Department of Biochemical Pharmacology, School Of Pharmacy And Pharmaceutical Sciences, SUNY, Buffalo
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Chapter Description:
Drugs that affect ion channels represent one of the most important class of therapeutic agents and also toxic agents secreted by venomous animals. This presentation will attempt to define what ion channels are, how they work and how drugs interact with these channels. Together with the literature recommendations provided you should have a good introduction to this important class of biological effectors.
Drugs that affect ion channels represent one of the most important class of therapeutic agents and also toxic agents secreted by venomous animals. This presentation will attempt to define what ion channels are, how they work and how drugs interact with these channels. Together with the literature recommendations provided you should have a good introduction to this important class of biological effectors.|~|/files/powerpoints_images/node28648/Slide1.JPG|~|563|~|422|~|0
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Drugs that affect ion channels represent one of the most important cl...
The cell membrane represents a formidable barrier to the passage of charged materials, including ions. Accordingly specific mechanisms are available in cell membranes that facilitate the trans-membrane passage of ions. Ion channels are one important mechanism.|~|/files/powerpoints_images/node28648/Slide2.JPG|~|563|~|422|~|0
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The cell membrane represents a formidable barrier to the passage of c...
A variety of mechanisms are employed to facilitate the movement of solutes across cell membranes: such processes are necessary because of the basic impermeability of cell membranes. Ion channels ( right hand side ) are but one of these processes and the principal topic of this session. However, it must be noted that ion channels are but one mechanism for the translocation of ions.A useful review on this topic is by DeFelice and Goswami, “Transporters as Channels”. Ann. Rev. Physiol. 69: 87-112, 2007.|~|/files/powerpoints_images/node28648/Slide3.JPG|~|563|~|422|~|0
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A variety of mechanisms are employed to facilitate the movement of so...
Under physiological conditions cellular ion channels play in harmony and make beautiful music. Defects in ion channels are associated with a number of disease states and pathologies from cardiac arrrhythmias to cystic fibrosis.|~|/files/powerpoints_images/node28648/Slide4.JPG|~|563|~|422|~|0
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Under physiological conditions cellular ion channels play in harmony ...
The cardiac action potential represents a particularly complex set of ionic interactions responsible for maintaining cardiac contractility and automaticity. You will find an interesting discussion of cardiac ion channels in: Chapter 5, “The Rhythm Section: The Heartbeat and other Rhythms”, from The Music of Life, by Denis Noble, OUP, 2007.|~|/files/powerpoints_images/node28648/Slide5.JPG|~|563|~|422|~|0
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The cardiac action potential represents a particularly complex set of...
One important therapeutic group of drugs affecting ion channels is the class of CALCIUM CHANNEL BLOCKERS. This class of drugs has major cardiovascular applications and represents a multi-billion dollar market.|~|/files/powerpoints_images/node28648/Slide6.JPG|~|563|~|422|~|0
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One important therapeutic group of drugs affecting ion channels is th...
Drugs that affect ion channels have important therapeutic and toxic properties.|~|/files/powerpoints_images/node28648/Slide7.JPG|~|563|~|422|~|0
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Drugs that affect ion channels have important therapeutic and toxic p...
Ion channels have several important properties that make them attractive targets for drug interaction.|~|/files/powerpoints_images/node28648/Slide9.JPG|~|563|~|422|~|0
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Ion channels have several important properties that make them attract...
Ion channels are very efficient signaling devices, capable of providing significant amplification of incoming signals in a very short period of time.|~|/files/powerpoints_images/node28648/Slide10.JPG|~|563|~|422|~|0
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Ion channels are very efficient signaling devices, capable of providi...
Cells maintain a highly asymmetric distribution of ions across plasma and intracellular membranes. This distribution is maintained by two principal factors - the relative impermeability of plasma membranes to inorganic ions, and the ability of metabolically-driven (ATP-dependent) ion pumps. In the absence of energy to maintain these pumps ( ischemia etc) this ionic distribution is dissipated and cell death eventually occurs. The equilibrium potential for each ion represents that cellular potential at which the net flux of ions (influx and efflux) are exactly balanced.|~|/files/powerpoints_images/node28648/Slide11.JPG|~|563|~|422|~|0
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Cells maintain a highly asymmetric distribution of ions across plasma...
