0205 - Gastric Secretions Flashcards
What are the primary active constituents of saliva?
Lingual lipase and salivary amylase, secreted by tongue and salivary glands respectively.
Also contains: mucins (lubrication) lysozyme (break down bacterial cell wall) lactoferrin (bind iron, bacteriostatic) Proline rich proteins, (protect teeth and bind tannins)
What are the functions of saliva?
Lubricate food to form a bolus and facilitate swallowing.
Act as a solvent to facilitate taste
Act as a lubricant to facilitate speech.
Where does saliva come from?
Secreted by salivons, crypts in the glands.
Parotid = 25%, watery secretion
Sublingual = 5%, viscous secretion
Submandibular = 70%, mixed secretion
Why does saliva osmolarity increase with production rate?
Primary saliva has a similar composition to plasma, and is altered to become hypotonic.
As production rates increase, the process to make it hypotonic lags behind the production rate, thereby increasing osmolarity.
What is the electrolyte composition of saliva?
Predominantly Na and Cl, with HCO3- and K+ as well.
Na and Cl are resorbed into the cell, and HCO3- and K+ are excreted. However, final saliva is still much higher in NaCl than the other two.
What are the most important cells for gastric secretions? What is the location and function of each?
Parietal cells (Corpus only) - H+ and Intrinsic Factor
Chief Cells (Corpus and antrum) - Pepsinogens
Endocrine Cells (mostly antrum, some corpus) - Gastrin (G cells, antrum only), Somatostatin (D cells, corpus and antrum)
Briefly outline stomach parietal cells.
Produce acid and intrinsic factor.
Abundant mitochondria (lots of ATP needed for acid)
When activated, open canaliculi greatly increase surface area to allow more pumps to secrete acid into lumen.
What is intrinsic factor?
Glycoprotein that binds to vitamin B12 (cyanocobalamin), and is essential for endocytosis of B12 in the small intestine.
Thus, gastrectomy or autoimmune destruction of cells may eventually (years) lead to Vit B12 deficiency and pernicious anaemia.
Describe the process of stomach acid secretion
Takes place in the Corpus only.
H+/K+ ATPase (active exchanger) in the luminal membrane pumps out H+ in exchange for K+, which returns to lumen via a luminal K+ channel.
H2O dissociates, providing H+, and CA forms HCO3- with CO2. HCO3- exchanges with extracellular Cl, which then exits to lumen (giving HCl).
Other H+ from H2O exits via Na/H exchanger, and Na leaves via Na/K-ATPase, providing K required by H/K-ATPase.
How much gastric juice is produced per day?
Stomach glands produce 2L/day of isotonic fluid.
Acid production takes place in the corpus only.
What 3 substances induce gastric acid secretion? How do they work?
ACh (from ENS) - directly and indirectly
Gastrin (from G-cells) - directly and indirectly
Histamine (from ECL cells) - directly.
Directly - straight onto parietal cell.
Indirectly - onto ECL cell, stimulating histamine.
How is gastric acid secretion regulated?
Secretagogues:
ACh, Histamine (via H2), and Gastrin (via CCKb) all act as direct secretagogues. ACh and gastrin both also act indirectly by activating ECL cells to produce histamine.
Inhibitors:
Somatostatin (from endocrine D cells in corpus and antrum) is primary inhibitor, working both directly and indirectly.
Name two drugs that can reduce acid secretion.
Omeprazole (blocks H/K-ATPase)
Cimetidine (antagonist of H2 histamine receptor) - ACh and Gastrin still have an effect.
What is the role of Somatostatin?
Major inhibitory mechanism of gastric acid secretion. Made by endocrine D-cells in corpus and antrum. Acts directly and indirectly.
Directly - Binds to SS receptors on parietal cells (inhibiting effect of histamine). Is both endocrine (if produced in antrum) and paracrine (if from Corpus), as parietal cells are only found in Corpus.
Indirectly - In corpus, reduces histamine release from ECL cells. In antrum, reduces Gastrin release from G-cells.
Briefly outline gastrin.
Purpose is to stimulate acid secretion and gastric mucosal growth. Released in the antrum and duodenum via gastrin-releasing peptide. Release controlled by ENS, as well as stomach wall stretching and protein fragments in stomach.
Briefly outline the gastric phase of gastric acid secretion.
Entry of food causes 2 stimuli for H+ secretion:
- Distension of gastric wall - (corpus and antrum) activates vagovagal reflex (same as cephalic response) and local ENS reflex (ACh-mediated activation of parietal cell).
- Partially digested proteins (peptones) stimulate antral G-cells.
Briefly outline the intestinal phase of gastric acid secretion.
Stimulated by peptones and amino acids in proximal small intestine.
- Peptones stimulate duodenal G-cells (gastrin release)
- “Entero-oxyntin” stimulates parietal cells.
- Absorbed amino acids stimulate H+ secretion by an unknown mechanism.
