Neural and Neuroendocrine Control of Digestion: Regulation of Gastrointestinal Hormone Secretion

1. Neural and Neuroendocrine Control of Digestion: Regulation of Gastrointestinal Hormone Secretion (Chapter 11 in Moyers & Schulte: “Principles of Animal Physiology”) (Chapter 15 in “Eckert Animal Physiology) (Chapter 10 in Hadley: “Endocrinology”)

2. General Concepts Covered: — neural and neuroendocrine integration of a complex physiological process — co-ordination of endocrine and exocrine secretion — exocrine secretion affecting endocrine status — endocrine influences on neural, muscle and secretory functions — paracrine and autocrine effects — local influences — positive and negative “feed-back” actions — enzymatic cascades — integration of carbohydrate, fat and protein digestion and appetite control

3. General Introduction — digestion requires co-ordination of movement of food particles, mechanical and chemical digestion of food — opening and closing of sphincter muscles to contain food particles in certain gut compartments for appropriate processing, also control passage through compartments — proper sequential enzymatic processing From Wikimedia commons General X-sectional diagram of gut

4. General Introduction (cont.) — peristaltic action to move food through gut; wave of circular muscle contraction and relaxation down the length of the intestine; co-ordination with relaxation and contraction of longitudinal smooth muscle Animation: From Wikimedia commons

5. General Introduction (cont.) — peristaltic action to move food through gut — segmentation: hard contraction of circular muscles to create small bolus

6. General Introduction (cont.) — peristaltic action to move food through gut — segmentation: hard contraction of circular muscles to create small bolus — repeated contraction of circular muscles at different locations along a segment create kneading movement to mix food particles with enzymes and to expose new surfaces for digestion, as well as for absorption

7. Secretin - first proposed hormone — 1902, WM Baylist and EH Starling (observations in dogs) — addition of food or acid in stomach is usually followed by pancreatic enzyme secretion into intestine — denervate and tie off duodenum from stomach, then 1) add acid to stomach - no pancreatic enzyme secretion 2) add acid to blood - no pancreatic enzyme secretion 3) add partly digested food from stomach to duodenum - pancreatic enzyme secretion 4) add acid to duodenum - pancreatic enzyme secretion 5) tie off blood supply to and from duodenum and add acid to duodenum - no pancreatic enzyme secretion — hypothesis: duodenum exposed to acid from stomach releases a blood-borne factor to cause pancreatic enzyme secretion

8. Secretin - first proposed hormone (continues) — scrap off mucosal layer of duodenum (i.e., preparation of epithelial cells of the duodenum) add acid — extraction of soluble products secreted from the resulting mixture — inject crude purified material into blood-stream of an animal under controlled conditions - pancreatic enzyme secretion — conclusion: hormone released from duodenum to stimulate pancreatic enzyme secretion — hormone named SECRETIN

9. Quick Review of Enzymatic Digestion - General — enzymatic digestion usually involves hydrolysis of chemical bonds, separation of building units of the substrate, and the release of energy — some enzymes are very specific, acting on a limited range of substrates; others may be less specific — different conditions (e.g., pH) required for different enzymatic steps — many of these enzymatic steps in digestion take place extracellularly so that bigger molecules are broken down to small units that can be absorbed by the epithelial cells lining the gut — once absorbed, final metabolic processing can occur intracellularly

10. Quick Review of Enzymatic Digestion – Carbohydrate 1 — carbohydrates are sugar molecules — complex polysaccharides digested by polysaccharidase; e.g., amylase in saliva and pancreatic secretions breaks down plant and animal carbohydrates (starch and glycogen) into disaccharides (e.g., maltose, sucrose and lactose) Branching more extensive in glycogen than starch Glycogen & Starch From Wikimedia commons

11. Quick Review of Enzymatic Digestion – Carbohydrate 2 — maltase, sucrase and lactase in small intestine further reduces these disaccharides to monosaccharides — monosaccharides (pentose and hexose) can then be absorbed by the gut epithelium via carrier mediated transport systems Monosaccharides Disaccharides Sucrose Fructose Glucose Maltose From Wikimedia commons

12. Starch Disaccharides Glycogen Salivary amylase Mouth (alkaline) Starch Glycogen Oligosaccharides Stomach (acidic) Starch Glycogen Oligosaccharides Intestine (alkaline) Pancreatic amylase Disaccharides Disaccharidases Monosaccharides Summary of major carbohydrate digestion steps in vertebrates Modified from “Principles of Animal Physiology” by Moyes & Schulte, 2nd edition

