Regulation of pancreatic excretory function by ion channels

2015

Viktoria Venglovecz

Morphology of the pancreas

Composition of pancreatic juice

• 1–2 liters of pancreatic juice per day

• acini secrete isotonic, plasma-like fluid

• pancreatic duct absorbs most of the Cl- and secrete HCO3

+

• Human, 140 mM HCO3+

• Mouse and rat, 50–70mM HCO3+

• The cation composition of the juice is nearly constant

Fluid and electrolyte secretion by acinar cells

Na+/K+ ATPase

• hydrolyzes ATP to exchange 3 Na+ in for 2 K+ out

• generate the transcellular Na+ and K+ gradients

• provides the electrochemical gradient

• determines the membrane potential.

 

K+ channels

• Ca2+ -activated K+ channel

• BK channels

• IK channels

Na+/K+/2Cl- channels

• NKCC1 is ubiquitous

• activated by cell shrinkage

• restore cell volume

• inhibited by the diuretics furosemide and bumetanide

• mediates 70% of the Cl- uptake

• provide most of the Na+ necessary to fuel the Na+/K+ pump• 

Na+/H+ and Cl-/HCO3+

• NHE1 and AE2

• NHE1 and AE2 are ubiquitous

• housekeepers of cytoplasmic pH

• activated by small changes in intracellular pH

• NHE1 is activated by acidic pHi

• AE2 is activated by alkaline pHi

• NHE1 and AE2 are involved in many cellular functions including mediation of acinar cell fluid and electrolytesecretion

TMEM16A

• apical Ca2+ -activated Cl- channel

AQUAPORINS

• AQP family consists of 13 members

• water channels• glyceroporins

• AQPs functions: cell adhesion, proliferation, migration, and cell survival

• Diseases: Sjögren’s syndrome and pancreatitis

• Deletion of AQP5 disrupted the integrity of the tight junction and reduced the paracellular water permeability

Fluid and electrolyte secretion by acinar cells

• Secretion is fueled by the basolateral Na+/K+ ATPase pump

• NKCC1 and NHE1 are the main routes of Na+ influx

• Ca2+ -activated K+ channels set the membrane potential at 50 to 60 mV

• NKCC1 is the major route of Cl- influx

• NHE1 and AE2 set cytoplasmic pH at 7.2

• Transcellular Na+ flux

Fluid and electrolyte secretion by ductal cells

TRANSPORTERS AT THE BASOLATERAL MEMBRANE

Na+/H+ exchanger 1 (NHE1)

• electroneutral 1Na+/1H+ exchangers

• NHE1 • ubiquitous• pHi homeostasis • localized at the basolateral membrane• provide a portion of HCO3

+ influx

• NHE2 and NHE3• localized at the luminal membrane • salvage HCO3

+

•

TRANSPORTERS AT THE BASOLATERAL MEMBRANE

Na+/HCO3+ exchanger (NBC)

• expressed at the BASOLATERAL membrane

• electroGENIC transporter

• 1 Na+-2 HCO3+ stoichiometry

• mediates the bulk of basolateral HCO3

+ entry

• Acid-base homeostasis, • Regulation of cell volume and intracellular pH

NBC1 NBC3

• expressed at the LUMINAL membrane

• electroNEUTRAL transporter

• 1 Na+-1 HCO3+ stoichiometry

• Salvage HCO3+

TRANSPORTERS AT THE BASOLATERAL MEMBRANE

TRANSPORTERS AT THE BASOLATERAL MEMBRANE

TRANSPORTERS AT THE LUMINAL MEMBRANE

CFTR

MECHANISM OF HCO3+ SECRETION

MECHANISM OF HCO3+ SECRETION

RELATIONSHIP BETWEEN ACINAR AND DUCTAL CELLS

fluid and HCO3-

enzymes

activating enzymes

Physiology PathophysiologySTRESS

AUTODIGESTION

INFLAMMATION

CELL DEATH

DIGESTION

Alcohol 45%Bile 45%Others 10%

Development of Acute Pancreatitis

InducingFactor

- Bile acid- Ethanol-Other factors

IntracinarEvents

-Pathological Ca2+ signal-Colocalization of lysosomal enzymes and zymogens

-Intrapancreatic  trypsinogen activation

-Autodigestion, Inflammation

ImmunResponse

-Leukocyte activation-Cytocines (IL-6, TNFα, etc.)-ROS

ACUTE PANCREATITIS

NO SPECIFICTHERAPY

The Pancreas

Ductal cells Acinar cells

Insufficient electrolyte and fluid secretion

by ductal cells in CF

Destruction of acinar cells

Primary defect in membrane trafficking

of zymogens Correction of the luminal pH reverses the membrane

trafficking defects and largely restores the membrane dynamics

Pancreatic duct obstruction

Freedman SD, Gastroenterology, 2001

Mislocalization of CFTR in AIP

Decreased pancreatic ductal function Reversal by corticosteroid treatment

Ko SB, Gastroenterology, 2010

c

Model of post-ERCP pancreatitis in rats

pH 6.9

pH 7.3

pH 6.0 Pancreatic damage

Noble MD, Gut, 2008

Lower extracellular pH

enhances secretagogue-induced zymogen activation

Bhoomagoud M , Gastroenterology, 2009

Ductal cells Acinar cells

alterations in pancreatic ductal fluid and bicarbonate secretion can increase

patients’ risk to pancreatitis

restoration of pancreatic ductal bicarbonate and fluid secretion may have therapeutic

benefits

Alcohol

Gallstone (bile acid)

Trypsinogen → Trypsin

?

ETIOLOGICAL FACTORS IN ACUTE PANCREATITIS

Venglovecz V et al. Gut 2008;57:1102-12.Ignath I. et al. Pancreas 2009;38:921-9.Venglovecz V, et al. Gut 2011;60:361-9.

Maleth J et al. Gut 2011;60:136-8.

Hegyi P. et al. Gut 2011;60:544-52.Maléth J. et al.unpiblished

The trypsin vicious cycle

Pallagi et al, Gastroenterology 2011 Dec;141(6):2228-2239 

HCO3-

HCO3-

HCO3-

HCO3-

To neutralize the acid content secreted by acinar cells

HCO3-

What are the roles of bicarbonate secretion?

HCO3-

To curtail trypsinogen autoactivation within the pancreatic ductal system

To neutralise the acid chyme entering the duodenum from the stomach

To defend the pancreas by washing out the toxic agents

The main pancreatitis-inducing factors (bile acids and ethanol) are strong inhibitors of pancreatic ductal bicarbonate secretion