2Department of Physiology and Biophysics, Finch University of Health Sciences/Chicago Medical School, North Chicago, Illinois 60064; and 1Department of Physiology, Gyeongsang National University School of Medicine, Chinju 660-751, Korea
Submitted 26 December 2002 ; accepted in final form 22 April 2003
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ABSTRACT |
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tandem-pore potassium channel; background potassium channel; pancreas; pH
Studies over the past 5 years have revealed the electrophysiological and pharmacological properties of most of the functional K2P channels. Native K+ channels that might be functional correlates of several, but not all, cloned K2P channels have also been identified in various cell types. Thus, native K+ channels with properties similar to TASK-1, TASK-3, TREK-1, and TREK-2 were recently reported in various regions of the brain and in other tissues (4, 7, 15, 17, 19, 22, 37). The identification of native K+ channels that are functional correlates of cloned K2P channels has been possible because of the detailed knowledge of their single-channel kinetics and sensitivity to various pharmacological agents. A few of the K2P channels such as TALK and THIK, however, have yet to be characterized at the single-channel level to help identify their native counterparts.
The diversity of functional K2P channels can be increased by heteromultimerization among different K2P channel subunits. For example, coexpression of TASK-1 and TASK-3 in oocytes enabled the formation of a functional heteromeric channel (6, 36). So far, there is no conclusive evidence showing that any native K2P channel is a heteromer. Another way that nature has increased the number of functional K2P channels is through alternative splicing. Two splice variants of TASK-1, two variants of TREK-1, and three variants of human TREK-2 have been identified (3, 10, 14, 22). Therefore, it is likely that splice variants of other K2P channels also exist. The detailed characterization of single-channel properties of all splice variants is necessary to help identify the native K2P channel forms in different tissues and to study their physiological roles.
The aim of this study was therefore to identify potential splice variants of TALK-1, determine which variants are functional, and characterize in detail the single-channel kinetics of all functional variants. Because TALK-1a originally cloned from human pancreas is activated by alkaline conditions, we also tested whether any of the splice variants is also sensitive to changes in pHo. Here we report isolation of three novel splice variants of TALK-1 from human pancreas and single-channel characterization of two functional variants, TALK-1a and TALK-1b. Our results show that both TALK-1a and TALK-1b are highly sensitive to pHo near the physiological range.
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MATERIALS AND METHODS |
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Cloning of cDNAs. Human pancreas total RNA (Clontech, Palo Alto,
CA) was reverse-transcribed to prepare first-strand cDNA with the Superscript
preamplification system (Life Technologies, Rockville, MD). Specific primers
[5'-ATA TGC CCA GTG CTG GGC TCT GCA GCT G-3' (sense) and
5'-TCA GCT TCC CAG TCC TTT CTT GGA TAT GGG GA-3' (antisense)] for
TALK-1a were used in a polymerase chain reaction (PCR) with ExTaq
polymerase. PCR conditions were 94°C for 3 min, 30 cycles of 94°C (40
s)-50°C (60 s)-72°C (2 min), and finally 72°C for 10 min.
Amplified products (0.8 to
1.2 kb) were subcloned into pcDNA3.1 TOPO
vector (Invitrogen, Carlsbad, CA) by TA cloning. To isolate another potential
TALK-1 variant homologous to the mouse TALK-1, two specific primers
[5'-ATA TGC CCA GTG CTG GGC TCT GCA GCT G-3' (sense) and
5'-TCA TGC AGA GAT GGG GAT TTT CTG TG-3' (antisense)] were used in
a PCR with human pancreas cDNA as template. An amplified DNA product (
0.9
kb) was detected and subcloned into the pcDNA3.1 TOPO vector by TA cloning.
Plasmid DNA was isolated from three separate colonies from each cloning
procedure, and the subcloned DNA fragments were sequenced on both strands with
the dideoxy termination method.
