Review Article - Journal of Physiology and Pharmacology

JOURNAL OF PHYSIOLOGY AND PHARMACOLOGY 2003, 54, 3, 293–317
www.jpp.krakow.pl
Review Article
1
S.J. KONTUREK, 1J. PEPERA, 2K. ZABIELSKI, 3P.C. KONTUREK, 4T. PAWLIK,
1
A. SZLACHCIC, 3E.G. HAHN
BRAIN-GUT AXIS IN PANCREATIC SECRETION
AND APPETITE CONTROL
1
Department of Physiology, Jagiellonian University School of Medicine, Kraków, Poland
The Kielanowski Institute of Animal Physiology and Nutrition, Polish Academy of Sciences,
Jablonna, Poland, 3First Department of Medicine, University Erlangen-Nuremberg, Erlangen,
Germany 4Department of Gerontology, Jagiellonian University School of Medicine,
Kraków, Poland
2
The stimulation of exocrine pancreatic secretion that has been attributed by Pavlov
exclusively to various reflexes (nervism), was then found that it depend also on
numerous enterohormones, especially cholecystokinin (CCK) and secretin, released by
duodeno-jejunal mucosa and originally believed to act via an endocrine pathway.
Recently, CCK and other enterohormones were found to stimulate the pancreas by
excitation of sensory nerves and triggering vago-vagal and entero-pancreatic reflexes.
Numerous neurotransmitters and neuropeptides released by enteric nervous system
(ENS) of gut and pancreas have been also implicated in the regulation of exocrine
pancreas. This article was designed to review the contribution of vagal nerves and
entero-hormones, especially CCK and other enterohormones, involved in the control of
appetitive behavior such as leptin and ghrelin and pancreatic polypeptide family
(peptide YY and neuropeptide Y). Basal secretion shows periodic fluctuations with
peals controlled by ENS and by motilin and Ach. Plasma ghrelin, that is considered as
hunger hormone, increases under basal conditions, while plasma leptin falls to the
lowest level. Postprandial pancreatic secretion, classically divided into cephalic, gastric
and intestinal phases, involves predominantly CCK, which under physiological
conditions acts almost entirely by activation of vago-vagal reflexes to stimulate the
exocrine pancreas, being accompanied by the fall in plasma ghrelin and increase of
plasma leptin, reflecting feeding behavior. We conclude that the major role in
postprandial pancreatic secretion is played by vagus and gastrin in cephalic and gastric
phases and by vagus in conjunction with CCK and secretin in intestinal phase. PP, PYY
somatostatin, leptin and ghrelin that affect food intake appear to participate in the
feedback control of postprandial pancreatic secretion via hypothalamic centers.
K e y w o r d s : pancreas, cholecystokinin, secretin, gastrin releasing peptide, calcitonin gene
related peptide, motilin, leptin, ghrelin, pancreatic polypeptide, peptide YY,
neuropeptide Y.
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INTRODUCTION
Historical background
Although Renier de Graf (1641-1679) had constructed first pancreatic and
biliary fistula in the dog, it was not until Claude Bernard (1813-1878)(1, 2)
emphasized the important role of pancreas in digestion of fat and attributed
metabolic activities to liver, the discoveries which led him to formulate the
concept of the stability of internal environment - milieu interieur - as a condition
of health and well-being (Fig. 1). However, another 50 years passed before active
physiology research on digestive system resumed starting at the turn of 19th and
during early part of 20th century with discoveries by Ivan P. Pavlov of Russia of
neural stimulation of digestive glands (3), attributing the mechanisms of this
stimulation exclusively to neural reflexes (nervism). Soon, W.M. Bayliss and
E.H. Starilng of England, discovered secretin in 1902 (4), a first hormonal
substance, stimulating pancreatic secretion in response to duodenal acidification,
while J.S. Edkins identified in 1905 (5) gastrin, another non-nervous stimulant,
implicated in the stimulation of gastric acid secretion by food. These two later
discoveries are millstones in development of modern endocrinology and for the
first time the term hormone, originally invented by Hardy was applied to secretin.
*C. BERNARD (1856) demonstrated that „pancreas plays an
unique role in fat digestion”.
* I.P. PAVLOV (1876) in his book: „The Work of the Digestive
Gland” asserted that exocrine pancreatic secretion is
exclusively regulated by neural reflex mechanism („nervism”).
CNS
*W.M. BAYLISS & E.H. STARLING (1902) discovered secretin,
first hormone, released by H+ in gut to stimulate pancreas.
*W. BOLDYREFF (1911) discovered motor and secretory cycles
during interdigestive period and explained them by neuroreflexes.
+
-
*J. MELLANBY (1925) postulated „dual hormonqal and nervous
control; secretin controlling water and bicarbonate and vagus enzyme secretion.
*A.A. HARPER & H.S. RAPER (1943) found „pancreozymine”
the hormone that stimulates pancreatic enzyme secretion.
