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CFTR functions as a bicarbonate channel in pancreatic duct cells.

Ishiguro H, Steward MC, Naruse S, Ko SB, Goto H, Case RM, Kondo T, Yamamoto A - J. Gen. Physiol. (2009)

Bottom Line: Apical HCO(3)(-) fluxes activated by cyclic AMP were independent of Cl(-) and luminal Na(+), and substantially inhibited by the CFTR blocker, CFTR(inh)-172.From the changes in pH(i), membrane potential, and buffering capacity, the flux and electrochemical gradient of HCO(3)(-) across the apical membrane were determined and used to calculate the HCO(3)(-) permeability.This suggests that CFTR functions as a HCO(3)(-) channel in pancreatic duct cells, and that it provides a significant pathway for HCO(3)(-) transport across the apical membrane.

View Article: PubMed Central - PubMed

Affiliation: Human Nutrition, Nagoya University Graduate School of Medicine, Nagoya 466-8550, Japan. ishiguro@htc.nagoya-u.ac.jp

ABSTRACT
Pancreatic duct epithelium secretes a HCO(3)(-)-rich fluid by a mechanism dependent on cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane. However, the exact role of CFTR remains unclear. One possibility is that the HCO(3)(-) permeability of CFTR provides a pathway for apical HCO(3)(-) efflux during maximal secretion. We have therefore attempted to measure electrodiffusive fluxes of HCO(3)(-) induced by changes in membrane potential across the apical membrane of interlobular ducts isolated from the guinea pig pancreas. This was done by recording the changes in intracellular pH (pH(i)) that occurred in luminally perfused ducts when membrane potential was altered by manipulation of bath K(+) concentration. Apical HCO(3)(-) fluxes activated by cyclic AMP were independent of Cl(-) and luminal Na(+), and substantially inhibited by the CFTR blocker, CFTR(inh)-172. Furthermore, comparable HCO(3)(-) fluxes observed in ducts isolated from wild-type mice were absent in ducts from cystic fibrosis (Delta F) mice. To estimate the HCO(3)(-) permeability of the apical membrane under physiological conditions, guinea pig ducts were luminally perfused with a solution containing 125 mM HCO(3)(-) and 24 mM Cl(-) in the presence of 5% CO(2). From the changes in pH(i), membrane potential, and buffering capacity, the flux and electrochemical gradient of HCO(3)(-) across the apical membrane were determined and used to calculate the HCO(3)(-) permeability. Our estimate of approximately 0.1 microm sec(-1) for the apical HCO(3)(-) permeability of guinea pig duct cells under these conditions is close to the value required to account for observed rates of HCO(3)(-) secretion. This suggests that CFTR functions as a HCO(3)(-) channel in pancreatic duct cells, and that it provides a significant pathway for HCO(3)(-) transport across the apical membrane.

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Cl− and Na+ dependence of HCO3− fluxes across the apical membrane of guinea pig pancreatic ducts. (A) Membrane potential–evoked changes in pHi in the absence of Cl−. To deplete intracellular Cl−, the bath and lumen were perfused with the Cl−-free, HEPES-buffered solution in the presence of dbcAMP for 30 min before the measurements. Experimental conditions were similar to those in Fig. 1 D, but with Cl− replaced by glucuronate in both the bath and luminal perfusates. Representative of four experiments. (B) Membrane potential–evoked changes in pHi in the absence of luminal Na+. Experiments were similar to those shown in Fig. 1, but with Na+ replaced by NMDG in the luminal perfusate. Representative of four experiments.
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fig2: Cl− and Na+ dependence of HCO3− fluxes across the apical membrane of guinea pig pancreatic ducts. (A) Membrane potential–evoked changes in pHi in the absence of Cl−. To deplete intracellular Cl−, the bath and lumen were perfused with the Cl−-free, HEPES-buffered solution in the presence of dbcAMP for 30 min before the measurements. Experimental conditions were similar to those in Fig. 1 D, but with Cl− replaced by glucuronate in both the bath and luminal perfusates. Representative of four experiments. (B) Membrane potential–evoked changes in pHi in the absence of luminal Na+. Experiments were similar to those shown in Fig. 1, but with Na+ replaced by NMDG in the luminal perfusate. Representative of four experiments.

Mentions: Electrodiffusion through an anion channel such as CFTR provides one possible explanation for these results. However, alternative pathways might account for membrane potential–induced HCO3− movements across the apical membrane. These include electrogenic Na+-nHCO3− cotransporters and electrogenic SLC26 transporters that mediate Cl−-nHCO3− exchange, such as SLC26A6 (Ko et al., 2002; Shcheynikov et al., 2006). To evaluate their contribution, we next examined whether the apical fluxes of HCO3− induced by membrane potential changes were dependent on the presence of Cl− (Fig. 2 A) or luminal Na+ (Fig. 2 B).


