Lecture 16: GI Secretory Functions, Digestion, and Absorption Flashcards
What are the 4 different types of glands in the GI system?
- Unicellular mucous
- Crypts of Lieberkuhn
- Tubular glands
- Complex glands
List 3 examples of where complex glands are found in the body.
- Salivary glands
- Pancreas
- Liver
List the 4 different mechanisms that stimulate glands in the GI system.
- Food contact and local epithelial stimulation.
- Autonomic stimulation (mostly parasympathetic).
- Higher brain centers.
- Hormonal stimulation.
What are the functions of secreted mucous?
> Adheres to food and other particles.
> Spreads thin film over surfaces.
> Coats wall of gut, preventing actual contact of food.
> Causes fecal particles to adhere to one another.
> Resistant to digestion by GI enzymes.
> Has amphoteric properties making it useful for buffering small amounts of acids and bases.
What are the functions of saliva?
> Initial starch digestion (alpha-amylase) and initial triglyceride digestion (lingual lipase).
> Lubrication of food and protection of mouth and esophagus.
Which enzyme starts initial starch digestion in the mouth?
alpha-amylase
Which enzyme is responsible for initial triglyceride digestion in the mouth?
lingual lipase
What is the composition of saliva?
> High potassium ion and bicarbonate concentrations.
> Low sodium and chloride ion concentrations.
> Hypotonicity
> Presence of alpha-amylase, lingual lipase, and kallikrein.
What are the characteristics of saliva that has the lowest flow rate?
- Lowest osmolarity.
- Lowest sodium, chloride, and bicarbonate ion concentrations.
- Highest potassium ion concentration.
Does saliva composition of a high or low flow rate have a higher potassium concentrations?
Lowest flow rate saliva has the highest potassium ion concentration.
Is the saliva secretions of the parotid gland mixed or serous?
Almost entirely serous.
Is the saliva secretions of the submandibular and sublingual glands serous or mixed?
Mixed Secretions
Where does the first stage of salivary secretion of ions occur?
In acini.
What enzyme does the first stage of salivary secretion of ions contain?
ptyalin (alpha-amylase)
Is the composition of the first stage of salivary secretion of ions isotonic, hypotonic, or hypertonic?
Isotonic, with ionic concentration similar to plasma.
Where does the second stage of salivary secretion of ions occur?
Salivary ducts.
In the second stage of salivary secretion of ions, what happens to sodium ions?
Active reabsorption.
During the second stage of salivary secretion of ions, are potassium ions actively secreted r reabsorbed?
Active secretion of potassium ions.
During the second stage of salivary secretion of ions, are bicarbonate ions secreted or reabsorbed?
Active/Passive secretion of bicarbonate ions.
During the second stage of salivary secretion of ions, are chloride ions secreted or reabsorbed?
Passive reabsorption of chloride ions due to -70 mV in ducts.
What is saliva production controlled by?
Mostly parasympathetic system, but also by sympathetic system - BOTH result in increase in saliva production.
True or False:
Both the parasympathetic and sympathetic system result in the increase of saliva production.
True
Which cranial nerves are involved in the regulation of salivary secretion?
CN 7 and 9
Which type of receptor, found on both the acini and ducts, signal parasympathetic function in the regulation of salivary secretion?
muscarinic cholinergic receptor
What is the second messenger in parasympathetic activation in the regulation of salivary secretion?
> inositol 1,4,5-triphosphate (IP3)
> increased [Ca2+]
Which receptor is involved in the sympathetic signaling in the regulation of salivary secretion?
beta-adrenergic receptors
What is the second messenger in sympathetic activation in the regulation of salivary secretion?
cAMP
List factors that increase saliva production.
- food in mouth (via parasympathetic activation)
- smells
- conditioned reflexes
- nausea
You know that food in the mouth increases saliva production, but is this regulated by parasympathetic or sympathetic activation?
parasympathetic activation
List factors that decrease saliva production.
> via inhibition of parasympathetic system:
- sleep
- dehydration
- fear
- anticholinergic drugs
What is inhibited to decrease saliva production?
Inhibition of parasympathetic system.
What are the 3 things that make up the primary secretion of saliva from the acini?
- Ptyalin
- Mucus
- Extracellular fluid
What kind of transport is happening to the 4 different ions during the second stage of saliva secretion?
