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Departments of Medicine and of Physiology, Tulane University School of Medicine, New Orleans, Louisiana 70112
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The effect of extracellular acidification was tested on the native epithelial Na+ channel (ENaC) in A6 epithelia and on the cloned ENaC expressed in Xenopus oocytes. Channel activity was determined utilizing blocker-induced fluctuation analysis in A6 epithelia and dual electrode voltage clamp in oocytes. In A6 cells, a decrease of extracellular pH (pHo) from 7.4 to 6.4 caused a slow stimulation of the amiloride-sensitive short-circuit current (INa) by 68.4 ± 11% (n = 9) at 60 min. This increase of INa was attributed to an increase of open channel and total channel (NT) densities. Similar changes were observed with pHo 5.4. The effects of pHo were blocked by buffering intracellular Ca2+ with 5 µM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. In oocytes, pHo 6.4 elicited a small transient increase of the slope conductance of the cloned ENaC (11.4 ± 2.2% at 2 min) followed by a decrease to 83.7 ± 11.7% of control at 60 min (n = 6). Thus small decreases of pHo stimulate the native ENaC by increasing NT but do not appreciably affect ENaC expressed in Xenopus oocytes. These effects are distinct from those observed with decreasing intracellular pH with permeant buffers that are known to inhibit ENaC.
epithelial sodium channel; A6 epithelia; Xenopus oocytes; noise analysis; channel density
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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THE NA+ channel in the apical membrane of many native electrically tight Na+-absorbing epithelia is subjected to an environment with a highly dynamic extracellular pH (pHo). It is well established that large decreases (e.g., pH 2) of luminal pH (pHo) decrease Na+ absorption and that this inhibition is mediated via changes of intracellular pH (pHi), leading to inhibition of the apical Na+ channel (6-10, 16, 19, 20, 22, 25). Indeed, Palmer and Frindt (20) found that channel activity and presumably open probability (Po) is inhibited by pHi in excised membrane patches. Zeiske et al. (25) have also recently reported that open channel density (No) of the Na+ channel found in A6 epithelia is inhibited by decreasing pHi.
Leaf et al. (16) have an unexplainable finding that small decreases of pHo (down to 5.5) causes a stimulation, rather than inhibition, of Na+ transport in the toad bladder. This stimulation was observed in the absence of detectable effects on pHi and was clearly distinct from the inhibitory effects observed with intracellular acidification. The mechanism for this stimulation, and whether such observation is applicable to other Na+-absorbing epithelia, remains undetermined.
The regulation of the cloned epithelial Na+ channel (ENaC) by pH has been recently investigated by Chalfant et al. (4). These authors found that decreases of pHi cause a rapid (within minutes) inhibition of ENaC expressed in Xenopus oocytes through effects on channel activity. Similar changes of pHo were without appreciable short-term (<10 min) effects on the channel. Thus it appears that the cloned ENaC has the capability to rapidly (<5 min) respond to changes of pHi and that these effects may represent a direct interaction with H+, because an inhibition of Po was found for ENaC incorporated into planar lipid bilayers. Moreover, channel activity was relatively unaffected by short-term (<10 min) deceases of pHo.
We have recently observed that a small decrease of pHo from 7.4 to 6.4 causes stimulation of the short-circuit current (Isc) in the Xenopus kidney cell line A6. This stimulation was not a direct effect of the pH change in that the increase of Isc was not immediate. Moreover, this effect was similar in its time course to that observed by Leaf et al. (16) in toad bladder. Because the apical Na+ channel (ENaC) is rate limiting to transepithelial transport, this stimulation is likely mediated via effects on the native ENaC. To determine the single channel basis of this stimulation, we utilized blocker-induced transepithelial fluctuation analysis. Similar experiments were also carried out on the cloned ENaC expressed in Xenopus oocytes to determine if prolonged extracellular acidification also stimulates this channel in this system.
We report that stimulation of the Isc observed by small apical acidification in polarized A6 epithelia is due to increases of total channel density (NT). A small decrease of the single channel current (iNa) was also observed and is likely due to apical membrane depolarization. This decrease of iNa slightly underestimated the stimulation of the macroscopic Isc. The changes of NT and Isc were mediated via intracellular Ca2+-dependent mechanisms, since pretreatment of A6 epithelia with an intracellular Ca2+ buffer [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)] prevented this stimulation.
