Alpha blocker

Class of pharmacological agents

Alpha blockers α-blockers
Neutral antagonist
Biological target	α-adrenoceptors
Legal status
In Wikidata

Historically, alpha-blockers were used as a tool for pharmacologic research to develop a greater understanding of the autonomic nervous system. Using alpha blockers, scientists began characterizing arterial blood pressure and central vasomotor control in the autonomic nervous system.[3] Today, they can be used as clinical treatments for a limited number of diseases.^[2]

Alpha blockers can treat a small range of diseases such as hypertension, Raynaud's disease, benign prostatic hyperplasia (BPH) and erectile dysfunction.^[2] Generally speaking, these treatments function by binding an α-blocker to α receptors in the arteries and smooth muscle. Ultimately, depending on the type of alpha receptor, this relaxes the smooth muscle or blood vessels, which increases fluid flow in these entities.^[2]

Classification

• α₁-blockers act on α₁-adrenoceptors
• α₂-blockers act on α₂-adrenoceptors

When the term "alpha blocker" is used without further qualification, it can refer to an α₁ blocker, an α₂ blocker, a nonselective blocker (both α₁ and α₂ activity), or an α blocker with some β activity.^[2] However, the most common type of alpha blocker is usually an α₁ blocker.

Non-selective α-adrenergic receptor antagonists include:

• Phenoxybenzamine
• Phentolamine
• Tolazoline
• Trazodone

Selective α₁-adrenergic receptor antagonists include:

• Alfuzosin^[4]
• Doxazosin^[5]
• Prazosin (inverse agonist)^[6]
• Tamsulosin^[7]
• Terazosin^[8]
• Silodosin^[9]

Selective α₂-adrenergic receptor antagonists include:

• Atipamezole
• Idazoxan
• Mirtazapine
• Yohimbine

Finally, the agents carvedilol and labetalol are both α and β-blockers.

Below are some of the most common drugs used in the clinic.

Drug Name	Common Brands	Structure	Mechanism of Action	Effects	Clinical Applications	Toxicity
Phenoxybenzamine	Dibenzyline		Nonselective covalent binding to α1 and α2 receptors. Irreversibly binds.[2][10]	Lowers blood pressure by decreasing peripheral resistance. Blocks alpha induced vasconstriction.[2]	Pheochromocytoma Excess catecholamine release [2]	Orthostatic hypotension Tachycardia Nausea Vomiting [2][10]
Phentolamine	Regitine		Competitive blocking of α1 and α2 receptors. 4 hours of action after initial administration.[2][10]	Reversal of epinephrine induced effects. Lowers blood pressure by decreasing peripheral resistance.[2][10]	Pheochromocytoma[2]	Reflex cardiac stimulation Tachycardia Arrythmia Anginal pain [10]
Prazosin	Minipress		Competitive blocking of α1 receptor.[10]	Lowers blood pressure.[2]	Hypertension Benign prostatic hyperplasia (BPH) Posttraumatic stress disorder (PTSD)[2][6][10]	First dose effect Orthostatic hypotension Nasal congestion Dizziness Lack of energy Headache Drowsiness [2][10]
Doxazosin	Cardura Cardura XL		Competitive blocking of α1 receptor.[10]	Lowers blood pressure.[2]	Hypertension Benign prostatic hyperplasia[5]	First dose effect Orthostatic hypotension Nasal congestion Dizziness Lack of energy Headache Drowsiness [2][10]
Terazosin	Hytrin		Competitive blocking of α1 receptor.[10]	Lowers blood pressure.[2]	Hypertension Benign prostatic hyperplasia (BPH)[8]	First dose effect Orthostatic hypotension Nasal congestion Dizziness Lack of energy Headache Drowsiness [2][10]
Tamsulosin	Flomax		A blocker that has slight selectivity for α1 receptors.[2]	Relaxation of prostatic smooth muscle.[2]	Benign prostatic hyperplasia[2][11]	Orthostatic hypotension [2]
Yohimbine	Yocon		Blocks α2 receptor, and increases norepinephrine release, thus increasing CNS activity.[2]	Raises blood pressure and heart rate.[2]	Sexual stimulation Male erectile dysfunction Hypotension[2]	May cause anxiety[12] Central Nervous System stimulation
Labetalol	Trandate		Blocks some α1 receptor activity, but binds more strongly to β receptors.[2]	Lowers blood pressure, increases heart rate slightly.[2]	Hypertension[2][13]	May cause tachycardia [2]
Carvedilol	Coreg Coreg CR		Blocks some α1 receptor activity, but binds more strongly to β receptors.[2]	Can interfere with noradrenergic mechanisms.[2]	Congestive heart failure(CHF)[2][14]	Fatigue [2]

