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Results for “Delayed Rectifier Potassium Channels”

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PubMed2001

Immunohistochemical study on the distribution of six members of the Kv1 channel subunits in the rat cerebellum.

Voltage-gated K(+) (Kv) channels are critical for a wide variety of processes, and play an essential role in neurons. In the present study, we have demonstrated a unique pattern of expression of the six Kv1 channel subunits in the rat cerebellum, for the first time. The greatest concentration of Kv1.2 was found in the basket cell axon plexus and terminal regions around the Purkinje cells. Relatively weak immunoreactivity for Kv1.1 was also found in this area. The somatodendritic Purkinje cell areas were intensely stained with anti-Kv1.5 antibodies. In the cerebellar nuclei, the cell bodies of cerebellar output neurons showed strong Kv1.5 and Kv1.6 immunoreactivities in the nucleus medialis, interpositus and lateralis. Interestingly, Kv1.2 immunoreactivity was found in some neurons with their processes. Our immunohistochemical results may support the notion that the formation of heteromultimeric Kv channels possibly represents an important contribution to the generation of Kv channel diversity in the brain, especially in the cerebellum.

Animals↗

PubMed2001

Effects of bupivacaine and a novel local anesthetic, IQB-9302, on human cardiac K+ channels.

We have studied and compared the effects of bupivacaine with those induced by a new local anesthetic, IQB-9302, on human cardiac K+ channels hKv1.5, Kv2.1, Kv4.3, and HERG. Both drugs have a close chemical structure, only differing in their N-substituent (n-butyl and cyclopropylmethyl, for bupivacaine and IQB-9302, respectively). Both drugs blocked Kv2.1, Kv4.3, and HERG channels similarly. Bupivacaine inhibited these channels by 48.6 +/- 3.4, 45.4 +/- 12.4, and 43.1 +/- 9.1%, respectively, and IQB-9302 by 48.1 +/- 3.3, 36.1 +/- 3.7, and 50.3 +/- 6.6%, respectively. However, bupivacaine was 2.5 times more potent than IQB-9302 to block hKv1.5 channels (EC(50) = 8.9 +/- 1.4 versus 21.5 +/- 4.7 microM). Both drugs induced a time- and voltage-dependent block of hKv1.5 and Kv2.1 channels. Block of Kv4.3 channels induced by either drug was time- and voltage-dependent at membrane potentials coinciding with the activation of the channels. IQB-9302 produced an instantaneous block of Kv4.3 and hKv1.5 channels at the beginning of the depolarizing pulse that can be interpreted as a drug interaction with a nonconducting state. Bupivacaine and IQB-9302 induced a similar degree of block of HERG channels and induced a steep voltage-dependent decrease of the relative current. These results suggest that 1) bupivacaine and IQB-9302 block the open state of hKv1.5, Kv2.1, Kv4.3, and HERG channels; and 2) small differences at the N-substituent of these drugs do not affect the drug-induced block of Kv2.1, Kv4.3, or HERG, but specifically modify block of hKv1.5 channels.

Algorithms↗

PubMed1997

Characterization of delayed rectifier K+ currents in rabbit coronary artery cells near resting membrane potential.

Previous studies have suggested that delayed rectifier K+ (Kdr) channels contribute to the control of membrane potential in vascular smooth muscle. To explore this hypothesis further, we investigated the characteristics of Kdr channels in the negative voltage range in the rabbit coronary artery. The Kdr channel blocker 4-aminopyridine (1 mM) contracted intact vessels and depolarized them from -52 to -37 mV, suggesting that these channels significantly contribute to the maintenance of resting membrane potential. In contrast, the ATP-sensitive K+ channel blocker glybenclamide (3 microM) had little effect on resting tone and did not alter the contraction elicited with 4-aminopyridine. K+ currents in isolated cells were then investigated by using whole-cell patch-clamp techniques. Increasing extracellular K+ concentration ([K+]o) from 5 to 135 mM resulted in the appearance of large inward currents at potentials between -60 and 0 mV. The voltage dependence of conductance for inward K+ currents was steeper and shifted toward more negative potentials when compared with outward K+ currents in 5 mM [K+]o solution. Various blockers of Kdr channels, i.e., 4-aminopyridine (3 mM), phencyclidine (0.1 mM), and intracellular tetraethylammonium (10 mM), nearly abolished currents in high [K+]o solution. In contrast, Ba2+ (0.1 mM) was without effect. These results suggest that the inward currents detected at potentials between -60 and 0 mV in high [K+]o solution are Kdr currents. Our results suggest that Kdr channels physiologically contribute to the control of membrane potential in the rabbit coronary artery.

