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1 Department of Physiology and Biophysics, Chicago Medical School, Finch University of Health Sciences, North Chicago, Illinois 60064; and 2 Department of Physiology, Chung-Ang University, Seoul 156-756, Korea
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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A mammalian
K+ channel subunit (TBAK-1/TASK-1)
containing two pore domains and four transmembrane segments and whose
mRNA is highly expressed in the heart has been cloned recently. TBAK-1 and TASK-1 are identical except for the additional nine amino acids in
the NH2 terminus of TBAK-1. We
examined their kinetic properties, pH sensitivity, and regional cardiac
mRNA expression and determined whether a native cardiac
K+ channel with similar kinetic
properties was present. When TBAK-1 or TASK-1 was transiently expressed
in COS-7 cells, time- and voltage-independent whole cell currents were
observed. Single-channel conductances of TBAK-1 and TASK-1 were 14.6 ± 1.0 and 13.8 ± 2.8 pS, respectively, at 
80 mV in 140 mM extracellular K+, and the mean
open times were 0.8 ± 0.1 and 0.6 ± 0.1 ms, respectively. Both
TBAK-1 and TASK-1 were highly sensitive to extracellular pH such that a
decrease from 7.2 to 6.4 reduced their open probability (Po) by 81 ± 14% and 80 ± 16%, whereas a decrease in intracellular pH
from 7.2 to 6.4 reduced the
Po by 42 ± 10% and 47 ± 12%, respectively. TBAK-1/TASK-1 mRNA was expressed
in all regions of the rat heart, with the highest level of expression
in the right atrium. A 14-pS K+
channel with kinetic properties similar to those of TBAK-1/TASK-1 was
identified in rat atrial and ventricular cells. These results indicate
that TBAK-1/TASK-1 represents a functional native
K+ channel in the rat heart.
two-pore potassium channel; atrial cell; pH
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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A NEW FAMILY of K+ channel subunits containing two pore domains and four transmembrane (4TM) segments was first identified in the nematode Caenorhabditis elegans (18). At least five 4TM K+ channel subunits have subsequently been cloned from mammalian tissues (2-4, 8, 10, 11, 14). The DNA and amino acid homology among the cloned subunits are generally low (<40%), and their mRNA expression patterns in mammalian tissues are also not similar, suggesting that each subunit belongs to a separate group within the subfamily of 4TM K+ channels. All of the cloned mammalian 4TM K+ channel subunits have been reported to form functional K+ channels when expressed in either Xenopus oocytes or mammalian cell lines. Some form a K+ channel with properties of a background K+ current, whereas others are activated by free fatty acids and stretch (4, 13). Despite their interesting characteristics, the native K+ channels encoded by these 4TM K+ channel subunits and their physiological functions remain largely unknown.
Recently, a 4TM K+ channel subunit named TBAK-1 or TASK-1 was cloned from mouse and rat tissues, and its mRNA was shown to be expressed predominantly in the heart tissue (2, 8, 10). Full-length mouse TBAK-1 (mTBAK-1) and mouse TASK-1 (mTASK-1) are identical except that mTBAK-1 has an additional nine amino acids in the NH2 terminus. The amino acid sequence of rat TASK-1 (rTASK-1) is 99% identical to that of mTASK-1. When mTBAK-1 was expressed in HEK cells, it was found to have rapid kinetics of channel opening and closing with a mean open time of ~1 ms (8). However, when rTASK-1 was expressed in oocytes, it was found to open in long bursts with durations lasting several seconds (10). Therefore, we speculated that the difference in the opening kinetics of rTASK-1 and mTBAK-1 may be due to the presence of the additional stretch of nine amino acids in the NH2 terminus of mTBAK-1. However, it is possible that the observed difference in kinetics may be due to differences in the experimental conditions and types of cells used for expression. To identify a native cardiac K+ channel with properties similar to those of TBAK-1 or TASK-1, such differences in the channel gating kinetics must first be determined. An interesting property of TASK-1 is its high sensitivity to extracellular pH (pHo) such that lowering pHo from 7.6 to 6.4 results in ~90% inhibition of the current (2, 10). TBAK-1 may possess the same pHo sensitivity as TASK-1, because extracellular domains of the two subunits are identical; however, this has not been directly studied.
