GI Secretion: Salivary Glands and Stomach Flashcards
Exocrine is (multicellular/unicellular)
both
Saliva is made up of 99% water (1-1.5 L/day) What else? (~8)
lipase, amylase, mucus, immunoglobulins (IgA), High Ca2, Phosphate, lysozymes
Serous cells secrete ____ while mucus cells secrete _____
enzymes + fluid; mucus
What are the three major salivary glands? Are they serous or mucus glands?
Parotid: serous
Submandibular: mixed, primarily serous
Sublingual: mixed, primarily mucus
What is role of duct cells?
They modify the primary fluid
All exocrine glands secrete (isosmotic/hyperosmotic) fluid
isosmotic
Saliva is always (isotonic/hypertonic/hypotonic)
hypotonic
Basal secretion is more (hypertonic/hypotonic) and (basic/acidic) than stimulated secretion
hypotonic, acidic
stimulated secretion is thus less hyptonic and basic
Saliva secretion is stimulated by (sympathetic/parasympathetic) innervation
parasympathetic
in humans, submandibular gland secretion is also stimulated by sympathetic!
Postganglionic neurostransmitter of parasympathetic neurons is usually _____. What is it’s receptor? What is signaling pathway l eading to fluid secretion?
Acetylcholine –> M3 receptor –>Gq—> DAG + IP3 –> PKC + increase Ca2+
What are the prime stimulators of gastric mucus secretion?
Prostaglandins
What is the cephalic phase of digestion?
Stimulating your saliva production when you see, smell and are in the presence of a delicious soft pretzel
Parasympathetic efferents in which two cranial nerves stimulate fluid and enzyme secretion?
CN VII (facial) and CN IX (glossopharyngeal)
When salivary flow is stimulated, the pH of the saliva changes from acid to basic. This happens because stimlation:
increases ductal HCO3 secretion
The most potent direct stimulus for the secretion of saliva (human) is:
Parasympathetic stimulation
In the parietal cell, what is the most efficient way to inhibit acid secretion?
Proton pump inhibitors! This pump on the apical side pumps out protons in exchange for K; Cl leaves through own channel and joins to make HCL in the lumen of the GI tube.
Name four important regulators of HCL secretion and their receptors and means of regulation. Which is the most important? Which signaling pathway does it cause?
- Acetylcholine—-M3, neurotransmitter
- Histamine—-H2, paracrine
- Gastrin—-CCKB, endocrine
- Somatostatin—-n/a, paracrine
HISTAMINE IS MOST IMPORTANT. Causes cAMP –> PKA pathway
The mechanism behind how histamine stimulates gastric acid secretion is similar to how ADH works to stimulate water reabsorption. Explain this.
- the resting cell has small number of proton pumps
- when histamine is secreted by ECL and acts on parietal cell, it inserts more proton pumps into the membrane pocket to do their job
- gastrin and Ach potentiate this reaction, but they need histamine present to do any good.
Gastrin secretion is (low/high) when the stomach is empty. What regulator is responsible for this?
Low. D-cell secrete somatostatin to inhibit G-cell so it can’t respond to any food by secreting gastrin
When food is in the stomach, what happens to somatostatin regulation?
D-cell stops secreting somatostatin so G-cell is no longer inhibited. Now can secrete gastrin
What happens when gastrin is secreted?
Will enter the blood stream to reach the parietal cel and ECl cell will secrete histamine which will encourage insertion of pumps for acid secretion
During the emptying state, which cell is very important to keep acid secretion at a minimum? Explain how it works
D-cell. Senses pH! Negative feedback system. When pH of antrum is too low, release somatostatin. Paracrine action on neighboring G cells to decrease release of gastrin and thus gastric acid secretion
Activation of vagal parasympathetic preganglionic outflow to the stomach acts in three ways to stimulate gastric acid secretion. Name them.
- Direct neural innervation and activation of parietal cells via Ach release which act on parietal cells muscarinic receptors
- Activate ECL cell to stimulate release of histamine
- Neurons release gastrin-releasing peptides that stimulate the G-cells in gastric gland in gastric antrum. Stimulates release of gastrin
What are enterogastrones? Name 4 important ones
Peptides that are secreted by the gut wen it is full and doesnt want more stuff. GLP-1, CCK, secretin, and GIP
What are the three phases of HCL secretion? Do they inhibit or stimulate? What are they mediated by? Which phase is most important?
- Cephalic –stimulates, mediated by vagus
- Gastric–stimulates, mediated by vago-vagal reflex, gastrin, and histamine
- Intestinal–inhibits, mediated by enterogastrones
Gastric is most important.
How does CCK work as an entergastrone?
CCK binds to the same receptor as gastrin, so it can displace gastrin and acid secretion is minimized. (CCK binding is not as effective as when gastrin binds though)
Does the cephalic phase of HCL secretion contribute a lot to total acid secretion?
No, it contributes very little
In which phase is the bulk of HCL secreted?
Gastric phase. Food in stomach.
How much HCL is secreted in the intestinal phase?
A small amount as along as food is still in stomach
Gastrin stimulates _____ secretion
histamine secretion
G cells sense ______ and dump gastrin full force
food in stomach
What cell secretes pepsinogen? What is the primarily regulator of its secretion?
