mechanism:
- These agents have both direct receptor agonist activity and the ability to stimulate the release of endogenous catecholamines.
Key Points:
- They provide a dual mechanism of action (direct activation + indirect release of catecholamines).
- This can lead to a broader range of effects compared to purely direct-acting or indirect-acting agents.
Examples:
- Ephedrine (directly activates α and β receptors & releases norepinephrine)
- Metaraminol (direct α₁ agonist & causes norepinephrine release, used in hypotension)