Alpha receptors can be classified as a subtype of adrenergic receptor, their activation triggers a complex range of autonomic responses that will be detailed in the following article.  We recommend our readers to check our article on beta receptors for further information on autonomic pharmacology.

Article contents:

Alpha 1 receptors

The image below shows the synaptic cleft, presynaptic and postsynaptic membranes. Alpha 1 receptors are located on the postsynaptic membrane of effector organs.

Activation of alpha 1 receptors increases vascular smooth muscle contraction, producing increases in blood presure. These receptors also influence activity of genitourinary smooth muscle.

alpha _1_receptors
Effects of alpha 1 receptor activation on different tissues and systems:

Alpha 2 receptors

Although alpha 2 receptors are found on both presynaptic neurons and postynaptic cells, they work mainly as autoreceptors to mediate feedback inhibition of sympathetic transmission.

In addition to neurons, alpha 2 receptors are located in other regions, like pancreatic beta cells and platelets.
The diagram below shows how when activated, these receptors act as inhibitory autoreceptors (they inhibit norepinephine release from adrenergic neurons) and as inhibitory heteroreceptors (they inhibit acetylcholine release from cholinergic neurons).
In addition, activation of alpha 2 receptors on pancreatic beta cells membranes inhibits insulin release.

Drugs affecting alpha receptors

Alpha 1 agonists

Because of their properties as vasoconstrictive agents they are used to reduce edema and inflammation. Common decongestant drugs include naphazoline, phenylephrine and propylhexedrine.

Alpha 1 antagonists

These drugs act by causing vasodilation and decreased peripheral resistance, therefore they are used in the treatment of hypertension (prazosin). Treatment of benign prostatic hyperplasia is another clinical use of alpha 1 antagonists: tamsulosin is a subtype-selective alpha 1A receptor that has more specificity toward smooth muscle in genitourinary tract; thus tamsulosin has lower incidence of orthostatic hypotension.

Alpha 2 agonists

By activating central alpha 2 receptors, alpha 2 adrenergic agonists inhibit sympathetic outflow from CNS. Clonidine is used for the rteatment of hypertension and symptoms associated with opioid withdrawal. Alpha-methyldopa is a precursor to the alpha 2 agonist alpha-methylnorepinephrine, it is the drug of choice for the treatment of hypertension during pregnancy.

Alpha 2 antagonists

Yohimbine blockade of alpha 2 receptors leads to increased release of norepinephrine with susequent stimulation of cardiac beta 1 receptors and peripheral vasculature alpha 1 receptors. Yohimbine was used in the past to treat erectile dysfunction.

References and further reading

Golan, David E (editor). “Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy”, 2nd edition. LWW: 2008.

Katzung, B. “Basic & Clinical Pharmacology”, 10th Edition. Mc Graw Hill Medical: 2007

Harvey, Richard; Champe, Pamela (series editors). “Lippincott illustrated reviews: Pharmacology”, 4th edition. LWW: 2009

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