.
`
`Basig 8: Clinicai
`
`Pharmaeology
`
`Bertram G. Katzung
`
`I .
`I
`
`i.
`
`INTELGENX 1034
`
`

`
`Schedule of Controlled Drugs
`
`SCHEDULE I (All nonresearch use illegal.)
`Narcotics: Heroin a~1d many non marketed synthetic
`narcotics
`Hallucinogens:
`LSD
`MDA, STP, DMT, DET, mescaline, peyote, bufotenine,
`ibogaine, psilocybin, phencyclidine (PCP; veterinary
`drug only)
`Marijuana
`Methaqualone
`
`SCHEDULE II (No telephone prescriptions, no refills.)
`Opioids:
`Opium
`Opium alkaloids and derived phenanthrene alkaloids:
`morphine, hydromorphone (Dilaudid) , oxymorphone
`(Numorphan), oxycodone (dihydroxycodeinone, a
`component of Percodan, Percocet, Roxicodone,
`Tylox)
`Designated synthetic drugs: levomethadyl (Orlaam),
`meperidine (Demerol), methadone, levorphanol
`(Levo-Dromoran}, fentanyl (Sublimaze, Duragesic),
`alphaprodine, alfentanil (Aifenta), sufentanil (Sufenta)
`Stimulants:
`Coca leaves and cocaine
`Amphetamine
`Amphetamine complex (Biphetamine)
`Dextroamphetamine (Dexedrine)
`Methamphetamine (Desoxyn)
`Phenmetrazine (Preludin)
`Methylphenidate (Ritalin)
`Above in mixtures with other controlled or uncontrolled
`drugs
`Depressants:
`Amobarbital (Amytal)
`Pentobarbital (Nembutal)
`Secobarbital (Seconal)
`Mixtures of above (eg, Tuinal)
`Cannabinoids:
`Dronabinol (Marino!)
`
`SCHEDULE IV (Prescnption must be rewritten after 6
`months or 5 refills; differs from Schedule Ill in penalties
`for illegal possession.)
`Opioids:
`Difenoxin (Motofen)
`Pentazocine (Talwin)
`Propoxyphene (Darvon)
`Stimulants:
`Diethylpropion (Tenuate)
`Mazindol (Sanorex)
`Phentermine (lonamin)
`Fenfluramine (Pondimin)
`Pemoline (Cylert)
`Depressants:
`Benzodiazepines
`Alprazolam (Xanax)
`Chlordiazepoxide (Librium)
`Clonazepam (Kionopin)
`Clorazepate (Tranxene)
`Diazepam (Valium)
`Estazolam (ProSom)
`Flurazepam (Dalmane)
`Halazepam (Paxipam)
`Lorazepam (Ativan)
`Midazolam (Versed)
`Oxazepam (Serax)
`Prazepam (Centrax)
`Quazepam (Dora!)
`Temazepam (Restoril)
`Triazolam (Halcion)
`Chloral hydrate
`Ethchlorvynol (Piacidyl)
`Ethinamate (Valmid)
`Meprobamate (Equanil, Miltown, etc)
`Mephobarbital (Mebaral)
`Methohexital (Brevital)
`Methyprylon (Noludar)
`Paraldehyde
`Phenobarbital
`Zolpidem (Ambien)
`
`SCHEDULE V (As any other nonopioid prescription drug;
`may also be dispensed without prescription unless
`additional state regulations apply.)
`Opioids:
`Diphenoxylate (not more than 2.5 mg and not less than
`0 .025 mg of atropine per dosage unit, as in Lomotil)
`The following drugs in combination with other active
`nonopioid ingredients and provided the amount per
`100 ml or 100 g does not exceed that shown:
`Codeine: 200 mg
`
`l:: l ,yre:cnodeine: 100 rng
`
`OE.C 0 ~ 1994
`
`SCHEDULE Ill (Prescription must be rewritten after 6
`months or 5 refills. 1)
`Opioids: The following opioids in combination with one or
`more active nonopioid ingredients , provided the
`amount does not exceed that shown:
`Codeine and dihydrocodeine: not to exceed 1800 mg/dl
`or 90 mg/tablet or other dosage unit
`Dihydrocodeinone (hydrocodone in Hycodan, Vicodin,
`and Lortab): not to exceed 300 mg/dl or 15 mg/tablet
`Opium: 500 mg/dl m 25 mg/5 ml or other dosage unit
`(pai·egoric)
`Stimulants:
`Benzphetamine (Didrex)
`Phendimetrazine (Piegine)
`Depressants:
`Schedule II barbiturates in mixtures with noncontrollea
`drugs or in suppository dosage fmm
`Aprobarbital (Aiurate}
`Butabarbital (Butisol)
`Glutethimide (Doriden)
`Metharbital (Gemonil)
`Talbuta.l (Lotusate)
`Thiamylal (Surital)
`Thiopental (Pentothal)
`11n some states (eg; California), anabolic steroids have been classified as Schedule Ill.
