MB2080: Topic 1 - Gastrointestinal physiology and disease

Is the pancreas an endocrine or exocrine gland

both

What is the exocrine function of the pancreas

production of enzymes to aid the digestion of macromolecules

What is the endocrine function of the pancreas

blood glucose homeostasis by the production of the hormones insulin and glucagon

Sphincter of Oddi

a muscular valve that can be contracted to seal the junction or relaxed to allow flow of pancreatic juice and bile into the duodenum to aid digestion

Islets of Langerhans

clusters of pancreatic cells responsible for producing essential hormones such as insulin and glucagon

Acini

pancreatic cells responsible for producing, storing, and secreting digestive enzymes to aid digestion

Zymogen granules

specialised membrane-bound secretory organelles in pancreatic cells which store inactive digestive enzymes

Zymogen

an inactive precursor of digestive enzymes that need to be cleaved to be activated

Zymogen examples and their derivatives

• trypsinogen → trypsin

• chymotrypsinogen → chymotrypsin

• proelastase → elastase

• procarboxypeptidase → carboxypeptidase

What enzymes cleaves trypsinogen to form trypsin and where does this occur

enterokinases on the duodenum brush border

Proteolytic cleavage cascade

the activation of of trypsinogen to produce trypsin which sequentially activates other proteolytic zymogens

Function of amylase

digestion of polysaccharides

Function of lipase

digestion of triglycerides

Function of phospholipase A₂

digestion of fatty acids

Function of ribonuclease

digestion of nucleic acids

Function of deoxyribonuclease

digestion of nucleic acids

Function of Trypsin inhibitor (SPINK1) serine protease inhibitor Kazal type 1

inhibits activation of trypsin within the pancreas

Function of Non-digestive proteases in the zymogen granule

proteolytic degradation of active trypsin in the zymogen granule

Function of GP2 (glycosylphosphatidylinositol-anchored protein)

abundant protein which helps zymogens bind to the membrane for exocytosis

How do zymogen granules function as an intracellular protective mechanism

1. stores the enzymes

2. contain enzyme inhibitors

3. contain non-digestive proteases to inactivate trypsin

4. zymogen granule pH is below the optimum for trypsin and lipase activity

What else does the pancreas secrete

• Na⁺, Cl^- and H₂O

• Ca²⁺

• Lithostathine

Function of pancreatic secretion of Ca²⁺

binds to GP2 to trigger the release of zymogens

Function of pancreatic secretion of lithostathine (pancreatic stone protein)

inhibits formation of caliculi formation

Function of pancreatic secretion of Na⁺, Cl^- and H₂O

hydration of secretions

Caliculi

stones made of unfolded, misfolded, digested proteins and calcium carbonate precipitate

How does lithostathine prevent calculi formation

inhibits the nucleation of calcium carbonate crystals by binding to calcium

How much pancreatic fluid is secreted per day

1.5 L

Process of secretion of NaCl rich pancreatic fluid from acinar cells

• basolateral Na⁺/K⁺/Cl^- cotransporter drives net Cl^- uptake into the acinar cell

• intracellular Cl^- accumulation establishes electrochemical gradient that drives Cl^- secretion into the acinar lumen through Cl^- channels on the apical channels

• negative lumen charge created by Cl^- influx

• extracellular Na+ enters the lumen to balance charge and brings water with it

What % of pancreatic fluid is released by acinar cells

20%

What % of fluid is released by duct cells

80%

Process of duct cells secreting pancreatic fluid

1. Cl^-/HCO₃^- exchanger on the apical surface secretes HCO₃^- into the lumen

2. some of the HCO₃^- enters the cell across the Na⁺/HCO₃^- transporter on basolateral side

3. water and CO₂ that enter the cell are converted to HCO₃^- by carbonic anhydrase to provide yet more HCO₃^-

4. excess H⁺ formed from the carbonic anhydrase is taken out of the cell through Na⁺/H⁺ exchangers in the apical membranes

Acute pancreatitis

a clinical syndrome resulting from acute inflammation and destructive autodigestion of the pancreas and pancreatic tissue

Prevalence of acute pancreatitis

5-50 per 100,000 per year

Acute pancreatitis diagnosis

• blood test to find pancreatic enzymes in the blood

• hyperlipasemia

• hyperamylasaemia

• serum lipase or amylase concentration 3x greater than normal

Hyperlipasemia (definition)

an elevated level of lipase in the blood

Hyperamylasaemia (definition)

an elevated level of amylase in the blood

Mild symptoms of acute pancreatitis

• acute and constant abdominal pain

• nausea and vomiting

• fever if there is tissue damage and an inflammatory response

• Steatorrhea

Steatorrhea (definition)

excretion of excess fat in faeces, characterised by loose, pale, foul-smelling stool

Severe symptoms of acute pancreatitis

• coagulopathy

• shock

Coagulopathy (definition)

peripheral blood clotting due to proteolytic activation of plasma coagulation cascade