In contrast, decreasing pHo in ENaC-expressing oocytes caused a small transient stimulation of the amiloride-sensitive conductance followed by recovery to below control levels. Thus additional Ca2+-dependent mechanisms may be present in tight epithelia and may be responsible for the sustained stimulation of NT with an acidic luminal environment. We speculate that the increase of NT observed in A6 cells may serve as a protective mechanism whereby an epithelium subjected to large acid loads, which would normally inhibit Na+ transport through changes of pHi, is more capable of resuming its Na+ reabsorption after recovery from this acidic environment.
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MATERIALS AND METHODS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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A6 epithelia.
Cells were obtained from American Type Culture Collection (ATCC,
Manassas, VA). Cells were cultured to confluency in
75-cm2 flasks and were subcultured on permeable polyester
membrane inserts (Transwell Clear; Costar). Cells were trypsinized, and
the equivalent of 1/360th of cells from a single flask were seeded on a
single insert (area 4.7 cm2). Cell polarity was assessed by
their ability to develop a transepithelial potential difference
(Voc) and appreciable transepithelial resistance (RT). With the use of the solutions described
below, monolayers exhibited Voc in the range of
40 to 60 mV and RT in the range of 7-12
k
· cm2 within 10-14 days after plating.
These values were stable for ~2 mo.
Oocyte isolation and injection. Toads were obtained from Xenopus Express (Beverly Hills, FL) and were kept in dechlorinated tap water at 18°C. Conditions for oocyte removal, processing, injection, and cRNA synthesis were as previously described (2). Injected oocytes were incubated at 18°C for 1-3 days until recording. All recordings were performed at 19-21°C.
Solutions and chemicals. All solutions and chemicals were as previously described by Awayda and Subramanyam (3). ND-96 (96 mM NaCl, 1.8 mM CaCl2, 1 mM MgCl2, 2 mM KCl, and 5 mM HEPES) at pH 7.4 was used for initial recording from both oocytes and A6 epithelia. HCl was used to alter pH in ND-96 to pH 6.4 or 5.4. Amiloride was a gift from Merck-Sharp & Dohme (Rahway, NJ). BAPTA-AM was obtained from Molecular Probes (Eugene, OR). All other chemicals were of the highest grade and were obtained from Sigma Chemical (St. Louis, MO).
Dual electrode clamp.
Defoliculated Xenopus oocytes were injected with cRNAs for
rat 
-, 
-, and 
-ENaC (rENaC). Injected oocytes were
cultured as previously described (3). Whole cell currents
were recorded in oocytes held at 0 mV and pulsed from 100 to +40 mV.
Slope conductance (gm) was summarized between
80 and 100 mV (2). By convention, inward flow of
cations is designated as inward current (negative current), and all
voltages are reported with respect to ground or bath. Except where
noted, all data are reported as means ± SE.
Fluctuation analysis. Membranes were placed in a modified Ussing chamber, and the transepithelial voltage was clamped to 0 mV using a low-noise, direct current-powered, four-electrode voltage clamp. Short 2-mV pulses were used to measure the transepithelial resistance.
Noise or fluctuation analysis was carried out as previously described by Helman et al. (13). After Isc was allowed to stabilize, noise analysis was conducted using the uncharged amiloride analog 6-chloro-3,5-diamino-2-pyrazinecarboxamide (CDPC). CDPC was pulsed into the apical side of the Ussing chamber at concentrations of 20 and 80 µM or at 15 and 40 µM. Current noise was filtered at Nyquist frequency (~1,900 Hz). The filtered signal was amplified and stored digitally. Signals were Fourier transformed to yield the power-density spectra. pH changes were made to the apical side of the tissue while the pH of the basolateral side was held constant. All solutions used a HEPES-based buffer and were therefore not expected to affect pHi. When used, BAPTA-AM was added to both sides of the tissue at a final concentration of 5 µM in 0.05% DMSO (tissue culture grade).pHi. These measurements used the pH-sensitive dye 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). BCECF was purchased from Molecular Probes in its membrane-permeable BCECF-AM form. BCECF-AM was dissolved in DMSO on the day of experiments and was used at a final concentration of 10 µM in ND-96. Cells were allowed at least 60 min to uptake the dye.
