GI Secretion I
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- Average volume of secreted and ingested fluid entering body daily?
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8.5 liters
(largest contributor - stomach) - Average volume of secreted and ingested fluid that is absorbed by body daily?
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8.3 liters
(only 200 ml not absorbed!) - Name the two types of secretions and what their functions are.
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1) serous - water and electrolytes to provide proper environ for digestion
2) protein - enzymes and mucus to digest and lubricate/buffer respectively - List the types of glands involved in secretion.
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1) Mucous - single cell (goblet)
2) Tubular - invaginations of epithelium that contain secretory cells (gastric pits, intestinal crypts)
3) Compound - outside of gut wall, acini and ducts, primary secretion into center of acinus (ex. salivary, pancreatic) - Describe the difference between primary and seconday secretion that occurs in compound glands.
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Primary - secretion into center of acinus
Secondary - cells lining ducts modify the secretion by adding electrolyte and water components directly into the duct as it proceeds toward the lumen of the gut - List the two mechanisms of intracellular mediators of secretion:
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1) G protein - AC - cAMP - secretion (B adrenergic agents, VIP, secretin family, cholera toxin)
2) PIP2 splits - IP3 - intracellular Ca2+ release - secretion (ACh, CCK, TKs) - Name three glands that provide salivary secretion and the type of secretion.
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1) parotid - serous only
2) submaxillary - serous and mucous
3) sublingual - serous and mucus, but mostly mucous - Name the functions of saliva:
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1) mucous - lubricates and keeps pH normal
2) enzyme - ptyalin (alpha amylase) digests CHO and lingual lipase digests lipids
3) taste, speech, preventing oral infections - What makes up the secretion produced in primary secretion of a salivary gland?
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water, electrolytes (Na, K, Cl, HCO3) mucus, enzyme
* electrolytes secreted are isotonic - What makes up the secretion produced in secondary secretion of a salivary gland?
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modification of primary secretion, extract Na and Cl from the soltn and add K and HCO3 (more electrolyte is removed than added)
*solution is now hypotonic - How does flow rate affect salivary secretion?
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Fast flow rate - not much time for secondary secretion - secretion most resembes primary secretion
Slow flow rate - more time for secondary secretion and exchange - What is the tonicity of a salivary secretion that had a SLOW flow rate?
- Hypotonic (low in NaCl and HCO3
- What is the tonicity of a salivary secretion that had a FAST flow rate?
- Isotonic
- What happens to electrolyte levels as the salivary secretion flow rate increases to above ~2 ml/min?
- they level off (except Cl- still increasing)
- Is control of salivary secretion hormonal, neural, or both?
- neural only -- there is NO hormonal control
- Are sympathetic NS and parasympathetic NS excitatory or inhibitory in salivary glands?
- both are excitatory and stimulate secretion
- Name the NTs in neural control of salivary secretion, and tell about their mechanisms.
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Para - releases ACh, and VIP - stimulate acinars directly - since BVs dilated by VIP, secretion high in ptyalin and serous component ** most potent secretion stimulator
Symp - NE activates alpha and beta receptors - serous component decreased - high mucous concentrations - What is the optimal pH for ptyalin (salivary amylase) and where does it exist?
- pH 7 (range pH 4-11) - begins in mouth, and if attached to bolus or inside bolus, can keep alkaline pH through stomach, intestine, and rest of GI tract and continue to digest CHO
- What is the optimal pH for lingual lipase?
- pH 4 (range pH 2-8) - continues to digest thru stomach
- Name the 5 types of gastric secretion.
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1) H+ (parietal/oxyntic cells)
2) Pepsinogen (chief cells)
3) Lipase (chief cells)
4) Intrinsic factor (parietal/oxyntic cells)
5) Mucus (mucous cells) - What do parietal (oxyntic) cells secrete?
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1) H+
2) Intrinsic factor - What is H+ responsible for in the stomach?