4-Aminopyridine↗

PubMed2000

Differential targeting of Shaker-like potassium channels to lipid rafts.

Ion channel targeting within neuronal and muscle membranes is an important determinant of electrical excitability. Recent evidence suggests that there exists within the membrane specialized microdomains commonly referred to as lipid rafts. These domains are enriched in cholesterol and sphingolipids and concentrate a number of signal transduction proteins such as nitric-oxide synthase, ligand-gated receptors, and multiple protein kinases. Here, we demonstrate that the voltage-gated K(+) channel Kv2.1, but not Kv4.2, targets to lipid rafts in both heterologous expression systems and rat brain. The Kv2.1 association with lipid rafts does not appear to involve caveolin. Depletion of cellular cholesterol alters the buoyancy of the Kv2.1 associated rafts and shifts the midpoint of Kv2.1 inactivation by nearly 40 mV without affecting peak current density or channel activation. The differential targeting of Kv channels to lipid rafts represents a novel mechanism both for the subcellular sorting of K(+) channels to regions of the membrane rich in signaling complexes and for modulating channel properties via alterations in lipid content.

Animals↗

PubMed2005

Expression and biophysical properties of Kv1 channels in supragranular neocortical pyramidal neurones.

Potassium channels are extremely diverse regulators of neuronal excitability. As part of an investigation into how this molecular diversity is utilized by neurones, we examined the expression and biophysical properties of native Kv1 channels in layer II/III pyramidal neurones from somatosensory and motor cortex. Single-cell RT-PCR, immunocytochemistry, and whole cell recordings with specific peptide toxins revealed that individual pyramidal cells express multiple Kv1 alpha-subunits. The most abundant subunit mRNAs were Kv1.1 > 1.2 > 1.4 > 1.3. All of these subunits were localized to somatodendritic as well as axonal cell compartments. These data suggest variability in the subunit complexion of Kv1 channels in these cells. The alpha-dendrotoxin (alpha-DTX)-sensitive current activated more rapidly and at more negative potentials than the alpha-DTX-insensitive current, was first observed at voltages near action potential threshold, and was relatively insensitive to holding potential. The alpha-DTX-sensitive current comprised about 10% of outward current at steady-state, in response to steps from -70 mV. From -50 mV, this percentage increased to approximately 20%. All cells expressed an alpha-DTX-sensitive current with slow inactivation kinetics. In some cells a transient component was also present. Deactivation kinetics were voltage dependent, such that deactivation was slow at potentials traversed by interspike intervals during repetitive firing. Because of its kinetics and voltage dependence, the alpha-DTX-sensitive current should be most important at physiological resting potentials and in response to brief stimuli. Kv1 channels should also be important at voltages near threshold and corresponding to interspike intervals.

Animals↗

PubMed2001

Genetic manipulation of cardiac K(+) channel function in mice: what have we learned, and where do we go from here?

In the mammalian myocardium, potassium (K(+)) channels control resting potentials, action potential waveforms, automaticity, and refractory periods and, in most cardiac cells, multiple types of K(+) channels that subserve these functions are expressed. Molecular cloning has revealed the presence of a large number of K(+) channel pore forming (alpha) and accessory (beta) subunits in the heart, and considerable progress has been made recently in defining the relationships between expressed K(+) channel subunits and functional cardiac K(+) channels. To date, more than 20 mouse models with altered K(+) channel expression/functioning have been generated using dominant-negative transgenic and targeted gene deletion approaches. In several instances, the genetic manipulation of K(+) channel subunit expression has revealed the role of specific K(+) channel subunit subfamilies or individual K(+) channel subunit genes in the generation of myocardial K(+) channels. In other cases, however, the phenotypic consequences have been unexpected. This review summarizes what has been learned from the in situ genetic manipulation of cardiac K(+) channel functioning in the mouse, discusses the limitations of the models developed to date, and explores the likely directions of future research.