To confirm the expression of TALK-1 variants in human pancreas, additional reverse transcriptase (RT)-PCRs were carried out with specific primers designed from exons 4, 5, and 6, where the splicing occurred. The primers used were from exons 4 and 6 (5-'GGGACGCTGGTCATTCTCATCTTCC-3' and 5'-TTGGACTCCTCTTGCTGCTGTAGAGCC-3') for TALK-1a and -1d, from exon 5' (5'-GGGACGCTGGTCATTCTCATCTTCC-3' and 5'-CGCTCATGCAGAGATGGGGATTTTCT-3') for TALK-1b, and from exon 6' (5'-CCACCCCCTTAACTTCATCACTCCCTC-3' and 5'-TTGGACTCCTCTTGCTGCTGTAGAGCC-3') for TALK-1c. The same amount of cDNA was used for all reactions. All PCR conditions were 94°C for 3 min, 30 cycles of 94°C (30 s)-50°C (20 s)-72°C (1 min), and finally 72°C for 10 min. A densitometer was used to assess the relative expression of TALK-1a and TALK-1d. DNA bands were extracted from the gel, amplified again with the same primers, and sequenced to confirm the presence of each variant. These results were reproduced in three different cDNA preparations.
Northern blot analysis. Multiple human tissue Northern blots
consisting of 15 different tissues were purchased from OriGene Technologies
(Rockville, MD). Membranes were pre-hybridized for 30 min at 65°C and
hybridized for 1 h at 65°C in ExpressHyb solution (Clontech) with
32P-labeled TALK-1b cDNA following the manufacturer's protocol. The
membrane were rinsed with solution containing 2x SSC (3.0 M sodium
chloride and 0.3 M sodium citrate) and 0.05% sodium dodecyl sulfate (SDS) for
30 min at room temperature. A second washing was performed in solution
containing 0.1x SSC and 0.1% SDS for 40 min at 50°C. The membrane
was exposed to X-ray film and developed 50 h later. The membrane was probed
again with 32P-labeled -actin DNA.
Transfection in COS-7 cells. Human (h)TALK-1 variants subcloned into pcDNA3.1 vector were tested for functional expression in COS-7 cells. COS-7 cells were seeded at a density of 2 x 105 cells per 35-mm dish 24 h before transfection in 10% bovine serum in Dulbecco's modified Eagle's medium (DMEM). COS-7 cells were cotransfected with pcDNA3.1/TALK-1 and pcDNA3.1/green fluorescent protein (GFP) with LipofectAMINE and OPTI-MEM I reduced-serum medium (Life Technologies). Green fluorescence from cells expressing GFP was detected with the aid of a Nikon microscope equipped with a mercury lamp light source. Cells were used 23 days after transfection.
Electrophysiological studies. Electrophysiological recording was
performed with a patch-clamp amplifier (Axopatch 200; Axon Instruments, Union
City, CA). All recordings were performed at room temperature (24°C).
Single-channel currents were digitized with a digital data recorder (VR10,
Instrutech, Great Neck, NY) and stored on videotape. The recorded signal was
filtered at 5 kHz with an eight-pole Bessel filter (3 dB; Frequency
Devices, Haverhill, MA) and transferred to a computer (Dell) with the Digidata
1200 interface (Axon Instruments) at a sampling rate of 20 kHz. Threshold
detection of channel openings was set at 50%. Single-channel data were
analyzed to obtain duration histogram, amplitude histogram, and relative
channel activity (relative NPo, where N is the
number of channels in the patch, and Po is the probability
of a channel being open) with the pCLAMP program (version 7).
NPo was determined from 1 min of current recording.
Single-channel current tracings shown in Figs.
6 and
7 were filtered at 2 kHz. In
experiments using excised patches, pipette and bath solutions contained (in
mM) 150 KCl, 1 MgCl2, 5 EGTA, and 10 HEPES (pH 7.3). For whole-cell
recordings, bath solution contained (in mM) 135 NaCl, 5 KCl, 1
MgCl2, 1 CaCl2, and 10 HEPES (pH 7.3). Arachidonic acid
was dissolved by sonicating for 5 min (Heat Systems-Ultrasonics W-380,
Farmingdale, NY) in bath recording solution at the desired concentration. All
other chemicals were purchased from Sigma (St. Louis, MO). For statistics,
analysis of variance and Student's t-test were used, with P
< 0.05 as a criterion for significance. Data are presented as means
± SD.
