ANSC 311 - Midterm 1 - Lectures 1-3

William Beaumont

• Physician

• Alexis St-Martin shot in the stomach, did research on him

  • Left partially open to look inside

  • Tie bits of food on string and put in stomach

  • Saw secretion of things - HCl

  • See how fast things are digested

• Started field of understanding digestion

• Experiments and observations on the gastric juice, and the physiology of digestion

motility, secretion, digestion, absorption, communication, immune system, detoxification and modification via metabolism

Jobs of the gastrointestinal tract

mouth, pharynx, esophagus, forestomach (rumen, reticulum, omasum), stomach (abomasum), small intestine (duodenum, jejunum, ileum), cecum, large intestine

Components of the GI tract

teeth, tongue, salivary glands, liver, gall bladder, pancreas

Accessory organs of the GI tract

10-15 m

Length of small intestine in humans

Jejunum (6-7 m)

Longest section of the small intestine

Ileum (3-5 m)

Middle length section of the small intestine

Site of most absorption

Duodenum (12 inches)

Shortest section of small intestine

Tolerant to acid from stomach, buffered by base

Body size

• Small intestinal transit rate is the same regardless of ________ of mature dog

Stratified squamous

Esophagus has a multi-layer _______ ________ epithelium

• Thick bcs pokey things that haven’t been digested yet

• Beyond esophagus everything is 1 cell layer

Gastric pits

Stomach has ____ ____ to protect against stomach acid

• Small surface area to not absorb so much

Crypts of Lieberkuhn

The small intestine has vili to increase surface area for absorption with _____ __ _______ in between

• Site of Paneth cells

Mucosa

Top Layer of the Intestinal Wall

• Top of villi till first muscle layer

• Epithelium

• Lamina propria

  • Connective tissue and muscles

  • Capillaries, nerve fibers, smooth muscle (muscularis mucosa), lymphatic vessels (immune cells)

Lamina propria

Part of the mucosal layer of the intestinal wall

• Made of Connective tissue and muscles

• Capillaries, nerve fibers, smooth muscle (muscularis mucosa), lymphatic vessels (immune cells)

Submucosa

Second Layer of the Intestinal Wall

• Nerves

• Blood vessels

• Contains glands (if they are present)

Muscularis externa

Third Layer of the Intestinal Wall

• 2 layers of muscle - for contraction to allow to move along tract)

  • Circular muscle (around)

  • Longitudinal muscle (along)

• Inside goes around outside goes up and down

• Nerves and muscle

Serosa

Fourth/Bottom Layer of the Intestinal Wall

• Connective tissue

• Peritoneum

Mucosa, Submucosa, Muscularis externa, Serosa

4 Layers of the Intestinal Wall

Columnar Absorptive Cells

Type of Epithelial Cell of GI Tract

• Absorption

• Most of cells

Mucous/Goblet Cells

Type of Epithelial Cell of GI Tract

• Secrete mucous

• Mucous = barrier to protect intestinal surface

Enteroendocrine cells

Type of Epithelial Cell of GI Tract

• Release hormones into blood circulation to communicate locally and systemically

• Communicate with body and brain, but also locally to produce more cells, secrete more, etc.

• Communication

Paneth cells

Type of Epithelial Cell of GI Tract

• Remain at the crypt base

• Secrete antimicrobial peptides/molecules to provide host defense against gut microbes

  • Found in the crypt (in the loop between vili)

  • Keeps an antimicrobial sterile environment

  • Protect intestinal stem cells

Intestinal Stem Cells

Type of Epithelial Cell of GI Tract

• Give rise to above four cells

• Divide and become all other cell types

• Stem cells near bottom of villi either migrate up or down (paneth cells)

Peyer’s patches

• Especially in ileum

• Lymphoid structures (immune), where there is sampling through M-cells and dendritic cells to expose to immune cells

• Not found in large intestine (no longer sampling)

Dendritic cells

•  Sample environment in gut and educates immune system

• Work with M-cells in Peyer’s patches

3-5

Cells replaced every ______ days

• Replication occurs from intestinal stem cells

mechanical, chemical, enzymatic, microbial

4 types of digestive processes

Transcellular

Type of Absorption

• Going across the cells (through the apical and basolateral membranes)

  • Most of what will be discussed 

Paracellular

Type of Absorption

• Going between the cells (through tight junctions)

Chemical

Ek > Vm - _____ force is greater and combined electrochemical force is in the direction of the ______ force

