Secretions of the GI tract and the pancreas Flashcards
What salivary gland is the largest and has only serous cells and makes 25% of the saliva and has AMYLASE?
Parotid glands
Which glands are the mixed glands that contain both serous and mucous cells, secrete glycoprotein, and secrete most of the saliva?
Submaxillary and sublingal glands
Where does blood supply come from in the salivary glands?
from the branches of the external carotid artery
What cells secrete the initial saliva?
the acinar cells
Which cells have motile extensions, contract to eject saliva, and contain an actinomycin?
myoepithelial cells
What contains myoepithelial cels and has saliva that is similar in ionic composition to plasma?
the intercalated duct
What is lined by columnar epithelial (ductal) cells that modify the inital saliva to produce the final hypotonic saliva?
Striated duct
Is saliva a simple ultra filtrate of plasma?
no
What are the electrolyte conentrations in saliva?
high K and HCO3
low Na and Cl
What are the two main steps int he formation of slaiva?
- formation of isotonic, plasma-like solution by acinar cells
- modification of the isotonic solution by the ductal cells
What is the combined action at the salivary gland?
absorption of Na and Cl and secretion of K and HCO3
*net absorption of solute** more NaCl absorbed than KHCO3 secreted
How does HCO3 leave the cell?
through CFTR channels or the cl HCO3 exchanger both at the apical membrane
What do CF patients lack?
the CFTR (cAMP gated)
How does saliva become hypotonic as it flows through the ducts?
the ductal cells are water impermeable
What is the concentration of saliva like at a high flow rate
like plasma or initial saliva
Saliva at a low flow rate concentrations?
low NaCl and high K
What does not apply to HCO3 in the saliva duct?
the contact time thing….. HCO3 is always pretty high in the saliva
Where does the sympathetic innervation for the saliva gland go?
the periarterial spaces
How does vasopressin and aldosteron modif the composition of saliva?
by decreasing its Na and increasing K concentrations
describe the parasympathetic pathway in saliva?
Ach binds mAchR which activates IP3 and Ca2+
describe the sypmathetic pathway of saliva things?
NE binds B adrengergics and makes cAMP
*both ps and symp stimulate the salivary cells
Which thing dominates, parasympathetic or sympathetic?
Parasympathetic always
What are 2 unusual feature in the regulation of salivary secretion?
- it’s exculisvely under the control of the ANS
- it’s increased by BOTH parasymp and symp stimulation
What hydrolyzes all infested protein if HCl is absent?
pancreatic enzymes
What are the components of gastric juice?
HCl, pepsingoen, mucus, intrinsic factor, H2O
What is intrinsic factor needed for?
the absorption of vit. B12 in the ileum
Describe the oxyntic gland?
located in the prox 80% of stomach
secretes acid
Describethe pylroic gland?
in the antrum
synthesizes and releases GASTRIN
What does Gastrin do?
goes into the circulation and goes to parietal cells and inhibits them from making more IF and HCl
What do mucus cells do?
secrete mucus, HCO3 , and pepsinogen
What is th main function of low gastric pH?
to convert pepsinogen to pepsin
Where is Hcl formed?
at the villus-like membrane of the canaliculi which are continuous with the lumen
How is H+ secreted into the gastric lumen?
the K/H antiporter
- remember that Cl will follow H into the lumen
- result is net secretino of Hcl and net absorption of HCO3
What does histamine do?
stimulates Hcl secretion
- released from ECL cells
- binds to H2 receptors on parietal cells (cimetidine blocks this)
What stimulates histamine release?
Gastrin and Ach
Which regulatory thing of Hcl secretion is blocked by atropine?
Ach the neurotransmitter
What does Ach even do?
- stimulates HCl secretion
- released from vagus n.
- binds to M3 mAchR on parietal cells
- atropine blocks it
- IP3/ Ca cascade
- indirectly stimulates ECL cells (they release histamine)
What does gastrin do?
- hormone
- stimulates HCl secretion
- secreted by G cells in the antrum and delivered back to stomach via circulation
- stimulates ECL cells which release histamine
What does gastrin bind?
CCKB receptors on parietal cells—>IP3/Ca2+
-CCKA is specific for CCK remember
What stimulates gastrin release?
gastric distension and presence of small peptides and aa’s and stimulation of the vagus N.
Does atorpine block the vagal stimulation of G cells?
NO!
What does Somatostatin do?
inhibits HCL secretion
-released from D cells in the antrum
What does somatostatin bind?
SSTR2 on parietal cells
-inhibits AC so no H+ secretion
What is potentiation?
when the combined response of two stimulants exceeds the sum of their individual responses
What were the examples of potentiation that we talked about?
histamine potentiates action sof Ach and gastrin
Ach potentiates the actions of histamine and gastrin
What does omeprazole do?
blocks the H+/K+ ATPase
What does Cimetidine do?
antagonist of H2 receptors, used to treat duodenal and gastric ulcers and GERD
As pH falls, what happens to gastrin release?
it gets inhibited
-passive feedback mechanism
What are the 2 different pathways that the vagus nerve can go in the stomach?