Action Potentials↗

PubMed2000

Solution structure of hanatoxin1, a gating modifier of voltage-dependent K(+) channels: common surface features of gating modifier toxins.

The three-dimensional structure of hanatoxin1 (HaTx1) was determined by using NMR spectroscopy. HaTx1 is a 35 amino acid residue peptide toxin that inhibits the drk1 voltage-gated K(+) channel not by blocking the pore, but by altering the energetics of gating. Both the amino acid sequence of HaTx1 and its unique mechanism of action distinguish this toxin from the previously described K(+) channel inhibitors. Unlike most other K(+) channel-blocking toxins, HaTx1 adopts an "inhibitor cystine knot" motif and is composed of two beta-strands, strand I for residues 19-21 and strand II for residues 28-30, connected by four chain reversals. A comparison of the surface features of HaTx1 with those of other gating modifier toxins of voltage-gated Ca(2+) and Na(+) channels suggests that the combination of a hydrophobic patch and surrounding charged residues is principally responsible for the binding of gating modifier toxins to voltage-gated ion channels.

Amino Acid Motifs↗

PubMed1998

The 1997 Stevenson Award Lecture. Cardiac K+ channel gating: cloned delayed rectifier mechanisms and drug modulation.

K+ channels are ubiquitous membrane proteins, which have a central role in the control of cell excitability. In the heart, voltage-gated delayed rectifier K+ channels, like Kv1.5, determine repolarization and the cardiac action potential plateau duration. Here we review the broader properties of cloned voltage-gated K+ channels with specific reference to the hKv1.5 channel in heart. We discuss the basic structural components of K+ channels such as the pore, voltage sensor, and fast inactivation, all of which have been extensively studied. Slow, or C-type, inactivation and the structural features that control pore opening are less well understood, although recent studies have given new insight into these problems. Information about channel transitions that occur prior to opening is provided by gating currents, which reflect charge-carrying transitions between kinetic closed states. By studying modulation of the gating properties of K+ channels by cations and with drugs, we can make a more complete interpretation of the state dependence of drug and ion interactions with the channel. In this way we can uncover the detailed mechanisms of action of K+ channel blockers such as tetraethylammonium ions and 4-aminopyridine, and antiarrhythmic agents such as nifedipine and quinidine.

4-Aminopyridine↗

PubMed1994

Modulation by protein kinase A of a cloned rat brain potassium channel expressed in Xenopus oocytes.

A potassium channel from rat brain was expressed in Xenopus oocytes in order to study modulation of channel function by phosphorylation via protein kinase A. Application of 8-Br-cAMP to oocytes expressing the drk1 channel (with the first 139 amino acids of the N terminus deleted, delta Ndrk1) caused a voltage-independent elevation of current amplitude, which was not seen for endogenous currents or for wild-type full-length drk1 channel. This effect on delta Ndrk1 was blocked by pre-injection of oocytes with Walsh-peptide protein kinase A inhibitor, suggesting mediation via protein kinase A. The protein kinase inhibitor also reduced both delta Ndrk1 and full-length drk1 currents. Substitution of the serine residues by alanine at one or both of the two consensus protein kinase A phosphorylation sites on the C terminus (residues 440 and 492) of delta Ndrk1 resulted in a loss of function of the expressed channels. These results indicate that phosphorylation via protein kinase A modulates drk1 channel function and that both consensus phosphorylation sites seems to be essential for channels to function.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed2001

Gene transfer and metabolic modulators as new therapies for pulmonary hypertension. Increasing expression and activity of potassium channels in rat and human models.

