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

III. Gastric Secretions
415
4 . Lipase
Lingual lipase is secreted by Von Ebner’s gland of the
tongue and is important in the digestive processes of the
human newborn, rats, and preruminant calves ( Cook
et al., 1994 ; Plucinski et al., 1979 ).
C . Functions of Saliva
Saliva continuously bathes the oral cavity, which protects
the surface epithelium. Ingested food is moistened and lubricated
by saliva, thereby facilitating mastication and swallowing.
Saliva also protects teeth from decay by washing food
particles from the surfaces of the teeth and using its buffering
capacity to neutralize the organic acids produced by bacteria
normally present in the mouth. Saliva is necessary for vocalization,
and, in some species that groom themselves, saliva
promotes cooling as it evaporates. Additionally, it may be a
source of pheromones. Salivary glands contain large numbers
of growth factors, vasoactive serine proteases, and regulatory
peptides ( Cook et al., 1994 ). There is reason to believe that
these glandular constituents affect a wide range of biological
functions not necessarily limited to the alimentary system.
Ruminants produce much greater quantities of saliva
than do simple-stomached animals, and their saliva has a
higher pH and bicarbonate ion concentration. In ruminants,
saliva serves several unique functions ( Phillipson, 1977 ). It
is required for maintenance of the fluid composition of the
contents of the rumen. The great buffering capacity of ruminant
saliva is necessary to neutralize the large amounts of
organic acids that are end products of rumen fermentation.
Rumen bacteria for protein synthesis can utilize the
urea in saliva. Protein synthesized in the rumen is then
used to meet dietary protein requirements. In this way, urea
nitrogen can be “ recycled ” through the amino acid pool of
the body, and in ruminants it need not be considered an
end stage in protein catabolism. The ability to reutilize
urea has also been demonstrated in the horse, and this may
be of particular benefit during periods of protein deficiency
( Houpt and Houpt, 1971 ).
III . GASTRIC SECRETIONS
The stomach is divided into two main regions on the basis
of secretory function. The oxyntic gland area corresponds
approximately to the body of the stomach in most species
of domestic animals and also to the fundus in the dog
and cat. The oxyntic glands contain (1) oxyntic or parietal
cells that produce hydrochloric (HCl) acid, (2) peptic
(zymogenic, chief) cells that produce pepsinogen, and (3)
mucous cells. The pyloric gland area contains mucus-producing
pyloric glands whose secretion is slightly alkaline.
This area also contains the G cells, which produce the
polypeptide hormone, gastrin.
A . Composition of Gastric Secretions
1 . Basal versus Stimulated Secretion
There are two components of gastric secretion. The surface
epithelial cells and other mucus-producing cells continuously
secrete the basal component. This component is neutral
or slightly alkaline pH. The electrolyte composition
is similar to that of an ultrafiltrate of plasma ( Table 14-2 ).
The basal secretion contains large amounts of mucus,
which has a cytoprotective effect on the epithelium. The
secretory component produced by the oxyntic gland cells
in response to stimulation contains free HCl and pepsinogen,
the principal enzyme of gastric digestion.
The composition of gastric juice depends on the relative
amounts of the basal and secretory components in the
juice and, in turn, is a function of the flow rate of each. In
the dog, gastric juice is produced in the resting state at a rate
of approximately 5 ml/h. The composition is similar to that
of the basal component, containing practically no peptic
activity or HCl. When the flow of gastric juice is stimulated
maximally, the dog may produce 80 ml or more per hour
of a secretion containing large amounts of peptic activity
and HCl. Na , the principal cation in the basal secretion, is
replaced to a large extent by H ion. The concentration of K
is similar in both basal and stimulated secretions and, therefore,
remains relatively constant at the various rates of flow.
HCl and pepsinogen are secreted by separate mechanisms,
but their production appears closely linked under
physiological conditions. Stimulation of the vagus nerve or
intravenous injection of gastrin increases pepsinogen and
HCl levels together. Other stimuli may affect the two processes
differently; for example, in the dog histamine infusion
stimulates HCl production maximally but appears to
inhibit pepsinogen secretion ( Emas and Grossman, 1967 ).
TABLE 14-2 Composition of Parietal and Nonparietal
Secretions of Canine Gastric Mucos a
Component
Parietal
Secretion a
(mmol/liter)
Nonparietal
Secretion a
(mmol/liter)
Na — 155.0 138.0
H 159.0 — —
K 7.4 7.4 4.0
Ca 2 — 3.7 5.0
Cl 166.0 133.0 117.0
pH 1.0 7.54 c 7.42
Nonparietal
Secretion b
(mmol/liter)
a
Determined in vivo using dogs with gastric fi stulas ( Gray and Bucher, 1941 ).
b
Determined in vitro with isolated gastric mucosa ( Altamirano, 1963 ).
c
Calculated from bicarbonate concentration assuming pCO 2 of 40 Torr.