Fluorescence studies were carried out on an inverted Nikon Diaphot-TMD microscope as previously described (14). Fluorescence ratio was measured using the Nikon/PTI Photoscan II system, with excitation at 490 and 440 nm and emission at 530 nm. Background fluorescence was measured in BCECF-free cells and was <10% of the fluorescence observed in BCECF-loaded cells. All experimental data were corrected for background fluorescence. pHi experiments were also carried out on polarized A6 monolayers grown on the same filters used for the noise analysis experiments to allow us to correlate these two measurements. Cells were studied in special chambers designed to allow pHi measurements in cultured polarized cells and offered separate access to the apical and basolateral compartments in addition to a means of rapid solution exchange in each of these compartments (17). Because the cells were grown on transparent membrane supports, it was expected that the filter material would not obstruct the pHi measurements. However, to circumvent any potential problems, the filter and accompanying cells were inverted in the chamber so that the apical membrane and the cells directly faced the excitation source. Statistical analysis was carried out using paired Student's t-test where appropriate. Significance was determined at the 95% confidence level (P < 0.05).| |
RESULTS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Effects of decreasing pHo on the macroscopic properties
of the native ENaC.
Confluent A6 cells were short circuited according to conventional
methods in symmetrical ND-96 at pH 7.4. In these cells, Isc is essentially all attributed to
Na+ transport through the apical native ENaC and is blocked
by 10 µM amiloride. To noninvasively calculate the single channel
properties, we used blocker-induced fluctuation analysis. This protocol
is similar to that used by Helman et al. (13) and allows
the assessment of the time course of changes of the single channel
parameters. To utilize these methods, cells were continuously perfused
in Ringer solution containing a low blocker concentration (20 or 15 µM CDPC) and were periodically (every 10 min) pulsed for ~3 min
with solution containing a higher blocker concentration (80 or 40 µM
CDPC). This protocol is shown in Fig. 1
along with a continuous recording of the Isc and
the effect of a decrease of apical pHo. It is evident from
the examples in Fig. 1 that decreasing pHo to 6.4 or 5.4 caused a gradual stimulation of the Isc and presumably the apical Na+ channels.
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Effects of decreasing pHo on the macroscopic properties
of the cloned channel.
Figure 3 is a representative example of
the whole cell currents in an ENaC-expressing oocyte and their block by
10 µM amiloride. Amiloride causes a decrease of the whole cell
currents to levels not different from those observed in control
water-injected oocytes. The whole cell currents between 100 and 80
mV were used to calculate the inward gm. As
evident from Fig. 3, the majority of the gm is
amiloride sensitive and is attributed to ENaC.
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Effects of decreasing pHo on the single channel
properties of the native channel.
In the absence of blockers, the spontaneous rates of ENaC transition
are too slow for the resolution of noise analysis. This is indeed
consistent with single channel patch-clamp data that indicate open and
close times on the order of seconds [see Garty and Palmer
(9)]. These slow kinetics are manifested by the absence
of a spontaneous Lorentzian function in the power density spectrum
(Fig. 5A). To resolve channel
properties, a blocker is used to interact with the channel and speed up
its rates of opening and closing. CDPC is used as it is uncharged and
results in small inhibition of macroscopic currents. The on and off
rates for CDPC are also sufficiently fast enough and allow for better
resolution of the blocker-induced Lorentzian (Fig. 5B). The
corner frequency of the Lorentzian function exhibits a linear
relationship with CDPC concentration, as expected from a first-order
reaction (Fig. 5C). The corner frequencies, macroscopic
currents, and power plateaus are used to calculate the channel
properties as described by Helman et al. (13). The effects
of pHo 6.4 and 5.4 on the channel properties are summarized
below.
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Role of intracellular Ca2+
concentration in the observed stimulation.