UNLABELLED: Chronic Hypoxic Pulmonary Hypertension (CH-PHT) is characterized by pulmonary artery (PA) vasoconstriction and cell proliferation/hypertrophy. PA smooth muscle cell (PASMC) contractility and proliferation are controlled by cytosolic Ca++ levels, which are largely determined by membrane potential (E(M)). E(M) is depolarized in CH-PHT due to decreased expression and functional inhibition of several redox-regulated, 4-aminopyridine (4-AP) sensitive, voltage-gated K+ channels (Kv1.5 and Kv2.1). Humans with Pulmonary Arterial Hypertension (PAH) also have decreased PASMC expression of Kv1.5 and Kv2.1. We speculate this "K+-channelopathy" contributes to PASMC depolarization and Ca++ overload thus promoting vasoconstriction and PASMC proliferation. We hypothesized that restoration of Kv channel expression in PHT and might eventually be beneficial. METHODS: Two strategies were used to increase Kv channel expression in PASMCs: oral administration of a metabolic modulator drug (Dichloroacetate, DCA) and direct Kv gene transfer using an adenovirus (Ad5-Kv2.1). DCA a pyruvate dehydrogenase kinase inhibitor, promotes a more oxidized redox state mimicking normoxia and previously has been noted to increase K+ current in myocytes. Rats were given DCA in the drinking water after the development of CH-PHT and hemodynamics were measured approximately 5 days later. We also tested the ability of Ad5-Kv2.1 to increase Kv2.1 channel expression and function in human PAs ex vivo. RESULTS: The DCA-treated rats had decreased PVR, RVH and PA remodeling compared to the control CH-PHT rats (n=5/group, p<0.05). DCA restored Kv2.1 expression and PASMC Kv current density to near normoxic levels. Adenoviral gene transfer increased expression of Kv2.1 channels and enhanced 4-AP constriction in human PAs. CONCLUSION: Increasing Kv channel function in PAs is feasible and might be beneficial.

Animals↗

PubMed2002

Pulmonary vascular K+ channel expression and vasoreactivity in a model of congenital heart disease.

K+ channels play an important role in mediating pulmonary vasodilation caused by increased oxygen tension, nitric oxide, alkalosis, and shear stress. To test the hypothesis that lung K+ channel gene expression may be altered by chronic increases in pulmonary blood flow, we measured gene and protein expression of calcium-sensitive (K Ca ) and voltage-gated (Kv2.1) K+ channels, and a pH-sensitive K+ channel (TASK), in distal lung from fetal lambs in which an aortopulmonary shunt was placed at 139 days gestation. Under baseline conditions, animals with an aortopulmonary shunt showed elevated pulmonary artery pressure and pulmonary blood flow compared with twin controls. Hypoxia caused a greater increase in pulmonary vascular tone in shunt animals compared with controls. Alkalosis caused pulmonary vasodilation in control but not shunt animals. To determine lung K+ channel mRNA levels, we performed quantitative RT-PCR. In comparison with control animals, lung K Ca channel mRNA content was increased in shunt animals, whereas TASK mRNA levels were decreased. There was no difference in Kv2.1 mRNA levels. Channel protein expression was consistent with these findings. We conclude that, in the presence of elevated pulmonary blood flow, K Ca channel expression is increased and TASK is decreased.

Alkalosis↗

PubMed1999

Differential inhibition of glial K(+) currents by 4-AP.

Spinal cord astrocytes express four biophysically and pharmacologically distinct voltage-activated potassium (K(+)) channel types. The K(+) channel blocker 4-aminopyridine (4-AP) exhibited differential and concentration-dependent block of all of these currents. Specifically, 100 microM 4-AP selectively inhibited a slowly inactivating outward current (K(SI)) that was insensitive to dendrototoxin (< or = 10 microM) and that activated at -50 mV. At 2 mM, 4-AP inhibited fast-inactivating, low-threshold (-70 mV) A-type currents (K(A)) and sustained, TEA-sensitive noninactivating delayed-rectifier-type currents (K(DR)). At an even higher concentration (8 mM), 4-AP additionally blocked inwardly rectifying, Cs(+)- and Ba(2+)-sensitive K(+) currents (K(IR)). Current injection into current-clamped astrocytes in culture or in acute spinal cord slices induced an overshooting voltage response reminiscent of slow neuronal action potentials. Increasing concentrations of 4-AP selectively modulated different phases in the repolarization of these glial spikes, suggesting that all four K(+) currents serve different roles in stabilization and repolarization of the astrocytic membrane potential. Our data suggest that 4-AP is an useful, dose-dependent inhibitor of all four astrocytic K(+) channels. We show that the slowly inactivating astrocytic K(+) currents, which had not been described as separate current entities in astrocytes, contribute to the resting K(+) conductance and may thus be involved in K(+) homeostatic functions of astrocytes. The high sensitivity of these currents to micromolar 4-AP suggests that application of 4-AP to inhibit neuronal A-currents or to induce epileptiform discharges in brain slices also may influence astrocytic K(+) buffering.

