Alpha blocker

[2] 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.

[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.

[19] In low doses, labetalol and carvedilol can decrease the peripheral resistance and block the effects of isoprenaline to reduce hypertensive symptoms.

[22] Yohimbine, an α2 blocker derived from the bark of the Pausinystalia johimbe tree, has been tested to increase libido and treat erectile dysfunction.

The proposed mechanism for yohimbine is blockade of the adrenergic receptors that are associated with neurotransmitters inhibition, including dopamine and nitric oxide, and thus aiding with penile erection and libido.

[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.

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.

[27] High doses of the α1 blocker, prazosin, have been efficacious in treating patients with PTSD induced nightmares due to its ability to block the effects of norepinephrine.

[29] This is a phenomenon in which patients with hypertension take an alpha blocker for the first time, and suddenly experience an intense decrease in blood pressure.

Ultimately, this gives rise to orthostatic hypotension, dizziness, and a sudden loss of consciousness due to the drastic drop in blood pressure.

[30] 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.

[2] 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.

[28][34] Priapism, an unwanted, painful long term erection not brought on by sexual arousal and lasting several hours has been associated with alpha blocker use.

[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] On the other hand, α2 receptors can be found in platelets, cholinergic nerve terminals, some vascular smooth muscle, postsynaptic CNS neurons, and fat cells.

Specific locations and functions of the α receptors. Image from Basic and Clinical Pharmacology by Bertram Katzung, et al. [ 1 ]
Schematic of G protein coupled receptor signaling, representing Gi GPCR signaling, Gs GPCR signaling, and Gq GPCR signaling.
An image of a patient with pheochromocytoma. In patients with this disease, a catecholamine-secreting tumor is formed, and causes excess CNS stimulation, such as excess sweating and tachycardia. Nonselective alpha blockers, such as phenoxybenzamine or phentolamine, can be used to mitigate this disease.
Benign prostate hyperplasia, a disease in which urinary retention becomes an issue. Alpha-1 blockers can be used, but it can result in side effects such as increased urination and retrograde ejaculation.
Patients with Raynaud's syndrome experience cut off blood flow from the fingers causing a large decrease in oxygen, which leads to the discoloration of the fingers. Using alpha blockers aids in restoring blood flow and treating the syndrome by stimulating the dilation of blood vessels.
Alpha-1 Receptor signaling cascade and antagonist signaling cascade.
Alpha-2 receptor signaling cascade and antagonist signaling cascade.