1 Department of Pediatrics, Our objective in
this study was to determine the effect of changes in luminal and
cytoplasmic pH on cystic fibrosis transmembrane regulator (CFTR)
Cl
sweat duct; phosphatase; kinase; chloride transport; chloride
conductance; fluid transport; electrolyte transport; sweat gland; cystic fibrosis
A PRIME PHYSIOLOGICAL function of the human sweat duct
is to reabsorb NaCl from the isotonic primary sweat secreted by the sweat gland secretory coil. As the primary sweat secreted by the secretory coil enters the lumen of the absorptive duct,
Na+ is reabsorbed down a
persistent electrochemical gradient via an amiloride-sensitive
Na+ channel in the apical membrane
(17, 18, 22). At least part of the control of absorption of NaCl from
the duct is exerted through regulation of cystic fibrosis transmembrane
conductance regulator (CFTR), the
Cl Recently, we demonstrated that the electrochemical driving force for
Cl Because the luminal pH (4.5-7.8) and salt concentration
[~10-100 mM (3)] decrease with decreasing sweat
secretory rates, one signal that might link changing luminal salt
concentration to CFTR
GCl activation
could be a change in extra- and/or intracellular pH. Acidic pH
was also shown to affect CFTR channel function in ex vivo model systems
(27). Therefore, we hypothesized that a change in luminal
and/or cytosolic pH after reduced luminal Cl The topic assumes added significance, since CFTR
Cl The basolaterally Tissue Acquisition
ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
References
conductance
(GCl). We
monitored CFTR
GCl in the apical
membranes of sweat ducts as reflected by
Cl
diffusion potentials
(VCl) and
transepithelial conductance
(GCl). We found
that luminal pH (5.0-8.5) had little effect on the
cAMP/ATP-activated CFTR
GCl, showing that
CFTR GCl is
maintained over a broad range of extracellular pH in which it functions
physiologically. However, we found that phosphorylation activation of
CFTR GCl is
sensitive to intracellular pH. That is, in the presence of cAMP and ATP [adenosine
5'-O-(3-thiotriphosphate)],
CFTR could be phosphorylated at physiological pH (6.8) but not at low
pH (~5.5). On the other hand, basic pH prevented endogenous
phosphatase(s) from dephosphorylating CFTR.After phosphorylation
of CFTR with cAMP and ATP, CFTR
GCl is normally
deactivated within 1 min after cAMP is removed, even in the presence of
5 mM ATP. This deactivation was due to an increase in endogenous
phosphatase activity relative to kinase activity, since it was reversed
by the reapplication of ATP and cAMP. However, increasing cytoplasmic
pH significantly delayed the deactivation of CFTR
GCl in a
dose-dependent manner, indicating inhibition of dephosphorylation. We
conclude that CFTR
GCl may be
regulated via shifts in cytoplasmic pH that mediate reciprocal control
of endogenous kinase and phosphatase activities. Luminal pH probably has little direct effect on these mechanisms. This regulation of CFTR
may be important in shifting electrolyte transport in the duct from
conductive to nonconductive modes.
INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
References
channel through which
Cl
passes during this
process.
(
µ
Cl
) across the apical and
basolateral membranes is a function of luminal salt concentration (18,
22), which can change rapidly from essentially isotonic to <15 mM in
vivo (18, 22). Accordingly, the
µ
Cl
may be favorable for
absorption at high luminal salt concentration (i.e., >50 mM) or
paradoxically unfavorable at low luminal salt concentration (i.e.,
<50 mM). A necessary consequence of such changes in
µ
Cl
across the duct
epithelium is that Cl
be
absorbed by electroconductive
Cl
channels when the salt
concentration is high enough to favor passive
Cl
diffusion across the
apical membrane through CFTR
Cl
channels and by other
transporter(s) or mechanisms when the salt concentration is low and the
µ Cl
is unfavorable
for passive Cl
diffusion
into the cell (22). Because intracellular
Cl
activity is above the
level required for passive distribution across the cell membranes at
physiologically low luminal salt concentration (22), the
electroconductive Cl
shunt
through CFTR must close to prevent secretion of
Cl
into the lumen
(22-24). However, little is known about the physiological processes that couple luminal salt concentration to
activation/deactivation of CFTR
Cl
conductance
(GCl).
