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.
Alpha 1 receptors
Activation of alpha 1 receptors increases vascular smooth muscle contraction, producing increases in blood presure. These receptors also influence activity of genitourinary smooth muscle.
Effects of alpha 1 receptor activation on different tissues and systems:
- Vascular smooth muscle. Activation of vascular smooth muscle receptors causes vasoconstriction, leading to an increase in peripheral resistance and in blood pressure. Alpha 1 antagonists are used in the treatment of hypertension because of their properties as peripheral vasodilators.
- Mydriasis. Mydriasis is mediated by both alpha 1 and 2 activation.
- Genitourinary tract smooth muscle. Alpha 1A receptors are located at the external sphincter of the bladder. Blockade of alpha 1A receptors decreases tone in the smooth muscle of the bladder neck and prostate, thus improving urinary flow. Tamsulosine is used for the treatment of benign prostatic hyperplasia because of its ability to selectively block alpha 1A receptors.
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
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