- release Ach onto parietal cells
- release GRP onto G-cells (atropine doesn’t block)
- remember that distension will also cause Ach to be released onto G-cells in the antrum
What is the cephalic phase of Gastric secretion?
30-% ofHcl secreted because of this
- just a response to a meal
- vagus n. to parietal cell
- vagus n. to gastrin to parietal cell
What would abolish the cephalic phase of gastric secretion?
A Vagotomy
What is the gastric phase of secretion?
60% of HCl made
- stim is distension of stomach and presence of aa’s and stuff
- we get the same stuff happening as in cephalic, but with distention and aa’s being a local reflex (gastrin to parietal cell
What is the intestine phase of secretion?
10% of HCl secreted
- distention of SI- stimulates acid secretion
- digested protein stimulates INTESTINAL G cell which to release gastrin to get tot he parietal cells
What does gastric juice composition depend on?
the secretion rate
What is the final juice like at low secretion rates?
a solution of NaCl
What happens at higher secretion rates for gastric juice?
the concentration of Na decreases and that of H+ increases
-at peak rates, it’s mainly HCl
What does the vagus nerve do to a SST cell?
inhibits it so that we can make some acid
What in the gastric lumen will stimulate the release of SST?
H+
-feedback
What secretes pepsinogen?
chief cells and mucus cells in the oxyntic glands
What nerve stimulation is the most imporatn stimulus for pepsinogen secretion?
vagus n.
What can pepsin do?
digest proteins and convert more pepsinogen to pepsin
What is the optimal pH of pepsin?
- 8-3.5
- the pH of the lumen is normally like 6
What is IF required for?
absorption of Vit. B12 in the ileum
*this is the only secretion by the stomach that is required
What does failure to secrete IF lead to
pernicious anemia
-but it won’t show up for a while because the liver stores a shit ton of B12
What protects the gastric mucosa?
HCO3, mucus, PG’s, mucosal blood flow, and growth factors
What damages the gastric mucosa?
acid, pepsin, NSAIDs, Helicobacter pylori, aspirin, alcohol, bile, and stress
Describe a gastric ulcer
forms primarily because of the mucosal barrier is defective
-the bacteria H pylori releases cytotoxins that breakdown the mucosal barrier and underlying cells
What enzyme allows the bacteria to colonize the gastric mucosa?
urease
- converts urea to NH3, which alkalinizes the local environment
- this is used for a diagnostic test
Describe a duodenal ulcer
- more common than gastric
- forms because H+ SECRETORY RATES ARE HIGHER THAN NORMAL
- role of H. pylori is indirect
- When the acid gets to the duodenum, it overwhelms the buffer capacity of HCO3 in pancreatic juice
What is the role of H. Pylori in the duodenal ulcer?
- inhibits SST secretion from D cells (inhibition of inhibition)
- it also spreads to the duodenum and inhibits duodenal HCO3 secretion
What is Zollinger ellison syndrome?
when acid secretory rates are the highest
- tumor usually in the pancreas that secretes gastrin like crazy
- low duodenal pH inactivates pancreatic lipases which results in Steatorrhea*
What are the treatment options for Zollinger-Ellison syndrome?
Cimetidine, omeprazole, and surgical removal of the tumor
What are the 2 main components of the exocrine pancreas’s secretion?
- aqueous solution containing HCO3
- enzymatic secretion
What cells of the pancreas secret the enzymatic portion?
Acinar cells
What cells of the pancreas secrete the HCO3 aqueous solution?
Ductal and centroacinar cells
Where do the sympathetic nerves to the pancreas come from?
the celiac and superior mesenteric plexuses
Where do the parasympathetic nerves to the pancreas come from?
the vagus nerve
- synapse in ENS
- postganglionic synpases on exocrin pancreas
What is the main difference between salivary glands and exocrine pancreas glands when it comes to innervation?
- in salivary glands, everything stimulates saliva production
- in pancreas, psymp stimulates it and sympathetic inhibits secretion
Which enzymes are secreted as active forms?
pancreatic amylases and lipases
Which enzymes are secreted as inactive forms?
pancreatic proteases
-convert to active forms in the lumen of the duodenum
What kind of tonic is the secretion of the pancreatic duct?
isotonic
How doe ductal cells modify the pancreatic secretion?
-net secretion of HCO3 and net absorption of H+
How does flow rate affect the pancreatic juice?
- high flow rate: HCO3 is highest and Cl is lowest
- Low flow rate: other way around
- Na is always high and K is always low
What maintains the reciprocal relationship of HCO3 and Cl- ions?
the HCO3/Cl- exchanger in the apical membrane of ductal cells
What mainly comes out of the cephalic and gastric phase of pancreatic secretion?
enzymatic stuff
What comes out of the intestinal phase of pancreatic secretion?
both enzymatic and aqueous secretions
-80% of pancreatic secretion
Where does CCK come from in the duodenum?
I cells
Where does secretin come from in the duodenum?
S cells
How does acute pancreatitis occur?
when pancreatic enzymes are activated in pancreatic tissue rather than in the lumen of the intestin
-results in autodigestion of pancreatic tissue
