1 NeuroSearch A/S, The human
intermediate-conductance,
Ca2+-activated
K+ channel (hIK) was identified by
searching the expressed sequence tag database. hIK was found to be
identical to two recently cloned
K+ channels, hSK4 and hIK1. RNA
dot blot analysis showed a widespread tissue expression, with the
highest levels in salivary gland, placenta, trachea, and lung. With use
of fluorescent in situ hybridization and radiation hybrid mapping,
hIK mapped to chromosome
19q13.2 in the same region as the disease Diamond-Blackfan anemia.
Stable expression of hIK in HEK-293 cells revealed single
Ca2+-activated
K+ channels exhibiting weak inward
rectification (30 and 11 pS at
intermediate-conductance calcium-activated potassium channel; charybdotoxin; clotrimazole; fluorescent in situ hybridization; radiation hybrid mapping; patch clamp; Diamond-Blackfan anemia
CALCIUM-ACTIVATED POTASSIUM channels are almost
ubiquitously distributed in mammalian cells and constitute a major link
between second messenger systems and the electrical activity of the
cell. On the basis of their electrophysiological characteristics, three major classes of Ca2+-activated
K+ channels have been described:
voltage-dependent, large-conductance channels (BK);
voltage-independent, small-conductance channels (SK); and inwardly
rectifying, voltage-independent, intermediate-conductance channels
(IK). BK (13) and SK (22) channels are widely distributed in excitable
cells as well as in some nonexcitable cells (e.g., Ref. 13). In
neurons, their activation underlies the generation of fast and slow
afterhyperpolarizations, respectively, indicating a major role in the
regulation of neuronal excitability. The human BK channel gene has been
cloned from brain and functionally expressed in both
Xenopus oocytes and mammalian cells
(36). Three genes encoding the class of SK channels have been cloned
from human brain (hSK1) and rat
brain (rSK2 and
rSK3) (22). Through the use of the
sequence information from these genes, human IK
(hIK1 or hSK4) has recently been cloned from
placenta (21) and pancreas (20). The predicted amino acid sequence of
hIK is related to but distinct from the neuronal SK channels.
IK channels are apparently absent in excitable tissues but are present
in various blood cells (15), endothelial cells (31, 34), and cell lines
of epithelial origin (7, 9). Physiologically, IK channels are strongly
activated by release of intracellularly stored
Ca2+, induced by agonists such as
ATP, bradykinin, and histamine. The activation of IK is followed by
long-lasting or oscillatory hyperpolarizations of the cell membrane,
which closely reflect the intracellular
Ca2+ activity. In the present
study, we define the chromosomal localization and describe the
expression level of hIK in 50 different human tissues. The pharmacology
and ion selectivity of the hIK channel after stable transfection of the
gene into mammalian HEK-293 cells is examined.
Cloning and Sequencing of hIK
ABSTRACT
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
100 and +100 mV, respectively).
Whole cell recordings showed a noninactivating, inwardly rectifying
K+ conductance. Ionic selectivity
estimated from bi-ionic reversal potentials gave the permeability
(PK/PX)
sequence K+ = Rb+ (1.0) > Cs+ (10.4)
Na+,
Li+,
N-methyl-D-glucamine (>51).
NH+4 blocked the channel completely. hIK was
blocked by the classical inhibitors of the Gardos channel charybdotoxin
(IC50 28 nM) and clotrimazole (IC50 153 nM) as well as by
nitrendipine (IC50 27 nM),
Stichodactyla toxin
(IC50 291 nM), margatoxin
(IC50 459 nM), miconazole
(IC50 785 nM), econazole
(IC50 2.4 µM), and cetiedil
(IC50 79 µM). Finally, 1-ethyl-2-benzimidazolinone, an opener of the T84 cell IK channel, activated hIK with an EC50 of 74 µM.
INTRODUCTION
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
MATERIALS AND METHODS
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
The cDNA of the hEST clone was sequenced bidirectionally by automated sequencing (ABI 377, Applied Biosystems) using a standard dideoxy chain-termination sequencing kit (Perkin-Elmer). Sequence assembly and analysis were carried out using Lasergene software (DNASTAR). The hEST clone contains a 1284-bp open reading frame, and when sequenced the hEST clone was found to correspond to hIK1/hSK4 (20, 21).
