What are the alpha-blockers?
Table of Contents:
Alpha-blockers are a class of medications used for the treatment of BPH. Alpha receptors are located within the bladder and the prostate and they modulate tone. Alpha receptors are found in the sympathetic nervous system and throughout the body. They control a variety of physiologic functions, not just voiding.
Alpha receptors respond to norepinephrine and, when norepinephrine binds (attaches) to the alpha receptors, it causes a change in the tone of smooth muscle fibers, increasing the tone. Alpha receptors are found in the smooth muscle of the prostate and the bladder and are important in regulating the prostate tone and the smooth muscle of the bladder.
When alpha receptors in the prostatic smooth muscle are stimulated by norepinephrine, the result is increased tone of the prostatic smooth muscle. A way to think of this is that the prostate squeezes around the urethra and tightens its grip, resulting in increased urethral resistance to urine flow from the bladder. This increased prostatic tone, therefore, can result in the symptoms of bladder outlet obstruction.
Because norepinephrine is the signal that acts on the alpha receptors to cause smooth muscle to contract, if you block part of that signal, you should decrease the muscle contraction. Alpha blockers block the binding of norepinephrine at the alpha adrenergic receptor and thus prevent contraction of the prostatic smooth muscle and thus are useful in treating BPH symptoms.
Over the past decade, an increasing number of alpha blockers have become available on the U.S. market (Table 12). They are generally divided into selective and nonselective alpha blockers, depending on which receptors
Table 12. BPH Drugs and Dosing
they block. Selective alpha blockers act primarily on the prostate with fewer systemic effects. Nonselective alpha blockers act both systemically and on the prostate. Both selective and nonselective alpha blockers are effective in treating BPH symptoms. A summary of the currently available alpha blockers and their characteristics appears in Table 12.
The choice of alpha blocker used may depend on your other medical conditions and medications that you are taking, your physician's preference, as well as your insurance plan coverage.
What are the side effects of alpha-blockers?
The side effects of alpha-blocker drugs are primarily caused by blockade of alpha receptor sites outside of the urinary tract. Commonly reported side effects of alpha-blocker drugs include dizziness, headache, asthenia (weakness), postural hypotension (decreased blood pressure with change of body position), rhinitis (inflammation of nasal mucous membranes), and ejaculatory dysfunction. These side effects occur in about 5 to 9% of patients taking the drugs and can be reversed by stopping the drugs.
Combined use of alpha-blockers and PDE-5 inhibitors [sildenafil (Viagra), vardenafil (Levitra), and tadalafil (Cialis)] can lead to significant decreases in blood pressure in some individuals.
Recently, there has been a concern regarding an increased risk of Intraoperative Floppy Iris Syndrome occurring during cataract surgery in individuals taking alpha-blockers. Thus, individuals considering cataract surgery who are on or have taken alpha-blockers should discuss their current/past use of alpha-blockers with the ophthalmologist.
How are they similar and how are they different?
All six of the alpha blockers commonly used today in the United States have been demonstrated to be beneficial in the treatment of BPH symptoms. Phenoxybenzamine (Dibenzyline) and Prazosin (Minipress) are not commonly used anymore because to be effective they have to be taken more than once per day. The medications vary in that some are single dose therapies and others are dosetitratable. Similarly, dosing recommendations with respect to timing in relation to meals varies with the different medications (see Table 12). Although all alpha-blockers have the potential to cause dizziness and postural hypotension, the incidence of these side effects are lower with the selective alpha-blockers such as tamsulosin and silodosin.
All of the alpha blockers have been proven to be more effective than placebo in treating the symptoms of BPH. More selective agents, such as tamsulosin and silodosin are thought to cause less systemic side effects, such as dizziness, hypotension, and fainting.