Gastrointestinal Physiology

1. Gastrointestinal Physiology (4) GI SECRETION • Salivary Secretion • Gastric Secretion • Pancreatic Secretion • Bile Secretion by the Liver • Secretions of the Small Intestine • Secretions of the Large Intestine

2. 1. Salivary Secretion a. Structure of the salivary glands b. Formation of saliva c. Regulation of salivary secretion

3. a. Structure of the salivary glands serous Three major salivary glands: Parotid glands Submandibular glands Sublingual glands mixed serous + mucous

4. Unusual feature : PSNS SNS both stimulate Saliva production Unusual feature : unusual high blood flow, > 10 times the blood flow to exercising skeletal muscle (when corrected for organ size)

5. b. Formation of saliva Step 1 - The acinar cells secrete the initial saliva. - The initial saliva is isotonic. - It has the same electrolyte composition as plasma. Step 2 - The ductal cells modify the initial saliva. - Absorption of Na+, CI- > secretion of K+ and HCO3- - The final saliva is hypotonic.

6. c. Regulation of salivary secretion • Salivary secretion is exclusively under neural control. • Both PSNS and SNS stimulate saliva production. PSNS is primary. • Conditioning, food, thought, and nausea etc. also stimulate salivary secretion. • Dehydration, fear, and sleep inhibit salivary secretion. Primary controller of salivation, large amount of watery saliva containing enzymes PSNS: SNS: Small volume of saliva, thick with mucus Because sympathetic stimulation accompanies frightening or stressful situations, the mouth may feel dry at such times.

7. Summary of Salivary Secretion Characteristics of saliva secretion: • lubrication • Protection thiocyanate ions, proteolytic enzymes (lysozyme), IgA etc. • α-amylase, lingual lipase • Kallikrein cleaves kininogen to produce bradykinin (a strong vasodilator, accounts for high salivary blood flow) • high volume (approx. 1 L/day) • high K+ and HCO3- concentrations • low Na+ and Cl- concentrations • hypotonicity • The composition of saliva varies with flow rate. • pH of 6.0 – 7.0 Functions of saliva:

8. 2. Gastric Secretion a. Structure and cell types of the gastric mucosa b. HCL secretion c. Pepsinogen secretion and activation d. Intrinsic factor secretion

9. a. Structure and cell types of the gastric mucosa Oxyntic glands surface epithelium mucous neck cells (mostly mucus but also pepsinogen) peptic or chief cells(pepsinogen) parietal or oxyntic cells(HCl and intrinsic factor) paracrine cells (histamine) Pyloric glands surface epithelium mucous cells(pepsinogen, mucus) G cells(gastrin)

10. Gastric pit The inner surface of the stomach has deep wells called gastric pits. Each pit leads to gastric glands.

11. [ Enterochromaffin-like (ECL)] • histamine

12. In the resting, nonstimulated state, tubulovesicular membranes are presented in the apical portion of the parietal cell. Upon stimulation, cytoskeletal rearrangement causes the tubulovesicular membranes to fuse into the canalicular membrane. The is a substantial increase (50-100 fold) in the surface area of the apical membrane of the parietal cell, as well as the appearance of microvilli. Parietal cell: resting and stimulated.

13. b. HCI secretion Omeprazole (-) Alkaline tide (in gastric venous blood) Figure 8-7 Mechanism of HCl secretion by gastric parietal cells. ATP, Adenosine triphosphate.

14. Summary of HCI Secretion • Intracellular fluid: • Carbonic anhydrase • Apical membrane • H+-K+ATPase, inhibited byomeprazole • CI- channel • Basolateral membrane • Cl-—HCO3- exchanger • alkline tide • Net secretion of HCl, net absorption of HCO3-

15. secrete histamine

16. STIMULATION OF GASTRIC H+ SECRETION • Vagal stimulation Direct path:  Vagus nerve innervates parietal cells  Ach is the neurotransmitter Indirect path:  Vagus nerve innervates G cells gastrin H+ secretion  GRP is the neurotransmitter Atropine blocks ________ path. Vagotomy eliminates __________ pathway(s)

17. STIMULATION OF GASTRIC H+ SECRETION 2. Gastrin • is released in response to eating a meal. • stimulates H+ secretion by interacting with the CCKB receptor on the parietal cells. • the second messenger for gastrin on the parietal cell is IP3/Ca2+ 3. Histamine • is released from enterochromaffin-like (ECL) cells in the gastric mucosa and diffuse to the nearby parietal cells. • stimulates H+ secretion by activating H2 receptor on the parietal cell membrane. • the second messenger for histamine is cAMP.