Electrical

Ek < Vm - ______ force is greater and combined electrochemical force is in the direction of the ______ force

Dextrins

• mixtures of polymers of glucose units linked by α-1,4 or α-1,6 glycosidic bonds (hydrolysis of starch)

1. Salivary amylase → di and trisaccharides, dextrins

2. Pancreatic amylase → di and trisaccharides, dextrins

3. Brush border enzymes → monosaccharides

4. SGLT1 absorbs glucose by secondary active transport and GLUT5 absorbs fructose by facilitated diffusion into cell

5. GLUT2 transports monosaccharides out of cell into bloodstream

Steps of Digestion and Absorption of Carbohydrates

a-dextrinase, glucoamylase

Brush border enzymes

• Break down starches into glucose which are sent to SGLT1 where glucose and sodium are absorbed into the cell

Sucrase

Brush border enzyme

• breaks down sucrose into a glucose that goes to SGLT1 and fructose that goes to GLUT5 to be absorbed into the cell

• Gains function as animal ages

Lactase

Brush border enzyme

• breaks down lactose into glucose and galactose which can both go through SGLT1

• Loses function as animal ages

SGLT1

Brush border carrier

• Allows glucose and galactose along with Na to pass from luminal space across the brush border membrane into the epithelial cell

• Can go against concentration gradient (secondary active transport)

GLUT5

Brush border carrier

• Allows fructose to pass from luminal space across the brush border membrane into the epithelial cell

• Facilitated diffusion

Luminal phase

Step 1 of carbohydrate digestion

• Have salivary and pancreatic amylase that breakdown large glucose complexes into smaller chunks (2-4 glucose units)

Brush border phase

Step 2 of carbohydrate digestion

• Occurs in duodenum, jejunum, and ileum

• Brush border enzymes (isomaltase, maltase, lactase, sucrase) convert to single glucose units and allow for absorption

Glucose, Fructose, Galactose

3 types of monosaccharides

sucrose (gl+fr), maltose(gl+gl), lactose(gl+ga)

3 types of disaccharides

Cellulose

• Polysaccharide

• B-1,4 glycosidic bonds

• Unlike starch, humans cannot digest

• Microbes contain enzymes to digest

Amylose

• Polysaccharide

• Starch

• has a-1,4 glycosidic bonds, but just a straight chain

Amylopectin

• Polysaccharide

• Starch

• has a-1,4 and a-1,6 glycosidic bonds, branched

GLUT2

Carrier

• Transports monosaccharides out of cell into bloodstream

• Through secondary active transport or during sugar-rich meals, facilitated diffusion

1. Gastric HCl denatures proteins down to primary structure

2. Pepsinogen is converted to Pepsin when exposed to HCl and Pepsin begins to break down the primary structure of proteins

3. Pancreatic proteases are activated by enterokinase on the brush border → large peptides, di and tripeptides, amino acids

4. Brush border peptidases further break down large peptides

5. Carriers allow amino acids, di, and tripeptides to cross the brush border membrane into the cell

6. Cytoplasmic peptidases break down di and tripeptides into aa (a small amount escape)

7. AA and di and tri peptides enter bloodstream through carriers via secondary active transport

Steps of Digestion and Absorption of Proteins

HCl

• Begins protein and nucleic acid digestion in the stomach

• Denatures proteins and linearizes it down to the primary structure

• Converts pepsinogen to active form pepsin

Pepsinogen

• a pro-enzyme that is converted to Pepsin when exposed to HCl

  • Pepsin starts to break down the primary structure of the proteins in the stomach

Pepsin

• Active form of pepsinogen

• Starts to break down the primary structure of the proteins in the stomach

Enterokinase

• On the brush border

• Converts pancreatic pro-enzymes to their active forms for further protein digestion

trypsin, chymotrypsin, elastase, carboxypeptidase A, carboxypeptidase B

5 pancreatic proteases (active forms)

trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase A, procarboxypeptidase B

5 pancreatic proteases (inactive forms → proenzymes)

Cytoplasmic peptidases

• Once proteins are in the cell ______ further break down di and tripeptides into amino acids

  • A small amount of di and trisaccharides can escape that and be brought out into circulation - important bcs a lot of bioactive peptides and need to be intact to have effect

Bile acids

• Emulsify lipids for digestion and absorption

• Produced by liver and stored in gallbladder

• One part is negatively charged (hydrophilic) and the other part is hydrophobic (act as a detergent)

