March 26, 2004
Pancreatic and Bile Secretion
Ron Lynch, PhD
626-2472
PANCREATIC SECRETIONS. (1200 ml/day).
Objectives:
1. Identify the components of pancreatic secretions, the cells from which each
component arise, and the function of each component.
a. Identify the basic constituents that provide the digestive and
protective functions of pancreatic secretions.
2. What is the basic cellular mechanism by which HCO3 is produced and secreted?
a. What are the key transport and enzymatic processes required for
activated HCO3 secretion?
3.
Determine how HCO3 secretion is regulated at the cellular level:
a. What are the primary agonists for activating HCO3
secretion?
b. What are the second messenger pathways through which the
activators of HCO3 secretion operate?
I. Functional Anatomy: The structure of the exocrine pancreas is very similar to that of the
salivary glands though more extensive. Lobules consist of groups of acini forming grape-like
structures. An extensive expanding ductal system collects secretions from the lobules and
carries it to common pancreatic ducts. Prior to reaching the upper duodenum, the common
pancreatic duct joins with the common bile duct then empties into the duodenum at the
sphincter of Oddi. Ultrafiltration occurs at the Acinus which provides a driving force for
fluid flux, and the digestive enzymes are produced in and secreted by acinar cells. However,
the acini are not highly vascularized like those of the salivary glands and the ductal cells
secrete HCO3- and the largest volume of fluid.
II.
Composition and Functions of Pancreatic Secretions: There are two general products
of the exocrine pancreas which are important in digestive function. First, bicarbonate
and the concommitant volumous aqueous secretion function to neutralize stomach
acid. The second important component of pancreatic secretions is the wide range of
digestive enzymes that are secreted upon activation, and are capable of digesting all
major foodstuffs.
III. Mechanism of Secretion: Secretions from the pancreas are dependent upon enzyme secretion
from the acini with addition of HCO3- and fluid in the extensive ductal system. Unlike the
salivary glands, increased fluid flux is not dependent on increasing blood flow around the
acinus. Protein is the primary component added by acinar cells, while unlike the salivary
glands, active HCO3- and water secretion occur at the level of the cells lining the extensive
ductal network.
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March 26, 2004
Pancreatic and Bile Secretion
Ron Lynch, PhD
626-2472
A. Enzymatic Component: Acinar Cells. The proteolytic enzymes of the pancreas are
synthesized in inactive forms; (e.g., trypsinogen, chymotrypsinogen) while amylase is
released in its active form. Enzyme containing granuals are expelled into the lumen of the
acinus by exocytosis following a stimulus (elevated Ca2+). The proteolytic enzymes are
activated only after reaching the intestine.
Enteropeptidase which is secreted by the intestinal mucosa cleaves the precursor
enzymes thereby activating them. The primary precursor upon which enteropeptidase acts
is trypsinogen. Once activated, trypsin activates the other precursor enzymes. Acinar cells
also secrete trypsin inhibitor to prevent premature activation of trypsin within the
pancreatic ducts. If flow from the pancreas is blocked, premature activation of trypsin can
occur within the ductal system. Subsequent activation of digestive enzymes leads to
breakdown of the pancreatic ducts and resultant pancreatitis.
B. Active Bicarbonate Secretion is carried out by the pancreatic ductal cells. This process
is carbonic anhydrase (CA) dependent. Since the ductal system in the pancreas is
extensive, a large surface area for HCO3- secretion exists.
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March 26, 2004
Pancreatic and Bile Secretion
Ron Lynch, PhD
626-2472
Carbonic Anhydrase once again is a key player in the HCO3- secretory mechanism. In ductal
cells, HCO3- produced by CA is transported across the apical membrane in exchange for Cl(HCO3- /Cl- anti-porter). The H+ produced by CA leaves the cell via a basolateral Na+/H+
exchanger. Thus, the Na+ gradient is used to drive H+ to the blood. Recall that secretion of HCl
into the stomach results in an "alkaline tide" or high HCO3- in the venous blood leaving the
stomach. Mixing of venous blood from the stomach with the relatively acidic blood from the
pancreas in the portal vein assures that blood pH is near neutral prior to entering the liver.
Another important component is the lumenal cAMP activated Cl- channel (CFTR). Secretin
increases cAMP in the ductal cells which allows Cl- to move into the lumen. The increased
lumenal Cl- is exchanged for HCO3 driving HCO3 secretion. In Cystic Fibrosis pancreatic fluid
and HCO secretion are dramatically decreased due to the absence of this C1- channel.
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IV. Regulation of Pancreatic Secretion:
Since the pancreas is not directly attached to the alimentary canal, inputs which regulate
secretion originate either from central reflex arcs or endocrine factors.
A. Neural regulation: Neural signals are short term initiating signals which arise centrally
(parasympathetic), and travel via the vagus nerve to the pancreas where they stimulate
secretion by primarily acinar cells, and to a lesser degree ductal cells. Thus, during the
cephalic phase, neural impulses initiate secretion of primarily enzymes into the acinus.