In this study, we examined in more detail the kinetic properties of mTASK-1 and mTBAK-1 under identical expression and experimental conditions and their modulation by both intracellular and extracellular pH. We studied their mRNA expression in different regions of the heart to determine whether the expression was homogeneous or localized in certain regions. Using isolated rat heart cells, we then examined whether a native cardiac K+ channel with properties similar to TASK-1 and TBAK-1 was present in certain regions of the heart. Our results showed that TBAK-1 and TASK-1 exhibit identical channel behavior, indicating that the additional nine amino acids in the NH2 terminus of TBAK-1 did not affect the intrinsic gating mechanism. The study also showed that a K+ channel with kinetic properties nearly identical to those of TBAK-1/TASK-1 was present in membrane patches of rat heart cells, although the density and open probability of the native TBAK-1/TASK-1-like K+ channels were generally low.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Expression of TBAK-1 and TASK-1 in COS-7 cells.
mTBAK-1 was originally cloned from a mouse heart cDNA library and
subcloned into pBluescript
SK
(8). mTASK-1 was also
cloned from a mouse heart cDNA library using mTBAK-1 sequence as a
probe. The coding regions of mTBAK-1 and mTASK-1 were subcloned into
pcDNA3 (Invitrogen, Carlsbad, CA) for expression into COS-7 cells.
Cells were transfected using LipofectAmine reagent (Life Technologies,
Grand Island, NY). pGFP (Clontech, Palo Alto, CA) was cotransfected
with pcDNA3-TBAK-1 or pcDNA3-TASK-1 to identify transfected cells.
COS-7 cells were grown for several days on glass coverslips treated
with polylysine before transfection and were used 2-4 days after transfection.
Northern blot analysis.
Commercially prepared rat multiple-tissue Northern blot nitrocellulose
membrane was purchased from Clontech Laboratories. Each lane was loaded
with 2 µg poly(A)+ RNA. To
determine mRNA expression within different regions of the heart, total
RNA was isolated from the right atrium, left atrium, left ventricle
(epicardium and endocardium separately), and right ventricle (RNeasy
kit, Qiagen, Valencia, CA). Total RNA (20 µg/lane) was loaded onto a
formaldehyde agarose gel (1.2%) and separated by electrophoresis. RNA
was transferred to a nitrocellulose paper and then cross-linked by
ultraviolet irradiation. A
32P-labeled fragment of rTASK-1
(690 bp) was used as a probe. This DNA fragment was obtained by RT-PCR
using total RNA from the rat ventricle and contained two pore regions
and most of the COOH terminus. Hybridization was performed according to
the instructions using ExpressHybe solution (Clontech). The blot was
probed again with human 
-actin DNA, which cross-hybridizes with
mouse and rat mRNA. After the film was exposed and developed, the
relative expression was determined using a molecular imager (Bio-Rad
GS-363 Imager and Gel Doc 1000).
RT-PCR. Total RNA from heart tissue prepared as described in Northern blot analysis was reverse transcribed using an oligo(dT) primer. PCR was carried out using two TBAK-1-specific primers (5'-AATGGTAGACGTGCAGAG- CTT-3' and 5'-TGCACACGATGAGAGCCAAC-3'). A plasmid containing TBAK-1 was used as control.
Isolated heart cell preparation. Single atrial and ventricular cells of adult rat heart were prepared by enzymatic digestion as described previously (6). Hearts were retrogradely perfused via the aorta in a Langendorff apparatus with a Ca2+-free, bicarbonate-buffered solution containing 0.05% collagenase (Worthington, Freehold, NJ) and 0.03% hyaluronidase (Sigma, St. Louis, MO) for 45 min. The atrial tissues were then cut into small strips and mechanically dissociated into single cells in the recording chamber. Bicarbonate-buffered solution contained (in mM) 118 NaCl, 4.7 KCl, 1 MgSO4, 1.2 KH2PO4, 25 NaHCO3, 10 HEPES, 10 pyruvate, and 10 glucose.
Electrophysiology.
