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Alpha-adrenergic agonist

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Alpha adrenergic agonist
Drug class
Phenylephrine
Skeletal structor formula of phenylephrine, a common nasal decongestant
Class identifiers
Use Decongestant, Hypotension, Bradycardia, Hypothermia etc.
ATC code N07
Biological target Alpha adrenergic receptors of the α subtype
External links
MeSH D000316
In Wikidata

Alpha-adrenergic agonists are a class of 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. Alpha-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 stimulates the membrane bound enzyme phospholipase C, and activation of α2 inhibits the enzyme adenylate cyclase. Inactivation of adenylate cyclase in turn leads to the inactivation of the secondary messenger cyclic adenosine monophosphate and induces smooth muscle and blood vessel constriction.

Classes

Norepinephrine (noradrenaline)

Although complete selectivity between receptor agonism is rarely achieved, some agents have partial selectivity. NB: the inclusion of a drug in each category just indicates the activity of the drug at that receptor, not necessarily the selectivity of the drug (unless otherwise noted).

α1 agonist

α1 agonist: stimulates phospholipase C activity. (vasoconstriction and mydriasis; used as vasopressors, nasal decongestants and during eye exams). Selected examples are:

α2 agonist

α2 agonist: inhibits adenylyl cyclase activity, reduces brainstem vasomotor center-mediated CNS activation; used as antihypertensive, sedative & treatment of opiate dependence and alcohol withdrawal symptoms). Selected examples are:

Nonspecific agonist

Nonspecific agonists act as agonists at both alpha-1 and alpha-2 receptors.

Undetermined/unsorted

The following agents are also listed as agonists by MeSH.

Clinical significance

Alpha-adrenergic agonists, more specifically the auto receptors of alpha 2 neurons, 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. Medications such as clonidine and dexmedetomidine target pre-synaptic auto receptors, therefore leading to an overall decrease in norepinephrine which clinically can cause effects such as sedation, analgesia, lowering of blood pressure and bradycardia. There is also low quality evidence that they can reduce shivering post operatively.

The reduction of the stress response caused by alpha 2 agonists were theorised to be beneficial peri operatively by reducing cardiac complications, however this has shown not to be clinically effective as there was no reduction in cardiac events or mortality but there was an increased incidence of hypotension and bradycardia.

See also

External links


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