The net driving force on an ion is determined by the balance of the electrical gradient and the concentration gradient for the ion. |~|/files/powerpoints_images/node28648/Slide12.JPG|~|563|~|422|~|0
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The net driving force on an ion is determined by the balance of the e...
The elementary considerations of the previous slides are summarized in schematic form in this slide. According to these principles, drugs - antagonists or agonists - acting at these channels can have different effects according to the channel that they modulate. Quite generally, drugs that depolarize will be cellular stimulants and drugs that hyperpolarize will be cellular stabilizers. Thus, a potassium channel activator will generally hyperpolarize the cell and inhibit function, whilst a potassium channel antagonist will generally depolarize the cell and stimulate function.|~|/files/powerpoints_images/node28648/Slide13.JPG|~|563|~|422|~|0
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The elementary considerations of the previous slides are summarized i...
Quite generally ion channels operate not singly, but multiply. Thus, in this pancreatic beta-cell the incoming signal - food intake - generates ATP that inhibits an ATP-dependent potassium channel. This channel is also the locus of action of the anti-diabetic drugs such as glyburide. Depolarization of the cell by potassium channel blockade activates a voltage-dependent calcium channel leading to calcium influx and activation of insulin release. In turn, the elevated level of intracellular calcium activates a calcium-dependent potassium channel leading to hyperpolarization and thus terminating calcium influx and insulin releaseAn example of such close coordination of ion channel function is provided by: Berkelfeld et al., BKCa-Cav channel complexes mediate rapid and localized Ca2 -activated K signaling. Science 314: 615, 2006. BKCa channels are activated by increases in cytosolic calcium and they occur in a complex with voltage-gated calcium channels of the CaV 1.2 (L), CaV 2.2 (N ) and CaV 2.1 (P/Q ) types.|~|/files/powerpoints_images/node28648/Slide14.JPG|~|563|~|422|~|0
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Quite generally ion channels operate not singly, but multiply. Thus,...
The principles of the previous slides are illustrated for the example of neuroprotection. Neuronal damage ( brought about by ischemia, hypoxia or cellular toxicants ) is associated with depolarization, the influx of calcium and the release of excitatory amino acids. Accordingly, several therapeutic routes are available that may generate neuroprotection - blockade of calcium influx, the blockade of sodium influx, the blockade of excitatory amino acid receptors, and the activation of potassium channels. |~|/files/powerpoints_images/node28648/Slide17.JPG|~|563|~|422|~|0
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The principles of the previous slides are illustrated for the example...
One of the most powerful electrophysiological approaches to the study of ion channels is the measurement of current flow under \'voltage-clamp\' conditions. In this technique, which can be applied to whole cells or portions of cell membranes that contain many ion channels or even a single ion channel, the membrane potential is maintained at a given level so that current can be measured. From such measurements itis possible to study both the effects of changes of membrane potential AND the effects of chemical perturbants on current flow. Additionally, from the electrophysioloigical data it is possible to calculate channel conductances, ionic permeabilities etc. A full discussion is provided in the text by Bertil Hille. |~|/files/powerpoints_images/node28648/Slide18.JPG|~|563|~|422|~|0
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One of the most powerful electrophysiological approaches to the study...
The voltage clamp is shown here applied to a portion of cell membrane containing ion channels. A depolarizing clamp is shown applied that opens in random fashion single channels: these openings are depicted as rectanglar downward deflections of the current trace.|~|/files/powerpoints_images/node28648/Slide19.JPG|~|563|~|422|~|0
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The voltage clamp is shown here applied to a portion of cell membrane...
The measured current I is determined by the product of the unitary current through a channel ( considered to a first approximation to be constant ), the number of functional channels in the preparation and the opening probability for the single channel. Ion channel opening and closing can be regarded as a probabilistic event.|~|/files/powerpoints_images/node28648/Slide20.JPG|~|563|~|422|~|0
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The measured current I is determined by the product of the unitary cu...