Gigaseals were formed using Sylgard-coated thin-walled borosilicate
pipettes (Kimax) with ~4-M
resistances. Channel currents were
recorded with an Axopatch 200B patch-clamp amplifier (Axon Instruments,
Foster City, CA), digitized with a digital data recorder (VR10,
Instrutech, Elmont, NY), and stored on videotape using a videotape
recorder. The recorded signal was filtered at 3 kHz using an eight-pole
Bessel filter (3 dB; Frequency Devices, Haverhill, MA) and
transferred to a computer (Dell) using the Digidata 1200 interface
(Axon Instruments) at a sampling rate of 20 kHz. The filter dead time
was ~100 µs (0.3/cutoff frequency); therefore, events
shorter than ~50 µs will be missed in our analysis. Continuous single-channel currents were analyzed with the pCLAMP program (version
6.0.3, Axon Instruments). Data were analyzed to obtain a duration
histogram, amplitude histogram, and channel activity (NPo), where
N is the number of channels in the
patch and Po is the probability of a channel being open.
NPo was
determined from 1-2 min of current recording. The pipette and bath
solutions contained 140 mM KCl, 2 mM
MgCl2, 10 mM HEPES, and 5 mM EGTA
(pH 7.2). To change solutions perfusing the cytosolic surface of
inside-out patches, the pipette with the attached membrane was brought
to the mouth of the polypropylene tubing through which the desired solution flowed at a rate of ~1 ml/min. Macroscopic membrane currents from COS-7 cells were recorded with the whole cell configuration. Membrane potential was held at 0 mV, and then a voltage step of 500 ms
in duration was applied from 80 mV to +80 mV in 20-mV increments
every 5 s. Leak current was not subtracted. Current tracings shown in
Figs. 2-6, 8, and 9 were filtered at 1 kHz. Data are
represented as means ± SD. Student's
t-test was used to test for
significance between two values at the level of 0.05.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Single-channel currents of mTBAK-1 and mTASK-1 expressed in COS-7
cells.
In this study, we have used mouse clones of TBAK-1 and TASK-1 to
examine and compare their channel properties. The amino acid sequence
of mTASK-1 has not been reported previously, and therefore we have
aligned it with that of mTBAK-1. Alignment of TBAK-1 and TASK-1 shows
that TBAK-1 is identical to TASK-1 except for the presence of an
additional nine amino acids in the
NH2 terminus of TBAK-1, suggesting
that they are splice variants from the same gene (Fig.
1). Despite their overall sequence
identity, they have been reported to exhibit markedly different channel
kinetics, suggesting that the NH2
terminus of TBAK-1 may play an important role in channel gating. To
confirm such a difference, we examined the single-channel kinetic
properties of both TBAK-1 and TASK-1 under identical expression and
ionic conditions.
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80
mV (n = 7; Fig.
2C). The conductance of the main (lower) level was 14.6 ± 1.0 pS (n = 7) at 80 mV and 9.1 ± 1.3 pS
(n = 7) at +80 mV. Thus the
current-voltage relationship showed a weak inward rectification in the
presence of 2 mM Mg2+ (Fig.
2D). Outward currents measured
at +80 mV were blocked by application of 3 mM
Ba2+ to the bath solution in
inside-out patches. Channel activity (NPo) was not
significantly affected by membrane potential (Fig. 2E). A change in intracellular
K+ concentration
([K+]i)
from 140 mM to 280 mM produced a 15 ± 1-mV
(n = 3) shift of the reversal
potential, as expected for
K+-selective channels. These
K+ channels were not present in
nontransfected COS-7 cells or in COS-7 cells not expressing green
fluorescent protein (GFP) (n = 10).
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80 mV are shown in Fig.
3A. The
amplitude and duration histograms indicated that the kinetics of TASK-1 were indistinguishable from those of mTBAK-1 (Fig.
3B). As with mTBAK-1, two
conductance levels for which the higher level was not a
multiple of the lower one were also present. The
single-channel conductances of main levels were 13.8 ± 2.8 pS at
80 mV and 10.1 ± 2.1 pS at +80 mV
(n = 6). The mean open time was 0.6 ± 0.1 ms (n = 6) at 80 mV.
The current-voltage relationship obtained using the amplitudes of the
main level for mTBAK-1 and mTASK-1 could be superimposed. Channel
activity (NPo)
was not significantly affected by membrane potential (Fig.