It is well known that large increases of the intracellular
Ca2+ concentration ([Ca2+]i)
inhibit ENaC (5, 7, 19, 20). However, Ca2+
is also involved in many second messenger-mediated signaling cascades, including those resulting in vesicular trafficking. To
determine the role of [Ca2+]i, A6 monolayers
were incubated with 5 µM BAPTA-AM, an intracellular Ca2+
chelator. This chelator is added in a membrane-permeable form that
allows it to enter the cell. Intracellular BAPTA is then deesterified,
which renders it membrane impermeant, and is trapped within the cell
where it binds free Ca2+ and buffers the changes of
[Ca2+]i. Figure
9 is a representative effect of BAPTA
followed by pHo 6.4 on the Isc.
Within minutes, the addition of BAPTA caused a marked decrease of
transport. These monolayers were challenged with pHo 6.4 1 h after the addition of BAPTA. In this example, there were no
changes of the Isc with pHo 6.4.
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Measurements of pHi.
It is highly possible that prolonged changes of pHo, even
in HEPES-buffered solutions, could lead to appreciable changes of pHi. To determine whether such a hypothesis is tenable in
the present experiments, we utilized fluorescence measurements of pHi in polarized A6 monolayers. As shown in the
representative example in Fig. 12,
decreasing apical pHo did not have any detectable changes
of the BCECF fluorescence ratio, indicating lack of effects on
pHi. On the other hand, an appreciable and reversible
effect could be observed with the permeable ion NH4. These
observations do not rule out small local changes of pHi but
indicate the lack of large changes of pHi.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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We used A6 epithelia and the Xenopus oocyte expression system to study the effects of extracellular acidification on ENaC. Consistent with a previous report by Leaf et al. (16) in the toad bladder, extracellular acidification caused a stimulation of Na+ transport in A6 epithelia. This response was sustained over 60 min. In contrast, currents in oocytes expressing the cloned ENaC were only slightly and transiently stimulated by extracellular acidification. The stimulation in A6 cells was reflected as a large increase of NT and a small compensatory decrease of iNa. These effects were dependent on [Ca2+]i, as pretreatment with BAPTA prevented the changes of INa, iNa, and NT.
Role of pHi. It is established in a variety of preparations that the native channel is inhibited by decreasing pHi (7-10, 16, 19, 20, 22). Moreover, recent evidence and our own unpublished observations indicate that the channel in cultured A6 epithelia is also inhibited by decreasing pHi (25). However, aside from a report in toad bladder (16), pHo is not thought to affect ENaC. Thus the present findings indicate that this phenomenon is not restricted to toad bladder and may be present in other Na+-absorbing epithelia. Our findings also provide the single channel basis for this increase along with potential mechanisms. The changes of pHo to 5.4 are well within the range of those encountered in the urinary bladder and in the distal nephron; thus, these findings represent an important physiological effect of external H+.
We cannot rule out with absolute certainty that the observed stimulation of No was due to small localized changes of pHi, despite the fact that we could not detect any changes of the BCECF fluorescence ratio and therefore bulk pHi. However, three additional lines of indirect evidence argue against this possibility. First, both pHo 6.4 and 5.4 were without effects on Po, which is shown to be rapidly inhibited by small intracellular acidification (4, 19, 20). Second, a decrease of pHi is expected to decrease No in A6 cells (25), which is opposite to our observed effects with decreasing pHo. Third, similar experiments in toad bladder found that a pHo down to 5.4 was also without detectable effects on pHi (16).Effects in oocytes vs. A6 cells. It is well established that many of the ENaC properties found in native and cultured epithelia are well reproduced for the cloned channel expressed in Xenopus oocytes. However, overexpression of the three cloned ENaC subunits may result in the formation of a channel that reproduces the basic native Na+ channel properties but lacks the regulation conferred by association with other endogenous proteins. One such example was proposed by Awayda et al. (2) to explain the lack of regulation of the cloned ENaC by protein kinase A in Xenopus oocytes. It is possible that this may also be applicable to the observed differences in the response to pHo. However, at the present time, we cannot distinguish whether the variance in the response to pHo is due to differences between the native and cloned ENaC or oocytes and A6 cells. In either case, a better understanding of the origins of these differences will ultimately depend on elucidation of the mechanisms for sensing pHo and/or the role of Ca2+.
