Pharmacology I - Drug Receptors & Pharmacodynamics PDF

Pharmacology I Drug Receptors & Pharmacodynamics Dr. Haneen Basheer 1st semester 2024/2023 RECEPTORS 1. Receptors are the specific molecules in a biologic system with which drugs interact to produce changes in the function of the system. 2. Receptors must be selective in their ligand-binding cha...

Pharmacology I Drug Receptors & Pharmacodynamics Dr. Haneen Basheer 1st semester 2024/2023 RECEPTORS 1. Receptors are the specific molecules in a biologic system with which drugs interact to produce changes in the function of the system. 2. Receptors must be selective in their ligand-binding characteristics (so as to respond to the proper chemical signal and not to meaningless ones). 3. Receptors must be modifiable when they bind a drug molecule (so as to bring about the functional change). Most of the receptors are proteins; a few are other macromolecules such as DNA. The receptor site (also known as the recognition site) for a drug is the specific binding region of the receptor macromolecule and has a relatively high and selective affinity for the drug molecule.  The interaction of a drug with its receptor is the fundamental event that initiates the action of the drug, and many drugs are classified on the basis of their primary receptor affinity. 2 © The McGraw-Hill Companies, Inc., 2010 Figure 2.1 (still) Chapter 2 MENU > 3 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins EFFECTORS  Effectors are molecules that translate the drug- receptor interaction into a change in cellular activity.  Some effectors are enzymes (eg. adenylyl cyclase) and others are a part of the receptor molecule (eg. sodium-potassium channel is the effector part of the nicotinic acetylcholine receptor) 4 © The McGraw-Hill Companies, Inc., 2010 GRADED DOSE-RESPONSE RELATIONSHIPS  Graded dose-response curve A graph of increasing response to increasing drug concentration or dose  The response is a graded effect, meaning that the response is continuous and gradual.  Plotting the same data on a semilogarithmic concentration axis usually results in a sigmoid curve, which simplifies the mathematical manipulation of the dose- response data. 5 © The McGraw-Hill Companies, Inc., 2010 Figure 2.7 (still) Chapter 2 MENU > 6 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins POTENCY  Potency denotes the amount of drug needed to produce a given effect.  In graded dose-response measurements, the effect usually chosen is 50% of the maximal effect and the dose causing this effect is called the EC50 (on the X-axis on dose-response curve)  Potency is determined mainly by: 1) the affinity of the receptor for the drug. 2) the number of receptors available.  Affinity: measure of the ability of the drug to bind to its molecular target. 7 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins Figure 2–1. The smaller the EC50 (or ED50), the greater the potency of the drug. 8 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins EFFICACY  Efficacy-often called maximal efficacy: is the greatest effect (Emax) an agonist can produce if the dose is taken to very high levels (on the Yaxis on dose-response curve)  Efficacy is determined mainly by: 1) the nature of the drug.( intrinsic activity) 2) the receptor and its associated effector system. It can be measured with a graded dose-response curve but not with a quantal dose-response curve. By definition, partial agonists have lower maximal efficacy than full agonists. The more potent drug does not mean it should have a higher efficacy 9 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins Figure 2–1. 10 © The McGraw-Hill Companies, Inc., 2010 11 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins GRADED DOSE-BINDING RELATIONSHIP & BINDING AFFINITY  It is possible to measure the percentage of receptors bound by a drug, and, by plotting this percentage against the log of the concentration of the drug, a graph similar to the dose-response curve is obtained.  The concentration of drug required to bind 50% of the receptor sites is denoted as Kd and is a useful measure of the affinity of a drug molecule for its binding site on the receptor molecule. The smaller the Kd, the greater the affinity of the drug for its receptor (Affinity is described as 1/KD. i.e. the inverse of equilibrium dissociation constant).  If the number of binding sites on each receptor molecule is known, it is possible to determine the total number of receptors in the system from the Bmax (Bmax: maximum number of receptor binding sites in the tissue preparation) 12 © The McGraw-Hill Companies, Inc., 2010 Figure 2–1. 