18. STIMULATION OF GASTRIC H+ SECRETION 4. Potentiating effects of Ach, histamine, and gastrin on H+ secretion • Potentiation occurs when the response to stimultaneous administration of two stimulants is greater than the sum of response to either agent given alone. • Histamine potentiates the actions of Ach and gastrin; • Ach potentiates the actions of histamine and gastrin.

19. INHIBITION OF GASTRIC H+ SECRETION • Low pH (< 3) in the stomach • inhibits gastrin secretion by negative feedback, thus inhibits further H+ secretion. • 2. Somatostatin • direct pathway: • indirect pathway: • 3. Prostaglandins via Gi protein  cAMP  inhibit histamine release from ECL cells inhibit gastrin release from G cells

20. c. Pepsinogen secretion and activation H+ pepsinogen pepsin

21. Why doesn’t pepsin digest your stomach? H+ alcohol, aspirin Helicobacter pylori (H. pylori) major causative factor of gastric ulcer Produce NH4+, damages mucosal barrier

23. GASTRIC AND PEPTIC ULCERS Peptic ulcers • Erosions of the gastric and duodenal mucosaproduced by action of HCl • Results from • Excessive acid secretion (i.e., Zollinger-Ellison syndrome - ↑ secretion of gastrin) • ↓ protective properties of the mucosal barrier (i.e., Helicobacter pylori -bacterium that resides in GI tract that liquefy and penetrate the barrier) • Treatment: Antibiotics, proton pump inhibitors, inhibitors of gastric secretion, selective vagotomy Gastritis • Bacterial infection of gastric mucosa • Histamine released by tissue damage and inflammation stimulate further acid secretion • Ingested irritant substances (i.e., alcohol, NSAID), smoking

24. d. Intrinsic factor secretion Intrinsic factor (IF): a mucoprotein, secreted by parietal cells along with HCl. • Vitamin B12 requires IF to be absorbed. • IF combines with vitamin B12 to form a complex that is absorbed in the terminal ileum. • Vitamin B12 is essential for maturation of red blood cells. The absence of IF prevent absorption of B12 and leads to abnormal production of RBCs, which causes pernicioius anemia. • . IF-B12

25. A Summary of Gastric Secretion Gastric mucosal epithelium is made entirely of secretary cells including exocrine, endocrine, and paracrine cells. Gastric juice: Thick alkaline mucus by surface epithelium Thin watery mucus by neck cells HCl by parietal cells Pepsinogen by chief cells Intrinsic factor by parietal cells ~ 1.5 L is secreted per day, pH: 0.8 – 3.5

26. 3. Pancreatic Secretion a. Structure of the pancreatic exocrine glands b. Formation of pancreatic secretion c. Regulation of pancreatic secretion

27. a. Structure of the pancreatic exocrine glands

28. b. Formation of pancreatic secretion amylase lipases proteases Acinar cells enzymes trypsinogen chymotrypsinogen procarboxypeptidase proelastase Exocrine pancreas aqueous secretion (HCO3-) Ductal cells Trypsin inhibitor is secreted by acini to prevent activation of trypsin. If the pancreas is damaged, large quantities of pancreatic secretion pools in the damaged areas, and trypsin inhibitor is overwhelmed. Pancreatic secretions can digest the pancreas, which is known as acute pancreatitis.

29. Figure 8-21 Mechanism of pancreatic secretion. The enzymatic component is produced by acinar cells, and the aqueous component is produced by centroacinar and ductal cells. ATP, Adenosine triphosphate.

