Peripherally Acting Sympatholytics
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Peripherally acting sympatholytics, or peripherally acting agents, are drugs that block the blood pressure increase action of nerves at the levels of the heart and the blood vessels. They work by preventing those nerves from sending signals to speed up heart rate and narrow blood vessels. As a result, the heart doesn't pump as hard and blood flows more easily through blood vessels.
As opposed to centrally acting sympatholytics, which are drugs that act at the brain level, peripherally acting sympatholytics act on the organs themselves.
General pharmacology
The sympathetic nervous system (part of the nervous system that becomes active during stress) can innervate the heart and blood vessels. When these nerves are activated by epinephrine (adrenaline) and norepinephrine, there is an increase of heart rate and blood pressure.
Peripherally acting sympatholytics block sympathetic activity around the heart and the blood vessels. This adrenergic activity is blocked when these drugs bind to proteins on nerve cells. These proteins are called a2 receptors, and when blocked, adrenaline cannot bind to them. This reduces heart rate and causes vasodilatation (widening and relaxing of blood vessels) which leads to a decrease of blood pressure.
Examples of peripherally acting sympatholytics
Examples of peripherally acting sympatholytics include:
- Prazosin.
- Guanethidine.
- Indoramin (Baratol).
- Doxazosin (Cardura).
Therapeutic indications
Peripherally acting sympatholytics are prescribed in the treatment of hypertension (high blood pressure). However, they are not used firstly because of the side effects. They are usually administered in combination with a diuretic to prevent fluid accumulation, which increases blood volume and compromises the blood pressure lowering effect of the drugs.
Doctors recommend peripherally acting sympatholytics to treat:
- Hypertension (high blood pressure).
- Alcohol or drug withdrawal.
- Hot flashes.
- Attention-deficit/hyperactivity disorder (ADHD).
- Tourette syndrome.
Side effects
Peripherally acting sympatholytics can have strong adverse effects, so they aren't commonly used.
Common side effects of peripherally acting sympatholytics include:
- Extreme fatigue.
- Bradycardia (slow heart beat).
- Dry mouth.
- Headache.
- Nausea and stomach upset.
- Drowsiness or sedation.
- Depression.
- Dizziness.
- Impotence.
- Constipation.
- Weight gain.
- Impaired thinking.
Cautions
A sudden discontinuation of use of centrally acting sympatholytics can cause unexpected and dangerous increase in blood pressure. It’s very important not to stop using these drugs, especially if beta blockers are also taken, without speaking to a doctor first.
Major drug groups
Cardiovascular system
References
Sélection Reader's Digest, Le Guide canadien des médicaments, 2002.
Wikipedia, the free encyclopedia (www.wikipedia.org).
Mayo Clinic, medical information and tools for better living (www.mayoclinic.com).
CV Pharmacology, cardiovascular pharmacology concepts (www.cvpharmacology.com).
The information above should be used as a reference only. Any medical decision should not be done before consulting a health care professional.
Last update: 29th of July 2009.