concentration would
deactivate CFTR Cl
channels, facilitating another nonconductive process for
Cl
absorption.
channels are expressed
in other membranes that also may experience pH environments ranging
from the extremely acidic compartments of intracellular organelles such
as endosomes (1, 15, 29) to the highly basic lumen of pancreatic ducts
(10, 26). In addition, sweat duct cells, which are extremely rich in
CFTR (17, 21), seem to present a unique opportunity to investigate pH effects, since large fluctuations in pH occur in the luminal and cytosolic compartments, depending on the status of physiological stimulation (7). The effect of these in vivo pH environments on the
functional properties of CFTR in a native epithelial membrane is
unknown. Therefore, we sought to 1)
determine possible physiological mechanisms for deactivating the
Cl
shunt in the sweat duct
and 2) better understand the effect
of such extreme pH changes on CFTR
GCl.
-toxin-permeabilized sweat duct preparation offers
a good opportunity to study pH regulation of apical CFTR
GCl, because
1) the apical membrane is rich in
CFTR, which comprises most, if not all,
GCl, such that
changes in membrane GCl can be
directly attributed to the activity of CFTR
GCl, and 2) the cytoplasmic pH can be
directly and freely manipulated through the
-toxin pores in the
basolateral membrane. We show that, in the native sweat duct, luminal
pH changes seem not to directly affect CFTR
GCl but
cytoplasmic pH changes may modulate CFTR
GCl through a
reciprocal control of relative kinase phosphorylation and phosphatase
dephosphorylation of CFTR.
METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
References
Selective Permeabilization of the Basolateral Membrane
The basolateral membrane of the sweat duct was selectively permeabilized with the pore-forming agentIntracellular pH
Qualitative changes in intracellular pH in response to luminal ClElectrical Measurements
Electrical setup. After the lumen of the sweat duct was cannulated with a double-lumen cannula made from theta glass (1.5 mm diameter, Clark Electromedical Instruments, Reading, UK), a constant-current pulse of 50-100 nA for a duration of 0.5 s was applied through one barrel of the cannulating pipette containing NaCl-Ringer solution. The other barrel of the cannulating pipette served as an electrode for measuring transepithelial potential (Vt) with respect to the contraluminal bath and as a cannula for perfusing the lumen of the duct with selected solutions. Vt was monitored continuously using one channel of a dual electrometer (model WPI-700) referenced to the contraluminal bath. Transepithelial conductance (Gt) was measured as described earlier (17, 21, 23) from the amplitude of transepithelial voltage deflections in response to 50- to 100-nA transepithelial constant-current pulses using a cable equation.
Apical GCl.
Cl diffusion potential
(VCl) and
GCl were
monitored to indicate the level of activation of
GCl. After
-toxin permeabilization of the basolateral membrane, the epithelium
is simplified to a single (apical) membrane with parallel
Na+ and
Cl
conductances (19, 20,
23). Application of amiloride further limited the system to that of a
predominantly Cl
-selective
membrane. The composition of Ringer solution in bath and lumen was
designed to create single ion permeation of the membrane for
Cl
[140 mM potassium
gluconate (bath)/150 mM NaCl + 10
5 M amiloride
(lumen)]. Under these conditions the
Vt and
Gt can be
regarded essentially as
VCl and
GCl,
respectively.
