PHY 503: GI EXAM STUDY GUIDE QUESTIONS WITH
COMPLETE SOLUTIONS
What gastrin does besides acid secretion -- Answer ✔✔ Besides acid secretion, gastrin
relaxes the ileocecal sphincter, increases pyloric sphincter tone, and reduces gastric
emptying by lowering stomach motility.
Somatostatin function, mechanism of action (GPCR), difference between SST from
antral D cells vs. from corpus D cells -- Answer ✔✔ Somatostatin is secreted from D
cells in response to antral acidification, and it inhibits further acid and gastrin release. To
do this, it binds to an inhibitory GPCR on the basolateral membrane of the parietal cell
and inhibits adenylyl cyclase to functionally antagonize histamine's stimulatory effect and
lead to a decrease in acid secretion.
SST in corpus D cells adjacent to parietal and ECL cells has paracrine action on these
parietal and ECL cells to decrease acid secretion and histamine release. SST in antral D
cells has endocrine action on corpus parietal cells to inhibit acid secretion and paracrine
action on neighboring G cells to inhibit gastrin secretion.
How atropine affects gastric acid secretion -- Answer ✔✔ Atropine blocks the M3
muscarinic AChR on parietal cells, and can reduce acid secretion that is done in this
manner. Atropine does not affect G cell secretion of gastrin or ECL cell secretion of
histamine, so these mechanisms can still function to secrete gastrin and acid.
, Events that increase acid secretion due to vagal stimulation -- Answer ✔✔ Vagal
postganglionic nerves in the corpus release ACh, which acts on mAChRs and directly
stimulates secretion of acid. ACh also triggers histamine release from ECL cells to
stimulate acid secretion. Peptidergic postganglionic vagal neurons and other enteric
neurons release GRP, which stimulates gastrin release and will ultimately result in acid
secretion. The vagus nerve also inhibits SST release from D cells to reduce any
background inhibition of gastrin release in the corpus or antrum.
Why the stomach pH needs to drop after a meal -- Answer ✔✔ When a meal is
ingested, the stomach pH must drop because pepsinogens are only activated when the
pH falls below 3. The pH drop activates pepsin and also ensures that any bacteria that
may have been consumed are not able to survive in the stomach.
How H2 blockers work -- Answer ✔✔ H2 blockers attenuate the secretory responses
to both ACh and gastrin by blocking the H2 receptor on the basolateral membrane of
the parietal cell. This reduces acid secretion by this mechanism, but does not have a
specific effect on the direct activity of gastrin or ACh on acid secretion.
What the salivary ductal cells absorb and secrete -- Answer ✔✔ The salivary ductal cells
actively absorb Na+, passively absorb Cl-, actively secrete K+, and also secrete HCO3-.
Hypergastrinemia from long-term acid suppression -- Answer ✔✔ Hypergastrinemia can
result from long-term suppression of acid secretion. Gastrin is normally inhibited by the
presence of acid in the antrum, so when acid secretion is continually suppressed, serum
gastrin will be chronically high to continue trying to stimulate acid secretion. The
negative feedback mechanism is not working properly in this situation to stop gastrin
secretion.
COMPLETE SOLUTIONS
What gastrin does besides acid secretion -- Answer ✔✔ Besides acid secretion, gastrin
relaxes the ileocecal sphincter, increases pyloric sphincter tone, and reduces gastric
emptying by lowering stomach motility.
Somatostatin function, mechanism of action (GPCR), difference between SST from
antral D cells vs. from corpus D cells -- Answer ✔✔ Somatostatin is secreted from D
cells in response to antral acidification, and it inhibits further acid and gastrin release. To
do this, it binds to an inhibitory GPCR on the basolateral membrane of the parietal cell
and inhibits adenylyl cyclase to functionally antagonize histamine's stimulatory effect and
lead to a decrease in acid secretion.
SST in corpus D cells adjacent to parietal and ECL cells has paracrine action on these
parietal and ECL cells to decrease acid secretion and histamine release. SST in antral D
cells has endocrine action on corpus parietal cells to inhibit acid secretion and paracrine
action on neighboring G cells to inhibit gastrin secretion.
How atropine affects gastric acid secretion -- Answer ✔✔ Atropine blocks the M3
muscarinic AChR on parietal cells, and can reduce acid secretion that is done in this
manner. Atropine does not affect G cell secretion of gastrin or ECL cell secretion of
histamine, so these mechanisms can still function to secrete gastrin and acid.
, Events that increase acid secretion due to vagal stimulation -- Answer ✔✔ Vagal
postganglionic nerves in the corpus release ACh, which acts on mAChRs and directly
stimulates secretion of acid. ACh also triggers histamine release from ECL cells to
stimulate acid secretion. Peptidergic postganglionic vagal neurons and other enteric
neurons release GRP, which stimulates gastrin release and will ultimately result in acid
secretion. The vagus nerve also inhibits SST release from D cells to reduce any
background inhibition of gastrin release in the corpus or antrum.
Why the stomach pH needs to drop after a meal -- Answer ✔✔ When a meal is
ingested, the stomach pH must drop because pepsinogens are only activated when the
pH falls below 3. The pH drop activates pepsin and also ensures that any bacteria that
may have been consumed are not able to survive in the stomach.
How H2 blockers work -- Answer ✔✔ H2 blockers attenuate the secretory responses
to both ACh and gastrin by blocking the H2 receptor on the basolateral membrane of
the parietal cell. This reduces acid secretion by this mechanism, but does not have a
specific effect on the direct activity of gastrin or ACh on acid secretion.
What the salivary ductal cells absorb and secrete -- Answer ✔✔ The salivary ductal cells
actively absorb Na+, passively absorb Cl-, actively secrete K+, and also secrete HCO3-.
Hypergastrinemia from long-term acid suppression -- Answer ✔✔ Hypergastrinemia can
result from long-term suppression of acid secretion. Gastrin is normally inhibited by the
presence of acid in the antrum, so when acid secretion is continually suppressed, serum
gastrin will be chronically high to continue trying to stimulate acid secretion. The
negative feedback mechanism is not working properly in this situation to stop gastrin
secretion.
