Organisms and Pathophysiologyã The Trustee of the Wellcome Trust 1998Reviewed by: Professor I W Booth, Institute of Child Health, University of Birmingham,Professor B S Drašar, Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine and Dr W A M Cutting, Department of Child Life and Health, The University of Edinburgh, UKPicture: A child with severe dehydration due to diarrhoea.Copyright Image from United Nations Children's Fund, India.Image references ################ .\IMAGES\T23159.jpg Contents Click on the underlined text to jump tothat screen. Screen 3 Objectives 4Introduction 5The Physiology of Healthy Bowel 13 Assessment 14 Fluid and Electrolyte Flux at the Cellular Level 26 Assessments 28 Pathophysiological Mechanisms of Diarrhoea 35 Assessment 36 Mechanisms of Diarrhoea Caused by Common Pathogens 46 Assessments 48SummaryUnderlined text is interactive. Click on underlined text to view extra information or to jump to another screen. Picture: A bloody stool from a patient with bacillary dysentery.Copyright Copyright Image from Behrens RH.Image references ################ .\IMAGES\T25584c.jpg ObjectivesAt the end of this tutorial you should be able to: 1. describe the physiology of healthy bowel in terms of functional anatomy and fluid exchanges 2. summarize fluid and electrolyte transportin small bowel enterocytes and its regulation 3. describe the general pathophysiological mechanisms of: · secretory (enterotoxin induced) diarrhoea · inflammatory and non-inflammatory invasive diarrhoea· osmotic diarrhoea 4. summarize the mechanisms by which key pathogens cause diarrhoea Image references ################ Introduction Diarrhoea Diarrhoea is: · an increase in the number, volume and water content of stools · a major global cause of illness and death· a factor in childhood malnutrition This tutorial concentrates on acute infectious diarrhoea in the developing world. How does diarrhoea present? Picture: A watery stool from a patient with diarrhoea due to enterotoxigenic Escherichia coli.Copyright Image from Tubbs HR. Diarrhoea morbidity and mortalityEach year there are: · approximately 3.3 million deaths due to diarrhoea, 80% in children under 2 years of age · over 1 billion episodes of diarrhoea, most in the developing world · 5 - 10 million travellers affected by diarrhoeaHow does diarrhoea present?Infectious diarrhoea presents as three clinical syndromes:· acute watery diarrhoea · acute bloody diarrhoea or dysentery · persistent diarrhoea Acute diarrhoea by definition lasts less than 14 days, often less than 5 - 7 days. Image references ################ .\IMAGES\T22792.jpg The Physiology of Healthy Bowel The Physiology of Healthy BowelImage references ################ The Physiology of Healthy Bowel - 1Why study healthy bowel?Understanding how healthy bowel works helps to explain the: · pathophysiology of diarrhoea · basis for treatment with oral rehydration salts solution (see picture) Picture: Oral rehydration treatment being given for diarrhoea in Bangladesh.Copyright Image from Cutting WAM. Pathophysiology of diarrhoeaDiarrhoea reflects a disturbance in the transport across the bowel wall (flux) of: · electrolytes, eg. Na+, Cl-, K+ · water Understanding what controls these fluxes in healthy bowel helps explain the mechanisms of diarrhoea.Oral rehydration treatment Understanding normal bowel function helps show why this simple treatment is so effective. Refer to the tutorial Diarrhoeal Diseases: Rehydration and Early Feeding. Image references ################ .\IMAGES\T44894.jpg The Physiology of Healthy Bowel - 2Absorption of Nutrients by the Bowel Food type Absorbed digestion product Examples Carbohydrates Monosaccharides Glucose, galactose,fructose Proteins Amino acids, dipeptides Alanine, aspartate Lipids Monoglycerides, fatty Short and longacids, glycerol chain fatty acids Absorption by healthy bowel A major role of the intestine is absorption of: · nutrients (see table) · electrolytes · water (see screen 8)Absorption of electrolytesElectrolytes (eg. Na+, Cl-, K+) enter the bowel from: · gastrointestinal (GI) secretions · ingested food and liquid Image references ################ The Physiology of Healthy Bowel - 3 Saliva (1 litre) Ingested fluids (2 litres) Gastric juice (2 litres) Bile (1 litre) Pancreatic juice (2 litres) Intestinal juice (1 litre) Total 9 litres AbsorptionSmall bowel (8 litres) Large bowel (0.9 litres) Ingestion and secretion Fluid exchanges in healthy bowel Fluid enters the GI tract from: · ingestion of liquids · GI secretions (eg. saliva, intestinal juice) Fluid is absorbed into the blood by the: · small bowel (90% of total fluid) · large bowel (10% of total fluid) Diarrhoea occurs when the fluid volume excreted in the stool exceeds 100 - 200 ml/day. This occurs when the bowel shows: · increased secretion and/or · decreased absorption Picture: Daily fluid exchanges along the healthy adult GI tract. Excreted in stool (0.1 litre) Total 8.9 litres Copyright Image from The Wellcome Trust modified from Tortora GJ, Grabowski SR. The digestive system. In: Principles of anatomy and physiology. 7th ed. New York: Harper Collins, 1993:765-820.Fluid exchanges in the GI tractThe values shown in the picture: · are for an adult in a temperate climate · may be affected by other water losses due to sweating and breathing In children the volumes: · are lower at all sites · show a broadly similar pattern between sites Image references ################ .\IMAGES\Gitract.gif The Physiology of Healthy Bowel - 4Functional anatomy of the small bowel Four structural features of the small bowel maximize the surface area for absorption: 1. length 2. circular folds3. villi (see picture)4. microvilli (see screen 10) Features 2 - 4 increase the absorptive area by about 600 times compared with a simple cylinder of the same dimensions.A section through the small bowel wall. Villus Mucosa Crypt Submucosa Muscularis Serosa Copyright Image from The Wellcome Trust modified from Tortora GJ, Grabowski SR. The digestive system. In: Principles of anatomy and physiology. 7th ed. New York: Harper Collins, 1993:765-820.Length of Segments of the Small BowelSegment Typical length in an adult (m) Duodenum 0.25Jejunum 2.5 Ileum 3.6 Total 6.35Circular foldsThe circular folds: · are permanent ridges in the mucosa · enhance absorption by making intestinal content spiral along the bowel Image references ################ .\IMAGES\Sbowel.jpg The Physiology of Healthy Bowel - 5Structure of the villus. The functional unit of the small bowel is the villus. Absorption from the small bowel lumen occurs: 1. through absorptive epithelial cells (enterocytes) 2. into the villous capillary network and lacteal Enterocytes have a brush border membrane of microvilli to increase their absorptive area. Within each villus, enterocytes: · arise continuously from stem cells in the crypt region· migrate toward the villus apex · differentiate as they migrate from crypt to apex · are shed from the apex into the bowel lumen Which types of cell occur in the villous epithelium?Capillaries Lamina propria Enterocyte Lacteal Goblet cell Copyright Image from The Wellcome Trust modified from Tortora GJ, Grabowski SR. The digestive system. In: Principles of anatomy and physiology. 