Solutions
The luminal perfusion Ringer solutions contained (in mM) 150 NaCl, 5 K, 3.5 PO4, 1.2 MgSO4, 1.0 Ca2+, and 0.01 amiloride (pH 7.4). The cytoplasmic/bath solution contained (in mM) 145 K, 140 gluconate, 3.5 PO4, and 1.2 MgSO4 and 260 µM Ca2+ buffered with 2.0 mM EGTA (Sigma Chemical) to 80 nM free Ca2+ (pH 6.8). The effect of luminal or cytoplasmic pH on CFTR GCl was evaluated by directly manipulating bath (cytoplasmic) or luminal pH from 4.5 to 9.0 under activated (0.1 mM cAMP/5 mM ATP) and deactivated conditions (with ATP but without cAMP, with cAMP but without ATP, or without both ATP and cAMP).Data Analysis
Values are means ± SE (where n is the number of ducts from ![]() |
RESULTS |
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Effect of Luminal pH on CFTR GCl
We investigated the effect of changing luminal pH on the apical CFTR GCl. While maintaining constant pH in the cytoplasmic bath, changing luminal pH between 5.0 to 8.5 had virtually no effect on activated (0.1 mM cAMP and 5 mM ATP) or nonactivated CFTR GCl (Fig. 1).
|
Effect of Luminal Cl
Concentration on Cytoplasmic pH
|
Effect of Cytoplasmic pH on CFTR GCl
Changes in cytoplasmic pH had little effect on nonactivated CFTR (i.e., in the absence of cAMP and ATP). However, changes in cytoplasmic pH had significant effects on cAMP/ATP-activated CFTR GCl. The dose-response relationship of the magnitude of activated CFTR GCl vs. cytosolic pH clearly showed that the CFTR GCl activity is a function of cytosolic pH. We found that acidic pH inhibited, while higher pH enhanced, CFTR GCl (Figs. 3 and 4). Although we used a wide range of pH to maximize the effect, CFTR GCl activity is clearly sensitive to cytosolic pH changes within a narrower, more physiological range (Fig. 4). An earlier study (7) revealed that, unlike most cell types, the cytosolic pH of sweat duct cells seems to fluctuate over a wide range (~1 pH unit) depending on the status of stimulation. Therefore, although the experimental range of intracellular pH changes imposed on the tissue in these studies was large, it may not be aphysiological.
|
|
Effect of Intracellular pH on Phosphorylated CFTR
Activation of CFTR GCl in the sweat duct requires protein kinase A (PKA) phosphorylation and a physiological concentration of ATP (19, 23). However, once CFTR is irreversibly phosphorylated, ATP alone can activate CFTR GCl. Therefore, activation of CFTR GCl by ATP without cAMP is a clear indication that CFTR has been irreversibly phosphorylated. CFTR can be irreversibly phosphorylated using different pharmacological tools (2, 23, 24). In these experiments we achieved irreversible phosphorylation of CFTR by 1) using phosphatase-resistant adenosine 5'-O-(3-thiotriphosphate) (ATP
|
|
Effect of Lowering Cytoplasmic pH on Phosphorylation/Dephosphorylation of CFTR
We investigated the effect of lowering cytoplasmic pH on endogenous PKA phosphorylation and endogenous phosphatase dephosphorylation of CFTR.Effect of Lowering pH on PKA Phosphorylation
To test whether PKA phosphorylation of CFTR was affected by acidic pH, we incubated the apical membranes in a phosphatase-inhibiting cocktail containing 0.01 mM cAMP and 5 mM ATP
|
|
|
Effect of Elevating Cytoplasmic pH on CFTR
Alkalinizing cytoplasmic pH resulted in a pH-dependent increase in the magnitude of the cAMP/ATP-activated CFTR GCl revealed as significantly larger increases in VCl and GCl (Fig. 10). The following experiments were designed to test the effect of high pH on phosphorylation/dephosphorylation of CFTR.