3E). Outward mTASK-1 current measured at +80 mV was also blocked by 3 mM
Ba2+ in the bath solution in
inside-out patches. Thus these results showed that the stretch of nine
amino acids in the NH2 terminus of
mTBAK-1 had no effect on the gating kinetics of the
K+ channel.
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Effect of pH on mTBAK-1 and mTASK-1.
TASK-1 has been reported to be particularly sensitive to
pHo (2, 10). Because TBAK-1 is
essentially identical to TASK-1, TBAK-1 is likely to be
pHo-sensitive as well. We wanted
to confirm this view and also to examine whether the effect of
pHo on the current was due to the
accompanying change in intracellular pH (pHi). We also studied the
effect of pHo on mTBAK-1 and
mTASK-1 single-channel currents to determine whether pH affected the
conductance and/or the frequency of opening. First, we studied the
effect of pHo on the whole cell
currents of mTBAK-1 expressed in COS-7 cells. Cell membrane potential
was held at 0 mV and then stepped to potentials ranging from 80
mV to +80 mV in 20-mV increments for 500 ms. Figure
4A shows
whole cell currents at three pHo
values in the same cell and illustrates the marked sensitivity of the TBAK-1 current to pHo,
particularly in the acidic range. The effect of 3 mM
Ba2+ in the bath solution on the
whole cell current at pH 7.2 in the same cell is also shown.
Ba2+-sensitive currents were then
plotted as a function of voltage to better illustrate the effect of
pHo (Fig.
4B). Averaged currents at different
pHo values at 80 and +80 mV
were determined from three experiments and plotted in Fig.
4C. The results showed that TBAK-1 had
a high sensitivity to pHo,
particularly between 7.2 and 6.4. Changing
pHo from 7.2 to 6.4 resulted in a
73 ± 16% and 72 ± 17% reduction of the mTBAK-1 current for
inward and outward currents, respectively.
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80 and +80 mV were not significantly affected by changes in
pHi (Fig.
6B). However, channel activity was
reduced at pH 6.4 and augmented at pH 8.0 compared with that observed
at pH 7.2. The reduction of channel activity by a change in
pHi from 7.2 to 6.4 was 42 ± 10% compared with that produced by the same change in
pHo (81 ± 14%,
n = 4). Thus these results indicate
that TBAK-1 is more sensitive to
pHo than to
pHi in the pH range from 7.2 and
6.4. In the pH range from 7.2 to 8.0, the effect of
pHo and
pHi on TBAK-1 activity was not
significantly different (50 ± 15% vs. 44 ± 17%,
P < 0.05). The results also show
that the effect of pHo is not due
to a change in pHi, because a
decrease in pHo from 7.2 to 6.4 reduces pHi by <0.2 pH unit in
cardiac cells (7).
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Expression of TBAK-1/TASK-1 in heart tissues.
In mouse tissues, TASK-1 was expressed abundantly in
the heart and at low levels in the lung (2, 8). Our Northern blot analysis using the TBAK/TASK probe also showed a strong ~4.5-kb band
in the heart and a weak band in the lung of the adult rat (Fig.
7A).
TBAK-1/TASK-1 distribution within the heart tissue was then examined to
determine whether it was expressed in any specific region of the rat
heart. As shown in Fig. 7B,
TBAK-1/TASK-1 mRNA was expressed in all five regions studied. Relative
expression of TBAK-1/TASK-1 was assessed as a fraction of -actin
expression in the same blot using a densitometer. If the expression in
the right atrium is taken as 1.0, the expression in the left atrium, right ventricle, and left ventricle (epicardium and endocardium) was
0.8 ± 0.2, 0.7 ± 0.1, 0.9 ± 0.2, and 0.7 ± 0.2, respectively (n = 3 each). The results
show that the expression of TBAK-1/TASK-1 in different regions of the
heart is relatively even. To help distinguish the expressions of TBAK-1
and TASK-1, we performed RT-PCR using primers specific for TBAK-1. No
PCR product (112 bases) for TBAK-1 was detected in any of the five
heart regions, whereas PCR product for TASK-1/TBAK-1 was clearly
present (n = 3 each). The PCR product
was sequenced to confirm that it was the correct DNA fragment. A
typical result with right atrial tissue is shown in Fig.