RNA Dot Blot Analysis
A 32P-labeled cDNA probe was generated using random priming (Amersham, Little Chalfont, UK) and used to probe a commercially obtained human RNA master blot (Clontech, lot 7061008). Prehybridizations were for 30 min at 38°C in ExpressHyb (Clontech). Hybridization of the dot blot was for 16 h at 68°C with 109 dpm/µg probe in ExpressHyb. The blot was washed for 45 min at 25°C in 2× saline sodium citrate (SSC)-0.05% SDS, for 40 min at 50°C in 0.1× SSC-0.1% SDS, and finally for 30 min at 65°C in 0.1× SSC-0.1% SDS and was analyzed by autoradiography.Radiation Hybrid Mapping
DNA from the subset of 86 GeneBridge4 clones for radiation hybrid (RH) mapping (18) was obtained from the Human Genome Mapping Project Resource Center (United Kingdom Medical Research Council). Primers (T-A-G-Copenhagen) located at positions 1474-1690 in the 3' untranslated region of the hIK gene and amplifying a 238-bp fragment were used for RH mapping. PCR amplification was performed for 40 cycles in a PTC-225 DNA engine tetrad thermocycler (MJ Research) at 95, 59, and 72°C. PCR-generated fragments were separated on a 2% agarose gel, and amplification products of correct length were scored as positives, whereas absent products or PCR products of incorrect length were scored as ambiguities or negatives. Mapping was performed by direct submission to the RH mapping facility at The Whitehead Institute/Massachusetts Institute of Technology Center for Genome Research.1 After mapping, neighboring markers of the RH data vector were identified using the ENTREZ genome database at NCBI, and the chromosomal location was documented.Fluorescent In Situ Hybridization
Fluorescent in situ hybridization (FISH) with corresponding 4',6'-diamidine-2'-phenylindole dihydrochloride (DAPI)-banding and measurement of the relative distance from the long arm telomere to the signals [fractional length from the short arm telomere (FLpter value)] was performed essentially as described previously (25), using 100 ng biotin-labeled 1.9-kb hIK. The metaphases were visualized on a Leica DMRB epifluorescence microscope equipped with a Sensys 1400 charge-coupled device camera (Photometrics) and IPLab Spectrum imaging software (Vysis).Stable Expression
hIK was excised from pT3T7 using EcoR I and Not I and subcloned into the mammalian expression vector pNS1Z (NeuroSearch), a custom-designed derivative of pcDNA3Zeo (InVitrogen). HEK-293 cells were grown in DMEM (Life Technologies) supplemented with 10% FCS (Life Technologies) at 37°C in 5% CO2. One day before transfection, 106 cells were plated in a cell culture T25 flask (Nunc). Cells were transfected with 2.5 µg of the plasmid pNS1Z_hIK using Lipofectamine (Life Technologies) according to the manufacturer's instructions. Cells transfected with pNS1Z_hIK were selected in media supplemented with 0.25 mg/ml Zeocin. Single clones were picked and propagated in selection media until sufficient cells for freezing were available. Thereafter the cells were cultured in regular medium without selection agent. Expression of functional hIK channels was verified by patch-clamp measurements.Electrophysiology
Experiments were performed at room temperature with an EPC-9 amplifier (HEKA Electronics, Lambrecht, Germany). Pipettes (1.5-3.0 MA coverslip with transfected HEK-293 cells was transferred to the perfusion chamber and continuously superfused at a rate of 1 ml/min with an extracellular K+ solution with a composition (in mM) of 144 KCl, 2 CaCl2, 1 MgCl2, and 10 HEPES (pH 7.4). The pipettes were filled with intracellular solutions consisting of (in mM) 144 KCl, 10 HEPES, 1 or 10 EGTA, 9 or 0 nitrilotetraacetic acid, and MgCl2 and CaCl2 in the concentrations calculated (EqCal, Cambridge, UK) to give free Ca2+ concentrations of 0.1, 0.3, and 3 µM, respectively. Free Mg2+ concentration was always 1 mM, and pH was adjusted to 7.2 before experiments. These intracellular solutions were also used in the bath in inside-out experiments.
In the selectivity study, extracellular solutions consisting of 150 mM
of the desired cation (Cl
salts) and 2 mM CaCl2, 1 mM
MgCl2, and 10 mM HEPES [pH
adjusted to 7.4 with 10 mM N-methyl-D-glucamine
(NMDG+)] were used.
