adrenergic receptors

receptor /re·cep·tor/ (-ter)

1. a molecule on the surface or within a cell that recognizes and binds with specific molecules, producing a specific effect in the cell; e.g., the cell-surface receptors for antigens or cytoplasmic receptors for steroid hormones.

2. a sensory nerve ending that responds to various stimuli.

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α-adrenergic receptors  adrenergic receptors that respond to norepinephrine and to such blocking agents as phenoxybenzamine. They are subdivided into two types: α1, found in smooth muscle, heart, and liver, with effects including vasoconstriction, intestinal relaxation, uterine contraction and pupillary dilation, and α2, found in platelets, vascular smooth muscle, nerve termini, and pancreatic islets, with effects including platelet aggregation, vasoconstriction, and inhibition of norepinephrine release and of insulin secretion.

adrenergic receptors  receptors for epinephrine or norepinephrine, such as those on effector organs innervated by postganglionic adrenergic fibers of the sympathetic nervous system. Classified as α and β.

alpha-adrenergic receptors  α.

β-adrenergic receptors , beta-adrenergic receptors adrenergic receptors that respond particularly to epinephrine and to such blocking agents as propranolol. They are subdivided into two basic types: β1, in myocardium and causing lipolysis and cardiac stimulation, and β2, in smooth and skeletal muscle and liver and causing bronchodilation and vasodilation. The atypical type β3 may be involved in lipolysis regulation in adipose tissue.

cholinergic receptors  cell-surface receptor molecules that bind the neurotransmitter acetylcholine and mediate its action on postjunctional cells.

complement receptors  cell-surface receptors for products of complement reactions, playing roles including recognition of pathogens, phagocytosis, adhesion, and clearance of immune complexes. The best characterized are C1–C4, which bind C3 fragments already bound to a surface.

cutaneous receptor  any of the various types of sense organs found in the dermis or epidermis, usually a mechanoreceptor, thermoreceptor, or nociceptor.

cytokine receptors  membrane-spanning proteins that bind cytokines via extracellular domains, acting to convert an extracellular signal to an intracellular one.

H1 receptors , H2 receptors see histamine.

joint receptor  any of several mechanoreceptors that occur in joint capsules and respond to deep pressure and to other stimuli such as stress or change in position.

muscarinic receptors  cholinergic receptors that are stimulated by the alkaloid muscarine and blocked by atropine; they are found on automatic effector cells and on central neurons in the thalamus and cerebral cortex.

muscle receptor  a mechanoreceptor found in a muscle or tendon.

nicotinic receptors  cholinergic receptors that are stimulated initially and blocked at high doses by the alkaloid nicotine and blocked by tubocurarine; they are found on automatic ganglion cells, on striated muscle cells, and on spinal central neurons.

nonadapting receptor  a mechanoreceptor, such as a nociceptor, that responds to stimulation with a continual steady discharge and little or no accommodation over time.

olfactory receptor  a chemoreceptor in the nasal epithelium that is sensitive to stimulation, giving rise to the sensation of odors.

opiate receptor , opioid receptor any of a number of receptors for opiates and opioids, grouped into at least seven types on the basis of their substrates and physiological effects.

orphan receptor  a protein identified as a putative receptor on the basis of its structure but without identification of possible ligands or evidence of function.

pain receptor  nociceptor.

rapidly adapting receptor  a mechanoreceptor that responds quickly to stimulation but that rapidly accommodates and stops firing if the stimulus remains constant.

sensory receptor  receptor (2).

slowly adapting receptor  a mechanoreceptor that responds slowly to stimulation and continues firing as long as the stimulus continues.

stretch receptor  a sense organ in a muscle or tendon that responds to elongation.

tactile receptor  a mechanoreceptor for the sense of touch.

thermal receptor  thermoreceptor.

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adrenergic (ad´rinur´jik),

adj 1. transmitted by norepinephrine or activated by norepinephrine or the other sympathomimetic agents.
n 2. a term applied to nerve fibers that liberate epinephrine or norepinephrine at a synapse when a nerve impulse passes.
n 3. a drug that mimics the action of adrenergic nerves.

adrenergic agonists,

n.pl drugs that mimic the actions of norepinephrine, a neurotransmitter, resulting in stimulation of the sympathetic nervous system.

adrenergic blocking agent,

n See agent, adrenergic blocking.

adrenergic fibers,

n.pl See fibers, adrenergic.

adrenergic receptors,

n.pl See receptors, adrenergic.

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receptor(s) (rēsep´tur),

n a site or location within a cell or its membrane that combines with a haptophore group of a toxin, drug, enzyme, hormone, or other substance and that may elicit a specific or general response; a sensory nerve terminal that responds to stimuli of various kinds.

receptor, sensory,

n receptor system built on the theory that receptor organs are specialized and respond to the law of specific nerve energies; i.e., each type of end-organ, no matter what stimulus is applied, will respond (if it responds) with only a single appropriate type of sensation. Common experience shows this to be true; e.g., when a person receives a blow to the eye, light is experienced as a consequence of the blow. Another factor is that the impulse will travel in only one direction, from the receptor organ back to the central nervous system. The receptor system is thus the summation in the brain of all the sensory stimuli that come from the special senses, general senses, mucous membrane, skin, and deeper tissues and is the basis for instruction sent to the musculoskeletal system for action, as in the masticatory phenomenon.

receptors, adrenergic,

n alpha and beta “units” associated with sympathetic neuroeffectors that react with sympathomimetic drugs to elicit the response of the effector cells.

receptors, beta,

n sites on some cell membranes in the sympathetic nervous system that either activate or block the normal functions of the cell by binding with epinephrine (adrenaline). Cells with beta receptors normally control physiologic responses such as heart rate.

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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.

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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.