Receptors

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The vast majority of homeostatic systems require cell-to-cell communication via chemical messengers. The first step in the action of any intercellular chemical messenger is the binding of the messenger to specific target-cell proteins known as receptors. In the general language of Chapter 4, a chemical messenger is a "li-gand," and the receptor is a "binding site." The binding of a messenger to a receptor then initiates a sequence of events in the cell leading to the cell's response to that messenger.

The term "receptor" can be the source of confusion because the same word is used to denote the "detectors" in a reflex arc, as described earlier in this chapter. The reader must keep in mind the fact that "receptor" has two totally distinct meanings, but the context in which the term is used usually makes it quite clear which is meant.

What is the nature of the receptors with which intercellular chemical messengers combine? They are proteins (or glycoproteins) located either in the cell's plasma membrane or inside the cell, mainly in the nucleus. The plasma membrane is the much more common location, applying to the very large number of messengers that are lipid-insoluble and so do not traverse the lipid-rich plasma membrane. In contrast, the much smaller number of lipid-soluble messengers pass through membranes (mainly by diffusion but, in some cases, by mediated transport as well) to bind to their receptors inside the cell.

Plasma-membrane receptors are transmembrane proteins; that is, they span the entire membrane thickness. A typical plasma-membrane receptor is illustrated in Figure 7-10. Like other transmembrane proteins, a plasma-membrane receptor has segments within the membrane, one or more segments extending out from the membrane into the extracellular fluid, and other segments extending into the intracellular fluid. It is to the extracellular portions that the mes senger binds. Like other transmembrane proteins, a receptor is often composed of two or more nonidentical subunits bound together.

It is the combination of chemical messenger and receptor that initiates the events leading to the cell's response. The existence of receptors explains a very important characteristic of intercellular communication—specificity (see Table 7-3 for a glossary of terms concerning receptors). Although a chemical messenger (hormone, neurotransmitter, paracrine/autocrine agent, or plasma-membrane-bound messenger) may come into contact with many different cells, it influences only certain cells and not others. The explanation is that cells differ in the types of receptors they contain. Accordingly, only certain cell types, frequently just one, possess the receptor required for combination with a given chemical messenger (Figure 7-11). In many cases, the receptors for a group of messengers are closely related structurally; thus, for example, endocrinologists refer to "superfamilies" of hormone receptors.

Where different types of cells possess the same receptors for a particular messenger, the responses of the various cell types to that messenger may differ from each other. For example, the neurotransmitter norepi-nephrine causes the smooth muscle of blood vessels to contract, but via the same type of receptor, norepi-nephrine causes endocrine cells in the pancreas to secrete less insulin. In essence, then, the receptor functions as a molecular "switch" that elicits the cell's response when "switched on" by the messenger binding to it. Just as identical types of switches can be used to turn on a light or a radio, a single type of receptor can be used to produce quite different responses in different cell types.

Similar reasoning explains a more surprising phenomenon: A single cell may contain several different receptor types for a single messenger. Combination of the messenger with one of these receptor types may produce a cellular response quite different from,

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

PART TWO Biological Control Systems

Extracellular fluid

Extracellular fluid

Alpha Helix Protein Cell Membrane

FIGURE 7-10

Structure of a human receptor that binds the hormone epinephrine. The seven clusters of amino acids in the plasma membrane represent hydrophobic portions of the protein's alpha helix. Note that the binding site for the hormone includes several of the segments that extend into the extracellular fluid. The amino acids denoted by black circles represent sites at which the receptor can be phosphorylated, and thereby regulated, by intracellular substances.

Adapted from Dohlman et al.

FIGURE 7-10

Structure of a human receptor that binds the hormone epinephrine. The seven clusters of amino acids in the plasma membrane represent hydrophobic portions of the protein's alpha helix. Note that the binding site for the hormone includes several of the segments that extend into the extracellular fluid. The amino acids denoted by black circles represent sites at which the receptor can be phosphorylated, and thereby regulated, by intracellular substances.

