Lecture 16: GI secretory Functions, Digestion, Absorption Flashcards
Describe the first and second stages of salivary gland secretion and compare the products
- First stage of salivary secretion of ions:
- Occurs in acini
- Secretion contains ptyalin (alpha-amylase)
- Composition is isotonic with ionic concentration similar to plasma
- Second stage of salivary secretion of ions:
- Occurs in salivary ducts
- Active reabsorption of sodium ions
- Active secretion of potassium ions
- Active/passive secretion of bicarbonate ions
- Passive reabsorption of chloride ions due to -70mV in ducts
What is the final ionic concentration of saliva normally
- concentrations of both sodium and chloride ions are several times less in saliva than in plasma
- 15 mEq/L each, about one seventh to one tenth their concentrations in plasma
- potassium ions is about 30mEq/L, seven times as great as plasma
- Bicarbonate ions= 50-70 mEq/L, about 2-3 times that of plasma
What is the concentration of ions during maximal secretion of saliva
- leads to less sodium and chloride reabsorption thus only about 1/2 to 2/3 that of plasma, and the potassium concentration rises to only 4 times that of plasma
list factors that stimulate or inhibit salivary secretions
- Stimulaiton
- Food in mouth (via parasympathetic activation)
- Smells
- Conditioned reflexes
- Nausea
- Inhibition
- Via inhibition of parasympathetic system
- Sleep
- dehydration
- fear
- anticholinergic drugs
- Via inhibition of parasympathetic system
Be able to diagram/explain the mechanism by which parietal cells secrete HCl into the gastric lumen
- Dissociation of water inside the cell into H+ and OH-
- OH- + CO2 + HCO3- + H+ (involves carbonic anhydrase)
- HCO3- is exchanged for Cl- ions (alkaline tide)
- HCO3- increases blood pH and will eventually eventually be secreted by pancreas to neutralize H+ in duodenum
- Cl- ions are secreted through chloride channels into canaliculi
- Hydrogen ion is pumped out of the cell in exchange for K+
- K+ leaks outside the cell but is transported back in via a H+- K+ ATPase pump
- Na+ is reabsorbed into the cell due to Na+-K+ basolateral pump
Describe the regulation of salivary secretion
- mainly by parasympathetics
- from the superior and inferior salivatory nuclei
Describe the mechanism by which parietal cells secrete hydrogen ions
Describe the direct and indirect pathways by which the parasympathetic system increases hydrogen ion secretion. This also includes neurotransmitters, receptors, and second messenger systems
- Parasympathetic (vagal) stimulaiton:
- Increases H+ secretion by two pathways:
- Direct:
- CN X iinervates parietal cells
- Stimulates H+ secretion directly
- Utilizes Ach and muscarinic (M3) receptor
- Indirect:
- CN X innervates G cells
- stimulates gastrin secretion
- Gastrin stimulates enterochromaffin-like cells to release histamine
- stimulates H+ secretion
- Neurotransmitter is GRP (Gastrin-releasing peptide)
- second messenger on parietal cell is IP3/Ca2+
-
Histamine
- receptors on parietla cell membrane
- H2
- Second messenger cAMP
- stimulates H+ secretion
- receptors on parietla cell membrane
- Direct:
- Increases H+ secretion by two pathways:
Describe the mechanisms by which somatostatin and prostaglandins inhibit gastric secretions
- Somatostatin:
- Inhibits gastric H+ secretions by direct and indirect pathway:
- Direct:
- Somatostatin binds to receptors on parietal cell that are coupled to adenyl cyclase via a Gi protein
- Antagonistic to stimulatory action of histamine
- Indirect:
- Inhibits the release of histamine and gastrin
- Direct:
- Inhibits gastric H+ secretions by direct and indirect pathway:
- Prostaglandins:
- Inhibits gastric H+ by activating Gi protein, inhibiting adenyl cyclase
List the major digestive enzymes and substrates secreted by the pancreas
- pancreatic digestive enzymes for proteins
- Trypsin (note they secrete trypsin inhibitor by grnadular cells to prevent trypsin action on pancreatic tissues)
- chymotrypsin
- carboxypolypeptidase
- Pancreatic digestive enzymes for carbohydrates
- Pancreatic amylase
- Pancreatic digestive enzymes for fat
- Pancreatic lipase
- cholesterol esterase
- phospholipase
Be able to describe the mechanism by which the pancreas secretes sodium bicarbonate
- secreted mainly by epithelial cells of the ductules and ducts that lead from the acini
- carbon dioxide diffuses to the interior of the cell form the blood and, under the influence of carbonic anhydrase, combines with water to form carbonic acid. Carbonic acid dissociates into bicarbonate ions and hydrogen ions . actively transferred into duct lumen
Descrbe the roles of acetylcholin, cholecystokinin, and secretin in regulating pancreatic secretion
- Acetylcholine
- Released in response to hydrogen ions, small peptides, amino acids, fatty acids in duodenum
- stimulates enzyme secretion by acinar cells and potentiates effect of secretin
- Cholecystokinin
- Release is stimulated by presence of food in upper intestine, especially small peptides, amino acids and fatty acids
- fxn:
- Results in dramatic increase in secretion of pancreatic enzymes
- Potentiates effect of secretin on ductal cells to stimulate bicarbonate secretion
- Secretin
- stimulated by presence fo acidic foods in upper intestine
- fxn:
- stimulates release of large amounts of sodium bicarbonate by ductal cells
What factors stimulate the release of bile from the gall bladder
- presence of fatty food in duodenum
Describe the formation of bile
- Primary biles acids:
- cholic acid and chenodeoxycholic acid are synthesized from cholesterol by hepatocytes
- bacteria in the intestine convert the primary bile acids into secondary bile acids (deoxycholic and lithocholic acid)
- Bile acids are conjugated with glycine or taurine
- Electrolytes and water are added to bile
- Bile is concentrated in the gallbladder as a result of isomotic absorption of solutes and water
List factors that contribute to the formation of gall stones
- Too much absorption of water from bile
- Too much absorption of bile acids from bile
- Too much cholesterol in bile
- Inflammation of epithelium