13 © The McGraw-Hill Companies, Inc., 2010 SPARE RECEPTORS  Spare receptors are said to exist if the maximal drug response (Emax) is obtained at less than maximal occupation of the receptors (Bmax ).  In practice, the determination is usually made by comparing the concentration for 50% of maximal effect (EC50) with the concentration for 50% of maximal binding (Kd). If the EC50 is less than the Kd, spare receptors are said to exist.  This might result from 1 of 2 mechanisms: 1) the duration of the activation of the effector may be much greater than the duration of the drug-receptor interaction. Thus the receptor could remain activated after the agonist departs, allowing one agonist molecule to activate several receptors. 2) the actual number of receptors may exceed the number of effector molecules available. the cell signaling pathways could allow for significant amplification of a relatively small signal, and activation of only a few receptors could be sufficient to produce a maximal response. 14 © The McGraw-Hill Companies, Inc., 2010 15 © The McGraw-Hill Companies, Inc., 2010 SPARE RECEPTORS..Cont. “The term spare receptors is widely misunderstood with some readers thinking that the “spare” receptors are nonfunctional. “Although all receptors may not be needed for a maximal response, all receptors contribute to the measured responses, thus the potency of full agonists (and often the physiological agonists) is enhanced by the presence of the spare receptors” The presence of spare receptors increases sensitivity to the agonist because the likelihood of a drug-receptor interaction increases in proportion to the number of receptors available. Note: In the literature, spare receptors may be found to be named “receptors reserve” which is essentially the same. 16 From: International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification. XXXVIII. Update on Terms and Symbols in Quantitative Pharmacology © The McGraw-Hill Companies, Inc., 2010 SPARE RECEPTORS  Experimentally, spare receptors may be demonstrated by using irreversible 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. 17 © The McGraw-Hill Companies, Inc., 2010 Figure 2–2. 18 © The McGraw-Hill Companies, Inc., 2010 Quantal dose-response curve • Quantal dose-response curve A graph of the fraction of a population that shows a specified response at progressively increasing doses 19 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins QUANTAL DOSE-RESPONSE RELATIONSHIPS  When the minimum dose required to produce a specified response is determined in each member of a population, the quantal dose response relationship is defined.  For example, a blood pressure-lowering drug might be studied by measuring the dose required to lower the mean arterial pressure by 20 mm Hg in 100 hypertensive patients. When plotted as the percentage of the population that shows this response at each dose versus the log of the dose administered, a cumulative quantal dose-response curve, usually sigmoid in shape, is obtained. 20 © The McGraw-Hill Companies, Inc., 2010 QUANTAL DOSE-RESPONSE RELATIONSHIPS  Median effective dose (ED50): the dose at which 50% of     individuals exhibit the specified quantal effect. Median toxic dose (TD50): the dose required to produce a particular toxic effect in 50% of animals. Median lethal dose (LD50): similar to TD50 if the toxic effect is death of the animal. How can we use quantal dose response curve in determining potency? Example: if the ED50 s of two drugs for producing a specified quantal effect are 5 and 500 mg, respectively, then the first drug can be said to be 100 times more potent than the second for that particular effect 21 © The McGraw-Hill Companies, Inc., 2010 QUANTAL DOSE-RESPONSE RELATIONSHIPS Important notes:  Because the magnitude of the specified effect is arbitrarily determined, the ED50 determined by quantal dose-response measurements has no direct relation to the ED50 determined from graded dose-response curves. Potency can be determined by both graded and quantal dose- response curves, while efficacy can only be determined by graded dose-response curve  Quantal dose-response data provide information about the variation in sensitivity to the drug in a given population, and if the variation is small, the curve is steep. 22 © The McGraw-Hill Companies, Inc., 2010 THERAPEUTIC INDEX & THERAPEUTIC WINDOW  The therapeutic index is the ratio of the TD50 (or LD50) to the ED50, determined from quantal dose-response curves.  