Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

422
Chapter | 14 Gastrointestinal Function
TABLE 14-5 Gastrointestinal Peptide Hormones
Hormone Source Action
Gastrin G cells of pyloric antrum Gastric acid secretion
Secretin S cells of duodenum and jejunum Pancreatic fluid and HCO 3
secretion, bile secretion
Cholecystokinin
(pancreozymin)
Somatostatin
Enteroglucagon
Gastric inhibitory
polypeptide
Duodenal and jejunal mucosa;
myenteric plexus
D cells of pancreas, CNS, gastric
and intestinal mucosa
L cells of small intestine, canine
stomach
Duodenal and jejunal mucosa
Pancreatic enzyme secretion, gallbladder contraction, and
sphincter of Oddi relaxation
Inhibits effect of gastrin on gastric secretion, inhibits
pancreatic enzyme secretion, stimulates ileal water and NaCl
Control of intestinal cell growth
Inhibits gastric secretion and stimulates intestinal secretion
Motilin Upper small intestinal mucosa Stimulates gastrointestinal motility
Stimulation of the vagus nerve causes a significant rise
in pancreatic enzyme concentration. This type of response
also is produced by cholecystokinin (pancreozymin), another
polypeptide hormone produced by the duodenal mucosa,
which also causes contraction of the gallbladder. The
C-terminal pentapeptide of cholecystokinin-pancreozymin
is exactly the same as that of gastrin. This fascinating relationship
suggests that gastrin and cholecystokinin-pancreozymin
participate in some integrated yet poorly understood
system of digestive control.
Several molecular forms of cholecystokinin (CCK)
exist ( Ward and Washabau, 2005 ). CCK-33, CCK-39, and
CCK-59 are the predominant forms that account for most
of the gastrointestinal hormone responses. Endocrine cells
in the duodenum and jejunum secrete CCK in response
to intraduodenal fatty acids, amino acids, and H ion.
CCK-8 is particularly important in cats. Intraluminal distension
will activate CCK-8 containing neurons, resulting
in acetylcholine release from the myenteric plexus and subsequent
peristaltic reflexes in the ileum and colon. CCK-8
neurons in the brain are involved in mediating the satiety
response following eating.
VI . OTHER GASTROINTESTINAL
HORMONES
A large number of polypeptides have been isolated from
the gastrointestinal mucosa and have been classified as
gut hormones ( Table 14-5 ). Some of these substances have
not yet met all the rigid physiological requirements of true
hormones. Some may have paracrine rather than endocrine
activities—that is, their actions are on cells and tissues in
the immediate vicinity of the cells of origin rather than
being released into the vascular system.
A . Motilin
Motilin is a polypeptide containing 22 amino acids that
was originally isolated from porcine duodenal mucosa
( Brown et al., 1971 ). The amino acid composition and
sequence have been described ( Brown et al., 1972, 1973 ).
Immunoreactive motilin has been found in the enterochromaffin
cells of the duodenum and jejunum of several
species ( Polak et al., 1975 ), and, by means of radioimmunoassay,
motilin has been identified in the plasma of dogs
( Dryburgh and Brown, 1975 ). Motilin has been shown to
stimulate pepsin output and motor activity of the stomach
( Brown et al., 1971 ) and to induce lower esophageal
sphincter contractions ( Jennewein et al., 1975 ). Studies by
Itoh et al. (1978) suggest that motilin plays an important
role in initiating interdigestive gastrointestinal contractions,
which are referred to as the interdigestive motility
complex or the migrating motility complex (MMC).
The cyclic release of motilin from the intestinal mucosa
coordinates gastric, pancreatic, and biliary secretions
with phase III of the MMC ( Ward and Washabau, 2005 ).
Erythromycin has been shown to induce an MMC similar
to motilin and, along with other macrolide-like antibiotics,
might be useful in selected cases with motility disorders.
B . Somatostatin
Somatostatin, which is named for its activity of inhibitory
release of growth hormone from the pituitary gland, has
been purified from ovine and bovine hypothalamus. The
hypothalamic hormone is composed of 14 amino acids.
Somatostatin also has been demonstrated in the stomach,
pancreas, and intestinal mucosa in concentrations higher
than in the brain ( Pearse et al., 1977 ). Somatostatin from
porcine intestine has been isolated and sequenced, and it