adrenergic receptors

ad·re·ner·gic re·cep·tors

reactive components of effector tissues, most of which are innervated by adrenergic postganglionic fibers of the sympathetic nervous system. Such receptors can be activated by norepinephrine and/or epinephrine and by various adrenergic drugs; receptor activation results in a change in effector tissue function, such as contraction of arteriolar muscles or relaxation of bronchial muscles; adrenergic receptors are divided into α-receptors and β-receptors, on the basis of their response to various adrenergic activating and blocking agents.

Synonym(s): adrenoceptor, adrenoreceptors

ad·re·ner·gic re·cep·tors

(ad'rĕ-nĕr'jik rĕ-sep'tŏrz)

Reactive components of effector tissues, most of which are innervated by the sympathetic nervous system. Such receptors can be activated by norepinephrine, epinephrine, and adrenergic drugs; receptor activation results in a change in effector tissue function, such as contraction of arteriolar muscles or relaxation of bronchial muscles.
Synonym(s): adrenoreceptor.

adrenergic receptors 

Receptors which are stimulated by the catecholamines adrenaline (epinephrine) and noradrenaline (norepinephrine). These receptors belong to a family of G protein coupled receptors and are found in the central nervous system and many tissues innervated by the peripheral nervous system. There are two types of adrenergic receptors: (1) α-receptors, which are mainly excitatory to smooth muscles and gland cells but cause relaxation of intestinal smooth muscles; (2) β-receptors of which there are (at least) two types, β₁ and β₂. Generally, stimulation of β-receptors produces an inhibitory response, although in some cases the effect is excitatory (e.g. in the heart). Example: the dilator pupillae muscle contains mainly α-adrenergic receptors and stimulation (e.g. with adrenaline) produces mydriasis. On the other hand, there are drugs that block the effect of catecholamines on α- or β-adrenergic receptors and are called α- or β-blockers (or sympatholytic drugs or adrenergic receptor agonists). Example: the ciliary epithelium contains mainly β-receptors and a β-blocker such as timolol inhibits the secretion of aqueous humour, thus reducing intraocular pressure. Syn. adrenoceptor. See alpha-adrenergic agonists; alpha-adrenergic antagonists; beta-blocker; miotics; mydriatic; sympatholytic drugs; sympathomimetic drugs; autonomic nervous system.

ad·re·ner·gic re·cep·tors

(ad'rĕ-nĕr'jik rĕ-sep'tŏrz)

Reactive components of effector tissues, most of which are innervated by adrenergic postganglionic fibers of the sympathetic nervous system. Such receptors can be activated by norepinephrine and/or epinephrine and by various adrenergic drugs.
Synonym(s): adrenoceptor.

adrenergic

1. activated by, characteristic of, or secreting epinephrine or substances with activities similar to those of epinephrine. The term is applied to those nerve fibers of the sympathetic nervous system that release norepinephrine (and possibly small amounts of epinephrine) at a synapse when a nerve impulse passes.

2. an agent that acts like epinephrine. Called also sympathomimetic.

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adrenergic agents

sympathomimetic amines which exert their effects on adrenergic receptors of effector cells innervated by the sympathetic nervous system. The administration of these adrenergic agonists mimics the physiological effects of sympathoadrenal discharge.

adrenergic alpha-blockers, beta-blockers

see adrenergic blockade.

adrenergic amines

these are the sympathomimetic amines. They have similar but not identical structures and actions. Epinephrine, norepinephrine and isoproterenol are catecholamines but differ in their effects. Norepinephrine is primarily an activator of alpha-receptors whereas isoproterenol is a selective beta-receptor agonist. Epinephrine is an active agonist for both alpha- and beta-receptors. Ephedrine is the classical noncatecholamine sympathetic agonist.

adrenergic blockade

adrenergic blocking agents prevent the activation of adrenergic receptors. They may be alpha-blockers, e.g. ergot, or beta-blockers such as propranolol.

adrenergic blocking agent

a drug that blocks the secretion of epinephrine and norepinephrine at the postganglionic nerve endings of the sympathetic nervous system. By blocking these adrenergic substances, which cause constriction of blood vessels and increased cardiac output, adrenergic blocking agents produce a dilatation of the blood vessels and a decrease in cardiac output.

adrenergic nerves

see adrenergic (1) (above).

adrenergic nervous system

see sympathetic nervous system.

adrenergic receptors

class of receptors named after the action of adrenalin(e), the alternative name for epinephrine. Alpha receptors, which are stimulated by norepinephrine and blocked by agents such as phenoxybenzamine, are categorized into two classes, α₁ and α₂, which have different actions. α₁ adrenergic actions include contraction of the iris, decreased motility in the intestine, and potassium and water secretions from the salivary glands. α₂ adrenergic receptors inhibit adenylate cyclase, rather than activating it. Beta receptors, which are stimulated by epinephrine and blocked by agents such as propranolol, are also categorized into two types; β₁ adrenergic receptors, which produce lipolysis and cardiostimulation, and β₂ adrenergic receptors, which produce bronchodilatation and vasodilatation.