Physiology 3120 Lecture Notes - Pancreatic Duct, Pancreatic Juice, Acinus

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Published on 26 Nov 2011
Human Physiology
Friday, March 26, 2010
“Gastro IV”
Down-regulation of gastric acid secretion
Very close to the secretory cells of the pyloric gland area is another cell that releases somatostatin
(normally released by pancreas, but has a somewhat ubiquitous secretion)
Also under cholinergic control, but ACh inhibits somatostatin release
Somatostatin is inhibitory to acid secretion
As stimuli that activate cholinergic mechanisms, the cholinergic nerves are not activated, and have less
ACh release
Less release of acid (from oxyntic) and gastrin (from G cell)
Somatostatin is disinhibited; and inhibits the release of gastrin
Eliminate the potentiated response of acid secretion
Acid itself has a stimulatory effect on somatostatin release; excess acid increases somatostatin,
which decreases gastrin and therefore turns down acid secretion
But, you need acid for other things (i.e. intrinsic factor, pepsinogen pepsin), so having it immediately
turn itself off would be detrimental
The somatostatin cell is pH-sensitive
pH in stomach before eating is somewhere around 4-6 (foods such as proteins or carbs
are relatively good buffers)
Eventually, so much acid is secreted that the buffering capacity is overwhelmed and the
pH begins to drop; eventually, you reach the pH switch (between 2.4-2.7) that turns on
the somatostatin cell, which inhibits gastrin
Antrum = archaic term for pyloric gland area
Pancreatic Secretion
Pancreas composed of exocrine (95%) and endocrine tissues
Empties into the duodenum (the proximal part of small intestine)
Stomach is able to resist its own acid; but the mucosa of any other part of the GI tract is very
sensitive to acid/fragile
2 major functions of exocrine pancreas
a) Production & secretion of digestive enzymes (i.e. proteins, carbohydrates, and fats)
b) Secrete a bicarbonate-rich juice; why is bicarbonate important?
Buffer for intestines; need to protect mucosa from ulceration by acid
Restore pH to the optimal range for the digestive enzymes from point (a) above
Structurally very similar to the salivon (i.e. acinus with converging ducts)
Does not have a striated duct system; epithelial cells all along the duct look exactly the same
Different division of labour
In salivon, everything originated in acinus (juice & organic compounds); in pancreatic
duct, acinar cells responsible exclusively for formation/secretion of digestive enzymes,
and the ductular cells are responsible for the inorganic components, especially HCO3
Differentiate ductal cells by their proximity to the surrounding sheath
All the tissue within the sheath is intralobar tissue; anything outside the sheath is
extralobar tissue
Intralobar ductal cell resting HCO3 secretion
Extralobar ductal cell stimulated HCO3 secretion
Resting HCO3 secretion
Specific HCO3 ATPase in the intralobar ductal cells; they are unregulated, so they function
regardless of whether the system is stimulated or resting (albeit at a low rate); pump HCO3 into
lumen; concentration gets relatively high
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