*A.C. IVY & E. OLBERG (1928) isolated from intestines
„cholecystokinin” the hormone that contracts the gallbladder
and relaxes sphincter of Oddi.
*E. JORPES & V. MUTT(1966) proved that pancreozymine and
cholecystokin are the same substance and called it
„cholecystokinin”.
ENS
*Numerous Investigators; discovered various gastro-enteric
peptides (GEP) such as somatostatin, GRP, VIP, PP, PYY, NPY,
galanin etc. released by neurons or endocrine-paracrine cells
(APUD) in the gut and brain „Brain-Gut Axis”.
Rene Dubois (1901): ”The past is not dead history, it is living material of which man buils its future”
Fig. 1. Major discoveries in pancreatic physiology and brain-gut axis.
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New discipline of gastrointestinal endocrinology flourished in the second part of
20th century when modern techniques of peptide biochemistry became available
and successfully applied to identify major gut hormones such as gastrin, secretin,
cholecystokinin (CCK), gastric inibitory peptide (GIP) and motilin, which have
been isolated, chemically characterized and synthesized. In addition, numerous
hormonal peptides have been recently discovered but still they lack full hormonal
status such as pancreatic polypeptide (PP), peptide YY, neuropeptide Y (NPY),
somatostatin, gastrin-releasing peptide (GRP), galanin, vasoactive intestinal
peptide (VIP), pituitary adenylyl associated peptide (PACAP), substance P and
other tachykinins, leptin and ghrelin (Fig. 2)(6).
Recent findings that some of the gut hormones such as CCK, PP, PYY or NPY,
somatostatin, originally believed to act on the target digestive organs via
endocrine pathway, involve neural pathway to exert their biological action
provide support for the concept of neuroendocrinology of the digestive system.
Thus, the old Pavlovian nervism has been “married” with endocrinology and
found to act together in full harmony to control the digestive activities. Enteric
portion of autonomic nervous system (ENS) forms in the digestive system so
called the “brain of gut” consisting of about 100 milion of neurons that cooperate
with enterohormones, each of them contributing to the motor, secretory,
absorptive, excretory, trophic and circulatory functions of this system. Most of the
gut hormones originally identified in the central nervous system (CNS) were then
also localized in the gut and vice versa, the brain neuropeptides and
neutrotransmitters were later on identified in the gut forming so called the “brain-
Stimulation
* Cholecystokinin (CCK)
Secretin
* Gastrin
* Gastrin-Releasing Peptide (GRP)
* Insulin
* Vasoactive Intestinal peptide (VIP)
* Cyclase-Activating peptide
* Substance P and other tachykinines
* Adenosine 5’-triphospate (ATP)
Uridine triphosphate (UTP)
* Histamine
Panacreatic phospholipase A2
Inhibition
* Pancreatic Polypeptide (PP)
* Leptin
* Ghrelin
Peptyde YY
* Neuropeptide Y
* Calcitonin Gene-Related Peptide
* Somatostatin
Glucagon
Glucagon-Like Peptide -1 (GLP-1)
* Thyrotropin-Releasing Hormone
* Enkephalin (Met- or Leu-)
* Nitric Oxide (NO)
* Dopamine
*Present both in the brain and gut
Acting through neural pathway
Fig. 2. List of stimulatory and inhibitory hormones, neuropeptides and neurotransmitters involved
in brain-gut axis, present in the gut and brain and acting through neural pathway
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gut axis” involved in the control of digestive activities and feeding behavior
under normal and pathological conditions (Fig. 3).
Structural basis of pancreatic secretion
The pancreas is both an exocrine and endocrine organ. The exocrine pancreas
combines two main functional elements; the acini comprizing about 85% of the
total gland mass and the ductal and centroacinar cells (called also principal cells)
that include only about 5% of gland cell mass. The acini of spheric or tubular
shape consist of acinar cells that are specialized to synthesize and store digestive
enzymes in secretory vesicles (zymogen granules) and release the enzymes in
response to various secretagogues. On their basolateral membrane various
receptors for secretagogues including CCK and neurotransmitters such as
acetylcholine (Ach), gastrin releasing peptide (GRP) and vasoactive intestinal
peptide (VIP) have been identified and found in in vitro studies on isolated
cultured acinar cells to stimulate enzyme secretion (7).
The centro-acinar and duct cells, that are largely responsible for the HCO3- and
water secretion, become more columnar further down the ductal tree and are
joined by other specialized cell types that are capable of secreting mucus and
certain peptides such as trifoil peptides. Numerous unmyelinated nerve fibers are
CNS
Cerebral cortex,
Limbic system
Brain stem & Hypothalamus
Vagal
pathway
Splanchnic
pathway
Enteric afferents
& interneurons
Sensors
SECRETION
MOTILITY
BLOOD FLOW
Afferent terminals
Enteric Nervous
System (ENS)
Neurotrasmitters,
Neuropeptides,
Other chemical and
Mechanical stimuli
Fig. 3. Functional organization of enteric nervous system (ENS) with its sensory neurons,
interneurons and effector neurons affecting the motility, secretion and blood flow to the gut and
pancreas (on left) and the control of ENS activity by CNS affecting the effector organs in the gut
and pancreas (on right).