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RESULTS |
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All four TALK-1 transcript variants were derived from a gene located on chromosome 6p21 (GenBank accession no. AL136087 [GenBank] ) on the basis of the comparison of cDNAs clones and genomic sequences obtained from GenBank. The TALK-1a transcript consists of six exons and five intervening regions, whereas the TALK-1b, TALK-1c, and TALK-1d transcripts consist of five exons and four intervening regions, all within the chromosomal location at 6p21.121.2 (Fig. 2A). As shown in Fig. 2B in more detail, TALK-1b mRNA was generated as a result of skipping of the donor splice site in the 3' end of exon 5 until a stop codon was reached. TALK-1c mRNA was formed by skipping of the entire exon 5 but including exon 6', which has an acceptor splice site located before another acceptor splice site at the beginning of exon 6. This produced a frameshift in exon 6. TALK-1d was formed by skipping of exon 5. All of the splice sites show the commonly found donor-acceptor splice consensus sequences (GT... AG) in the exonintron boundaries. Partial DNA sequences of all four TALK-1 splice variants were found in the human EST database. Thus multiple copies (>5) of TALK-1a and -1c, two copies of TALK-1b, and one copy of TALK-1d were found. Mouse TALK-1 (accession no. XM138942) was an ortholog of human TALK-1b.
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The hydropathy plots of the four TALK-1 variants showed major differences
in the COOH terminus (Fig.
2C). TALK-1b is identical to TALK-1a except for the last
30 amino acids of the COOH terminus and has 10 fewer amino acids than
TALK-1a in this region. According to the hydropathy plot, TALK-1c does not
possess the fourth transmembrane segment although the COOH terminus is longer
than that of TALK-1a or -1b. TALK-1d is identical in amino acid sequence to
TALK-1a except that the exon 5 that encodes 53 amino acids is deleted from the
M4/COOH terminus region. Therefore, only TALK-1a and TALK-1b showed complete
four transmembrane segments and two pore regions.
Tissue expression of TALK-1. To confirm that mRNAs of all four
TALK-1 variants are expressed in the pancreas, we carried out additional
RT-PCR with primers specific to each variant. The primers were designed from
DNA sequences in exons 4, 5, and 6. As shown in
Fig. 3A, correct PCR
products of TALK-1 variants were obtained in three separate experiments in
which first-strand cDNAs were prepared with oligo(dT) as primer. Although not
quantitative, these results show that all four variants are expressed in the
pancreas. For TALK-1a and TALK-1d, we were able to estimate relative mRNA
expressions from the density of DNA bands in the gel, as the same primers and
cDNA were used under an identical condition. As shown in
Fig. 3A, mRNA
expression of TALK-1a was significantly (6-fold) greater than that of
TALK-1d, as judged by measurement of band intensity. However, this is a rough
estimate because DNA content was not measured during the exponential phase of
PCR amplification. Using the same set of primers (in exons 4 and 6) that
detected TALK-1a and TALK-1d, we expected to obtain a PCR product for TALK-1c;
however, a PCR product for TALK-1c was not observed. Using a different set of
primers that binds to sequences within exon 6', we were able to detect a
PCR product for TALK-1c (Fig.
3A).
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The tissue expression of TALK-1a mRNA was studied previously by Northern blot analysis that showed a high level of expression in human pancreas (12). In our recent study, TASK-5 mRNA was also expressed in human pancreas and in addition it was found in other endocrine tissues such as adrenal glands (18). Therefore, we tested the possibility that TALK-1 is expressed in tissues that have not been examined previously, such as other endocrine tissues, small intestine, and stomach. Our Northern blot data showed that one or more transcripts of TALK-1 (2.43 kb) are present in pancreas but not in any of the other 14 tissues examined (Fig. 3). Because we used full-length TALK-1b DNA as probe and the four variants share high nucleotide identity, we expect all four variants to be detected if they are present. Our results from RT-PCR and Northern blot analyses strongly suggest that pancreas is the only major tissue that expresses TALK-1 and its variants in humans.
Functional expression of TALK-1b. The aim of this part of the
study was to determine whether any splice variants of TALK-1a are capable of
forming a functional channel. To determine functional expression, whole cell
currents were recorded from COS-7 cells expressing a TALK-1 variant in a
physiological solution containing 5 mM KCl. Cell membrane potential was held
at 80 mV, and a ramp pulse was applied from 120 to +60 mV for 1
s. In cells expressing GFP alone, very small currents of <100 pA (90
± 7 pA at +60 mV; n = 5) were recorded
(Fig. 4A). In cells
transfected with TALK-1a/GFP DNAs, the same voltage ramp produced a large
outwardly rectifying current (723 ± 185 pA at +60 mV; n = 14),
in agreement with an earlier study
(12). TALK-1a current was
partially inhibited by 1 mM Ba2+ (30 ± 7%;
n = 7) applied to the bath solution. In cells transfected with
TALK-1b/GFP DNAs, the same voltage ramp produced a similar outwardly
rectifying current (721 ± 221 pA at +60 mV; n = 13) that was
also partially inhibited by 1 mM Ba2+ (31 ± 8%;
n = 7). In cells transfected with TALK-1c/GFP DNAs, very small
currents of 100 pA (113 ± 28 pA at +60 mV; n = 10) were
recorded (Fig. 4A).
