I would greatly appreciate references regarding the effect of barium (200mM and lower concentrations) on the different potassium currents in mammalian neurons.
Hi Pacha Mama
check the following articles
this second article is not on the mammalian system but perhaps it may help you as well..
Effects of Barium on Delayed Rectifier Potassium Current in Bullfrog Sympathetic Neurons Pretreated with Wortmannin
Chie INOMOTO and Takayuki TOKIMASA
Department of Physiology, Tokai University School of Medicine
(Received August 19, 1998; Accepted September 9, 1998)
The effect of barium (1 mM) on a delayed rectifier-type potassium current was examined in
bullfrog sympathetic neurons. An M-type potassium current was eliminated by pretreatment
of the cells with a microbial product, wortmannin (10Ж M). An A-type potassium current was
continuously inactivated by setting a holding potential at ?65 mV. In treated cells (n?10),
the delayed rectifier at 0 mV averaged 2200?107 pA in the presence of barium (1 mM) as
compared to 2308?110 pA in the controls, and 2085?103 pA after washing out the barium.
It is concluded that the delayed rectifier is insensitive to barium blockage in amphibian auto-
Key Words: Potassium current, Delayed rectifier, M-current, Barium, Wortmannin,
1: J Neurosci. 2009 Jul 1;29(26):8551-64.
TWIK-1 and TREK-1 are potassium channels contributing significantly to astrocyte
passive conductance in rat hippocampal slices.
Zhou M, Xu G, Xie M, Zhang X, Schools GP, Ma L, Kimelberg HK, Chen H.
Ordway Research Institute, Albany, New York 12208, USA. eval(unescape('%64%6f%63%75%6d%65%6e%74%2e%77%72%69%74%65%28%27%3c%61%20%68%72%65%66%3d%22%6d%61%69%6c%74%6f%3a%6d%7a%68%6f%75%40%6f%72%64%77%61%79%72%65%73%65%61%72%63%68%2e%6f%72%67%22%3e%6d%7a%68%6f%75%40%6f%72%64%77%61%79%72%65%73%65%61%72%63%68%2e%6f%72%67%3c%2f%61%3e%27%29%3b'))
Expression of a linear current-voltage (I-V) relationship (passive) K(+) membrane
conductance is a hallmark of mature hippocampal astrocytes. However, the
molecular identifications of the K(+) channels underlying this passive
conductance remain unknown. We provide the following evidence supporting
significant contribution of the two-pore domain K(+) channel (K(2P)) isoforms,
TWIK-1 and TREK-1, to this conductance. First, both passive astrocytes and the
cloned rat TWIK-1 and TREK-1 channels expressed in CHO cells conduct significant
amounts of Cs(+) currents, but vary in their relative P(Cs)/P(K) permeability,
0.43, 0.10, and 0.05, respectively. Second, quinine, which potently inhibited
TWIK-1 (IC(50) = 85 microm) and TREK-1 (IC(50) = 41 microm) currents, also
inhibited astrocytic passive conductance by 58% at a concentration of 200 microm.
Third, a moderate sensitivity of passive conductance to low extracellular pH
(6.0) supports a combined expression of acid-insensitive TREK-1, and to a lesser
extent, acid-sensitive TWIK-1. Fourth, the astrocyte passive conductance showed
low sensitivity to extracellular Ba(2+), and extracellular Ba(2+) blocked TWIK-1
channels at an IC(50) of 960 microm and had no effect on TREK-1 channels.
Finally, an immunocytochemical study showed colocalization of TWIK-1 and TREK-1
proteins with the astrocytic markers GLAST and GFAP in rat hippocampal stratum
radiatum. In contrast, another K(2P) isoform TASK-1 was mainly colocalized with
the neuronal marker NeuN in hippocampal pyramidal neurons and was expressed at a
much lower level in astrocytes. These results support TWIK-1 and TREK-1 as being
the major components of the long-sought K(+) channels underlying the passive
conductance of mature hippocampal astrocytes.
Research Support, Non-U.S. Gov't
Research Support, U.S. Gov't, Non-P.H.S.
PMID: 19571146 [PubMed - indexed for MEDLINE]
2: Am J Physiol Lung Cell Mol Physiol. 2007 May;292(5):L1304-12. Epub 2007 Feb 2.
A Ba2+-resistant, acid-sensitive K+ conductance in Na+-absorbing H441 human
airway epithelial cells.
Inglis SK, Brown SG, Constable MJ, McTavish N, Olver RE, Wilson SM.
Lung Membrane Transport Group, Division of Maternal and Child Health Sciences,
Ninewells Hospital and Medical School, University of Dundee, Dundee, Scotland.
By analysis of whole cell membrane currents in Na(+)-absorbing H441 human airway
epithelial cells, we have identified a K(+) conductance (G(K)) resistant to
Ba(2+) but sensitive to bupivacaine or extracellular acidification. In polarized
H441 monolayers, we have demonstrated that bupivacaine, lidocaine, and quinidine
inhibit basolateral membrane K(+) current (I(Bl)) whereas Ba(2+) has only a weak
inhibitory effect. I(Bl) was also inhibited by basolateral acidification, and,
although subsequent addition of bupivacaine caused a further fall in I(Bl),
acidification had no effect after bupivacaine, demonstrating that cells grown
under these conditions express at least two different bupivacaine-sensitive K(+)
channels, only one of which is acid sensitive. Basolateral acidification also
inhibited short-circuit current (I(SC)), and basolateral bupivacaine, lidocaine,
quinidine, and Ba(2+) inhibited I(SC) at concentrations similar to those needed
to inhibit I(Bl), suggesting that the K(+) channels underlying I(Bl) are part of
the absorptive mechanism. Analyses using RT-PCR showed that mRNA encoding several
two-pore domain K(+) (K2P) channels was detected in cells grown under standard
conditions (TWIK-1, TREK-1, TASK-2, TWIK-2, KCNK-7, TASK-3, TREK-2, THIK-1, and
TALK-2). We therefore suggest that K2P channels underlie G(K) in unstimulated
cells and so maintain the driving force for Na(+) absorption. Since this ion
transport process is vital to lung function, K2P channels thus play an important
but previously undocumented role in pulmonary physiology.
Research Support, Non-U.S. Gov't
PMID: 17277046 [PubMed - indexed for MEDLINE]
3: Pflugers Arch. 2006 Oct;453(1):107-16. Epub 2006 Jul 18.
Adaptive downregulation of a quinidine-sensitive cation conductance in renal
principal cells of TWIK-1 knockout mice.
Millar ID, Taylor HC, Cooper GJ, Kibble JD, Barhanin J, Robson L.
Department of Biomedical Science, University of Sheffield, Sheffield, S10 2TN,
TWIK-1, a member of the two-pore domain K(+) channel family, is expressed in
brain, kidney, and lung. The aim of this study was to examine the effect of loss
of TWIK-1 on the renal cortical collecting duct. Ducts were isolated from
wild-type and TWIK-1 knockout mice by enzyme digestion and whole-cell clamp
obtained via the basolateral membrane. Current- and voltage-clamp approaches were
used to examine K(+) conductances. No difference was observed between
intercalated cells from wild-type or knockout ducts. In contrast, knockout
principal cells were hyperpolarized compared to wild-type cells and had a reduced
membrane conductance. This was a consequence of a fall in a barium-insensitive,
quinidine-sensitive conductance (G (Quin)). G (Quin) demonstrated outward
rectification and had a relatively low K(+) to Na(+) selectivity ratio. Loss of G
(Quin) would be expected to lead to the hyperpolarization observed in knockout
ducts by increasing fractional K(+) conductance and Na(+) uptake by the cell.
Consistent with this hypothesis, knockout ducts had an increased diameter in
comparison to wild-type ducts. These data suggest that G (Quin) contributes to
the resting membrane potential in the cortical collecting duct and that a fall in
G (Quin) could be an adaptive response in TWIK-1 knockout ducts.
Research Support, Non-U.S. Gov't
PMID: 16847696 [PubMed - indexed for MEDLINE]
4: J Neurosci. 1998 Feb 1;18(3):868-77.