7th ed. New York: Harper Collins, 1993:765-820.Brush border membranePicture: The brush border membrane. Note that this picture is an SEM at high magnification. Image from Booth IW. The luminal border of each enterocyte is formed into elongated microvilli (the brush border membrane). This greatly increases the surface area for absorption. Cells of the villous epitheliumThe villous epithelium contains absorptive enterocytes and these non-absorptive cells: · goblet cells, which secrete mucus · enteroendocrine cells, which secrete neuropeptides · Paneth cells, which secrete lysozyme and other protective factorsMigration of enterocytesMigration of enterocytes from crypt to apex takes about 5 days. Image references ################ .\IMAGES\Gvillus.jpg .\IMAGES\T44890p.jpg The Physiology of Healthy Bowel - 6Overview of fluid and electrolyte flux in the villus Net fluid and electrolyte exchanges vary along the villus (see picture). Water flows across the epithelium by osmosis, driven by two ions in particular. What are these ions in: · absorption? · secretion? In healthy bowel absorption dominates over secretion. Net fluid and electrolyte fluxes in the villus. Mature enterocytes Villus tip Net absorption of fluid and electrolytes Bowel lumen Net secretionof fluid and electrolytes Crypt Immature enterocytes Copyright Image from The Wellcome Trust modified from World Health Organization. Readings on diarrhoea. Student manual. Geneva: WHO, 1992. Variation in fluxThese differences reflect the differentiation of the enterocyte as it migrates along the villus.AbsorptionAbsorption of water: · follows uptake of Na+ from the bowel lumen· is characteristic of mature apical villus enterocytes Na+ H2O SecretionLoss of water: · follows secretion of Cl-into the bowel lumen· is characteristic of immature crypt enterocytes Cl- H2OImage references ################ .\IMAGES\Abssec.gif The Physiology of Healthy Bowel - 7A section through the large bowel wall.Absorption by the large bowel The major roles of the large bowel are absorption of water and electrolytes, and formation of the stool. The colonic mucosa (see picture): · has crypts but no villi · contains two epithelial cell types: - columnar absorptive cells (colonocytes) - goblet cells, which secrete mucus Colonocytes: · have rudimentary microvilli shorter than those in enterocytes · differentiate as they migrate up the crypt (like enterocytes) · absorb water and electrolytes at the luminal surface Opening of intestinal gland Colonocyte Mucosa Submucosa Muscularis Serosa Copyright Image from The Wellcome Trust modified from Tortora GJ, Grabowski SR. The digestive system. In: Principles of anatomy and physiology. 7th ed. New York: Harper Collins, 1993:765-820.Absorption Compared with the small bowel, the large bowel: · absorbs a much smaller volume of water · is working against a greater osmotic gradient Image references ################ .\IMAGES\Lgebowel.jpg Introduction and the Physiology of Healthy Bowel: Assessment Are the following statements about diarrhoea and normal bowel function true or false?To return to the start of the section. Click on the True or False button for each statement. 1. Diarrhoea causes 34 million deaths in developing countries each year.2. Small bowel enterocytes mature as they migrate from the crypts of Lieberkühn to the apical villus.3. Absorption by the large bowel is increased by the presence of villi.4. Net fluid flux in the apical part of the small bowel villus is absorption and in the crypt region net flux is secretion. Correct Text explaining the answer (11-pt plain blue) Incorrect The estimated global impact of diarrhoeal disease each year is over: · 3 million deaths · 1 billion episodesIncorrect Text explaining the answer (11-pt plain blue) Correct The estimated global impact of diarrhoeal disease each year is over: · 3 million deaths · 1 billion episodesCorrect Small bowel enterocytes: · arise continuously from stem cells in the crypt region· migrate toward the villus apex · differentiate as they migrateIncorrect Text explaining the answer (11-pt plain blue) Incorrect Small bowel enterocytes: · arise continuously from stem cells in the crypt region· migrate toward the villus apex · differentiate as they migrateCorrect Text explaining the answer (11-pt plain blue) Correct Text 11 pt Arial dark blue goes here Incorrect Villi are: · characteristic of the small bowel · absent from the large bowelIncorrect Text 11 pt Arial dark blue goes here Correct Villi are: · characteristic of the small bowel · absent from the large bowelCorrect Enterocytes in the: · villous apex are mature and absorptive · crypts are immature and secretoryIncorrect Text 11 pt Arial dark blue goes here Incorrect Enterocytes in the: · villous apex are mature and absorptive · crypts are immature and secretoryCorrect Text 11 pt Arial dark blue goes hereImage references ################ Fluid and Electrolyte Flux at the Cellular LevelFluid and Electrolyte Flux at the Cellular Level Image references ################ Fluid and Electrolyte Flux at the Cellular Level - 1 Electrochemical gradients and transport across membranes An electrochemical gradient of a molecule across a membrane consists of: 1. a chemical gradient or difference in transmembrane concentration 2. an electrical gradient or difference in transmembrane electrical potential Cellular movement of molecules can be of various types: · passive transport · active transport: - primary active transport - secondary active transport ·solvent drag For further definitions.Copyright Image and animations from The Wellcome Trust. Electrical gradientsElectrical gradients exist only for charged molecules or atoms (ions).In passive transport the molecule moves downhill, eg. from high to low concentration. In active transport the molecule moves uphill, eg. from low to highconcentration.In primary active transport the uphill movement uses energy from ATP. In secondary active transport the uphill movement of one molecule () uses energy from the downhill movement of another ().In solvent drag, solutes in high luminal concentration are carried through tight junctions by the flow of water (). Further definitions Transport across membranes can be further classified as: · cotransport · exchange Cotransport is defined as the linked movement of two molecules in the same direction. Exchange is defined as the linked movement of two molecules in opposite directions. It is carried out by a symport. Image references ################ .