The therapeutic index represents an estimate of the safety of a drug, because a very safe drug might be expected to have a very large toxic dose and a much smaller effective dose. For exampl, the ED50 is approximately 3 mg, and the LD50 is approximately 150 mg. The therapeutic index is therefore approximately 150/3, or 50 in mice.  Obviously, a full range of toxic doses cannot be ethically studied in humans. Furthermore, factors such as the varying slopes of dose-response curves make this estimate a poor safety index even in animals.  The therapeutic window, a more clinically useful index of safety, describes the dosage range between the minimum effective therapeutic concentration or dose, and the minimum toxic concentration or dose. For example, if the average minimum therapeutic plasma concentration of theophylline is 8 mg/L and toxic effects are observed at 18 mg/L, the therapeutic window is 8-18 mg/L.  Both the therapeutic index and the therapeutic window depend on the specific toxic effect used in the determination. 23 © The McGraw-Hill Companies, Inc., 2010 Figure 2.13 (still) Chapter 2 MENU > 24 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins AGONISTS, PARTIAL AGONISTS, INVERSE AGONISTS, and neutral antagonist  A full agonist is a drug capable of fully activating the effector system when it binds to the receptor. A full agonist has high affinity for the activated receptor conformation, and sufficiently high concentrations result in all the receptors achieving the activated state.  A partial agonist produces less than the full effect, even when it has saturated the receptors, presumably by combining with both receptor conformations, but favoring the active state. (In the presence of a full agonist, a partial agonist acts as an inhibitor. )  Inverse agonist is an agent that binds to the same receptor as an agonist but induces a pharmacological response opposite to that agonist.  Neutral antagonist an agent that has no activity in the absence of an agonist or inverse agonist but can block the activity of either 25 © The McGraw-Hill Companies, Inc., 2010 ANTAGONISTS A. Pharmacolgical antagoinsts Competitive antagonists: are drugs that bind to, or very close to, the agonist receptor site in a reversible way without activating the effector system for that receptor.  In the presence of a competitive antagonist, the log dose- response curve for an agonist is shifted to higher doses (ie, horizontally to the right on the dose axis), but the same maximal effect is reached. The agonist, if given in a high enough concentration, can displace the antagonist and fully activate the receptors. 26 © The McGraw-Hill Companies, Inc., 2010 Figure 2–3. 27 © The McGraw-Hill Companies, Inc., 2010 ANTAGONISTS  Irreversible antagonist: causes a downward shift of the maximum, with no shift of the curve on the dose axis unless spare receptors are present. 28 © The McGraw-Hill Companies, Inc., 2010 Figure 2–3. 29 © The McGraw-Hill Companies, Inc., 2010 Comparison Competitive antagonist Noncompetitive antagonist Binding site Binds to, or very close to, the agonist receptor site in a reversible way Binds to at an allosteric site of the receptor reversibly or irreversibly. If it acts at the receptor site, it binds irreversibly. Effect on EC50 on log doseresponse curve for an agonist Increases it (Shifts log No effect (unless spare dose-response curve to the receptors are present) right, i.e higher doses of the agonist is needed) Effect on maximal effect on No effect log dose-response curve for an agonist How to overcome their effect. Reduces it. Can be overcome by adding Cannot be overcome by more agonist adding more agonist 30 © The McGraw-Hill Companies, Inc., 2010 31 © The McGraw-Hill Companies, Inc., 2010 ANTAGONISTS B. Physiologic Antagonists:  A physiologic antagonist binds to a different receptor molecule, producing an effect opposite to that produced by the drug it antagonizes.  Thus, it differs from a pharmacologic antagonist, which interacts with the same receptor as the drug it inhibits. A familiar example of a physiologic antagonist is the antagonism of the bronchoconstrictor action of histamine (mediated at histamine receptors) by epinephrine's bronchodilator action (mediated at β adrenoceptors). Similarly, glucagon (acting at glucagon receptors) can antagonize the cardiac effects of an overdose of propranolol (acting at β receptors). C. Chemical Antagonists:  A chemical antagonist interacts directly with the drug being antagonized to remove it or to prevent it from binding to its target.  