Similarly, in cells transfected with TALK-1d/GFP DNAs, small currents (110
± 16 pA at +60 mV; n = 10) not significantly different from
those observed with GFP-transfected cells (control) were recorded
(Fig. 4A).
Figure 4B shows
expressed currents of spliced variants of TALK-1 in COS-7 cells at +60 mV.
These results show that TALK-1a and TALK-1b, but not TALK-1c and TALK-1d, can
be functionally expressed in COS-7 cells.
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Figure 5A shows activation of currents after step changes in membrane potential ranging from 120 mV to +40 mV. Activation of TALK-1a and TALK-1b by step depolarization was rapid and showed no time-dependent inactivation. Ion selectivity of TALK-1b was studied by changing the concentration of K+ ([K+]) in the bath solution from 5 to 150 mM while maintaining the pipette [K+] constant at 150 mM. As shown in Fig. 5B, the reversal potential shifted to the right as [K+]in the bath solution was elevated progressively, as expected of an ion channel that is permeable to K+ but not to Cl. The plot of the reversal potentials as a function of extracellular [K+] ([K+]o) showed that the slope was 51 ± 2 mV (n = 4) per 10-fold change in [K+]o, close to the calculated Nernst value of 58 mV (Fig. 5C). These results confirm that TALK-1b is a relatively K+-selective ion channel, similar to TALK-1a (12).
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The effects of various pharmacological agents were examined on the TALK-1b current with whole cell configuration from COS-7 cells. TALK-1b was insensitive to 1 mM tetraethylammonium (TEA). TALK-1b was inhibited by 1 mM Ba2+ (31 ± 8% inhibition; n = 7) and 1 mM quinidine (71 ± 8% inhibition; n = 8). Neither 4-aminopyridine (4-AP; 100 µM) nor Cs+ (1 mM) affected the TALK-1b current significantly (P > 0.05; n = 8). Arachidonic acid (20 µM), which activates K2P channels such as TREK and TRAAK (25, 30) and inhibits TASK (21, 23), failed to alter the TALK-1b current (n = 6).
Single-channel properties of TALK-1a and TALK-1b. The
single-channel properties of TALK-1a have not yet been characterized in any
cell system. To study the single-channel properties of TALK-1a and TALK-1b,
cell-attached patches were formed on COS-7 cells transfected with plasmids
containing one variant of TALK-1 and GFP. The concentration of KCl in the
pipette and bath solution was 150 mM. Nearly all COS-7 cells expressing
TALK-1a and TALK-1b exhibited robust channel activity that did not change
significantly after 5 min (P > 0.05; n = 12). Inside-out
patches showed similar channel activity, and no rundown was observed. COS-7
cells transfected with GFP alone showed no such channel activity (n
> 20), consistent with the results of whole cell studies. Channel openings
of TALK-1a and TALK-1b at different membrane potentials in cell-attached
patches when both pipette and bath solutions contained 150 mM KCl are shown in
Fig. 6, A and
C. Channel openings of TALK-1a and TALK-1b were extremely
brief such that the mean open time was <0.2 ms
(Fig. 6, B and
D). At a given membrane potential, the single-channel
current amplitude of TALK-1a and TALK-1b varied markedly, producing a broad
Gaussian curve in the amplitude histogram
(Fig. 6, B and
D). The amplitude variability of TALK-1a and TALK-1b were
less at +60 mV than at 60 mV, as judged by the broadness of the
histograms. The marked variability of current amplitude is likely to be due to
the limitation of our recording system, because we were unable to record at a
filter band pass higher than 5 kHz. Under these conditions, the estimated
apparent single-channel conductances of TALK-1a and TALK-1b were 23 ± 3
and 21 ± 2 pS at 60 mV and 11 ± 2 and 10 ± 2 pS at
+60 mV, respectively (n = 4). These values are underestimates of the
true conductances because of the limited resolution of channel openings. The
single-channel current-voltage relationships show that TALK-1a and TALK-1b are
weak inwardly rectifying K+ channels in high-K+ solution
(Fig. 6E). Again, our
interpretation of the electrophysiological data is simply based on what we
obtained under our recording conditions and may not reflect the intrinsic
properties of the channel. In inside-out patches, application of 1 mM ATP or
100 µM guanosine 5'-O-(3-thiotriphosphate) (GTPS) to
the cytoplasmic side of the membrane produced no significant effect on TALK-1b
activity (P > 0.05; n = 4 each).
