|Alpha Adrenergic agonist|
Skeletal structor formula of phenylephrine, a common nasel decongestant
|Use||Decongestant, Hypotension, Bradycardia, etc.|
|Biological target||Alpha adrenergic receptors of the α subtype|
An adrenergic alpha-agonist (or alpha-adrenergic agonists) are a class sympathomimetic agents that selectively stimulates alpha adrenergic receptors. The alpha-adrenergic receptor has two subclasses α1 and α2. Alpha 2 receptors are associated with sympatholytic properties. α Adrenergic agonists have the opposite function of alpha blockers. Alpha adrenoreceptor ligands mimic the action of epinephrine and norepinephrine signaling in the heart, smooth muscle and central nervous system, with norepinephrine being the highest affinity. The activation of α1 and α2 inhibits the enzyme Adenylate cyclase. This in turn leads to the inactivation of the secondary messenger Cyclic adenosine monophosphate and induces smooth muscle and blood vessel constriction.
Although complete selectivity between receptor agonism is rarely achieved, some agents have partial selectivity.
α1 agonists 
α2 agonists 
α2 agonists: inhibits adenylyl cyclase activity. (reduce brainstem vasomotor center-mediated CNS activation; used as antihypertensives, sedatives & treatment of opiate dependence and alcohol withdrawal symptoms). Selected examples are:
- Clonidine (mixed alpha2-adrenergic and imidazoline-I1 receptor agonist)
- Guanfacine, (preference for alpha2A-subtype of adrenoceptor)
- Guanabenz (most selective agonist for alpha2-adrenergic as opposed to imidazoline-I1)
- Guanoxabenz (metabolite of guanabenz)
- Guanethidine (peripheral alpha2-receptor agonist)
Clinical significance 
Alpha-adrenergic agonists are used in the treatment of glaucoma by decreasing the production of aqueous fluid by the ciliary bodies of the eye and also by increasing uveoscleral outflow.
See also 
- Declerck I, Himpens B, Droogmans G, Casteels R (September 1990). "The alpha 1-agonist phenylephrine inhibits voltage-gated Ca2(+)-channels in vascular smooth muscle cells of rabbit ear artery". Pflugers Arch. 417 (1): 117–9. doi:10.1007/BF00370780. PMID 1963492.
- Sagvolden T (2006). "The alpha-2A adrenoceptor agonist guanfacine improves sustained attention and reduces overactivity and impulsiveness in an animal model of Attention-Deficit/Hyperactivity Disorder (ADHD)". Behav Brain Funct 2: 41. doi:10.1186/1744-9081-2-41. PMC 1764416. PMID 17173664.
- Atalik KE, Sahin AS, Doğan N (April 2000). "Interactions between phenylephrine, clonidine and xylazine in rat and rabbit aortas". Methods Find Exp Clin Pharmacol 22 (3): 145–7. PMID 10893695.
- MeSH list of agents 82000316
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