• Eventually reabsorbed by secondary active transport in the end of the small intestine in the ileum

1. Lipids are emulsified by bile acids

2. Pancreatic lipase, supported by colipase, break down large droplets into smaller micelles

3. Monoglycerides and free fatty acids pass through brush border membrane into cell via simple diffusion

4. Reform into triglycerides in the cell

5. Smooth ER packages triglycerides into chylomicrons

6. Golgi apparatus puts chylomicrons into vesicles and ships them out of the cell via exocytosis

7. Enter lacteal

8. Travels up to thoracic duct and enters bloodstream

Steps of Digestion and Absorption of Lipids

Pancreatic lipase, colipase

• _______ supported by _____, break down large droplets into smaller micelles

• Begins lipid digestion

Chylomicrons

• Combination of triglycerides, cholesterol, lipoproteins, and phospholipids

• Formed in the smooth ER of intestinal epithelial cells

• Bring triglycerides from intestinal epithelial cells into lacteal

1. Denatured by gastric acid

2. Broken down into nucleotides by pancreatic nucleases (ribonuclease and deoxyribonuclease)

3. Phosphatase on brush border cleaves phosphate ion

4. Nucleosidase breaks bond btw nitrogenous base and pentose sugar

5. Pentose sugar absorbed through facilitated diffusion, nitrogenous bases absorbed through active transport

Steps of Digestion and Absorption of Nucleic Acids

histones, nucleosomes

Strands of DNA wrap around a protein (_____) forming _______

chromatin, chromosomes

Nucleosomes coil together forming _______, which supercoils to form ________

Pancreatic nucleases (ribonuclease, deoxyribonuclease)

• Break down denatured nucleic acids into individual nucleotides

Phosphatase

• Once broken down into nucleotides by pancreatic nucleases

• ______ on brush border cleaves off phosphate ion

Nucleosidase

• Once broken down into nucleotides by pancreatic nucleases

• Phosphatase on brush border cleaves off phosphate ion

• Then, ____ catalyze breaking of covalent bond between nitrogenous base and pentose sugar

volatile fatty acids (acetate, proprionate, butyrate), microbial protein, B vitamins

End products of microbial fermentation

• Fluid environment

• Neutralized environment

• Continuous removal of the end-products by absorption

• Long retention time of digesta

Requirements for microbial digestion (4)

• Non-Ionic Diffusion: Protonated then can diffuse across membrane

• Apical membrane SCFA-HCO3 exchange:

  • HCO3- into lumen

  • SCFA into cell

Mechanisms of SCFA Absorption (2)

Pinocytosis

Mechanism of Passive Transfer

• Absorption of immunoglobulins in the first 12-24 hours of life

HCO3- + H+ <-> H2CO3 <-> H2O + CO2

HCO3- formula

Buffer (HCO3-, H2PO4-), Na, Cl, Amylase

Salivary Secretion - Composition

• Medulla

• acetylcholine, mechanoreceptor activation in mouth and stomach

Salivary Secretion - Control

• Location of salivary center:

• Secretion increased by:

Acinar Cells

• Produce salivary secretion in salivary glands

• Produce enzymes and proenzymes for pancreatic secretions in pancreas and store them in vesicles until release

Duct Cells

• Produce bicarbonate and mucus for pancreatic secretion

Intrinsic factor

______ ______ produced by the stomach

Important for Vit B12 absorption later on in tract

Parietal Cells

Stomach Cell

• In proper gastric (fundus and corpus) of stomach

• Produce HCl

• Produce intrinsic factors that facilitate Vit B12 absorption in the ileum

Chief Cells

Stomach Cell

• In proper gastric (fundus and corpus) of stomach

• Produce pepsinogen

Enterochromaffin cells (ECL cells)

Stomach Cell

• Produce histamine to stimulate release of HCl from parietal cell

1. Esophageal (stratified squamous)

2. Cardia (non-glandular)

3. Proper gastric (fundus and corpus)

4. Pylorus

4 Parts of the Stomach

HCl

Part of Gastric Secretion

• Secreted by parietal cells

• Breakdown of collagen 

• Protein uncoiling 

• Bacteriocidal 

• Activates pepsinogen 

• Optimum pH (~2.0)

• Acetylcholine

  • Neural

  • Distension of stomach wall → neural signal → ACh activates parietal cell

• Gastrin

  • Endocrine

  • Presence of peptides/aa → activates G-cell → gastrin release → ECL cell activation → histamine release → Parietal cell activation