Ion channels can be differentiated by a variety of criteria: the type of ion permeated ( cation vs anion, sodium vs potassium ), the voltage- and chemical-sensitivity of the channel and the nature of drugs that affect the channel. Channels are also differentiatable by the size of the channel - conductance - and the kinetics of channel opening and closing. Here we show the kinetics of opening and closing of the voltage-sensitive sodium and potassium channels in the squid giant axon. |~|/files/powerpoints_images/node28648/Slide21.JPG|~|563|~|422|~|0
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Ion channels can be differentiated by a variety of criteria: the type...
Ion channels are differentially sensitive to drugs and toxins. a, b: tetrodotoxin and saxitoxin block sodium channels (the basis of their toxic properties ); c, tetraethylammonium blocks many potassium channels; d. nifedipine blocks one class of voltage-sensitive calcium channel and this is the basis of its cardiovascular actions.|~|/files/powerpoints_images/node28648/Slide22.JPG|~|563|~|422|~|0
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Ion channels are differentially sensitive to drugs and toxins. a, b:...
All ion channels are characterized by chemical sensitivity. Indeed, a prominent characteristic of ion channels is that they are typically sensiitve to multiple classes of chemical agents. Represented here is the GABA-sensitive chloride channel activated by the action of the neurotransmitter GABA and several synthetic analogs. This channel is also sensitive to antagonists such as bicuculline and is also modulated by therapeutically important agents such as the benzodiazepines. Agents such as valium serve to interact at discrete receptor sites associated with GABA receptors to potentiate the actions of this inhibitory transmitter.|~|/files/powerpoints_images/node28648/Slide23.JPG|~|563|~|422|~|0
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All ion channels are characterized by chemical sensitivity. Indeed, ...
Similarly, the ligand-gated excitatory amino acid glutamate receptor is also sensitive to a variety of chemically distinct ligands interacting at topographically distinct sites.|~|/files/powerpoints_images/node28648/Slide24.JPG|~|563|~|422|~|0
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Similarly, the ligand-gated excitatory amino acid glutamate receptor ...
An excellent review is: Ramsey, Delling and Clapham, An Introduction to TRP Channels, Ann. Rev. Physiol. 68: 619-647, 2006.Gharat and Szallasi, Medicinal chemistry of the vanilloid ( capsaicin ) TRPV1 receptor: Current knowledge and future perspective. Drug Dev Res 68: 477, 2007.See also: Bautista et al., The menthol receptor TRPM8 is the principal detector of environmental cold, Nature 448: 204, 2007.Story and Cruz-Orengo, Feel the burn, American Scientist, 95: 326, 2007.|~|/files/powerpoints_images/node28648/Slide25.JPG|~|563|~|422|~|0
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An excellent review is: Ramsey, Delling and Clapham, An Introduction ...
Schnell and Chou, Structure and mechanism of the M2 proton channel of influenza A virus, Nature 451: 591, 2008.Stouffer et al., Structural basis for the function and inhibition of an influenza virus protein channel, Nature 451: 596, 2008Clapham, Coughing up flu’s proton channels, Nature 451: 532, 2008.|~|/files/powerpoints_images/node28648/Slide26.JPG|~|563|~|422|~|0
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Schnell and Chou, Structure and mechanism of the M2 proton channel of...
A schematic representation of ion channels showing the presence of the PORE ( through which ions permeate ), GATES in the channel that are opened and closed in response to a variety of incoming chemical and electrical signals, SENSORS ( both intra- and extracellular ) that recognize the incoming signals: a SENSOR site is also shown as topographically distinct from the channel: this site may communicate with the channel through a variety of membrane signals and signaling pathways, including G-proteins.An example whereby a biochemical event is linked to the gating process is found in Suh et al. “rapid biochemically induced changes of PtdIns(4,5)P2 gate KCNQ ion channels. Science 314: 1454, 2006. Current through this channel falls when phosphatidylinositol 3,5-bisphosphonate is depleted and rises when this chemical is overproduced.|~|/files/powerpoints_images/node28648/Slide27.JPG|~|563|~|422|~|0
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A schematic representation of ion channels showing the presence of th...
