with gut tissues. The presence of acid in
the duodenum sensitizes the mucosa to
release the hormone, secretin. Secretin
stimulates the exocrine pancreas to secrete
approximately isotonic bicarbonate solu-
tion into the lumen of the intestine. A pH-
neutral environment is essential to
maintain the activity of the digestive
enzymes present in the intestinal tract and
to allow the fragile surface necessary for
absorption. Peptides generated by gastric
digestion of dietary proteins or lipids are
necessary in the digesta to stimulate the
intestinal mucosa to release the hormone,
cholecystokinin (CCK). CCK stimulates the
exocrine pancreas to release digestive
enzymes, some in the form of zymogens,
into the lumen of the intestine. The
pancreatic secretions are carried to the
intestine via the pancreatic duct and
common bile duct. CCK also causes relaxa-
tion of the sphincter of Oddi, an area of
increased tonation where the common bile
duct enters the duodenum, and which
prevents backflux of intestinal contents
into the liver and pancreas.
Pancreatic amylase enters the intestinal
lumen in an active form where it mixes
with the digesta and continues catalysing
hydrolysis of the 1–4 glycosidic bonds in
starch and dextrins in a random manner as
initiated by salivary amylase. The end-
products of starch digestion by amylase are
maltose (disaccharide of glucose with 1–4
linkage), maltotriose (trisaccharide of
glucose with 1–4 linkage) and -limit
dextrins. The -limit dextrins have various
structures. They contain the 1–6 linkage
of the branch points of amylopectin. The
number of glucose units before and after
the branch point and the length of the
branch vary. -Amylase does not hydrolyse
the glucose unit immediately before or
until two glucose units after branch points
(Newsholme and Leech, 1989). The
minimum size, therefore, of -limit dextrin
is five glucose units, but the average size is
eight glucose units. No free glucose is
released by -amylase-catalysed hydrolysis
of starch (Alpers, 1987).
Completion of starch digestion is
catalysed by saccharidases attached to the
brush border of the small intestine. There
is some disagreement as to the relative
importance of the saccharidases (Gray et
al., 1979; Taraval et al., 1983; Rodriguez et
al., 1984), but for swine it appears that
-limit dextrinase is most active in
hydrolysing oligosaccharides. Limit dextrins
are hydrolysed to multiple units of glucose
and isomaltose (disaccharide of glucose
linked by a 1–6 bond). The isomaltose
then is hydrolysed to glucose by isomaltase,
although isomaltase could cleave the 1–6
bond first, generating substrate for maltase
and glucoamylase. Maltose and maltotriose
are hydrolysed to glucose by maltase.
Longer 1–4 oligosaccharides are hydro-
lysed one glucose unit at a time from the
non-reducing end by glucoamylase.
Dietary disaccharides are hydrolysed
by brush border disaccharidases. Sucrose,
plentiful in fruits and cell solubles, is
hydrolysed by sucrase to fructose and
glucose. Sucrose should not be included in
the diet of neonatal pigs and most other
animals because sucrase activity is not
expressed in most young animals (Kidder
and Manners, 1980). Lactose, the sugar
found in milk, is hydrolysed to galactose
and glucose by lactase. Trehalose, plentiful
in algae and fungi and the main circulating
carbohydrate of insects, is hydrolysed to
glucose by trehalase.
The pH optimum for these enzymes is
6.0. Intestinal pH is maintained near
neutral by the secretin–pancreas system
described above. The Kmof the enzymes
for sugars range from 1 to 20 mM(Alpers,
1987). The difference in Kms for -limit
dextrins between isomaltase and -limit
dextrinase may explain some of the contro-
versy concerning their importance. The
difference is about an order of magnitude,
so isomaltase activity may appear more
important when -limit dextrin is plentiful
and -limit dextrinase more important
when substrate is less plentiful.
With the exception of lactase in
mature animals and sucrase in young
animals, the capacity for digestion exceeds
the capacity for absorption. After a meal
containing disaccharides, monosaccharides
accumulate in the intestinal lumen (Alpers,126 R.W. Russell and S.A. Gahr