Adrenergic agonist

Overview
An adrenergic is a drug, or other substance, which has effects similar to, or the same as, epinephrine (adrenaline). Thus, they are a kind of sympathomimetic agents. Alternatively, it may refer to something which is susceptible to epinephrine, or similar substances, such as a biological receptor (specifically, the adrenergic receptors).

Beta blockers block the action of epinephrine and norepinephrine in the body. Adrenergic drugs either stimulate a response (agonists) or inhibit a response (antagonists). The five categories of adrenergic receptors are: α1, α2, β1, β2, and β3, and agonists vary in specificity between these receptors, and may be classified respectively. However, there are also other mechanisms of adrenergic agonism. Epinephrine and norepinephrine are endogenous and broad-spectrum. More selective agonists are more useful in pharmacology.

α1 agonists
α1 agonists: stimulates phospholipase C activity. (vasoconstriction and mydriasis; used as vasopressors, nasal decongestants & eye exams). Selected examples are:
 * Methoxamine
 * Methylnorepinephrine
 * Oxymetazoline
 * Phenylephrine

α2 agonists
α2 agonists: inhibits adenylyl cyclase activity. (reduce brainstem vasomotor center-mediated SNS activation; used as antihypertensives, sedatives & treatment of opiate & 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 (periferal alpha2-receptor agonist)

β1 agonists
β1 agonists: stimulates adenylyl cyclase activity; opening of calcium channel. (cardiac stimulants; used to treat cardiogenic shock, acute heart failure, bradyarrhythmias). Selected examples are:
 * Dobutamine
 * Isoproterenol (β1 and β2)

β2 agonists
β2 agonists: stimulates adenylyl cyclase activity; closing of calcium channel (smooth muscle relaxants; used to treat asthma and COPD). Selected examples are:
 * salbutamol (albuterol in USA)
 * Fenoterol
 * Formoterol
 * Isoproterenol (β1 and β2)
 * Metaproterenol
 * Salmeterol
 * Terbutaline
 * Clenbuterol

Other mechanisms

 * Indirect action
 * Amphetamine
 * Tyramine
 * Mixed action
 * Ephedrine