A chemical antagonist does not depend on interaction with the agonist's receptor (although such interaction may occur). Pralidoxime, which combines avidly with the phosphorus in organophosphate cholinesterase inhibitors, is another type of chemical antagonist. 32 © The McGraw-Hill Companies, Inc., 2010 Figure 2–4. 33 © The McGraw-Hill Companies, Inc., 2010 Figure 2–15. 34 © The McGraw-Hill Companies, Inc., 2010 SIGNALING MECHANISMS • Once an agonist drug has bound to its receptor, some effector mechanism is activated. • The receptor, its cellular target, and any intermediary molecules are referred to as a receptor–effector system or signal-transduction pathway. • The receptor-effector system may be an enzyme in the intracellular space (eg, cyclooxygenase, a target of nonsteroidal anti-inflammatory drugs) or in the membrane or extracellular space (eg, acetylcholinesterase). The receptors for many drugs are neurotransmitter reuptake transporters (eg, the norepinephrine transporter, NET, and the dopamine transporter, DAT, targets for cocaine). 35 © The McGraw-Hill Companies, Inc., 2010 SIGNALING MECHANISMS • For the largest group of drug-receptor interactions, the drug is present in the extracellular space while the effector mechanism resides inside the cell and modifies some intracellular process. • These represent the classic drug-receptor interactions and involve signaling across the membrane. • Five major types of transmembrane signaling mechanisms for receptor-effector systems have been defined. 36 © The McGraw-Hill Companies, Inc., 2010 Figure 2–5. 37 © The McGraw-Hill Companies, Inc., 2010 SIGNALING MECHANISMS A. Intracellular Receptor: • Some drugs, especially more lipidsoluble or diffusible agents (eg, steroid hormones, nitric oxide), may cross the membrane and combine with an intracellular receptor that affects an intracellular effector molecule. The receptor and effector may or may not be the same molecule, but no specialized transmembrane signaling device is required. 38 © The McGraw-Hill Companies, Inc., 2010 Figure 2.2 (part 4) Chapter 2 MENU > 39 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins Figure 2–6. The mechanism used by hormones that act by regulating gene expression has two therapeutically important consequences: 1. All of these hormones produce their effects after a characteristic lag period of 30 minutes to several hours—the time required for the synthesis of new proteins. 2. The effects of these agents can persist for hours or days after the agonist concentration has been reduced to zero. The persistence of effect is primarily due to the relatively slow turnover of most enzymes and proteins, which can remain active in cells for hours or days after they have been synthesized. 40 © The McGraw-Hill Companies, Inc., 2010 SIGNALING MECHANISMS B. Receptors Located on Membrane-Spanning Enzymes • These receptors are polypeptides consisting of an extracellular hormone-binding domain and a cytoplasmic enzyme domain, which may be a protein tyrosine kinase, a serine kinase, or a guanylyl cyclase. • In all these receptors, the two domains are connected by a hydrophobic segment of the polypeptide that crosses the lipid bilayer of the plasma membrane. • For example, insulin acts on a tyrosine kinase that is located in the membrane. The insulin receptor site faces the extracellular environment, and the effector enzyme catalytic site is on the cytoplasmic side. When activated, such receptors dimerize and phosphorylate specific intracellular protein substrates. 41 © The McGraw-Hill Companies, Inc., 2010 Figure 2–7. 42 © The McGraw-Hill Companies, Inc., 2010 The receptor tyrosine kinase signaling pathway begins with binding of ligand to the receptor’s extracellular domain. results in changing the receptor conformation. causes two receptor molecules to bind to one another (dimerize) and thus brings together the tyrosine kinase domains, which become enzymatically active phosphorylate one another as well as additional downstream signaling proteins. Activated receptors catalyze phosphorylation of tyrosine residues on different target signaling proteins, thereby allowing a single type of activated receptor to modulate a number of biochemical processes. 