Effects on NT (role of Ca2+). The observed effects of pHo on NT were gradual, and appreciable changes were observed up to 30 min at pHo 5.4 and 40 min at pHo 6.4. This time course rules out an effect of external H+ on the channel, leading to direct activation of electrically silent but membrane-resident channels. However, it is possible that external H+ activates the channel via indirect effects on channel-related regulatory mechanisms. Decreasing pHi may protonate an accessory or a regulatory protein, e.g., a kinase or a phosphatase, that may be involved in channel trafficking. Alternatively, ENaC itself may be modified to alter its membrane residency to decrease its rates of endocytosis. At present, we are not able to select a likely mechanism among these; however, the finding that these changes were dependent on [Ca2+]i may indicate the potential involvement of a cell signaling cascade in the observed increase of NT.
Stimulation of Na+ transport in A6 epithelia by various mechanisms has been linked to Ca2+ mobilization. Hayslett and colleagues (11, 12) found that stimulation by adenosine and by vasopressin is linked to Ca2+ mobilization, since this effect could be blocked by BAPTA pretreatment. These authors measured the equivalent Isc; therefore, their methods did not allow for an assessment of the single channel properties and channel density. Nevertheless, our data are consistent with their observations and establish a role for channel trafficking or channel activation by [Ca2+]i. This may occur via a simple dependence of the cellular trafficking machinery on [Ca2+]i similar to that observed in many exocytic fusion events. Alternatively, it may involve a more complicated second messenger cascade. In any case, the potential roles of Ca2+, as delineated by buffering with BAPTA, should be distinguished from those observed with large and sometimes pharmacological increases of Ca2+ with ionophores that are known to inhibit ENaC. Assuming that pHi is not altered, how do we envision an increase of external H+ concentration causing an increase of [Ca2+]i? There are no known H+-sensing proteins in A6 cells. However, these cells are thought to contain an external Ca2+ sensor (15). This sensor is mildly Ca2+ selective in that it can also respond to Mg2+ and other multivalent cations (1, 21). Moreover, is it also known that this sensor is coupled to intracellular Ca2+-signaling cascades and to G protein-coupled cascades (1). It is unclear if such a sensor is also affected by extracellular H+ concentration; however, such a process could account for the effects of pHo on NT and the involvement of [Ca2+]i. A notable alternative hypothesis worth mentioning is that pHo may not alter [Ca2+]i but may affect the interaction of intracellular Ca2+ with ENaC. To our knowledge, such a mechanism has not been described previously. However, a relevant mechanism was described by Garty and colleagues (7). Using toad bladder vesicles, these investigators found that pHi affects the interaction of the Na+ channel with [Ca2+]i. In these experiments, the ability of Ca2+ to inhibit Na+ uptake was greatly reduced by decreasing pHi from 7.4 to 7.0. In this case, it would be expected that a decrease of pHi may relieve the inhibition of the channel by Ca2+ and cause its stimulation. It is unclear if a similar process occurs with changes of pHo, as this requires that protonation of an externally accessible site on the channel or associated protein leads to changes in the interaction with internal Ca2+. The above hypotheses await further experimental testing.Potential physiological significance. We demonstrated that extracellular acidification caused a more than twofold increase of channel density. In a native Na+-absorbing epithelium, such as the cortical collecting duct, this could cause major changes in Na+ reabsorption. If the present findings can be extended to native epithelia, it is possible that this mechanism may prime the principal cells to increase their capacity for Na+ transport and allow for a better recovery after inhibition by a large acid load. A second hypothesis was proposed by Leaf et al. (16) who made the original observation of stimulation of Na+ transport by pHo in the toad bladder. They remarked that many clinically encountered conditions associated with excess plasma acidosis and increased urinary H+ secretion are also accompanied by the need for Na+ conservation. In this case, the stimulation of Na+ transport by luminal acidity would constitute an intrinsic mechanism that conserves Na+. This mechanism may also serve to prevent excess Na+ loss through Na+/H+ exchangers in the presence of increased luminal H+.
pHo was found to activate Na+ transport in A6 epithelia. This activation was primarily due to an increase of NT. The increase of NT was [Ca2+]i dependent and was prevented by buffering [Ca2+]i with BAPTA. This stimulation may represent an intrinsic mechanism of channel regulation leading to increased Na+ reabsorption. It is unclear if the cloned channel behaves in a similar manner, since the currents in oocytes expressing the cloned ENaC were only transiently stimulated by pHo.| |
ACKNOWLEDGEMENTS |
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We thank Dr. Willy Van Driessche (K.U. Leuven) for helpful discussions regarding the pKa of CDPC.