|
Role of Dephosphorylation in Activating CFTR GCl by Basic Intracellular pH
Progressively elevating cytosolic pH not only increased the magnitude of activation of CFTR GCl, but results also suggested that the deactivation of CFTR GCl after cAMP washout became progressively slower at higher pH values (Fig. 10). Prolonged exposure of CFTR to cytoplasmic alkaline pH (~10 min at pH >7.5) resulted in an irreversible phosphorylation of CFTR, because CFTR GCl did not fall as long as ATP was present. In fact, phosphorylation of CFTR was so stable that it could be repeatedly activated and deactivated by addition and deletion of ATP alone, even after pH is lowered to 6.8 (Figs. 5 and 6).Parenthetically, we also found that the magnitude of cAMP/ATP-activated CFTR GCl responses at pH 6.8 was spontaneously very low in some ducts (VCl ~15 mV compared with that of most preparations, which show a change of about +50 mV; Fig. 11). Previously, we discarded such ducts, assuming that they were damaged and physiologically not viable. However, we now find that increasing the cytoplasmic pH (>7.5 pH) of what we interpret as undamaged ducts often activates CFTR GCl to normal levels (Fig. 11).
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DISCUSSION |
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CFTR GCl Is Insensitive to Changes in Luminal pH
In the sweat duct, where CFTR is richly expressed, the luminal pH undergoes extreme changes from alkaline (pH 7.8) to highly acidic values (pH <5.0), depending on the sweat secretory rates (3). Therefore, we sought to determine the role of luminal pH in regulating CFTR GCl. We tested the effect of lowering luminal pH from 8.5 to 5.0 on CFTR GCl. We were surprised that such extreme changes in luminal pH had little effect on the magnitude of cAMP- and ATP-activated CFTR GCl (Fig. 1). These results are in contrast to previous reports that extracytosolic acidic pH significantly reduced the open probability of CFTR ClIt has been reported that CFTR
Cl channels in planar lipid
bilayers are significantly affected by pH changes in the cytoplasmic side (27). Cytosolic pH is also known to regulate a variety of
epithelial transport mechanisms, including
Na+-K+
pump activity (9, 12, 28), basolateral
K+ conductance (4, 8, 13, 28), and
other epithelial Cl
channels (6). Therefore, we asked whether changes in intracellular pH
could in fact be a factor that determines when CFTR
GCl should open
and close as a function of luminal
Cl
concentration.
To be relevant, we expect that 1)
cytosolic pH should be a function of luminal
Cl concentration and
2) such a change in cytosolic pH
activates and deactivates CFTR
GCl when luminal
Cl
concentration changes
(22). Preliminary studies support this possibility, in that cytosolic
pH is coupled to luminal Cl
concentration. As shown in Fig. 2, a decrease in luminal
Cl
concentration
significantly and reversibly decreased the intracellular pH. Even
though we did not quantify the magnitude of these changes in
intracellular pH, the change is qualitatively consistent with acidifying and deactivating CFTR
GCl when luminal
Cl
concentration falls, as
found ex vivo (27). Therefore, we sought to determine whether and how
cytosolic pH might regulate endogenous CFTR
GCl in vivo.
Cytosolic pH Modulates CFTR GCl
We determined that cytosolic pH has little effect on deactivated CFTR (no cAMP/ATP). However, intracellular pH had a significant effect on CFTR GCl activated by cAMP/ATP. Lowering intracellular pH inhibited, while elevating intracellular pH enhanced, CFTR GCl (Figs. 3 and 4). These observations suggested that cytosolic pH might not be directly affecting, but rather might be modulating, previously activated CFTR GCl. Because CFTR GCl is regulated by ATP and phosphorylation/dephosphorylation (2, 11, 13, 19, 23, 24), we sought to determine which of these components of activation is specifically affected by changes in cytosolic pH.Does Intracellular pH Affect ATP Regulation of CFTR GCl?