43 © The McGraw-Hill Companies, Inc., 2010 Figure 2.2 (part 3) Chapter 2 MENU > 44 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins SIGNALING MECHANISMS C. Receptors Located on Membrane-Spanning Molecules That Bind Separate Intracellular Tyrosine Kinase Molecules • Like receptor tyrosine kinases, these receptors have extracellular and intracellular domains and form dimers. • However, after receptor activation by an appropriate drug (often a cytokine) at the extracellular receptor site, associated but separate tyrosine kinase molecules that are bound noncovalently to the receptor (Janus kinases; JAKs) are activated, resulting in phosphorylation of STAT molecules (signal transducers and activators of transcription) . • STAT dimers (the effectors) then travel to the nucleus, where they regulate transcription. 45 © The McGraw-Hill Companies, Inc., 2010 Figure 2–8. 46 © The McGraw-Hill Companies, Inc., 2010 SIGNALING MECHANISMS D. Receptors Located on Membrane Ion Channels • Some drugs work by mimicking or blocking the actions of endogenous ligands that regulate the flow of ions through plasma membrane channels. • Receptors that regulate membrane ion channels may directly cause the opening of the channel (eg, acetylcholine at the nicotinic receptor) or modify the ion channel's response to other agents (eg, benzodiazepines at the GABA-activated chloride channel). The channel molecule acts as both receptor and effector, and the result is a change in transmembrane electrical potential. 47 © The McGraw-Hill Companies, Inc., 2010 Figure 2-9. acetylcholine causes the opening of the ion channel in the nicotinic acetylcholine receptor (nAChR), which allows Na + to flow down its concentration gradient into cells, producing a localized excitatory postsynaptic potential—a depolarization. 48 © The McGraw-Hill Companies, Inc., 2010 SIGNALING MECHANISMS E. Receptors Linked to Effectors via G Proteins • A very large number of drugs bind to receptors that are linked by coupling proteins to intracellular or membrane-bound effectors. • The best-defined examples of this group are the sympathomimetic drugs, which activate or inhibit adenylyl cyclase (formerly called adenylate cyclase) by a multistep process: activation of the receptor (located in the membrane with the binding site facing the extracellular side) by the drug results in activation of separate G proteins (located in the intracellular face of the membrane), which either stimulate or inhibit the cyclase. – Thus, the receptor and effector are linked through the Gcoupling protein. 49 © The McGraw-Hill Companies, Inc., 2010 SIGNALING MECHANISMS E. Receptors Linked to Effectors via G Proteins…Cont. • When G-coupled receptors bind agonist, the G protein is activated. This process involves replacement of the GDP that is bound to the protein with GTP and subsequent dissociation of the trimeric G protein complex into a GTP-alpha moiety and a beta-gamma moiety. • The GTP-alpha portion is the primary player in most interactions with effector molecules, but in some, the beta-gamma moiety is the activator. 50 © The McGraw-Hill Companies, Inc., 2010 Figure 2.3 (part 1) Chapter 2 MENU > 51 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins Figure 2.3 (part 3) Chapter 2 MENU > 52 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins Table 2-1. 53 © The McGraw-Hill Companies, Inc., 2010 Well-Established Second Messengers A. Cyclic Adenosine Monophosphate (cAMP): cAMP mediates the mobilization of stored energy (the breakdown of carbohydrates in liver), conservation of water by the kidney (mediated by vasopressin), Ca +2 homeostasis (regulated by parathyroid hormone), increased rate and contractile force of heart muscle (β-adrenomimetic catecholamines), the production of adrenal and sex steroids etc. 54 © The McGraw-Hill Companies, Inc., 2010 Figure 2–13. 55 © The McGraw-Hill Companies, Inc., 2010 Well-Established Second Messengers B. Phosphoinositides and Calcium: Phospholipase C (PLC), membrane enzyme, which splits a minor phospholipid component of the plasma membrane, phosphatidylinositol-4,5-bisphosphate (PIP 2 ), into two second messengers, diacylglycerol (DAG) and inositol- 1,4,5-trisphosphate (IP 3 or InsP 3 ). DAG: is confined to the membrane, where it activates a phospholipid- and calcium sensitive protein kinase called protein kinase C. IP 3: is water-soluble and diffuses through the cytoplasm to trigger release of Ca+2 by binding to ligand-gated calcium channels in the limiting membranes of internal storage vesicles. 