4-Aminopyridine↗

PubMed1999

Determinants of potassium channel assembly localised within the cytoplasmic C-terminal domain of Kv2.1.

The C-terminal domain of the voltage-gated potassium channel Kv2.1 is shown to have a role in channel assembly using dominant negative experiments in Xenopus oocytes. Kv2.1 channel polypeptides were co-expressed with a number of polypeptide fragments of the cytosolic C-terminus and the assembly of functional channel homotetramers quantified electrophysiologically using the two electrode voltage clamp technique. Co-expression of C-terminal polypeptides corresponding to the final 440, 318, 220 and 150 amino acid residues of Kv2.1 all resulted in a significant reduction in the functional expression of the full-length channel. A truncated version of Kv2.1 lacking the final 318 amino acids of the C-terminal domain (Kv2. 11-535) exhibited similar electrophysiological properties to the full-length channel. Co-expression with either the 440 or 318 residue polypeptides resulted in a reduction in the activity of the truncated channel. In contrast, the 220 and 150 residue C-terminal fragments had no effect on Kv2.11-535 activity. These data demonstrate that C-terminal interactions are important for driving Kv2.1 channel assembly and that distinct regions of the C-terminal domain may have differential effects on the formation of functional tetramers.

Animals↗

PubMed2004

Cellular localization of Shab and Shaw potassium channels in the lobster stomatogastric ganglion.

The motor pattern generated by the 14 neurons composing the pyloric circuit in the stomatogastric ganglion (STG) of the spiny lobster, Panulirus interruptus, is organized not only by the synaptic connections between neurons, but also by the characteristic intrinsic electrophysiological properties of the individual cells. These cellular properties result from the unique complement of ion channels that each cell expresses, and the distribution of those channels in the cell membranes. We have mapped the STG expression of shab and shaw, two genes in the Shaker superfamily of potassium channel genes that encode voltage-dependent, non-inactivating channels. Using antibodies developed against peptide sequences from the two channel proteins, we explored the localization and cell-specific expression of the channels. Anti-Shab and anti-Shaw antibodies both stain all the pyloric neurons in the somata, as well as their primary neurites and branch points of large neurites, but to varying degrees between cell types. Staining was weak and irregular (Shaw) or absent (Shab) in the fine neuropil of pyloric neurons, where most synaptic interactions occur. There is a high degree of variability in the staining intensity among neurons of a single cell class. This supports Golowasch et al.'s [J Neurosci 19 (1999) RC33; Neural Comput 11 (1999) 1079] hypothesis that individual cells can have similar firing properties with varying compositions of ionic currents. Both antibodies stain the axons of the peripheral nerves as they enter foregut muscles. We conclude that both Shab and Shaw channels are appropriately localized to contribute to the noninactivating potassium current in the stomatogastric nervous system.

Analysis of Variance↗

PubMed1998

Enhancement of outward potassium current may participate in beta-amyloid peptide-induced cortical neuronal death.

In light of recent evidence implicating the upregulation of outward K+ current in mediating several forms of neuronal apoptosis, we tested the hypothesis that such an upregulation might specifically contribute to the pathogenesis of beta-amyloid peptide (A beta)-induced neuronal death. Exposure to A beta fragment 25-35 (20 microM) or 1-42 (20 microM) enhanced the delayed rectifier K+ current IK, shifting its activation voltage relationship toward hyperpolarized levels and increasing maximal conductance, but did not affect the transient K+ current IA or charybdotoxin-sensitive BK current. Reducing IK by adding the channel blocker tetraethylammonium (TEA, 5 mM) or raising extracellular K+ to 25 mM attenuated A beta-induced neuronal death, even in the presence of nifedipine or gadolinium to block associated increases in Ca2+ influx. The IA blocker 4-aminopyridine (4-AP, 5 mM) and the CI- channel blocker anthracene-9-carboxylic acid (ACA, 500 microM) were not neuroprotective. These data raise the intriguing possibility that manipulations aimed at reducing outward K+ current may provide an approach to reducing neuronal degeneration in patients with Alzheimer's disease.

Amyloid beta-Peptides↗

PubMed2000

Beta(3)-adrenergic stimulation and insulin inhibition of non-selective cation channels in white adipocytes of the rat.

