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