13. Intestinal villi structure increases surface area for absorption Intestinal Wall From Wikimedia Commons

14. Carbohydrate 3: Carbohydrate absorption/uptake at the level of the intestine Lumen Low Glucose High Glucose Glucose Fructose Galactose Apical glucose uptake Low [glucose] condition: SGLT High [glucose] condition: GLUT2 also recruited to microvilli on apical membrane Galactose/ Glucose Fructose Fructose Galactose/ Glucose Na+ Na+ apical Na+/K+ ATPase (Active transport) Tight junction Enterocyte Enterocyte SGLT = sodium-glucose co-transporter GLUT = glucose transporter (diffusion transport) GLUT5 GLUT2 (facilitated diffusion transport) Glucose Fructose Galactose Glucose Fructose Galactose basolateral K+ Na+ K+ Na+ Intestinal Extracellular Fluid

15. Quick Review of Enzymatic Digestion – Carbohydrate 4 — other structural carbohydrates require other considerations — cellulose - cellulase (present in bacteria, snails, arthropods) — in many vertebrate herbivores, microbiol communities of the gut (bacterial, fungi, yeasts & protists) are important for the digestion of cellulose; specialized fermentation chambers of non-acidic environment required — foregut fermenters (ruminants: cows, sheep); midgut fermenters (herbivorous fishes); hindgut fermenters (in cecum and/or colon: rabbits, horses, elephants) — products of cellulose are often short-chain fatty acids (volatile fatty acid), CO2 and methane gas — short-chain fatty acids are usually easily absorbed since they are both water and fat soluble — chitin - chitinase (present in frogs and other insect eating animals)

16. Chitin: repeated units of N-acetylglucosamine in b-1,4 linkages Cellulose From Wikimedia Commons

17. Starch Cellulose Disaccharides Glycogen Salivary amylase Mouth (alkaline) Starch Glycogen Oligosaccharides Stomach (acidic) Starch Glycogen Oligosaccharides Intestine (alkaline) Pancreatic amylase Disaccharides Short-chain fatty acids Gut microbs cellulase Disaccharidases Special gut compartment (not always present) Monosaccharides Cellulose Summary of major carbohydrate digestion steps in vertebrates Modified from “Principles of Animal Physiology” by Moyes & Schulte, 2nd edition

18. Example of a foregut fermenter (e.g., cow, sheep) Rumen: from Wikimedia Commons (From: Moyes & Schulte)

19. Examples of hindgut fermenters (e.g., pony, rabbit) comparison with the human cecum from Wikimedia Commons (From: Moyes & Schulte) NOTE: Large/long cecum relative to other parts of the intestine

20. Other roles of gut microflora in addition to helping in digestion? — Metabolites and secretion products of these microflora have been shown to regulate the immune system of the host leading to decreased inflammatory and hypersensitivity responses. — Lowering of cancer rate, infection rate, blood pressure have also been proposed. — Concept of food containing probiotics (e.g., beneficial bacteria) and prebiotics (e.g., nutrients for enhancing the growth of beneficial bacteria)

21. Quick Review of Enzymatic Digestion – Lipid 1 — triglycerides with a glycerol backbone and three fatty acid side-chains or a phosphosugar plus two fatty acid side-chains Glycerol Triglyceride (From: Wikimedia Commons)

22. Fat globule Quick Review of Enzymatic Digestion – Lipid 2 lipase Lipase Triglyceride Diglyceride Monoglyceride Glycerol Fatty Acid Bile salt Chylomicron droplets micelles Glycerol, fatty acids and monoglycerides in micelles freely enter cells at the brush border Bile salt SER – triglyceride reassembled Golgi stacks Exocytosis of chylomicrons to extracellular fluid Water soluble fatty acids and glycerol can enter the blood Blood vascular system Lymphatic system Processing in liver

23. Quick Review of Enzymatic Digestion – Lipid 3 — extracellular digestion of lipid usually occurs in the intestinal lumen — small lipid droplets required for digestion - emulsifier needed (e.g., bile salts) — small lipid droplets hydrolyzed by lipases (secreted by pancreas in vertebrates) into glycerol and fatty acid components — formation of micelle (single layer of fatty acids, monoglycerides, glycerol and bile salts with hydrophilic heads outside and the hydrophobic tails inside) — components of micelle can easily pass through plasma membrane of enterocytes at the microvillus — eventual formation of chylomicrons in cells — chylomicrons (small lipid droplets coated in proteins) formed in epithelial cells of intestine — chylomicrons are carried via the lymphatic system to the venous blood and further processed in the liver