The electrode potentials were always zeroed with the open pipette in a
high-K+ solution. No zero current
or leak current subtraction was performed during the experiments. In
whole cell experiments, the cell capacitance and the series resistance
(Rs) were
updated before each pulse application.
Rs values ranged
from 3 to 6 M and remained stable during all experiments (10-30
min). At least 80% compensation of
Rs was obtained.
The current signals were low-pass filtered at 3 kHz (whole cell
recordings) or 0.5-1 kHz (single-channel recordings) and digitized
at a sample rate of at least three times the filter cutoff frequency.
All analyses and drawings were performed with IGOR software
(WaveMetrics, Lake Oswego, OR).
The IC50 values reported for the tested compounds were calculated from the kinetics of the block. The time course of the decrease in current (I) was fitted to the equation
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Clotrimazole, charybdotoxin, iberiotoxin, apamin, econazole, and miconazole were from Sigma. Ketoconazole was from Research Biochemicals International. 1-Ethyl-2-benzimidazolinone (1-EBIO) was from Aldrich. Kaliotoxin, margatoxin, and Stichodactyla toxin were from Alomone Lab. Stock solutions were prepared in DMSO (clotrimazole, econazole, miconazole, ketoconazole, and 1-EBIO) or water (peptides) and diluted to final concentrations in the appropriate salt solutions. BSA (0.01% wt/vol) was present in all external solutions when peptides, clotrimazole, or clotrimazole analogs were used during the experiment. None of the vehicles had any effect on the recorded currents.
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RESULTS |
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Cloning and Sequencing of hIK
The cloning of hIK was based on the identification of clones in the GenBank EST database with low but significant homology to hSK1. Although most of the retrieved ESTs with homology to hSK1 were identical to or highly similar to the channels SK1, SK2, and SK3, the rest exhibited rather low homology (<50%) to known K+ channels and were potentially members of a new branch of the six-transmembrane K+ channel family. The majority of these ESTs are part of the IMAGE Consortium Washington University-Merck EST project (27). Three of the hESTs with low homology to hSK1 were obtained, and two pancreatic hESTs were found to be partial clones by sequencing. The last of these three hESTs was from placenta, and this cDNA clone includes a full-length open reading frame encoding a polypeptide of 427 amino acids. The initiating methionine was assigned to an in-frame ATG within a strong Kozak consensus site (GXXGTissue Expression of hIK
Northern blot analysis of eight human tissues revealed a prominent 2.3-kb and a 2.9-kb transcript expressed in placenta (not shown). To estimate the tissue distribution of hIK, the expression of hIK in 50 different human tissues was examined by RNA dot blot analysis (Fig. 1). Significant amounts of transcript were evident in several nonexcitable tissues, including salivary gland, placenta, trachea, and lung, but not in any neuronal tissue (Fig. 1). The widespread distribution of hIK in tissues rich in epithelia suggests that hIK may represent an isoform primarily involved in secretion and absorption in salt-transporting epithelia.
|
Chromosomal Localization of hIK
Mapping by RH analysis. To define the chromosomal localization of the hIK gene, PCR was performed on DNA from an RH panel that included 86 cell lines (18). Using RH mapping, the hIK-encoding gene was mapped to within 2.74 centiRays (lod score >3.0) of marker WI-6526 and centromeric to D19S420 (17) and is thus contained within chromosomal region 19q13.2-19q13.3.
Mapping by FISH. Specific FISH signals were observed on the distal part of the long arm of chromosome 19, with 58 of 60 analyzed metaphases (97%) displaying at least one specific signal. In total, 139 of the 240 chromatids (58%) were labeled; there were 2 cells with 0, 5 cells with 1, 28 cells with 2, 19 cells with 3, and 5 cells with 4 labeled chromatids. The FLpter value derived from measurements of 37 signal-bearing chromosomes was 0.132 ± 0.021, corresponding to a localization at 19q13.13-19q13.31 (3). The localization of the FISH signals on elongated metaphase chromosomes suggested a finer localization to the border of bands 19q13.13-19q13.2 (Fig. 2).
