Gastric Secretion 31-Jul-19 gastric secretion

Gastric Secretion Gastric juice HCl Electrolytes Mucus, Water Intrinsic factor Bicarbonate 31-Jul-19 gastric secretion

Gastric Gland Gastric (oxyntic) gland cell types Gastric lumen Gastric (oxyntic) gland cell types Surface epithelia cells (mucus cells) Alkaline mucus- viscid Mucous neck cells (neck chief cells) Soluble mucus pepsinogen Columnar mucus cells Mucous neck cell Parietal cell Peptic cell From Physiology Book by H.T. Sherrief 31-Jul-19 gastric secretion

Gastric Gland Parietal cell Produce a solution containing Gastric lumen Parietal cell Produce a solution containing Approx. 160 mmol of HCl/l; pH 0.8 (H+ conc. = 3 to 4 million time that in plasma) Energy cost = 1500 calories/l of HCl Intrinsic factor (vit. B12 - R factor binding macromolecule) Columnar mucus cells Mucous neck cell Parietal cell Peptic cell From Physiology Book by H.T. Sherrief 31-Jul-19 gastric secretion

Gastric Gland Parietal cell also produce Gastric lipase Gastric lumen Parietal cell also produce Gastric lipase Gastric amylase Gelatinase Columnar mucus cells Mucous neck cell Parietal cell Peptic cell From Physiology Book by H.T. Sherrief 31-Jul-19 gastric secretion

Gastric Gland Chief [peptic] cells (zymogen cells) produce Pepsinogen Gastric lumen Chief [peptic] cells (zymogen cells) produce Pepsinogen Contain Pepsinogen( PG I, PG II) Small volume of electrolyte similar to ECF Some individuals secrete ABO blood group antigen in their gastric juice Columnar mucus cells Mucous neck cell Parietal cell Peptic cell From Physiology Book by H.T. Sherrief 31-Jul-19 gastric secretion

Gastric Gland Enterochromaffin cells Specialized cells serotonin Gastric lumen Enterochromaffin cells serotonin Histamine Specialized cells Vasoactive intestinal peptide (VIP) Substance P Glucagon Columnar mucus cells Mucous neck cell Parietal cell Peptic cell From Physiology Book by H.T. Sherrief 31-Jul-19 gastric secretion

Pyloric Gland Similar structure like the oxtntic gland Have G-cells Gastric lumen Similar structure like the oxtntic gland Contain few peptic (chief) cells No parietal cells Contain mucus cells Have G-cells Columnar mucus cells Mucous neck cell G-cells Peptic cell From Physiology Book by H.T. Sherrief 31-Jul-19 gastric secretion

Pyloric Gland secrete G cells produce Gastrin Gastric lumen secrete Mucus and electrolytes G cells produce Gastrin Peptic cells produce pepsinogen Columnar mucus cells Mucous neck cell G-cells Peptic cell From Physiology Book by H.T. Sherrief 31-Jul-19 gastric secretion

HCL Secretion Function of HCL Participates in the breakdown of proteins Provide optimal pH for pepsin Hinders growth of pathogenic bacteria 31-Jul-19 gastric secretion

Mechanisms of HCL Secretion Chloride ions are actively secreted into lumen of canaliculi Against concentration gradient creates a negative potential -40 to -70 mv in canaliculi passive diffusion of K+, Na+ KCl, NaCl 31-Jul-19 gastric secretion

Mechanism of Acid Secretion The H+ conc. In parietal cells 3 to 4 million fold higher than in blood Cl- is secreted against Concentration and electrical gradient Ability of parietal cell to secrete acid Dependent on active transport 31-Jul-19 gastric secretion

Mechanism of Acid Secretion Key player in acid secretion H+/K+ATPase or “proton pump” Mg++ dependent 31-Jul-19 gastric secretion

Mechanisms of Acid Secretion Dissociation of H2O H2O  H+ + OH - The H+ is then actively secreted into the lumen by active pump in exchange of K+ Proton pump H+/K+ ATPase 31-Jul-19 gastric secretion

Mechanism of Acid Secretion Na+ is actively reabsorbed by a separate pump Thus most of the Na+, K+ that had diffused into canaliculi Reabsorbed into the cell H+ take their place Giving strong solution of HCL 31-Jul-19 gastric secretion

Mechanism of Acid Secretion The OH - combine with CO2 to form HCO3- Carbonic Anhydrase The HCO3 - diffuses to ECF and into blood Alkaline tide 31-Jul-19 gastric secretion

Mechanism of Acid secretion H2O passes through cell and into canaliculi by OSMOSIS Final secretion from the canaliculi contain HCL at a conc of 150 to 160 mEq/L KCl at conc of 15 mEq/L Small amount of NaCl 31-Jul-19 gastric secretion