7C. Therefore, these results indicate
that in the rat heart, TASK-1 is the predominant form.
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A native K+
channel with kinetic properties similar to TBAK-1/TASK-1.
The relatively high expression of TBAK-1/TASK-1 mRNA in the heart
tissue suggested that the small-conductance (14 pS)
K+ channel observed in transfected
COS-7 cells may be present in rat heart cells. To identify such a
K+ channel, we prepared single
dissociated cells from the right atrium and from the left ventricular
epicardium of rat heart. In cell-attached patches, we looked for a
channel that has kinetics of opening similar to that of TBAK-1
expressed in COS-7 cells. Once such a channel was considered to be
present, inside-out patches were formed and currents recorded at
different membrane potentials in symmetric 140 mM KCl. Figure
8 shows current recordings from inside-out
patches of a right atrial cell and a COS-7 cell expressing TBAK-1,
respectively. For the atrial K+
channel, the open time duration histogram could be fitted with a
single-exponential function with a time constant of 0.8 ± 0.1 ms
(n = 3). The mean current amplitudes
of the main open level at 80 and +80 mV were 1.08 ± 0.04 and
0.77 ± 0.02 pA, respectively, similar to the values for
mTBAK-1/mTASK-1 in COS-7 cells. The single-channel conductances at
80 and +80 mV were 13.5 ± 0.5 and 9.6 ± 0.3 pS,
respectively (n = 3). The
current-voltage relationship obtained using the amplitudes of the main
open level is shown in Fig.
8E. These channels were
K+ selective, as judged by a
31.6 ± 2.8-mV shift after switch of the KCl in the pipette
solution from 140 mM to 35 mM (expected shift from Nernst equation is
36 mV) and by the absence of these K+ channel openings (inward
current) when 3 mM Ba2+ was
present in the pipette solution (n = 5). Although nearly all patches examined showed channel openings with
similar kinetics, the open probability was always very low. In right
atrial cells, only ~20% of the patches studied showed channel
activity such as that shown in Fig. 8A
(P < 0.012;
n = 3).
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80 mV, respectively (n = 3). The mean open time averaged from ~100 openings was 0.8 ± 0.1 ms. For comparison, channel openings in COS-7 cells transfected with
TBAK-1 are also shown (Fig. 9B).
Figure 9C shows an inward current
recording from an inside-out patch in which opening of ATP-sensitive
K+ channels (72 ± 2 pS;
n = 3) was present. After rundown of
the ATP-sensitive K+ channel, we
could see openings of the small 14-pS channel, as indicated by the
arrows in Fig. 9C. Similarly, Fig.
9D shows a recording from an
inside-out patch in which one open background K+ channel
(IK1) with a
single-channel conductance of 24 ± 2 pS (n = 3) was present. We could see the
small ~14-pS channel on top of
IK1, as indicated
by the arrows in Fig. 9D. Thus,
despite their low open probability, TBAK-1-like channels were observed in both atrial and ventricular cells. However, because of the extremely
low open probability, we were unsuccessful in studying the effect of
pHo on these native
K+ channels. Nevertheless, our
results show that the K+ channel
with kinetic properties nearly identical to those of TBAK-1 is present
in heart cells.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Among cloned mammalian genes that encode K+ channel subunits possessing two pore-forming domains and four transmembrane segments, only TBAK-1/TASK-1 have been found to be expressed primarily in the heart tissue (2, 8). In this study, we characterized the kinetic behavior of TASK-1 and TBAK-1 to identify a native K+ channel with similar kinetic properties. Our study showed that TBAK-1 and TASK-1 have identical single-channel kinetics and pH sensitivity. TBAK-1/TASK-1 mRNA was found to be relatively evenly expressed throughout the heart. We have identified a K+ channel in rat atrial and ventricular cells with kinetic properties nearly identical to those of TBAK-1/TASK-1.
