Regulation of Organic Metabolism and Energy Balance
573
scope of this book, but the overall pattern is shown in
Figure
16–6
for reference and to illustrate several principles. The
essential information to understand about insulin’s actions is
the target cells’ ultimate responses—that is, the material sum-
marized in Figure 16–4. Figure 16–6 shows some of the spe-
cifi c biochemical reactions that underlie these responses.
A major principle illustrated by Figure 16–6 is that,
in each of its target cells, insulin brings about its ultimate
responses by multiple actions. Take, for example, its effects
on muscle cells. In these cells, insulin favors glycogen forma-
tion and storage by (1) increasing glucose transport into the
cell, (2) stimulating the key enzyme (
glycogen synthase
) that
catalyzes the rate-limiting step in glycogen synthesis, and
(3) inhibiting the key enzyme (
glycogen phosphorylase
) that
catalyzes glycogen catabolism. Thus, insulin favors glucose
transformation to and storage as glycogen in muscle through
three pathways. Similarly, for protein synthesis in muscle
cells, insulin (1) increases the number of active plasma mem-
brane transporters for amino acids, thereby increasing amino
acid transport into the cells, (2) stimulates the ribosomal
enzymes that mediate the synthesis of protein from these
amino acids, and (3) inhibits the enzymes that mediate pro-
tein catabolism.
Control of Insulin Secretion
The major controlling factor for insulin secretion is the plasma
glucose concentration. An increase in plasma glucose concen-
tration, as occurs after a meal, acts on the beta cells of the islets
of Langerhans to stimulate insulin secretion, whereas a decrease
in plasma glucose removes the stimulus for insulin secretion.
The feedback nature of this system is shown in
Figure 16–7
:
Following a meal, the increase in plasma glucose concentration
stimulates insulin secretion. The insulin stimulates the entry of
glucose into muscle and adipose tissue, as well as net uptake,
rather than net output, of glucose by the liver. These effects
eventually reduce the blood concentration of glucose to its pre-
meal level, thereby removing the stimulus for insulin secretion
and causing it to return to its previous level.
In addition to plasma glucose concentration, there are
many other factors that control insulin secretion (
Figure 16–8
).
For example, elevated amino acid concentrations stimulate insu-
lin secretion. This is another negative feedback control: Amino
+
Begin
V
es
icl
cle
E
n
En
En
dosome
ndosome
ndosome
ndosome
ndosome
ndosome
ndosome
ndosome
ndosome
ndosome
ndosome
ndosome
dosome
dosom
dosom
dosom
dosom
dosom
dosom
dosom
dosom
dosom
dosom
osom
osom
osom
osom
osom
osom
osom
osom
Glucose transporter
Plasma
membrane
FACILITATED DIFFUSION
OF GLUCOSE
Extracellular
fluid
Insulin
Insulin
receptor
S
i
g
n
a
l
t
r
a
n
s
d
u
c
t
i
o
n
p
a
t
h
w
a
y
d
fluid
ular fluid
racellular fluid
Intracellular flui
Intracellular
Intracellu
Nucleus
Figure 16–5
Stimulation by insulin of the translocation of glucose transporters from cytoplasmic vesicles to the plasma membrane in muscle cells and
adipose tissue cells. Note that these transporters are constantly recycled by endocytosis from the plasma membrane back through endosomes
into vesicles. As long as insulin levels are elevated, the entire cycle continues, and the number of transporters in the plasma membrane stays
high. This is how insulin decreases the plasma level of glucose. In contrast, when insulin levels decrease, the cycle is broken, the vesicles
accumulate in the cytoplasm, and the number of transporters in the plasma membrane decreases. Thus, without insulin the plasma glucose level
would increase, because glucose transport from plasma to cells would be reduced.
Figure 16–5
physiological
inquiry
What advantage is there to having insulin-dependent glucose transporters pre-packaged in a cell?
Answer can be found at end of chapter.
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