`
`lL
`
`l .
`
`. "·s"\ 1
`~. ~ ~
`
`INTELGENX 1034
`
`

`
`Basic & Clinical
`Pharmacology
`
`INTELGENX 1034
`
`

`
`-
`
`a lANGE medical book
`Basic & Clinical
`Pharmacology
`
`sixth edition
`
`Edited by
`
`Bertram G. Katzung, MD, PhD
`Professor of Pharmacology
`Department of Pharmacology
`University of California, San Francisco
`
`APPLETON & LANGE
`Norwalk, Connecticut
`
`INTELGENX 1034
`
`

`
`Notice: The authors and the publisher of this volume have taken care to
`make certain that the doses of drugs and schedules of treatment are correct
`and compatible with the standards generally accepted at the time of
`publication. Nevertheless, as new information becomes available, changes in
`treatment and in the use of drugs become necessary. The reader is advised to
`carefully consult the instructions and information included in the
`package insert of each drug or therapeutic agent before administration.
`This advice is especially important when using new or infrequently used drugs.
`The author and publisher disclaim any liability, loss, injury, or damage incurred as
`a consequence, directly or indirectly, of the use and application of any of
`the contents of this volume.
`
`Copyright© 1995 by Appleton & Lange
`Paramount Publishing Business and Professional Group
`Previous editions© 1992, 1989, 1987 by Appleton & Lange
`
`All rights reserved. This book, or any parts thereof, may not be used or
`reproduced in any manner without written permission. For information,
`address Appleton & Lange, 25 VanZant Street, East Norwalk, Connecticut 06855.
`
`95 96 97 98 99
`
`I 10 9 8 7 6 5 4 3 2
`
`Prentice Hall International (UK) Limited, London
`Prentice Hall of Australia Pty. Limited, Sydney
`Prentice Hall Canada, Inc., Toronto
`Prentice Hall Hispanoamericana, S.A., Mexico
`Prentice Hall of India Private Limited, New Delhi
`Prentice Hall of Japan, Inc., Tokyo
`Simon & Schuster Asia Pte. Ltd., Singapore
`Editora Prentice Hall do Brasil Ltda., Rio de Janeiro
`Prentice Hall, Englewood Cliffs, New Jersey
`
`ISBN 0-8385-0619-4
`ISSN 0891 - 2033
`
`Acquisitions Editor: John Dolan
`Production Editor: Christine Langan
`Art Coordinator: Becky Hainz-Baxter
`
`PRINTED IN THE UNITED STATES OF AMERICA
`
`ISBN
`
`9
`
`0-8385-0619-4
`90000
`
`I IIIII
`196
`
`1111111111111111
`
`INTELGENX 1034
`
`

`
`Drug Receptors &
`Pharmacodynamics
`
`Henry R. Bourne, MD, & James M. Roberts, MD
`
`2
`
`The therapeutic and toxic effects of drugs result from
`their interactions with molecules in the patient. In
`most instances, drugs act by associating with specific
`macromolecules in ways that alter their biochemical or
`biophysical activity. This idea, now almost a century
`old, is embodied in the terms receptive substance and
`receptor: the component of a cell or organism that in(cid:173)
`teracts with a drug and initiates the chain of biochemi(cid:173)
`cal events leading to the drug's observed effects.
`Initially, the existence of receptors was inferred
`from observations of the chemical and physiologic
`specificity of drug effects. Thus, Ehrlich noted that
`certain synthetic organic agents had characteristic an(cid:173)
`tiparasitic effects while other agents did not, though
`their chemical structures differed only slightly.