CO2 CO2 H20 CO2 + OH- H+ HCO3- HCO3- K+ H+ K+ K+ K+ Na+ Na+ K+ K+ Na+ Na+ Cl- Cl- Cl- H20 H20 H20

Pepsinogen Secretion Precursor of pepsin Produced by the Chief Cells Stored in secretory granules Vagal stimulation, Histamine, Gastrin Cause preformed granules to be released into lumen 31-Jul-19 gastric secretion

Intrinsic Factor Mucoprotein Essential for vitamin B12 absorption Secreted by parietal cells 31-Jul-19 gastric secretion

Control of Gastric Acid Secretion Basal secretion During fasting gastric juice secretion occurs Postprandial Cephalic, Gastric and Intestinal phases 31-Jul-19 gastric secretion

Cephalic Phase Before food gets into the stomach Sight , smell, thought, taste, hunger,appetite center Cause reflex stimulation of gastric secretion This phase of secretion Accounts for 20% of total gastric secretion associated with eating a meal 31-Jul-19 gastric secretion

Gastric Phase Presence of food in the stomach excite Stretch receptors, chemo-receptors Local enteric reflexes Gastrin production All these lead to stimulation of gastric secretion Gastric phase of secretion Account for 70% of total gastric secretion associated with eating a meal 31-Jul-19 gastric secretion

Intestinal Phase Presence of food in the intestines Release of Gastrin by duodenum Continue to stimulate secretion of some gastric juice 31-Jul-19 gastric secretion

Control of Gastric Acid Secretion Neural mechanisms Stimulation of vagus Stimulates intramural plexuses Which then stimulates PARIETAL, CHIEF, MUCUS cells Neurotransmitter is Acetylcholine 31-Jul-19 gastric secretion

Control of Gastric Acid Secretion Vagal stimulation increases secretion by Stimulating histamine releasing cells Histamine then diffuses to stimulate parietal cells Vagal stimulation activates antral G – cells Produce Gastrin Gastrin acts on parietal and chief cells 31-Jul-19 gastric secretion

Control of Gastric Acid Secretion Humoral Gastrin Secreted by G - cells Peptide G34, G17 G17 more abundant and potent than G34 for acid secretion 31-Jul-19 gastric secretion

Control of Gastric Stimulation Stimulus for Gastrin secretion Vagus Secretagougues (AA,peptides, alcohol,caffeine,Ca++ Distension of stomach Gastrin release intracellular Ca++ Promote proton pump Cl- channel insertion 31-Jul-19 gastric secretion

Control of Gastric Acid Secretion Histamine Enterochromaffin cells Mast cells Acts on H2 receptors Increases cAMP This then promote proton pump function Increases H+ secretion H2 receptor blockers Cimetidine etc Prostaglandin E1, E2 inhibit Histamine release Pentagastrin stimulates HCl and Gastrin release 31-Jul-19 gastric secretion

Ca++ Gastrin receptor Ca++ Ca++ Ach receptor H+ K+ cAMP Histamine receptor H2 31-Jul-19 gastric secretion

+ + - - + - + + Ach Gastrin Histamine Somatostatin ECL Cells D- Cells H2 receptor Histamine ATP - + Adenyl cyclase - D- Cells cAMP + Prot kinase K+ Somatostatin H+ G - Receptor  Ca++ Ach - Receptor +

Inhibition of Gastric Juice Secretion Acidity of gastric juice to pH <3.0 Gastrin mechanism is blocked Enterogastric reflex Presence of food in SI Distension, acidity, FA, AA Secretin, cholecystokinin Oppose stimulatory effect of Gastrin 31-Jul-19 gastric secretion

Enterogastric Reflex CCK GIP gastric inhibitory peptide Initiates enterogastric reflex Slows gastric emptying Inhibit gastric secretion GIP gastric inhibitory peptide Inhibit Histamine VIP vasoactive Intestinal peptide; somatostatin Have moderate inhibitory effects on gastric secretion 31-Jul-19 gastric secretion

Gastric Mucosal Barrier Protect the gastric lining cells from Damage by intra-luminal HCL Autodigestion 31-Jul-19 gastric secretion

Gastric Mucosal Barrier Compact arrangement of epithelial cells with TIGHT JUNCTION Epithelial cells secrete bicarbonate (under influence of prostaglandin) Thick layer of adherent MUCUS (alkaline mucus) 1 mm thick 31-Jul-19 gastric secretion

Gastric Mucosal Barrier Regenerative capacity of mucosa Trophic effect of Gastrin High turn over rate: Entire mucosa replaced within 3 days 31-Jul-19 gastric secretion