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present in the pancreas and innervate the glandular cells as well as the vessels.
These fibers may belong to cholinergic, adrenergic or peptidergic as well as
sensory neurons and interneurons of the ENS, which transmit the signals from
efferents fibers of external autonomic nerves to ENS or from the ENS in intestinal
mucosa to the glandular cells of the pancreas through short enteropancreatic
reflexes or through long vago-vagal reflexes with intergrative centers in the CNS.
The nerve fibers in the pancreas are distributed through the interlobular
connective tissue, where intrinsic ganglia, representing peripheral integrative
centers are also found.
The endocrine portion of the pancreas is located in the islets of Langerhans
that contain A-, B-, D- and F-cells, releasing, respectively, glucagon, insulin,
somatostatin, PP and many others. The islet hormones first perfuse the
surrounding acinar cells by the insulino-acinar portal system and regulate
digestive enzyme synthesis and transport, as well as the proliferation and growth
of these cells. The disorders of this endocrine portion such as diabetes mellitus,
resulting either from the lack of insulin production because of viral or
autoimmune damage of B-cells (type I) or genetic defect that produces insensitive
insulin receptors throughout the body (type II), greatly affect exocrine portion of
the pancreas but this is beyond this review. The islet hormones reach general
circulation with pancreatic blood flow and some of them (e.g. PP, NPY, CGRP,
somatostatin) may also affect the pancreatic secretion via hypothalamic centers in
CNS, were the receptors for these hormones have been identified (Fig. 4)
Cellular regulation of HCO3– secretion
Studies on pancreatic HCO3- secretion and volume flow in various species
revealed many species-dependent variations in secretion of this component of
secretion by ductal and centroacinar cells that are specifically equipped with
receptors such as for secretin and VIP. They contain high activity of carbonic
anhydrase, an enzyme which is important for their ability to secrete HCO3-. While
human, canine and feline pancreas secretion is small under basal conditions, it
respond with abundant amounts of HCO3- in response to secretin stimulation and
little in response to CCK or vagal stimulation. In rats, which are so often used in
physiological and pharmacological studies, the spontaneous pancreatic secretion
is relatively higher, but secretin, CCK and vagal stimulation result in small
increment of this secretion (8). Pigs, like humans, secrete little under basal
conditions but respond with abundant secretion when secretin, CCK or vagus are
stimulated ( 8). Maximum HCO3- secretion, which in humans or pigs may reach
up to 150 mM/L, is only about 70 mM/L in rats. Whether this variation in HCO3water secretion reflects a variation in composition of primary fluid secretion or
the rate of ductal cell HCO3/Cl– exchange is unknown but surely the findings
obtained from animals may not be relevant to those in humans an should be
interpreted with caution. Secretin and VIP act on receptors to increase the
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Fig. 4. Gastrointestinal hormones neurotransmitters inhibitng pancreatic secretion by acting on the
hypothalamus and brain stem.
concentration of cyclic adenosine monophosphate (cAMP) in ductal and
centroacinar cells (8). The rise in cAMP increases HCO3- secretion by primary
activation of a Cl– channel on the luminal membrane resulting in the increase of
chloride secretion into the lumen of acinus (9). The increased chloride transport
in the lumen is coupled with Cl–/HCO3- antiport resulting in an exchange of Cl–
for HCO3- at the lumanl membrane. This is the reason for the reciprocal
relationship between pancreatic juice HCO3- and Cl– concentrations that could be
explained in part by the difference in water secretion in pancreatic juice. As the
flow rate of secretion increases so does HCO3- concentrations, while Cl–
concentration declines. On baso-lateral membrane of the duct cell are a Na+/H +
antiport, a Na+/K+-ATPase, a H+-ATPase and K+ channels. Following secretin or
VIP stimulation, an apical Cl–/HCO3- antiport is activated resulting in increased
luminal formation of HCO3- that originates from the CO2, produced locally in the
cell as metabolic product, as well as CO2 released in extracellular fluid by the
action of H+ on plasma HCO3-. CO2 diffusing readily into the alkalinized ductal
cells combines with water, the step catalyzed by high activity of carbonic
anhydrase. The continued movement of H+ across the basolateral membrane leads
to build up of HCO3- and the movement of HCO3- across the apical membrane in
exchange for Cl–. As HCO3- is secreted into the duct lumen, Na+ also moves across
the epithelium to preserve electrical neutrality. Na+ moves mostly through the
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intercellular pathway and water then rushes into the lumen along its osmotic
gradient. With enhanced secretin or VIP stimulation of ductal and centroacinar
cells there is continued increase in the HCO3- concentration in pancreatic juice as
well as its pH combined with the fall in Cl– concentration (Fig. 5). It is interest
that CCK and vagal stimulation, though universally believed to promote
predominantly protein enzyme secretion from acinar cells and in most species
their effect on volume flow is negligible, however, in some species like pigs, rats,
guinea pigs and rabbits, often used for pharmacological studies, both CCK and
vagal nerves are potent stimulants of fluid secretion. Furthermore, in pigs and
guinea pigs large number of VIP-ergic neurons are present in the pancreas and
during vagal stimulation, these fibers release large amounts of VIP, which evokes
HCO3- secretion in manner analogous to secretin (8,10-13 ).