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Stable Expression of hIK in HEK-293 Cells
Functional expression of hIK channels was revealed by single-channel as well as whole cell recordings. Figure 3A shows single-channel recordings from an inside-out patch exposed to symmetrical K+-solutions with 0.3 µM free Ca2+ at the intracellular side. Single-channel openings were observed at both positive and negative membrane potentials, and the gating showed no significant voltage dependency. However, the channel exhibited clear open-channel inward rectification. In the experiment depicted in Fig. 3, the unitary inward current fluctuations estimated at
|
Figure 4A
shows a typical whole cell experiment with a slightly inwardly
rectifying current developing during the first 30 s after break-in to
the whole cell mode (Fig. 4A). Only
a linear leak (trace
1) of a maximum of 100 pA was seen
when a voltage ramp (100 to +100 mV, 200 ms in duration) was
applied at the very moment that the whole cell configuration was
obtained. The hIK current (trace
2 and
inset) was activated as the cytosol
was exchanged by the pipette solution due to the increase in
intracellular Ca2+ concentration
(3 µM in pipette solution). The reversal potential (Vr) shifted
from 0 to
91 ± 3 mV (mean ± SE,
n = 12;
trace
3) on reduction of the extracellular
K+ concentration from 144 to 4 mM,
indicating a high
K+/Na+
selectivity (see also below). Furthermore, with the physiological ion
gradient, the
I-V
curve became almost linear in the range from
100 to 50 mV, and a
decrease in current was obtained at potentials >50 mV, as previously
described for IK currents in human red blood cells (8).
|
Figure 4B shows the immediate appearance of a noninactivating whole cell hIK current as voltage steps were applied. The observed whole cell inward rectification therefore represents an instantaneous property of the channels (open channel rectification or voltage-dependent block) and is not related to voltage-dependent gating, in accordance with the single-channel data.
Selectivity of hIK
The selectivity of the hIK channel was addressed in bi-ionic whole cell experiments. Voltage ramps (
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Pharmacology of hIK
The sensitivity of the expressed hIK channel to three K+ channel blockers is shown in Fig. 6. Ramp currents were elicited every 5 s, and the time course of the current at +80 and
|
To strengthen this view further, we extended the pharmacological study to include 1) other peptides with well-known K+ channel blocker profiles, 2) imidazole antimycotics considered to be analogs of clotrimazole, and 3) various other compounds previously reported to modulate native IK channels. Figure 7 illustrates the pharmacological experiments. Among the peptides, Stichodactyla toxin (100 nM; Fig. 7A) and margatoxin (100 nM; Fig. 7B) blocked the current, although less potently than charybdotoxin. The mean IC50 values obtained were 291 ± 50 nM (n = 3) for Stichodactyla toxin and 459 ± 34 nM (n = 3) for margatoxin. Also shown in Fig. 7B is the lack of effect of the SK channel blocker dequalinium chloride (1 µM) (6). Furthermore, kaliotoxin (50 nM, n = 3) and iberiotoxin (300 nM, n = 5) had no discernible effects (Fig. 7C). This potency order is consistent with that obtained by Brugnara et al. (4) on Ca2+-activated K+ transport in erythrocytes, although the IC50 values obtained with erythrocyte suspensions were somewhat lower than the values obtained in the present study.
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In addition to clotrimazole, the Ca2+-dependent K+ channels in human erythrocytes are blocked by several imidazole antimycotics (1). Figure 7D shows that hIK is blocked by econazole (IC50 = 2.4 ± 0.5 µM, n = 7), whereas ketoconazole is without effect (5 µM; n = 4). Miconazole blocked hIK with an IC50 = 785 ± 107 nM (n = 6).
Several classical Ca2+ channel antagonists were found to inhibit the Gardos channel (11), and the dihydropyridine nitrendipine is the most potent compound in this chemically diverse group of compounds. The effect of 300 nM nitrendipine is shown in Fig. 7E. The IC50 for the block by nitrendipine was 27 ± 6 nM (n = 6); nifedipine, a nitrendipine analog, was much less potent (1.5 ± 0.2 µM, n = 4), as were verapamil (72 ± 20 µM, n = 4) and diltiazem (154 ± 22 µM, n = 4).
Finally, cetiedil, a compound previously tested for its effect in sickle cell anemia, blocked the current with an IC50 value of 79 ± 44 µM (Fig. 7F; n = 4). Figure 7F also shows an activation of hIK by 10 µM 1-EBIO, which has been reported to activate Ca2+-activated K+ currents in colonic T84 epithelial cells and to stimulate short-circuit current and secretion in epithelia (10).