`Langley noted that curare did not prevent electrical
`stimulation of muscle contraction but did block con(cid:173)
`traction triggered by nicotine. From these simple be(cid:173)
`ginnings, receptors have now become the central fo(cid:173)
`cus of investigation of drug effects and
`their
`mechanisms of action (pharmacodynamics). There(cid:173)
`ceptor concept, extended to endocrinology, immunol(cid:173)
`ogy, and molecular biology, has proved essential for
`explaining many aspects of biologic regulation. Drug
`receptors are now being isolated and characterized as
`macromolecules, thus opening the way to precise un(cid:173)
`derstanding of the molecular basis of drug action.
`In addition to its usefulness for explaining biology,
`the receptor concept has immensely important practi(cid:173)
`cal consequences for the development of drugs and
`for making therapeutic decisions in clinical practice.
`These consequences-explained more fully in later
`sections of this chapter-form the basis for under(cid:173)
`standing the actions and clinical uses of drugs de(cid:173)
`scribed in every chapter of this book. They may be
`briefly summarized as follows:
`{1} Receptors largely determine the quantita=
`Uve reijations between dose or comcentration of
`dmg and pharmacologic effects. The receptor's
`affinity for binding a drug determines the concentra(cid:173)
`tion of drug required to form a significant number of
`drug-receptor complexes, and the total number of re(cid:173)
`ceptors often limits the maximal effect a drug may
`produce.
`{2) Receptors are responsible for se~ectlvUy
`
`of drug action. The molecular size, shape, and elec(cid:173)
`trical charge of a drug determine whether~and with
`what avidity-it will bind to a particular receptor
`among the vast array of chemically different binding
`sites available in a cell, animal, or patient. Accord(cid:173)
`ingly, changes in the chemical structure of a drug can
`dramatically increase or decrease a new drug's affini(cid:173)
`ties for different classes of receptors, with resulting
`alterations in therapeutic and toxic effects.
`(3) Receptors mediate the actions of phar(cid:173)
`macologic antagonists. Many drugs and endo(cid:173)
`genous chemical signals, such as hormones, regulate
`the function of receptor macromolecules as agonists;
`ie, they change the function of a macromolecule as a
`more or less direct result of binding to it. Pure phar(cid:173)
`macologic antagonists, however, bind to receptors
`without directly altering the receptors' function.
`Thus, the effect of a pure antagonist on a cell or in a
`patient depends entirely upon its preventing the bind(cid:173)
`ing of agonist molecules and blocking their biologic
`actions. Some of the most useful drugs in clinical
`medicine are pharmacologic antagonists.
`
`MACROMOLECULAR NATURE
`OF DRUG RECEPTORS
`
`Until recently, the chemical structures and even the
`existence of receptors for most drugs could only be
`inferred from the chemical structures of the drugs
`themselves. Now, however, receptors for many drugs
`have been biochemically purified and characterized.
`The accompanying box describes some of the methods
`by which receptors are discovered and defined. Most
`receptors are proteins, presumably because the struc(cid:173)
`tures of polypeptides provide both the necessary di(cid:173)
`versity and the necessary specificity of shape and
`electrical charge.
`The best-characterized drug receptors are regula(cid:173)
`tory proteins, which mediate the actions of endo(cid:173)
`genous chemical signals such as neurotransmitters,
`autacoids, and hormones. This class of receptors me(cid:173)
`diates the effects of many of the most useful thera(cid:173)
`peutic agents. The molecular structures and bio(cid:173)
`chemical mechanisms of these regulatory receptors
`
`9
`
`INTELGENX 1034
`
`

`
`10 I CHAPTER 2
`
`HOW ARE RECEPTORS DISCOVERED?
