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autocrine, or endocrine manner. Paracrine or autocrine secretions that stimulate cells to divide or differentiate are called growth factors. Pharmacologists refer to all of these secretions as autocoids. This array of terms arose from the independent discovery of chemical messages by workers in different disciplines. However, the terminology should not obscure the fact that, from the perspective of the target cell, all of the foregoing are merely chemical signals regardless of where they originate or how they reach their targets. In the ensuing sections we consider all of these secretions simply as chemical signals to emphasize the generality of the cellular processes involved.

Responses of Target Cells

The general sequence of events that occurs in cells when a signal is received is shown in Fig. 8. The input may be a physical signal or any of the classes of chemical signals listed in Table 1. To perceive a signal, the target cell must have a receptor for it. A receptor is a specialized molecule or complex of molecules that is capable of recognizing a specific signal and triggering the chain of events that produces a characteristic response. Interaction with a signal is thought to change the configuration of the receptor and thereby change the way the receptor molecule interacts with nearby molecules in the response pathway. Transducer is the term used to describe the molecular mechanism for

FIGURE 8 Events in cellular communication. Input signals are recognized by specific receptors and translated into a biochemical change by a transducer mechanism. The biochemical signal then acts on the cellular apparatus or effector to produce a physiological response or output. The output may feed back directly or indirectly to affect the source of the input signal and increase or decrease its intensity. The output may also act directly or indirectly to modulate the cellular response to a signal by augmenting or damping events at the level of the receptor, the transducer, or the effector apparatus.

FIGURE 8 Events in cellular communication. Input signals are recognized by specific receptors and translated into a biochemical change by a transducer mechanism. The biochemical signal then acts on the cellular apparatus or effector to produce a physiological response or output. The output may feed back directly or indirectly to affect the source of the input signal and increase or decrease its intensity. The output may also act directly or indirectly to modulate the cellular response to a signal by augmenting or damping events at the level of the receptor, the transducer, or the effector apparatus.

converting the receptor-signal interaction into a biochemical change within the cell. The effector describes the cellular machinery that produces the cellular response(s). Examples of cellular responses include secretion, contraction, relaxation, phagocytosis, cell division, or cell differentiation. Defective, unneeded, or unwanted cells may also receive signals that initiate reactions that lead to their death. The process of programmed cell death is called apoptosis.

Responses may be achieved by activation or inhibition of the enzymatic apparatus already present in the target cell or may require production of new enzymes, secretory products, or structures through changes in gene transcription or translation. A response or output from one cell may directly or indirectly become the input signal to another cell. Cellular input and output arranged in series may produce a feedback effect to shut down (negative feedback) or reinforce (positive feedback) production of the initial signal (see Chapter 37). Output from other cells may modulate the response of a target cell by modifying some aspect of the receptor, the transducer, or the effector.

Complex series of biochemical reactions referred to as signaling pathways or transduction pathways connect receptors with responses. Although thousands of signals produce highly specific responses in their target cells, relatively few families of signaling pathways may be utilized in different combinations in different cell types. In addition, the presence of multiple isoforms of key intermediate molecules contributes to the uniqueness of transduction pathways for different signals. By isoforms we mean closely related molecules that perform similar functions. Different isoforms of a protein may be products of different genes, or they may be products of the same gene and arise from alternative splicing of RNA or posttranslational protein processing. Even very small structural differences can confer significant differences in regulatory properties or in the specificity of interaction with other molecules.

Characteristics of Receptors

Because most chemical signals reach their target cells by way of extracellular fluids, they may be accessible to many different types of cells, but only certain cells respond to certain signals. This selectivity resides in the receptors; only those cells that have receptors for a signal can respond to it. All known receptors are proteins or glycoproteins. To function as a receptor, a molecule must have a domain that binds to the signal molecule with a high degree of selectivity and a domain that is sufficiently altered by agonist binding that its ability to interact with other molecules is changed. Similar considerations hold for photoreceptors and mechanoreceptors, for which photons of a specific wavelength or a mechanical perturbation produce a characteristic conformational change.

Photo-, mechano-, and most chemoreceptors reside on the cell surface and thus provide unimpeded access to signal molecules; however, receptors for some chemical signals are also found within the cytosol or the cell nucleus. To interact with receptors that are located in the cell interior, signal molecules must be able to cross the plasma membrane readily and resist degradation by intracellular enzymes. Receptors have very limited tolerance for variations in the structure or nature of signals. This property confers specificity and ensures that cells recognize and respond only to certain appropriate signals. Receptors for chemical signals can recognize and bind their agonists even though most agonists are present at very low concentrations, usually less than 0.1 ^M (10~7 M). In addition to high affinity, receptors have a limited capacity and hence may become saturated when the concentration of agonist is high. Affinity and capacity of receptors for their agonists are two important determinants of the range of responses that a given cell can express.

The number of receptors in a cell is not fixed. Some receptors may only be expressed at certain stages of the life cycle of a cell or after a cell has been stimulated by other signals. Many cells adjust the number of receptors they express in accordance with the abundance of the signal that activates them. Frequent or intense stimulation may cause a cell to decrease the number of receptors expressed. This phenomenon is called down-regulation. Conversely, cells may upregulate receptors in the face of rare or absent stimulation by their agonists or in response to another signal. Consequences of changes in receptor abundance are discussed in detail in Chapter 37. In addition, cells may temporarily activate or inactivate receptors by adding or removing phosphate or by sequestering them in vesicles to prevent access to agonists. Inactivation of receptors, called adaptation or desensitization, is called homologous desensitization when produced by its own ligand, and heterologous desensitization when caused by other agonists acting through their receptors.

Receptors that reside in the plasma membrane may be uniformly distributed over the entire surface of a cell or they may be confined to some discrete region such as the neuromuscular junction or the basal surface of renal tubular epithelial cells. Receptors in many cells are not fixed in place by attachments to the cytoskeleton, and can migrate laterally in the plane of the membrane. Some cell surface receptors are concentrated in specialized surface invaginations called caveolae. In some cells, receptors that are occupied by their ligands may cluster at one pole, a phenomenon known as capping. Clustering may be important for initiating some cellular responses. Also, membrane receptors are internalized either alone or bound to their ligands by a process called receptor-mediated endocytosis. Some cells recycle receptors between the plasma membrane and internal membranes and can vary the rate of transfer and hence the relative abundance of receptors on the cell surface. Receptors, like other cellular proteins, are broken down and replaced many times during the lifetime of a cell.

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