The activation of hIK by 1-EBIO was further elucidated by running cumulative dose-response experiments like the one shown in Fig. 8. In these experiments, the intracellular Ca2+ concentration was buffered at 0.1 µM, which is too low for significant activation of the channels. However, superfusion with 1-EBIO dose dependently activated the hIK current. The time course of a typical experiment is shown in Fig. 8A; the corresponding I-V curves are plotted in Fig. 8B. From the dose-response curve in Fig. 8C, an EC50 value of 74 ± 11 µM (n = 3) was estimated.
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DISCUSSION |
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In summary, we have cloned, expressed, and functionally characterized an hIK from human placenta. The gene was localized at chromosome 19 at region 19q13.2-19q13.3, and mRNA expression was detected at the highest density in salivary gland, placenta, lung, and trachea and in smaller quantities in liver, colon, thymus, kidney, and bone marrow. It was absent from excitable tissues, such as brain and skeletal and heart muscle, as well as from most embryonic tissues except liver. Recently, data on the cloning and expression of a pancreatic IK channel with similar characteristics were presented by Ishii et al. (20), whereas Kaczmarek and co-workers (21) designated the very same channel hSK4 due to the similarity of hIK to the SK channel family (~45% identity on the amino acid level). However, the functional and pharmacological characteristics, as well as the tissue distribution of hIK expression, strongly suggest that the cloned channel is identical to the well-defined type of IK channel present in a number of nonexcitable cells, such as T and B lymphocytes (14, 32), red blood cells (8, 15), and cells derived from epithelia (7, 35) and endothelia (34). Logsdon et al. (28) recently cloned an IK channel from T lymphocytes that is identical to hIK.
The unifying functional characteristics of this group of native
channels are the single-channel conductance, which exhibits weak inward
rectification in symmetrical K+
(30-40 pS at 100 mV; 10-15 pS at 100 mV), the weak or
absent voltage dependency of gating, and the high and steep sensitivity to intracellular Ca2+ (8, 10, 15,
31, 35). In comparison, the estimated single-channel chord conductance
for the cloned channel is 30 pS (present study) or ~33 pS (20).
The cloned channel was found to be highly selective for K+, with demonstrable permeability (PX/PK) to Rb+ (1.0) and Cs+ (10.4), whereas Li+, Na+, and NMDG+ were largely impermeable (>51). The permeability sequence obtained for hIK closely resembles that of the prototype channel within this group, the erythrocyte K+ channel (8). The notable differences are 1) that Cs+ in the present study was demonstrated to conduct inward current, 2) the powerful block of the cloned channel by NH+4, and 3) a tendency of hIK toward decreased selectivity after prolonged exposure to K+-free solutions. Regarding difference 1, inward single-channel currents conducted by Cs+ were impossible to demonstrate in the erythrocyte channel study, most likely due to very small single-channel currents. However, the extrapolated value of the Vr in that study corresponds to the value determined here. Regarding difference 2, the erythrocyte channel was not completely blocked by NH+4, and genuine permeability measurements were possible at the single-channel level (PK/PX = 8.5). However, the gating of the erythrocyte channel was pronouncedly changed in NH+4 solutions (short openings, low open state probability) (2). Qualitatively, the complete block seen in the present study may reflect the same phenomenon. Regarding difference 3, after a longer period in Na+, Li+, or NMDG+, the decrease of the outward current at positive potentials became more prominent, a small inward current developed at very negative potentials, and the Vr shifted toward less negative potentials. This could indicate that the channels become permeable to these ions in the total absence of external K+, as described by Korn and Ikeda (23) for Kv1.5. However, although mechanistically interesting, the phenomenon has not been further elucidated in this study.
In addition to the difference in open channel properties, the SK and IK channels can be distinguished pharmacologically. It is well known that charybdotoxin blocks IK channels with high affinity (low nanomolar range, e.g., Ref. 4), whereas SK channels are not affected. Conversely, apamin blocks certain SK channels (equivalent to the cloned SK2 and SK3), whereas there is no effect on IK channels. Because the cloned hIK is potently blocked by charybdotoxin and is insensitive to apamin [and to dequalinium, which binds to the apamin-site (6)], the basic pharmacology points toward a classical IK channel. The experiments with the K+ channel-blocking peptides clearly show that charybdotoxin is the most potent, with a potency order exactly as described for erythrocytes. Clotrimazole, nitrendipine, cetiedil, and 1-EBIO are classical modulators of IK channels, but their selectivity properties are largely unknown. However, the overall correlation between the potency of these compounds (and their analogs) on native IK channels and the cloned channel supports the conclusion of a closely related pharmacology.