\IMAGES\Aniback.bmp Fluid and Electrolyte Flux at the Cellular Level - 2The key electrochemical gradient in membrane is the Na+ - K+-ATPase (‘sodium pump’). This exchanger: · imports 2 K+ ions· converts ATP to ADP to provide energy To runthe animation. What type of transport is this? These processes set up electrochemical gradients for Na+ across the: · basolateral membrane · brush border membraneThese electrochemical Na+ gradients drive the key transport processes in the enterocyte.the enterocyte is due to Na+ A key transporter in the enterocyte basolateral · exports 3 Na+ ions Sodium pump Copyright Image and animation from The Wellcome Trust. Type of transportThe Na+ - K+-ATPase: · moves Na+ up its electrochemical gradient · uses ATP for energy This is thus primary active transport. Electrochemical gradients The electrical potentials due to the sodium pump are: · 0 mV in the lumen · - 40 mV inside the enterocyte · + 10 mV in the lamina propria Image references ################ .\IMAGES\Sodp0001.bmp Fluid and Electrolyte Flux at the Cellular Level - 3Absorption by the apical villus cell The apical villus cell absorbs: · monosaccharides and Na+ · amino acids and Na+· Cl-and water through tight junctions (see screen 20) All these processes are driven by the Na+ gradients set up by the Na+ - K+-ATPase. Animation: Absorption of monosaccharides and amino acids by the apical villus cell. To run the animation. Glucose or amino acid Across the brush border membrane. Cotransporter Sodium pump Copyright Image and animation from The Wellcome Trust. Uptake across the brush border membraneSugars and amino acids are taken up: · as digestion products after a meal · by specific carriers (eg. glucose - Na+ and glycine - Na+ cotransporters) · using secondary active transportUptake through tight junctionsThe forces driving these paracellular processes are: · for Cl-, a positive electrical potential (+10 mV) · for water, a high osmotic tensionExtracellular fluid compared with bowel lumen.Image references ################ .\IMAGES\Uptk0001.bmp Fluid and Electrolyte Flux at the Cellular Level - 4 Na+ absorption by the apical villus cell between meals After a meal: · nutrients such as monosaccharides and amino acids are present in the bowel lumen · Na+ is absorbed by nutrient - Na+ cotransporters (see screen 17) Between meals: · nutrients are absent from the bowel lumen · Na+ is absorbed by a different carrier, the Na+-Cl- cotransporter To run the animation. Animation: Absorption of Na+ and Cl- by the Na+ - Cl-cotransporter.Sodium pump Cotransporter Copyright Image and animation from The Wellcome Trust. Na+ - Cl- cotransporter The Na+ - Cl- cotransporter: · is driven by the Na+ gradient across the brush border membrane · maintains electroneutrality and pH homoeostasis Na+ - Cl- cotransporter The ‘Na+ - Cl- cotransporter’ in the brush border membrane is in fact two transporters: · a Na+ - H+ exchanger · a Cl- - HCO3- exchanger The combined action of this double exchange is the same as that of a single Na+ - Cl- cotransporter. Image references ################ .\IMAGES\Ctrn0001.bmp Fluid and Electrolyte Flux at the Cellular Level - 5 Secretion by the crypt cell The crypt cell secretes Cl- by maintaining this ion at a high intracellular concentration. 1. A Na+ - K+ - 2Cl-cotransporter in the basolateral membrane imports Cl-.2. A Cl-channel in the brush border membrane opens to allow passive efflux of Cl-. 3. Cl-secretion drives paracellular Animation: Secretion of Cl-, Na+ and water by the crypt cell.To run the animation.Cotransporter Cl- channel Sodium pump secretion of Na+ and water via tight junctions. Copyright Image and animation from The Wellcome Trust. Na+ - K+- 2Cl- cotransporterThis transporter: · is electroneutral (involves no net charge transfer) · moves Cl- into the cell against its gradient by secondary active transportParacellular secretion Tight junctions between crypt cells have greater permeability than tight junctions between enterocytes in the apical villus.Image references ################ .\IMAGES\Crpt0001.bmp Fluid and Electrolyte Flux at the Cellular Level - 6The structure of a tight junction.Tight junctions and the paracellular route Diffusion through tight junctions, the paracellular route, is a major mechanism of electrolyte and water flux. Tight junctions are: · present near the luminal surface of neighboring enterocytes (see picture) · the route for passive: - absorption of Cl- and water between apical villus enterocytes - secretion of Na+ and water between crypt enterocytes · important in maintaining membrane asymmetry· regulated by nutrients and toxinsPlasma membrane Occludin Enterocyte Other proteins Spectrin Cingulin Actin filaments Copyright Image from The Wellcome Trust modified from Ballard ST, Hunter JH, Taylor AE. Annu Rev Nutr 1995;15:35-55. Membrane asymmetryBy restricting movement of proteins within the plasma membrane, tight junctions help keep: · nutrient - Na+ cotransporters in the brush border membrane · Na+ - K+-ATPase and adenylate cyclase in the basolateral membrane Keeping the enterocyte membranes distinct is central to cell function.Regulation by nutrientsAfter a meal: · luminal glucose causes the permeability of tight junctions to increase · absorption of Cl- and water by the paracellular route is favoured Image references ################ .\IMAGES\Tightjn.gif Fluid and Electrolyte Flux at the Cellular Level - 7Apical cell Fluid and electrolyte flux in enterocytes: a summary Monosaccharide K+ Na+ Fluid and Electrolyte Flux in Enterocytes Apical villusCrypt enterocyte enterocyte Membrane Absorption of Na+ Secretion of transporters and nutrientsCl- TightAbsorption of Secretion of junctions Cl- and water Na+and water Na+ Amino acid Na+ H2O Cl- Crypt cell K+ Cl- Cl- Na+ K+ Na+ Na+ Pictures: A summary of key transporters in apical and crypt cells. H2O Copyright Image from The Wellcome Trust.Image references ################ .\IMAGES\Apicryp.gif Fluid and Electrolyte Flux at the Cellular Level - 8Regulation of electrolyte transport in the small bowel - 1 What controls electrolyte transport? Factors that have been identified include: · hormones · neurotransmitters · inflammatory mediators · neurohumoral factors · pharmacological agents · nutrients · bacterial toxins Each of these acts initially by binding to a specific receptor in the enterocyte brush border membrane. Regulation by nutrientsFor example, the permeability of tight junctions rises in the presence of intraluminal glucose after a meal. Image references ################ Fluid and Electrolyte Flux at the Cellular Level - 9Regulatory substances LUMEN Regulation of electrolyte transportin the small bowel - 2 Binding of regulatory substances to their receptors activates a complex cascade involving: · activation of regulatory proteins such as: - stimulatory GTP binding (Gs) proteins - adenylate cyclase - guanylate cyclase - phospholipase C (PLC) · production of second messengers such as: - cAMP (see screen 24) and cGMP - inositol triphosphate (IP3) - Ca2+ - diacyl glycerol (DAG) · activation of protein kinases by second messengers (eg. cAMP, DAG)Protein kinase C R R PLC Adenylate cyclase G protein IP3 DAG cAMP Ca2+ Electrolyte/nutrient transporters Picture: The cascade of regulation of electrolyte transport. The diagram is highly simplified and shows only a selection of key interactions. R, receptor. Second messengers are shown in red text.Copyright Image from The Wellcome Trust. Guanylate cyclaseGuanylate cyclase converts guanosine triphosphate (GTP) into cyclic guanosine monophosphate (cGMP).Adenylate cyclaseAdenylate cyclase converts adenosine triphosphate (ATP) into cyclic adenosine monophosphate (cAMP). Protein kinases Activated protein kinases phosphorylate key amino acids intarget proteins, leading to inhibition or stimulation of membrane transporters.Image references ################ .