To test whether ATP regulation of CFTR GCl is dependent on intracellular pH, we irreversibly phosphorylated CFTR, as described in METHODS (Figs. 5-9; see below). We found that, even after irreversible phosphorylation, ATP regulation of CFTR GCl was dependent on intracellular pH (Fig. 6). These results suggested that at least some of the effects of intracellular pH on CFTR GCl may be independent of phosphorylation/dephosphorylation events and involve an effect on nucleotide interaction with CFTR molecule. However, we do not know whether such regulation is caused by a direct effect of pH on CFTR molecular conformation and/or the protonation status of ATP.Reciprocal Regulation of CFTR Phosphorylation/Dephosphorylation by Intracellular pH
We previously showed that pharmacological inhibition of endogenous phosphatases did not enhance cAMP-activated baseline CFTR GCl in sweat duct (24). These results suggested that the phosphatase that is responsible for dephosphorylating CFTR may be subject to coordinated inhibition synchronous with the activation of PKA to optimize CFTR phosphorylation (24). The following results show that intracellular pH can exert a reciprocal effect on the activities of endogenous kinases and phosphatases that regulate CFTR GCl.Intracellular pH-Dependent PKA Phosphorylation
We found that a basic cytosolic pH was favorable, whereas an acidic pH was inhibitory, for PKA phosphorylation of CFTR. We know that CFTR can be irreversibly phosphorylated in the presence of ATPIntracellular pH-Dependent Phosphatase Dephosphorylation
Previously, we showed that the rate of deactivation of CFTR GCl after cAMP washout is a function of endogenous phosphatase dephosphorylation (24). Inhibition of endogenous phosphatases by fluoride and okadaic acid markedly delayed or completely abolished the spontaneous deactivation of CFTR after cAMP washout (24). We found that increasing intracellular pH can also abolish the spontaneous deactivation of CFTR GCl that normally follows cAMP washout (Fig. 10). These results indicated that the endogenous phosphatase responsible for dephosphorylation deactivation of CFTR GCl is more active at acidic pH and inhibited at alkaline pH (Fig. 10). Furthermore, incubating permeabilized ducts in Ringer solution at pHPhysiological Significance of pH Regulation of CFTR
Recently, we showed that the transapical electrochemical gradient is unfavorable for passive ClThis system begs the question, How could a reduction in the luminal
Cl concentration trigger
deactivation of CFTR
GCl? Early data
suggested that intracellular pH may follow changes in luminal
Cl
concentration
(Fig. 2). High luminal
Cl
concentration raises
cytoplasmic pH, resulting in an increase in PKA phosphorylation with a
concomitant decrease in the phosphatase dephosphorylation of CFTR.
These reciprocal effects should result in increased phosphorylation
(activation) of CFTR, which maximally conducts
Cl
down an electrochemical
gradient (22). On the other hand, low luminal
Cl
concentration decreases
cytosolic pH, which supports an increase in phosphatase
dephosphorylation with a concomitant decrease in PKA phosphorylation of
CFTR to close
GCl. Deactivation
of CFTR GCl at
low luminal Cl
concentration should facilitate carrier transport of
Cl
against the electrical
gradient as described earlier (18, 22). We recognize that acidifying
the lumen with a proton pump to drive the anion exchange is not
expected to acidify the cytoplasm, but we also are aware that low
cytoplasmic concentrations are expected to acidify the cell by virtue
of anion exchanger in the basal membrane. A coordinated interaction
between luminal Cl
,
cytosolic pH, PKA, endogenous phosphatase, and CFTR will be essential
for efficient reabsorption of salt from the primary sweat.
Conclusions
We conclude that cytosolic pH could be a factor regulating CFTR GCl activity by exerting reciprocal control over phosphorylation/dephosphorylation of CFTR. Acidic pH inhibits PKA phosphorylation and activates phosphatase dephosphorylation. In contrast, basic pH activates PKA phosphorylation and inhibits phosphatase dephosphorylation of CFTR. Inhibiting CFTR GCl at acidic pH may prevent backleak of Cl ![]() |
ACKNOWLEDGEMENTS |
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We are grateful to Kirk Taylor for expert technical assistance and the numerous subjects who volunteered for skin biopsy.