Na - active absorption
Cl - passive absorption
K - active secretion
HCO3 - secretion
Which cranial nerve goes to the parotid gland for parasympathetic control?
CN 9 (glossopharyngeal)
Which cranial nerve goes to the submandibular gland for parasympathetic control?
CN 7 (facial nerve)
What are the 3 secretory cell types in gastric glands?
- Mucous neck cells
- Chief (peptic) cells
- Parietal (oxyntic) cells
What is secreted by chief cells?
Pepsinogen (inactive pepsin):
> proteolytic enzyme
> pH range of activity = 1.8 to 3.5
What is the pH range of activity for pepsin?
1.8 to 3.5
What stimulates the release of pepsiongen by chief cells?
> Ach from vagus nerves or gastric enteric nervous plexus.
> Response to acid in stomach.
Besides pepsiongen, what else is secreted by chief cells?
intrinsic factor
What is released by parietal cells?
HCl
What is the mechanism of how HCl is the secretory product of parietal cells.
> Dissociation of water inside the cell into H and OH.
> OH + CO2 -> HCO3 + H (involves carbonic anhydrase)
> HCO3 is exchanged for Cl ions (alkaline tide)
- HCO3 increases blood pH and will eventually be secreted by pancreas to neutralize H in duodenum.
> Cl ions are secreted through chloride channels into the canaliculi.
> Hydrogen ion is pumped out of the cell in exchange for K.
> K leaks outside the cell but is transported back in via a H - K ATPase pump.
> Na is reabsorbed into the cell due to Na - K basolateral pump.
What are the two pathways in which parasympathetic (vagal) stimulation of gastric secretions increases H+ secretions?
> Direct:
- CN X innervates parietal cells
- Stimulates H secretion directly
- utilizes Ach and muscarinic (M3) receptor
Indirect:
- CN X innervates G cells
- Stimulates gastrin secretion
- Gastrin stimulates secretion of H+
- NT is GRP (gastrin-releasing peptide)
What is the indirect pathway in parasympathetic (vagal) stimulation for increasing H+ secretion?
> CN X innervates G cells.
Stimulates gastrin secretion.
Gastrin stimulates secretion of H+.
NT is GRP (gastrin-releasing peptide)
What is the direct pathway in parasympathetic (vagal) stimulation of increasing H+ secretion?
> CN X innervates parietal cells.
Stimulates H secretion directly.
Utilizes Ach and muscarinic (M3) receptor.
Which cell is innervated by CN 10 in the direct pathway for increasing H+ gastric secretion by parasympathetic (vagal) stimulation?
parietal cells
Which cell is innervated by CN 10 in the indirect pathway for increasing H+ gastric secretion by parasympathetic (vagal) stimulation?
G cells
How does the direct pathway in the stimulation of gastric secretions of H+ get its name?
It directly stimulates H+ secretion.
How does the indirect pathway in the stimulation of gastric secretions of H+ get its name?
G cells first stimulated to secrete gastrin, which then stimulates secretion of H+.
What are the two forms of secreted gastrin?
G-34
G-17 (most abundant)
What is gastrin released in response to?
Presence of protein in pylorus.
What does the presence of protein in the pylorus cause?
release of gastrin from gastrin (G) cells in pyloric glands
What type of gland is found in the pylorus?
Pyloric glands
What does the release of gastrin cause?
Causes enterochromaffin-like cells to release histamine.
What does the release of histamine from enterochromaffin-like cells stimulate the secretion of?
Stimulates H+ secretion.
What are the second messengers that act on parietal cells?
IP3/Ca2+
You know that enterochromaffin-like cells release histamine due to the presence of gastrin, which stimulates H+ secretion. How exactly does histamine stimulate H+ secretion
> Stimulates secretion of H+ by activating H2 receptors on parietal cell membrane.
> H2 receptor is coupled to adenyl cyclase via Gs protein.
> Second messenger is cAMP.
What inhibits gastric secretion of H+?
Somatostatin
What are the two pathways in which somatostatin inhibits gastric H2 secretion?
Direct
Indirect
What is the direct pathway in the inhibition of gastric H2 secretions by somatostatin?
> Somatostatin binds to receptors on parietal cell that are coupled to adenyl cyclase via a Gi protein.
> Antagonistic to stimulatory action of histamine.
What does the indirect pathway in the inhibition of gastric H2 secretions by somatostatin inhibit?