\IMAGES\Regulat.gif Fluid and Electrolyte Flux at the Cellular Level - 10The role of cAMP in the enterocyte.cAMP in the enterocyte cAMP is important in diarrhoea pathogenesis. 1. A regulatory substance binds to a receptor. 2. Inactive Gs protein becomes activated, liberating the a subunit (Gs-a) bound to GTP. 3. Dissociated Gs-a diffuses to the basolateral membrane. 4. Gs-a stimulates adenylate cyclase, causingit to produce more cAMP. 5. High cAMP levels act via protein kinase to: - open the crypt cell Cl-channel - inhibit the apical villus cell Na+ - Cl-cotransporter 6. These processes lead to net secretion of Cl-, and loss of Na+ and water.1. Regulatory substance Receptor 3. 2. bg a Gs protein Gs-a Adenylate cyclase 5. cAMP 4. 6. Cl- Na+ Cl- Copyright Image from The Wellcome Trust. Gs-aActive Gs-a is converted back to inactive Gs-a by automatic hydrolysis of bound GTP.Image references ################ .\IMAGES\Campreg2.gif Fluid and Electrolyte Flux at the Cellular Level - 11Brush border enzymes in the apical villus cell Enterocytes in the apical villus have several digestive enzymes inserted into the brush border membrane. These include: · maltase · sucrase · lactase · peptidases to digest proteins· nucleosidases and phosphatases to digest nucleic acids. A deficiency of disaccharidases such as lactase can be important in the pathogenesis of diarrhoea (see screen 34). Picture: Microvilli in the brush border membrane.Convert disaccharides into monosaccharides. Copyright Image from Booth IW. MonosaccharidesThe monosaccharide is then absorbed by a Na+ cotransporter. Image references ################ .\IMAGES\T44890.jpg Correctly identify the transporters labelled 1 - 4 in the pictures. Click your mouse on a box below.Hold the mouse down and drag the box to the correct number. Apical cell Monosaccharide 3. K+ Na+ 1. Na+ Amino acid 1. 2. 3. 4. Na+ H2O Cl- Crypt cell K+ Cl- Cl- 2. Na+ K+ 1. Na+ 4. Na+ H2O To return to the start of the section. Fluid and Electrolyte Flux at the Cellular Level: Assessment - 1Na+-glucose cotransporterTight junction Cl- channel Sodium pump Yes. That's right. Yes. That's right. Yes. That's right. Yes. That's right. No. That's wrong. Try again. Well done. You have now finished this assessment.Image references ################ .\IMAGES\Apicryp.gif Fluid and Electrolyte Flux at the Cellular Level: Assessment - 2Are the following statements about fluid and electrolyte flux true or false? Secretion of water into the small bowel lumenis favoured by: Click on the True or False button for each statement. To return to the start of the section. 1. opening of the Cl- channel in the crypt cell brush border membrane2. activation of adenylate cyclase in the basolateral membrane3. eating a meal rich in carbohydrates4. an excess of crypt like enterocytes Correct The crypt cell will: · secrete more Cl- · lose more Na+ and water by the paracellular routeIncorrect Text 11 pt Arial dark blue goes here Incorrect The crypt cell will: · secrete more Cl- · lose more Na+ and water by the paracellular routeCorrect Text 11 pt Arial dark blue goes here Correct A high cAMP level favours secretion through protein kinase: · opening of the Cl- channel in crypt cells · inhibition of the Na+ - Cl- cotransporter in apical villus cellsIncorrect Text explaining the answer (11-pt plain blue) Incorrect A high cAMP level favours secretion through protein kinase: · opening of the Cl- channel in crypt cells · inhibition of the Na+ - Cl- cotransporter in apical villus cellsCorrect Text explaining the answer (11-pt plain blue) Correct Text 11 pt Arial dark blue goes here Incorrect The bowel lumen will contain monosaccharides, causing apical villus cells to absorb: · Na+ and monosaccharides across the brush border membrane · water and Cl-through tight junctionsIncorrect Text 11 pt Arial dark blue goes here Correct The bowel lumen will contain monosaccharides, causing apical villus cells to absorb: · Na+ and monosaccharides across the brush border membrane · water and Cl-through tight junctionsCorrect An excess of secretory (crypt like) enterocytes over absorptive (apical) enterocytes favours net secretion of water into the lumen. Incorrect Text 11 pt Arial dark blue goes here Incorrect An excess of secretory (crypt like) enterocytes over absorptive (apical) enterocytes favours net secretion of water into the lumen. Correct Text 11 pt Arial dark blue goes hereImage references ################ Pathophysiological Mechanisms of DiarrhoeaPathophysiological Mechanisms of Diarrhoea Image references ################ Pathophysiological Mechanisms - 1 Secretory diarrhoea Na+ Cl- The three basic mechanisms of diarrhoea Diarrhoea reflects: · increased secretionand/or · decreased absorption Mechanisms of acute infectious diarrhoea can be divided into three basic types: · secretory diarrhoea · invasive diarrhoea· osmotic diarrhoea Some pathogens cause diarrhoea by a combination of mechanisms. Picture: The basic mechanisms of diarrhoea. Toxin H2O Blood Invasive diarrhoea Exudate Pus Cell death Na+ Cl- InflammatoryNon-inflammatory H2O Na+ Cl- H2O Osmotic diarrhoea Osmoticpull Na+ Cl- H2O Copyright Image from The Wellcome Trust. Secretory diarrhoeaIn secretory diarrhoea: · excess secretion of fluid and electrolytes is caused by an enterotoxin · the pathogen does not invade enterocytes Invasive diarrhoea In invasive diarrhoea, reduced absorption is caused by the pathogen invading and damaging enterocytes and colonocytes. Invasive diarrhoea can be: · inflammatory · non-inflammatory Osmotic diarrhoea In osmotic diarrhoea, reduced absorption is caused by an osmotically active non-absorbed substance in the bowel lumen.Image references ################ .\IMAGES\Diarmech.gif Pathophysiological Mechanisms of Diarrhoea - 2Pathophysiology of secretory diarrhoea Secretory diarrhoea (see picture) is caused by toxins released by the pathogen entering the cell. These toxins induce active secretion by the small bowel. 1. A pathogen adheres to the enterocyte. 2. The pathogen releases a soluble toxin (enterotoxin), which binds to a receptor. 3. The enterotoxin is taken up by the cell and modifies the Gs protein, locking it in the active (GTP binding) state.4. Gs-a activates adenylate cyclase, causing it to produce high levels of cAMP. 5. cAMP activates protein kinases, causing: - opening of Cl- channels in crypt cells - inhibition of Na+ - Cl-cotransporters in apical villus cells6. Massive secretion of Cl-, and loss of Na+ and water follows. 