Flashcards in Gastric Physiology Deck (65):
What are the five major GI hormones?
Gastrin, Cholecystokinin, Secretin, Glucose-dependent Insulinotropic Polypeptide, Motilin
Gastrin is produced by which cells? Where?
G cells, in the antrum of the stomach (some in the duodenum as well.)
What is the main signal for gastrin secretion?
Gastric lumen oligopeptides and amino acids.
Minor signal (oligopeptides and amino acids in the duodenal lumen d/t G cells there.)
What is gastrin's pathway for action? What are its main target tissues?
It is endocrine.
It's target tissues are enterochromaffin-like cells (ECL) and parietal cells of the gastric corpus.
What are gastrin's main actions?
Increase gastric acid secretion and gastric motility.
-Stimulate ECL cells to secrete histamine (indirect effect on parietal cells)
-Stimulate parietal cells to secrete HCl rich gastric juice (direct effect)
-Increase gastric motility but also increase pyloric constriction
Since gastrin increases pyloric constriction, what is the net effect?
Increased mechanical digestion and decreased gastric emptying.
What produces CCK? Where is it produced?
Produced by I cells, in the duodenum and the jejunum.
What is the main signal for CCK secretion?
Fatty acids in the duodenal/jejunal lumen
(Minor signal: oligopeptides and amino acids in the duodenal lumen.)
What is CCK's pathway for action? What are its main target tissues?
It is endocrine, but has an indirect effect via the ANS.
It's main target tissues are ANS sensory neurons, GB smooth muscle, pancreatic acinar cells; possibly parietal cells?
What are CCK's main actions?
Stimulating GB contraction, bile secretion; reduce gastric secretions and emptying. Stimulates pancreatic enzyme secretion.
What produces secretin? Where is it produced?
Secretin is produced by S cells, mostly in the duodenum (some in the jejunum.)
What is the main signal for secretion of secretin?
Acid (pH<4.5) in the duodenal lumen.
(Minor signal is acid in the jejunal lumen.)
What is secretin's main pathway for action? What are secretin's main target tissues?
Secretin acts via endocrine pathway.
It's main target tissues are pancreatic ducts, bile ducts, pancreatic acinar cells (Minor target: vagal afferent fibers.)
What are secretin's main actions?
Increase secretion of bicarb into the duodenum.
-Stimulate pancreatic and bile ducts to increase bicarb secretion.
-Stimulate pancreatic enzyme secretion from pancreatic acinar cells.
Where is GIP produced?
GIP is produced by K cells in the duodenum and jejunum.
What are the main signals for GIP secretion?
Carbs, proteins, and lipids in the intestinal lumen.
What is GIP's pathway for action and what are its main target tissues?
GIP is endocrine. It's target tissues are pancreatic islet cells, parietal cells, and G cells.
What are GIP's main actions?
Potentiate insulin secretion.
Inhibit gastric secretion and motility.
Where is motilin produced?
Motilin is produced by M cells in the duodenum and jejunum.
What is the main signal for motilin secretion?
(We don't know, but likely lack of nutrients.) If there is food in the stomach/intestines, it will not be secreted.
What is motilin's pathway for action?
Motilin is endocrine.
What are motilin's main target tissues?
Motilin targets smooth muscle, from the stomach to the colon.
What are the main actions of motilin?
Motilin stimulates the most intense contractions of the gastric/intestinal smooth muscle in the Migrating Motor Complex in order to clear non-nutritive components out of the stomach and intestine. (How we get FB out.)
What are the functions of gastric motility? (3)
Reservoir creation (fundus)
Gastric emptying (pylorus/antrum/proximal duodenum.)
What are the main functions of the LES and cardia? (3)
Prevention of reflux.
Entry of food.
Regulation of belching.
What are the main functions of the fundus and the body? (2)
Acting as a reservoir.
Providing tonic force during emptying.
What are the main functions of the antrum and pylorus? (4)
Regulation of emptying.
What is the purpose of gastric accommodation and receptive relaxation?
Make it easy to store food in the fundus.
Keep the antrum free of stored food so it can begin grinding/digestion.
How does churning work?
Moves solid particles towards the antrum.
Closes the pylorus.
Propels particles towards the pylorus.
Retropulsion returns most of the particles to the body while a small amount is pushed through the pylorus.
Which hormone potently affects churning and why?
Gastrin. Because it increases gastric motility in the body/antrum and increases resistance to emptying at the pyloric sphincter.
What types of things slow emptying? (3)
Fed state (large chunks, acids/fats)
Gastrin in the stomach.