The voltage-gated calcium channel is a heteromeric structure composed of several distinct subunits. The principal and largest subunit is the alpha-subunit that contains both the channel functional machinery and the major drug binding sites. The properties and the expression of this subunit are modified by the presence of the auxiliary subunits, in particular the beta-subunit. Currently there are, in addiiton to splice variants of each subunit, 10 alpha-subunits, five beta-subunits, four alpha2delta-subunits, and five gamma-subunits. However, the exact number of combinations seen physiologically and their tissue distribution has not been established.|~|/files/powerpoints_images/node28648/Slide29.JPG|~|563|~|422|~|0
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The voltage-gated calcium channel is a heteromeric structure composed...
The classification of voltage-gated calcium channels in terms of structure, function and pharmacology.|~|/files/powerpoints_images/node28648/Slide30.JPG|~|563|~|422|~|0
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The classification of voltage-gated calcium channels in terms of stru...
The subunit heteromeric construction of GABA-gated chloride channels (receptors) makes possible the expression of a variety of functionally and pharmacologically distinct receptor subtypes.|~|/files/powerpoints_images/node28648/Slide31.JPG|~|563|~|422|~|0
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The subunit heteromeric construction of GABA-gated chloride channels ...
GABA receptors have distinct pharmacological properties and function according to their subunit composition (and localization).|~|/files/powerpoints_images/node28648/Slide32.JPG|~|563|~|422|~|0
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GABA receptors have distinct pharmacological properties and function ...
The preceding considerations indicate that, as indicated earlier, ion channels can be regarded as a class, albeit a large and diverse class, of pharmacological receptors with the properties indicated above. However, they also have a number of distinguishing properties that are of critical significance to the understanding of drug action and that are related directly to the the fact that ion channels function through conformationally distinct states that are associated with their opening and closing processes. These properties are illustrated in the following slides. |~|/files/powerpoints_images/node28648/Slide33.JPG|~|563|~|422|~|0
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The preceding considerations indicate that, as indicated earlier, ion...
The HERG channel can interact with many drug classes, but rather than maintaining separate receptor sites for each drug class, rather these drugs interact at a single promiscuous site.|~|/files/powerpoints_images/node28648/Slide34.JPG|~|563|~|422|~|0
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The HERG channel can interact with many drug classes, but rather than...
Tyr-652 and Phe-656 are residues critical in drug interaction at HERG. From Fernandez et al., Physicochemical features of the HERG channel drug binding site, J Biol. Chem 279: 1010-10127, 2004.|~|/files/powerpoints_images/node28648/Slide35.JPG|~|563|~|422|~|0
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Tyr-652 and Phe-656 are residues critical in drug interaction at HERG...
Schematic representation of the potassium channel alpha1 subunit binding topography. Four of these units form the functional channel with beta-subunits.|~|/files/powerpoints_images/node28648/Slide36.JPG|~|563|~|422|~|0
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Schematic representation of the potassium channel alpha1 subunit bind...
Examples of drug classes that interact at the HERG channel, with examples of drugs withdrawn from the market because of these interactions.Further details are found in:M A Crouch, L. Limon and A T Cassano, Clinical relevance and management of drug-related QT interval prolongation. Pharmacotherapy 23: 881-908, 2003.M L De Bruin et al., Anti-HERG activity and the risk of drug-induced arrhythmias and sudden death. Eur Heart J 26: 590-597, 2005.M. Recanatini et al., QT prolongation through HERG channel blockade. Current knowledge and strategies for the early prediction during drug development. Med Res Revs 25: 133-166, 2005.|~|/files/powerpoints_images/node28648/Slide37.JPG|~|563|~|422|~|0
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Examples of drug classes that interact at the HERG channel, with exam...
