Chapter 16
The Endocrine System
619
16
1.
Catalyze the oxidation of glucose for ATP production
2.
Join glucose molecules together to form glycogen
3.
Convert glucose to fat (particularly in adipose tissue)
As a rule, energy needs are met first, followed by glycogen for-
mation. Finally, if excess glucose is still available, it is converted
to fat. Insulin also stimulates amino acid uptake and protein
synthesis in muscle tissue. In summary, insulin “sweeps” glu-
cose out of the blood, causing it to be used for energy or con-
verted to other forms (glycogen or fats); insulin also promotes
protein synthesis and fat storage.
Factors That Influence Insulin Release
Elevated blood glu-
cose levels are the chief factor that stimulates pancreatic beta
cells to secrete insulin. Other stimuli include rising blood levels
of amino acids and fatty acids, and release of acetylcholine by
parasympathetic nerve fibers. As body cells take up glucose and
Enhances membrane transport of glucose (and other simple
sugars) into most body cells, especially muscle and fat cells.
Inhibits the breakdown of glycogen to glucose.
Inhibits the conversion of amino acids or fats to glucose.
±ese inhibiting effects counter any metabolic activity that
would increase plasma levels of glucose.
Insulin is
not
needed for glucose entry into liver, kidney, and
brain tissue, all of which have easy access to blood glucose re-
gardless of insulin levels. However, insulin does have impor-
tant roles in the brain—it participates in neuronal development,
feeding behavior, and learning and memory.
Insulin activates its receptor (a tyrosine kinase enzyme), which
phosphorylates specific proteins, beginning the cascade that pro-
motes glucose uptake and insulin’s other effects. A²er glucose en-
ters a target cell, insulin binding triggers enzymatic activities that
BALANCE:
Normal blood glucose level (about 90 mg/100 ml)
IMBALANCE
IMBALANCE
Liver
Glycogen
Glucose
Glycogen
Glucose
Liver
Tissue cells
Stimulates glucose
uptake by cells
Stimulates
glycogen
formation
Pancreas
Pancreas
Insulin
Blood
glucose
falls to
normal
range.
Stimulates
glycogen
breakdown
Blood
glucose
rises to
normal
range.
Glucagon
Stimulus
Blood
glucose level
Stimulus
Blood
glucose level
Figure 16.19
Insulin and glucagon from the pancreas regulate blood glucose levels.
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