Chief cell, vagus (Ach). Gastrin and other peptides can also stimulate
What secretes mucus? What is mucus necessary for? What stimulates secretion? (3)
Surface epithelial cells. Protects gastric mucosa. Vagus, presence of food in stomach, and prostaglandins.
Gastric mucus is (hydrophilic/hydrophobic). What secretes it? What stimulates secretion? (3)
Hydrophobic. Mucus and mucus neck cells. Vagal activity, prostaglandins, and gastrin
What inhibits gastric mucus secretion?
NSAIDs (i.e. aspirin)
What are the two damaging effects NSAIDs have on gastric mucosa?
- decrease mucus secretion
2. destroy hydrophobic interface
Patient is taking an NSAID to help with pain from osteoarthritis and develops a peptic ulcer. What is the connection between the ulcer and NSAID?
NSAID inhibits mucus secretion.
During the intestinal phase of digestion, the small intestine secretes hormones that have 4 major effects. What are they?
- Stimulate pancreatic secretion of enzymes, fluids and electrolytes
- Stimulate gallbladder contraction + relaxation of Sphincter of Oddi
- Stimulate intestinal fluid secretion
- Inhibit gastric acid secretion and gastric emptying and food intake
Which two phases of digestion regulate pancreatic secretion?
Gastric and Intestinal phase
The amount of pancreatic secretion is __L/day and is rich in _____
1-1.5 L/day
HCO3- rich
Of all the enzymes that make up the composition of pancreatic secretion list:
- 1 lipolytic
- 1 amyolytic
- 2 nucleases
- 1 other important
lipolytic-lipase
amyolytic-alpha amylase
nucleases-DNAses and RNAses
other-SPINK1 (trypsin inhibitor)
note: this is exocrine pancreas function
(endocrine is beta cells and alpha cells for insulin etc.)
Most pancreatic enzymes are synthesized as inactive pro-enzymes. Where are they stored and with what?
Secretory granules, with protease inhibitors
What is a characteristic of secretory granules to prevent enzyme activation?
Acid intra-granular pH
The enzymes are released by exocytosis from secretory granules in response to _____
an increase in intracellular calcium concentration
The acinar cell secretes ___ and ___ into the duct system
enzymes and isosmotic NaCl
During pancreatic enzyme secretion, where is the electrolyte concentration modified?
In the ducts (water is also added)
What activates the pro-enzymes int he duodenum?
Trypsin
What converts trypsinogen to trypsin? Where is this enzyme released?
Enterpeptidase (from intestinal epithelial cell)
What are the main two stimulus for pancreatic secretion? (control of secretion) Where does this occur?
Neuronal—Ach (M3 receptor)
Hormonal—CCK (CCKA) [note CCKB is in parietal cells in stomach]
Occurs in acinus
Is the pH more acidic or basic when you reach the duodenum? Importance of this?
Not very acidic since pancreatic duct added HCO3. Trypsin is now ready to activate everything so that digestion can occur
What releases CCK?
I cells
What stimulates release of CCK? What DOES NOT stimulate its release?
Presence of FA, aa, and peptones (larges peptides) in the duodenum. Sugar and carbs do not stimulate CCK
CCK can stimulate acinar secretion by what two mechanisms?
- direct: circulation —> CCKA receptor
2. Paracrine stimulation of a GVA near I cell –> dorsal motor X nucleus —> Ach —> M3 on acinar cell
HCO3 gets secreted in the pancreatic duct. What are two mechanisms?
- HCO3/Cl antiporter on apical side
2. CFTR channel (cystic fibrosis transmembrane regulator)
Name a disease that causes decreased secretion of fluids and bicarbonate by the pancreatic duct.
impacts CFTR receptor-patients tend to lose their pancreas. Need CFTR to regulate movement of chloride, and HCO3 in pancreatic duct
What is the main controller of secretion in the acinus? What stimulates it?
CCK (cholecystokinin); luminal food content
What is the main controller of secretion in the pancreatic duct? What stimulates it?
Secretin; low luminal pH
The secretion of pancreatic amylase after a meal containing pure starch (amylose) is possible because
the cephalic phase of digestion can contribute up to 50% of maximal pancreatic enzyme secretion
What prevents auto-digestion of the pancreas in view of the fact that pancreatic enzymes are stored for a considerable amount of time?
Pro-enzymes are stored together with Ca2+ and SPINK-1 which in high concentrations are strong inhibitors.
while acidic intracullar pH is helpful, enzymes can still be activated a little, so not best answer
Salivary gland secretion is maximal during which phase?
Cephalic phase
Gastric HCL is maximal during which phase?
Gastric Phase
Pancreatic secretion is maximal during which phase?
Intestinal phase
Incretin role/function. What secretes it?
Secreted by intestinal neuroendocrine cells. Potentiates glucose-stimulated insulin secretion
Enterogastrone role/function. What secretes it?
Hormone secreted by intestinal neuroendocrine cells and inhibits acid secretion and gastric emptying
True or false: many hormones acts as incretins AND enterogastrones
True.
