Pharmacology of Alpha Blockers
Alpha blockers are used in the treatment of benign prostatic hyperplasia (enlarged prostate). Here, we review its members, how they work, what side effects they have as well as what clinical features they’re associated with.
Alpha blockers are divided into three classes:
- Non-selective alpha blockers – phenoxybenzamine, phentolamine
- Alpha-1 selective – doxazosin, alfuzosin, terazosin, tamsulosin
- Alpha-2 selective – yohimbine, atipamezole
In this study guide, we predominantly focus on selective alpha-1 blockers – medicines used to treat benign prostatic hyperplasia and, in some cases, hypertension.
However, before we continue to study that class in more detail, let’s briefly review the other two classes and what conditions they are used to treat.
Phenoxybenzamine and phentolamine are non-selective antagonists at both alpha-1 and alpha-2 receptors. Both medicines are used to treat pheochromocytoma (PCC); a neuroendocrine tumor of the medulla of the adrenal glands. Diaphoresis, or excessive sweating, is associated with this condition. Both phenoxybenzamine and phentolamine may be used to treat this symptom, as well as hypertension caused by this specific illness.
Yohimbine is a selective antagonist at adrenergic alpha-2 receptors. Yohimbine is derived from the bark of the Pausinystalia johimbe tree, a tree common to central Africa. It is mostly used for veterinary purposes where it may be used to reverse sedation in dogs and deer (in cases where the animal had been treated with the pro-sedative drug, xylazine; an alpha-2 receptor agonist).
That leaves us with the selective alpha-1 blockers, medicines used to treat:
- Benign prostatic hyperplasia – to control symptoms
- Resistant hypertension – when first and second-line agents prove insufficient
With these indications in mind, let’s quickly review how alpha blockers – specifically the alpha-1 antagonists – are used to treat both conditions.
Mechanism of action
Alpha-1 receptors are predominantly found on:
- Smooth muscle of blood vessels
- Smooth muscle of the urinary tract
It is no surprise, then, that receptor blockade causes vasodilation and consequent reduction in blood pressure, as well as relaxation of the muscles of the bladder neck and prostate.
Side effects with alpha blockers include:
- Postural hypotension – particularly after the first dose
- Nasal congestion
- Lack of energy
Generally, alpha blockers are well tolerated drugs.
When we talk about the clinical pharmacology of alpha blockers, we need to think about the following factors:
- That due to their first-dose effect, caution is warranted in patients with existing postural hypotension. First-dose postural hypotension is often worse with doxazosin.
- That alpha blockers are not recommended as first-line antihypertensive agents. Preferred drugs include calcium channel blockers, ACE inhibitors and thiazide diuretics. They are generally used when other antihypertensive agents have been exhausted.
- That taking alpha blockers along with other blood pressure lowering agents only amplifies that effect.
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