Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

VI. Insulin and Carbohydrate Metabolism
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Blood glucose is the primary regulator of both insulin
release and its biosynthesis. This is a highly selective process,
and only insulin, C-peptide, and proinsulin are released
and released rapidly. The insulin response curve to a glucose
load (IVGTT) exhibits 2 peaks in humans, the early 5-min
peak representing release and the second 10- to 30-min
peak representing de novo insulin synthesis and release.
This bimodal curve is not clear in dogs (see Fig. 3-14 , presented
later) but it is likely to occur.
During proinsulin hydrolysis, C-peptide also accumulates
in the granules. Therefore, when the granule contents
are released by glucose stimulation, insulin, C-peptide, and
proinsulin all appear in plasma and each can be measured
by radioimmunoassay (RIA). Whereas studies in humans
have focused on all three, in animals, the focus has been
on insulin, and little is known of proinsulin or C-peptide in
health or disease.
The influence of the various gastrointestinal hormones
on insulin secretion is of considerable interest because
plasma insulin levels are higher at a given plasma glucose
level after an oral glucose load as compared to an
intravenous load. The oral glucose tolerance test (OGTT)
is known to elicit larger total insulin response than the
intravenous glucose tolerance test (IVGTT). A number of
GI hormones are known to influence insulin secretion to
varying degrees and are sufficient to form an entero-insular
axis ( Buchanan, 1975 ). The hormones implicated are
secretin, cholecystokinin-pancreozymin (CCK-PZ), gastrin,
glucagon-like activity (GLA) of the gut, and gastric
inhibitory peptide (GIP). GIP is a powerful stimulator of
insulin secretion in humans and dogs and this is associated
with a rise in blood glucose ( Ross et al ., 1977 ) . Thus, GIP
is central to the entero-insular axis.
B . Insulin Transport
Insulin is transported in the circulation bound to a β -globulin.
At a tissue, insulin binds to receptors on the cell membrane.
The insulin receptor is a very large glycoprotein on
the surface of virtually all cells, including liver, kidney,
fat, muscle, erythrocytes, and monocytes. The receptor is a
posttranslational derivative of a gene product and is a tetramer
of 2 α and 2 β subunits. The internal β subunit of the
receptor anchors the receptor to the membrane. As a result,
insulin moves through the plasma membrane and into the
cytoplasmic compartment, but the mechanism is unclear.
All cells, in particular liver and kidney, are able to inactivate
insulin by reductive cleavage of the disulfide bonds.
Liver inactivates about 50% of the total insulin.
C . Glucose Transport
Insulin binding also activates receptors both on the plasma
membrane surface and in the cytoplasm. This activation
induces a variety of reactions—for example, phosphorylations—but
the details and their implications are not yet
known. However, the end result of these interactions—that
is, glucose transport across the membrane and into the cell—
is defined. Glucose transport proteins (glucose transporters
[GLUT-1-7]) are characterized. They are small membrane
proteins, 40 to 50kd, and the different transporters are distributed
in different cells; GLUT-1 is widely distributed
(brain, RBC, placenta, kidney), as are GLUT-2 (liver, pancreatic
β cells, mucosal cells), GLUT-3, (brain), and GLUT-4
(skeletal muscle, heart muscle, fat). GLUT-5 is in the
intestine, GLUT-6 is not available, and GLUT-7 is within
cell organelles ( Winter and Signorino, 2002 ). GLUT-4
is the only insulin-responsive glucose transporter, and for
this reason has been studied extensively. Insulin mobilizes
GLUT-4 to the membrane, thereby facilitating glucose
transport into the cell. Glucose transport activity was studied
in the erythrocytes of trained and untrained racehorses
(Arai et al ., 1994 ). Horses in training had glucose transport
activities 2 to 3.5 times greater than those of untrained
horses. The specific glucose transporter was not identified
but presumably is GLUT-1 as in other animals.
D . Insulin Action on Biochemical Systems
The principal sites of insulin action are in the initial phases
of glucose metabolism. Insulin first binds to insulin receptors
of the target cell plasma membranes and then facilitates
glucose entry into cells such as muscle and fat by
activation of glucose transporters, in this case GLUT-4.
There is also a high degree of stereo-specificity because
D-glucose is transported but L-glucose is not. With
increased accumulation of glucose in the cells, the movement
of glucose into the metabolic scheme is enhanced and
glucose utilization increases.
Insulin influences the metabolism of glucose by the
liver cells, the central organ of glucose homeostasis, but
with a slightly different focus. GLUT-2 is not significantly
regulated by insulin, so the liver cell is freely permeable
to glucose. Therefore, the major action of insulin in liver
is after the initial transport step. The principal step is the
first phosphorylation of glucose to form G-6-P in the reaction
catalyzed by glucokinase (GK). This GK reaction is
rate limiting and GK activity is influenced by insulin.
Additionally, the effect of insulin on other key unidirectional
phosphorylative steps directs glucose metabolism
toward utilization and FA synthesis. An important effect
of insulin is to increase the activity of the pyruvate dehydrogenase
(PD) system, which increases AcCoA, thereby
promoting increased FA synthesis and oxidation to CO 2
via the Krebs TCA cycle. These and other reactions are
described in Section VII.C. Thus, there are two major
roles for insulin, promoting (1) glucose transport across
the membranes of muscle and fat cells and (2) glucose