Medical uses

While there are limited clinical α-blocker uses, in which most α-blockers are used for

Another treatment for hypertension is using drugs that have both α₁ blocking activity, as well as nonselective β activity, such as Labetalol or carvedilol.^[19] In low doses, labetalol and carvedilol can decrease the peripheral resistance and block the effects of isoprenaline to reduce hypertensive symptoms.^[19]

Pheochromocytoma

Pheochromocytoma is a disease in which a catecholamine secreting tumor develops.^[2][20] Specifically, norepinephrine and epinephrine are secreted by these tumors, either continuously or intermittently.^[21] The excess release of these catecholamines increases central nervous system stimulation, thus causing blood vessels to increase in vascular resistance, and ultimately giving rise to hypertension.^[20] In addition, patients with these rare tumors are often subject to headaches, heart palpitations, and increased sweating.^[2]

Phenoxybenzamine, a nonselective α₁ and α₂ blocker, has been used to treat pheochromocytoma.^[21] This drug blocks the activity of epinephrine and norepinephrine by antagonizing the alpha receptors, thus decreasing vascular resistance, increasing vasodilation, and decreasing blood pressure overall.^[21]

Congestive heart failure

Blockers that have both the ability to block both α and β receptors, such as

Phentolamine, a non-selective alpha blocker, has also been tested to treat erectile dysfunction. By reducing vasoconstriction in the penis, there appears to be increased blood flow that aids in penile erection. Side effects associated with phentolamine include headache, flushing, and nasal congestion.^[23]

Phenoxybenzamine, a non-competitive α₁ and α₂ blocker was used by Dr. Giles Brindley in the first intracavernosal pharmacotherapy for erectile dysfunction.^[24]

Benign prostatic hyperplasia

In benign prostatic hyperplasia (BPH), men experience urinary obstruction and are unable to urinate, thus leading to urinary retention.^[2] α₁ specific blockers have been used to relax the smooth muscle in the bladder and enlarged prostate.^[25] Prazosin, doxazosin, and terazosin have been particularly useful for patients with BPH, especially in patients with hypertension.^[2] In such patients, these drugs can treat both conditions at the same time.^[2] In patients without hypertension, tamsulosin can be used, as it has the ability to relax the bladder and prostate smooth muscle without causing major changes in blood pressure.^[25]

Raynaud's disease

Both α₁ blockers and α₂ blockers have been examined to treat Raynaud's disease. Although α₁ blockers, such as prazosin, have appeared to give slight improvement for the sclerotic symptoms of Raynaud's disease, there are many side effects that occur while taking this drug. Conversely, α₂ blockers, such as yohimbine, appear to provide significant improvement of the sclerotic symptoms in Raynaud's Disease without excessive side effects.^[26]

Post traumatic stress disorder

Patients with

Although alpha blockers have the ability to reduce some disease pathology, there are some side effects that come with these alpha blockers.[28] However, because there are several structural compositions that make each alpha blocker different, the side effects are different for each drug. Side effects that arise when taking alpha blockers can include the first dose effect, cardiovascular side effects, genitourinary side effects, as well as other side effects.^[28]

First dose effect

One of the most common side effects with alpha blockers is the

Alpha blockers that possess these side effects include prazosin, doxazosin, and terazosin.^[30]

Cardiovascular side effects

There are some alpha blockers that can give rise to changes in the cardiovascular system, such as the induction of reflex tachycardia, orthostatic hypotension, or heart palpitations via alterations of the QT interval.^[28][31]

