Pharmacodynamics describes the actions of a drug on the body and the influence of drug concentrations on the magnitude of the response. Most drugs exert their effects, both beneficial and harmful, by interacting with receptors (that is, specialized target macromolecules) present on the cell surface or within the cell. The drug–receptor complex initiates alterations in biochemical and/or molecular activity of a cell by a process called signal transduction (Figure 2.1).
Figure 2.1. The recognition of a drug by a receptor triggers a biologic response.
Drugs act as signals, and their receptors act as signal detectors. Many receptors signal their recognition of a bound ligand by initiating a series of reactions that ultimately result in a specific intracellular response. [Note: The term “ligand” refers to a small molecule that binds to a site on a receptor protein. The ligand may be a naturally occurring molecule or a drug.] “Second messenger” molecules (also called effector molecules) are part of the cascade of events that translates ligand binding into a cellular response.
A. The drug–receptor complex
Cells have different types of receptors, each of which is specific for a particular ligand and produces a unique response. The heart, for example, contains membrane receptors that bind and respond to epinephrine or norepinephrine as well as muscarinic receptors specific for acetylcholine. These receptors dynamically interact to control the heart’s vital functions. The magnitude of the response is proportional to the number of drug–receptor complexes:
This concept is closely related to the formation of complexes between enzyme and substrate or antigen and antibody. These interactions have many common features, perhaps the most noteworthy being specificity of the receptor for a given ligand. However, the receptor not only has the ability to recognize a ligand, but can also couple or transduce this binding into a response by causing a conformational change or a biochemical effect.
Most receptors are named to indicate the type of drug/chemical that interacts best with it. For example, the receptor for histamine is called a histamine receptor. Although much of this chapter will be centered on the interaction of drugs with specific receptors, it is important to be aware that not all drugs exert their effects by interacting with a receptor. Antacids, for instance, chemically neutralize excess gastric acid, thereby reducing the symptoms of “heartburn.”
B. Receptor states
Classically, the binding of a ligand was thought to cause receptors to change from an inactive state (R) to an activated state (R*). The activated receptor then interacts with intermediary effector molecules to produce a biologic effect. This induced-fit model is a simple and intuitive scheme and is used in the illustrations in this chapter. More recent information suggests that receptors exist in at least two states, inactive (R) and active R* states that are in reversible equilibrium with one another. In the absence of an agonist, R* typically represents a small fraction of the total receptor population (that is, the equilibrium favors the inactive state).
Drugs occupying the receptor can stabilize the receptor in a given conformational state. Some drugs may cause similar shifts in equilibrium between R to R* as an endogenous ligand. For example, drugs acting as agonists bind to the active state of the receptors and, thus, rapidly shift the equilibrium from R to R*. Other drugs may induce a change that may be different from the endogenous ligand. These changes render the receptor less functional or nonfunctional.
Pharmacology defines a receptor as any biologic molecule to which a drug binds and produces a measurable response. Thus, enzymes, nucleic acids, and structural proteins can be considered to be pharmacologic receptors. However, the richest sources of therapeutically exploitable pharmacologic receptors are proteins that are responsible for transducing extracellular signals into intracellular responses.
These receptors may be divided into four families: 1) ligand-gated ion channels, 2) G protein–coupled receptors, 3) enzyme–linked receptors, and 4) intracellular receptors (Figure 2.2). The type of receptor a ligand will interact with depends on the chemical nature of the ligand. Hydrophilic ligands interact with receptors that are found on the cell surface (Figures 2.2A, B, C). In contrast, hydrophobic ligands can enter cells through the lipid bilayers of the cell membrane to interact with receptors found inside cells (Figure 2.2D).
Figure 2.2. Transmembrane signaling mechanisms.
A. Ligand binds to the extracellular domain of a ligand-gated channel. B. Ligand binds to a domain of a transmembrane receptor, which is coupled to a G protein. C. Ligand binds to the extracellular domain of a receptor that activates a kinase enzyme. D. Lipid-soluble ligand diffuses across the membrane to interact with its intracellular receptor. R = inactive protein.