`
`Because today's new receptor sets the stage for
`tomorrow' s new drug, it is important to know how
`new receptors are discovered. The discovery proc(cid:173)
`ess follows a few key steps, summarized in Figure
`2-1. As presented in greater detail elsewhere in
`this chapter, the process of defining a new receptor
`(stage 1 in Figure 2-1) begins by studying there(cid:173)
`lations between structures and activities of a group
`of drugs on some conveniently measured re(cid:173)
`sponse. Binding of radioactive ligands defines the
`molar abundance and binding affinities of the pu(cid:173)
`tative receptor and provides an assay to aid in its
`biochemical purification. Analysis of the pure re(cid:173)
`ceptor protein tells us the number of its subunits,
`its size, and (sometimes) provides a clue to how it
`works (eg, agonist-stimulated autophosphoryla(cid:173)
`tion on tyrosine residues, seen with receptors for
`insulin and many growth factors).
`These "classic" steps in receptor identification
`now serve as a warming-up exercise for a power(cid:173)
`ful new experimental strategy aimed at molecular
`cloning of the segment of DNA that encodes the
`receptor (stages 2-5 in Figure 2- 1). The core of
`this strategy is the ability to identify a putative re(cid:173)
`ceptor DNA sequence in a representative popula(cid:173)
`tion of cDNAs (DNA sequences complementary
`to the messenger RNAs expressed in an appropri(cid:173)
`ate cell or tissue are obtained by means of reverse
`transcriptase). To do so (stage 2), investigators use
`biochemical and functional features of the recep(cid:173)
`tor protein as handles for picking out the corre(cid:173)
`sponding DNA. Thus, an antibody raised against
`the pure receptor protein or nucleic acid sequences
`based on its amino acid sequence may distinguish
`a bacterial colony containing putative receptor
`eDNA
`from
`colonies
`containing
`irrelevant
`cDNAs, by binding to receptor antigen expressed
`in the bacterium (2a) or by hybridizing to receptor
`DNA (2b), respectively. Alternatively, the popula(cid:173)
`tion of cDNAs may be expressed as proteins in
`frog oocytes or vertebrate cells, and the putative
`receptor eDNA can then be detected by virtue of
`the protein's signaling function (2c) or its ability
`to bind a specific ligand (2d).
`Once the putative receptor eDNA has been
`
`identified, it is "validated" by carefully comparing
`the function and biochemical properties of the re(cid:173)
`combinant protein with those of the endogenous
`receptor that originally triggered the search (3a).
`The base sequence of the receptor DNA is also de(cid:173)
`termined (3b ), so that the amino acid sequence of
`the complete receptor protein can be deduced and
`compared with sequences of known receptors. Based
`on these criteria, it may then be possible to announce
`the identification of a new receptor (step 4).
`A much greater quantity and quality of informa(cid:173)
`tion flows from molecular cloning of the eDNA
`encoding a new receptor than from identifying a
`receptor in the "classic" way. The deduced amino
`acid sequence almost always resembles those of
`previously known receptors. Investigators can im(cid:173)
`mediately place the new receptor into a specific
`class of known receptors, and the structural class
`tells us how the receptor works-whether it is a
`receptor tyrosine kinase, a seven-transmembrane
`region receptor coupled to G proteins, etc. The
`DNA sequence provides a probe to identify cells
`and tissues that express messenger RNA encoding
`the new receptor. Expression of the eDNA in cul(cid:173)
`tured cells gives the pharmaceutical chemist an
`unlimited supply of recombinant receptor protein
`for precise biochemical analysis, tests of agonist
`and antagonist binding, and development of new
`drugs.
`Finally (step 5), the receptor DNA itself pro(cid:173)
`vides a tool for identifying yet more receptors. Re(cid:173)
`ceptors within a specific class or subclass contain
`highly conserved regions of similar or identical
`amino acid (and therefore DNA) sequence. The
`DNA sequences corresponding to these conserved
`regions can be used as probes to find sequences of
`related but potentially novel receptors, either by
`DNA-DNA hybridization (2b) or as primers in a
`polymerase chain reaction (PCR) designed to am(cid:173)
`plify receptor DNA sequences (2e). These probes
`may lead to cloning DNA encoding a receptor
`whose ligand is unknown (an "orphan" receptor);
`the appropriate ligand is then sought by testing for
`functional and binding interactions with the re(cid:173)
`combinant receptor.
`
`are described in a later section entitled Signaling
`Mechanisms and Drug Action.
