Lecture 7 Gastric Secretion Flashcards
What is the Antrum
This is the wider part of the pylorus, which is the narrower part of the stomach. It resides upstream from the pyloric canal and its junction of the pyloric sphincter to the duodenum
Define pepsinogens
Precursor protein digesting enzymes
Mucous neck cells in the gastric glands secrete
Mucus
Chief cells in the gastric glands secrete
pepsinogens
Parietal cells in the gastric glands secrete
HCl
Intrinsic factor
What is the concentration of HCl in the parietal cells compared to the blood
3-million fold higher
What type of gradient is chloride secreted against
Concentration and electrical
What is the ability of parietal cells to secrete acid dependent on
Active transport using the H+/ATPase or proton pump
Where is the proton pump located
near the lumen of the gastric pit in the canalicular membrane
How are hydrogen ions generated
Dissociation of water
How is bicarbonate ion formed in the parietal cell
- Hydrogen ions are generated within the parietal cell from dissociation of water
- The hydroxyl ions formed in this process rapidly combine with carbon dioxide to form bicarbonate ion, a reaction catalysed by carbonic anhydrase
What is the alkaline tide and what is its purpose
- Bicarbonate is transported out of the basolateral membrane in exchange for chloride
- The outflow of bicarbonate into blood results in slight elevation of blood pH known as “alkaline tide”. This process serves to maintain intracellular pH in the parietal cell
What is necessary for the secretion of acid
• Chloride and potassium ions are transported into the lumen of the canaliculus by conductance channels
How does outward diffusion of water occur from the parietal cell
- Hydrogen ion is pumped out of the cell, into the lumen, in exchange for potassium through the action of the proton pump and potassium is recycled
- Accumulation of osmotically active hydrogen ion in the canaliculus generates an osmotic gradient across the membrane that results in outward diffusion of water
How is gastric acid secretion controlled
- Neurocrine – Vagus/local reflexes
- Endocrine – gastrin
- Paracrine- histamine
What is histamine’s effect on the parietal cells
Activates adenylate cyclase which leads to elevation of cAMP concentrations and activates protein kinase A
PKA phosphorylates cytoskeleton involved in transport of the H+/k+ ATPase pump from cytoplasm to plasma membrane
What is the action of acetylcholine and gastrin binding onto receptors on parietal cells
Elevation of intracellular calcium
This leads to activation of PKA
What neuroendocrine cells can Gastrin and Acetylcholine act on to cause the release of histamine
Enterochromaffin Cells
What a G cells
a type of cell in the stomach and duodenum that secretes gastrin. It works in conjunction with gastric chief cells and parietal cells.
Where a G cells found
G cells are found deep within the pyloric glands of the stomach antrum, and occasionally in the pancreas and duodenum
Where are G cells found
Deep within pyloric glands of the stomach antrum and occasionally in pancreas and duodenum
Describe the gastric phase of secretion
When there is distension in the stomach the vagal/enteric reflexes are activated leading to the release of Act.