The primary goal of pancreatic HCO3- secretion in response to secretin in man
or dogs is the neutralization of gastric acid entering the duodenum. In this
process, HCO3- originating from the pancreatic and biliary duct cells and
duodenal mucosa provide optimal intraduodenal pH for protein, lipid and
carbohydrate digestion by pancreatic and intestinal brush border enzymes and for
the absorption of digestion products. Duodenal pH in the major regulator of secretin
release and the threshold value for secretin release and stimulation of pancreatic
HCO3- is 4.5 (12-14). Below this threshold the amount of secreted HCO3- is related
to the increment in plasma secretin and this increment depends upon the total
B
A
C
R
VIP
Fig. 5. Mechanisms of secretion of HCO3– by centro-acinar and ductal cells (A), The relation
between HCO3– and Cl– secretion as related to the pancreatic volume flow (B) and the acinar fluid
and plasma levels of major electrolytes (C).
300
amounts of titratable acid delivered to the duodenum (14, 15). Chey and his
coworkers (16) obtained an evidence that the release of secretin occurring
postprandially plays a crucial role in the secretion of HCO3- and fluid because
immunoneutralization of secretin by specific antibody actually blocked the increase
in the secretion of fluid and HCO3-. Li at al (17,18) found that concentrated acid
perfusate in duodenum results in the rise of plasma secretin that is mediated by
secretin-releasing peptides (S-RP) present in the proximal intestinal lumen or in the
pancreatic juice (Fig. 6). Partially purified S-RP infused into the duodenum of
anesthetized rats increased pancreatic fluid and HCO3- secretion and raised plasma
secretin level. These secretory effects were blocked by vagotomy, tetradotoxin and
capsaicin, suggesting neutral control of S-RP release by intestinal mucosa (17-20).
This peptide has some homology to pancreatic phospholipase, therefore, it is not
excluded that phospholipase A2 is capable of specific receptor-mediated action on
secretin-releasing cells and this may represent an alternative function of the enzyme
on intestinal endocrine cells (21-23).`
Fig. 6. Schematic presentation of the mechanism of release of secretin by acid in duodenum and
mediation of secretin-releasing peptide (S-RP) in this release (on left) and pancreatic bicarbonate
outputs as related to plasma levels of secretin alone or to combined with acetylcholine or CCK or
to acetylcholine or CCK alone (on right) [modified from Li et al (17, 18)].
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Cellular regulation of enzyme secretion
Pancreatic enzymes are synthesized, packaged, stored and released from acinar
cells by the process of exocytosis which involves multiple subcellular organelles.
Most of the packaging of newly synthesized enzymatic protein occurs in the Golgy
stack. A G protein facilitates the transfer of enzymatic protein from endoplasmic
vesicles into the cisternae. Secretory granules exist in the cytoplasm or migrate to
apical cell membrane to be released into the lumen of acinus (Fig. 7).
Hormones such as CCK and secretin and neuro-transmitters such as Ach or
GRP induced the fusion of zymogen granules with the apical plasma membrane of
acinar cells resulting in the release by exocytosis of digestive enzymes into the
pancreatic duct system. The process of binding of secretagogues to membrane
receptors, ultimately lead to exocytosis includes the receptor-mediated generation
of intracellular messengers and exocytosis (24-27). Receptors for pancreatic
secretagogues are believed to belong to the receptor family characterized
structurally by seven hydrophobic transmembrane domains and functionally by
Fig. 7. Stimuli for acinar cells bind to membrane receptors and initiate intracellular events that
increase enzyme secretion. Schematic presentation of stimulus-secretioin coupling of pancreatic
acinar cell protein secretion.
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their interaction with G-proteins. The CCKA-, muscarinic M3- and GRP-receptors
interact with heterotrimetric G-protein complex leading to the stimulation of
phosphoinositide-specific phospholipase Cβ (PLCβ) activity (Fig. 7).