`Other classes of proteins that have been clearly
`identified as drug receptors include enzymes, which
`may be inhibited (or, less commonly, activated) by
`binding a drug (eg, dihydrofolate reductase, the re(cid:173)
`ceptor for the antineoplastic drug methotrexate);
`transport proteins (eg, Na+JK+ ATPase, the mem-
`
`brane receptor for cardioactive digitalis glycosides);
`and structural proteins (eg, tubulin, the receptor for
`colchicine, an anti-inflammatory agent).
`This chapter deals with three aspects of drug recep(cid:173)
`tor function, presented in increasing order of com(cid:173)
`plexity: (1) The first aspect is their function as deter(cid:173)
`minants of the quantitative relation between the
`concentration of a drug and the pharmacologic re-
`
`INTELGENX 1034
`
`

`
`G?Response ___ • .,. Binding----1•.,. Purify protein)
`
`Putative receptor eDNA
`A. Express, test function
`B. Sequence
`
`Identify
`A. Receptor
`class
`B. Common
`DNA sequence
`
`\____j 4
`
`New receptor! __ }
`
`Figure 2-1. Methods used in the discovery and descrip(cid:173)
`tion of receptors. (See box: How Are Receptors Discov(cid:173)
`ered?)
`
`sponse. From this point of view, receptors are simple
`entities, principally characterized by their affinity for
`binding drug ligands and their abundance in target
`cells or tissues. (2) The second aspect is their function
`as regulatory proteins and components of chemical
`signaling mechanisms that provide targets for impor(cid:173)
`tant drugs. Here receptors are considered as complex
`molecules whose structures and biochemical func(cid:173)
`tions help to explain key features of concentration-ef(cid:173)
`fect relations, as well as pharmacologic selectivity.
`(3) The third aspect is their function as key elements
`of the therapeutic and toxic effects of drugs in pa(cid:173)
`tients. At this highest level of complexity, we discuss
`the crucial roles receptors play in determining selec(cid:173)
`tivity of drug action, the relation between the dose of
`
`DRUG RECEPTORS & PHARMACODYNAMICS I 11
`
`a drug and its effects, and the therapeutic usefulness
`of a drug (ie, therapeutic effectiveness versus tox(cid:173)
`icity).
`
`RELATION BETWEEN DRUG
`CONCENTRATION & RESPONSE
`
`The relation between dose of a drug and the clini(cid:173)
`cally observed response may be quite complex. In
`carefully controlled in vitro systems, however, the re(cid:173)
`lation between concentration of a drug and its effect
`is often simple and can be described with mathemati(cid:173)
`cal precision. We will analyze this idealized relation
`first because it underlies virtually all of the more
`complex relations between dose and effect that occur
`when drugs are given to patients.
`
`Concentration-Effect Curves
`& Receptor Binding of Agonists
`Even in intact animals or patients, responses to low
`doses of a drug usually increase in direct proportion
`to dose. As doses increase, however, the incremental
`response diminishes; finally, doses may be reached at
`which no further increase in response can be
`achieved. In idealized or in vitro systems, the relation
`between drug concentration and effect is described by
`a hyperbolic curve (Figure 2-2A) according to the
`following equation:
`
`Emax X C
`E = --=.:=::.. __
`C + ECSO
`
`where E is the effect observed at concentration C, Emax
`is the maximal response that can be produced by the
`drug, and ECSO is the concentration of drug that pro(cid:173)
`duces 50% of maximal effect.
`This hyperbolic relation resembles the mass action
`law, which predicts association between two mole(cid:173)
`cules of a given affinity. This resemblance suggests
`
`1.0
`
`A
`
`1.0
`
`B
`
`---~
`"" 2
`
`~
`~
`t::
`~
`0
`-<::> .:.
`2
`at-
`" ~
`
`0.5
`
`I
`I
`I
`I
`:~Ko
`---L-----
`
`Drug concentratio •~ (C )
`
`--1'/-
`
`Drug concentration (C)
`
`Figure 2-2. Relation s between drug concentration and drug effect (A) or receptor-bound drug (B). The drug concentra(cid:173)
`tions at which effect or receptor occupancy is half-maximal are denoted EC50 and K0 , respectively.