1. Transmembrane ligand-gated ion channels
The first receptor family comprises ligand-gated ion channels that are responsible for regulation of the flow of ions across cell membranes (see Figure 2.2A). The activity of these channels is regulated by the binding of a ligand to the channel. Response to these receptors is very rapid, enduring for only a few milliseconds. These receptors mediate diverse functions, including neurotransmission, cardiac conduction, and muscle contraction.
For example, stimulation of the nicotinic receptor by acetylcholine results in sodium influx, generation of an action potential, and activation of contraction in skeletal muscle. Benzodiazepines, on the other hand, enhance the stimulation of the ?-aminobutyric acid (GABA) receptor by GABA, resulting in increased chloride influx and hyperpolarization of the respective cell. Although not ligand-gated, ion channels, such as the voltage-gated sodium channel, are important drug receptors for several drug classes, including local anesthetics.
2. Transmembrane G protein–coupled receptors
A second family of receptors consists of G protein–coupled receptors. These receptors comprise a single ? helical peptide that has seven membrane-spanning regions. The extracellular domain of this receptor usually contains the ligand-binding area (a few ligands interact within the receptor transmembrane domain). Intracellularly, these receptors are linked to a G protein (Gs, Gi, and others) having three subunits, an ? subunit that binds guanosine triphosphate (GTP) and a ?? subunit (Figure 2.3). Binding of the appropriate ligand to the extracellular region of the receptor activates the G protein so that GTP replaces guanosine diphosphate (GDP) on the ? subunit.
Dissociation of the G protein occurs, and both the ?-GTP subunit and the ?? subunit subsequently interact with other cellular effectors, usually an enzyme, a protein, or an ion channel. These effectors then activate second messengers that are responsible for further actions within the cell. Stimulation of these receptors results in responses that last several seconds to minutes. G protein–coupled receptors are the most abundant type of receptors, and their activation accounts for the actions of most therapeutic agents. Important processes mediated by G protein–coupled receptors include neurotransmission, olfaction, and vision.
Figure 2.3. The recognition of chemical signals by G protein–coupled membrane receptors triggers an increase (or, less often, a decrease) in the activity of adenylyl cyclase.
PPi = inorganic pyrophosphate.
a. Second messengers
These are essential in conducting and amplifying signals coming from G protein–coupled receptors. A common pathway turned on by Gs, and other types of G proteins, is the activation of adenylyl cyclase by ?-GTP subunits, which results in the production of cyclic adenosine monophosphate (cAMP)—a second messenger that regulates protein phosphorylation. G proteins also activate phospholipase C, which is responsible for the generation of two other second messengers, namely inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 is responsible for the regulation of intracellular free calcium concentrations, and of other proteins as well. DAG activates several enzymes such as protein kinase C (PKC) within the cell leading to a myriad of physiological effects.
3. Enzyme-linked receptors
A third major family of receptors consists of a protein that spans the membrane once and may form dimers or multisubunit complexes. These receptors also have cytosolic enzyme activity as an integral component of their structure and function (Figure 2.4). Binding of a ligand to an extracellular domain activates or inhibits this cytosolic enzyme activity. Duration of responses to stimulation of these receptors is on the order of minutes to hours. Metabolism, growth, and differentiation are important biological functions controlled by these types of receptors.
The most common enzyme-linked receptors (epidermal growth factor, platelet-derived growth factor, atrial natriuretic peptide, insulin, and others) are those that have a tyrosine kinase activity as part of their structure. Typically, upon binding of the ligand to receptor subunits, the receptor undergoes conformational changes, converting kinases from their inactive forms to active forms. The activated receptor autophosphorylates and then phosphorylates tyrosine residues on specific proteins (see Figure 2.4). The addition of a phosphate group can substantially modify the three-dimensional structure of the target protein, thereby acting as a molecular switch.
For example, when the peptide hormone insulin binds to two of its receptor subunits, their intrinsic tyrosine kinase activity causes autophosphorylation of the receptor itself. In turn, the phosphorylated receptor phosphorylates target molecules (insulin-receptor substrate peptides) that subsequently activate other important cellular signals, such as inositol triphosphate and the mitogen-activated protein (MAP) kinase system. This cascade of activations results in a multiplication of the initial signal, much like that which occurs with G protein–coupled receptors.
Figure 2.4. Insulin receptor.