The physiological significance of IK channels has been described in
various tissues. The erythrocyte
K+ channel was the first
Ca2+-activated
K+ channel ever described, but
despite this the function of IK channels in normal erythrocytes is
obscure, whereas pathological activation is known to be critical in the
development of crises in sickle cell anemia (5). The
hIK gene is mapped to chromosome
19q13.2 at the same localization as Diamond-Blackfan anemia (16), a disease with the clinical hallmarks of a selective decrease in erythroid precursors and anemia. In some salt (and water) secreting epithelia, activation of basolateral IK channels is essential for
maintaining the driving force for luminal
Cl efflux (10). In the
lung, where hIK is expressed relatively abundantly, a
charybdotoxin-sensitive IK channel is thought to hyperpolarize the
airway epithelia in response to agonists that elevate intracellular
cAMP and Ca2+, thus facilitating
apical Cl
secretion (29,
37). Possibly a pharmacological activator of hIK might therefore be of
clinical interest in the treatment of mild forms of cystic fibrosis.
Furthermore, hIK is expressed in colon (Fig. 1), and activation of
Ca2+-activated
K+ channels has been shown to be
coupled to the apical Cl
secretion (9, 33). Blockers of hIK may thus prove efficient in the
treatment of secretory diarrhea. The high level of mRNA expression detected in salt-transporting tissues (Fig. 1) may indicate
a universal role of IK channels in epithelial function. In T and B
lymphocytes, the number of IK channels is upregulated on activation
with antibodies or the protein kinase C activator phorbol 12-myristate
13-acetate (14, 32), suggesting important functions of this class of
channels in the activation of the immune system.
A number of native channels with characteristics different from those of the erythrocyte-type IK channel have traditionally been classified as IK channels. Thus the IK channel group is quite heterogeneous, comprising channels that differ widely in single-channel conductances (20-150 pS), degree of rectification, voltage dependency of gating, pharmacology, and Ca2+ sensitivity (19, 26, 30). Many of these channels deviate strongly from the channel described in the present paper, and some are even expressed in excitable cells. It is therefore possible that a whole family of IK channels exists and that more types of Ca2+-activated K+ channels with intermediate conductance will be cloned in the future.
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ACKNOWLEDGEMENTS |
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We gratefully acknowledge L. G. Larsen and H. Z. Andresen for excellent technical assistance.
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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.
1 RH mapping can be performed via the World Wide Web (http://www-genome.wi.mit.edu/cgi-bin/contig/rhmapper.pl).
Address for reprint requests: B. S. Jensen, NeuroSearch A/S, 26B Smedeland, DK-2600 Glostrup, Denmark.
Received 2 March 1998; accepted in final form 8 May 1998.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Alvarez, J.,
M. Montero,
and
J. García-Sancho.
High affinity inhibition of Ca2+-dependent K+ channels by cytochrome P-450 inhibitors.
J. Biol. Chem.
267:
11789-11793,
1992
2.
Bennekou, P.,
and
P. Christophersen.
The gating of human red cell Ca2+-activated K+ channels is strongly affected by the permeant cation species.
Biochim. Biophys. Acta
1030:
183-187,
1990[Medline].
3.
Bray-Ward, P.,
J. Menninger,
J. Lieman,
T. Desai,
N. Mokady,
A. Banks,
and
D. C. Ward.
Integration of the cytogenetic, genetic, and physical maps of the human genome by FISH mapping of CEPH YAC clones.
Genomics
32:
1-14,
1996[Medline].
4.
Brugnara, C.,
C. C. Armsby,
L. De Franceschi,
M. Crest,
M.-F. Martin Euclaire,
and
S. L. Alper.
Ca2+-activated K+ channels of human and rabbit erythrocytes display distinctive patterns of inhibition by venom peptide toxins.
J. Membr. Biol.
147:
71-82,
1995[Medline].
5.
Brugnara, C.,
B. Gee,
C. C. Armsby,
S. Kurth,
M. Sakamoto,
N. Rifai,
S. L. Alper,
and
O. Platt.
Therapy with oral clotrimazole induces inhibition of the gardos channel and reduction of erythrocyte dehydration in patients with sickle cell disease.
J. Clin. Invest.
97:
1227-1234,
1996
6.
