Gastric Secretion

• In the stomach, how does somatostatin act?

  It acts as a paracrine inhibitor of acid secretion by suppressing the release of gastrin from G cells and histamine from enterochromaffin cells (ECL).
• Where is gastrin secreted from?

  G cells
• Where is histamine secreted from?

  Enterochromaffin Cells
• How does histamine act in the stomach?

  Histamine functions as a primary paracrine stimulator of acid secretion by directly activating parietal cells to produce acid. Act as an "accelerator" of acid production.
• How is somatostatin released in the stomach?

  It is released locally to nearby cells when stimulated by high levels of gastrin or stomach acidity.
• How is histamine secreted in the stomach?

  It is released locally to stimulated nearby parietal cells via H2 receptors.
• What is the overall function of somatostatin?

  It inhibits the release of both gastrin (from G cells) and histamine (from ECL cells) thereby indirectly reducing gastric acid secretion.
• What is the overall function of histamine?

  It stimulates acid secretion by directly activating parietal cells to produce acid.
• What are the protein components of chief cells?

  The protein component of chief cell secretions is pepsinogen, which is an inactive precursor to pepsin.
• What is an alkaline tide?

  An alkaline tide is a temporary increase in the alkalinity of the body's fluids after eating a meal.
• When does an alkaline tide in the hepatic portal venous system occur?

  It occurs after a meal when the stomach parietal cells actively secret hydrochloric acid into the stomach lumen.
• What effect on the blood pH does an alkaline tide have?

  The influx of bicarbonate ions temporarily raises the pH of the blood in the hepatic portal vein, creating the alkaline tide
• What role does the liver play in regulating an alkaline tide?

  The liver plays a key role in regulating this alkaline tide by rapidly removing the excess bicarbonate from the blood and utilizing it to produce bile, which is necessary for digestion in the small intestine.
• What role does hydrochloric acid play in the gastric digestion of carbohydrates, proteins, and fats?

  Hydrochloric acid (HCI) plays a crucial role in protein digestion in the stomach by creating an acidic environment that activates the enzyme pepsin, which breaks down proteins into smaller peptides. It does not directly digest carbohydrates or fats, but it does help prepare the gastric environment for their further breakdown in the small intestine by denaturing proteins and creating the optimal pH for enzymatic activity.
• What stimuli cause pepsinogen release?

  Food ingestion, vagal nerve stimulation, gastrin hormone release
• What mechanism activates pepsinogen?

  The activation of pepsinogen occurs when it encounters the hydrochloric acid secreted by parietal cells, which causes a conformational change in the pepsinogen molecule, converting it into the active enzyme pepsin.
• What are the digestion products of pepsin activity?

  Pepsin breaks down proteins into smaller peptides and amino acids through the hydrolysis of peptide bonds, with a preference for cleaving near aromatic amino acids like phenylalanine.
• How is the H+/K+ ATPase pump regulated?

  It is regulated by a mechanism involving its intracellular storage in vesicles. These vesicles fuse with the apical plasma membrane upon stimulation by secretagogues, effectively translocating the pump to the luminal membrane where it actively pumps protons (H+) out of the cell in exchange for potassium (K+) ions. Thus facilitating stomach acid secretion.
• What is the key stimuli for the activation of H+/K+ ATPase?

  The key stimuli for activation include the binding of gastrin, histamine, and acetylcholine to their respective receptors, triggering a signaling cascade that leads to vesicle fusion with the plasma membrane.
• What stimuli increase gastrin release?

  It is stimulated by products of protein digestion, stomach distention, and vagal stimulation via GRP or bobesin.
• What stimuli inhibit gastrin release?

  It is inhibited by the low pH of the gastric contents.
• What mechanism allows for gastric H+ generation and secretion?

  Gastric H+ generation and secretion primarily occurs within parietal cells in the stomach lining, where carbon dioxide (CO2) is converted to carbonic acid by the enzyme carbonic anhydrase.  It then dissociated into H+ and bicarbonate ions.
• How are H+ ions transported into the stomach lumen?

  The H+ ions are then actively transported across the apical membrane of the parietal cell into the stomach lumen in exchange for potassium ions (K+) by a specialized enzyme called the H+-K+ ATPase (proton pump), resulting in the secretion of hydrochloric acid (HCl).
• What catalyzes the reaction of carbon dioxide with water in the parietal cell?

  Carbonic anhydrase
• What is formed when carbon dioxide reacts with water in the parietal cell?

  Carbonic acid
• What ions does carbonic acid dissociate into within the parietal cell?

  H+ and HCO3- ions
• How are H+ ions transported into the stomach lumen?

  They are actively pumped via the H+/K+ ATPase
• What happens to K+ ions during the active transport of H+ ions into the stomach lumen?

  K+ ions are transported back into the parietal cell
• How do chloride ions contribute to the formation of hydrochloric acid in the stomach lumen?

  They passively follow the secreted H+ ions through channels
• What role does the enterochromaffin-like (ECL) cell play in gastric acid secretion?

  It releases histamine to stimulate parietal cells
• What primarily controls the regulation of gastric acid secretion?

  The interplay between gastrin and somatostatin
• What stimulates ECL cell activation?

  Gastrin secreted by G cells
• What happens when gastrin binds to ECL cell receptors?

  It triggers the release of histamine
• How does histamine affect gastric acid secretion?

  It acts on H2 receptors on parietal cells to increase secretion
• What role does vagal stimulation play in gastric acid secretion?

  It activates ECL cells and enhances histamine secretion
• What is the function of somatostatin in gastric acid regulation?

  It inhibits gastrin secretion and histamine release
• What triggers the release of hormones like secretin and cholecystokinin (CCK) in the duodenum?

  Presence of acid, fat, and solutions of high osmolarity
• What is the effect of secretin on gastric acid secretion?

  It inhibits gastrin release from G cells
• How does CCK affect gastric acid secretion?

  It directly inhibits gastric acid secretion by acting on parietal cells
• What happens when high osmolarity is detected in the duodenum?

  It activates osmoreceptors, triggering a neural reflex that releases inhibitory hormones