Alpha blockers that may have these side effects include yohimbine, phenoxybenzamine, and phentolamine.^[2]

Genitourinary side effects

When alpha blockers are used to treat BPH, it causes vasodilation of blood vessels on the bladder and the prostate, thus increasing urination in general.^[32] However, these alpha blockers can produce the exact opposite side effect, in which edema, or abnormal fluid retention, occurs.^[33]

In addition, due to the relaxation of the prostate smooth muscle, another side effect that arises in men being treated for BPH is impotence, as well as the inability to ejaculate.^[32][34] However, if any ejaculation activity does occur, oftentimes, it results in a phenomenon called retrograde ejaculation, in which semen flows into the urinary bladder instead of exiting through the urethra.^[34]

Drugs that may produce such side effects include prazosin, terazosin, tamsulosin, and doxazosin.^[34]

Other side effects

Finally, there are other general side effects that can be caused by most alpha blockers (however, more frequently in alpha-1 blockers). Such side effects include

There is only one compelling indication for alpha blockers, which is for benign prostatic hyperplasia.^[33] Patients who need alpha blockers for BPH, but have a history of hypotension or postural heart failure, should use these drugs with caution, as it may result in an even greater decrease in blood pressure or make heart failure even worse.^[35][36] The most compelling contraindication is urinary incontinence and overall fluid retention.^[35][36] To combat such fluid retention, patients can take a diuretic in combination with the alpha-blocker.^[36]

In the absence of compelling indications or contraindications, patients should take alpha blockers as a step 4 therapy to reduce blood pressure, but only if the use of ACE inhibitors, angiotensin-II receptor blockers, calcium channel blockers, or thazide diuretics (in full dose or in combinations) have not been efficacious.^[33][35][36]

Drug interactions

As with any drug, there are drug interactions that can occur with alpha blockers. For instance, alpha blockers that are used for the reduction of blood pressure, such as phenoxybenzamine or phentolamine can have synergy with other drugs that affect smooth muscle, blood vessels, or drugs used for erectile dysfunction (i.e. sildenafil, tamsulosin, etc.). This stimulates exaggerated hypotension.^[2]

Alternative alpha blockers, such as prazosin, tamsulosin, doxazosin, or terazosin can have adverse interactions with beta blockers, erectile dysfunction drugs, anxiolytics, and antihistamines.^[2] Again, these interactions can cause dangerous hypotension. Furthermore, in rare cases, drug interactions can cause irregular, rapid heartbeats or an increase blood pressure.^[2]

Yohimbine can interact with stimulants, hypertension drugs, naloxone, and clonidine. Interactions with such drugs can cause either an unintended increase in blood pressure or potentiate an increase in blood pressure.^[2]

Finally, in drugs with both alpha and beta blocking properties, such as carvedilol and labetalol, interactions with other alpha or beta blockers can exaggerate a decrease in blood pressure.^[2] Conversely, there are also drug interactions with carvedilol or labetalol in which blood pressure is increased unintentionally (such as with cough and cold medications).^[2] Finally, there may also be some alpha/beta blocker drug interactions that can worsen previous heart failure.^[2]

Mechanism of action

Alpha blockers work by blocking the effect of nerves in the sympathetic nervous system. This is done by binding to the alpha receptors in smooth muscle or blood vessels.^[37] α-blockers can bind both reversibly and irreversibly.^[2]

There are several α receptors throughout the body where these drugs can bind. Specifically, α₁ receptors can be found in most vascular smooth muscle, the pupillary dilator muscle, the heart, the prostate, and pilomotor smooth muscle.^[2] On the other hand, α₂ receptors can be found in platelets, cholinergic nerve terminals, some vascular smooth muscle, postsynaptic CNS neurons, and fat cells.^[2]

The structure of α receptors is a classic

Because the α₁ and α₂ receptors have different mechanisms of action, their antagonists also have different effects.

DAG

to decrease calcium release, thus, decreasing overall signaling. On the other hand, α₂ blockers prevent the reduction of cAMP, thus leading to an increase in overall signaling.

See also

• Beta blocker
• Adrenergic antagonist

References