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chapter 12
Gastrointestinal Digestion and Absorption
esophagus is replaced not with the normal squamous cells
but with specialized columnar cells. Columnar epithelial
cells normally are found in the small intestine. Hence,
this process is known as intestinal metaplasia or
Barrett’s
esophagus
and is regarded as a premalignant condition.
Cholecystokinin
Cholecystokinin (CCK) is found throughout the small in-
testine but is located predominantly in the mucosal I cells
of the duodenum and jejunum. In the ileum and colon, it
is localized in nerve endings, and it is widely distributed
throughout the peripheral and central nervous systems.
CCK consists of 33 amino acid residues (CCK-33,
Table 12-2), and shows macro- and microheterogeneity.
Several other forms are known: CCK-58, CCK-39, and
CCK-
8
. Naturally occurring CCK has a sulfated tyrosyl
residue at position 27, and removal of sulfate changes
the biological activity to that of gastrin. The C-terminal
tetrapeptide is identical with that of gastrin. A syn-
thetic C-terminal octapeptide is about three times more
potent than CCK-33. Cerulein, a decapeptide present
in the skin and GI tract of certain amphibians, has a
C-terminal octapeptide sequence identical to that of CCK
(Table 12-2). These two peptides have similar biological
properties,
and cerulein
is clinically useful
for the
stimulation of gallbladder contraction. CCK is secreted
as a result of stimuli caused by the products of digestion
of proteins and lipids. The secretion of CCK is terminated
when these digestion products are absorbed or migrate
into the lower portions of the GI tract. The principal
physiological actions of CCK are to stimulate gallbladder
contraction, to relax the sphincter of Oddi, and to stimulate
secretion of pancreatic juice rich in digestive enzymes.
Other functions are stimulation of bicarbonate-rich fluid
secretion, insulin secretion, and intestinal motility. CCK
can induce satiety in laboratory animals and humans, the
gastric vagal fibers being necessary for this effect.
Secretin
Secretin is synthesized by the S cells of the duode-
num and jejunum and is also present in the brain. Its
amino acid sequence is similar to that of glucagon, VIP,
and GIP (Table 12-3). All 27 amino acid residues are
required for biological activity. Chyme (pH < 4.5) in
the duodenum stimulates release of secretin. Secretin
stimulates the secretion of pancreatic juice rich in bi-
carbonate, which neutralizes the acid chyme and in-
hibits further secretion of the hormone. This action
appears to be mediated by membrane-bound adenylate
F o o d in th e sto m ach
I
Z
G astrin s ec re tio n
(pH ~ 1.5)
In c re a se d acid a n d » .
p e p sin sec re tio n
In c re a se d m otility
a n d blood flow
P artially d ig e ste d food
a n d a cid into d u o d e n u m
- - ■
C C K a n d
- - -
s e c re tin s ec re tio n
-----
1
,
G IP
1
Insulin
s e c re tio n
P a n c re a tic a n d biliary
s ec re tio n into th e
sm all in testin e
N eutralization of a d d
- a n d in testin al d ig estio n
a n d ab so rp tio n
FIGURE 12-6
Integrated function of gastrointestinal hormones in regulation of digestion
and absorption of food. Dashed arrows indicate inhibition.
cyclase and increased concentrations of intracellular
cAMP.
Gastric Inhibitory Peptide
Gastric inhibitory peptide (GIP) is found in the K cells of
the duodenum and jejunum: It consists of 43 amino acid
residues and occurs in multiple molecular forms. Its se-
cretion is stimulated by the presence of glucose and lipids
in the duodenum. It has two main functions:
1. To stimulate insulin secretion that prepares the
appropriate tissues for the transport and metabolism of
nutrients obtained from the GI tract (Chapter 22), and
2. To inhibit gastric secretion and motility.
The physiological functions of the GI hormones dis-
cussed above are integrated and coordinated to facilitate
the digestion and absorption of food (Figure 12-6).
12.3 Digestion and Absorption of
Major Food Substances
Carbohydrates
In most diets, carbohydrates are a major source of the
body’s energy requirements. The predominant digestible
carbohydrates are starches (amylose and amylopectin).
Depending upon the diet, the digestible carbohydrates may
