Regulation and Integration of the Body
Located partially behind the stomach in the abdomen, the sof,
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.
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-
islets of Langerhans
), tiny cell
clusters that produce pancreatic hormones
islets contain two major populations oF hormone-producing
cells, the glucagon-synthesizing
and the more
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
hormone, whereas insulin
. Some islet cells also
synthesize other peptides in small amounts, including
statin, pancreatic polypeptide
), and others. However, here we
will Focus on glucagon and insulin.
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-
Breakdown oF glycogen to glucose (
) (±igure 16.19)
Synthesis oF glucose From lactic acid and From noncarbohy-
drate molecules (
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
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.
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
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.
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
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
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.
Compare and contrast the effects of the two major
Photomicrograph of differentially stained
A pancreatic islet is surrounded by acinar cells,
which produce the exocrine product (enzyme-rich pancreatic juice).
cells of the islets that produce insulin have cytoplasm that
stains pale pink, and the
cells that produce glucagon have bright
pink cytoplasm (190