Single-channel currents were recorded from the plasma membrane of white adipocytes of 6-8-week-old male Sprague-Dawley rats. In outside-out patches (high K(+), no Ca(2+) in pipette), a voltage-dependent K-channel (delayed rectifier) with a single-channel conductance (gamma) of 16 pS (24 degrees C) in modified Ringer's was active at a density of 0.5/microm(2). It was blocked by TEA (IC(50)=1.5 mM). A Ca(2+)-activated non-selective cation channel (NSC-channel) appeared at a mean density of 1/microm(2) in inside-out patches ([Ca(2+)](i)=1.2 mM). gamma was 28 pS (24 degrees C). The NSC showed weak voltage dependence and was blocked by mefenamic acid and by internal ATP. In the cell-attached mode spontaneous activity could be blocked reversibly by 100 nM insulin. Noradrenaline (NA, 100 nM) induced a flickering activity of the NSC-channels. Isoproterenol (100 nM) caused activity of the NSC-channel as well. After 1 microM propranolol even 1 microM NA did not induce any activity. The alpha-antagonist phentolamine had no effect on isoproterenol- or on NA-induced currents. The beta(3)-agonists BRL 37344 and BRL 35135A induced activity of the NSC-channel at 100 nM as well. We conclude that white adipocytes express ion channels which are comparable to those in brown adipocytes and that beta-receptor activation opens NSC-channels thus allowing for Na(+) entry into white adipocytes.

Adenosine Triphosphate↗

PubMed2002

Molecular cloning and characterization of Kv6.3, a novel modulatory subunit for voltage-gated K(+) channel Kv2.1.

We report identification and characterization of Kv6.3, a novel member of the voltage-gated K(+) channel. Reverse transcriptase-polymerase chain reaction analysis indicated that Kv6.3 was highly expressed in the brain. Electrophysiological studies indicated that homomultimeric Kv6.3 did not yield a functional voltage-gated ion channel. When Kv6.3 and Kv2.1 were co-expressed, the heteromultimeric channels displayed the decreased rate of deactivation compared to the homomultimeric Kv2.1 channels. Immunoprecipitation studies indicated that Kv6.3 bound with Kv2.1 in co-transfected cells. These results indicate that Kv6.3 is a novel member of the voltage-gated K(+) channel which functions as a modulatory subunit.

Amino Acid Sequence↗

PubMed2003

Syntaxin 1A binds to the cytoplasmic C terminus of Kv2.1 to regulate channel gating and trafficking.

Voltage-gated K(+) (Kv) 2.1 is the dominant Kv channel that controls membrane repolarization in rat islet beta-cells and downstream insulin exocytosis. We recently showed that exocytotic SNARE protein SNAP-25 directly binds and modulates rat islet beta-cell Kv 2.1 channel protein at the cytoplasmic N terminus. We now show that SNARE protein syntaxin 1A (Syn-1A) binds and modulates rat islet beta-cell Kv2.1 at its cytoplasmic C terminus (Kv2.1C). In HEK293 cells overexpressing Kv2.1, we observed identical effects of channel inhibition by dialyzed GST-Syn-1A, which could be blocked by Kv2.1C domain proteins (C1: amino acids 412-633, C2: amino acids 634-853), but not the Kv2.1 cytoplasmic N terminus (amino acids 1-182). This was confirmed by direct binding of GST-Syn-1A to the Kv2.1C1 and C2 domains proteins. These findings are in contrast to our recent report showing that Syn-1A binds and modulates the cytoplasmic N terminus of neuronal Kv1.1 and not by its C terminus. Co-expression of Syn-1A in Kv2.1-expressing HEK293 cells inhibited Kv2.1 surfacing, which caused a reduction of Kv2.1 current density. In addition, Syn-1A caused a slowing of Kv2.1 current activation and reduction in the slope factor of steady-state inactivation, but had no affect on inactivation kinetics or voltage dependence of activation. Taken together, SNAP-25 and Syn-1A mediate secretion not only through its participation in the exocytotic SNARE complex, but also by regulating membrane potential and calcium entry through their interaction with Kv and Ca(2+) channels. In contrast to Ca(2+) channels, where these SNARE proteins act on a common synprint site, the SNARE proteins act not only on distinct sites within a Kv channel, but also on distinct sites between different Kv channel families.

Animals↗