Phospholipase C activity leads to hydrolysis of phosphatidylinositol-4,5biphosphate (PI-P2) and the formation of inositol 1,4,5 triphosphate (IP3) and 1,2diacylglycerol (DAG). This occurs within seconds in response to high
secretagogue concentrations. IP3 then binds intracellular receptors forming
channels activating the release of Ca2+ from intracellular stores. The DAG
produced by phosholipase C (PLC) activates protein kinase C (PKC). The major
intracellular messengers involved in the regulation of pancreatic secretion are IP3,
Ca2+, DAG and cAMP (8). The first three are predominant in the acinar cells and
increase after the activation of phosphoinositide specific phosphalipase C by CCK+
or Ach while cAMP in the predominant messenger in centro-acinar and duct cells
originating from ATP due to the action of adenylyl cyclase (AC) and activated by
secretin. The central role in stimulus-secretion coupling in acinar cells is played by
Ca2+ that originates both from the intracellular stores followed by a small sustained
increase mediated by entrance of Ca2+ through the plasma membrane channels.
Physiological concentrations of secretagogues induce only transient and repetitive
increase in Ca2+ named Ca2+ oscillations or spikes. The increase in Ca2+ spreads
around the acini with both Ca2+ and IP3 traversing through gap-junctions from one
acinar cell to another (24). Ca2+ activates then kinases and phosphatases and this
activation involves calmodulin (CAM) as a Ca2+ receptor and a catalytic kinase or
phosphatase domain. Ca2+ can also activate protein phosphatase 2B (PP) which is
identical to calcineurin. Secretin and VIP binds with the membrane receptors to
increase intracellular cAMP which in turn stimulates protein kinase A (Fig. 8).
Fig. 8. Functional unit of pancreas consisting of the acinus and draining pancreatic duct. The site of
action of hormones and neuropeptides mediating the secretory response of acinar and ductal cells.
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The pancreas produces several important digestive enzymes that are responsible
for about half of overall digestion of nutrients in the gastrointestinal tract. Alphaamylase digests starch, the most prevalent carbohydrate in the human diet. Lipase
cleaves the ester linkages of dietary fats, resulting in the conversion of triglycerides
to diglycerides and then to 2-monoglycerides and fatty acids that are absorbed from
the intestines. Trypsin, secreted primarily as trypsinogen, a proenzyme, is converted
to its active form, trypsin, that in turn is responsible for conversion of other
proenzymes to active enzymes. Carboxypeptidase is also secreted as a proenzyme,
procarboxypeptidase, which is converted to active enzyme by trypsin to cleave Cterminal linkages of proteins into amino acids that can be absorbed from the gut.
Pancreatic juice also contains a low concentration of trypsin inhibitor, a polypeptide
that at pH 3 to 7 combines with trypsin and inactivates this enzyme in a 1:1 ratio. It
also partially inhibits chymotrypsin. The presence of trypsin inhibitor in the
pancreas is thought to protect the pancreas against autodigestion by small amounts
of active trypsin within the organ. Several studies demonstrated that the relative
synthesis of specific digestive enzymes change as a function of dietary intake. The
mechanism of this adaptation is not understood but for example the amylase gene
expression was reported to be regulated by both insulin and gene (28).
Interdigestive pancreaticl secretion
Pancreatic enzyme secretion occurs continuously in the interdigestive
(fasting) state when upper g.i. tract is cleared of food as well as after meal. The
interdigestive secretion in humans and in other species (carnivore) is cyclic and
follows the pattern of the migrating myoelectric complex (MMC) (Fig. 9). The
patterns recur every 90-100 min with burst of motor activity and pancreatic
enzyme secretion temporally associated with phase II and III of MMC in the
duodenum. The underlying mechanism of this cycling is not clear but since
atropine blocked the responses it is believed that cholinergic activation is
involved and accompanied by the rise in plasma motilin and pancreatic
polypeptide (PP). Motilin by itself is capable of activating premature MMC cycle
and this does not depend upon cholinergic blockade so it may be that motilin
release is not dependent upon the cholinergic activity but its action on the
musculature of the gut requires this enhanced motor activity (29). The role of
fluctuations of pancreatic secretion is not known but it may be related to the
“housekeeping” function of MMC. Fluctuations in basal pancreatic secretion are
maintained in early phase of acute pancreatitis in humans (30). Basal pancreatic
secretion is relatively small and the enzymatic component does not exceed 1015% of maximal response of the organ to exogenous secretagogue.
Posprandial pancreaticl secretion
Posprandially, the pancreatic secretion starts almost immediately after the
meal (Figs. 9 and 10) and it is accompanied by the significant elevation of plasma
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Fig. 9. Interdigestive and postprandial pancreatic trypsin secretion and related to phases of MMC
(34). The fluctuations in pancreatic enzyme secretion under basal conditions probably depend upon
the alterations in the activity of ENS and in the release of motilin (11).
Pancreatic secretion
Fig. 10. Pancreatic protein and bicarbonate secretion before and after meat meal in dogs as
compared to maximal CCK-induced protein secretion and maximal secretin-induced bicarbonate
secretion in these animals (unpublished data).