4. 1. Gs-a Adenylate cyclase cAMP Gs 3. 2. Toxin 5. Protein kinases Cl- 6. Na+ Cl- Copyright Image from The Wellcome Trust. Adhesion to the brush borderAdhesion of the pathogen is usually by a specific interaction.Modification of the Gs proteinIntracellular enterotoxin catalyses irreversible modification (ADP-ribosylation) of Gs-a.Receptor The enterotoxin binds to a specific receptor on the enterocyte brush border membrane.Image references ################ .\IMAGES\Secrdiar.gif Pathophysiological Mechanisms of Diarrhoea - 3Pathophysiology of secretory diarrhoea Note that the pathogen does not: · invade the enterocyte · cause gross structural damage to the epithelium These are important differences between secretory and invasive diarrhoea. Clinical features Secretory diarrhoea is acute watery diarrhoea with:· stools of high volume but without blood (see picture) · dehydration (loss of fluid and electrolytes) · vomiting · fever in some casesPicture: Secretory diarrhoea - ‘rice water’ diarrhoea of cholera. This opaque watery stool looks like the water above cooking rice. What are the classical examples ofsecretory diarrhoea?Copyright Image from Behrens RH. Classical examplesThe classical examples of secretory diarrhoea are: · cholera toxin of Vibrio cholerae O1 · heat labile toxin of enterotoxigenic E. coli Refer to the tutorial Diarrhoeal Diseases: Acute Watery Diarrhoea.Duration of secretory diarrhoea Non-fatal secretory diarrhoea is typically short lived (about 3 - 7 days) because new enterocytes replace those containing the enterotoxin. Image references ################ .\IMAGES\T45352.jpg Pathophysiological Mechanisms of Diarrhoea - 4 Pathophysiology of inflammatory invasive diarrhoea 1. Invasive bacteria infect epithelial cells of the distal small bowel and colon. 2. Bacteria multiply within mucosal cells and spread between cells, killing them. 3. Extensive inflammatory damage is caused to the mucosa. An acute bloody diarrhoea results, caused by:· reduced absorption of the damaged mucosa · exudation of fluid from the mucosa What are the classical examples of inflammatory invasive diarrhoea? Picture: Haemorrhagic colitis due to Shigella in the Rhesus monkey.Copyright Image from Centers for Disease Control and Prevention. Mucosal damageGross damage to the mucosa includes: · mucosal inflammation and erosion · mucosal ulceration· formation of microabscesses· production of exudate containing blood, mucus and pus This inflammatory reaction: · is regulated by a wide range of cytokines and other mediators · may extend beyond the mucosa to deeper muscle layers Clinical features of acute bloody diarrhoeaThe clinical features of inflammatory invasive diarrhoea are: · stools containing blood (dysentery), often with mucus and pus · frequent stools of relatively low volume · fever · abdominal or rectal pain (tenesmus) on defaecation Refer to the tutorial Diarrhoeal Diseases: Acute Bloody Diarrhoea.Classical examples The classical examples of inflammatory invasive diarrhoea are: · Shigella (see picture) · Campylobacter jejuniPopup1 heading Bulleted popup body text: · text1 · text2Image references ################ .\IMAGES\T33790.jpg Pathophysiological Mechanisms of Diarrhoea - 5Mechanism of non-inflammatory invasive diarrhoea due to rotavirus. Pathophysiology of non- inflammatory invasive diarrhoea 1. A pathogen (eg. rotavirus) invades mature enterocytes in the proximal small bowel. 2. Intracellular multiplication of the pathogen damages enterocytes. 3. The bowel responds by rapidly replacing damaged epithelial cells (), causing: - repopulation of epithelium by immature crypt like cells - crypt hyperplasia to form more enterocytes - atrophy of villi An acute watery diarrhoea results, caused by reduced absorption due to the: · low absorptive capacity of immature crypt like enterocytes · smaller surface area of stunted villi Shed enterocytes 1. 2. 3. Crypt hyperplasia Rotavirus Villous atrophy Villus Crypt Copyright Image from The Wellcome Trust. Clinical features of acute watery diarrhoeaClinical features of non-inflammatory invasive diarrhoea are: · watery stools of high volume but without blood · dehydration (loss of fluid and electrolytes) · vomiting · fever in some cases These are very similar to the clinical features of secretory diarrhoea. Refer to the tutorial Diarrhoeal Diseases: Acute Watery Diarrhoea. Repopulation of epitheliumRemember how enterocytes: · arise by division of stem cells in the crypt region as an immature secretory stage· differentiate into a mature absorptive stage as they migrate Villous atrophyVillous atrophy reflects the increased rate of enterocyte loss and proliferation. Image references ################ .\IMAGES\Invadiar.gif Pathophysiological Mechanisms of Diarrhoea - 6Pathophysiology of osmotic diarrhoea Osmotic diarrhoea is caused by the pathogen damaging brush border enzymes. This causes: · osmotically active substances in the lumen· reduced absorption 1. A pathogen causes damage to the enterocyte brush border. 2. Brush border enzymes, notably lactase, are inactivated (lactase deficiency).3. Undigested lactose builds up in the small bowel lumen, reducing absorption by its high osmotic tension.4. Bacteria in the colon break down lactose into short chain fatty acids, causing anacidic stool.Mechanism of osmotic diarrhoea. 1. Bacteria Lactase deficiency 2. Lactose 3. Gas Short chain fatty acids 4. Osmotic pull keeps water in the lumen Copyright Image from The Wellcome Trust modified from Cutting WA. Diarrhoeal diseases of children in the tropics. In: Lawson DH, ed. Current medicine 4.Edinburgh: Churchill Livingstone, 1994:133-61.Causes of osmotic diarrhoeaNon-infectious causes of osmotic diarrhoea include: · ingestion of large amounts of non-absorbed osmotically active solutes, eg. magnesium sulphate · low activity of brush border enzymes from malnutrition, drugs and genetic deficiencies Acidic stoolThe short chain fatty acids also contribute to the osmotic load in the lumen, and hence to the diarrhoea. Osmotic diarrhoea is: · characteristically acute watery diarrhoea with acidic stools · a factor in diarrhoea caused by several pathogens · often seen before damage to the mucosa is apparentLactase deficiencyLactase seems to be particularly easily damaged as it is: · located at the tip of the microvillus · vulnerable to proteolysisReduced absorptionRemember that absorption of water requires a favourable osmotic gradient. Image references ################ .\IMAGES\Osmodiar.gif Pathophysiological Mechanisms of Diarrhoea: AssessmentAre the following statements about the mechanisms of diarrhoea true or false?To return to the start of the section. Click on the True or False button for each statement. 