CFTR functions as a bicarbonate channel in pancreatic duct cells.

Ishiguro H, Steward MC, Naruse S, Ko SB, Goto H, Case RM, Kondo T, Yamamoto A - J. Gen. Physiol. (2009)

Cl− and Na+ dependence of HCO3− fluxes across the apical membrane of guinea pig pancreatic ducts. (A) Membrane potential–evoked changes in pHi in the absence of Cl−. To deplete intracellular Cl−, the bath and lumen were perfused with the Cl−-free, HEPES-buffered solution in the presence of dbcAMP for 30 min before the measurements. Experimental conditions were similar to those in Fig. 1 D, but with Cl− replaced by glucuronate in both the bath and luminal perfusates. Representative of four experiments. (B) Membrane potential–evoked changes in pHi in the absence of luminal Na+. Experiments were similar to those shown in Fig. 1, but with Na+ replaced by NMDG in the luminal perfusate. Representative of four experiments.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC2654087&req=5

fig2: Cl− and Na+ dependence of HCO3− fluxes across the apical membrane of guinea pig pancreatic ducts. (A) Membrane potential–evoked changes in pHi in the absence of Cl−. To deplete intracellular Cl−, the bath and lumen were perfused with the Cl−-free, HEPES-buffered solution in the presence of dbcAMP for 30 min before the measurements. Experimental conditions were similar to those in Fig. 1 D, but with Cl− replaced by glucuronate in both the bath and luminal perfusates. Representative of four experiments. (B) Membrane potential–evoked changes in pHi in the absence of luminal Na+. Experiments were similar to those shown in Fig. 1, but with Na+ replaced by NMDG in the luminal perfusate. Representative of four experiments.
Mentions: Electrodiffusion through an anion channel such as CFTR provides one possible explanation for these results. However, alternative pathways might account for membrane potential–induced HCO3− movements across the apical membrane. These include electrogenic Na+-nHCO3− cotransporters and electrogenic SLC26 transporters that mediate Cl−-nHCO3− exchange, such as SLC26A6 (Ko et al., 2002; Shcheynikov et al., 2006). To evaluate their contribution, we next examined whether the apical fluxes of HCO3− induced by membrane potential changes were dependent on the presence of Cl− (Fig. 2 A) or luminal Na+ (Fig. 2 B).

Bottom Line: Apical HCO(3)(-) fluxes activated by cyclic AMP were independent of Cl(-) and luminal Na(+), and substantially inhibited by the CFTR blocker, CFTR(inh)-172.From the changes in pH(i), membrane potential, and buffering capacity, the flux and electrochemical gradient of HCO(3)(-) across the apical membrane were determined and used to calculate the HCO(3)(-) permeability.This suggests that CFTR functions as a HCO(3)(-) channel in pancreatic duct cells, and that it provides a significant pathway for HCO(3)(-) transport across the apical membrane.

View Article: PubMed Central - PubMed

Affiliation: Human Nutrition, Nagoya University Graduate School of Medicine, Nagoya 466-8550, Japan. ishiguro@htc.nagoya-u.ac.jp

ABSTRACT
Pancreatic duct epithelium secretes a HCO(3)(-)-rich fluid by a mechanism dependent on cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane. However, the exact role of CFTR remains unclear. One possibility is that the HCO(3)(-) permeability of CFTR provides a pathway for apical HCO(3)(-) efflux during maximal secretion. We have therefore attempted to measure electrodiffusive fluxes of HCO(3)(-) induced by changes in membrane potential across the apical membrane of interlobular ducts isolated from the guinea pig pancreas. This was done by recording the changes in intracellular pH (pH(i)) that occurred in luminally perfused ducts when membrane potential was altered by manipulation of bath K(+) concentration. Apical HCO(3)(-) fluxes activated by cyclic AMP were independent of Cl(-) and luminal Na(+), and substantially inhibited by the CFTR blocker, CFTR(inh)-172. Furthermore, comparable HCO(3)(-) fluxes observed in ducts isolated from wild-type mice were absent in ducts from cystic fibrosis (Delta F) mice. To estimate the HCO(3)(-) permeability of the apical membrane under physiological conditions, guinea pig ducts were luminally perfused with a solution containing 125 mM HCO(3)(-) and 24 mM Cl(-) in the presence of 5% CO(2). From the changes in pH(i), membrane potential, and buffering capacity, the flux and electrochemical gradient of HCO(3)(-) across the apical membrane were determined and used to calculate the HCO(3)(-) permeability. Our estimate of approximately 0.1 microm sec(-1) for the apical HCO(3)(-) permeability of guinea pig duct cells under these conditions is close to the value required to account for observed rates of HCO(3)(-) secretion. This suggests that CFTR functions as a HCO(3)(-) channel in pancreatic duct cells, and that it provides a significant pathway for HCO(3)(-) transport across the apical membrane.

Show MeSH
Related in: MedlinePlus