`
`I
`I
`I
`-L
`
`INTELGENX 1034
`
`

`
`12 I CHAPTER 2
`
`that drug agonists act by binding to ("occupying") a
`distinct class of biologic molecules with a charac(cid:173)
`teristic affinity for the drug receptor. With the advent
`of radioactive receptor
`ligands,
`including both
`agonists and antagonists, this occupancy assumption
`has been amply confirmed for a number of drug-re(cid:173)
`ceptor systems. In these systems, the relation between
`drug bound to receptors (B) and the concentration of
`free (unbound) drug (C) depicted in Figure 2-2B is
`described by an analogous equation:
`
`in which Bmax indicates the total concentration of re(cid:173)
`ceptor sites (ie, sites bound to the drug at infinitely
`high concentrations of free drug). K0 (the equilibrium
`dissociation constant) indicates the concentration of
`free drug at which half-maximal binding is observed.
`This constant characterizes the receptor's affinity for
`binding the drug in a reciprocal fashion: If the K0 is
`low, binding affinity is high, and vice versa.
`
`Note also that the ECSO and K0 may be identical
`but need not be, as discussed below.
`Graphic representation of dose-response data is
`frequently improved by plotting the drug effect (ordi(cid:173)
`nate) against the logarithm of the dose or concentra(cid:173)
`tion (abscissa). The effect of this purely mathematical
`maneuver is to transform the hyperbolic curve of Fig(cid:173)
`ure 2-2 into a sigmoid curve with a linear midportion
`(eg, Figure 2-3). This transformation makes it easier
`to compare different dose-response curves graphi(cid:173)
`cally because it expands the scale of the concentration
`axis at low concentrations (where the effect is chang(cid:173)
`ing rapidly) and compresses it at high concentrations
`(where the effect is changing slowly). This transfor(cid:173)
`mation has no special biologic or pharmacologic sig(cid:173)
`nificance.
`
`Receptor-Effector Coupling
`& Spare Receptors
`When a receptor is occupied by an agonist, the re(cid:173)
`sulting conformational change is only the first of
`many steps usually required to produce a pharma-
`
`]
`""' "' -~ 0 .5
`&
`
`<'{
`
`EC50(Al
`
`EC50 (8)
`
`EC50(C) EC50 (0 , E) :::: K 0
`Agonist concentration (C) (log scale)
`
`Figure 2-3. Experimental demonstration of spare receptors, using different concentrations of an irreversible antagonist.
`Curve A shows agonist response in the absence of antagonist. After treatment with a low concentration of antagonist
`(c urve B), the curve is sl'"iifted to the right; maximal responsiveness is preserved, however, because the remaining avail(cid:173)
`able receptors are still in excess of the number required. In curve C, produced after treatment with a larger concentration
`of antagonist, the available receptors are no longer "spare"; instead, they are just sufficient to mediate an undiminished
`maximal response. Still higher concentrations of antagonist (curves 0 and E) reduce the number of available receptors to
`the point that maximal response is diminished . The apparent EC50 of the agonist in curves 0 and E may approximate the
`K0 that characterizes the binding affinity of the agonist for the receptor.
`
`INTELGENX 1034
`
`

`
`cologie response. The transduction process between
`occupancy of receptors and drug response is often
`termed coupling. The relative efficiency of occu(cid:173)
`pancy-response coupling is partially determined by
`the initial conformational change in the receptor(cid:173)
`thus, the effects of full agonists can be considered
`more efficiently coupled to receptor occupancy than
`can the effects of partial agonists, as described below.
`Coupling efficiency is also determined by the bio(cid:173)
`chemical events that transduce receptor occupancy
`into cellular response.
`High efficiency of receptor-effector interaction
`may also be envisioned as the result of spare recep(cid:173)
`tors. Receptors are said to be "spare" for a given phar(cid:173)
`macologic response when the maximal response can
`be elicited by an agonist at a concentration that does
`not result in occupancy of the full complement of
`available receptors. Spare receptors are not qualita(cid:173)
`tively different from nonspare receptors. They are not
`hidden or unavailable, and when they are occupied,
`they can be coupled to response. Experimentally,
`spare receptors may be demonstrated by using irre(cid:173)
`versible antagonists to prevent binding of agonist to a
`proportion of available receptors and showing that
`high concentrations of agonist can still produce an
`undiminished maximal response (Figure 2--3). Thus,
`a maximal inotropic response of heart muscle to
`catecholamines can be elicited even under conditions
`where 90% of the beta receptors are occupied by a
`quasi-irreversible antagonist. Accordingly, myocar-
`
`DRUG RECEPTORS & PHARMACODYNAMICS I 13
`
`dium is said to contain a large proportion of spare re(cid:173)
`ceptors.
