Page 317 - Color Atlas of Biochemistry

308 Tissues and organs
Buffer function in organ meta-
bolism
All of the body’s tissues have a constant re-
quirement for energy substrates and nu-
trients. The body receives these metabolites
with food, but the supply is irregular and in
varying amounts. The liver acts here along
with other organs, particularly adipose tissue, postabsorbtive state quickly sets in. The pan-
as a balancing buffer and storage organ.
In the metabolism, a distinction is made
between the absorptive state (well-fed state)
immediately after a meal and the postabsorb-
tive state (state of starvation),which starts
later and can merge into hunger. The switch-
ing of the organ metabolism between the two
phases depends on the concentration of en-
ergy-bearing metabolites in the blood (plas-
ma level). This is regulated jointly by hor-
mones and by the autonomic nervous system.
A. Absorptive state
The absorptive state continues for 2–4 hours
after food intake. As a result of food digestion, serves themselves. When the liver’s glycogen
the plasma levels of glucose, amino acids, and
fats (triacylglycerols) temporarily increase.
The endocrine pancreas responds to this by
altering its hormone release—there is an in-
crease in insulin secretion and a reduction in
glucagon secretion. The increase in the insu-
lin/glucagon quotient and the availability of
substrates trigger an anabolic phase in the
tissues—particularly liver, muscle, and adi-
pose tissues.
The liver forms increased amounts of gly-
cogen and fats from the substrates supplied.
Glycogen is stored, and the fat is released into glucose.
the blood in very low density lipoproteins
(VLDLs).
Muscle also refills its glycogen store and
synthesizes proteins from the amino acids
supplied.
Adipose tissue removes free fatty acids
from the lipoproteins, synthesizes triacylgly-
cerols from them again, and stores these in
the form of insoluble droplets.
During the absorptive state, the heart and
neural tissue mainly use glucose as an energy
source, but they are unable to establish any
substantial energy stores. Heart muscle cells
arein a sense“omnivorous,” as they can also
use other substances to produce energy (fatty
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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acids, ketone bodies). By contrast, the central
nervous system (CNS) is dependent on glu-
cose. It is only able to utilize ketone bodies
after a prolonged phase of hunger (B).
B. Postabsorptive state
When the food supply is interrupted, the
creatic A cells now release increased amounts
of glucagon, while the B cells reduce the
amount of insulin they secrete. The reduced
insulin/glucagon quotient leads to switching
of the intermediary metabolism. The body
now falls back on its energy reserves. To do
this, it breaks down storage substances (gly-
cogen, fats, and proteins) and shifts energy-
supplying metabolites between the organs.
The liver first empties its glycogen store
(glycogenolysis; see p.156). It does not use
the released glucose itself, however, but sup-
plies the other tissues with it. In particular,
the brain, adrenal gland medulla, and eryth-
rocytes depend on a constant supply of glu-
cose, as they have no substantial glucose re-
reserves are exhausted after 12–24 hours,
gluconeogenesis begins (see p. 154). The pre-
cursors for this are derived from the muscu-
lature (amino acids) and adipose tissue (glyc-
erol from fat degradation). From the fatty
acids that are released (see below), the liver
starts to form ketone bodies (ketogenesis; see
p. 312). These are released into the blood and
serve as important energy suppliers during
the hunger phase. After 1–2 weeks, the CNS
also starts to use ketone bodies to supply part
of its energy requirements, in order to save
In muscle, the extensive glycogen reserves
are exclusively used for the muscles’ own
requirements (see p. 320). The slowly initi-
ated protein breakdown in muscle supplies
amino acids for gluconeogenesis in the liver.
In adipose tissue, glucagon triggers lipoly-
sis, releasing fatty acids and glycerol. The fatty
acids are used as energy suppliers by many
types of tissue (with the exception of brain
and erythrocytes). An important recipient of
the fatty acids is the liver, which uses them for
ketogenesis.