Castle, N. A.,
D. G. Haylett,
J. M. Morgan,
and
D. H. Jenkinson.
Dequalinium: a potent inhibitor of apamin-sensitive K+ channels in hepatocytes and of nicotinic responses in skeletal muscle.
Eur. J. Pharmacol.
236:
201-207,
1993[Medline].
7.
Christensen, O.,
and
E. K. Hoffmann.
Cell swelling activates K+ and Cl channels as well as nonselective, stretch-activated cation channels in Ehrlich ascites tumor cells.
J. Membr. Biol.
129:
13-36,
1992[Medline].
8.
Christophersen, P.
Ca2+-activated K channel from human erythrocyte membranes: single channel rectification and selectivity.
J. Membr. Biol.
119:
75-83,
1991[Medline].
9.
Devor, D. C.,
and
R. A. Frizzell.
Calcium-mediated agonists activate an inwardly rectified K+-channel in colonic secretory cells.
Am. J. Physiol.
265 (Cell Physiol. 34):
C1271-C1280,
1993
10.
Devor, D. C.,
A. K. Singh,
R. A. Frizzell,
and
R. J. Bridges.
Modulation of Cl secretion by benzimidazolones. I. Direct activation of a Ca2+-dependent K+ channel.
Am. J. Physiol.
271 (Lung Cell. Mol. Physiol. 15):
L775-L784,
1996
11.
Ellory, J. C.,
S. J. Culliford,
P. A. Smith,
M. W. Wolowyk,
and
E. E. Knaus.
Specific inhibition of Ca-activated K channels in red cells by selected dihydropyridine derivatives.
Br. J. Pharmacol.
111:
903-905,
1994[Abstract].
12.
Francke, U.
Digitized and differentially shaded human chromosome ideograms for genomic applications.
Cytogenet. Cell Genet.
65:
206-219,
1994[Medline].
13.
Gribkoff, V. K.,
J. E. Starrett, Jr.,
and
S. I. Dworetzky.
The pharmacology and molecular biology of large-conductance calcium-activated (BK) potassium channels.
Adv. Pharmacol.
37:
319-348,
1997[Medline].
14.
Grissmer, S.,
A. N. Nguyen,
and
M. D. Cahalan.
Calcium-activated potassium channels in resting and activated human T lymphocytes.
J. Gen. Physiol.
102:
601-630,
1993[Abstract].
15.
Grygorczyk, R.,
and
W. Schwarz.
Properties of the Ca2+-activated K+ conductance of human red cells as revealed by the patch clamp technique.
Cell Calcium
4:
499-510,
1983[Medline].
16.
Gustavsson, P.,
T.-N. Willig,
A. Haeringen,
G. Thernia,
I. Dianzani,
M. Donner,
G. Elinder,
J.-I. Henter,
P.-G. Nilsson,
L. Gordon,
G. Skeppner,
L. Veer-Korthof,
A. Kreuger,
and
N. Dahl.
Diamond-Blackfan anaemia: genetic homogeneity for a gene on chromosome 19q13 restricted to 1.8 Mb.
Nat. Genet.
16:
368-371,
1997[Medline].
17.
Gyapay, G., J. Morissette, A. Vignal, C. Dib, C. Fizames, P. Millasseau, S. Marc, G. Bernadi, M. Lathrop, and J. Weissenbach.
The 1993-94 Genethon human genetic linkage map.
Nat. Genet. 246-339, 1994.
18.
Gyapay, G.,
K. Schmitt,
C. Fizames,
H. Jones,
N. Vega-Czarny,
D. Spillett,
D. Muselet,
J.-F. Prud'Homme,
C. Dib,
C. Auffray,
J. Morissette,
J. Weissenbach,
and
P. N. Goodfellow.
A radiation hybrid map of the human genome.
Genomics
5:
339-346,
1996.
19.
Hay, M.,
and
D. L. Kunze.
An intermediate conductance calcium activated potassium channel in rat visceral sensory afferent neurons.
Neurosci. Lett.
167:
179-182,
1994[Medline].
20.
Ishii, T. M.,
C. Silvia,
B. Hirschberg,
C. T. Bond,
J. P. Adelman,
and
J. Maylie.
A human intermediate conductance calcium-activated potassium channel.
Proc. Natl. Acad. Sci. USA
94:
11651-11656,
1997
21.
Joiner, W. J.,
L. Wang,
M. D. Tang,
and
L. K. Kaczmarek.
hSK4, a member of a novel subfamily of calcium-activated potassium channels.