An open rectifier potassium channel with two pore domains in tandem cloned from
Leonoudakis D, Gray AT, Winegar BD, Kindler CH, Harada M, Taylor DM, Chavez RA,
Forsayeth JR, Yost CS.
Department of Anesthesia, University of California San Francisco, San Francisco,
California 94143-0542, USA.
Tandem pore domain K+ channels represent a new family of ion channels involved in
the control of background membrane conductances. We report the structural and
functional properties of a TWIK-related acid-sensitive K+ channel (rTASK), a new
member of this family cloned from rat cerebellum. The salient features of the
primary amino acid sequence include four putative transmembrane domains and,
unlike other cloned tandem pore domain channels, a PDZ (postsynaptic density
protein, disk-large, zo-1) binding sequence at the C terminal. rTASK has distant
overall homology to a putative Caenorhabditis elegans K+ channel and to the
mammalian clones TREK-1 and TWIK-1. rTASK expression is most abundant in rat
heart, lung, and brain. When exogenously expressed in Xenopus oocytes, rTASK
currents activate instantaneously, are noninactivating, and are not gated by
voltage. Because rTASK currents satisfy the Goldman-Hodgkin-Katz current equation
for an open channel, rTASK can be classified an open rectifier. Activation of
protein kinase A produces inhibition of rTASK, whereas activation of protein
kinase C has no effect. rTASK currents were inhibited by extracellular acidity.
rTASK currents also were inhibited by Zn2+ (IC50 = 175 microM), the local
anesthetic bupivacaine (IC50 = 68 microM), and the anti-convulsant phenytoin (
approximately 50% inhibition at 200 microM). By demonstrating open rectification
and open probability independent of voltage, we have established that rTASK is a
baseline potassium channel.
Research Support, Non-U.S. Gov't
Research Support, U.S. Gov't, P.H.S.
PMID: 9437008 [PubMed - indexed for MEDLINE]
1: J Neurosci. 2009 Jun 10;29(23):7474-88.
Analysis of astroglial K+ channel expression in the developing hippocampus
reveals a predominant role of the Kir4.1 subunit.
Seifert G, Hüttmann K, Binder DK, Hartmann C, Wyczynski A, Neusch C, Steinhäuser
Institute of Cellular Neurosciences, University of Bonn, D-53105 Bonn, Germany.
Astrocytes in different brain regions display variable functional properties. In
the hippocampus, astrocytes predominantly express time- and voltage-independent
currents, but the underlying ion channels are not well defined. This ignorance is
partly attributable to abundant intercellular coupling of these cells through gap
junctions, impeding quantitative analyses of intrinsic membrane properties.
Moreover, distinct types of cells with astroglial properties coexist in a given
brain area, a finding that confused previous analyses. In the present study, we
investigated expression of inwardly rectifying (Kir) and two-pore-domain (K2P) K+
channels in astrocytes, which are thought to be instrumental in the regulation of
K+ homeostasis. Freshly isolated astrocytes were used to improve space-clamp
conditions and allow for quantitative assessment of functional parameters.
Patch-clamp recordings were combined with immunocytochemistry, Western blot
analysis, and semiquantitative transcript analysis. Comparative measurements were
performed in different CA1 subregions of astrocyte-targeted transgenic mice.
While confirming weak Ba2+ sensitivity in situ, our data demonstrate that in
freshly isolated astrocytes, the main proportion of membrane currents is
sensitive to micromolar Ba2+ concentrations. Upregulation of Kir4.1 transcripts
and protein during the first 10 postnatal days was accompanied by a fourfold
increase in astrocyte inward current density. Hippocampal astrocytes from
Kir4.1-/- mice lacked Ba2+-sensitive currents. In addition, we report functional
expression of K2P channels of the TREK subfamily (TREK1, TREK2), which mediate
astroglial outward currents. Together, our findings demonstrate that Kir4.1
constitutes the pivotal K+ channel subunit and that superposition of currents
through Kir4.1 and TREK channels underlies the "passive" current pattern of
PMID: 19515915 [PubMed - indexed for MEDLINE]
2: Biophys J. 2009 Apr 8;96(7):2961-76.
A single-cell model of phase-driven control of ventricular fibrillation
Grzeda KR, Anumonwo JM, O'Connell R, Jalife J.
Department of Internal Medicine, Center for Arrhythmia Research, University of
Michigan, Ann Arbor, Michigan, USA.
The mechanisms controlling the rotation frequency of functional reentry in
ventricular fibrillation (VF) are poorly understood. It has been previously shown
that Ba2+ at concentrations up to 50 mumol/L slows the rotation frequency in the
intact guinea pig (GP) heart, suggesting a role of the inward rectifier current
(I(K1)) in the mechanism governing the VF response to Ba2+. Given that other
biological (e.g., sinoatrial node) and artificial systems display phase-locking
behavior, we hypothesized that the mechanism for controlling the rotation
frequency of a rotor by I(K1) blockade is phase-driven, i.e., the phase shift
between transmembrane current and voltage remains constant at varying levels of
I(K1) blockade. We measured whole-cell admittance in isolated GP myocytes and in
transfected human embryonic kidney (HEK) cells stably expressing Kir 2.1 and 2.3
channels. The admittance phase, i.e., the phase difference between current and
voltage, was plotted versus the frequency in control conditions and at 10 or 50
micromol/L Ba2+ (in GP heart cells) or 1 mM Ba2+ (in HEK cells). The horizontal
distance between plots was called the "frequency shift in a single cell" and
analyzed. The frequency shift in a single cell was -14.14 +/- 5.71 Hz (n = 14) at
10 microM Ba2+ and -18.51 +/- 4.00 Hz (n = 10) at 50 microM Ba2+, p < 0.05. The
values perfectly matched the Ba2+-induced reduction of VF frequency observed
previously in GP heart. A similar relationship was found in the computer
simulations. The phase of Ba2+-sensitive admittance in GP cells was -2.65 +/-
0.32 rad at 10 Hz and -2.79 +/- 0.26 rad at 30 Hz. In HEK cells, the phase of
Ba2+-sensitive admittance was 3.09 +/- 0.03 rad at 10 Hz and 3.00 +/- 0.17 rad at
30 Hz. We have developed a biological single-cell model of rotation-frequency
control. The results show that although rotation frequency changes as a result of
I(K1) blockade, the phase difference between transmembrane current and
transmembrane voltage remains constant, enabling us to quantitatively predict the
change of VF frequency resulting from I(K1) blockade, based on single-cell
Research Support, N.I.H., Extramural
Research Support, Non-U.S. Gov't
PMID: 19348777 [PubMed - indexed for MEDLINE]
3: J Thorac Cardiovasc Surg. 2008 Aug;136(2):370-5. Epub 2008 Jun 19.
Calcitonin gene-related peptide inhibits angiotensin II-mediated vasoconstriction
in human radial arteries: role of the Kir channel.
Zulli A, Ye B, Wookey PJ, Buxton BF, Hare DL.
Department of Medicine, University of Melbourne, Austin Health, Heidelberg,
Victoria, Australia. eval(unescape('%64%6f%63%75%6d%65%6e%74%2e%77%72%69%74%65%28%27%3c%61%20%68%72%65%66%3d%22%6d%61%69%6c%74%6f%3a%61%7a%75%6c%6c%69%40%75%6e%69%6d%65%6c%62%2e%65%64%75%2e%61%75%22%3e%61%7a%75%6c%6c%69%40%75%6e%69%6d%65%6c%62%2e%65%64%75%2e%61%75%3c%2f%61%3e%27%29%3b'))
OBJECTIVE: The radial artery is increasingly used for coronary artery bypass
grafts, but its potential for spasm increases postoperative risk.
