Sympathomimetic drugs (adrenergic agonists; adrenergic drugs; adrenergic amines) are stimulant drugs that activate the adrenergic receptors to produce a response similar to the sympathetic nervous system. The general effects of these drugs are to increases the heart rate, increase the force of cardiac contraction and raise blood pressure.
Four mechanisms of activation of adrenergic receptors:
1) Binds to receptor for direct action as it mimics the natural neurotransmitter
2) Promotes the release of neurotransmitters from terminal of sympathetic nerves
3) Inhibits the reuptake of the neurotransmitter
4) Inhibits inactivation or breakdown of the neurotransmitter
Two chemical types of sympathomimetic drugs:
1) Catecholamines:
• contain a catechol and an amine group
• have short half lives and can not be taken orally (quickly destroyed by enzymes located in liver and intestinal wall – monoamine oxidase (MAO) and catechol-o-methyltransferase)
• can not cross blood-brain barrier (polar molecules)
• eg adrenaline/epinephrine, dopamine, dobutamine, isoproternol
2) Noncatecholamines:
• do not contain the catechol part
• not broken down by catechol-o-methyltransferase (COMT) and slowly metabolised by monoamine oxidase so have much longer half lives and can be taken orally
• less polar, so can cross blood brain barrier
• eg ephedrine, pseudoephedrine
Effects of activation of adrenergic receptors
Sympathomimetic drugs are classified based on the type of receptors that the drugs act on:
a) Alpha1-adrenergic receptors:
• vasoconstriction, mydriasis (dilation of pupil)
• used for haemostasis; nasal decongestion; delay local anaesthetic absorption; elevate blood pressure
• adverse effects – hypertension, bradycardia
• activated by epinephrine, ephedrine, norepinephrine, phenylephrine, dopamine
b) Alpha2-adrenergic receptors:
• inhibits release of norepinephrine in receptors located in the periphery
• in CNS – stimulation of alpha2 receptors reduces of sympathetic outflow to heart and blood vessels
• activated by epinephrine, ephedrine, norepinephrine,
c) Beta1-adrenergic receptors:
• used for cardiac arrest (activation of receptors initiates contraction if heart has stopped beating); heart failure (activation increases force of contraction); shock; AV heart block
• activated by epinephrine, ephedrine, norepinephrine, isoproterenol, dobutamine, dopamine
d) Beta2-adrenergic receptors:
• clinically relevant receptors are located in lungs and uterus
• used for asthma – activation of receptors promote vasodilation
• activated by epinephrine, ephedrine, isoproterenol, terbutaline
General Adverse effects of sympathomimetic drugs:
As these drugs mimic sympathetic adrenergic stimulation they can lead to hypertension, excessive cardiac stimulation and cardiac arrhythmias.
Examples of Sympathomimetic drugs (Adrenergic Agonists):
| Vasopressor agents: | Cardiac Stimulants: | CNS Stimulants: | Bronchodilators: | Anoretics: |
| Noradrenaline Ephedrine Phenylephrine Dopamine | Adrenaline Dobutamine Isoprenaline | Amphetamine Methamphetamine Dexaamphetamine | Isoprenaline Salbutamol Salmeterol Terbutaline | Amphetamine Fenfluramine |
| Uterine Relaxants: | Nasal Decongestants: | |||
| Salbutamol Terbutaline Ritodrine | Xylometazoline Pseudoephedrine |
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