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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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Changes in membrane potential and HCO3− fluxes across the apical membrane of guinea pig pancreatic ducts. (A) Experimental conditions used for the measurement of apical HCO3− permeability in microperfused interlobular ducts. In this case, the bath perfusate contained 25 mM HCO3− and was equilibrated with 5% CO2. This ensured that pCO2 was constant throughout the system. All other conditions were the same as in Fig. 1 A. (B) Changes in basolateral membrane potential Vb recorded with a conventional microelectrode when the bath K+ concentration was switched between 1, 5, and 70 mM. Representative of five experiments. (C) Changes in transepithelial potential difference Vt with the tip of the electrode advanced into the duct lumen. Representative of four experiments. (D) Membrane potential–evoked changes in pHi recorded under the same conditions as in B. Representative of five experiments.
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fig5: Changes in membrane potential and HCO3− fluxes across the apical membrane of guinea pig pancreatic ducts. (A) Experimental conditions used for the measurement of apical HCO3− permeability in microperfused interlobular ducts. In this case, the bath perfusate contained 25 mM HCO3− and was equilibrated with 5% CO2. This ensured that pCO2 was constant throughout the system. All other conditions were the same as in Fig. 1 A. (B) Changes in basolateral membrane potential Vb recorded with a conventional microelectrode when the bath K+ concentration was switched between 1, 5, and 70 mM. Representative of five experiments. (C) Changes in transepithelial potential difference Vt with the tip of the electrode advanced into the duct lumen. Representative of four experiments. (D) Membrane potential–evoked changes in pHi recorded under the same conditions as in B. Representative of five experiments.

Mentions: As before, the lumen was perfused with the high-HCO3− solution (24 mM Cl−, 125 mM HCO3−, 5% CO2), and the cells were depolarized or hyperpolarized by manipulation of [K+]B (Fig. 5 A). Fig. 5 B shows a representative trace of the basolateral membrane potential Vb measured with a conventional microelectrode during one of these experiments. The addition of H2DIDS to the bath perfusate caused a significant depolarization, most probably due to the inhibition of HCO3− uptake via the basolateral Na+-HCO3− cotransporter. Thereafter, Vb changed rapidly and reproducibly as [K+]B was stepped through a sequence of changes between 70, 5, and 1 mM (Fig. 5 B). The mean values of Vb measured at these three K+ concentrations were −38.5 ± 2.7 (n = 8), −49.4 ± 1.7 (n = 11), and −58.7 ± 2.7 mV (n = 6), respectively.


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)

Changes in membrane potential and HCO3− fluxes across the apical membrane of guinea pig pancreatic ducts. (A) Experimental conditions used for the measurement of apical HCO3− permeability in microperfused interlobular ducts. In this case, the bath perfusate contained 25 mM HCO3− and was equilibrated with 5% CO2. This ensured that pCO2 was constant throughout the system. All other conditions were the same as in Fig. 1 A. (B) Changes in basolateral membrane potential Vb recorded with a conventional microelectrode when the bath K+ concentration was switched between 1, 5, and 70 mM. Representative of five experiments. (C) Changes in transepithelial potential difference Vt with the tip of the electrode advanced into the duct lumen. Representative of four experiments. (D) Membrane potential–evoked changes in pHi recorded under the same conditions as in B. Representative of five experiments.
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fig5: Changes in membrane potential and HCO3− fluxes across the apical membrane of guinea pig pancreatic ducts. (A) Experimental conditions used for the measurement of apical HCO3− permeability in microperfused interlobular ducts. In this case, the bath perfusate contained 25 mM HCO3− and was equilibrated with 5% CO2. This ensured that pCO2 was constant throughout the system. All other conditions were the same as in Fig. 1 A. (B) Changes in basolateral membrane potential Vb recorded with a conventional microelectrode when the bath K+ concentration was switched between 1, 5, and 70 mM. Representative of five experiments. (C) Changes in transepithelial potential difference Vt with the tip of the electrode advanced into the duct lumen. Representative of four experiments. (D) Membrane potential–evoked changes in pHi recorded under the same conditions as in B. Representative of five experiments.
Mentions: As before, the lumen was perfused with the high-HCO3− solution (24 mM Cl−, 125 mM HCO3−, 5% CO2), and the cells were depolarized or hyperpolarized by manipulation of [K+]B (Fig. 5 A). Fig. 5 B shows a representative trace of the basolateral membrane potential Vb measured with a conventional microelectrode during one of these experiments. The addition of H2DIDS to the bath perfusate caused a significant depolarization, most probably due to the inhibition of HCO3− uptake via the basolateral Na+-HCO3− cotransporter. Thereafter, Vb changed rapidly and reproducibly as [K+]B was stepped through a sequence of changes between 70, 5, and 1 mM (Fig. 5 B). The mean values of Vb measured at these three K+ concentrations were −38.5 ± 2.7 (n = 8), −49.4 ± 1.7 (n = 11), and −58.7 ± 2.7 mV (n = 6), respectively.

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