Single-channel kinetics of TBAK-1 and TASK-1. The single-channel TBAK-1/TASK-1 currents showed at least two levels of openings in which the channel with higher conductance (22 pS) was not a simple multiple of that with a smaller conductance (14 pS). The amplitude histograms for each level showed a broad peak, indicating the large fluctuation of current amplitudes. This could be due to channel openings whose short durations (<100 µs) are not fully detected by the limited resolution of our recording system (rise time 100 µs). At a low level of expression, when the patch showed one or two channel openings, the 14-pS channel was always predominant. Therefore, we used the current amplitudes of this level to plot all current-voltage relationships for comparison among two-pore K+ channels studied here. Our results showed that the kinetic properties of mTASK-1 are indistinguishable from those of mTBAK-1. This was surprising because of the earlier report that rTASK-1 showed long open bursts that lasted for several seconds, interrupted by closings that also lasted for seconds (10). The amino acid sequence of mTASK-1 is 99% identical to that of rTASK-1, and no differences are found within the pore and membrane-spanning regions. Furthermore, the same channel homologs from different species normally show identical behavior when expressed heterologously, despite small differences in the amino acid sequence. Therefore, it is highly unlikely that the channel kinetics of mTASK-1 and rTASK-1 would be different. In any case, our results showed clearly that mTASK-1 and mTBAK-1 have identical channel kinetics, indicating that the short stretch of nine amino acids in the NH2 terminus of TBAK-1 does not affect channel gating.
Sensitivity of TBAK-1 to pHo and
pHi.
TASK-1 has been shown to be very sensitive to
pHo, particularly in the
6.4-7.4 range. This could be due to an effect from the
intracellular side, because a change in
pHo causes a change in
pHi. Our results showed that
TBAK-1 is sensitive to both pHi and pHo in a qualitatively similar
way. Quantitatively, the same change in pH from 7.2 to 6.4 was twice as
effective in blocking the current from the extracellular as that from
the intracellular side. Therefore, the region of TBAK-1/TASK-1 that
confers sensitivity to pHo is on
the extracellular side. The pHo
effect on the channel current showed a very weak voltage dependence,
suggesting that H+ probably acts
at sites peripheral to the mouth of the pore. Our calculation of the
electric distance (
) in the membrane at which H+ interacts with the channel
protein from the external side was 0.05.
A rat cardiac
K+ channel with
kinetic behavior similar to that of TBAK-1/TASK-1.
The presence of TBAK-1/TASK-1 mRNA in single rat ventricular and atrial
cells has recently been confirmed by RT-PCR (8). Thus
K+ channels encoded by
TBAK-1/TASK-1 would be expected to be present in the cardiac cell
membrane, because they can form functional channels in COS-7 cells. To
our knowledge, a cardiac K+
channel with kinetic properties similar to those of TBAK-1/TASK-1 has
not been previously reported in any species. A voltage-dependent K+ channel with 14-pS conductance
in physiological solution has been reported in guinea pig ventricular
cells (21). However, long openings that occur only at depolarized
potentials indicate that this K+
channel is not TBAK-1/TASK-1. Therefore, this is the first study to
identify and report a TBAK-1/TASK-1-like
K+ channel in the rat heart. One
reason why such a channel was not detected previously may be that the
open probability was too low for the channel to be recorded in any
consistent manner, as our study indicates. Another reason may be that
the channel conductance was small relative to other inwardly rectifying
K+ channels that have higher
conductances and open at rest with much greater open probability, thus
overshadowing the small-amplitude channels. The estimated macroscopic
current of the 14-pS K+ channel at
80 mV would be ~13 pA in an atrial cell with a capacitance of
~50 pF, assuming that cell capacitance is ~50 pF and that there are
~1,000 channels in the membrane. This is probably too small to be of
physiological significance.
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ACKNOWLEDGEMENTS |
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We thank Dr. Yoshihisa Kurachi for providing the mouse TBAK-1 and TASK-1 clones, which were cloned by Dr. Donghee Kim in Dr. Kurachi's laboratory during a sabbatical.
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FOOTNOTES |
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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. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: D. Kim, Dept. of Physiology and Biophysics, Chicago Medical School, 3333 Green Bay Rd., N. Chicago, IL 60064 (E-mail: donghee.kim{at}finchcms.edu).
Received 28 January 1999; accepted in final form 24 May 1999.
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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