Page 451 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Fluid and Electrolyte Disturbances In Gastrointestinal and Pancreatic Disease 439

concentration and stimulates pancreatic secretion in effects. Morphine also may cause increased muscular tone
dogs, suggesting that a positive feedback mechanism in the sphincter of Oddi and a similar effect also may
may be involved in the regulation of pancreatic secre- occur in the pancreatic duct. 135
tion. 122 Secretin is the principal mediator of pancreatic Pancreatic duct cells also produce intrinsic factor (IF),
fluid and electrolyte secretion (Figure 18-5) and is which is a protein necessary for the absorption of cobala-
6
released in response to acidification of the proximal small min (vitamin B 12 ). Dogs, unlike cats, also produce IF in
intestine. Secretin and cholecystokinin have synergistic the stomach. 5,6,39,116 Although the IF that originates in
effects on fluid and electrolyte secretion. Bicarbonate is the stomach is considered partially responsible for
5
responsible for solubilizing zymogens within the pancre- mediating the absorption of cobalamin, the amount of
atic ductular system and neutralizing gastric acid in the IF produced by the stomach of dogs is considered mini-
duodenum to provide an optimal pH for pancreatic mal compared with the amount produced by the pancreas
enzyme activity. Feeding is the most important factor (possibly fivefold to tenfold less). 6
controlling pancreatic secretions. Some drugs (e.g., mor- Classically, the pancreatic response to a meal has been
phine) have been implicated in halting pancreatic bicar- divided into cephalic, gastric, and intestinal phases.
bonate secretion and protein response to feeding. 64 During normal feeding conditions, these phases overlap
Acetylcholine inhibition due to morphine administration and occur simultaneously, but the intestinal phase appears
has been one of the mechanisms implicated in these to be quantitatively most important. Pancreatic secretion
occurs not only in response to a meal but also cyclically
throughout the day. Peaks in interdigestive secretion
Na
160 HCO 3 are accompanied by an increase in biliary secretion and
Ion concentration (mmol/L) 80 Cl housekeeping function by flushing digestive products,
intestinal motility. These cycles are thought to be
mediated by motilin and may serve an intestinal
120
cell debris, and bacteria along the intestinal tract. Inhibi-
tion of exocrine pancreatic secretion has not been studied
as extensively as stimulation, but glucagon and somato-
40
statin appear to decrease pancreatic secretion.
0
0 4 8 12 16 K 20 Intestine
Figure 18-4 Ionic composition of pancreatic juice secreted at Net absorption of fluid and electrolytes in the intestine
different flow rates in the anesthetized cat in response to secretin. reflects a balance between absorption and secretion,
(From Argent BE, Case RM. Pancreatic ducts: cellular mechanism and the final outcome in the healthy intestine represents
and control of bicarbonate. In: Johnson LR, editor. Physiology of the a victory for absorption. The ability of the intestine to
gastrointestinal tract, 3rd ed. New York: Raven Press, 1994: 1473.) absorb fluid and electrolytes varies according to site.
In a 20-kg dog, approximately 2.70 L of fluid (oral

16 1.0
14 0.8
Volume of secretion (mL) 10 Trypsinogen (mU) 0.6
12

8
0.4
6
4
2 0.2
0 0.0
CCK-8 SEC CCK-8&SEC CCK-8 SEC CCK-8&SEC
Figure 18-5 Output of trypsinogen and fluid in pancreatic juice during intravenous infusion of
cholecystokinin (CCK-8), secretin (Sec), and CCK-8 and Sec together. Data (mean SE) for eight dogs are
expressed as total output per 15 minutes during 45-minute infusion periods. Order of secretagogues was
varied, and there was a 15-minute rest period between secretagogues. (From Simpson KW, Alpers DH,
DeWille J, et al. Cellular localization and hormonal regulation of pancreatic intrinsic factor secretion in dogs.
Am J Physiol 1993;265:G178–G188.)