Proc. Natl. Acad. Sci. USA
94:
11013-11018,
1997
22.
Köhler, M.,
B. Hirschberg,
C. T. Bond,
J. M. Kinzie,
N. V. Marrion,
J. Maylie,
and
J. P. Adelman.
Small-conductance, calcium-activated potassium channels from mammalian brain.
Science
273:
1709-1714,
1996
23.
Korn, S. J.,
and
S. R. Ikeda.
Permeation selectivity by competition in a delayed rectifier potassium channel.
Science
269:
410-413,
1995[Medline].
24.
Kozak, M.
An analysis of 5'-noncoding sequences from 699 vertebrate RNAs.
Nucleic Acids Res.
15:
8125-8148,
1987[Abstract].
25.
Kozyraki, R.,
M. Kristiansen,
A. Silahtaroglu,
C. Hansen,
C. Jacobsen,
N. Tommerup,
P. J. Verroust,
and
S. K. Moestrup.
The human intrinsic factor-vitamin B12 receptor, cubilin: molecular characterization and chromosomal mapping of the gene to 10p within the autosomal recessive megaloblastic anemia (MGA1) region.
Blood
91:
3593-3600,
1998
26.
Latorre, R.,
A. Oberhauser,
P. Labarca,
and
O. Alvarez.
Varieties of calcium-activated potassium channels.
Annu. Rev. Physiol.
51:
385-399,
1989[Medline].
27.
Lennon, G.,
C. Auffray,
M. Polymeropoulos,
and
M. B. Soares.
The I.M.A.G.E. consortium: an integrated molecular analysis of genomes and their expression.
Genomics
33:
151-152,
1996[Medline].
28.
Logsdon, N. J.,
J. Kang,
J. A. Togo,
E. P. Christian,
and
J. Aiyar.
A novel gene, hKCa4, encodes the calcium-activated potassium channel in human T lymphocytes.
J. Biol. Chem.
272:
32723-32726,
1997
29.
McCann, J. D.,
J. Matsuda,
M. L. Garcia,
G. J. Kaczorowski,
and
M. J. Welsh.
Basolateral K+ channels in airway epithelia. I. Regulation by Ca2+ and block by charybdotoxin.
Am. J. Physiol.
258 (Lung Cell. Mol. Physiol. 2):
L334-L342,
1990
30.
McManus, O. B.
Calcium-activated potassium channels: regulation by calcium.
J. Bioenerg. Biomembr.
23:
537-560,
1991[Medline].
31.
Olesen, S.-P.,
and
M. Bundgaard.
ATP-dependent closure and reactivation of inward rectifier K+ channels in endothelial cells.
Circ. Res.
73:
492-495,
1993[Abstract].
32.
Partiseti, M.,
D. Choquet,
A. Diu,
and
H. Korn.
Differential regulation of voltage- and calcium-activated potassium channels in human B lymphocytes.
J. Immunol.
148:
3361-3368,
1992
33.
Sandle, G. L.,
C. M. McNicholas,
and
R. B. Lomax.
Potassium channels in colonic crypts.
Lancet
343:
23-25,
1994[Medline].
34.
Sauvé, R.,
L. Parent,
C. Simoneau,
and
G. Roy.
External ATP triggers a biphasic activation process of a calcium-dependent K+ channel in cultured bovine aortic endothelial cells.
Pflügers Arch.
412:
469-481,
1988[Medline].
35.
Sauvé, R.,
C. Simoneau,
R. Monette,
and
G. Roy.
Single-channel analysis of the potassium permeability in HeLa cancer cells: evidence for a calcium-activated potassium channel of small unitary conductance.
J. Membr. Biol.
92:
269-282,
1986[Medline].
36.
Tseng-Crank, J.,
N. Godinot,
T. E. Johansen,
P. K. Ahring,
D. Strøbæk,
R. Mertz,
C. D. Foster,
S. P. Olesen,
and
P. H. Reinhart.
Cloning, expression, and distribution of a Ca2+-activated K+ channel -subunit from human brain.
Proc. Natl. Acad. Sci. USA
93:
9200-9205,
1996
37.
Welsh, M. J.,
and
J. D. McCann.
Intracellular calcium regulates basolateral potassium channels in a chloride-secreting epithelium.
Proc. Natl. Acad. Sci. USA
82:
8823-8826,
1985[Abstract].