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FOOTNOTES |
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This study was supported by grants from Cystic Fibrosis Research and the National Cystic Fibrosis Foundation.
Address for reprint requests: P. M. Quinton, UCSD School of Medicine,
Dept. of Pediatrics0831, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0831.
Received 17 November 1997; accepted in final form 14 July 1998.
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REFERENCES |
---|
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---|
1.
Al-Alwqati, Q.,
J. Barasch,
and
D. Landry.
Chloride channels of intracellular organelles and their potential role in cystic fibrosis.
J. Exp. Biol.
173:
245-266,
1992.
2.
Anderson, M. P.,
H. A. Berger,
D. P. Rich,
R. J. Gregory,
A. E. Smith,
and
M. J. Welsh.
Nucleoside triphosphates are required to open the CFTR chloride channel.
Cell
67:
775-784,
1991[Medline].
3.
Bijman, J.,
and
P. M. Quinton.
Lactate and bicarbonate transport in the sweat duct of cystic fibrosis and normal subjects.
Pediatr. Res.
21:
79-82,
1987[Abstract].
4.
Cook, C. U.,
M. Ikeuchi,
and
W. Y. Fujimoto.
Lowering of pHi inhibits Ca2+ activated K+ channels in pancreatic -cells.
Nature
311:
269-271,
1984[Medline].
5.
Cox, S.,
and
S. S. Tayler.
Kinetic analysis of cAMP dependent protein kinase: mutations at histidine 87 affect peptide binding and pH dependence.
Biochemistry
34:
16203-16209,
1995[Medline].
6.
Cuppoletti, J.,
A. M. Baker,
and
D. H. Malinowska.
Cl channels of gastric parietal cells that are active at low pH.
Am. J. Physiol.
264 (Cell Physiol. 33):
C1609-C1618,
1993
7.
Dearborn, D. G.,
R. L. Waller,
and
I. Yike.
The human sweat duct: cytosolic pH changes in response to autonomic agents.
In: Cellular and Molecular Basis of Cystic Fibrosis, edited by G. Mastella,
and P. M. Quinton. San Francisco, CA: San Francisco Press, 1988, p. 141-149.
8.
Duffey, M. E.,
and
D. C. Devor.
Intracellular pH and membrane potassium conductance in rabbit distal colon.
Am. J. Physiol.
258 (Cell Physiol. 27):
C336-C343,
1990
9.
Eaton, M. E.,
K. L. Hamilton,
and
K. E. Jones.
Intracellular acidosis blocks the basolateral Na-K pump in the rabbit urinary bladder.
Am. J. Physiol.
246 (Renal Fluid Electrolyte Physiol. 15):
F946-F954,
1984
10.
Gray, M. A.,
A. Harris,
L. Coleman,
J. R. Greenwell,
and
B. E. Argent.
Two types of chloride channels on duct cells cultured from human fetal pancreas.
Am. J. Physiol.
257 (Cell Physiol. 26):
C240-C251,
1989
11.
Gunderson, K. L.,
and
R. R. Kopito.
Effects of pyrophosphate and nucleotide analogs suggest a role for ATP hydrolysis in cystic fibrosis transmembrane regulator channel gating.
J. Biol. Chem.
269:
19349-19353,
1994
12.
Harvey, B. J.,
S. R. Thomas,
and
J. Ehrenfeld.
Intracellular pH controls cell membrane conductances and ion transport in frog skin epithelium.
J. Gen. Physiol.
92:
767-791,
1988[Abstract].
13.
Hwang, T.-C.,
G. Nagel,
A. C. Nairn,
and
D. C. Gadsby.
Regulation of CFTR Cl channels by phosphorylation and ATP hydrolysis.
Proc. Natl. Acad. Sci. USA
91:
4698-4702,
1994[Abstract].
14.
Keller, S. K.,
T. J. Jentsch,
M. Koch,
and
M. Widerholt.
Interactions of pH and potassium conductances in cultured bovine retinal pigment epithelial cells.