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FOOTNOTES |
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This work was supported by a Grant-In-Aid from the Louisiana American Heart Association and by a Louisiana Education Quality Support Fund grant from the Louisiana Board of Regents to M. S. Awayda.
Address for reprint requests and other correspondence: M. S. Awayda, Dept. of Medicine, SL 35, Tulane Univ. School of Medicine, New Orleans, LA 70112 (E-mail: mawayda{at}tulane.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 20 January 2000; accepted in final form 6 July 2000.
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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 K. Sobczak, A. Willing, K. Kusche, N. Bangel, and W.-M. Weber Amiloride-sensitive sodium absorption is different in vertebrates and invertebrates Am J Physiol Regulatory Integrative Comp Physiol, June 1, 2007; 292(6): R2318 - R2327. [Abstract] [Full Text] [PDF]
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 N. A. Tobey, C. M. Argote, M. S. Awayda, X. C. Vanegas, and R. C. Orlando Effect of luminal acidity on the apical cation channel in rabbit esophageal epithelium Am J Physiol Gastrointest Liver Physiol, March 1, 2007; 292(3): G796 - G805. [Abstract] [Full Text] [PDF]
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 D. Cucu, J. Simaels, J. Eggermont, W. Van Driessche, and W. Zeiske Opposite effects of Ni2+ on Xenopus and rat ENaCs expressed in Xenopus oocytes Am J Physiol Cell Physiol, October 1, 2005; 289(4): C946 - C958. [Abstract] [Full Text] [PDF]
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W. Shao, R. C. Orlando, and M. S. Awayda Bisphosphonates stimulate an endogenous nonselective cation channel in Xenopus oocytes: potential mechanism of action Am J Physiol Cell Physiol, August 1, 2005; 289(2): C248 - C256. [Abstract] [Full Text] [PDF]
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D. Jans, J. Simaels, E. Lariviere, P. Steels, and W. Van Driessche Extracellular Ca2+ regulates the stimulation of Na+ transport in A6 renal epithelia Am J Physiol Renal Physiol, October 1, 2004; 287(4): F840 - F849. [Abstract] [Full Text] [PDF]
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 H.-L. Ji and D. J. Benos Degenerin Sites Mediate Proton Activation of {delta}{beta}{gamma}-Epithelial Sodium Channel J. Biol. Chem., June 25, 2004; 279(26): 26939 - 26947. [Abstract] [Full Text] [PDF]
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S. Sheng, J. B. Bruns, and T. R. Kleyman Extracellular Histidine Residues Crucial for Na+ Self-inhibition of Epithelial Na+ Channels J. Biol. Chem., March 12, 2004; 279(11): 9743 - 9749. [Abstract] [Full Text] [PDF]
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 S. A. Simon Interactions between Salt and Acid Stimuli: A Lesson in Gustation from Simultaneous Epithelial and Neural Recordings J. Gen. Physiol., November 25, 2002; 120(6): 787 - 791. [Full Text] [PDF]
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V. Lyall, R. I. Alam, T.-H. T. Phan, O. F. Russell, S. A. Malik, G. L. Heck, and J. A. DeSimone Modulation of Rat Chorda Tympani NaCl Responses and Intracellular Na+ Activity in Polarized Taste Receptor Cells by pH J. Gen. Physiol., November 25, 2002; 120(6): 793 - 815. [Abstract] [Full Text] [PDF]
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M. S. Awayda, J. D. Platzer, R. L. Reger, and A. Bengrine Role of PKCalpha in feedback regulation of Na+ transport in an electrically tight epithelium Am J Physiol Cell Physiol, October 1, 2002; 283(4): C1122 - C1132. [Abstract] [Full Text] [PDF]
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N. L. Nakhoul, K. S. Hering-Smith, S. M. Abdulnour-Nakhoul, and L. L. Hamm Ammonium interaction with the epithelial sodium channel Am J Physiol Renal Physiol, September 1, 2001; 281(3): F493 - F502. [Abstract] [Full Text] [PDF]
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