Peptides in the lumen cause the relate of gastrin from G cells and Act and G cells act on ECL cells to release histamine and all 3 act on parietal cell
Define the gastric phase
Occurs while food is in the stomach. 60% of total secretion
Vagal and enteric nerves
Paracrine (local) hormones (histamine) and endocrine hormones (gastrin)
What inhibits the secretion of gastrin
Lumen pH <2
How does the intestinal phase end
Acid in the duodenum leads to aplnchnic reflex that caused a decreased of gastrin secretion and the release of GIP which also reduces gastrin secretion and HCl
What are the mechanisms of the cephalic phase
The sight, smell and taste of food causes an increase in activity of the vagus nerve which stimulates the release of Ach and G cells which release gastrin
Gastrin and Ach act on ECL cells which release histamine
What are the mechanisms for the gastric phase
Distension in the stomach is detected by vagal/enteric reflexes which leads to the release of Ach
peptides in lume G cells release Gastrin
Gastrin and Ach act on ECL which release gastrin
What stops the cephalic phase of gastric secretion
Decrease in pH (increase in HCl) which reduces the amount of gastrin
What stops the intestinal phase
Splanchnic reflex, secretin release and GIP release
What are Enterogastrones
Hormones released from gland cells in duodenal mucosa- secretin, cholecystokinin (CCK), GIP
What are enterogastrones
Hormones released from gland cells in duodenal mucosa- secretin, cholecystokinin (CCK), GIP
What is the function of enterogastrones
• Released in response to acid, hypertonic solution, fatty acids or monoglycerides in duodenum
• Act collectively to prevent further acid build up in duodenum
• Two strategies
o Inhibit gastric secretion
o Reduce gastric emptying (inhibit motility/contract pyloric sphincter)
What is pepsinogen and what is it secreted by
(zymogen = inactive precursor) secreted by chief cells
Why is pepsin kept as a zymogen
Prevents cellular digestion and is activated at low pH (more acidic)
What is the role of gastric mucus
produced by surface epithelial cells and mucous neck cells. protect mucosal surface from mechanical injury and gastric acid corrosion and pepsin digestion
Where are intrinsic factors produced
Parietal cells
Why are intrinsic factors essential
Vitamin B12 absorption
What happens when there is a defect in Intrinsic factor and Vitamin B12 absorption
Pernicious Anaemia (failure of erythrocyte maturation)
Describe the Cephalic phase of gastric acid secretion
When there is sight, smell or taste of food the vagus nerve activity increases leading to the release of ch and Gastrin from G cells. Gastrin and School bind to ECL cells to release Histamine and all three bind to parietal cells
What are Enterochromaffin Cells
distinctive type of neuroendocrine cell in the gastric mucosa underlying the epithelium that release histamine
What are G cells
a type of cell in the stomach and duodenum that secretes gastrin. It works in conjunction with gastric chief cells and parietal cells
Where are G cells found
Deep within pyloric glands of the stomach antrum and occasionally in pancreas and duodenum
Describe the gastric phase of secretion
When there is distension in the stomach the vagal/enteric reflexes are activated leading to the release of Act.
Peptides in the lumen cause the relate of gastrin from G cells and Act and G cells act on ECL cells to release histamine and all 3 act on parietal cell
How does the Cephalic phase end
Decrease in vagal activity
How does the gastric phase end
Decrease in pH (increase in HCl) and a decrease in Gastrin
How does the intestinal phase end
Acid in the duodenum leads to aplnchnic reflex that caused a decreased of gastrin secretion and the release of GIP which also reduces gastrin secretion and HCl
When does the cephalic phase occur
Before food enters stomach and is 30% of secretion
When does the gastric phase occur
When the food is in the stomach >60% of secretion
When does the intestinal phase occur
Occurs after food enters the small intestine and is largely hormonal
What stimulates the intestinal phase
Gastrin
What inhibits the intestinal phase
GIP, secretin, cholecystokinin
What are enterogastrones
Hormones released from gland cells in duodenal mucosa- secretin, cholecystokinin (CCK), GIP
What is the function of enterogastrones
• Released in response to acid, hypertonic solution, fatty acids or monoglycerides in duodenum
• Act collectively to prevent further acid build up in duodenum
• Two strategies
o Inhibit gastric secretion
o Reduce gastric emptying (inhibit motility/contract pyloric sphincter)
What is pepsinogen
A zymogen that is secreted by chief cells
Why is pepsin kept as a zymogen
Prevents cellular digestion and is activated at low pH (more acidic)
What is the role of gastric mucus
produced by surface epithelial cells and mucous neck cells. protect mucosal surface from mechanical injury and gastric acid corrosion and pepsin digestion
What is Intrinsic Factor
Produced by paritetal cells and are required for Vitamin B12 absorption from ileum
What occurs if vitamin B12 is unable to be absorbed
Pernicious anaemia (failure of rbc maturation)