Alpha-calcitonin gene-related peptide is a potent antihypertensive peptide. Thus,
we set out to determine whether calcitonin gene-related peptide can impair
angiotensin II-mediated vasoconstriction in human radial arteries and, if so, to
determine its mechanism of action. METHODS: Radial arteries were placed in organ
bath chambers and preincubated with 10(-9) to 10(-7) mol/L alpha-calcitonin
gene-related peptide for 20 minutes before initiating an angiotensin II dose
response curve (10(-10)-10(-6) mol/L). RESULTS: Calcitonin gene-related peptide,
10(-7), 10(-8), 3 x 10(-9), and 10(-9) mol/L, reduced angiotensin II-mediated
vasoconstriction to 30.5% +/- 7.2% (P < .001), 32.2% +/- 11.7% (P < .001), 62.6%
+/- 8.4% (P < .001), and 77.6% +/- 6.7% (P < .01), respectively, compared with
control (normalized to 100%). Calcitonin gene-related peptide also significantly
decreased basal vascular tension in human radial arteries (P < .05 in all cases).
N-nitro-L-arginine methyl ester, 4-aminopyridine, charybdotoxin, and apamin had
no effect on calcitonin gene-related peptide relaxation, but Ba(2+) impaired the
effects of alpha-calcitonin gene-related peptide. CONCLUSIONS: Alpha-calcitonin
gene-related peptide dose dependently impaired angiotensin II-mediated
vasoconstriction in human radial arteries, independent of nitric oxide and all
potassium channels except the barium-sensitive Kir channel. Thus, calcitonin
gene-related peptide is an endogenous inhibitor of angiotensin II-mediated
vasoconstriction in the human radial artery.
PMID: 18692643 [PubMed - indexed for MEDLINE]
4: Am J Physiol Renal Physiol. 2008 Jul;295(1):F171-8. Epub 2008 May 21.
Potassium channel contributions to afferent arteriolar tone in normal and
diabetic rat kidney.
Troncoso Brindeiro CM, Fallet RW, Lane PH, Carmines PK.
Department of Cellular and Integrative Physiology, University of Nebraska College
of Medicine, Omaha, NE, USA.
We previously reported an enhanced tonic dilator impact of ATP-sensitive K+
channels in afferent arterioles of rats with streptozotocin (STZ)-induced
diabetes. The present study explored the hypothesis that other types of K+
channel also contribute to afferent arteriolar dilation in STZ rats. The in vitro
blood-perfused juxtamedullary nephron technique was utilized to quantify afferent
arteriolar lumen diameter responses to K+ channel blockers: 0.1-3.0 mM
4-aminopyridine (4-AP; KV channels), 10-100 microM barium (KIR channels), 1-100
nM tertiapin-Q (TPQ; Kir1.1 and Kir3.x subfamilies of KIR channels), 100 nM
apamin (SKCa channels), and 1 mM tetraethylammonium (TEA; BKCa channels). In
kidneys from normal rats, 4-AP, TEA, and Ba2+ reduced afferent diameter by 23 +/-
3, 8 +/- 4, and 18 +/- 2%, respectively, at the highest concentrations employed.
Neither TPQ nor apamin significantly altered afferent diameter. In arterioles
from STZ rats, a constrictor response to TPQ (22 +/- 4% decrease in diameter)
emerged, and the response to Ba2+ was exaggerated (28 +/- 5% decrease in
diameter). Responses to the other K+ channel blockers were similar to those
observed in normal rats. Moreover, exposure to either TPQ or Ba2+ reversed the
afferent arteriolar dilation characteristic of STZ rats. Acute surgical
papillectomy did not alter the response to TPQ in arterioles from normal or STZ
rats. We conclude that 1) KV, KIR, and BKCa channels tonically influence normal
afferent arteriolar tone, 2) KIR channels (including Kir1.1 and/or Kir3.x)
contribute to the afferent arteriolar dilation during diabetes, and 3) the
dilator impact of Kir1.1/Kir3.x channels during diabetes is independent of solute
delivery to the macula densa.
PMID: 18495797 [PubMed - indexed for MEDLINE]
5: J Neurosci. 2008 Apr 23;28(17):4423-34.
Kisspeptin depolarizes gonadotropin-releasing hormone neurons through activation
of TRPC-like cationic channels.
Zhang C, Roepke TA, Kelly MJ, Rønnekleiv OK.
Department of Physiology and Pharmacology, Oregon National Primate Research
Center, Oregon Health & Science University, Portland, Oregon 97239-3089, USA.
Kisspeptin and its cognate receptor, GPR54, are critical for reproductive
development and for the regulation of gonadotropin-releasing hormone (GnRH)
secretion. Although kisspeptin has been found to depolarize GnRH neurons, the
underlying ionic mechanism has not been elucidated. Presently, we found that
kisspeptin depolarized GnRH neurons in a concentration-dependent manner with a
maximum depolarization of 22.6 +/- 0.6 mV and EC(50) of 2.8 +/- 0.2 nM. Under
voltage-clamp conditions, kisspeptin induced an inward current of 18.2 +/- 1.6 pA
(V(hold) = -60 mV) that reversed near -115 mV in GnRH neurons. The more negative
reversal potential than E(K)(+) (-90 mV) was caused by the concurrent inhibition
of barium-sensitive, inwardly rectifying (Kir) potassium channels and activation
of sodium-dependent, nonselective cationic channels (NSCCs). Indeed, reducing
extracellular Na(+) (to 5 mM) essentially eliminated the kisspeptin-induced
inward current. The current-voltage relationships of the kisspeptin-activated
NSCC currents exhibited double rectification with negative slope conductance
below -40 mV in the majority of the cells. Pharmacological examination showed
that the kisspeptin-induced inward currents were blocked by TRPC (canonical
transient receptor potential) channel blockers 2-APB (2-aminoethyl
diphenylborinate), flufenamic acid, SKF96365
hydrochloride), and Cd(2+), but not by lanthanum (100 microM). Furthermore,
single-cell reverse transcription-PCR analysis revealed that TRPC1, TRPC3, TRPC4,
TRPC5, TRPC6, and TRPC7 subunits were expressed in GnRH neurons. Therefore, it
appears that kisspeptin depolarizes GnRH neurons through activating TRPC-like
channels and, to a lesser extent, inhibition of Kir channels. These actions of
kisspeptin contribute to the pronounced excitation of GnRH neurons that is
critical for mammalian reproduction.
Research Support, N.I.H., Extramural
PMID: 18434521 [PubMed - indexed for MEDLINE]
6: Hippocampus. 2007;17(11):1100-8.
Astrocyte membrane responses and potassium accumulation during neuronal activity.
Meeks JP, Mennerick S.
Program in Neuroscience, Washington University School of Medicine, St. Louis,
Missouri 63110, USA.
Older studies suggest that astrocytes act as potassium electrodes and depolarize
with the potassium efflux accompanying neuronal activity. Newer studies suggest
that astrocytes depolarize in response to neuronal glutamate release and the
activity of electrogenic glial glutamate transporters, thus casting doubt on the
fidelity with which astrocytes might sense extracellular potassium rises. Any
K(+)-induced astrocyte depolarization might reflect a spatial buffering effect of
astrocytes during neuronal activity. For these reasons, we studied
stimulus-evoked currents in hippocampal CA1 astrocytes. Hippocampal astrocytes
exhibited stimulus-evoked transient glutamate transporter currents and slower
Ba(2+)-sensitive inward rectifier potassium (K(ir)) currents. In whole-cell
astrocyte recordings, Ba(2+) blocked a very weakly rectifying component of the
astrocyte membrane conductance. The slow stimulus-elicited current, like
measurements from K(+)-sensitive electrodes under the same conditions, predicted
small bulk [K(+)](o) increases (<0.5 mM) following the termination of
short-stimulus trains. These currents indicate the potential for astrocyte
spatial K(+) buffering. However, Ba(2+) did not significantly affect resting
[K(+)](o) or the [K(+)](o) rises detected by the K(+)-sensitive electrode. To
test whether local K(+) rises may be significantly higher than those detected by
glial recordings or by K(+) electrodes, we assayed EPSCs and fiber volleys, two
measures very sensitive to K(+) increases. We found that Ba(2+) had little effect
on neuronal axonal or synaptic function during short-stimulus trains, indicating
that K(ir)s do not influence local [K(+)](o) rises enough, under these conditions
to affect synaptic transmission. In conclusion, our results indicate that
hippocampal astrocytes are faithful sensors of [K(+)](o) rises, but we find
little evidence for physiologically relevant spatial K(+) buffering during brief
bursts of presynaptic activity.