1. Secretory diarrhoea is due to a pathogen entering the cell and releasing a toxin that disturbs electrolyte and water secretion.2. Invasive pathogens can cause non-inflammatory or inflammatory diarrhoea. 3. Inflammatory invasive diarrhoea is characterized by extensive damage to the bowel mucosa.4. The most common type of osmotic diarrhoea is caused by lactic acid deficiency.Correct Text explaining the answer (11-pt plain blue) Incorrect Secretory diarrhoea is caused by a toxin entering the enterocyte. The pathogen does not invade the epithelium.Incorrect Text explaining the answer (11-pt plain blue) Correct Secretory diarrhoea is caused by a toxin entering the enterocyte. The pathogen does not invade the epithelium.Correct Different invasive pathogens can cause:·non-inflammatory diarrhoea - clinically watery· inflammatory diarrhoea - clinically bloody Incorrect Text explaining the answer (11-pt plain blue) Incorrect Different invasive pathogens can cause:·non-inflammatory diarrhoea - clinically watery· inflammatory diarrhoea - clinically bloody Correct Text explaining the answer (11-pt plain blue) Correct Inflammatory invasive diarrhoea is characterized by: · mucosal inflammation, erosion and mucosal ulceration· formation of micro- abscesses· an exudate containing blood, mucus and pusIncorrect Inflammatory invasive diarrhoea is characterized by: · mucosal inflammation, erosion and mucosal ulceration· formation of micro- abscesses· an exudate containing blood, mucus and pusIncorrect Inflammatory invasive diarrhoea is characterized by: · mucosal inflammation, erosion and mucosal ulceration· formation of micro- abscesses· an exudate containing blood, mucus and pusCorrect The most common cause of osmotic diarrhoea is deficiency of the enzyme lactase.Correct Incorrect The most common cause of osmotic diarrhoea is deficiency of the enzyme lactase. Incorrect Text explaining the answer (11-pt plain blue) Correct The most common cause of osmotic diarrhoea is deficiency of the enzyme lactase.Image references ################ Mechanisms of Diarrhoea Caused by Common PathogensMechanisms of Diarrhoea Caused by Common Pathogens Image references ################ Mechanisms of Diarrhoea Caused by Common Pathogens - 1Pathogens in the stools ofyoung children with acute diarrhoea in developing countries. Data from WHO 1992.Aetiological agents of diarrhoea in young children The five most important diarrhoea pathogens worldwide are shown in the pie chart. A sixth pathogen, the bacterium Vibrio cholerae O1, is not shown but is important because it causes: · serious epidemics · 5 - 10% of all cases of acute diarrhoea worldwide For a table of other commonly isolated pathogens. Rotavirus 15 - 25 % Enterotoxigenic E. coli 10 - 20% No pathogen found 20 - 30% Campylobacterjejuni 10 - 15% Shigella 5 - 15% Cryptosporidium 5 - 15% No pathogen found Viruses Bacteria ProtozoaCopyright Image from The Wellcome Trust based on data in World Health Organization. Readings on diarrhoea. Student manual. Geneva: WHO, 1992. Less Common Pathogens in Childhood Acute DiarrhoeaGroup Species Viruses Small round structured viruses, eg. Norwalk agent Enteric adenoviruses Bacteria Other types of E. coli Other types of V. cholerae,eg. V. cholerae O139Non-typhoid Salmonella Protozoa Giardia lamblia Entamoeba histolyticaIsolation of a pathogen from the stool is not proof that the pathogen caused that episode of diarrhoea.Pathogens in young children These agents are the most important causes of diarrhoea worldwide because most diarrhoea morbidity and mortality is in this age group.Image references ################ .\IMAGES\Agendiar.gif Mechanisms of Diarrhoea Caused by Common Pathogens - 2Summary of the Mechanisms of Diarrhoea in Key Pathogens Mechanism of diarrhoea InvasiveInvasive Pathogen Secretory inflammatory non-inflammatoryOsmotic Rotavirus - - +++ ++ Vibrio cholerae O1 +++ - - - Enterotoxigenic E. coli+++ - - - Campylobacter jejuni ++ ++ - - Shigella- +++ - - Cryptosporidium ++ - ++ - +++, major; ++, significant; -, insignificant. E. coliOther strains of E. coli: · cause diarrhoea by different mechanisms or · are not pathogenic at all Image references ################ Mechanisms of Diarrhoea Caused by Common Pathogens - 3Rotavirus: non-inflammatory invasive and osmotic diarrhoea In rotaviral diarrhoea: 1. virus invades mature enterocytes in the proximal small bowel 2. viral multiplication damages enterocytes, causing increased enterocyte shedding and proliferation 3. resulting changes to the small bowel are: - crypt hyperplasia- stunted villi withdamaged microvilli- fall in disaccharidaseactivity (causes osmotic diarrhoea) The major mechanism of rotaviral diarrhoea is probably repopulation of the villi with immature crypt like enterocytes (see screen 33).Picture: Particles of rotavirus (EM). Rotavirus is a double stranded RNA reovirus that causes acute watery diarrhoea. What are the clinical features?Cause non-inflammatory invasive diarrhoea. Copyright Image from Centers for Disease Control and Prevention. Clinical featuresThe clinical features of rotavirus infection are: · profuse watery diarrhoea without blood or mucus · dehydration with metabolic acidosis · vomiting · fever Refer to the tutorial Diarrhoeal Diseases: Acute Watery Diarrhoea. Image references ################ .\IMAGES\T33755.jpg Mechanisms of Diarrhoea Caused by Common Pathogens - 4Vibrio cholerae O1: secretory diarrhoea V. cholerae O1: · is a Gram-negative bacillus· causes cholera, characterized by a watery diarrhoea of high volume · produces two main toxins, described below What are the clinical features? 1. Cholera toxin (CT) (see picture): · enters the cell as a fragment of the toxin A subunit · causes secretory diarrhoea by the mechanism described (see screen 30)2. Zonula occludens toxin (ZOT):· causes secretory diarrhoea by increasing permeability of tight junctions · leads to paracellular electrolyte/water loss The mode of action of CT in secretory diarrhoea. Cholera toxin (A and B subunits) A B Ganglioside GM1 Receptor A2 A1 a bg G protein A1 GTPGDP Copyright Image from The Wellcome Trust modified from Booth IW, McNeish AS. Baillieres Clin Gastroenterol 1993;7:215-42. Cholera toxinCholera toxin is the classical example of secretory diarrhoea. Its action through this route is amplified by effects of cholera toxin on the enteric nervous system, which may be mediated by vasoactive intestinal peptide.Clinical features The clinical features of cholera are: · extremely profuse ‘rice water’ diarrhoea (up to 20 litres/day in an adult) · severe dehydration with hypovolaemia · metabolic acidosis · depletion of sodium, chloride and potassium · vomiting · fever in some cases Fluid and electrolyte loss can rapidly cause death from shock or circulatory collapse. Refer to the tutorial Diarrhoeal Diseases: Acute Watery Diarrhoea. ZOTSome strains of V. cholerae O1: · lack a functional gene for CT· cause mild diarrhoea through ZOT alone Image references ################ .