Regulation of TALK-1b current by pH. TALK-1a has been shown to be activated at alkaline pH and inhibited at acidic pH (12). To study whether TALK-1b is also pH sensitive, whole cell currents of COS-7 cells expressing TALK-1b were recorded at different pHo values. TALK-1b current increased 32 ± 5% (n = 5; +60 mV) at pH 8.3 compared with that at pH 7.3 and increased further only slightly at pH 8.8 (10 ± 6%; n = 5). Similarly, TALK-1b current decreased 48 ± 6% (n = 5; +60 mV) at pH 6.3 compared with that at pH 7.3 and did not decrease further at pH 5.8 (Fig. 7A). Although not shown in Fig. 7A, TALK-1b currents at pH 6.8 and 7.8 in relation to pH 7.3 were also tested in separate cells. Averaged currents at +60 mV were determined from five experiments, normalized to the mean current determined at pH 8.8, and plotted as a function of pH (Fig. 7B). The data were fitted to a modified Boltzmann equation: y = a + {b/[1 + exp(xxo)/s]}, where a is the minimum value (0.28), b is the difference between the maximum and minimum values (0.72), x is the [H+] at which the channel activity was measured, xo is the [H+] at which half-maximal effect occurs, and s is the slope factor (0.52). At +60 mV, the half-maximal effect was observed at pH 7.2. Similar experiments were carried out on COS-7 cells expressing TALK-1a, and pH sensitivity was compared with that of TALK-1b. The results show that TALK-1a and TALK-1b possess nearly identical sensitivity to pHo near the physiological range. TALK-1c and -1d were not activated by alkaline pHo (pH 8.8), consistent with their lack of functional expression.
To study in more detail the effect of changes in pHo and pHi on TALK-1b kinetics, we recorded single-channel currents from outside-out and inside-out patches, respectively, at different pH values. Figure 7C shows channel openings from an outside-out patch kept at 60 mV in response to changes in the pHo of the bath solution from 6.3 to 8.3 at 0.5-pH unit intervals. Relative current was estimated by multiplying NPo and single-channel current amplitude at 60 mV and then plotted as a function of pHo. Single-channel current amplitudes were 1.0 ± 0.1, 1.0 ± 0.1, 1.1 ± 0.1, 1.2 ± 0.1, and 1.3 ± 0.2 pA at five pH values starting with pH 6.3. Figure 7D shows that TALK-1b is particularly sensitive to changes in pHo ranging from 6.3 to 8.3, in keeping with the whole cell current data. The single-channel amplitude was decreased only slightly (<10%) by changing the pH of the medium (Fig. 7C). These results show that the marked changes in channel activity at different pH values are largely due to apparent changes in the frequency of opening. Figure 7D also shows results obtained from inside-out patches in which the pH of the bath solution (pHi) was sequentially changed from 5.8 to 8.8. Changing pHi did not significantly affect single-channel conductance (1.0 ± 0.1 and 1.1 ± 0.1 pA at pH 5.8 and 8.8, respectively). At pH 5.8, 6.3, 8.3, and 8.8, TALK-1b currents were 75 ± 12, 84 ± 5, 120 ± 5, and 140 ± 20%, respectively, of that observed at pH 7.3. These results show that TALK-1b is much more sensitive to pHo than pHi and that the effect of pHo is not mediated via changes in pHi.