30. Enzymes often secreted in an inactive form, and activated near the wall of gastrointestinal tract - so food is broken down where it can be transported into the blood stream.

31. c. Regulation of pancreatic secretion Figure 8-23 Regulation of pancreatic secretion. ACh, Acetylcholine; cAMP, cyclic adenosine monophosphate; CCK, cholecystokinin; IP3, inositol 1,4,5-triphosphate.

32. The Cl channel is encoded by the cystic fibrosis gene product CFTR. Thus patients with cystic fibrosis, who lack a functional Cl channel have defective duct transport. The ducts get clogged with precipitated enzymes and mucus and the pancreas undergoes a fibrosis (hence the name of the disease). The physiological significance of this model is twofold, first the HCO delivered to the duodenal lumen neutralizes gastric acid and allows the digestive enzymes to operate at their pH optimum, close to neutral. Second, H which are produced in the duct cells when HCO is generated for secretion,leave via NaH exchange into the blood. The net effect is to neutralize the alkaline tide in the blood that was generated by gastric acid secretion.

33. A Summary of Pancreatic Secretion HCO3- : neutralize the contents from the stomach Enzymes: digestion of protein, carbohydrate, and fats Pancreatic juice is characterized by: - high volume (1 L/day) - virtually the same Na+ and K+ concentrations as plasma - much higher HCO3- concentration than plasma - much lower Cl- concentration than plasma - isotonicity - pancreatic lipase, amylase, and proteases - pH of 8.0 – 8.3

34. 4. Bile Secretion a. Overview of the biliary system b. Composition and functions of bile c. Formation of bile and function of the gallbladder d. Regulation of bile excretion from the gallbladder e. Enterohepatic circulation of bile salts f. Clinical correlation

35. a. Overview of the biliary system Figure 8-24 Secretion and enterohepatic circulation of bile salts.Light blue arrows show the path of bile flow; yellow arrows show the movement of ions and water. CCK, Cholecystokinin.

36. Hepatocytes secrete bile into the bile canaliculi and bile ductules. bile ductule

37. b. Composition and functions of bile CompositionFunctions Bile salts including bile acids (50%) Phospholipids (Lecithin, 40%) Bile pigments (2%): bilirubin Cholesterol (4%) Electrolytes (Na+, K+, Ca++, Cl-, HCO3-) Water aids in fat digestion aids in fat absorption emulsifying fat eliminating metabolic wastes

38. Bile salts Cholesterol Cholic acid Chenodeoxycholic acid (Primary bile acids) Glycine or taurine Glyco- or tauro-conjugatedbile acids (Bile salts) Intestinal bacteria Deoxycholic acid Lithocholic acid (Secondary bile acid)

39. c. Formation of bile and functions of the gallbladder Hepatocytes secrete: Bile ducts secrete: watery solution, Na+, HCO3- organic components (bile acids, cholesterol, bilirubin) Bile Gallbladder stores bile, concentrates bile, empties bile. Watery components are reabsorbed by the gallbladder mucosa. Organic constituents are highly concentrated (5 – 20 fold).

40. d. Regulation of bile excretion from the gallbladder CCK • is released in response to small peptides and fatty acids in the duodenum. • tells the gallbladder that fats need to be emulsified and absorbed – in other words, bile is needed. • causes contraction of the gallbladder smooth muscle. • causes relaxation of the sphincter of Oddi. ACh • also causes contraction of the gallbladder.

41. e. Enterohepatic circulation of bile salts diffuse Cholesterol 7α-hydroxylase (rate-limiting enzyme) 94% active transport Na+-bile salt cotransporter Figure 8-24 Secretion and enterohepatic circulation of bile salts.

