Joseph L Izzo - Hypertension Primer: The Essentials of High Blood Pressure: Basic Science, Population Science, and Clinical Management
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Hypertension Primer: The Essentials of High Blood Pressure: Basic Science, Population Science, and Clinical Management
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Now in its thoroughly updated Fourth Edition, the Hypertension Primer is a comprehensive, readable source of state-of-the-art scientific and clinical information on hypertension. The book contains 171 short chapters by distinguished experts that cover every aspect of hypertension and its pathogenesis, epidemiology, impact, and management. Highlights of this edition include updated JNC 7 findings regarding special population therapy and clinical management. Chapters are grouped into three well-organized sections--basic science, population science, and clinical management--and each chapter is cross-referenced to other relevant chapters. Each chapter is easily digestible and begins with a bulleted list of key points.
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Editors: Izzo, Joseph L.; Sica, Domenic A.; Black, Henry R.
Title: HypertensionPrimer: The Essentials of High Blood Pressure: Basic Science,Population Science, and Clinical Management, 4th Edition
Copyright 2008 Lippincott Williams & Wilkins
> Table of Contents > PART A - Basic Science > SECTION I - Ion Transport and SignalTransduction > CHAPTER A1 - Signal Transduction: Receptors
CHAPTER A1
Signal Transduction: Receptors
Greti Aguilera MD
Key Points
Receptors are protein complexes thatrecognize specific hormones and translate extracellular hormone levelsinto intracellular events.
The guanyl nucleotide protein (G protein)coupled receptor superfamily is the largest group of plasma membrane receptors.
Hormone ligand binding activates thereceptors, usually downregulates receptor number or affinity, andreduces tissue sensitivity.
Receptors modulate their biological activity by interacting at the proteinprotein or signaling levels.
See also Chapters A2 , A3 , A4 , and A5
Cell-to-cell communication, an essential component ofintegrated physiologic function in multicellular organisms, is mediatedlargely through informational molecules, such as hormones andneurotransmitters.
Major receptor types and functions
Extracellular signaling molecules, or first messengers,are recognized by specific receptor proteins in the target cell, whichare of two types. Type 1 includes receptors located in the plasmamembrane that recognize growth factors, catecholamines, insulin,cytokines, prostaglandins, and many other substances. Type 2 includesreceptors for steroids and iodothyronines that are located in thecytoplasm or nucleus of the cell and control gene expression.
Receptors have two major functions: (a) recognition of aspecific hormone ligand and (b) intracellular transmission ofinformation leading to modification of cell function. Modified hormoneanalogs capable of binding the receptor without activating transductionmechanisms act as receptor antagonists.
Receptor binding properties
The use of radiolabeled ligands has made possible theidentification and measurement of receptors for steroids, peptidehormones, and neurotransmitters in their various target tissues.
Receptor kinetics.
In general, each ligand-receptor interaction is rapidand reversible, consistent with the time course of the biologicaleffects of hormones. Binding kinetics depend on the rates ofassociation and dissociation of the ligand-receptor complexes, whichare affected by temperature and pH. The ratio between association anddissociation rates determines the association constant (Ka). The reciprocal of the association constant is the dissociation constant (Kd), which is usually expressed as the ligand concentration necessary to saturate the binding sites.
Receptor affinity.
Ligand-receptor binding exhibits high affinity, whichallows for significant binding despite low circulating levels ofhormones. Receptor binding is always saturable, indicating a limitednumber of binding sites. Receptor affinity usually correlates well withtissue sensitivity to the biological effect of the hormone, but in anumber of systems, full biological response is achieved with onlypartial receptor occupancy. The presence of excess or spare receptorsmay be important to maintain biological effects of hormones inphysiologic or pathologic conditions involving alterations of receptornumber.
P.2
Transduction mechanisms
In general, a requisite for a receptor molecule is theability to communicate information to effector molecules (secondmessengers) inside the cell. The informational transduction can becarried out by the receptor itself or through activation of one or moreintermediary signaling molecules or pathways.
Cell membrane (type 1) receptors.
Interaction of hormones or neurotransmitters withcell-surface receptors leads to modification of cell function through achain of events involving the generation of second-messenger molecules.Cell-surface receptors can be categorized into two major groups: (a)receptors with intrinsic enzymatic or ion channel activity and (b)receptors coupled to cellular effector molecules through a transductionprotein. Molecular cloning and characterization of these receptors showthat they are anchored to the cell membrane through one or severalhydrophobic amino acid sequences. In general, the structure of thesereceptors consists of an extracellular domain, transmembrane regions,and one or more intracellular regions responsible for catalyticactivity or coupling to intermediary proteins.
Intracellular (type 2) receptors.
Intracellular receptors, such as steroid and thyroidhormone receptors, are dimeric proteins consisting of a hormone-bindingsubunit and a regulatory subunit. After ligand (first-messenger)binding, the regulatory subunit dissociates from the complex and theactivated hormone-binding subunit interacts with DNA, influencing genetranscription. Activated receptors interact with DNA-responsiveelements in the form of homodimers. Receptor-DNA binding activity canbe modulated through formation of heterodimers with other transcriptionfactors, and by recruitment of coactivators or corepressors on bindingto responsive elements at the gene promoter level.
Type 1 receptors and intrinsic activity
Receptors with intrinsic activity
Tyrosine kinase dependent.
Growth factor and insulin receptors include a tyrosinekinase domain and one or more tyrosine phosphorylation sites that arestructural parts of the receptor molecule. Ligand interaction withthese receptors results in receptor autophosphorylation, which leads tobinding of phosphorylated receptor domains to signaling molecules.Examples include phosphatidylinositol kinase, guanosine triphosphatase(GTPase)-activating factor, phospholipase C, Src family kinases, orserine kinases.
Receptor-gated ion channels.
Receptor-gated ion channels, in which the receptor is astructural component of the ion channel, also exhibit intrinsicactivity. Examples are -aminobutyric acid (GABA)A receptors associated with Cl- and HCO3- transport; nicotinic acetylcholine receptors associated with Na+, K+, and Ca2+ transport; N -methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors associated with Na+, K+, and Ca2+ transport; 5- hydroxytryptamine (HT)3 receptors associated with Na+ and K+ transport; and channel-opening adenosine triphosphate (ATP) receptors associated with Ca2+, Na+, and Mg2+ transport.
Receptors without intrinsic activity.
A second group of cell-surface receptors lacks intrinsicactivity and uses an intermediary protein such as adenylyl cyclase,phospholipase C, ion channels, or tyrosine kinases for coupling toeffectors. Two major types belong to this group: (a) the cytokinereceptor superfamily, including growth hormone and prolactin receptors,which activate tyrosine kinases of the Janus kinase (JAK) family, and(b) the guanyl nucleotidebinding protein (G protein) receptorsuperfamily (GPCR).
G proteins are located on the intracellular side of theplasma membrane, where they can interact with the receptor and theeffector signaling system. G proteins consist of three subunits, , ,and , of which has GTP binding and GTPase activity properties.Occupancy of the receptor by its ligand causes conformational changesin the associated G protein, allowing binding of the subunit to GTPand dissociation from the -complex. The activated subunit activatesan effector molecule, such as adenylyl cyclase or phospholipase C. Thisprocess is rapidly reversible on degradation of bound GTP by theintrinsic GTPase activity of the subunit.
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