122
Chapter 5
required to bind a given chemical messenger (
Figure 5–2
). In
many cases, the receptor proteins for a group of messengers are
structurally related. Thus, for example, scientists who study hor-
mones refer to “superfamilies” of hormone receptors.
When 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 norepinephrine causes the smooth muscle of
certain blood vessels to contract, but, via the same type of recep-
tor, 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 mes-
senger 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 phenom-
enon: A single cell may contain more than one different recep-
tor type
for a single messenger.
When the messenger binds to
one of these receptor types, it may produce a cellular response
quite different from, indeed sometimes opposite to, that pro-
duced when the messenger combines with the other recep-
tors. For example, there are two distinct types of receptors
for the hormone epinephrine in the smooth muscle of certain
blood vessels. This hormone can cause either contraction or
relaxation of the muscle depending on the relative degrees of
binding to the two different types of receptors. The degree to
which the molecules of a particular messenger bind to differ-
ent receptor types in a single cell is determined by the
affi
nity
of the different receptor types for the messenger. A receptor
with high affi
nity will bind at lower concentrations of a mes-
senger than will a receptor of low affi nity.
You should not infer from these descriptions that a given
cell has receptors for only one messenger. In fact, a single cell
usually contains many different receptors for different chemi-
cal messengers.
Other characteristics of messenger-receptor interactions
are
saturation
and
competition.
These phenomena were
described in Chapter 3 for ligands binding to binding sites on
proteins and are fully applicable here (and are summarized in
Figure 5–3
). In most systems, a cell’s response to a messen-
ger increases as the extracellular concentration of messenger
increases, because the number of receptors occupied by mes-
senger molecules increases. There is an upper limit to this
responsiveness, however, because only a fi nite number of recep-
tors are available, and they become saturated at some point.
Figure 5–2
Specifi city of receptors for chemical messengers. Only cell A has the
appropriate receptor for this chemical messenger; therefore, it is the
only one among the group that is a target cell for the messenger.
Chemical messenger
Response
Receptor
Cell A
Cell B
Cell C
Secretory cell
Table 5–1
A Glossary of Terms Concerning
Receptors
Receptor
(receptor protein)
A specifi c protein in either the plasma
membrane or the interior of a target cell
that a chemical messenger combines with,
thereby invoking a biologically relevant
response in that cell.
Specifi city
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 messengers. If all are occupied, the
receptors are fully saturated; if half are
occupied, the saturation is 50 percent, and
so on.
Affi
nity
The strength with which a chemical
messenger binds to its receptor.
Competition
The ability of different molecules very similar
in structure to compete with each other 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.
Antihistamines are examples of antagonists.
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. Decongestants are
examples of agonists.
Down-regulation
A decrease in the total number of target-cell
receptors for a given messenger; may occur
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;
may occur in response to a chronic low
extracellular concentration of the messenger.
Supersensitivity
The increased responsiveness of a target cell
to a given messenger; may result from up-
regulation of receptors.
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