42. f. Clinical correlation Gallstone formation: precipitated cholesterol high-fat diet, prone to the development of gallstones too much absorption of water from bile inflammation of epithelium Ileal resection IF-Vitamin B12 complex cannot be absorbed. Steatorrhea: recirculation of bile via enterohepatic circulation is reduced. Most secreted bile acids are lost in feces. Oil droplets in the stool. Diarrhea: bile acids  cAMP-dependent Cl- secretion in colonic epithelium, Na+ and water follow Cl- into the lumen

43. Mechanisms of stones formation  absorption of water in the gallbladder  absorption of bile acids ( solubility of cholesterol)  cholesterol concentration (fatty diet) Inflammation of the epithelium Mechanisms  the risk of stones formation Secretion of H+ by the mucosa (acidification of bile)  Ca 2+ precipitation Absorption of large amounts (about 50%) of Ca 2+ Release of the inhibitors of Ca2+ and cholesterol precipitation Secretion of water and electrolytes during digestion which intermittently dilute the gallbladder content Combination of cholesterol with lecithin and bile salts (micelles)  water solubility of cholesterol Contractions prevent accumulation of microcrystal GALLSTONES • 2 types of stones: • Cholesterol stones • Calcium bicarbonate stones

44. CONSEQUENCIES OF ILEAL RESECTION (INFLAMMATION) • IF-Vitamin B12 complex cannot be absorbed • Steatorrhea: ↓ recirculation of bile via enterohepatic circulation → loss of bile acids and depletion of bile acids pool → malabsorption of fats, oil droplets in the stool; cholesterol gallstones • Diarrhea: bile acids  cAMP-dependent Cl- secretion in colonic epithelium, Na+ and water follow Cl- into the lumen

45. 5. Secretions of the Small Intestine Brunner’s glands An extensive array of compound mucus glands Located in the wall of duodenum. Secrete mucus and HCO3- Crypts of Lieberkühn Located over the entire surface of the small intestine. Goblet cell: secrete mucus Enterocytes: secrete and absorb water and electrolytes

46. 6. Secretions of the Large Intestine The large intestine has many crypts of Lieberkühn and secrets an alkline mucus solution containing bicarbonate and K+. The sole function of mucus is protection. It protects the large intestine wall from damage by acids formed in feces from attacking the intestinal wall. Acid and mechanical stimulation, mediated by both long and short reflexes, increase the secretion of mucus. the wall of the large intestine Acid passage of feces Neural reflexes (long and short) Mucus secretion a mucus layer lining the wall

47. Case: Zollinger-Ellison Syndrome Description of Case: A 52-year-old man visits his physician complaining of abdominal pain, nausea, loss of appetite, frequent belching, and diarrhea. The man reports that his pain is worse at night and is sometimes relieved by eating food or taking antacids containing HCO3-. GI endoscopy reveals an ulcer in the duodenal bulb. Stool samples are positive for blood and fat. His serum gastrin level is measured and found to be markedly elevated. A CT scan reveals a 1.5 cm mass in the head of the pancreas. The man is referred to a surgeon. While awaiting surgery, the man is treated with the drug omeprazole, which inhibits H+ secretion by gastric parietal cells. During a laparotomy, a pancreatic tumor is located and excised. After surgery, the man’s symptoms diminish, and subsequent endoscopy shows that the duodenal ulcer has healed. Explanation of Case: All of the man’s symptoms and clinical manifestations are caused, directly or indirectly, by a gastrin-secreting tumor of the pancreas. In Zollinger-Ellison syndrome, the tumor secretes large amounts of gastrin into the circulation. The target cell for gastrin is the gastric parietal cell, where it stimulates H+ secretion. The physiologic source of gastrin, the gastric G cells, are under negative feedback control. Thus, normally, gastrin secretion and H+ secretion are inhibited when the gastric contents are acidified (i.e., when no more H+ is needed). In Zollinger-Ellison syndrome, however, this negative feedback control mechanism does not operate: gastrin secretion by the tumor is not inhibited when the gastric contents are acidified. Therefore, gastrin secretion continues unabated, as does H+ secretion by the parietal cells.

