Regulation of Organic Metabolism and Energy Balance
571
Endocrine and Neural Control
of the Absorptive and
Postabsorptive States
We now turn to the endocrine and neural factors that control and
integrate these metabolic pathways. We will focus primarily on
the following questions, summarized in
Figure 16–3
: (1) What
controls net anabolism of protein, glycogen, and triglyceride in
the absorptive phase, and net catabolism in the postabsorptive
phase? (2) What induces the cells to utilize primarily glucose
for energy during the absorptive phase, but fat during the post-
absorptive phase? (3) What stimulates net glucose uptake by
the liver during the absorptive phase, but gluconeogenesis and
glucose release during the postabsorptive phase?
The most important controls of these transitions
from feasting to fasting, and vice versa, are two pancreatic
hormones—insulin and glucagon. Also playing a role are
the hormones epinephrine and cortisol from the adrenal
glands and the sympathetic nerves to liver and adipose tissue.
Insulin and glucagon are peptide hormones secreted
by the
islets of Langerhans
(or, simply, pancreatic islets),
clusters of endocrine cells in the pancreas. There are several
distinct types of islet cells, each of which secretes a different
Table 16–2
Summary of Nutrient Metabolism
During the Postabsorptive Period
1.
Glycogen, fat, and protein syntheses are curtailed, and net
breakdown occurs.
2.
Glucose is formed in the liver both from the glycogen
stored there and by gluconeogenesis from blood-borne
lactate, pyruvate, glycerol, and amino acids. The kidneys
also perform gluconeogenesis during a prolonged fast.
3.
The glucose produced in the liver (and kidneys) is released
into the blood, but its utilization for energy is greatly
reduced in muscle and other nonneural tissues.
4.
Lipolysis releases adipose tissue fatty acids into the blood,
and the oxidation of these fatty acids by most cells and of
ketones produced from them by the liver provides most of
the body’s energy supply.
5.
The brain continues to use glucose but also starts using
ketones as they build up in the blood.
Most cells
CO
2
+
H
2
O
+
energy
Most cells
CO
2
+
H
2
O
+
energy
Liver
Glycogen
Fat
Liver
Glucose
Proteins
Triglyceride
Glycogen
Glucose
Fatty
acids
Amino
acids
Glucose
Absorptive state
Postabsorptive state
α
-glycerol
phosphate
(1)
(2)
(3)
Fatty acids
and
ketones
Glucose
Pyruvate,
lactate,
glycerol,
and
amino acids
Proteins
Amino
acids
Glycogen
Triglyceride
Glucose
Glycerol
Fatty
acids
Figure 16–3
Summary of critical points in transition from the absorptive state to the postabsorptive state. The term
absorptive state
could be replaced with
actions of insulin,
and the term
postabsorptive state
with
results of decreased insulin.
The numbers at the left margin refer to discussion questions
in the text.
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