> Inhibits the release of histamine and gastrin.
What inhibits the release of histamine and gastrin, thus inhibiting gastric H2 secretion?
somatostain (indirect pathway)
How does prostaglandins inhibit gastric H2 secretions?
By activating Gi protein, thus inhibiting adenyl cyclase.
What are the two proteins that inhibit gastric secretions?
> somatostatin
> prostaglandins
What are the 3 phases of gastric secretions?
- cephalic phase
- gastric phase
- intestinal phase
What are the basic factors that stimulate gastric secretion?
> acetylcholine
gastrin
histamine
What is the cephalic phase of gastric secretion?
> Occurs even before food enters the stomach, especially while it is being eaten.
> Results from the sight, smell, thought, or taste of food, and greater the appetite, the more intense the stimulation is.
> Neurogenic signals that cause the cephalic phase of gastric secretion originate in the cerebral cortex and in the appetite centers of the amygdala and hypothalamus. They are transmitted through the dorsal motor nuclei of the vagi and thence through the vagus nerves to the stomach.
> This phase of secretion normally accounts for about 30 % of the gastric secretion associated with eating a meal.
What is the gastric phase of gastric secretions?
> Once food enters the stomach, it excites:
1) long vagovagal reflexes from the stomach to the brain and back to the stomach.
2) local enteric reflexes.
3) the gastrin mechanism, all of which cause secretion of gastric juice during several hours while food remains in the stomach.
> The gastric phase of secretion accounts for about 60% of total gastric secretion associated with eating a meal and therefore accounts for most of the total daily gastric secretion of about 1500 milliliters.
What is the intestinal phase of gastric secretions?
> Presence of food in the upper portion of the small intestine, particularly in the duodenum, will continue to cause stomach secretion of small amounts of gastric juice, probably partly because of small amounts of gastrin released by the duodenal mucosa.
> This secretion accounts for about 10% of the acid response to a meal.
Cephalic phase of gastric secretions.
Via vagus nerve - parasympathetics excite pepsin and acid production.
Gastric phase of gastric secretions.
1) Local nervous secretory reflexes.
2) Vagal reflexes.
3) Gastrin-histamine stimulation.
Intestinal phase of gastric secretions.
1) Nervous mechanisms.
2) Hormonal mechanisms. `
What are exocrine enzymes used for that are secreted from the pancreas?
Digestive enzymes for proteins, carbohydrates, and fats.
What digestive enzymes for proteins are secreted by the pancreas?
- Trypsin
- Chymotrypsin
- Carboxypolypeptidase
What type of pancreatic cell secretes a trypsin inhibitor?
Glandular Cells
Why is the secretion of trypsin inhibitor by glandular cells in the pancreas necessary?
To prevent the action of trypsin on the pancreatic tissues themselves.
What digestive enzymes for carbohydrates are secreted by the pancreas?
- Pancreatic amylase
What digestive enzymes for fats are secreted by the pancreas?
- Pancreatic lipase
- Choleterol esterase
- Phospholipase
Does the pancreas release bicarbonate ions?
Yes
> H2O + CO2 (from the blood) (involves carbonic anydrase) –> H2CO3 –> HCO3 + H.
> HCO3 + Na are actively transported into duct lumen.
> Hydrogen ions are exchanged for sodium ions.
List the characteristics of pancreatic secretions.
- High Volume
- ISOTONIC
- Same Na and K ion concentrations as plasma.
- Low flow rates -> isotonic fluid composed mostly of Na and Cl ions.
- High flow rates -> isotonic fluid compoased mostly of Na and HCO3 ions.
What ions are low flow rates of pancreatic secretions mostly composed of?
Na and Cl ions.
What ions are high flow rates of pancreatic secretions mostly composed of?
Na and HCO3 ions.
What do acinar cells do in the formation of pancreatic secretions?
Produce small volume of pancreatic secretion composed mainly of Na and Cl ions.
What is the role of ductal cells in the formation of pancreatic secretions?
> Secrete HCO3 ion and reabsorb Cl ion via a Cl - HCO3 exchange mechanism.
> Ducts are permeable to water, so water moves into ducts to make secretion isotonic.
How does acetylcholine regulate pancreatic secretions?
> From parasympathetic nerves and enteric nervous system.
> Released in response to hydrogen ions, small peptides, amino acids, fatty acids in duodenum.