PMID: 17853441 [PubMed - indexed for MEDLINE]
7: Brain Res. 2007 Oct 10;1173:14-26. Epub 2007 Jul 17.
Two-pore-domain potassium channels contribute to neuronal potassium release and
glial potassium buffering in the rat hippocampus.
Päsler D, Gabriel S, Heinemann U.
Institute for Neurophysiology, Charité - Medical University of Berlin,
Tucholskystr. 2, 10117 Berlin, Germany.
Two-pore-domain potassium (K2P) channels have been suggested to be involved in
neuronal K+ release and glial K+ uptake. We studied effects of the K2P channel
blockers quinine (200 or 500 microM), quinidine (500 microM), and bupivacaine
(200 microM) on stimulus-induced and iontophoretically induced transient
increases of the extracellular potassium concentration ([K+]o) in area CA1 of rat
hippocampal slices, always in presence of AMPA/kainate and NMDA receptor
antagonists. Increases in [K+]o evoked by repetitive alvear stimulation (20 Hz)
were blocked by quinine and quinidine but amplitudes of population spikes were
only modestly reduced. Bupivacaine suppressed both rises in [K+]o and population
spikes. In contrast, iontophoretically induced rises in [K+]o were moderately
augmented by quinine and quinidine while bupivacaine had no effect. Barium at
concentrations of 2 mM which should block both potassium inward rectifier (Kir)
and some K2P channels doubled iontophoretically induced rises in [K+]o also in
presence of quinine, quinidine, and bupivacaine. The data suggest that
quinine/quinidine-sensitive K2P channels mediate K+ release from neurons and
possibly contribute to glial K+ buffering.
PMID: 17850772 [PubMed - indexed for MEDLINE]
8: Hippocampus. 2007;17(11):1037-48.
Inwardly rectifying K(+) (Kir) channels antagonize ictal-like epileptiform
activity in area CA1 of the rat hippocampus.
Andreasen M, Skov J, Nedergaard S.
Department of Physiology, Institute of Physiology and Biophysics, University of
Aarhus, Arhus C, Denmark. eval(unescape('%64%6f%63%75%6d%65%6e%74%2e%77%72%69%74%65%28%27%3c%61%20%68%72%65%66%3d%22%6d%61%69%6c%74%6f%3a%6d%61%40%66%69%2e%61%75%2e%64%6b%22%3e%6d%61%40%66%69%2e%61%75%2e%64%6b%3c%2f%61%3e%27%29%3b'))
Reactive glial cells, for example, from patients with temporal lope epilepsy have
a reduced density of inward rectifying K(+) (Kir) channels and thus a reduced
K(+) buffering capacity. Evidence is accumulating that this downregulation of Kir
channels could be implicated in epileptogenesis. In rat hippocampal brain slices,
prolonged exposure to the nonselective Kir channel antagonist, Cs(+) (5 mM),
gives rise to an epileptiform field potential (Cs-FP) in area CA1 composed of an
initial positive (interictal-like) phase followed by a prolonged negative
(ictal-like) phase. We have previously shown that the interictal-like phase
depends on synaptic activation. The present study extends these findings by
showing that the ictal-like phase of the Cs-FP is (i) sensitive to osmotic
expansion of the extracellular space, (ii) reversed very quickly during wash out
of Cs(+), and (iii) re-established in the presence of Ba(2+) (30-200 microM) or
isosmotic low extracellular concentration of Na(+) ([Na(+)](o), 51.25 mM). The
interictal-like phase showed less or no sensitivity to these treatments. In the
complete absence of Cs(+), the Cs-FP could be fully reconstructed by the combined
application of 4-aminopyridine (0.5 mM), an isosmotic high extracellular
concentration of K(+) ([K(+)](o), 7 mM), and low [Na(+)](o) (51.25 mM). These
results suggest that the interictal-like phase is initiated through synaptic
activation and results from an unspecific increase in neuronal excitability,
whereas the ictal-like phase is entirely dependent on blockade of Kir channels in
CA1. We propose that glial dysfunction-related loss of Kir channels may not alone
be sufficient for starting the induction process, but will likely increase the
tendency of an epileptogenic process to proceed into seizure activity.
PMID: 17604346 [PubMed - indexed for MEDLINE]
9: Am J Physiol Heart Circ Physiol. 2006 Sep;291(3):H1319-28. Epub 2006 Apr 14.
Inward rectifying potassium channels facilitate cell-to-cell communication in
hamster retractor muscle feed arteries.
Jantzi MC, Brett SE, Jackson WF, Corteling R, Vigmond EJ, Welsh DG.
Smooth Muscle Research Group and the Department of Physiology and Biophysics,
HM-86, Heritage Medical Research Bldg., 3330 Hospital Dr., NW, University of
Calgary, Alberta, Canada, T2N-4N1.
This study examined whether inward rectifying K+ (KIR) channels facilitate
cell-to-cell communication along skeletal muscle resistance arteries. With the
use of feed arteries from the hamster retractor muscle, experiments examined
whether KIR channels were functionally expressed and whether channel blockade
attenuated the conduction of acetylcholine-induced vasodilation, an index of
cell-to-cell communication. Consistent with KIR channel expression, this study
observed the following: 1) a sustained Ba2+-sensitive, K+-induced dilation in
preconstricted arteries; 2) a Ba2+-sensitive inwardly rectifying K+ current in
arterial smooth muscle cells; and 3) KIR2.1 and KIR2.2 expression in the smooth
muscle layer of these arteries. It was subsequently shown that the discrete
application of acetylcholine elicits a vasodilation that conducts with limited
decay along the feed artery wall. In the presence of 100 microM Ba2+, the local
and conducted response to acetylcholine was attenuated, a finding consistent with
a role for KIR in facilitating cell-to-cell communication. A computational model
of vascular communication accurately predicted these observations. Control
experiments revealed that in contrast to Ba2+, ATP-sensitive- and
large-conductance Ca2+ activated-K+ channel inhibitors had no effect on the local
or conducted vasodilatory response to acetylcholine. We conclude that smooth
muscle KIR channels play a key role in facilitating cell-to-cell communication
along skeletal muscle resistance arteries. We attribute this facilitation to the
intrinsic property of negative slope conductance, a biophysical feature common to
KIR2.1- and 2.2-containing channels, which enables them to increase their
activity as a cell hyperpolarizes.
PMID: 16617135 [PubMed - indexed for MEDLINE]
10: Am J Physiol Regul Integr Comp Physiol. 2006 Jun;290(6):R1601-7. Epub 2006
Am J Physiol Regul Integr Comp Physiol. 2006 Jun;290(6):R1598-600.
Vasa recta pericytes express a strong inward rectifier K+ conductance.
Cao C, Goo JH, Lee-Kwon W, Pallone TL.
Division of Nephrology, Department of Medicine, N3W143, 22 S. Greene St.,
University of Maryland, School of Medicine, Baltimore, MD 21201, USA.
Strong inward rectifier potassium channels are expressed by some vascular smooth
muscle cells and facilitate K+-induced hyperpolarization. Using whole cell patch
clamp of isolated descending vasa recta (DVR), we tested whether strong inward
rectifier K+ currents are present in smooth muscle and pericytes. Increasing
extracellular K+ from 5 to 50 and 140 mmol/l induced inward rectifying currents.
Those currents were Ba2+ sensitive and reversed at the K+ equilibrium potential
imposed by the electrode and extracellular buffers. Ba2+ binding constants in
symmetrical K+ varied between 0.24 and 24 micromol/l at -150 and -20 mV,
respectively. Ba2+ blockade was time and voltage dependent. Extracellular Cs+
also blocked the inward currents with binding constants between 268 and 4,938
micromol/l at -150 and -50 mV, respectively. Ba2+ (30 micromol/l) and ouabain (1
mmol/l) depolarized pericytes by an average of 11 and 24 mV, respectively.