24. Quick Review of Enzymatic Digestion – Lipid 4 — once they enter the target cells from the blood stream, glycerol and fatty acids can be processed through the Kreb’s cycle (TCA cycle - tricarboxylic acid cycle) and beta-oxidation pathways — in addition to NADH and FADH2, beta-oxidation yields AcetylCoA which also enters the Kreb’s cycle — Note: bile is alkaline so this terminates acidic gastric digestion; also carries water-insoluble wastes filtered from the blood in liver (e.g., steroids, cholesterol, hemoglobin, lipid soluble drugs) Note: although the bulk of the digestion of lipids occurs in the intestine, lingual lipases (with optimal pH around 4.5 to 5) are secreted by glands in the tongue and thus can be present in small amounts in the saliva

25. Quick Review of Enzymatic Digestion – Protein 1 — proteases or peptidases - endopeptidases and exopeptidases, endopeptidases usually act first — endopeptidase - break bonds in the “middle” of peptides (e.g., pepsin in stomach secretion, trypsin and chymotrypsin in pancreatic secretion) — pepsin requires acidic environment; trypsin and chymotrypsin require a slightly alkaline environment — exopeptidases - cleaves off terminal amino acids (amino- and carboxy-peptidases) - produced by pancreas — short peptide fragments result from gastric and pancreatic digestion — other exopeptidases (tripeptidases and dipeptidases) finishes off the digestion of the small peptide fragments in the intestine — amino acids and some dipeptides are absorbed by the gut epithelium via carrier-mediated transports (e.g., amino acid-Na+ co-transport)

26. Proteins Mouth (alkaline) Proteins Stomach (acidic; mainly endopeptidases) Pepsin Mainly large polypeptides Large polypeptides Intestine (alkaline; endopeptidases and exopeptidases) Trpsin, chymotrpsin, carboxypeptidases Dipeptides Dipeptidases Amino acids Summary of major protein digestion steps in vertebrates Modified from “Principles of Animal Physiology” by Moyes & Schulte, 2nd edition

27. Quick Review of Enzymatic Digestion – Protein 2 Proline Glutamine Histidine Arginine Alanine Cysteine Glycine Serine Threonine Isoleucine Methionine Valine Glutamate aKG Pyruvate Succinyl CoA Citrate AcCoA Oxaloacetate Fumerate AcetoacetylCoA Phenylalanine Tyrosine Asparagine Aspartate Phenylalanine Tyrosine Leucine Lysine Tryptophan

28. Quick Review of Enzymatic Digestion – others — nucleases, nucleotidases and nucleosidases act on nucleic acids and their residues — esterases act on esters (C-O-C; compounds formed from an alcohol and an acid) — although these processes do not yield products that lead to direct energy production, these products are essential building blocks for other complex molecules in the body

29. Prevention of self-digestion of gut epithelium — thick layer of mucus secreted (sugar-based) — many proteases are secreted as inactive precursor molecules - proenzymes or zymogens — secretion of proenzymes also ensure that co-ordinated digestion is possible as they can be activated as needed, but only under correct conditions In Stomach: — production of the zymogen pepsinogen as the secreted compound — stomach cells also secrete acid — pepsinogen (inactive proenzyme) can be cleaved by acid to pepsin (active enzyme); pepsin can also do this acid Pepsinogen Pepsin

30. Prevention of self-digestion of gut epithelium In the small intestine: — production of trypsinogen and chymotrypsinogen as the secreted zymogens from the pancreas — pancreatic secretion is also alkaline — under alkaline conditions, enterokinase (also called enteropeptidase) released from the intestine (duodenum) cleaves off the 6 amino acids at the N-terminal of trypsinogen to produce trypsin; trypsin can also do this — trypsin also converts chymotrypsinogen to chymotrypsin High pH Enterokinase Trpsinogen Trypsin Chymotrypsinogen Chymotrypsin

31. Neural and Endocrine Control of Digestive Secretion General: — in vertebrates, control of digestive enzymes secretion is by both neural and endocrine means — neural - quick reflex responses — endocrine - more long-term control — many of these hormones are produced by endocrine cells dispersed throughout the epithelial lining of the gut (a diffused or dispersed endocrine system) — gastrointestinal hormones — also called the enterochromaffin-like cells system (entero- = gut; chromaffin - a cell-type stainable with silver salts) — many of these peptide hormones are also found in neurons in the CNS and in the peripheral nervous system (gut-brain peptides)

32. Digestion in the oral cavity — chewing mechanically breaks down food and mixes food with saliva — amylase present in saliva (slightly alkaline) — amylase action starts carbohydrate digestion — mixing action of chewing and muscular movement of tongue forms food particles into small bolus — saliva also contains mucin (glycoprotein; proteinaceous mucopolysaccharide) — mucin acts as a lubricant - helps swallowing and the bolus to slide down the esophagus with the help of peristaltic movements of smooth muscles lining the esophagus

