Gastrointestinal Physiology: Secretion I Flashcards by Joel Glotfelty

The largest contributor to GI secretion

-Secreted in the upper portion of the GI tract and reabsorbed in the lower portion

Water

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Serves a protective function for the epithelial surface all along the tract

Mucus

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In general, are secreted via 2nd messenger activated fusion of granules with the luminal membrane

Enzymes

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Initiated by active transport, primary or secondary, of one ion through the basolateral membrane and diffusion through the apical membrane or by active transport of one ion through the apical membrane

Electrolyte secretion

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The composition of saliva varies. But what are the two constants?

) Always hypotonic

2. ) K+ is higher than in plasma

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Saliva secretion declines with age. In children, we see an average saliva secretion of

1-1.5L per day

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The resting secretory rate for saliva is

25uL/min

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Concentration of electrolytes in saliva varies, but it is always

Hypotonic

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Relative to serum, saliva is always lower in concentration of

Na+ and Cl-

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Most of the time, saliva is rich in

HCO3-

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In the two-stage secretion model for saliva, primary secretions are performed by the

Acinar cells

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In the two-stage secretion model for saliva, primary secretions by the acinar cells is followed by modifications of the luminal fluid by the

Ductal Cells

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An isotonic ultrafiltrate of plasma that is secreted by the acinar cells

Primary secretion

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Modify the primary secretion fluid by reabsorbing Na+ and Cl- and secreting K+ and HCO3-

Ductular epithelial cells

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Here, there is a greater amount of Na+

and Cl- reabsorbed than

K+ and HCO3- secretion

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This reduction in the number of ions in the tubular fluid plus the impermeability of ducts leads to a

Hypotonic Saliva

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What two things increase during active saliva secretion?

Metabolic hyperemia and vasodilation

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Due to a local increase in the concentrations of metabolic end products

Metabolic hyperemia

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Mediated by vasoactive intestinal polypeptide (VIP) from non-cholinergic parasympathetic fibers and by bradykinin

Vasodilation

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Higher in Cl- and K+ than plasma, and much higher in H+

Gastric Juice

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Whatis the approximate amount of gastric juice secreted per day?

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Continued loss of saliva and gastric juice (vomiting or aspiration) can lead to water and electrolyte losses that result in

Dehydration, metabolic alkalosis, and severe electrolyte imbalance

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Functions to liquify the bolus and form chyme

Gastric secretions

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Gastric secretions provide intrinsic factor for absorption of

Vitamin B12

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Stabilize an absorbable form of some minerals (Ca+2, Fe+2)

Gastric Secretions

Surface columnar mucous and epithelial cells secrete mucus and an

Alkaline fluid

Fundic glands (straight) have which three types of cells?

1. ) Chief cells 2. ) Parietal cells 3. ) Neck Chief Cells

Secrete pepsinogens

Chief cells

Secrete HCl and IF (oxyntic)

Parietal cells

Secrete mucus & are the progenitors of luminal cells

Neck Chief Cells

Pyloric glands (branched) have which two secretory cell types?

Mucous cells and G cells

Secrete mucous and pepsinogens

Mucous cells

Release gastrin into the blood (not lumen)

G cells

States that there are two different secretions from different cells with different controls

Two component hypothesis

What are the two components of the two component hypothesis?

1. ) Oxyntic component (isotonic HCl) | 2. ) Non-oxyntic component (mucosal barrier)

Transported into the lumen in the resting oxyntic cell and the surface epithelial cell

Cl-

This movement is by HCO3-/Cl exchangers in the basolateral membrane and Cl channels in the

Luminal Membrane

A major contributor to the negative electrical potential difference (-70 to -80 mV) between the stomach lumen and blood (lumen negative)

Oxyntic component

Upon stimulation, oxyntic cells undergo structural changes and actively secrete which two things?

1. ) Isotonic HCl into lumen | 2. ) HCO3- into interstitial area

At the whole body level, stimulation of acid secretion is normally associated with

Meals

Inhibition of acid secretion is associated with loss of stimuli + specific inhibition of

Secretion

Paracellular movement and cellular secretion of an isotonic ultrafiltrate of plasma that is high in K+ (10-20 mM) and HCO3- (45 mM)

Non-oxyntic component

In the non-oxyntic component, HCO3- is produced in the epithelial cells, and transported from the blood into the cell by a

Basolateral HCO3-

This HCO3- is secreted at the luminal membrane by an electroneutral Cl/HCO3 exchanger, stimulated by

Glucagon

This HCO3- is also secreted at the luminal membrane by an electrogenic HCO3 transporter, stimulated by

Prostaglandin E2

In the non-oxyntic component, soluble mucus is released from the mucous neck cells in response to

PNS stimulation via ENS

In the non-oxyntic component, visible mucus is released from the surface cells in response to

Mechanical irritation and chemical stimulation

Local reflex, and both sympathetic and parasympathetic activity via the ENS play a minor role in

Visible mucous secretion

The non-oxyntic component secretions create the mucosal barrier. This barrier can neutralize about

20 mM of acid

Cytoprotection of gastric, small intestinal, liver and pancreatic cells has been experimentally observed with

Prostaglandins E2, F2, I2, and alpha

Effective concentrations of the prostaglandins are below those that inhibit acid secretion in the

Oxyntic cells

Five common agents known to cause disruption of the gastric mucosal barrier and epithelial cell destruction are?