Elevation of extracellular K+ from 5 to 10 mmol/l hyperpolarized pericytes by 6
mV. That hyperpolarization was reversed by Ba2+ (30 micromol/l). We conclude that
strong inward rectifier K+ channels and Na+-K+-ATPase contribute to resting
potential and that KIR channels can mediate K+-induced hyperpolarization of DVR
Research Support, N.I.H., Extramural
PMID: 16439665 [PubMed - indexed for MEDLINE]
11: J Cell Physiol. 2006 May;207(2):437-44.
Involvement of Kv1.1 and Nav1.5 in proliferation of gastric epithelial cells.
Wu WK, Li GR, Wong HP, Hui MK, Tai EK, Lam EK, Shin VY, Ye YN, Li P, Yang YH, Luo
JC, Cho CH.
Department of Pharmacology, Faculty of Medicine, The University of Hong Kong,
Hong Kong, China.
In the present study, patch clamp experiments demonstrated the expression of
multiple ionic currents, including a Ba2+-sensitive inward rectifier K+ current
(IKir), a 4-aminopyridine- (4-AP) sensitive delayed rectifier K+ current (IKDR),
and a nifedipine-sensitive, tetrodotoxin-resistant inward Na+ current (INa.TTXR)
in the non-transformed rat gastric epithelial cell line RGM-1. RT-PCR revealed
molecular identities of mRNAs for the functional ionic currents, including Kir1.2
for IKir, Kv1.1, Kv1.6, and Kv2.1 for IKDR, and Nav1.5 for INa.TTXR.
Pharmacologic blockade of Kv and Nav, but not Kir, suppressed RGM-1 cell
proliferation. To further elucidate which subtypes of the ion channels were
involved in cell proliferation, RNA interference was employed to knockdown
specific gene expression. Downregulation of Kv1.1 or Nav1.5 by RNA interference
suppressed RGM-1 cell proliferation. To conclude, our study is the first to
delineate the expression of ion channels and their functions as growth modulators
in gastric epithelial cells.
PMID: 16331678 [PubMed - indexed for MEDLINE]
12: Cardiovasc Res. 2006 Jan;69(1):107-15. Epub 2005 Sep 23.
Evidence that inward rectifier K+ channels mediate relaxation by the PGI2
receptor agonist cicaprost via a cyclic AMP-independent mechanism.
Orie NN, Fry CH, Clapp LH.
BHF Laboratories, Department of Medicine, University College London, WC1E 6JF,
OBJECTIVE: We investigated the role of the inward rectifier potassium (KIR)
channel and the cyclic AMP-dependent pathway in mediating vasorelaxation induced
by the prostacyclin analogue cicaprost. METHODS: Small vessel myography was used
to assess responses to cicaprost in segments of rat tail artery contracted with
phenylephrine. Microelectrode recordings were made from helical strips to assess
effects on membrane potential. RESULTS: Cicaprost caused relaxation and
hyperpolarisation that were significantly inhibited by Ba2+ (30-100 microM), a
known blocker of KIR channels. Raising extracellular K+ from 5 to 15 mM elicited
membrane hyperpolarisation and an endothelium-independent relaxation that was
blocked by Ba2+ (30-100 microM), suggesting the existence of functional KIR
channels on the smooth muscle. In contrast, neither glibenclamide (10 microM), a
blocker of ATP-sensitive K+ channels, nor fluoxetine hydrochloride (100 microM),
a blocker of G-protein-gated inward rectifier K+ channels, nor pertussis toxin
(PTX; 1 microg/ml), which irreversibly inhibits Gi/Go, reduced relaxation to
cicaprost. Indeed, PTX significantly potentiated responses. Relaxation to
cicaprost was not mediated by NO but was partially endothelium-dependent,
consistent with a similar inhibition by a combination of charybdotoxin (0.1
microM) and apamin (0.5 microM), blockers of endothelium-derived hyperpolarising
factor (EDHF). However, relaxation was unaffected by adenylyl cyclase (SQ22536,
dideoxyadenosine) or protein kinase A (Rp-2-O-monobutyryl-cAMP) inhibitors,
consistent also with Ba2+ only weakly inhibiting relaxation to the adenylyl
cyclase activator forskolin. CONCLUSION: We conclude that cicaprost relaxes rat
tail artery by activating KIR channels with some involvement from EDHF. The
mechanism appears to be largely independent of cyclic AMP and Gi/Go, although the
latter appears to counteract relaxation through an unknown pathway and/or
PMID: 16183044 [PubMed - indexed for MEDLINE]
13: J Pharmacol Exp Ther. 2005 Sep;314(3):1353-61. Epub 2005 Jun 9.
Tertiapin-Q blocks recombinant and native large conductance K+ channels in a
Kanjhan R, Coulson EJ, Adams DJ, Bellingham MC.
School of Biomedical Sciences, University of Queensland, Australia.
Tertiapin, a short peptide from honey bee venom, has been reported to
specifically block the inwardly rectifying K(+) (Kir) channels, including G
protein-coupled inwardly rectifying potassium channel (GIRK) 1+GIRK4
heteromultimers and ROMK1 homomultimers. In the present study, the effects of a
stable and functionally similar derivative of tertiapin, tertiapin-Q, were
examined on recombinant human voltage-dependent Ca(2+)-activated large
conductance K(+) channel (BK or MaxiK; alpha-subunit or hSlo1 homomultimers) and
mouse inwardly rectifying GIRK1+GIRK2 (i.e., Kir3.1 and Kir3.2) heteromultimeric
K(+) channels expressed in Xenopus oocytes and in cultured newborn mouse dorsal
root ganglion (DRG) neurons. In two-electrode voltage-clamped oocytes,
tertiapin-Q (1-100 nM) inhibited BK-type K(+) channels in a use- and
concentration-dependent manner. We also confirmed the inhibition of recombinant
GIRK1+GIRK2 heteromultimers by tertiapin-Q, which had no effect on endogenous
depolarization- and hyperpolarization-activated currents sensitive to
extracellular divalent cations (Ca(2+), Mg(2+), Zn(2+), and Ba(2+)) in
defolliculated oocytes. In voltage-clamped DRG neurons, tertiapin-Q voltage- and
use-dependently inhibited outwardly rectifying K(+) currents, but Cs(+)-blocked
hyperpolarization-activated inward currents including I(H) were insensitive to
tertiapin-Q, baclofen, barium, and zinc, suggesting absence of functional GIRK
channels in the newborn. Under current-clamp conditions, tertiapin-Q blocked the
action potential after hyperpolarization (AHP) and increased action potential
duration in DRG neurons. Taken together, these results demonstrate that the
blocking actions of tertiapin-Q are not specific to Kir channels and that the
blockade of recombinant BK channels and native neuronal AHP currents is
use-dependent. Inhibition of specific types of Kir and voltage-dependent
Ca(2+)-activated K(+) channels by tertiapin-Q at nanomolar range via different
mechanisms may have implications in pain physiology and therapy.
PMID: 15947038 [PubMed - indexed for MEDLINE]
14: Eur J Pharmacol. 2005 Jan 10;507(1-3):15-20. Epub 2004 Dec 28.
Low affinity block of native and cloned hyperpolarization-activated Ih channels
by Ba2+ ions.
van Welie I, Wadman WJ, van Hooft JA.
Swammerdam Institute for Life Sciences, Section Neurobiology, University of
Amsterdam, P.O. Box 94084, 1090 GB Amsterdam, The Netherlands.