> Stimulates enzyme secretion by acinar cells and potentiates effect of secretin.
How does cholecystokinin regulate pancreatic secretions?
> Release is stimulated by presence of food in upper intestine, especially small peptides, amino acids and fatty acids.
Secreted by duodenaland upper jejunal mucosal I cells.
Results in dramatic increase in secretion of pancreatic enzymes.
Potentiates effect of secretin on ductal cells to stimulate bicarbonate secretion.
Second messengers are IP3 and increased intracellular [Ca].
What is acetylcholine released in response to?
Hydrogen ions, small peptides, amino acids, fatty acids in duodenum.
What is cholecystokinin released in response to?
Release is stimulated by presence of food in upper intestine, especially small peptides, amino acids and fatty acids.
What is cholecystokinin secreted by?
Duodenal and upper jejunal mucosal I cells.
What does acetylcholine stimulate the secretion of in the regulation of pancreatic secretions?
Stimulates enzyme secretion by acinar cells and potentiates effect of secretin.
What does the release of CCK result in?
Results in dramatic increase in secretion of pancreatic enzymes.
What does CCK potentiate the effect of?
Potentiates effect of secretin on ductal cells to stimulate bicarbonate secretion.
How does secretin regulate pancreatic secretions?
> Release is stimulated by presence of acidic foods in upper intestine.
Secreted by duodenal and upper jejunal S mucosal cells.
Stimulates release of large amounts of sodium bicarbonate by ductal cells.
Second messenger is cAMP.
What is the second messenger of secretin?
cAMP
What are the second messengers of CCK?
IP3 and increased intracellular [Ca].
What does secretin stimulates the release of?
Large amounts of sodium bicarbonate by ductal cells.
What is secretin secreted by?
Secreted by duodenal and upper jejunal S mucosal cells.
What is the release of secretin by duodenal and upper jejunal S mucosal cells stimulated by?
Presence of acidic foods in upper intestine.
In the regulation of pancreatic secretions, what does a vagal stimulation cause a release of?
Enzymes into the acini.
True of False:
Bile is secreted continuously by hepatocytes in the liver and stored in the gall bladder until needed.
True
How is bile concentrated in the gallbladder?
In the gallbladder, bile is concentrated by active transport of Na followed by secondary absorption of Cl ions, water, and other diffusible constituents.
What factor stimulates the release of bile from the gallbladder?
Presence of fatty food in duodenum.
What are the functions of bile salts?
> Emulsification
complex with lipids to form micelles for absorption across intestinal mucosa:
- bile salts are positioned on the outside of the micelles.
- free fatty acids and monoglycerides are inside the micelle.
True or False:
Bile salts are hydrophobic and orient themselves around lipid droplets and keep them dispersed (emulsification).
False - Bile salts are amphipathic and orient themselves around lipid droplets and keep them dispersed (emulsification).
In the formation of bile, what synthesizes cholic acid and chenodeoxycholic acid?
Synthesized from cholesterol by hepatocytes.
What are bile acids conjugated with?
Glycine or taurine.
What is added to bile acids that are conjugated with glycine or taurine?
Electrolytes and water.
Bile in the gallbladder is concentrated in result of what?
Result of isomotic absorption of solutes and water.
What does CCK in the bloodstream result in?
- Gallbladder contraction.
2. Relaxation of sphincter of Oddi.
List the 4 causes of gallstones.
1) Too much absorption of water from bile.
2) Too much absorption of bile acids from bile.
3) Too much cholesterol in bile.
4) Inflammation of epithelium.
What are the different cell types that are found in the Crypts of Lieberkuhn?
> Goblet cells
Enterocytes (absorptive cells)
Paneth Cells (secrete antimicrobial proteins to limit bacteria-enterocyte contact)
Enteroendocrine cells (secrete peptide hormones controlling several GI system functions)
What is a condensation reaction?
Used to remove hydrogen ions and hydroxyl ions from building blocks in order to allow the bonding of monomers into polymers.
What is hydrolysis?
The reverse of condensation reactions, incorporating water molecules in such a way that polymers are broken down into monomers.
In the digestion of carbohydrates, which enzymes breakdown starches?
- Ptyalin (saliva) 20-40%
- Pancreatic amylase 50-80%
In the digestion of carbohydrates, which enzymes breakdown maltose?