\IMAGES\Ctoxmode.gif Mechanisms of Diarrhoea Caused by Common Pathogens - 5Enterotoxigenic E. coli (ETEC): secretory diarrhoea ETEC produces two main toxins. 1. Heat labile enterotoxin (LT): · causes secretory diarrhoea by the classical mechanism · has a mode of action very similar to CT 2. Heat stable enterotoxin (ST) is present as two types:· STa· STbPicture: Enterotoxigenic E. coli (EM).Note the fimbriae. ETEC is a Gram-negative bacillus. It causes watery diarrhoea of moderate volume and is the most common cause of traveller’s diarrhoea.What are the clinical features? Copyright Image from Knutton S. Clinical features The clinical features of ETEC infection are: · acute watery diarrhoea · dehydration · metabolic acidosis · vomiting· abdominal cramps · fever in some cases Refer to the tutorial Diarrhoeal Diseases: Acute Watery Diarrhoea. Mode of action of STaThe action of STa differs from that of CT in important details. These are: · STa binds to a different receptor· the second messenger is cGMP not cAMP· STa does not act through a G protein intermediate · the action of STa is reversibleToxinsDifferent strains of ETEC produce: · both types of toxin or · one type onlyLT compared with CT LT differs from CT because the A subunit of LT is not cleaved before it enters the cell. Image references ################ .\IMAGES\T45904.jpg Mechanisms of Diarrhoea Caused by Common Pathogens - 6Campylobacter jejuni: secretory or inflammatory invasive diarrhoea C. jejuni is an S shaped Gram-negative rod. Different strains cause: · watery diarrhoea· dysentery What are the clinical features? Secretory diarrhoea In the acute watery diarrhoea, an enterotoxin may: · be released by C. jejuni · cause secretory diarrhoea by the classical mechanism Picture: C. jejuni (EM). Note the two long flagella.Copyright Image from Skirrow MB courtesy of Purdham DR. Clinical featuresWatery diarrhoea due to C. jejuni is typically of a high volume. The clinical features of dysenteric infection with C. jejuni are: · severe bloody mucoid diarrhoea · abdominal pain · fever · nausea Refer to the tutorial Diarrhoeal Diseases: Acute Bloody Diarrhoea. Image references ################ .\IMAGES\T45525.jpg Mechanisms of Diarrhoea Caused by Common Pathogens - 7C. jejuni: inflammatory invasive diarrhoea In the inflammatory invasive diarrhoea (dysentery): 1. C. jejuni invades epithelial cells of the terminal ileum and colon 2. the bacterium produces a cytotoxin 3. invasion causes cell death with:- mucosal inflammation- mucosal ulceration - purulent exudate The precise role of the cytotoxin in the pathology of dysentery is unclear. Picture: Severe small bowel damage in Campylobacter infection. Note the deep crypts and loss of villi.Copyright Image from Department of Pathology and Laboratory Medicine, Mount Sinai Hospital, Toronto courtesy of Luk SC.Image references ################ .\IMAGES\T14925.jpg Mechanisms of Diarrhoea Caused by Common Pathogens - 8Shigella: inflammatory invasive diarrhoea In bacillary dysentery due to Shigella: 1. the bacterium invades and spreads between the epithelial cells of the distal ileum and colon 2. intracellular multiplication causes cell death with: - intense inflammatory reaction - haemorrhage - mucosal ulceration and erosion - microabscesses and purulent exudate In addition, Shigella produces: · Shiga toxin· other less well characterized factors Picture: Shigella invading cultured cells (TEM). This genus of Gram-negative bacilli characteristically cause a bloody diarrhoea (bacillary dysentery). What are the clinical features? Copyright Image from Sansonetti PJ. Clinical featuresThe clinical features of bacillary dysentery are: · frequent bloody stools with mucus and pus (stools can be watery at the start of an episode) · fever · abdominal pain and tenesmus · anorexia · extraintestinal complications, eg. renal failure from haemolytic-uraemic syndrome Refer to the tutorial Diarrhoeal Diseases: Acute Bloody Diarrhoea.The genus ShigellaThe four pathogenic species of Shigella are: · S. dysenteriae · S. flexneri · S. boydii · S. sonnei S. dysenteriae and S. flexneri: · are the most important species in developing countries · often cause acute bloody diarrhoea (the other two species more often cause watery diarrhoea) Inflammatory reactionThe acute inflammatory reaction is regulated by cytokines produced by cells such as macrophages and infected enterocytes. These cytokines include: · interleukin 1 · interleukin 6 · tumour necrosis factor a Refer to the tutorial Diarrhoeal Diseases: Defence Mechanisms.Shiga toxinShiga toxin is: · a cytotoxic, enterotoxic and neurotoxic protein · an inhibitor of protein synthesis · responsible for haemolytic-uraemic syndrome, an extraintestinal complication · poorly understood as regards the dysentery Image references ################ .\IMAGES\T45955.jpg Mechanisms of Diarrhoea Caused by Common Pathogens - 9Cryptosporidium: non-inflammatory ‘invasive’ and secretory diarrhoea In cryptosporidiosis: 1. the parasite attaches to, and partially invades, epithelial cells of the small and large bowel 2. parasite multiplication and reinvasion of neighboring enterocytes or colonocytes causes: - destruction of microvilli- villous atrophy - crypt hyperplasia3. an enterotoxin that causes secretory diarrhoea is also produced Picture: Oocysts of Cryptosporidium, which are 4 - 6 mm in diameter (SEM). Cryptosporidium is a genus of zoonotic coccidian protozoa that causes acute watery diarrhoea. What are the clinical features? Copyright Image from Centers for Disease Control and Prevention. Parasite multiplication Repopulation of the villus with immature crypt like epithelial cells is probably the major mechanism causing diarrhoea.Clinical featuresThe clinical features of Cryptosporidium infection (cryptosporidiosis) are: · high volume watery diarrhoea · fever · abdominal discomfort · nausea and vomiting Cryptosporidiosis is particularly important in people infected with human immunodeficiency virus (HIV). Refer to the tutorial Diarrhoeal Diseases: Persistent Diarrhoea. Image references ################ .\IMAGES\T39435.jpg To return to the start of the section. What are the mechanisms by which different pathogens cause diarrhoea ? Click your mouse on a pathogen box below.Hold the mouse down and drag the box to match the correct answer. Secretory diarrhoea Non-inflammatory invasive diarrhoea Inflammatory invasive diarrhoea Mechanisms of Diarrhoea: Assessment - 1Shigelladysenteriae Rotavirus V. cholerae O1 Yes. That's right. Yes. That's right. Yes. That's right. No. That's wrong. Try again. Well done. You have now finished this assessment.Image references ################ Mechanisms of Diarrhoea: Assessment - 2Are the following statements about the causes of acute diarrhoea true or false?To return to the start of the section. Click on the True or False button for each statement. 