Essential Cell Biology 5th edition

544 CHAPTER 16 Cell Signaling
TABLE 16–2 SOME FOREIGN SUBSTANCES THAT ACT ON CELL–SURFACE RECEPTORS
Substance Normal Signal Receptor Action Effect
Barbiturates and
benzodiazepines
(Valium and Ambien)
γ-aminobutyric acid
(GABA)
stimulate GABA-activated ion-channelcoupled
receptors
relief of anxiety; sedation
Nicotine acetylcholine stimulates acetylcholine-activated ionchannel-coupled
receptors
constriction of blood vessels; elevation
of blood pressure
Morphine and heroin
endorphins and
enkephalins
stimulate G-protein-coupled opiate
receptors
analgesia (relief of pain); euphoria
Curare acetylcholine blocks acetylcholine-activated ionchannel-coupled
receptors
Strychnine glycine blocks glycine-activated ion-channelcoupled
receptors
Capsaicin heat stimulates temperature-sensitive ionchannel-coupled
receptors
Menthol cold stimulates temperature-sensitive ionchannel-coupled
receptors
blockage of neuromuscular transmission,
resulting in paralysis
blockage of inhibitory synapses in spinal
cord and brain, resulting in seizures and
muscle spasm
induces painful, burning sensation;
prolonged exposure paradoxically leads
to pain relief
in moderate amounts, induces a cool
sensation; in higher doses, can cause
burning pain
of cells to react to acetylcholine in different ways, increasing contraction
in skeletal muscle and decreasing the rate of contractions in the heart
(see Figure 16–5A and C).
This plethora of cell-surface receptors also provides targets for many
foreign substances that interfere with our physiology, from heroin and
nicotine to tranquilizers and chili peppers. These substances either block
or overstimulate the receptor’s natural activity. Many drugs and poisons
act in this way (Table 16−2), and a large part of the pharmaceutical industry
is devoted to producing drugs that will exert a precisely defined effect
by binding to a specific type of cell-surface receptor.
Ion-Channel-Coupled Receptors Convert Chemical
Signals into Electrical Ones
Of all the types of cell-surface receptors, ion-channel-coupled receptors
(also known as transmitter-gated ion channels) function in the simplest
and most direct way. As we discuss in detail in Chapter 12, these receptors
are responsible for the rapid transmission of signals across synapses
in the nervous system. They transduce a chemical signal, in the form
of a pulse of secreted neurotransmitter molecules delivered to a target
cell, directly into an electrical signal, in the form of a change in voltage
across the target cell’s plasma membrane (see Figure 12−41). When the
neurotransmitter binds to ion-channel-coupled receptors on the surface
of a target cell, the receptor alters its conformation so as to open a channel
in the target cell membrane, rendering it permeable to specific types
of ions, such as Na + , K + , or Ca 2+ (see Figure 16–13A and Movie 16.1).
Driven by their electrochemical gradients, the ions rush into or out of the
cell, creating a change in the membrane potential within milliseconds.
This change in potential may trigger a nerve impulse or make it easier
(or harder) for other neurotransmitters to do so. As we discuss later, the
opening of Ca 2+ channels has additional important effects, as changes in
the Ca 2+ concentration in the target-cell cytosol can profoundly alter the
activities of many Ca 2+ -responsive proteins.