33. Control of Salivary Gland Secretion Visual, Olfactory, Auditory, Tactile & Other cues Thought Food in mouth Higher Brain Centers Chemosensory & Tactile cues Hypothalamus Brain Stem Salivatory Motor Center Salivary Gland Parasympathetic Ach neurons Salivary Secretion Increased Amylase and Mucin Content Carbohydrate Digestion Lubrication

34. Control of Gastric Secretions — gastric digestion occurs in stomach — carbohydrate, protein and some fat are digested — pepsin, amylase, lipase and tributyrase present in stomach enzymatic mix — three phases - cephalic, gastric and intestinal — cephalic phase - gastric digestion events initiated and controlled by neuronal input from the CNS — gastric phase - gastric digestion events initiated and controlled by gastric secretion(s) — intestinal phase - gastric digestion events initiated and controlled by intestinal secretions — events in these three phases of digestions interact and affect one another (From: Moyes & Schulte)

35. Abbreviations for regulators of gastric digestion Ach = Acetylcholine SS = Somatostatin GRP = Gastrin-releasing peptide GIP = Gastric-inhibitory peptide VIP = Vasoactive intestinal peptide CCK = Cholecystokinin

36. Control of Gastric Secretion D-cell (+) Ach (+) SS Ach, GRP Ach (+) (+) (–) Parietal Cell (+) Chief cell (–) (+) D-cell (+) (+) H+ H+ Blood Stream H+ Pepsinogen Pepsin amino acid H+ nutrient (+) Gastrin Food & resulting distention (–) (+) G-cell SS Ach (+) (–) (–) (–) Stretch receptor Ach Stomach muscle motility Gastrin release from duodenum VIP (–) (+) Peptides in chyme (relax) (contract) CCK gastrin histamine Mast cell gastrin gastrin Protein STOMACH (+) GRP CCK, GIP, secretin, bulbogastrone, VIP Pyloric sphincter Vagal/Vagal Reflex Food in small intestine Too much Food in gut

37. Control of Intestinal Digestion — completion of extracellular digestion of protein, starch and fat — pancreas - the major enzymatic secretion organ (amylase, trypsin, chymotrypsin, carboxypeptidases, aminopeptidases, lipases, cholesterol esterase, phospholipases, nucleases, trypsin inhibitor) — pancreas also site of alkaline fluid secretion (neutralization of acid from the stomach as well as to provide suitable pH environment for the above enzymes) — bile secretion also required for digestion of fat — co-ordination of secretion events and muscular events (peristaltic and kneading movements, as well as contraction of gall bladder and co-ordination of sphincter muscles) needed — hormone secreted from intestine regulate gastric digestion events, satiation, and secretion of other metabolic regulators

38. Control of Intestinal Digestion Gut movement, mucus secretion Vasodilation of gut blood vessels H+ H+ VIP bulbogastrone H+ ? Cell ? Ach Ach (+) (+) HCO3- Peptides Amino acids protein Gall bladder (+) IntestinalLumen Satiation Center of Brain Hormonal Control of Stomach H cell Starch & Complex Carbohydrate S cell Secretin Amylase Simple Sugars (glucose) HCO3- Pancreas enzymes Trypsin, Chymotrypsin, exopeptidases lipase I cell CCK Free Fatty Acid Bile salts Fat Hormonal control of gastric digestion & stimulation of insulin release K cell GIP

39. Other endocrine factors from the gut that control hunger and satiation • Ghrelin – an orexigenic hormone • — a peptide of the motilin family (motilin is a duodenal hormonal secretion that regulates gastrointestinal motility) • — stomach is the major source of ghrelin in the blood • — fasting and hypoglycemia elevates ghrelin mRNA expression, protein production and secretion from the stomach • — activity of ghrelin secretory cells is affected by neuroendocrine factors and by lumenal environment of the stomach • — secretion can also be entrained by regular feeding schedule • — levels of expression and secretion drops within a hour of feeding • — ghrelin acts on the brain the stimulate hunger • — can also act locally at the level of the stomach to stimulate gastric acid release and motility • — ghrelin also stimulates pituitary GH secretion and GH in turn inhibits ghrelin release

40. Other endocrine factors from the gut that control hunger and satiation 2) Leptin – an anorexigenic hormone — originally found to be produced from adipocytes — thought to be an indicator of the status of fat storage in adipocytes — acts on the brain to reduce hunger and increase satiation — more recently also found to be produced by the stomach and can be released by the stomach into the blood after feeding — endocrine secretion from stomach can be stimulated by the presence of gastric acid in the stomach lumen — can also act locally at the stomach to decrease gastric secretion and motility — essentially works with ghrelin as an antagonistic pair