1. A key factor in rotaviral diarrhoea is repopulation of the apical villus with immature crypt like enterocytes.2. V. cholerae O1 produces two types of toxin called heat labile (LT) and heat stable (ST).3. Mucosal damage in bacillary dysentery is caused by enterocytes and colonocytes taking up secreted Shiga toxin.4. The most important causes of acute diarrhoea globally are parasites. Correct The bowel responds to enterocyte damage by increased rates of enterocyte shedding and proliferation.Incorrect Text 11 pt Arial dark blue goes here Incorrect The bowel responds to enterocyte damage by increased rates of enterocyte shedding and proliferation.Correct Text 11 pt Arial dark blue goes here Correct Text 11 pt Arial dark blue goes here Incorrect V. cholerae O1 produces: · cholera toxin (CT) · zonula occludens toxin (ZOT) The LT and ST toxins are produced by enterotoxigenic E. coli. Incorrect Text 11 pt Arial dark blue goes here Correct V. cholerae O1 produces: · cholera toxin (CT) · zonula occludens toxin (ZOT) The LT and ST toxins are produced by enterotoxigenic E. coli. Correct Text 11 pt Arial dark blue goes here Incorrect Mucosal damage in Shigella infection: · follows multiplication of Shigella within epithelial cells (inflammatory invasive mechanism) · is related to Shiga toxin in a way that is not well understoodIncorrect Text 11 pt Arial dark blue goes here Correct Mucosal damage in Shigella infection: · follows multiplication of Shigella within epithelial cells (inflammatory invasive mechanism) · is related to Shiga toxin in a way that is not well understoodCorrect Incorrect Although parasites cause large numbers of cases of diarrhoea, more important agents are: · bacteria (E. coli, Shigella, Campylobacter) · viruses (especially rotavirus) Incorrect Text explaining the answer (11-pt plain blue) Correct Although parasites cause large numbers of cases of diarrhoea, more important agents are: · bacteria (E. coli, Shigella, Campylobacter) · viruses (especially rotavirus)Image references ################ Summary Click on the buttons below for summary information. Picture: A stool from a patient with Shigella infection. The stool contains blood but very little faecal material. The Physiology ofHealthy Bowel Fluid and Electrolyte Fluxat the Cellular Level Pathophysiological Mechanisms of Diarrhoea Mechanisms ofDiarrhoea Caused byCommon Pathogens Copyright Copyright Image from Tubbs HR. The Physiology of Healthy BowelUnderstanding how healthy bowel works is important to:· explain how pathogens cause diarrhoea · justify treatment by oral rehydration therapy The key to absorption is the small bowel with its villi, enterocytes and microvilli (the brush border). The villus shows: · net absorption in the apical villus by mature enterocytes · net secretion in the crypts of Lieberkühn by immature enterocytes The large bowel absorbs only 10% of the total fluid absorbed by the gut. In the colon: · the mucosa has crypts but no villi · there are absorptive colonocytes with rudimentary microvilliFluid & Electrolyte Flux at the Cellular LevelThe key electrochemical gradient in the enterocyte is generated by the Na+ - K+-ATPase (the ‘sodium pump’). Apical villus cells absorb: · nutrients and Na+ after meals through brush border cotransporters · Na+ and Cl-between meals through a brush border ‘cotransporter’ · Cl- and water through tight junctions Crypt cells secrete: · Cl-through a basolateral membrane cotransporter and a brush border Cl-channel · Na+ and water through tight junctions Enterocyte transporters are regulated by G proteins, adenylate and guanylate cyclases, cAMP and cGMP, and protein kinases.Pathophysiological Mechanisms of Diarrhoea Secretory diarrhoea is due to excess secretion by the small bowel. 1. A pathogen adheres to the enterocyte, and enterotoxin enters the cell. 2. cAMP levels are raised, activating protein kinases. 3. Cl- secretion is stimulated and Na+ - Cl- absorption is inhibited. 4. High volume watery diarrhoea results with loss of fluid and electrolytes. Inflammatory invasive diarrhoea is due to reduced absorption and an inflammatory exudate. 1. A pathogen invades the mucosa of the distal small bowel and large bowel. 2. Extensive inflammatory damage to the bowel mucosa occurs, causing: - loss of absorptive area - exudate of blood, pus and mucus 3. Bloody diarrhoea (dysentery) results. Non-inflammatory invasive diarrhoea is due to reduced absorption by the small bowel. 1. A pathogen invades enterocytes of the small bowel. 2. The bowel responds with increased enterocyte turnover, causing: - stunted villi, crypt hyperplasia and repopulation with immature enterocytes - reduced absorption through crypt like enterocytes and loss of absorptive area 3. High volume watery diarrhoea results. Osmotic diarrhoea is due to reduced absorption by the small bowel. 1. A pathogen damages brush border enzymes, eg. lactase. 2. Osmotically active substances (eg. lactose) are retained in the bowel lumen. 3. Acidic watery diarrhoea results.Diarrhoea Caused by Common PathogensRotavirus: · is an RNA virus · causes watery diarrhoea by a non-inflammatory invasive and osmotic mechanism Vibrio cholerae O1: · is a Gram-negative bacillus · causes high volume watery diarrhoea by a secretory mechanism (cholera toxin) · has a zonula occludens toxin that increases secretion through tight junctions Enterotoxigenic E. coli: · is a Gram-negative bacillus · causes watery diarrhoea by a secretory mechanism (two enterotoxin types) · LT is very like CT (raises cAMP); STa acts through cGMP to the same effect Campylobacter jejuni: · is a Gram-negative rod · causes watery diarrhoea probably by a secretory mechanism (an enterotoxin like CT) · can also cause bloody diarrhoea by inflammatory invasion of ileal and colonic mucosa Shigella: · is a Gram-negative bacillus (four species) · causes bloody diarrhoea (bacillary dysentery) by inflammatory invasion of ileal and colonic mucosa · contains Shiga toxin and other factors Cryptosporidium:· is a coccidian protozoan · causes watery diarrhoea by: - non-inflammatory partial invasion of small and large bowel mucosa - a secretory mechanism (enterotoxin) Image references ################ .\images\Lapaz.jpg .\IMAGES\T22821.jpg You have now finished the tutorial Organisms and Pathophysiologyã The Trustee of the Wellcome Trust, 1998 Further reading Further activities Restart tutorial Picture: Shigella invading a cultured epithelial cell. Copyright Image from Sansonetti PJ. Further reading Booth IW, McNeish AS. Mechanisms of diarrhoea. Baillieres Clin Gastroenterol 1993;7:215-42. Hart CA. Introduction to acute infective diarrhoea. In: Cook GC, ed. Manson’s tropical diseases. 20th ed. London: WB Saunders, 1996:817-23. Riedel BD, Ghishan FK. Acute diarrhea. In: Walker WA, Durie PR, Hamilton JR, Walker-Smith JA, Watkins JB, eds. Pediatric gastrointestinal disease: pathophysiology, diagnosis, management. Vol. 1. 2nd ed. St Louis: Mosby-Year Book, 1996:251-62. Further activities To look at pictures related to this tutorial, search the image collection using the following keywords: · pathology · organismsImage references ################ .\IMAGES\T45958.jpg