136
Chapter 5
Chapter 5 Test Questions
(Answers appear in Appendix A.)
Match the receptor feature (a–e) with the best choice (1–3; you can
use an answer more than once):
a. affi nity
b. saturation
c. competition
d. down-regulation
e. specifi city
1. defi nes the situation when all receptor binding sites are
occupied by a messenger
2. defi nes the strength of receptor binding to a messenger
3. refl ects the fact that a receptor normally binds only to a single
messenger
4. Which of the following intracellular or plasma-membrane
proteins require Ca
2+
for full activity?
a. calmodulin
b. Janus kinase (JAK)
c. protein kinase A
d. guanylyl cyclase
e. all of the above
5. Which is correct?
a. Protein kinase A phosphorylates tyrosine residues.
b. Protein kinase C is activated by cAMP.
c. The subunit of G
s
proteins that activates adenylyl cyclase is
the
β
-subunit.
d. Lipid-soluble messengers typically act on receptors in the cell
cytosol or nucleus.
e. The binding site of a typical plasma-membrane receptor
for its messenger is located on the cytosolic surface of the
receptor.
6. Inhibition of which enzyme(s) would inhibit the conversion of
arachidonic acid to leukotrienes?
a. cyclooxygenase
b. lipoxygenase
c. phospholipase A
2
d. adenylyl cyclase
e. both b and c
Chapter 5 Quantitative and Thought Questions
(Answers appear in Appendix A.)
1. Patient A is given a drug that blocks the synthesis of all
eicosanoids, whereas patient B is given a drug that blocks the
synthesis of leukotrienes but none of the other eicosanoids.
What are the enzymes these drugs most likely block?
2. Certain nerves to the heart release the neurotransmitter
norepinephrine. If these nerves are removed in experimental
animals, the heart becomes extremely sensitive to the
administration of a drug that is an agonist of norepinephrine.
Explain why, in terms of receptor physiology.
3. A particular hormone is known to elicit, completely by way of
the cyclic AMP system, six different responses in its target cell.
A drug is found that eliminates one of these responses but not
the other fi ve. Which of the following, if any, could the drug be
blocking: the hormone’s receptors, G
s
protein, adenylyl cyclase,
or cyclic AMP?
4. If a drug were found that blocked all calcium channels directly
linked to G proteins, would this eliminate the role of calcium as
a second messenger? Why or why not?
5. Explain why the effects of a fi rst messenger do not immediately
cease upon removal of the messenger.
Chapter 5 Answers to Physiological Inquiries
Figure 5–5
Expressing more than one type of receptor allows
a cell to respond to more than one type of fi rst messenger.
For example, one fi rst messenger might activate a particular
biochemical pathway in a cell by activating one type of
receptor and signaling pathway. By contrast, another fi rst
messenger acting on a different receptor and activating
a different signaling pathway, might inhibit the same
biochemical process. In this way, the biochemical process can
be tightly regulated.
Figure 5–8
Enzymes can generate large amounts of product
without being consumed. This is an extremely effi cient way to
generate a second messenger like cAMP. Enzymes have many
other advantages (Table 3–7) including the ability to have their
activities fi ne-tuned by other inputs (Figures 3–35 to 3–38).
This enables the cell to adjust its response to a fi rst messenger
depending on the other conditions present.
Figure 5–9
Not necessarily. In some cases, a kinase may
phosphorylate the same protein in many different types of cells.
However, many cells also express certain cell-specifi c proteins
that are not found in all tissues, and some of these proteins may
be substrates for cAMP-dependent protein kinase. Thus, the
proteins that are phosphorylated by a given kinase depend upon
the cell type, which makes the cellular response tissue-specifi c.
As an example, in the kidneys cAMP-dependent protein kinase
phosphorylates proteins that insert water channels in cell
membranes and thereby reduce urine volume, while in heart
muscle the same kinase phosphorylates calcium channels that
increase the strength of muscle contraction.
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