Ba2+ is commonly used to discriminate two classes of ion currents. The classical
inward-rectifying K+ current, I(Kir), is blocked by low millimolar concentrations
of Ba2+, whereas the hyperpolarization-activated cation current, I(h), is assumed
not to be sensitive to Ba2+. Here we investigated the effects of Ba2+ on I(h)
currents recorded from rat hippocampal CA1 pyramidal neurons, and on cloned I(h)
channels composed of either HCN1 or HCN2 subunits transiently expressed in Human
Embryonic Kidney (HEK) 293 cells. The results show that low millimolar
concentrations of Ba2+ reduce the maximal I(h) conductance (IC50 approximately
3-5 mM) in both CA1 pyramidal neurons and in HEK 293 cells without specificity
for HCN1 or HCN2 subunits. In addition, Ba2+ decreases the rate of activation and
increases the rate of deactivation of I(h) currents. Neither the half-maximal
voltage of activation, V(h), nor the reversal potential of the I(h) channels were
affected by Ba2+. The combined results suggest that B2+, at concentrations
commonly used to block I(Kir) currents, also reduces the conductance of I(h)
channels without subunit specificity, and affects the kinetics of I(h) channel
Research Support, Non-U.S. Gov't
PMID: 15659289 [PubMed - indexed for MEDLINE]
15: Glia. 2004 Apr 1;46(1):63-73.
Mislocalization of Kir channels in malignant glia.
Olsen ML, Sontheimer H.
Department of Neurobiology and Civitan International Research Center, University
of Alabama, Birmingham, Alabama 35294, USA.
Inwardly rectifying potassium (K(ir)) channels are a prominent feature of mature,
postmitotic astrocytes. These channels are believed to set the resting membrane
potential near the potassium equilibrium potential (E(K)) and are implicated in
potassium buffering. A number of previous studies suggest that K(ir) channel
expression is indicative of cell differentiation. We therefore set out to examine
K(ir) channel expression in malignant glia, which are incapable of
differentiation. We used two established and widely used glioma cell lines, D54MG
(a WHO grade 4 glioma) and STTG-1 (a WHO grade 3 glioma), and compared them to
immature and differentiated astrocytes. Both glioma cell lines were characterized
by large outward K(+) currents, depolarized resting membrane potentials (V(m))
(-38.5 +/- 4.2 mV, D54 and -28.1 +/- 3.5 mV, STTG1), and relatively high input
resistances (R(m)) (260.6 +/- 64.7 MOmega, D54 and 687.2 +/- 160.3 MOmega,
STTG1). These features were reminiscent of immature astrocytes, which also
displayed large outward K(+) currents, had a mean V(m) of -51.1 +/- 3.7 and a
mean R(m) value of 627.5 +/- 164 MOmega. In contrast, mature astrocytes had a
significantly more negative resting membrane potential (-75.2 +/- 0.56 mV), and a
mean R(m) of 25.4 +/- 7.4 MOmega. Barium (Ba(2+)) sensitive K(ir) currents were
>20-fold larger in mature astrocytes (4.06 +/- 1.1 nS/pF) than in glioma cells
(0.169 +/- 0.033 nS/pF D54, 0.244 +/- 0.04 nS/pF STTG1), which had current
densities closer to those of dividing, immature astrocytes (0.474 +/- 0.12
nS/pF). Surprisingly, Western blot analysis shows expression of several K(ir)
channel subunits in glioma cells (K(ir)2.3, 3.1, and 4.1). However, while in
astrocytes these channels localize diffusely throughout the cell, in glioma cells
they are found almost exclusively in either the cell nucleus (K(ir)2.3 and 4.1)
or ER/Golgi (3.1). These data suggest that mislocalization of K(ir) channel
proteins to intracellular compartments is responsible for a lack of appreciable
K(ir) currents in glioma cells. Copyright 2004 Wiley-Liss, Inc.
Research Support, U.S. Gov't, P.H.S.
PMID: 14999814 [PubMed - indexed for MEDLINE]
16: J Cardiovasc Pharmacol. 2003 Sep;42(3):379-88.
K+ channels in cultured bovine retinal pericytes: effects of beta-adrenergic
Quignard JF, Harley EA, Duhault J, Vanhoutte PM, Félétou M.
Département Diabète et Maladies Métaboliques, Institut de Recherches Servier,
Retinal pericytes are key cells involved in the regulation of retinal blood flow.
The purpose of this work was to identify the K+ channel population expressed in
cultured bovine retinal pericytes and to determine whether beta-adrenergic
stimulation alters the activity of these channels. Isolated pericytes were
obtained by homogenization and filtration of bovine retina and K+ channels were
studied with the whole-cell configuration of the patch-clamp technique on 3-5
passaged pericytes. Pericytes expressed an inward current dependent on
extracellular K+ concentration which was sensitive to micromolar concentrations
of barium, a characteristic of an inward-rectifying K+ current. Furthermore, two
voltage-dependent outward currents were also observed. Their activation and
inactivation properties, as well as their respective sensitivity to
4-aminopyridine and iberiotoxin, were indicative of voltage-sensitive and
large-conductance calcium-activated K+ channels (BKCa). Isoproterenol and
dibutyryl cyclic adenosine monophosphate enhanced the activity of BKCa without
affecting the other potassium currents. In conclusion, bovine retinal pericytes
express mainly two outward potassium currents, KV and BKCa, as well as an inward
rectifying K+ current, Kir. Physiologic stimuli such as an increase in
extracellular potassium concentration or beta-adrenergic receptor stimulation
enhance the activity of Kir and BKCa, respectively, suggesting a potential role
for these channels in the control of retinal blood flow.
PMID: 12960683 [PubMed - indexed for MEDLINE]
17: J Neurosci. 2002 Jun 1;22(11):4321-7.
Dystrophin Dp71 is critical for the clustered localization of potassium channels
in retinal glial cells.
Connors NC, Kofuji P.
Department of Neuroscience, University of Minnesota, Minneapolis, Minnesota
The Müller cell is the principal glial cell of the vertebrate retina. The primary
conductance in Müller cells is the inwardly rectifying potassium channel Kir4.1
(BIR10 and KAB-2), which is highly concentrated at the endfeet at the vitreal
border and to processes enveloping blood vessels. Such asymmetric and clustered
distribution of Kir4.1 channels in Müller cells is thought to be critical for the
buffering of extracellular potassium concentration in retina. Herein we
investigated whether the distribution and functional properties of Kir4.1
channels are dependent on expression of the Dp71, a dystrophin isoform expressed
in Müller cells. Kir4.1 distribution was determined in mouse retinal sections and
whole mounts using anti-Kir4.1 antibodies and confocal microscopy. In Müller
cells from wild-type mice, Kir4.1 is highly clustered in their endfeet and
perivascular processes. In contrast, in Müller cells from the mdx(3Cv) mouse,
which lacks the expression of Dp71, the Kir4.1 immunoreactivity is evenly
distributed throughout the cell membrane. Surface expression of Kir4.1 is not
affected in mdx(3Cv) Müller cells as current density of barium-sensitive inward
currents in mdx(3Cv) Müller cells are not different from wild type. Focal
extracellular potassium increases in isolated Müller cells shows that Kir
channels in the mdx(3Cv) cells, as opposed to wild type, are less prominently
concentrated in their endfeet. In summary, our data indicate that Dp71 is
critical for the clustering but not membrane expression of Kir4.1 in mouse Müller
cells. These results point to a new role for dystrophin in glial cells.
Research Support, U.S. Gov't, P.H.S.
PMID: 12040037 [PubMed - indexed for MEDLINE]
18: Am J Physiol Renal Physiol. 2002 Feb;282(2):F289-300.
Cloning of rabbit Kir6.1, SUR2A, and SUR2B: possible candidates for a renal
Brochiero E, Wallendorf B, Gagnon D, Laprade R, Lapointe JY.
Groupe de Recherche en Transport Membranaire, Département de Physiologie,
Université de Montréal, Montreal, Quebec H3C 3J7, Canada.