When food enters the stomach (gastric phase), stimulation of mechanoreceptors initiates
reflexes whose afferent signal initiate pancreatic secretions. At most 10-20% of the
secretory response is initiated during the Cephalic and Gastric phases via vagal signals.
B. Endocrine regulation: Hormones provide strong stimulatory signals of long duration
driving pancreatic secretion during the Intestinal phase. Secretin release from duodenal S
cells is stimulated by fat or H+ in the intestine. Secretin then primarily stimulates luminal
cells to secrete bicarbonate.
Cholecystokinin release from intestinal I cells is stimulated by peptides and fat. Cholecystokinin
stimulates enzyme secretion from acini. CCK also regulates gene expression of proteases and
lipases, thus the absence of fat in a meal causes downregulation of lipase and protease activities.
Although the exact mechanisms are unknown, the composition of ones diet modulates pancreatic
enzyme expression such that pure vegetarians often are observed to down-regulate expression of
certain proteases and lipases. At least 60% of all pancreatic secretion occurs during the
intestinal phase, and is driven by the endocrine factors.
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March 26, 2004
Pancreatic and Bile Secretion
Ron Lynch, PhD
626-2472
V. Receptor Coupling and Second Messenger Systems.
Signals for stimulation of HCO3- Secretion by Ductal Cells.
Secretin is the primary activator of secretion by ductal cells. Secretin activates adenylate
cyclase and cAMP formation. cAMP elicits opening of lumen Cl- channels and stimulates
production of CO2 which drives the carbonic anhydrase reaction producing carbonic acid.
CCK and acetylcholine can potentiate this response through Ca2+ mediated pathways but
do not activate HCO3- secretion by themselves. In this case, elevated cytosolic Ca2+
increases mitochondrial respiration, and thereby CO2 production. The elevated CO2
drives carbonic acid and bicarbonate production via CA. However, without activation of
the cAMP activated C1- channel (by secretin), HCO secretion is only mildly elevated.
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+
When both Ca2 and cAMP are elevated, maximal secretory rates can be achieved.
As an example, distension of the duodenum is known to potentiate the secretin mediated
response, and this is associated with Ach release in the vicinity of ductal cells through a
vagal mediated reflex. However, in the absence of the hormone secretin, intestinal
distension is largely ineffectual accounting for low HCO3- secretion during the cephalic
phase when Ach is also released from nerves.
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March 26, 2004
Pancreatic and Bile Secretion
Ron Lynch, PhD
626-2472
SECRETION OF BILE BY THE LIVER: Lipid Digestion and Absorption
Objectives:
1.
Understand the organization of the liver lobule, and biliary tract which
underlies the ability of hepatocytes to regulate bile levels.
2.
Determine the components of bile important for digestion, and the mechanism
for their cycling during a meal.
3.
Know the mechanisms which regulate bile secretion.
I. Functional Anatomy
The hepatic biliary system is the basic
structural and functional unit of the liver.
Hepatocytes receive blood supply from
the systemic circulation via the hepatic
artery, and also venous blood collected
from the gastrointestinal organs via the
portal vein. Venous blood moves from the portal vein into portal venules then past the
hepatocyets into central hepatic veins by
which it leaves the liver to enter the
systemic circulation. Hepatocytes are very
proficient in extracting compounds from the portal blood, and excreting them into the biliary
system. Bile canaliculi are minute channels between hepatocytes into which bile is secreted. Bile
ducts are channels that drain the canaliculi. Large biles ducts from each lobe of the liver combine
to form the common bile duct. An appendage to the common bile duct is the Gallbladder. The
gall bladder acts as a storage recepticle for bile where bile becomes highly concentrated during
periods of fasting. Bile movement to the intestine is through the common bile duct which
combines with the pancreatic duct just prior to the entrance to the intestine. There is a tonically
controlled region of smooth muscle at the entrance to the duodenum which is called the
Sphincter of Oddi. Activity in this sphincter controls movement of bile and pancreatic
secretions into small intestine. The gallbladder can hold up to 4 gms of bile, while the liver can
synthesize between 0.2 and 0.6 gms of bile per day. Average fat intake is about 65 gms per day.
To immulsify this amount of fat at least 35 gms of bile are required. Therefore, the amount of
bile stored and synthesized is not sufficient to digest this fat requirement. Our ability to digest
large amounts of fat is due to the ability of the intestine to reabsorb bile acids after digestion of
fat occurs, and the ability of hepatocytes to then extract the bile acids from portal vein blood with
subsequent secretion back into the biliary tract (enterohepatic circulation).
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March 26, 2004
II.
Pancreatic and Bile Secretion
Ron Lynch, PhD
626-2472
Composition of Bile
Bile is composed of a variety of organic and ionic constituents. Some of the constituents
do not serve a digestive purpose but rather are secreted by the liver into the alimentary
canal for excretion from the body. This excretory component includes the bile pigments
(e.g., bilirubin), as well as, a variety of possible hepatic metabolites including drug
metabolites. Bilirubin is an end product of hemoglobin degradation, and is actively
absorbed from blood by hepatocytes and secreted into canaliculi with the bile acids. This
compound provides much of the coloration of stool; blockage of the bile duct network
and loss of bilirubin excretion to the canal is the cause of increased blood bilirubin and
the coloration associated with jaundice.