Example of proteolytic activation of plasma coagulation cascade

fibrinogen → fibrin → cross-linked fibrin clot

Pathway for clotting cascade

• injury to blood vessel triggers the release of clotting factors

• prothrombin cleaved into thrombin by pancreatic enzymes

• thrombin cleaves fibrinogen into fibrin

• fibrin forms an insoluble clot in the blood vessel

What causes shock in severe acute pancreatitis

• hypovolemia (due to haemorrhage into the peritoneal space)

• acute respiratory distress syndrome (due to pancreatic fluid breaches the diaphragm and proteolytic enzymes damage the alveoli)

Pancreatic pseudocysts

cavities filled with plasma, pus and pancreatic juices

Pancreatic pseudocysts complications

• there may be no drainage due to blocked ducts

• acini continue to secrete pancreatic fluid so the pseudocysts continue to grow

• risk of rupture, hemorrhage, infection, obstruction of GI tract and abscess formation

Prophylaxis of severe symptoms of acute pancreatitis

• thrombo-prophylaxis e.g. warfarin inhibits synthesis of clotting factors

• fluid resuscitation to manage hypovolemia

• oxygen supply to manage acute respiratory distress syndrome

Treatment of acute pancreatitis

no treatment only management of symptoms

Main causes of acute pancreatitis

• alcohol abuse

• gall stones

Other causes of acute pancreatitis (name at least 3)

• pancreatic anatomical variants

• trauma

• tumour

• hereditary

• adverse drug reactions

• infection

• autoimmune

• scorpion sting

Chronic pancreatitis (definition)

long term inflammation of the pancreas causing severe abdominal pain and insufficient exocrine/endocrine function

Prevalence of chronic pancreatitis

• 30 in 100 000 yearly

• 22% patients die within 10 years

Symptoms of chronic pancreatitis and causes

• Steatorrhea and/or osmotic diarrhoea caused by malabsorption

• malnutrition caused by exocrine insufficiency

• diabetes mellitus caused by endocrine insufficiency

Chronic pancreatitis complications

• pseudocysts

• jaundice due to obstruction of bile duct or sphincter of Oddi blocking bilirubin excretion in faeces

Causes of chronic pancreatitis

• chronic alcoholism (70-80% of cases)

• recurrent acute pancreatitis

Treatment of chronic pancreatitis

• replacement of exocrine enzymes with granules containing pancreatic enzymes coated with an acid resistant polymer coat to prevent inactivation in the stomach

• insulin to treat diabetes

General organisation of the small intestine duodenum

• mucosa

• lamina propria

• muscularis mucosae

• submucosa (glands, blood & lymphatic vessels, nerve plexus of enteric nervous system)

• muscularis externa (inner circular, outer longitudinal)

• serosa

6 Functions of the GI tract

• absorption

• secretion

• motility

• digestion

• defence

• excretion

Functions of small intestine

• main site of digestion

• principle site for nutrient absorption (villi)

• site of Na⁺, Cl^- and K⁺ absorption

• secretion of fluid and HCO₃^- (crypts)

• water and electrolyte secretions flush bacterial toxins away from the epithelium plays a role in defence

Amount of water absorbed by the small intestine per day

8.5 L

Key features of tight junctions

• regulates paracellular pathway (water and ion flow between cells)

• tightness regulated

• leakiest in small intestine to allow massive water reabsorption

Types of water movement in the gut

• paracellular route (in-between cells through leaky tight junctions)

• transcellular route (through the cell water movement through aquaporins)

• isotonic

Paracellular water movement

• in-between cells through leaky tight junctions

• driven by the osmotic gradient

• can occur in either direction

• moves into the lumen if chyme is hypertonic

Transcellular water movement

• water movement through the cell

• through aquaporins (water channels in the plasma membrane)

• driven by the osmotic gradient

Isotonic water movement

• when chyme becomes isotonic with plasma there is no osmotic gradient to drive water absorption

• in the ileum isotonic water absorption occurs

• based on creating micro-hypertonic environments so that water continues to move out of the gut lumen

Secretion of sodium chloride and H₂O

• basolateral Na⁺/K⁺-ATPase pump mediates primary active transport to drive the osmotic gradient

• basolateral K⁺/Na⁺/Cl^- cotransporter mediates secondary active transport to accumulate intracellular Cl^-

• excess K+ leaves via basolateral K+ channels to maintain membrane potential

• Cl^- leaves via apical membrane Cl^- channels into the lumen

• attracting paracellular movement of Na⁺ and H2O

Regulation of fluid and electrolyte secretion

G_αs signalling increases cAMP that activates PKA that phosphorylates CFTR to increase conductance of Cl^-

Types of diarrhoea

• Secretory

• Osmotic

Example of secretory diarrhoea

Cholera

Cholera toxin as an example of secretory diarrhoea

• cholera subunit B binds to GM1 at the cell surface of intestinal epithelial cells

• enabling the entry cholera toxin subunit A , which has ADP-ribosyltransferase activity, into the cell

• ADP-ribosylation inhibits GTPase activity of G_αs causing it to remain GTP-bound

• leading to persistent activation of adenylyl cyclase which results in high [cAMP]