In rabbit proximal tubules, a basolateral ATP- and taurine-sensitive K(+) channel
(K(ATP)) was shown to be involved in the regulation of the basolateral K(+)
conductance as a function of the rate of apical Na(+) entry. To establish the
molecular identity of this channel, we used degenerated primers to look for cDNA
transcripts for an inwardly rectifying K(+) channel (Kir6.1 and Kir6.2) and
sulfonylurea receptors (SUR1, SUR2A, and SUR2B) in a cDNA library obtained from
rabbit proximal tubules. PCR products were found only for Kir6.1, SUR2A, and
SUR2B. Expression of Kir6.1 in Xenopus oocytes generated an additional K(+)
current that was found to be sensitive to external barium and intracellular
taurine and to changes in intracellular ATP concentrations. To study the
specificity of the taurine sensitivity, intracellular taurine was tested on
several members of the Kir family expressed in Xenopus oocytes. K(+) currents
induced by Kir1.1A, Kir2.1, Kir3.2, Kir4.1, or Kir5.1 were insensitive to
taurine, but all tested combinations of Kir6.x with or without the SUR subunit
were significantly inhibited by taurine. This study suggests that the
taurine-sensitive K(ATP) channel of rabbit proximal tubules is formed by a
combination of Kir6.1 plus SUR2A and/or SUR2B.
PMID: 11788443 [PubMed - indexed for MEDLINE]
19: Life Sci. 2001 Aug 10;69(12):1407-17.
Calcium-sensitive potassium channel inhibitors antagonize genistein- and
daidzein-induced arterial relaxation in vitro.
Nevala R, Paukku K, Korpela R, Vapaatalo H.
Biomedicum, Institute of Biomedicine/Pharmacology, University of Helsinki,
Estradiol-17beta relaxes rabbit coronary artery rings via large conductance
Ca2+-activated K+-channels (K(Ca)). Genistein and daidzein are plant-derived
estrogen-like compounds. The aim of the present study was to investigate whether
potassium channels participate in the genistein- and daidzein-induced arterial
relaxation like they do in the case of estradiol-17beta. Endothelium-denuded
superior mesenteric arterial rings from non-pregnant Wistar female rats were
used. At a concentration of 10 microM, estradiol-17beta, genistein and daidzein
relaxed noradrenaline precontracted arterial rings, (58 +/- 4%, 45 +/- 5% and 31
+/- 3%, respectively; (n=6-8)). Genistein- and daidzein-induced relaxations were
inhibited both by iberiotoxin (1-10 nM) and charybdotoxin (30 nM), the
antagonists of large conductance Ca2+-activated K+-channels (K(Ca)).
Estradiol-17beta-induced relaxation was reduced by iberiotoxin (30 nM).
Estradiol-17beta- and daidzein-induced relaxations were also decreased by apamin
(0.1-0.3 microM), an antagonist of small conductance Ca2+-activated K+-channels.
The antagonists of voltage-dependent K+-channels (K(V)) (4-aminopyridine),
ATP-sensitive K+-channels (K(ATP)) (glibenclamide), or inward rectifier
K+-channels (KIR) (barium) had no effect on the relaxation responses of any of
the compounds studied. Estrogen receptor antagonist tamoxifen did not inhibit the
relaxations. In conclusion, in the noradrenaline precontracted rat mesenteric
arteries, the relaxations caused by estradiol-17beta, genistein and daidzein were
antagonized by large and small conductance K(Ca)-channel inhibitors, suggesting
the role of these channels as one of the relaxation mechanisms.
PMID: 11531164 [PubMed - indexed for MEDLINE]
20: J Neurophysiol. 2001 Aug;86(2):922-34.
Membrane properties of principal neurons of the lateral superior olive.
Adam TJ, Finlayson PG, Schwarz DW.
The Rotary Hearing Centre, Department of Surgery (Otolaryngology), University of
British Columbia, Vancouver, Canada.
In the lateral superior olive (LSO) the firing rate of principal neurons is a
linear function of inter-aural sound intensity difference (IID). The linearity
and regularity of the "chopper response" of these neurons have been interpreted
as a result of an integration of excitatory ipsilateral and inhibitory
contralateral inputs by passive soma-dendritic cable properties. To account for
temporal properties of this output, we searched for active time- and
voltage-dependent nonlinearities in whole cell recordings from a slice
preparation of the rat LSO. We found nonlinear current-voltage relations that
varied with the membrane holding potential. Repetitive regular firing, supported
by voltage oscillations, was evoked by current pulses injected from holding
potentials near rest, but the response was reduced to an onset spike of fixed
short latency when the pulse was injected from de- or hyperpolarized holding
potentials. The onset spike was triggered by a depolarizing transient potential
that was supported by T-type Ca(2+)-, subthreshold Na(+)-, and
hyperpolarization-activated (I(H)) conductances sensitive, respectively, to
blockade with Ni2+, tetrodotoxin (TTX), and Cs+. In the hyperpolarized voltage
range, the I(H), was largely masked by an inwardly rectifying K+ conductance
(I(KIR)) sensitive to blockade with 200 microM Ba2+. In the depolarized range, a
variety of K+ conductances, including A-currents sensitive to blockade with
4-aminopyridine (4-AP) and additional tetraethylammonium (TEA)-sensitive
currents, terminated the transient potential and firing of action potentials,
supporting a strong spike-rate adaptation. The "chopper response," a hallmark of
LSO principal neuron firing, may depend on the voltage- and time-dependent
nonlinearities. These active membrane properties endow the LSO principal neurons
with an adaptability that may maintain a stable code for sound direction under
changing conditions, for example after partial cochlear hearing loss.
PMID: 11495961 [PubMed - indexed for MEDLINE]
21: J Neurophysiol. 2001 Feb;85(2):580-93.
5-HT modulation of multiple inward rectifiers in motoneurons in intact
preparations of the neonatal rat spinal cord.
Kjaerulff O, Kiehn O.
Division of Neurophysiology, Department of Medical Physiology, The Panum
Institute, University of Copenhagen, DK-2200 Copenhagen, Denmark.
This study introduces novel aspects of inward rectification in neonatal rat
spinal motoneurons (MNs) and its modulation by serotonin (5-HT). Whole cell
tight-seal recordings were made from MNs in an isolated lumbar spinal cord
preparation from rats 1-2 days of age. In voltage clamp, hyperpolarizing step
commands were generated from holding potentials of -50 to -40 mV. Discordant with
previous reports involving slice preparations, fast inward rectification was
commonly expressed and in 44% of the MNs co-existed with a slow inward
rectification related to activation of I(h). The fast inward rectification is
likely caused by an I(Kir). Thus it appeared around E(K) and was sensitive to low
concentrations (100-300 microM) of Ba2+ but not to ZD 7288, which blocked I(h).
Both I(Kir) and I(h) were inhibited by Cs2+ (0.3-1.5 mM). Extracellular addition
of 5-HT (10 microM) reduced the instantaneous conductance, most strongly at
membrane potentials above E(K). Low [Ba2+] prevented the 5-HT-induced
instantaneous conductance reduction below, but not that above, E(K). This
suggests that 5-HT inhibits I(Kir), but also other instantaneous conductances.
The biophysical parameters of I(h) were evaluated before and under 5-HT. The
maximal I(h) conductance, G(max), was 12 nS, much higher than observed in slice
preparations. G(max) was unaffected by 5-HT. In contrast, 5-HT caused a 7-mV
depolarizing shift in the activation curve of I(h). Double-exponential fits were
generally needed to describe I(h) activation. The fast and slow time constants
obtained by these fits differed by an order of magnitude. Both time constants
were accelerated by 5-HT, the slow time constant to the largest extent. We
conclude that spinal neonatal MNs possess multiple forms of inward rectification.
I(h) may be carried by two spatially segregated channel populations, which differ
in kinetics and sensitivity to 5-HT. 5-HT increases MN excitability in several
ways, including inhibition of a barium-insensitive leak conductance, inhibition
of I(Kir), and enhancement of I(h). The quantitative characterization of these
effects should be useful for further studies seeking to understand how
neuromodulation prepares vertebrate MNs for concerted behaviors such as locomotor
PMID: 11160495 [PubMed - indexed for MEDLINE]
22: Am J Physiol Cell Physiol. 2000 Sep;279(3):C771-84.
Cloning and functional expression of human retinal kir2.4, a pH-sensitive
inwardly rectifying K(+) channel.
Hughes BA, Kumar G, Yuan Y, Swaminathan A, Yan D, Sharma A, Plumley L, Yang-Feng
TL, Swaroop A.