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concentrations of stimulatory hormones such as gastrin, CCK, secretin and
slightly GRP (Fig. 11). Concurrently, there is an increase in plasma levels of
pancreatic inhibitory hormones such as PP, leptin and decrease in plasma ghrelin.
The postprandial stimulation of pancreatic secretion includes three
overlapping phases; cephalic, gastric and intestinal contributing, respectively, to
about 20%, 10% and 70% of total postprandial response (19). In the past the
postprandial phases have been entirely attributed to vagal stimulation with the
efferent nerves releasing Ach in the pancreas and to cholinergically released
gastrin that stimulate the acinar cells together with cholecystokinin (CCK) and
secretin released from duodeno-jejunal mucosa by products of protein and fat
digestion and gastric acid entering the duodeum (14, 15).
Cephalic phase, that can be induced in dogs by sham-feeding and in humans
by modified sham-feeding, is characterized by a prompt rise in pancreatic enzyme
protein secretion and small increase in HCO3- secretion (Fig. 12). It is
accompanied by the elevation of plasma concentrations of stimulatory hormones
such as gastrin and CCK and slight rise in plasma secretin. Among the inhibitory
Stimulating hormones
A
Inhibiting hormones
B
Ghrelin
Fig. 11. Major gut stimulatory hormones (CCK, secretin, gastrin and GRP) and inhibitory hormones
(PP, PYY, ghrelin and leptin) involved in the control of pancreatic secretion (unpublished data).
Inhibitory hormones
Stimulatory hormones
Fig. 12. Cephalic phase of gastric and pancreatic secretion in dogs with esophageal, gastric and pancreatic fistulas (on left) and plasma levels of
pancreatic stimulatory (gastrin, CCK and secretin) and inhibitory hormones (PP, leptin and ghrelin) (on right) (unpublished data).
Pancreatic secretion
Gastric secretion
306
307
hormones, both PP and leptin tend to increase, while ghrelin after short rise
declines (21). Similar changes in pancreatic secretion can be induced by insulin
hypoglycemia or glycocytopenia induced by 2-deoxy-D-glucose (2-DG) (Fig.
13). Since the rise in pancreatic secretion can be significantly reduced using either
atropine to clock muscarinic receptors or S-0509, a specific CCKB-receptors, it is
reasobnaly to conclude that the major stimulants of the cephalic phase induced by
sham-feeding, insulin or 2-DG include vagal-cholinergic stimulation and vagally
released gastrin and possibly GRP, acting directly on the acinar cells to stimulate
enzyme secretion.
The major part of the postprandial pancreatic secretion is usually attributed to
intestinal phase during which the major role has been attributed to CCK and
secretin, released from the endocrine I and S cells by protein and fat digestion
products and by gastric acid entering the duodenum. The question remains
whether CCK, the most importent stimulant of pancreatic enzyme secretion, acts
through the endocrine pathway following the release into the circulation and
stimulation of acinar cells via CCK2-receptors or through the neurocrine pathway
Fig. 13. Cephalic phase of gastric and pancreatic secretion in dogs with esophageal, gastric and
pancreatic fistulas in response to sham-feeding, insulin or 2-DG alone and after administration of
atropine (25 µg/kg sc), L-364,718 (CCK1-receptor blocker) or S-0509 (CCK2-receptor blocker)
(unpublished data). Schematic presentation of the mechanisms responsible for gastric and
pancreatic secretion in response to cephalic stimulation.
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via neural reflexes (Fig. 14). Classic concept of postprandial pancreatic secretion
that CCK acts primarily via endocrine pathway was changed with the discovery
that efferent vagal nerves to the pancreas release not only Ach but also other
neurotransmitters such as GRP and VIP (31). Moreover, CCK together with
secretin were found to play a major role in the postprandial pancreatic secretion
(32). Studies on rats with the diversion of pancreatic juice from the intestine
showed an increased pancreatic secretion (33, 34), the release of CCK appears to
be controlled by at least two CCK-releasing factors, one monitor peptide secreted
by acinar cells and another CCK-releasing peptide produced by enterocytes.
These releasing peptides are inactive in the interdigestive period because of their
digestion by free luminal trypsin. Both peptides appear to remain under neuronal
(cholinergic) and neurocrine (somatostatin) control. After meal, when trypsin is
bound by food products, the releasing peptides are secreted in large amounts
causing excessive release of CCK that may act via stimulation of afferent nerves
and via endocrine pathway on acinar cells. Actually, at present, not only the
pancreatic protein secretion but also other effects of CCK such as relaxation of
proximal stomach combined with increased antral and pyloric contraction,
increased duodenal motility, gall bladder contraction and sphincter Oddi
Fig. 14. The question remains whether CCK released from the I-cells is acting through the
endocrine or neurocrine pathway.
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relaxation are mediated by vago-vagal reflexes activated by stimulation of by
CCK of the CCKA-receptors at sensory nerve terminals in the digestive system.
