618
UNIT 3
Regulation and Integration of the Body
16
Located partially behind the stomach in the abdomen, the sof,
tadpole-shaped
pancreas
is a mixed gland composed oF both
endocrine and exocrine gland cells (see ±igure 16.1). Along with
the thyroid and parathyroids, it develops as an outpocketing
oF the epithelial lining oF the gastrointestinal tract.
Acinar cells
,
Forming the bulk oF the gland, produce an enzyme-rich juice
that is carried by ducts to the small intestine during digestion.
Scattered among the acinar cells are approximately a mil-
lion
pancreatic islets
(also called
islets of Langerhans
), tiny cell
clusters that produce pancreatic hormones
(Figure 16.18)
. Te
islets contain two major populations oF hormone-producing
cells, the glucagon-synthesizing
alpha (
a
) cells
and the more
numerous insulin-synthesizing
beta (β) cells
. Tese cells act as
tiny Fuel sensors, secreting glucagon and insulin appropriately
during the Fasting and Fed states.
Insulin and glucagon are intimately but independently in-
volved in regulating blood glucose levels. Teir effects are antag-
onistic: Glucagon is a
hyperglycemic
hormone, whereas insulin
is a
hypoglycemic
hormone
(Figure 16.19)
. Some islet cells also
synthesize other peptides in small amounts, including
somato-
statin, pancreatic polypeptide
(
PP
), and others. However, here we
will Focus on glucagon and insulin.
Glucagon
Glucagon
(gloo
9
kah-gon), a 29-amino-acid polypeptide, is an
extremely potent hyperglycemic agent: One molecule can cause
the release oF 100 million glucose molecules into the blood! Te
major target oF glucagon is the liver, where it promotes the Fol-
lowing actions:
Breakdown oF glycogen to glucose (
glycogenolysis
) (±igure 16.19)
Synthesis oF glucose From lactic acid and From noncarbohy-
drate molecules (
gluconeogenesis
)
Release oF glucose to the blood by liver cells, causing blood
glucose levels to rise
A secondary effect is to lower blood levels oF amino acids as
the liver cells sequester these molecules to make new glucose
molecules.
Humoral stimuli, mainly Falling blood glucose levels, prompt
the alpha cells to secrete glucagon. However, sympathetic nervous
system stimulation and rising amino acid levels (as might Follow
a protein-rich meal) are also stimulatory. Glucagon release is sup-
pressed by rising blood glucose levels, insulin, and somatostatin.
Insulin
Insulin
is a small (51-amino-acid) protein consisting oF two
amino acid chains linked by disulfide (–S–S–) bonds. It is syn-
thesized as part oF a larger polypeptide chain called
proinsulin
.
Enzymes then excise the middle portion oF this chain, releasing
Functional insulin. Tis “clipping” process occurs in the secre-
tory vesicles just beFore the beta cell releases insulin.
Insulin’s effects are most obvious when we have just eaten. Its
main effect is to lower blood glucose levels (±igure 16.19), but it
also influences protein and Fat metabolism. Circulating insulin
lowers blood glucose levels in three ways. It
In children, melatonin may have an antigonadotropic effect.
In other words, it may affect the timing oF puberty and inhibit
precocious (too early) sexual maturation.
Te
suprachiasmatic nucleus
oF the hypothalamus, an area
reFerred to as our “biological clock,” is richly supplied with
melatonin receptors, and exposure to bright light (known to
suppress melatonin secretion) can reset the clock timing. As
a result, changing melatonin levels may influence rhythmic
variations in physiological processes such as body temperature,
sleep, and appetite.
Check Your Understanding
13.
Synthetic melatonin supplements are available, although
their safety and efficacy have not been proved. What do you
think they might be used for?
For answers, see Appendix H.
Other Endocrine Glands
and Tissues
So Far, we’ve examined the endocrine role oF the hypothalamus
and oF glands dedicated solely to endocrine Function. We will
now consider a set oF organs that contain endocrine tissue but
also have other major Functions. Tese include the pancreas,
gonads, and placenta.
The Pancreas
Compare and contrast the effects of the two major
pancreatic hormones.
Pancreatic acinar
cells (exocrine)
Pancreatic islet
α
(Glucagon-
producing)
cells
β
(Insulin-
producing)
cells
Figure 16.18
Photomicrograph of differentially stained
pancreatic tissue.
A pancreatic islet is surrounded by acinar cells,
which produce the exocrine product (enzyme-rich pancreatic juice).
The
cells of the islets that produce insulin have cytoplasm that
stains pale pink, and the
cells that produce glucagon have bright
pink cytoplasm (190
3
).
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