4. Intracellular receptors
The fourth family of receptors differs considerably from the other three in that the receptor is entirely intracellular, and, therefore, the ligand must diffuse into the cell to interact with the receptor (Figure 2.5). This places constraints on the physical and chemical properties of the ligand, because it must have sufficient lipid solubility to be able to move across the target cell membrane. Because these receptor ligands are lipid soluble, they are transported in the body attached to plasma proteins such as albumin. The primary targets of these ligand-receptor complexes are transcription factors.
The activation or inactivation of these factors causes the transcription of DNA into RNA and translation of RNA into an array of proteins. For example, steroid hormones exert their action on target cells via this receptor mechanism. Binding of the ligand with its receptor follows a general pattern in which the receptor becomes activated because of the dissociation from a variety of proteins. The activated ligand–receptor complex migrates or translocates to the nucleus, where it binds to specific DNA sequences, resulting in the regulation of gene expression. The time course of activation and response of these receptors is much longer than that of the other mechanisms described above.
Because gene expression and, therefore, protein synthesis is modified, cellular responses are not observed until considerable time has elapsed (30 minutes or more), and the duration of the response (hours to days) is much greater than that of other receptor families. Other targets of intracellular ligands are structural proteins, enzymes, RNA, and ribosomes. For example, tubulin is the target of antineoplastic agents such as paclitaxel (see Paclitaxel and docetaxel), the enzyme dihydrofolate reductase is the target of antimicrobials such as trimethoprim (see Trimethoprim), and the 50s subunit of the bacterial ribosome is the target of macrolide antibiotics such as erythromycin (see Erythromycin).
Figure 2.5. Mechanism of intracellular receptors.
mRNA = messenger RNA.
D. Some characteristics of signal transduction
Signal transduction has two important features: 1) the ability to amplify small signals and 2) mechanisms to protect the cell from excessive stimulation.
1. Signal amplification
A characteristic of many receptors, particularly those that respond to hormones, neurotransmitters, and peptides, is their ability to amplify signal duration and intensity. The family of G protein–linked receptors exemplifies many of the possible responses initiated by ligand binding to a receptor. Specifically, two phenomena account for the amplification of the ligand–receptor signal. First, a single ligand–receptor complex can interact with many G proteins, thereby multiplying the original signal manyfold. Second, the activated G proteins persist for a longer duration than the original ligand–receptor complex.
The binding of albuterol, for example, may only exist for a few milliseconds, but the subsequent activated G proteins may last for hundreds of milliseconds. Further prolongation and amplification of the initial signal is mediated by the interaction between G proteins and their respective intracellular targets. Because of this amplification, only a fraction of the total receptors for a specific ligand may need to be occupied to elicit a maximal response from a cell.
Systems that exhibit this behavior are said to have spare receptors. Spare receptors are exhibited by insulin receptors, where it has been estimated that 99 percent of the receptors are “spare.” This constitutes an immense functional reserve that ensures that adequate amounts of glucose enter the cell. On the other end of the scale is the human heart, in which about 5 to 10 percent of the total ?-adrenoceptors are spare. An important implication of this observation is that little functional reserve exists in the failing heart, because most receptors must be occupied to obtain maximum contractility.
2. Desensitization and down-regulation of receptors
Repeated or continuous administration of an agonist (or an antagonist) may lead to changes in the responsiveness of the receptor. To prevent potential damage to the cell (for example, high concentrations of calcium, initiating cell death), several mechanisms have evolved to protect a cell from excessive stimulation. When repeated administration of a drug results in a diminished effect, the phenomenon is called tachyphylaxis. The receptor becomes desensitized to the action of the drug (Figure 2.6). In this phenomenon, the receptors are still present on the cell surface but are unresponsive to the ligand. Receptors can also be down-regulated in the presence of continual stimulation.
Binding of the agonist results in molecular changes in the membrane-bound receptors, such that the receptor undergoes endocytosis and is sequestered within the cell, unavailable for further agonist interaction. These receptors may be recycled to the cell surface, restoring sensitivity, or, alternatively, may be further processed and degraded, decreasing the total number of receptors available. Some receptors, particularly voltage-gated channels, require a finite time (rest period) following stimulation before they can be activated again. During this recovery phase they are said to be “refractory” or “unresponsive.”
Figure 2.6. Desensitization of receptors.