The major change in the mechanism of pancreatic secretion concerns the
action of CCK on exocrine pancreas through vago-vagal reflexes (32-36) (Fig.
15). The evidence for such neuronal rather than endocrine pathway of the action
of CCK on the pancreas stems from earlier studies in animals such as dogs in
which atropine was found to inhibit the pancreatic secretion induced by
endogenous stimulants of CCK such as leucine or tryptophan, but also by lower,
more physiological doses of CCK (34, 37). Our studies on humans performed in
collaboration with Gabryelewicz showed that pancreatic enzyme secretion
induced by CCK + secretin was inhibited not only by loxiglumide but also by
atropine indicating that neuronal pathway in men does play a role in the action of
postprandially released CCK on pancreatic enzyme secretion (38).
CCK release & action on pancreas
CCK release in peptides
Fig. 15. The release of CCK from duodenal I-cells by products of protein (amino acids) and fat
digestion (fatty acids) (upper panel) and by monitor peptide originating from the pancreas and by
CCK-releasing peptide produced by the enterocytes (lower panel). The physiological effects of
CCK on the pancreas (stimulation of enzyme secretion), the stomach (relaxation of proximal
stomach and inhibition of acid secretion), gall-bladder (contraction) and sphincter of Oddi
(relaxation) (not shown) are mediated by long vago-vagal and short entero-pancreatic reflexes.
310
Fig. 16. Schematic presentation of pancreatic enzyme secretion in response to CCK administered
intraduodenally that is mediated via neurocrine route acting through long vago-vagal and short
entero-pancreatic reflexes. Exogenous CCK in large dose may stimulate pancreatic secretion by
direct action on acinar cell receptors.
Owyang (32) using anesthetized rats demonstrated that atropine and
hexamethonium almost completely abolished the pancreatic protein response to
low but not high supraphysiological doses of CCK. Similar effects were obtained
using local capsaicin deactivation of afferent nerves or transection of afferent
nerves. Thus, there is strong evidence for neuronal rather than endocrine action
of CCK on pancreatic secretion but further studies are needed to clarify the
relative contribution of neuronal and endocrine pathway of the action of CCK on
exocrine pancreas. Zabielski et al (35 ) were first to find that intraintestinally
applied CCK-8 resulted in the stimulation of pancreatic secretion, the effect that
was abolished following blockade of muscarinic and CCK1-receptors, suggesting
that these receptors are involved in the mediation of the stimulatory effect of
luminal CCK on exocrine pancreatic secretion. These results could be
summarized that the major stimulant in the postprandial pancreatic secretion
originates from the action of CCK, that at large pharmacological doses applied
exogenously may stimulate the exocrine pancreas by direct activatiuon of the
acinar cells. However, under physiological conditions CCK acts primarily via
long vago-vagal and short intestino-pancreatic reflexes to release at the terminal
of postsynaptic nerves such neuropeptides as GRP and VIP and neurotransmitters
such as Ach (Fig. 15).
311
Fig. 17. Schematic presentation of the complex neuro-hormonal mechanisms involved in the
pancreatic stimulation (on left) and inhibition (on right). Nitric oxide is one of the factors released
by neurons and CGRP to increase pancreatic blood flow and secretion (left panel); Neurohormonal factors affecting food intake acting through endocrine and neuronal pathways on
hypothalamus to control the release and action of NPY and AgRP responsible for the appetitive
behavior. Leptin and ghrelin acting in opposite direction (“leptin-ghrelin tango”) on satiety and
appetite (right panel).
Recently the role of nitric oxide (NO) and CGRP has been suggested to play
a role in the control of pancreatic secretion and pancreatitis (39-43). NO is
produced in epithelium, endothelium and macrophages of the GI tract and the
pancreas from L-arginine by constitutive NO synthase (cNOS). GI tract is very
active in NO release, therefore attempts have been made to define the possible
role of NO in exocrine pancreatic secretion (Fig. 16). Using cNOS inhibitor, Lnitro-arginine derivative L-NNA, we found that such blockade of NOS in vivo
dogs resulted in the dose-dependent reduction in CCK + secretin stimulated
pancreatic secretion and this was accompanied by the reduction in insulin and PP
secretion, the effects that were reversed by addition of L-arginine. (42) These
results indicate that NO alters the exocrine pancreatic secretion and this alteration
is mediated by changes in pancreatic blood flow.
312
It should be mentioned that L-nitro-arginine (L-NMMA) was also highly
effective in the inhibition of pancreatic enzyme secretion in humans, the effect
that was also reversed by the addition of L-arginine to L-NMMA (43). In rats, we
have an evidence that endogenous NO may be released from afferent nerves by
stimulation with small dose of capsaicin and that such stimulation attenuates the
development of pancreatitis induced by caerulein (44,45). This is supported by
the fact that small dose of capsaicin that stimulates the release of CGRP and NO
or exogenously applied calcitonin-gene related peptide (CGRP) prevents the
damage of pancreatic acini provoked by caerulein overstimulation (45).