Signals from the duodenum. (low pH, lipids, amino acids, carbs.)
What serves to increase pyloric tone?
Intrinsic signals (distal to pylorus)
What serves to decrease pyloric tone?
Intrinsic signals (proximal to pylorus)
What serves to increase peristalsis?
What serves to decrease peristalsis?
Fat in the gastric lumen
What are the four functions of gastric secretions?
Digestion of proteins/lipids (pepsinogen, HCl, gastric lipase)
Protection of stomach from gastric acid (HCO3- and mucus)
Protect Vitamin B12 from intestinal digestion (Intrinsic factor)
Destroy bacteria and other micro-organisms (HCl)
What are the three things in "new" gastric juice?
Mucus, chief cells, parietal cells
Describe Chief Cell's secretion product.
High in NaCl, (not bicarb or acid)
Pepsinogen secreted via exocytosis
Higher proportion secreted at basal (lowest) flow rates
Describe Parietal Cell's secretion product.
High in HCl.
Intrinsic factor is secreted via exocytosis.
Much higher proportion secreted at maximal flow rates.
What is the function of the gastric mucosal barrier?
Protects gastric epithelial cells. (via mucus and bicarb.)
Chief cells are located where? Why??
At the base of the gastric pit. So they can wash out the more acidic parietal cell secretions.
What is important about resting v actively secreting parietal cells?
Resting parietal cells have a small apical surface area that contains tubulovesicles with H-K pumps. When they become active, the tubulovesicles fuse with the membrane and H-K pumps become available so that more H+ can be secreted.
Describe the alkaline tide.
In the parietal cells, carbonic anhydrase breaks down CO2 into HCO3- and H+. The HCO3- enters the blood via the Cl- in/HCO3- out antiporter on the basolateral side. This raises the pH of the blood so that it is more alkaline, thus it is called the alkaline tide.
What three major secretagogues stimulate parietal cell HCl secretion?
ACh, Gastrin, Histamine
Which receptor does the secretagogue, ACh, act on?
M3 on the enterochromaffin cells and on the M3 of the parietal cell.
Which receptor does the secretagogue, Gastrin act on?
CCK2 on the enterochromaffin cell and on the CCK2 of the parietal cell.
On which receptor does the secretagogue, Histamine, act?
Which pathway does M3 use?
phospholipase C, which produces IP3 and DAG. IP3 then stimulates Ca2+ release from ER or PKA increased activation of ATPase.
Which pathway does CCK2 use?
Phospholipase C, which produces IP3 and DAG. IP3 then stimulates Ca2+ release from ER or PKA increased activation of ATPase.
Which pathway does H2 use?
Gs increasing adenylyl cyclase which produces cAMP.
What do prostaglandins do?
(via Gi) they inhibit histamine's effect on adenylyl cyclase, so that less cAMP is produced.
Which three things act to secrete acid?
-Acid in the gastric lumen
Which three things act to stop acid secretion?
-Acid in the gastric lumen
Acid in the gastric lumen
What are the three phases of acid secretion?
Describe the cephalic phase: what brings it about? What is it mediated by? How much of the total acid secretion does it bring about?
Smell/taste/thought of food.
Mediated by vagal signals
30% of total acid secretion
Describe the gastric phase: what brings it about? What is it mediated by? How much of the total acid secretion does it bring about?
Feedback from the stomach, both ENS and PNS involved. G cells in the gastric antrum secrete gastrin.
60% of total acid secretion
Describe the intestinal phase: what brings it about? What is it mediated by? How much of the total acid secretion does it bring about?
Feedback from signals in the duodenum.
G cells in the duodenum secrete gastrin.
10% of total acid secretion.
Where does mechanical digestion begin?
In the mouth, continuing through the stomach and intestine.
Where does chemical digestion begin?
Begins in the stomach with pepsin.
It is helped by pancreatic juice which contains peptidases which are active in the intestine.
In the duodenum, these digest proteins into amino acids and dipeptides.
Where are many amino acids absorbed?
By the intestinal transporters. Then sent to the liver via portal blood.
At what pH does pepsinogen get converted to pepsin?
at pH<5.0 in the gastric lumen.
What is special about pepsinogen activation?
Pepsin is auto-activating.
Where is pepsin's optimal pH? At which pH is it inactivated?
It's optimal pH is 1.8-3.5. It is inactivated above 3.5.
If there is significant buffering of the stomach acid by food, which enzyme is still active?
Salivary amylase (ptyalin). pH optimum is >4. About 20-40% of starch is broken down by it in the oral cavity and stomach.