`How can we account for the phenomenon of spare
`receptors? In a few cases, the biochemical mecha(cid:173)
`nism is understood, such as for drugs that act on some
`regulatory receptors. In this situation, the effect of re(cid:173)
`ceptor activation-eg, binding of GTP by an interme(cid:173)
`diate-may greatly outlast the agonist-receptor inter(cid:173)
`action (see the following section called G Proteins
`and Second Messengers). In such a case,
`the
`"spareness" of receptors is temporal in that the re(cid:173)
`sponse initiated by an individual ligand-receptor
`binding event persists longer in time than the binding
`event itself.
`In other cases, where the biochemical mechanism
`is not understood, we imagine that the receptors are
`spare in number. If the concentration or amount of a
`cellular component other than the receptor limits the
`coupling of receptor occupancy to response, then a
`maximal response can occur without occupancy of all
`receptors. Figure 2-4 illustrates the notion of recep(cid:173)
`tors that are spare in this sense and helps to explain
`how the sensitivity of a cell or tissue to a particular
`concentration of agonist may depend not only on the
`affinity of the receptor for binding an agonist ( charac(cid:173)
`terized by the K0 ) but also on the total concentration
`of receptors. Sensitivity may be expressed in terms of
`EC50, the concentration of agonist that results in
`half-maximal response. The K0 of the agonist-recep(cid:173)
`tor interaction determines what fraction (BIBmaJ of
`
`A
`
`• :. •••
`• •• •
`• • •••
`••• • ••••
`•• • •
`••••
`••••• •••
`••• • • • ••
`
`B
`
`• •
`• •
`• •
`•
`
`Figure 2-4. Spare receptors
`increase
`sensitivity to drug. In panel A (left), the free
`concentration of agonist is equal to the K0
`concentration; this is sufficient to bind 50%
`of the four receptors present, resulting in
`the formation of two agonisHeceptor com(cid:173)
`plexes. (Note: When the agonist concen-
`tration is equal to the K0 , half the receptors
`will be occupied. Remember that B/Bmax =
`C/(C + K0 ).) Agonist occupancy of these
`two receptors changes their conformation
`so that they bind to and activate two effec(cid:173)
`tor molecules, resulting in a response. Be(cid:173)
`cause two of four effectors are stimulated
`by agonist-receptor complexes, the re ·
`sponse is 50% of maximum. In membrane
`B (right), the receptor concentration has
`been increased tenfold (not all receptors
`for binding of
`are shown), and the K0
`agonist to receptors remains unchanged .
`Now a very much smaller concentration of
`free agonist(= 0.05 times the K0 ) suffices
`to occupy
`two
`receptors and conse(cid:173)
`quently to activate two effector molecules .
`Thus, the response is 50% of maximum
`(just as in A), even though the agonist con(cid:173)
`centration is very much lower than the K0 .
`
`INTELGENX 1034
`
`

`
`14 I CHAPTER 2
`
`total receptors will be occupied at a given free con(cid:173)
`centration (C) of agonist, regardless of the receptor
`concentration:
`
`C
`B
`Bmax C + Ko
`
`Imagine a responding cell with four receptors and
`four effectors (as in Figure 2-4). Here the number of
`effectors does not limit the maximal response, and the
`receptors are not spare in number. Consequently, an
`agonist present at a concentration equal to the K0 will
`occupy 50% of the receptors, and half of the effectors
`will be activated, producing a half-maximal response
`(ie, two receptors stimulate two effectors). Now
`imagine that the number of receptors increases ten(cid:173)
`fold to 40 receptors but that the total number of effec(cid:173)
`tors remains constant. Now most of the receptors are
`spare in number. As a result, a very much lower con(cid:173)
`centration of agonist suffices to occupy two of the 40
`receptors (5% of the receptors), and this same low
`concentration of agonist is able to elicit a half-maxi(cid:173)
`mal response (two of four effectors activated). Thus,
`it is possible to change the sensitivity of tissues with
`spare receptors by changing the receptor concentra(cid:173)
`tion. (Note:. Changing the number of receptors does
`not usually change the free concentration of drug
`achieved by administering a given dose. This is be(cid:173)
`cause the concentration of receptors in a tissue is usu- .