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1) convert pepsinogen to pepsin, which digests proteins at low pH
2) maintain low pH for continued pepsin action and to prevent bacterial growth - Where are parietal cells located in the stomach?
- in the oxyntic glands located in the fundus and corpus of the stomach
- What do chief cells secrete?
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1) Pepsinogen
2) Gastric lipase - Where are cheif cells located?
- oxyntic gland of stomach
- What two kinds of lipases in the body liberate a single free fatty acid and a diglyceride from a TG and have acidic pH optimums?
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lingual lipase (3rd FA) and gastric lipase
* pancreatic lipase is different - For what purpose is intrinsic factor needed?
- to absorb vitamin B12 in the ileum
- Where is mucus secreted from in the stomach and what is its main purpose here?
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- secreted from mucous cells in surface epithelium and in neck of oxyntic gland
- protects stomach from autodigestion - What kind of glands are oxyntic glands and what kind of cells are located within these glands?
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- tubular glands (from gastric pits) in fundus and corpus areas
- parietal cells, chief cells, mucous neck cells - What cells are more numerous at the base of oxyntic glands?
- parietal cells
- What is the purpose of cannilculi formation in parietal cells upon their stimulation?
- canniliculi increase surface area of parietal cells for more H+/K+ ATPase (proton pumps) thus more H+ secretion (and K+ import)- final gastric pH <1
- What generates H+ inside of the parietal cell?
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Water.
(HCO3- formed from rxn diffuses into blood when exchanged with Cl-) - What is omeprazole and how does it work?
- a proton pump (H+/K+ ATPase) inhibitor - decreases amount of H+ secreted - increases pH in lumen -- used to treat gastric ulcer disease and gastroesophageal reflex (GERD)
- What is the net reaction in the parietal cell?
- H2O + CO2 + NaCl --> NaHCO3 (blood) + HCl (lumen)
- What is secreted from the basolateral membrane of the parietal cell during high rates of stimulation vs. low rates of stimulation?
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high stimulation - HCl secreted (via HCO3/Cl- exchanger)
low stimulation - NaCl secreted (Na+/K+ ATPase)
** hthe reason for this is that when there is low stimulation, less H+ out/K+ in, lower conc. of K+ in cell, want more K+ inside cell Na+/K+ going strong, Na+ out, combine with Cl- for NaCL - Name the 3 controls of acid secretion in parietal cells.
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1) neural (enteric and parasympathetic)
2) hormonal (gastrin)
3) paracrine (histamine, SS) - How does parasympathetic stimuation (vagal activation) increase acid secretion?
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1) ACh
2) Histamine release via enterochromaffin-like cells
3) Gastrin secretion by antral G cell (using GRP) - To what are type of receptors does gastrin bind, and what do the do when stimulatated?
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CCK-B
- when stimulated intracellular Ca2+ is elevated and HCl secretion occurs
- competitive antagonism also exists - What type of receptors in the stomach are activated by histamine and what happens when they are activated?
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- H2 receptors on parietal cells
- increases cAMP and HCl secretion - What does cimetidine do?
- - blocks H2 receptors to treat gastric ulcer disease (by decreasing HCl secreted)
- What kinds of receptors does SS bind to and what happens when they are stimulated?
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- SS receptors
- decreases cAMP and HCl secretion and inhibit gastric secretion - Name and describe the phases of acid secretion.
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1) Cephalic
(think about eating a meal)
2) Gastric
(presence of meal in stomach)
3) Intestinal
(presence of chyme in intestine) - What happens in the cephalic phase of acid secretion?
- - vagal reflex stimulates parietal cell directly and induces the release of gastrin from the G-cell
- What happens in the gastric phase of acid secretion?
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- maximal stimulation of acid secretion via neural stimulation of parietal cell
- G cell stimulation by distention, vagal, and aas - What happens in the intestinal phase of acid secretion?
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- inhibit acid secretion via fall of pH inhibiting gastrin
- neural reflexes due to presence of food in duodenum inhibits acid secretion (GIP, CCK, secretin, enterogastrone)