• cAMP increases activation of PKA (protein kinase A) and phosphorylates CFTR increasing the flow of Cl^- ions

• water follows the electrolytes

Cholera effect on secretion

massive increase in intestinal secretions

How much water is secreted by cholera-infected individuals?

up to 20 L/day

Main cholera therapy

oral rehydration

G_αs-ADP ribosylation

• ADP becomes covalently and therefore permanently bound to the G_αs

• ADP ribosylation of G_αs inhibits GTPase activity

• once G_αs is bound to ADP, its hydrolysing function through GAP no longer works

• so G_αs remains in the GTP bound state — and therefore permanently active

Cholera oral rehydration solution

• sodium - 75 mM

• glucose - 75 mM

• chloride - 65 mM

• potassium - 20 mM

• citrate - 10 mM

Importance of (Cholera oral rehydration solution) osmolarity

slightly hypotonic to stimulate water movement into the gut

Osomolarity of rehydration solution vs plasma osmolarity

• rehydration solution — 245 mM

• plasma — 300 mM

Purpose of sodium/glucose in rehydration

• sodium and glucose activate a sodium-glucose co-transporter

• they enter intestinal epithelial cells through the apical membrane and into the plasma through the basolateral membrane

• as they do they draw large amounts of water from the gut into the plasma

Future cholera therapies

• CFTR inhibitors (small intestine only)

• specific adenylyl cyclase inhibitors (small intestine only)

Osmotic diarrhoea

• non-digested carbs and lipids cannot be absorbed

• maldigested food is not absorbed so chyme maintains high osmolarity compared to plasma

• water moves into gut lumen down the osmotic gradient

• therefore leading to diarrhoea and steatorrhea

Causes of osmotic diarrhoea

• pancreatic disease

• lactose intolerance

Pancreatic disease in diarrhoea

loss of pancreatic enzyme secretion leads to maldigestion and non-absorbable osmotic load in the lumen

Lactose intolerance in osmotic diarrhoea

non-digested lactose from milk cannot be absorbed and creates an osmotic load in the lumen

Lactase

• an enzyme which converts lactose into absorbable monosaccharides, glucose and galactose at the brush border

• lactase expression is high in all infants

Lactose intolerance symptoms

• abdominal pain

• diarrhoea

• nausea

Cause of lactose intolerance symptoms

• unabsorbed lactose leads to an osmotic load in the gut so chyme retains water in lumen

• unabsorbed lactose ferments in the colon exacerbating symptoms

Lactose intolerance diagnosis

hydrogen breath test — unabsorbed lactose enters the colon where it is fermented by bacteria with a release of hydrogen which is given off in the breath

Lactose intolerance therapy

• abstinence from dairy products

• supplement diet with calcium and vitamin D

• lactase enzyme

Osmotic diarrhoea pharmacological application

osmotic agents such as non-digestible synthetic sugars, like lactulose can be used as laxatives to induce osmotic diarrhoea to treat constipation

Problem that leads to diarrhoea

not enough absorption or too much secretion in the small intestine

4 functions of the stomach

• secretion of acid, pepsinogen, intrinsic factor, mucus, bicarbonate and water

• motility - churning to reduce particle size and to mix the secretions with chyme

• beginning of digestion (pepsins and acid)

• defence (acids)

Hormones secreted by the stomach

• gastrin

• somatostatin

How much fluid does the stomach secrete daily

2 L/day

Cells of the gastric pit (and functions)

• superficial epithelial cells — secretes HCO₃^-

• mucous neck cells — secretes mucus

• stem/regenerative cells

• parietal (oxyntic) cells — secretes HCl and intrinsic factor

• chief cells — secretes pepsinogens

• endocrine cells (e.g. entero-chromaffin cells) — secretes histamine

Stimulation of HCl secretion

• vagus nerve stimulates parietal cell directly

• vagal stimulation of ECL cells increase histamine release which directly stimulates acid secretion

• vagal stimulation of G cells to release GRP triggering gastrin release

• gastrin directly stimulates parietal acid secretion and indirectly stimulates acid secretion by increasing histamine release

Key stimulatory receptors in HCl secretion

• gastrin receptors

• M3 receptor

• histamine receptors

Inhibition of HCl secretion

• luminal acid stimulates D cells to release somatostatin

• somatostatin decreases gastrin secretion from G cells

• somatostatin inhibits acid secretion directly at the parietal cell

• somatostatin inhibits acid secretion indirectly at the ECL cell

• prostaglandin E2 binds to PGE2 receptors on the gastric epithelial and inhibits acid secretion

Gastrinomas/Zollinger-Ellison Syndrome

• gastrin secreted from pancreatic G cell tumour

• unregulated gastrin secretion stimulates acid secretion both directly and indirectly

• high levels of acid are continuously secreted from parietal cells

Sign of gastrinoma

• high gastrin in blood

• high acid secretion (too acidic in stomach)

• hyperacidity can result in ulcer formation

Gastrinoma treatment

proton pump inhibitor

How does PGE2 protect the stomach from acid damage

• inhibition of acid secretion

• increases mucus secretion

• stimulates bicarbonate production