`ally very small relative to effective concentrations of
`drugs.)
`An important biologic consequence of spare recep(cid:173)
`tors is that they allow agonists with low affinity for
`receptors to produce full responses at low concentra(cid:173)
`tions, to the extent that EC50 is lower than K0 . This is
`important because ligands with low affinity (high K0 )
`dissociate rapidly from receptors, allowing rapid ter(cid:173)
`mination of biologic responses. High binding affinity
`(low K0 ), on the other hand, would result in slow dis(cid:173)
`sociation of agonist from receptor and correspond(cid:173)
`ingly slower reversal of a biologic response.
`
`Competitive & Irreversible
`Antagonists
`Receptor antagonists bind to the receptor but do
`not activate it. The effects of these antagonists result
`from preventing agonists (other drugs or endogenous
`regulatory molecules) from binding to and activating
`receptors. Such antagonists are divided into two
`classes depending on whether or not they reversibly
`compete with agonists for binding to receptors. The
`two classes of receptor antagonism produce quite dif(cid:173)
`ferent concentration-effect and concentration-binding
`curves in vitro and exhibit important practical differ(cid:173)
`ences in therapy of disease.
`In the presence of a fixed concentration of agonist,
`increasing concentrations of a competitive antago(cid:173)
`nist progressively inhibit the agonist response; high
`antagonist concentrations prevent response com-
`
`pletely. Conversely, sufficiently high concentrations
`of agonist can completely surmount the effect of a
`given concentration of the antagonist, ie, the Emax for
`the agonist remains the same for any fixed concentra(cid:173)
`tion of antagonist (Figure 2- SA). Because the antago(cid:173)
`nism is competitive, the presence of antagonist in(cid:173)
`creases the agonist concentration required for a given
`degree of response, and so the agonist concentration(cid:173)
`effect curve shifts to the right.
`The concentration (C') of an agonist required to
`produce a given effect in the presence of a fixed con(cid:173)
`centration ([I]) of competitive antagonist is greater
`than the agonist concentration (C) required to pro(cid:173)
`duce the same effect in the absence of the antagonist.
`The ratio of these two agonist concentrations (the
`"dose ratio") is related to the dissociation constant
`(K1) of the antagonist by the Schild equation:
`
`Pharmacologists often use this relation to deter(cid:173)
`mine the K1of a competitive antagonist. Even without
`knowledge of the relationship between agonist occu(cid:173)
`pancy of the receptor and response, the K1 can be de(cid:173)
`termined simply and accurately. As shown in Figure
`2- 5, concentration response curves are obtained in
`the presence and in the absence of a fixed concentra(cid:173)
`tion of competitive antagonist; comparison of the
`agonist concentrations required to produce identical
`degrees of pharmacologic effect in the two situations
`reveals the antagonist's K1. If C' is twice C, for exam(cid:173)
`ple, then [I]= K1. K1values derived from such experi(cid:173)
`ments agree with those determined by direct measure(cid:173)
`ments of binding of radiolabeled competitive
`antagonists to receptors.
`For the clinician, this mathematical relation has
`two important therapeutic implications:
`(1) The degree of inhibition produced by a com(cid:173)
`petitive antagonist depends upon the concentration of
`antagonist. Thus, the extent and duration of action of
`such a drug will depend upon its concentration in
`plasma and will be critically influenced by the rate of
`its metabolic clearance or excretion. Different pa(cid:173)
`tients receiving a fixed dose of propranolol, for exam(cid:173)
`ple, exhibit a wide range of plasma concentrations,
`owing to differences in clearance of the drug. As a
`result, the effects of a fixed dose of this competitive
`antagonist of norepinephrine may vary widely in pa(cid:173)
`tients, and the dose must be adjusted accordingly.
`{2) The equation defines another important source
`of variability in clinical response to a competitive an(cid:173)
`tagonist, ie, the concentration of agonist that is com(cid:173)
`peting for binding to receptors. Here also propranolol
`provides a useful example: When this