48. Case: Zollinger-Ellison Syndrome, explanation (cont.) The man’s diarrhea is caused by the large volume of fluid delivered from the stomach (stimulated by gastrin) to the small intestine; the volume is so great that it overwhelms the capacity of the intestine to absorb it. The presence of fat in the stool (steatorrhea) is abnormal, since mechanisms in the small intestine normally ensure that dietary fat is completely absorbed. Steatorrhea is present in Zollinger-Ellison syndrome for two reasons. 1) The first reason is that excess H+ is delivered from the stomach to the small intestine and overwhelms the buffering ability of HCO3--containing pancreatic juices. The duodenal contents remain at acidic pH rather than being neutralized, and the acidic pH inactivates pancreatic lipase. When pancreatic lipase is inactivated, it cannot digest dietary triglycerides to monoglycerides and fatty acids. Undigested triglycerides are not absorbed by intestinal epithelial cells, and thus, they are excreted in the stool. 2) The second reason for steatorrhea is that the acidity of the duodenal contents damages the intestinal mucosa (evidenced by the duodenal ulcer) and reduces the microvillar surface area for absorption. Treatment: While the man is awaiting surgery to remove the gastrin-secreting tumor, he is treated with omeprazole, which directly blocks the H+-K+-ATPase in the apical membrane of gastric parietal cells. This ATPase is responsible for gastric H+ secretion. The drug is expected to reduce H+ secretion and decrease the H+ load to the duodenum. Later, the gastrin-secreting tumor is surgically removed.

49. Case: Resection of the Ileum Description of Case: A 36-year-old woman has 75% of her ileum resected following a perforation caused by severe Crohn’s disease (chronic inflammatory disease of the intestine). Her postsurgical management included monthly injections of vitamin B12. After surgery, she experienced diarrhea and noted oil droplets in her stool. Her physician prescribed the drug cholestyramine to control her diarrhea, but she continues to have steatorrhea. Explanation of Case: The woman’s severe Crohn’s disease caused an intestinal perforation, which necessitated a subtotal ileectomy, removal of the terminal portion of the small intestine. Consequences of removing the ileum include decreased recirculation of bile acids to the liver and decreased reabsorption of the intrinsic factor-vitamin B12 complex. In normal persons with an intact ileum, 95% of the bile acids secreted in bile are returned to the liver, via the enterohepatic circulation, rather than being excreted in feces. This recirculation decreases the demand on the liver for the synthesis of new bile acids. In a patient who has had an ileectomy, most of the secreted bile acids are lost in feces, increasing the demand for synthesis of new bile acids. The liver is unable to keep pace with the demand, causing a decrease in the total bile acid pool. Because the pool is decreased, inadequate quantities of bile acids are secreted into the small intestine, and both emulsification of dietary lipids for digestion and micelle formation for absorption of lipids are compromised. As a result, dietary lipids are excreted in feces, seen as oil droplets in the stool (steatorrhea). This patient has lost another important function of the ileum, the absorption of vitamin B12. Normally, the ileum is the site of absorption of the intrinsic factor-vitamin B12 complex. Intrinsic factor is secreted by gastric parietal cells, forms a stable complex with dietary vitamin B12, and the complex then is absorbed in the ileum. The patient cannot absorb vitamin B12 and must receive monthly injections, bypassing the intestinal absorptive pathway. The woman’s diarrhea is caused, in part, by high concentrations of bile acids in the lumen of the colon (because they are not recirculated). Bile acids stimulate cAMP-dependent Cl- secretion in colonic epithelial cells. When Cl- secretion is stimulated, Na+ and water follow Cl- into the lumen, producing a secretory diarrhea (sometimes called bile acid diarrhea). Treatment: The drug cholestyramine, used to treat bile acid diarrhea, binds bile acids in the colon. In bound form, the bile acids do not stimulate Cl- secretion or cause secretory diarrhea. However, the woman will continue to have steatorrhea.

50. Contraction of the gallbladder is correctly described by which of the following statements? a. It is inhibited by a fat-rich meal b. It is inhibited by the presence of amino acids in the duodenum c. It is stimulated by atropine d. It occurs in response to cholecystokinin e. It occurs simultaneously with the contraction of the sphincter of Oddi D.