Am. J. Physiol.
250 (Cell Physiol. 19):
C124-C137,
1986
15.
Lukacs, G. L.,
X.-B. Chang,
N. Kartner,
O. D. Rotstein,
J. R. Riordan,
and
S. Grinstein.
The cystic fibrosis transmembrane regulator is present and functional in endosomes.
J. Biol. Chem.
267:
14568-14572,
1992
16.
Negulescu, P. A.,
and
T. E. Machen.
Intracellular ion activities and membrane transport in parietal cells measured with fluorescent dyes.
Methods Enzymol.
192:
38-81,
1990[Medline].
17.
Quinton, P. M.
Missing Cl conductance in cystic fibrosis.
Am. J. Physiol.
251 (Cell Physiol. 20):
C649-C652,
1986
18.
Quinton, P. M.,
and
M. M. Reddy.
Cl conductance and acid secretion in the human sweat duct.
Ann. NY Acad. Sci.
574:
438-446,
1989[Medline].
19.
Quinton, P. M.,
and
M. M. Reddy.
Control of CFTR chloride conductance by ATP levels through non-hydrolytic binding.
Nature
360:
79-81,
1992[Medline].
20.
Reddy, M. M.,
and
P. M. Quinton.
Intracellular potassium activity and the role of potassium in transepithelial salt transport in human reabsorptive sweat duct.
J. Membr. Biol.
119:
199-210,
1991[Medline].
21.
Reddy, M. M.,
and
P. M. Quinton.
cAMP activation of CF affected chloride conductance in both membranes of an absorptive epithelium.
J. Membr. Biol.
130:
49-62,
1992[Medline].
22.
Reddy, M. M.,
and
P. M. Quinton.
Intracellular Cl activity: evidence of dual mechanisms of Cl
absorption in human sweat duct.
Am. J. Physiol.
267 (Cell Physiol. 36):
C1136-C1144,
1994
23.
Reddy, M. M.,
and
P. M. Quinton.
Rapid regulation of electrolyte absorption in sweat duct.
J. Membr. Biol.
140:
57-67,
1994[Medline].
24.
Reddy, M. M.,
and
P. M. Quinton.
Deactivation of CFTR Cl conductance by endogenous phosphatases in the native sweat duct.
Am. J. Physiol.
270 (Cell Physiol. 39):
C474-C480,
1996
25.
Reddy, M. M.,
and
P. M. Quinton.
Hydrolytic and nonhydrolytic interactions in the ATP regulation of CFTR Cl conductance.
Am. J. Physiol.
271 (Cell Physiol. 40):
C35-C42,
1996
26.
Schulz, I.,
and
K. J. Ulrich.
Transport processes in the exocrine pancreas.
In: Membrane Transport in Biology, edited by G. Giebisch,
D. C. Tosteson,
and H. H. Ussing. New York: Springer Verlag, 1979, vol. IV, p. 811-852.
27.
Sherry, A. M.,
J. Cuppoletti,
and
D. H. Malinowska.
Differential acidic pH sensitivity of F508 CFTR Cl
channel activity in lipid bilayers.
Am. J. Physiol.
266 (Cell Physiol. 35):
C870-C875,
1994
28.
Stample, P.,
and
B. V. Bogind.
The Ca2+ sensitive K+ conductance of the human red cell membrane is strongly dependent on cellular pH.
Biochim. Biophys. Acta
815:
313-321,
1985[Medline].
29.
Webster, P.,
L. Vanacore,
A. C. Nairn,
and
C. R. Marino.
Subcellular localization of CFTR to endosomes in a ductal epithelium.
Am. J. Physiol.
267 (Cell Physiol. 36):
C340-C348,
1994
30.
Yoon, M. Y.,
and
P. F. Cook.
Chemical mechanism of the adenosine cyclic 3',5'-monophosphate dependent protein kinase from pH studies.
Biochemistry
26:
4118-4125,
1987[Medline].