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1. a molecule on the cell surface (cell-surface or membrane receptor) or within a cell, usually in its nucleus (nuclear receptor) that recognizes and binds with specific molecules, producing some effect in the cell; e.g., the cell-surface receptors of immunocompetent cells that recognize antigens, complement components, or lymphokines; or those of neurons and target organs that recognize neurotransmitters or hormones.
2. a sensory nerve ending that responds to various stimuli.
α-r's (α-adrenergic r's) alpha-adrenergic receptors.
adrenergic r's receptors for epinephrine or norepinephrine, such as those on effector organs innervated by postganglionic adrenergic fibers of the sympathetic nervous system. There are two types, alpha-adrenergic receptors and beta-adrenergic receptors.
alpha r's (alpha-adrenergic r's) adrenergic receptors found in cardiac muscle and vascular smooth muscle; they are stimulated by norepinephrine and blocked by agents such 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. Called also α-receptors and α-adrenergic receptors.
β-r's (β-adrenergic r's) beta-adrenergic receptors.
B cell antigen r's monomeric IgM, IgD, and (on memory cells only) IgG that is attached to the cell membrane of B lymphocytes (B cells); in conjunction with helper T cells, it triggers B cell activation on contact with antigen.
beta r's (beta-adrenergic r's) adrenergic receptors that are stimulated by epinephrine and blocked by agents such as propranolol. They are subdivided into two basic types: β1-receptors are found in the myocardium and cause lipolysis and cardiac stimulation, and β2-receptors are found in smooth and skeletal muscle and liver and cause bronchodilation and vasodilation. A third type, β3, is atypical; it is more sensitive to norepinephrine than to epinephrine, relatively resistant to propranolol blockade, and may be involved in lipolysis regulation in adipose tissue. Called also β-receptors and β-adrenergic receptors.
cell-surface receptor membrane receptor.
cholinergic r's membrane receptors on cells of effector organs, innervated by cholinergic nerve fibers and responsive to the acetylcholine secreted by these fibers. There are two types, muscarinic receptors and nicotinic receptors.
complement receptor a membrane receptor that can bind activated complement components. For example, component C3b binds to complement receptors of neutrophils, B lymphocytes, and macrophages.
estrogen receptor a cellular regulatory protein that binds estrogenic hormones, found particularly in estrogen-sensitive tissues such as the uterus and breast. Cytoplasmic levels are measured in surgically removed breast carcinomas; high levels indicate that a positive response to endocrine therapy is likely.
Fc r's specific membrane receptors for antigen-antibody complexes or aggregated immunoglobulins that bind a site in the Fc portion of the immunoglobulin molecule and may exhibit specificity for particular immunoglobulin classes. Fc receptors are found on B cells, K cells, macrophages, neutrophils, and eosinophils, and, during some developmental stages, on T cells.
histamine r's receptors for histamine. H1-receptors mediate contraction of smooth muscles and dilation of capillaries, causing effects such as bronchoconstriction and contraction of the intestine; they are blocked by antihistamines such as pyrilamine or chlorpheniramine. H2-receptors mediate acceleration of the heart rate and produce gastric acid secretion; they are blocked by agents such as cimetidine.
IgE r's membrane receptors for IgE, found on mast cells and basophils.
insulin r's a type of membrane receptors specific for insulin, found on target cells.
LDL r's (low-density lipoprotein r's) specific receptors for low-density lipoproteins found in coated pits on the surface of mammalian cells. The coated pits are internalized forming coated vesicles from which the receptors are recycled back to the plasma membrane while particles of low-density lipoprotein are transferred to lysosomes where they are degraded, releasing free cholesterol, phospholipids, and amino acids. Genetic defects in LDL receptors are responsible for familial hypercholesterolemia.
membrane receptor a receptor located on or in the membrane of a cell. Called also cell-surface receptor.
muscarinic r's cholinergic receptors on autonomic effector cells (and also on some autonomic ganglion cells and in some central neurons) that are stimulated by muscarine and parasympathomimetic drugs and blocked by atropine.
nicotinic r's cholinergic receptors of autonomic ganglion cells and motor end-plates of skeletal muscle that are stimulated by low doses of nicotine and blockaded by high doses of nicotine or by tubocurarine.
olfactory receptor a specialized sensory nerve-ending sensitive to stimulation giving rise to the sensation of odors; called also osmoreceptor.
opiate r's (opioid r's) receptors that combine with particular opiates to create analgesia and certain other effects. Several different ones have been identified and assigned Greek letters; the μ receptor gives superior analgesia, and the κ receptor is associated with a low degree of physical dependency.
stretch receptor a sense organ in a muscle or tendon that responds to elongation.
T cell r's the characteristic marker of T lymphocytes (T cells) that recognize specific foreign antigens as well as self MHC antigens; both must be seen simultaneously to trigger T cell activation (see also lymphocyte activation). The receptor is not a complete immunoglobulin molecule but may contain heavy and light chain variable regions.
See muscle spindle.