Page 295 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Carbohydrate Metabolism and glucose concentration as constant as possible
Pancreatic Hormones (! A); and (4) promote growth.
Glucose is the central energy carrier of the Insulin
human metabolism. The brain and red blood Synthesis. Insulin is a 6 kDa peptide (51 amino acids,
cells are fully glucose-dependent. The plasma AA) formed by the C chain cleaved from proinsulin
glucose concentration (blood sugar level) is (84 AA), the precursor of which is preproinsulin, a pre-
determined by the level of glucose production prohormone. Insulin contains two peptide chains (A
and consumption. and B) held together by disulfide bridges Degrada-
tion: Insulin has a half-life of about 5–8 min and is de-
graded mainly in liver and kidneys.
The following terms are important for proper under-
Hormones and Reproduction conversion of glucose to lactate (! p. 72). This oc- "! glucose oxidation "! cytosolic ATP
standing of carbohydrate metabolism (! A, C):
Secretion. Insulin is secreted in pulsatile
1. Glycolysis generally refers to the anaerobic
bursts, mainly in response to increases in the
blood levels of glucose (! B right), as fol-
curs in the red blood cells, renal medulla, and skeletal
lows: plasma glucose "! glucose in B cells
muscles (! p. 72). Aerobic oxidation of glucose oc-
curs in the CNS, heart, skeletal muscle and in most
other organs.
+
K
"! closure
channels
ATP-gated
of
2. Glycogenesis, i.e., the synthesis of glycogen
! depolarization ! opening of voltage-gated
from glucose (in liver and muscle), facilitates the
channels ! cytosolic Ca ". The rising
Ca
2+
2+
plasma glucose concentration. Glycogen stored in a
+
and (b) re-opening of K channels (deactivated
muscle can only be used by that muscle.
by feedback control). Stimulation. Insulin
3. Glycogenolysis is the breakdown of glycogen
11 storage of glucose and helps to maintain a constant Ca 2+ in B cells leads to (a) exocytosis of insulin
secretion is stimulated mainly during food
to glucose, i.e., the opposite of glycogenesis.
4. Gluconeogenesis is the production of glucose digestion via acetylcholine (vagus nerve),
(in liver and renal cortex) from non-sugar molecules gastrin, secretin, GIP (! p. 234) and GLP-1
such as amino acids (e.g., glutamine), lactate (pro- (glucagon-like peptide = enteroglucagon), a
duced by anaerobic glycolysis in muscles and red peptide that dissociates from intestinal pro-
cells), and glycerol (from lipolysis).
5. Lipolysis is the breakdown of triacylglycerols glucagon. Certain amino acids (especially ar-
into glycerol and free fatty acids. ginine and leucine), free fatty acids, many
6. Lipogenesis is the synthesis of triacylglycerols pituitary hormones and some steroid hor-
(for storage in fat depots). mones also increase insulin secretion. Inhibi-
tion. Epinephrine and norepinephrine (α 2-
Islets of Langerhans in the pancreas play a pri-
mary role in carbohydrate metabolism. Three adrenoceptors; ! A, B), SIH (! p. 273 B) and
cell types (A, B, D) have been identified so far the neuropeptide galanin inhibit insulin secre-
tion. When hypoglycemia occurs due, e.g., to
(! p. 273 B). 25% of all islet cells are type A (α) fasting or prolonged physical exercise, the low
cells that produce glucagon, 60% are B (") cells
that synthesize insulin, and 10% are D (δ) cells blood glucose concentration is sensed by cen-
that secrete somatostatin (SIH). These hor- tral chemosensors for glucose, leading to reflex
mones mutually influence the synthesis and activation of the sympathetic nervous system.
secretion of each other (! p. 273 B). Islet cells The insulin receptor is a heterotetramer (α 2" 2) con-
in the pancreas head synthesize pancreatic sisting of two extracellular α subunits and two trans-
polypeptide, the physiological function of membranous " subunits. The α subunits bind the
which is not yet clear. High concentrations of hormone. Once the " subunits are autophosphory-
these hormones reach the liver by way of the lated, they act as receptor tyrosine kinases that
phosphorylate insulin receptor substrate-1 (IRS-1). In-
portal venous circulation. tracellular proteins with SH2 domains are phospho-
Function. Pancreatic hormones (1) ensure rylated by IRS-1 and pass on the signal (! p. 277 C3).
that ingested food is stored as glycogen and fat
(insulin); (2) mobilize energy reserves in re- Action of insulin (! A, B, C). Insulin has ana-
sponse to food deprivation, physical activity or bolic and lipogenic effects, and promotes the
storage of glucose, especially in the liver, where
282 stress (glucagon and the non-pancreatic hor- it activates enzymes that promote glycolysis
mone epinephrine); (3) maintain the plasma
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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