1. ) Aspirin 2. ) Nonsteroid anti-inflammatory agents 3. ) Ethanol 4. ) Bile acids 5. ) H. Pylori

Causes intracellular acidification and inhibits prostaglandin synthesis, and the secretion of mucus and HCO3-

Aspirin

Inhibit prostaglandin synthesis, and mucus and HCO3- secretion

Nonsteroidal Anti-Inflammatory Drugs (NSAIDS)

Inhibits intracellular enzymes

Ethanol

Solubilizes the plasma membrane

Bile acids

Induces inflammation and an immune response through production of NH4+ and other virulence factors

H. Pylori

Secreted by oxyntic cells -The control appears similar to acid secretion

Intrinsic factor

Secreted by chief and mucous cells -secretion is correlated with acid secretion

Pepsinogen

The common secretogues of pepsinogen are

ACh and Secretin

Secretion of pepsinogen is potentiated by luminal acid via a

Local reflux

The composition of pancreatic juice depends on the number and type of secreting cells, and the

Secretory rate

Up to 2 L of pancreatic juices are secreted daily, mostly in response to

Eating

Has essentially no pancreatic amylase activity, negligible lipase activity, low protease activity, and the pancreas does not respond to CCK stimulation

Newborns

Lipases from milk, lingual and gastric sources and salivary amylase play an important role in

Infant digestion

Normal pancreatic function develops by age

Functions to provide enzymes for intraluminal digestion of most foods. protect the mucosa and provide an alkaline environment for optimal enzymatic activity

Pancreas

Secrete proteins via fusion of zymogen granules with the apical membrane

Acinar cells

Acinar cells also secrete a small amount of fluid that is essentially

Isotonic HCl

Secrete a large volume of alkaline fluid, pH 7.6 - 8.2, that is high in HCO3-

Pancreatic extralobular ductule cells

The proposed fluid secretion mechanism of the pancreas is the two component hypotheses, plus HCO3-/Cl exchange in the main collecting ducts that transports HCO3- in and Cl- out of the

Ductal cell

With disfunctional CFTR, cystic fibrosis patients have insufficient -at higher risk for pancreatitis

Pancreatic secretion

During the interdigestive period, basal secretion from the pancreas accounts for what percentage of 1. ) HCO3- 2. ) Enzymes

1. ) 2-3% | 2. ) 10-15%

Accounts for 10-15% of the volume & 25% of the enzymes released from the pancreas in response to a meal

Cephalic phase

The cephalic phase is under what control?

Vagal

A continuation of the cephalic with greater neural stimulation due to gastric distention

Gastric phase of pancreatic secretion

Accounts for 70-80% of a meal response (CCK, secretin & reflexes)

Intestinal phase of pancreatic secretion

There is 500-1500 ml/day of an electrolyte solution coming from the small intestinal epithelial and

Crypt cells

Functions to maintain isotonicity and fluidity of luminal contents, this aids absorption and protect the epithelial surface

Intestinal secretions

In intestinal secretion, ion movement is initiated by active transport of Cl and/or HCO3-, and other ions and H2O follow via

Passive diffusion

H2O also enters the intestinal lumen in response to an increase in the concentration of osmotically active particles produced by

Gastric emptying and digestion

Cl- enters the intestinal lumen through

CFTR

Activate CFTR and induce diarrhea

Cholera toxin and other microbial products

Protected from diarrheal pathogens due to their insufficiency in intestinal fluid secretion

Cystic fibrosis patients

External fluid circuit concept of intestinal secretion is absorption at the tip of villi and secretion at the base or in

Crypts

Fluid reabsorption can be significantly upregulated by the addition of

Glucose and NaCl

One way to treat people with severe diarrhea is administering

Glucose and NaCl

Functions to emulsify fats and solubilize hydrophobic substances (cholesterol in the gallbladder and biliary tract, plus free fatty acids and fat soluble vitamins in the small intestine)

Hepatic Secretions (excretion)

Augment pancreatic HCO3- secretions to maintain an alkaline pH in the luminal of the small intestine

Hepatic Secretions

Excrete lipid soluble substances and heavy metals from the body

Hepatic secretions

A mixture of two solutions, one from the hepatocyte (organic substances and an isotonic filtrate of plasma), and one from the epithelial cells in the ductules and ducts (HCO3- enriched fluid)

Bile

Secrete organic substances

Hepatocytes

Amphipathic molecules that form micelles -secreted by hepatocytes

Bile Salts

Secrete phospholipids (lecithin), cholesterol, bile pigments (bilirubin), miscellaneous proteins, inorganic ions (Na+, K+, Ca2+, Cl and heavy metals) and H2O

Hepatocytes

Secretion of the organic component depends on the availability of

Bile acids (salts)

Rates of hepatocyte organic compound secretion are high during the intestinal phase due to return of

Bile acids (salts)

The transporters for hepatocyte secretion belong to the

ATP-Binding Cassette (ABC) family

The hepatic ductal cells secrete alkaline fluid. What stimulates HCO3- secretion?

Secretin

Functions to reabsorb Na+, Cl, H2O, and HCO3- concentrates and acidifies the bile

Gall bladder

Free Na+, and Cl and/or HCO3- are transported out of the lumen of the gall bladder and into the

Epithelial cells