• Histamine

  • Paracrine

  • Release from ECL activates parietal cell

Stimulatory Molecules of Gastric Secretion

(brief description of mechanism)

Somatostatin

• Low pH → somatostatin-releasing cell activated → somatostatin release → parietal cell inhibited

Inhibitory Molecule of Gastric Secretion

(brief description of mechanism)

Atropine

Used to block positive regulation of Acetylcholine (neural) on gastric secretion

Cimetidine

Used to block positive regulation of Histamine (paracrine) on gastric secretion

Proglumide

Used to block positive regulation of Gastrin (endocrine on gastric secretion

Draw out process of production of HCl via parietal cell using H2CO3 equation

Helicobacter pylori

• Organism that causes gastric ulcers in humans

• Buries into mucus layer of stomach

• 80% of people asymptomatic

• Requires acid and increases stomach pH

Helicobacter suis

• Organism that causes gastric ulcers in pigs

Nutrition, physical diet, management, microbial

Factors that impact gastric ulcer development in pigs

Trypsin(ogen), Chymotrypsin(ogen), (Pro)elastase

Pancreatic Enzyme(s)

• Substrate: Proteins, peptides

• Effect: Cleaves interior peptide bonds

(Pro)carboxypeptidase

Pancreatic Enzyme(s)

• Substrate: Proteins, peptides

• Effect: Releases amino acids at the carboxylic terminal

Lipase

Pancreatic Enzyme(s)

• Substrate: Triglycerides

• Effect: Cleaves ester bonds at the 1- and 3- positions producing free fatty acids and monoglycerides

Phospholipase

Pancreatic Enzyme(s)

• Substrate: Phospholipids

• Effect: Cleaves the ester bond at the 2-position of phospholipids

Amylase

Pancreatic Enzyme(s)

• Substrate: Polysaccharides

• Effect: Cleaves starch and glycogen to maltose and short chains

Ribonuclease

Pancreatic Enzyme(s)

• Substrate: RNA

• Effect: Cleaves RNA to nucleotides

Deoxyribonuclease

Pancreatic Enzyme(s)

• Substrate: DNA

• Effect: Cleaves DNA to nucleotides

Exocrine Pancreatic Insufficiency

• Inability to produce enough pancreatic enzymes

• More common in German Shepherd and Rough Collies 

• Pancreatic acinar atrophy 

• Symptoms 

  • Weight loss 

  • Greasy, foul smelling diarrhea 

  • Dry, dandruffy coat 

• Testing 

  • Blood test for trypsin-like immunoreactivity 

  • Stool test for protease and elastase (old school)

• Treatment 

  • Pancreatic enzymatic replacement therapy (PERT) 

  • Sometimes given with antacids

• Causes

  • Genetic in dogs, but not cats (autosomal recessive) 

  • Chronic pancreatitis (most common) 

  • Obstruction of pancreatic duct (cancer/tumor) 

  • Bacterial overgrowth in small intestine

  • Autoimmune pancreatitis 

  • Often associated with bacterial overgrowth

Draw pancreatic bicarbonate secretion by duct cell

1. Digesta enters → Stretch

2. Stimulates acetylcholine release

3. ACh activates G-protein coupled receptor

4. G-protein released → activates PL-A enzyme

5. PL-A releases IP3

6. IP3 stimulates endoplasmic reticulum to release Ca ions

7. Ca interacts with calmodulin

8. Calmodulin activates chloride transporter

9. Cl out of cell

10. Na follows, water follows

Crypt Enterocyte Secretion of Water Step-by-Step - Normal Conditions

1. Oxidative stress in gut activates Prostaglandin E

2. Activates G-protein coupled receptor

3. G-protein released → activates PL-A enzyme

4. PL-A releases IP3

5. IP3 stimulates endoplasmic reticulum to release Ca ions

6. Ca interacts with calmodulin

7. Calmodulin activates chloride transporter

8. Cl out of cell

9. Na follows, water follows

Crypt Enterocyte Secretion of Water Step-by-Step - Secretory Diarrhea in Response to Damage of Reactive Oxygen Species

1. Toxins → activate enteroendocrine cells

2. Enteroendocrine cells release serotonin

3. Serotonin stimulates neighboring cells to bring Ca in

4. Extra Ca binds to calmodulin

5. Calmodulin activates chloride transporter

6. Cl out of cell

7. Na follows, water follows

Crypt Enterocyte Secretion of Water Step-by-Step - Secretory Diarrhea in Response to Toxins