The functional secretory component consists primarily of bile salts, cholesterol, lecithin
and fatty acids. Bile acids are the major component. The primary bile acids, cholic and
chenodeoxycholic acid, are synthesized from cholesterol in hepatocytes. Bile acids are
then conjugated with glycine or taurine and secreted as Na+ salts which are more soluble
than the acid forms. Secondary bile acids are formed by dehydroxylation of primary bile
acids by bacteria in the colon. The secondary bile acids are deoxycholic acid and
lithocholic acid. The epithelial cells lining the bile canaliculi also contribute a watery
HCO3- containing secretion similar to that elaborated by pancreatic ductal cells.
III.
Composition of Liver Bile Relative to Gall Bladder Bile.
The Gall Bladder stores bile during periods of inactivity. While in the gall bladder, water
and specific anions are removed. The resulting gall bladder bile is concentrated (net
volume loss) with respect to the concentration of bile salts, and the other organic
components. The primary components removed to drive water loss are NaCl and HCO3-
Bile salts
Bile Pigments
Cholesterol
Lecithin
Na+
K+
Ca+
ClHCO3-
Composition of Bile
Liver Bile (mM)
Gallbladder
Bile (mM)
35
310
0.8
3.2
3.0
25
1.0
8.0
165
280
5
10
2.5
12
90
45
15
8
6
Fold change
In Bile
10x increase
4x increase
8x increase
8x increase
1.7x increase
2x increase
4.8x increase
6.0x decrease
5.6x decrease
March 26, 2004
IV.
Pancreatic and Bile Secretion
Ron Lynch, PhD
626-2472
Functions of Bile Secretions.
Bile salts provide a detergent reaction within the intestine where they act to emulsify fat
droplets contained in food. This action breaks up fat particles into smaller globules and
increases and modifies the surface area of the lipid which digestive enzymes can attack.
V.
Enterohepatic Circulation of Bile Salts
After bile performs its function within the intestine, it is absorbed at the level of the ileum
into the portal blood. As the portal blood passes through the liver sinusoids, bile is
absorbed by hepatocytes and then secreted again into the biliary network. The entire bile
pool is circulated 2-3 times per meal.
The cycling of bile between intestine and blood is referred to as the entero-hepatic
circulation. Little bile salt reabsorption occurs in duodenum or jejunum which allows
bile salts to aid fat metabolism without being reabsorbed. 90-95% of bile is absorbed
from the ileum by specific transporters. Bile which enters the large intestine is converted
to secondary bile acids which can be absorbed passively to a limited extent. The bile salts
are returned to liver via portal blood, and are taken up by hepatocytes. Also recycled by
the enterohepatic pathway are compounds like the lipid soluble vitamins (e.g., Vitamin D)
and many drugs such as cardiac glycosides and indomethacin.
Liver
Cholesterol
7-hydroxylase
Cholesterol
Newly synthesized
bile acids
(0.6 g/24 h)
Portal
Vein
Excrete
d
(0.6
bileg/24 h)
VI. acids
Regulation of Bile Secretion.
Colon
2-4g
bile-acid pool
: circulated 6 – 10
times in 24 h
Bile
Duct
s
Stomach
Small
intestine
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March 26, 2004
Pancreatic and Bile Secretion
Ron Lynch, PhD
626-2472
Between meals most of the bile is stored ni the gall bladder where excess fluid volume is
reabsorbed and the bile is concentrated. Mechanisms which regulate bile secretion act to
either alter the rate of bile producition/secretion, or to contract the gall bladder.
Neural reflexes activated by sensory receptors for taste, smell, or sight of food mildly
activate gall bladder contraction (Cephalic Phase). However, little bile reaches the small
intestine because the Sphincter of Oddi remains closed. Intestinal distention increases the
contractile activity of the duodenum which causes the Sphincter of Oddi to relax resulting
in small amounts of bile being squirted into the duodenum with each wave of
contractility. Thus, during the Cephalic phase, limited amounts of pancreatic enzymes and
bile are secreted into the ductal system, from where they can then be released into the
intestine immediately upon arrival of chyme into the duodenum.
The primary stimulus for bile "secretion" to the intestinal lumen is hormonal.
Cholecystokinin is released from intestinal I cells by the products of fat digestion. Free
fatty acids and triglycerides within the intestinal lumen provide strong stimuli for
cholecystokinin release which in turn causes strong contraction of the gall bladder and
also increased bile salt secretion by hepatocytes.
VIII.
Defects in Bile Circulation.
A. Bile Duct Obstructions: Obstructions lead to buildup of bilirubin in the blood
causing symptomatic jaundice. Furthermore, the absence of bile for digestion leads to
high fat content in stool. Since the color of stool is due to pigments in bile, the
absence of bile leads to white colored stools.
B. Ileal Resection. Since bile is absorbed from the alimentary canal in the ileum,
resection leads to limited enterohepatic circulation of bile salts. Bile salts which
remain in the colon act both as an osmotic agent and also as a secretory stimulant; i.e.,
cause diarrhea (Lecture 10).
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