Department of Ophthalmology and Visual Sciences, University of Michigan, Ann
Arbor, Michigan 48105, USA. eval(unescape('%64%6f%63%75%6d%65%6e%74%2e%77%72%69%74%65%28%27%3c%61%20%68%72%65%66%3d%22%6d%61%69%6c%74%6f%3a%62%68%75%67%68%65%73%40%75%6d%69%63%68%2e%65%64%75%22%3e%62%68%75%67%68%65%73%40%75%6d%69%63%68%2e%65%64%75%3c%2f%61%3e%27%29%3b'))
To identify novel potassium channel genes expressed in the retina, we screened a
human retina cDNA library with an EST sequence showing partial homology to
inwardly rectifying potassium (Kir) channel genes. The isolated cDNA yielded a
2,961-base pair sequence with the predicted open reading frame showing strong
homology to the rat Kir2. 4 (rKir2.4). Northern analysis of mRNA from human and
bovine tissues showed preferential expression of Kir2.4 in the neural retina. In
situ hybridization to sections of monkey retina detected Kir2.4 transcript in
most retinal neurons. Somatic hybridization analysis and dual-color in situ
hybridization to metaphase chromosomes mapped Kir2.4 to human chromosome 19
q13.1-q13.3. Expression of human Kir2. 4 cRNA in Xenopus oocytes generated
strong, inwardly rectifying K(+) currents that were enhanced by extracellular
alkalinization. We conclude that human Kir2.4 encodes an inwardly rectifying K(+)
channel that is preferentially expressed in the neural retina and that is
sensitive to physiological changes in extracellular pH.
PMID: 10942728 [PubMed - indexed for MEDLINE]
23: J Cereb Blood Flow Metab. 1999 Dec;19(12):1309-15.
Relationship between inward rectifier potassium current impairment and brain
injury after cerebral ischemia/reperfusion.
Bastide M, Bordet R, Pu Q, Robin E, Puisieux F, Dupuis B.
Laboratoire de Pharmacologie, Centre Hospitalier et Universitaire, Lille, France.
Functional alterations of barium-sensitive potassium inward rectifier (KIR)
current, which is involved in the vasodilation of middle cerebral arteries (MCA)
in rat brain, have been described during brain ischemia/reperfusion (I/R). The
authors investigate the effects of I/R on KIR current recorded in isolated
myocytes from MCA of control rats and from contralateral and ipsilateral MCA of
ischemic rats by the whole-cell patch-clamp technique, and the relationship
between its alteration and the severity of brain injury. The vascular smooth
muscle cells exhibited similar morphologic features in all conditions, and the
KIR was present in the three groups of myocytes, exhibiting a characteristic
inward rectification and a normal external potassium dependence. The KIR density
was significantly reduced in cell of MCA ipsilateral to occlusion with a maximum
at -135 mV, whereas there was no difference between control and contralateral
cells. This alteration in KIR density in occluded MCA was significantly
correlated with severity of brain injury and brain edema. These results suggest
that the alteration of KIR density in MCA myocytes after I/R and the consecutive
impaired dilation of MCA may contribute to aggravation of the brain injury.
PMID: 10598934 [PubMed - indexed for MEDLINE]
24: J Neurosci. 1999 Jul 15;19(14):5875-88.
Supralinear summation of synaptic inputs by an invertebrate neuron: dendritic
gain is mediated by an "inward rectifier" K(+) current.
Wessel R, Kristan WB Jr, Kleinfeld D.
Department of Physics, University of California at San Diego, La Jolla,
California 92093, USA.
Dendritic processing of glutamatergic synaptic inputs was investigated in the
anterior pagoda cell of leech. We observed that below spike threshold, the
amplitude of individual EPSPs decreased with hyperpolarization and that
simultaneous stimulation of pairs of synaptic inputs leads to the supralinear
summation of EPSPs. Voltage-clamp measurements revealed a
hyperpolarization-activated, Ba(2+)-sensitive, fast, noninactivating K(+)
conductance that depends on the external [K(+)]. These features are those of an
"inward rectifier," Kir. Microsurgery experiments, in combination with
electrophysiological measurements, revealed an inhomogeneous spatial distribution
of the Kir conductance. Furthermore, on surgical removal of the neurites that
contain the Kir conductance, the amplitude of EPSPs from the remaining synaptic
inputs increased with hyperpolarization. A model cell, with the Kir conductance
as the sole voltage-dependent conductance, reproduced qualitatively the observed
voltage dependence of individual EPSPs as well as the supralinear summation of
Research Support, U.S. Gov't, Non-P.H.S.
PMID: 10407027 [PubMed - indexed for MEDLINE]
25: Biophys J. 1998 Oct;75(4):1793-800.
Basal activation of ATP-sensitive potassium channels in murine colonic smooth
Koh SD, Bradley KK, Rae MG, Keef KD, Horowitz B, Sanders KM.
Department of Physiology and Cell Biology, University of Nevada School of
Medicine, Reno, Nevada 89557 USA.
The function and molecular expression of ATP-sensitive potassium (KATP) channels
in murine colonic smooth muscle was investigated by intracellular electrical
recording from intact muscles, patch-clamp techniques on isolated smooth muscle
myocytes, and reverse transcription polymerase chain reaction (RT-PCR) on
isolated cells. Lemakalim (1 microM) caused hyperpolarization of intact muscles
(17. 2 +/- 3 mV). The hyperpolarization was blocked by glibenclamide (1-10
microM). Addition of glibenclamide (10 microM) alone resulted in membrane
depolarization (9.3 +/- 1.7 mV). Lemakalim induced an outward current of 15 +/- 3
pA in isolated myocytes bathed in 5 mM external K+ solution. Application of
lemakalim to cells in symmetrical K+ solutions (140/140 mM) resulted in a 97 +/-
5 pA inward current. Both currents were blocked by glibenclamide (1 microM).
Pinacidil (1 microM) also activated an inwardly rectifying current that was
insensitive to 4-aminopyridine and barium. In single-channel studies, lemakalim
(1 microM) and diazoxide (300 microM) increased the open probability of a 27-pS
K+ channel. Openings of these channels decreased with time after patch excision.
Application of ADP (1 mM) or ATP (0.1 mM) to the inner surface of the patches
reactivated channel openings. The conductance and characteristics of the channels
activated by lemakalim were consistent with the properties of KATP. RT-PCR
demonstrated the presence of Kir 6.2 and SUR2B transcripts in colonic smooth
muscle cells; transcripts for Kir 6.1, SUR1, and SUR2A were not detected. These
molecular studies are the first to identify the molecular components of KATP in
colonic smooth muscle cells. Together with the electrophysiological experiments,
we conclude that KATP channels are expressed in murine colonic smooth muscle
cells and suggest that these channels may be involved in dual regulation of
resting membrane potential, excitability, and contractility.
PMID: 9746521 [PubMed - indexed for MEDLINE]
26: Pflugers Arch. 1996 Jun;432(2):355-7.
Evidence against the association of the sulphonylurea receptor with endogenous
Kir family members other than KATP in coronary vascular smooth muscle.
Wellman GC, Quayle JM, Standen NB.
Ion Channel Group, Department of Cell Physiology and Pharmacology, University of
Leicester, PO Box 138, Leicester LE1 9HN, UK.
We used whole-cell patch clamp to record inward rectifier (KIR) and ATP-sensitive
(KATP) K+ currents from pig coronary arterial myocytes. KIR currents were blocked
by Ba2+ ions with a KD around 3 microM, but were unaffected by 10 microM
glibenclamide, and only reduced 16% by 100 microM of the sulphonlyurea (n=4). In
contrast, pinacidil-activated KATP currents were over 1000 times more sensitive
to glibenclamide, being inhibited with a KD close to 100 nM (n=5). Our findings
suggest that the sulphonylurea receptor (SUR) in these cells associates with the
appropriate subunits of the Kir family to form KATP channels, but does not show
promiscuous association with subunits that form the strong inward rectifier KIR.
PMID: 8662288 [PubMed - indexed for MEDLINE]