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

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Chapter | 14 Gastrointestinal Function
In those species that produce salivary amylase (e.g., pig
and human), the secretory granules are the zymogen
precursors of this enzyme. A third cell type is found lining
the striated ducts. The striations along the basal borders
of these cells are caused by vertical infoldings of the
cell membrane, a characteristic of epithelial cells involved
in rapid movement of water and electrolytes. The primary
secretion of the acinar cells is modified by active transport
processes of the ductal epithelium.
The distribution of the different types of secretory
cells in the salivary glands varies among species. The
parotid glands of most animals are serous glands, which
produce a secretion of low-specific gravity and osmolality
containing electrolytes and proteins including certain
hydrolytic enzymes. The mandibular (submaxillary) and
sublingual glands are mixed salivary glands that contain
both mucous and serous types of cells and produce a more
viscous secretion that contains large amounts of mucus
( Dukes, 1955 ).
B . Composition of Saliva
1 . Mucus
Mucus is an aqueous mixture of proteoglycans and glycoproteins.
One of the most completely studied glycoproteins
is mucin. Salivary mucins are O-glycosylated and consist
of peptides with many oligosaccharides linked covalently
to the hydroxyamino acid serine or threonine. The carbohydrate
portion of submaxillary mucin from sheep is a
disaccharide of N-acetylneuraminic acid (sialic acid) and
N-acetylgalactosamine ( Carlson et al., 1973 ). The enzymes
that link protein with hexosamine have been purified from
the mandibular glands of sheep ( Carlson et al., 1973 ) and
swine ( Schachter et al., 1971 ).
The physiological functions of mucin are related to its
high viscosity. N-acetylneuraminic acid is the component
responsible for the formation of viscous aqueous solutions
and, at physiological pH, causes expansion and stiffening of
the mucin molecule. The resistance of mucin to enzymatic
breakdown is also due to the presence of disaccharide residues.
Removal of terminal N-acetylneuraminic acid residues
by action of neuraminidase significantly increases the
susceptibility of peptide bonds to trypsin.
2 . Electrolytes
The principal inorganic constituents of saliva are sodium,
potassium, chloride, and bicarbonate, which, with the
exception of bicarbonate, originate directly from the plasma.
Rates of salivary flow vary depending on stimulation, and
there are wide variations in electrolyte concentration. Saliva
is formed by a process that initially requires uptake of
sodium and other electrolytes from the interstitium of the
terminal structural unit of the salivary gland, the acinus or
end piece. Water flows passively. This primary or precursor
fluid has a sodium concentration similar to plasma, and
the potassium concentration is similar to or slightly higher
than plasma. As the primary fluid passes from the acinus
along the duct system, the concentration of sodium, potassium,
and other electrolytes changes. In most species, there
is net sodium absorption and potassium secretion. Wide
variations in electrolyte composition may occur depending
on the flow rate ( Young and Schneyer, 1981 ), the salivary
gland of origin, and the species ( Table 14-1 ).
3 . Amylase
The saliva of rodents contains the α -amylase, ptyalin,
but this enzyme activity is absent in the saliva of dogs,
cats, horses, cattle, and sheep ( Dukes, 1955 ; Young and
Schneyer, 1981 ). Salivary amylase splits the α 1,4-glucosidic
bonds of various polysaccharides. Salivary amylase is
similar in major respects to pancreatic α -amylase, which is
described in Section V.B. Salivary amylase initiates digestion
of starch and glycogen in the mouths of those species
that secrete the enzyme. The optimal pH for amylase activity
is approximately 7, so that this activity ceases when the
enzyme mixes with acidic gastric contents.
TABLE 14-1 Electrolyte Concentration of Mandibular Gland and Parotid Gland Saliva Observed during
Maximum Rates of Secretion (mmol/l)
Mandibular Gland Parotid Gland
Na K HCO 3 Na K HCO 3
Sheep 20 7 23 160–175 9–10 113–140
Dog 70–100 12–15 10–30 80–110 6–14 50
Cat 40–51 9–10 26 — — —
Rabbit 50–100 10–40 25 110–140 10 12–30