Quaternary ammonium ions block potassium channels. However,this blockade is kinetically complex since blockade increases with increasing channel open time: this suggested that the ammonium ions can access their receptor site preferentially after the channel has opened. This process is referrred to generically as \'state-dependent\' blockade.|~|/files/powerpoints_images/node28648/Slide38.JPG|~|563|~|422|~|0
In the interaction of a local anesthetic ( lidocaine - an antiarrhythmic agent functioning through blockade of sodium channels ) with the sodium channel blockade increases with increasing frequency of channel opening. This indicated that the anesthetic - through \'frequency-dependent\' blockade preferentially interacts with channels in the open/inactivated state.|~|/files/powerpoints_images/node28648/Slide39.JPG|~|563|~|422|~|0
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In the interaction of a local anesthetic ( lidocaine - an antiarrhyth...
A highly schematized representation of an ion channel cycling through three states ( or families of states ) - closed, open and inactivated. Drugs may preferentially interact ( or not ) with any of these states. Accordingly, a drug that interacts prefernentially with any specific state will have an apparently increased affinity in any condition that increases the fraction of availability of that state. Additionally, according to the hydrophilic/hydrophobic balance structure of the drug receptor site access mayoccur through distinct pathways as indicated. See SLIDE 38 for a simple equation indicating this.|~|/files/powerpoints_images/node28648/Slide40.JPG|~|563|~|422|~|0
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A highly schematized representation of an ion channel cycling through...
Ligand-gated ion also go through similar conformational change. In this illustration two of the subunits of the pentameric nicotininic acetylcholine receptor are shown interacting with Ach molecules in the resting, open and inactivated states.For a recent review of nicotinic ligands see: Faghih, Gopalakrishnan and Briggs, Allosteric modulators of the a7 nicotinic acetylcholine receptor, J. Med. Chem. 51: 701, 2008.|~|/files/powerpoints_images/node28648/Slide41.JPG|~|563|~|422|~|0
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Ligand-gated ion also go through similar conformational change. In t...
There are important implications from this concept of state-dependent interactions for therapeutic applications, for drug design AND for the interpretation of experimental data from ion channel assays.|~|/files/powerpoints_images/node28648/Slide42.JPG|~|563|~|422|~|0
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There are important implications from this concept of state-dependent...
A simple equation for a 2-state model of an ion channel indicating how the APPARENT affinity of a drug is determined by the different microscopic affinities of the drug for the two states of the channel AND by the equilibrium concentrations of these two states.|~|/files/powerpoints_images/node28648/Slide43.JPG|~|563|~|422|~|0
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A simple equation for a 2-state model of an ion channel indicating ho...
An illustration of state-dependent interactions for the local anesthetic lidocaine interacting at the sodium channel. The drug has a significantly higher affinity for the state of the channel favored by depolarization than for the resting state of the channel. |~|/files/powerpoints_images/node28648/Slide44.JPG|~|563|~|422|~|0
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An illustration of state-dependent interactions for the local anesthe...
The chiral local anesthetic exhibits different stereoselectivity according to the state of the channel. Thus, there is no stereoselectivity when the drug interacts with the resting state of the channel, but increasing stereoselectivity with the open and inactivated states. This slide and the previous slide illustrate the difficulty of establishing SAR with drugs that interact with a \'moving\' target.|~|/files/powerpoints_images/node28648/Slide45.JPG|~|563|~|422|~|0
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The chiral local anesthetic exhibits different stereoselectivity acco...
An increasing number of such \'channelopathies\' are known.|~|/files/powerpoints_images/node28648/Slide46.JPG|~|563|~|422|~|0
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An increasing number of such "channelopathies" are known.
The most common ion channelopathy in the Caucasian population is cystic fibrosis - a defect in the cystic fibrosis transmembrane regulator CFTR - a member of a group of transporter proteins and which is respeonsible for chloride transport in epithelial tissues. |~|/files/powerpoints_images/node28648/Slide48.JPG|~|563|~|422|~|0
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The most common ion channelopathy in the Caucasian population is cyst...
Drugs that affect ion channels represent one of the most important class of therapeutic agents and also toxic agents secreted by venomous animals. This presentation will attempt to define what ion channels are, how they work and how drugs interact with these channels. Together with the literature recommendations provided you should have a good introduction to this important class of biological effectors.
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