With recent discovery of ghrelin (46,47) and leptin (48), that are involved in
appetitive behavior, ghrelin as “hunger hormone” and leptin as “satiety hormone”
(47), the concept of negative interaction between these peptides on food intake so
called “ghrelin-leptin tango” has been proposed (49) to explain their contribution
in the release of hypothalamic neuropeptide Y (NPY) and agouti-related protein
(AgRP), controlling food intake (Fig. 17).
Using the technique of feeding with liquid meal and measuring the amounts
of food ingested by collection of the meal dropping from the large gastric fistula,
we showed that under basal conditions, ghrelin prevails and its plasma level in
fasted animals reaches the peak, while plasma leptin is strongly attenuated,
indicating that ghrelin diminishes the release of leptin from the stomach and
other sources such as adipocytes. Following food intake, there is an immediate
Fig. 18. Changes in food intake and plasma ghrelin after feeding and administration of leptin
without and with specific antibody against leptin in conscious rats with gastric fistula to collect and
measure the amounts of ingested liquid food (unpublished data).
313
Fig. 19. Changes in food intake and plasma leptin after feeding and administration of ghrelin
without and with specific antibody against ghrelin in conscious rats with gastric fistula to collect
and measure the amount of ingested liquid food (unpublished data).
fall in plasma ghrelin, whereas plasma leptin increases during and after feeding
indicating that circulating leptin suppresses ghrelin release and plays a role of
the satiety signal (Figs 18 and 19). Immunonetralization of either peptide leads
to the changes in eating behavior and similar effects can be obtained with
reduction of NPY activity using specific antagonist of NPY. These changes
result also in alterations in exocrine pancreatic secretion, but both ghrelin and
leptin appear to inhibit exocrine pancreatic secretion (50-54). Both ghrelin and
leptin appear to act through normal endocrine pathway to affect the release of
NPY as well as via activation of peripheral, namely gastric receptors and
signaling to the hypothalamic feeding centers via the neural pathway. This
double signaling routes, one endocrine and another neuronal, emphasizes the
importance of the feeding behavior in the homeostasis. Furthermore, both
peptides contribute not only to the control of food intake but also to the
adjustment of gastric and pancreatic secretion to the anticipation and ingestion
of food to provide the optimal conditions for the digestion and absorption of
ingested food (54-55).
In addition to the control of food intake both ghrelin and leptin were shown
to affect gastric mucosal integrity by enhancing the mucosal blood flow and
protection against topical irritants (57). Both hormonal peptides were shown to
reduce pancreatic damage in the course of acute pancreatitis (50-52).
314
CONCLUSIONS
In conclusion, the mechanism of pancreatic secretion under basal conditions
and following physiological stimulus such as meal is very complex. The major
role appears to be played by vagal nerves and CCK but other neuromediators
including Ach, GRP, CGRP, NO are also involved as modulators of this secretion,
partly by affecting pancreatic blood flow. Under physiological conditions
cholinergic afferent pathways rather than pancreatic acinar cells represent the
primary targets on which gut peptides such as CCK affects the postprandial
pancreatic secretion. This supports an old Pavlovian concept that the neural
system is the major regulator of pancreatic enzyme secretion. Pancreatic
bicarbonate secretion appears to be dependent upon the pH of duodenal content
and duodenal acid loads and the mediator of this secretion is mainly secretin
acting by endocrine way on centro-acinar or duct cells of the pancreas. A
feedback system involving the release from the pancreas and the gut of peptides
stimulating the release of CCK from I cells and secretin from S cells has been
postulated to control of pancreatic enzyme and bicarbonate secretion but its
physiological role in the overall regulation of exocrine pancreas, at least in
humans, has not been fully elucidated. Reflex pancreatic stimulation involving
CGRP and NO and induced by capsaicin in the duodenum indicates that
enteropancreatic reflexes are also operating to control exocrine pancreatic
secretion. Feeding behavior is accompanied by the release from gastric mucosa of
two opposing peptides, ghrelin activating the hunger mechanisms and leptin
responsible for the satiety mechanisms. Ghrelin and leptin interact one on another
creating a kind of “tango” to control the hypothalamic release of NPY and AgRP
that are directly responsible for the feeding behavior. These peptides are
implicated in the control of exocrine pancreas to adjust the rate of secretion to
optimal state for the digestion and assimilation of the ingested food.
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R e c e i v e d : July 8, 2003
A c c e p t e d : July 31, 2003
Author’s address: Prof. Dr S.J. Konturek, Department of Physiology Univ. Med. College,
ul. Grzegorzecka 16, 31-531 Krakow, Poland. Tel: +48-12-4211006, fax: 48-12-4211578.
E-mail; [email protected]