- Maltase (intestine)
- alpha-dextrinase (intestine)
What does maltase and alpha-dextrinase (intestine) breakdown maltose into?
Glucose
What does sucrase (intestine) breakdown sucrose into?
Fructose and Glucose
What does lactase (intestine) breakdown lactose into?
Galactose and Glucose
What are starches brokendown into by ptyalin (saliva) and pancreatic amylase?
Maltose and 3-9 glucose polymers.
What does pepsin break proteins into?
- Proteoses
- Peptones
- Polypeptides
What does trypsin, chymotrypsin, carboxypolypeptidase, and proelastase breakdown proteoses, peptones, and polypeptides into?
Polypeptides + Amino Acids
What enzyme breaks down polypeptides + amino acids into amino acids?
peptidases
What is digestion of fats, going from fat -> fatty acids and 2-monoglycerides?
Fat -(Bile + Agitation)-> Emusified fat -(Pancreatic lipase)-> fatty acids and 2-monoglycerides
Where does the absorption of glucose and galactose take place?
Small Intestine
What is the mechanism of glucose and galactose being absorbed in the small intestine?
Active transport of sodium ions (Na-K pump in basolateral membrane) followed by secondary active transport involving a sodium co-transport mechanism (SGLT 1) in luminal membrane.
What is the absorption of fructose facilitated by?
Absorption of fructose is entirely by facilitated diffusion in small intestine.
Cannot be absorbed against a concentration gradient
What does the absorption of lactose require in the brush border in the small intestine?
Absorption of lactose requires lactase in brush border.
> lactase breaks lactose down into glucose and galactose.
> Absence of lactase results in lactose intolerance, causing lactose to remain in lumen and cause osmotic diarrhea.
True of False:
Protein digestive products are absorbed as amino acids, dipeptides, and tripeptides in the small intestine.
True
How are peptides or amino acids absorbed in the small intestine?
Sodium-amino acid co-transport mechanism in luminal membrane.
facilitated transport from enterocyte into blood
What are the four separate transporters that are involved in the absorption of peptides or amino acids in the small intestine?
- neutral amino acids
- acidic amino acids
- basic amino acids
- imino amino acids
** so different charged amino acids have different transporter**
What type of co-transporters are found on the luminal membrane help in the breakdown into amino acids for the absorption into the small intestine?
H ion-dipeptide/tripeptide co-transporters on luminal membrane; cytoplasmic peptidases then convert them to amino acids.
What role does CCK play in the absorption of lipids in the intestine?
CCK slows gastric emptying, allowing adequate time for digestion and absorption in the intestine.
What do bile acids do to lipids in the small intestine?
Bile acids emulsify lipids in small intestine.
What role does pancreatic lipases have in the absorption of lipids in the intestine?
Pancreatic lipases hydrolyze lipids to fatty acids, monoglycerides, cholesterol, and lysolecithin:
- pancreatic lipase
- cholesterol ester hydrolase
- phopholipase A2
What is the property of products from lipids after they have undergone enzymatic digestion?
Products are hydrophobic and are solubilized in micelles by bile acids.
What is the role of micelles in the absorption of lipids in the intestine?
Micelles bring the products of digestion into contact with cell membrane of enterocyte and contents diffuse into cells.
Intracellularly, what are the products of lipid digestion converted into?
Triglycerides, cholesterol ester, and phospholipids.
What are triglycerides, cholesterol ester, and phospholipids combined with to form chylomicrons?
apoproteins
What happens to chylomicrons after they are formed by the combining of apoproteins + triglycerides, cholesterol ester, and phospholipids?
Are exocytosed and transferred to lymph vessels.
What are the different mechanisms in which NaCl is absorbed in the intestines?
> passive diffusion of Na ions through Na channels.
Na-Glucose or Na-amino acid cotransporters.
Na ion-H ion exchange
Passive absorption of Cl ion.
Na ion-Cl ion cotransport.
Cl ion-HCO3 ion exchange
What are the different mechanisms in which K is absorbed in the intestines?
> K ion can be absorbed passively via paracellular route.
> K secretion in colon is stimulated by aldosterone.
What are the characteristics of water being absorbed in the intestines?
> Secondary to solute absorption.
Isomotic in small intestine and gallbladder.
Permeability to water is much lower in colon than in small intestine – feces may be hypertonic.