Adapted from Dohlman et al.

indeed sometimes opposite to, that produced when the messenger combines with the other receptors. For example, as we shall see in Chapter 14, there are two distinct types of receptors for the hormone epineph-rine in the smooth muscle of certain blood vessels, and this hormone can cause either contraction or relaxation of the muscle depending on the relative degrees of binding to the two different types. The degree to which the molecules of a particular messenger bind to different receptor types in a single cell is determined by the affinity of the different receptor types for the messenger.

It should not be inferred from these descriptions that a cell has receptors for only one messenger. In fact, a single cell usually contains many different receptors for different chemical messengers.

Other characteristics of messenger-receptor interactions are saturation and competition. These phenomena were described in Chapter 4 for ligands binding to binding sites on proteins and are fully applicable here. In most systems, a cell's response to a messenger increases as the extracellular concentration of messenger increases, because the number of receptors occupied by messenger molecules increases. There is an upper limit to this responsiveness, however, because only a finite number of receptors are available, and they become saturated at some point.

Competition is the ability of different messenger molecules that are very similar in structure to compete with each other for a receptor. Competition occurs physiologically with closely related messengers, and it also underlies the action of many drugs. If researchers

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Homeostatic Mechanisms and Cellular Communication CHAPTER SEVEN

TABLE 7-3 A Glossary of Terms Concerning Receptors

Receptor

A specific protein in either the plasma membrane or interior of a target cell with which a chemical messenger combines.

Specificity

The ability of a receptor to bind only one type or a limited number of structurally related types of chemical messengers.

Saturation

The degree to which receptors are occupied by a messenger. If all are occupied, the receptors are fully saturated; if half are occupied, the saturation is 50 percent, and so on.

Affinity

The strength with which a chemical messenger binds to its receptor.

Competition

The ability of different molecules very similar in structure to combine with the same receptor.

Antagonist

A molecule that competes for a receptor with a chemical messenger normally present in the body. The antagonist binds to the receptor but does not trigger the cell's response.

Agonist

A chemical messenger that binds to a receptor and triggers the cell's response; often refers to a drug that mimics a normal messenger's action.

Down-regulation

A decrease in the total number of target-cell receptors for a given messenger in response to chronic high extracellular concentration of the messenger.

Up-regulation

An increase in the total number of target-cell receptors for a given messenger in response to a chronic low extracellular concentration of the messenger.

Supersensitivity

The increased responsiveness of a target cell to a given messenger, resulting from up-regulation.

or physicians wish to interfere with the action of a particular messenger, they can administer competing molecules, if available, that bind to the receptors for that messenger without activating them. This prevents the messenger from binding and does not trigger the cell's response. Such drugs are known as antagonists with regard to the usual chemical messenger. For example,

FIGURE 7-11

Specificity of receptors for chemical messengers. Only cell A has the appropriate receptor for this chemical messenger and, therefore, is a target cell for the messenger.

so-called beta-blockers, used in the treatment of high blood pressure and other diseases, are drugs that antagonize the ability of epinephrine and norepinephrine to bind to one of their receptors—the beta-adrenergic receptor (Chapter 8). On the other hand, some drugs that bind to a particular receptor type do trigger the cell's response exactly as if the true chemical messenger had combined with the receptor; such drugs are known as agonists and are used to mimic the messenger's action. For example, the decongestant drug ephedrine mimics the action of epinephrine.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • scolastica
    What is meant by the term Membrane bound receptor?
    8 years ago
  • Mario
    Which chemical binds a receptor site without triggering the cell's response?
    8 years ago
  • Savanna
    What reflects the fact that a receptor normally binds only to a single messenger?
    8 years ago
  • mandy sankt
    Are receptors located in extra celular fluid?
    7 years ago
  • lionel
    What are receptors in a human body?
    7 years ago
  • sian morrison
    How are protein receptors composed in terms of chemestry?
    2 years ago
  • Averardo
    How many chemical receptors in the body?
    2 years ago
  • Ileana
    Which type of proteins can function as chemical messengers or as receptors in the plasma membrane?
    10 months ago

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