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DISCUSSION |
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To identify native K+ channels that are functional correlates of
K2P channels, it is necessary to characterize their single-channel
kinetics. We describe here the first single-channel measurement of TALK-1a and
TALK-1b expressed in COS-7 cells. We
(2,
15,
2022)
and others (26,
29) previously described
single-channel properties of functional K2P channels such as TREK,
TRAAK, and TASK. The single-channel kinetics of TALK-1 is somewhat reminiscent
of TRAAK kinetics as both show very short open time durations and spiky
openings with variable current amplitudes. However, the two can be
distinguished easily by a large difference in single-channel conductance
(22 pS for TALK-1 and 62 pS for TRAAK at 60 mV) as well as by
distinctly different responses to various modulators. For example, TALK-1a and
-1b are insensitive to arachidonic acid and applied pressure whereas TRAAK is
activated by fatty acids and mechanosensitive
(20,
26). The gating kinetics of
both variants of TALK-1 is also very different from those of TREK and TASK
channels. Thus the single-channel properties of TALK-1 are unique among
K2P channels and perhaps among all K+ channels. This
will allow us to identify a similar K+ channel in the pancreas in
the future and to study its potential physiological role.
All four TALK-1 transcript variants are derived from a gene located within the human chromosome 6p21.121.2 region. The process of formation of four splice variants of TALK-1 is reminiscent of that of KCNK7 mRNAs that are produced by excluding exons (exon skipping) within the COOH terminus to generate several transcripts (34). In this regard, it is interesting to note that mRNA variants of several other K2P channels such as TASK-1, TREK-1, and TREK-2 are formed as a result of alternative splicing at the NH2 terminus (3, 14, 22). At present, we do not know whether TALK-1c and -1d are translated into proteins and whether they are expressed along with TALK-1a and -1b in the same pancreatic cell. If they are, the nonfunctional variants may produce a negative effect on the expression of functional TALK-1. In a preliminary test, COS-7 cells were cotransfected with plasmids containing DNAs that encode TALK-1b and TALK-1c or plasmids containing DNAs that encode TALK-1b and TALK-1d. As a control, a nonfunctional K2P channel, TASK-5 (1, 18), was coexpressed with TALK-1b. In cell-attached patches, coexpression with TALK-1c or -1d did not alter the single-channel kinetics of TALK-1b. At the whole cell level, TALK-1c or -1d also failed to alter the expression level when normalized to control. These findings suggest that TALK-1c and -1d mRNAs may not be translated into proteins or that the proteins, if synthesized, do not interact with functional forms of the variants. It is also possible that different splice variants are also expressed in distinct cell types in the pancreas. Further studies are needed to address this issue.
TALK-1a and TALK-1b share the same channel properties that define them as background K+ channels. Like other K2P channels (TASK, TREK), they both show rapid activation on voltage changes and little or no inactivation. They are open across the physiological range of membrane potentials and are generally insensitive to well-known K+ channel blockers such as TEA, 4-AP, and Cs+. However, they are blocked by high concentrations of quinidine. Although we do not yet know which types of cells in the pancreas express TALK-1a and/or -1b, these K+ channels are likely to contribute to the background K+ conductance and to regulate the resting membrane potential in those cells that express them. Islets of Langerhans in the pancreas secrete insulin, glucagon, somatostatin, and pancreatic polypeptide. The amount of hormone secretion from such cells is critically dependent on the cell membrane potential that determines Ca2+ influx and subsequently the hormone secretion. For example, insulin secretion is enhanced by increased plasma glucose concentration that elevates intracellular ATP, which in turn inhibits ATP-sensitive K+ channels. Sulfonylureas such as tolbutamide and glyburide act directly to block ATP-sensitive K+ channels. In both cases, cells depolarize because of inhibition of outward K+ flux and ultimately result in increased insulin secretion (27, 35). TALK-1 may help to regulate hormone secretion via a negative feedback mechanism, as membrane depolarization would increase TALK-1 current and drive the membrane potential toward the K+ equilibrium potential.
In addition to its putative role as a background K+ channel, TALK-1 may also serve as a signal transducer. Because TALK-1 is highly sensitive to changes in pHo, its activity would be dependent on plasma pH. Acidosis would inhibit TALK-1 current and cause depolarization, whereas alkalosis would tend to hyperpolarize the cells. Depolarization caused by acidosis would tend to enhance hormone secretion. Thus TALK-1 may have a role that is similar to those of TASK channels that are present in type I cells of carotid body and respiration-sensitive cells in the brain stem (4, 37). By sensing changes in pHo and regulating the cell membrane potential, TASK and TALK-1 channels may modulate nerve activity and hormone secretion in their respective environments. The roles of two functional splice variants of TALK-1 in hormone secretion remain to be investigated. The detailed characterization of TALK-1 variants provided here should help to identify the native K+ channel with similar properties and help to study its behavior in the native system.
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DISCLOSURES |
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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. Section 1734 solely to indicate this fact.
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