56 © The McGraw-Hill Companies, Inc., 2010 Phosphoinositides and Calcium Elevated cytoplasmic Ca+2 concentration resulting from IP 3 -promoted opening of these channels promotes the binding of Ca+2 to the calcium-binding protein calmodulin, which regulates activities of other enzymes, including calcium-dependent protein kinases. 57 © The McGraw-Hill Companies, Inc., 2010 Phosphoinositides and Calcium Signal termination: • IP 3 is inactivated by dephosphorylation; • DAG is either phosphorylated to yield phosphatidic acid, which is then converted back into phospholipids, or it is deacylated to yield arachidonic acid. • Ca+2 is actively removed from the cytoplasm by Ca+2 pumps. 58 © The McGraw-Hill Companies, Inc., 2010 Figure 2-14. 59 © The McGraw-Hill Companies, Inc., 2010 60 © The McGraw-Hill Companies, Inc., 2010 Well-Established Second Messengers C. Cyclic Guanosine Monophosphate (cGMP): Ligands detected by cell surface receptors stimulate membrane-bound guanylyl cyclase to produce cGMP, and cGMP acts by stimulating a cGMP-dependent protein kinase. The actions of cGMP in these cells are terminated by enzymatic degradation of the cyclic nucleotide and by dephosphorylation of kinase substrates. 61 © The McGraw-Hill Companies, Inc., 2010 http://www.reading.ac.uk/nitricoxide/intro/no/cgmp.htm 62 © The McGraw-Hill Companies, Inc., 2010 63 © The McGraw-Hill Companies, Inc., 2010 RECEPTOR REGULATION • • • • • Receptors are dynamically regulated in number, location, and sensitivity. Changes can occur over short times (minutes) and longer periods (days). Frequent or continuous exposure to agonists often results in short-term diminution of the receptor response, sometimes called tachyphylaxis. Several mechanisms are responsible for this phenomenon. First, intracellular proteins may block access of a G protein to the activated receptor molecule. For example, the molecule β-arrestin has been shown to bind to an intracellular loop of the β adrenoceptor when the receptor is continuously activated. Beta-arrestin prevents access of the Gs-coupling protein and thus desensitizes the tissue to further β agonist activation within minutes. Removal of the β agonist results in removal of β-arrestin and restoration of the full response after a few minutes or hours. Second, agonist-bound receptors may be internalized by endocytosis, removing them from further exposure to extracellular molecules. The internalized receptor molecule may then be either reinserted into the membrane (eg, morphine receptors) or degraded (eg, β adrenoceptors, epidermal growth factor receptors). In some cases, the internalizationreinsertion process may actually be necessary for normal functioning of the receptor-effector system. Third, continuous activation of the receptor-effector system may lead to depletion of some essential substrate required for downstream effects. For example, depletion of thiol cofactors may be responsible for tolerance to nitroglycerin. In some cases, repletion of the missing substrate (eg, by administration of glutathione) can reverse the tolerance. 64 © The McGraw-Hill Companies, Inc., 2010 RECEPTOR REGULATION • Long-term reductions in receptor number (downregulation) may occur in response to continuous exposure to agonists. The opposite change (upregulation) occurs when receptor activation is blocked for prolonged periods (usually several days) by pharmacologic antagonists or by denervation. 65 © The McGraw-Hill Companies, Inc., 2010 Figure 2–12. 66 © The McGraw-Hill Companies, Inc., 2010 Revision Pharmacodynamic principles  A receptor molecule may have several binding sites. 67 © The McGraw-Hill Companies, Inc., 2010 Figure 1–4. A receptor which is capable of producing a biological response in the absence of a bound ligand is said to display "constitutive activity“, which as shown can be blocked by inverse agonist 68 © The McGraw-Hill Companies, Inc., 2010 Figure 2.10 (still) Chapter 2 MENU > 69 © The McGraw-Hill Companies, Inc., 2010 Intrinsic activity: Is the maximal effect (Emax) of a ligand relative to a reference "standard" agonist in a given system. The intrinsic activity of the reference agonist is considered as 1, and the intrinsic activity for the ligand of concern that produces 50% that of the reference agonist, then its intrinsic activity is 0.5, while that of the antagonist is = 0. Figure 2.11 (wide version) Chapter 2 MENU > 70 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins 71 © The McGraw-Hill Companies, Inc., 2010 Figure 2.12 (still) Chapter 2 MENU > 72 Copyright © 2012 Wolters Kluwer Health│Lippincott Williams & Wilkins 73 © The McGraw-Hill Companies, Inc., 2010