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Outward K+ Current (outward + k+_current)
Kinds of Outward K+ Current Selected AbstractsDevelopment of ionic currents of zebrafish slow and fast skeletal muscle fibersDEVELOPMENTAL NEUROBIOLOGY, Issue 3 2006Christopher A. Coutts Abstract Voltage-gated Na+ and K+ channels play key roles in the excitability of skeletal muscle fibers. In this study we investigated the steady-state and kinetic properties of voltage-gated Na+ and K+ currents of slow and fast skeletal muscle fibers in zebrafish ranging in age from 1 day postfertilization (dpf) to 4,6 dpf. The inner white (fast) fibers possess an A-type inactivating K+ current that increases in peak current density and accelerates its rise and decay times during development. As the muscle matured, the V50s of activation and inactivation of the A-type current became more depolarized, and then hyperpolarized again in older animals. The activation kinetics of the delayed outward K+ current in red (slow) fibers accelerated within the first week of development. The tail currents of the outward K+ currents were too small to allow an accurate determination of the V50s of activation. Red fibers did not show any evidence of inward Na+ currents; however, white fibers expressed Na+ currents that increased their peak current density, accelerated their inactivation kinetics, and hyperpolarized their V50 of inactivation during development. The action potentials of white fibers exhibited significant changes in the threshold voltage and the half width. These findings indicate that there are significant differences in the ionic current profiles between the red and white fibers and that a number of changes occur in the steady-state and kinetic properties of Na+ and K+ currents of developing zebrafish skeletal muscle fibers, with the most dramatic changes occurring around the end of the first day following egg fertilization. © 2005 Wiley Periodicals, Inc. J Neurobiol, 2006 [source] Molecular cloning, genomic organization and functional characterization of a new short-chain potassium channel toxin-like peptide BmTxKS4 from Buthus martensii Karsch(BmK)JOURNAL OF BIOCHEMICAL AND MOLECULAR TOXICOLOGY, Issue 4 2004Sheng Jiqun Abstract Scorpion venom contains many small polypeptide toxins, which can modulate Na+, K+, Cl,, and Ca2+ ion,channel conductance in the cell membrane. A full-length cDNA sequence encoding a novel type of K+ -channel toxin (named BmTxKS4) was first isolated and identified from a venom gland cDNA library of Buthus martensii Karsch (BmK). The encoded precursor contains 78 amino acid residues including a putative signal peptide of 21 residues, propeptide of 11 residues, and a mature peptide of 43 residues with three disulfide bridges. BmTxKS4 shares the identical organization of disulfide bridges with all the other short-chain K+ -channel scorpion toxins. By PCR amplification of the genomic region encoding BmTxKS4, it was shown that BmTxKS4 composed of two exons is disrupted by an intron of 87 bp inserted between the first and the second codes of Phe (F) in the encoding signal peptide region, which is completely identical with that of the characterized scorpion K+ -channel ligands in the size, position, consensus junctions, putative branch point, and A+T content. The GST-BmTxKS4 fusion protein was successfully expressed in BL21 (DE3) and purified with affinity chromatography. About 2.5 mg purified recombinant BmTxKS4 (rBmTxKS4) protein was obtained by treating GST-BmTxKS4 with enterokinase and sephadex chromatography from 1 L bacterial culture. The electrophysiological activity of 1.0,M rBmTxKS4 was measured and compared by whole cell patch-clamp technique. The results indicated that rBmTxKS4 reversibly inhibited the transient outward K+ current (Ito), delayed inward rectifier K+ current (Ik1), and prolonged the action potential duration of ventricular myocyte, but it has no effect on the action potential amplitude. Taken together, BmTxKS4 is a novel subfamily member of short-strain K+ -channel scorpion toxin. © 2004 Wiley Periodicals, Inc. J Biochem Mol Toxicol 18:187,195, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jbt.20026 [source] Changes in Left Ventricular Repolarization and Ion Channel Currents Following a Transient Rate Increase Superimposed on Bradycardia in Anesthetized DogsJOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Issue 6 2000MICHAEL RUBART M.D. Electrical Remodeling of the Heart due to Rate. Introduction: We previously demonstrated in dogs that a transient rate increase superimposed on bradycardia causes prolongation of ventricular refractoriness that persists for hours after resumption of bradycardia. In this study, we examined changes in membrane currents that are associated with this phenomenon. Methods and Results: The whole cell, patch clamp technique was used to record transmembrane voltages and currents, respectively, in single mid-myocardial left ventricular myocytes from dogs with 1 week of complete AV block; dogs either underwent 1 hour of left ventricular pacing at 120 beats/min or did not undergo pacing. Pacing significantly heightened mean phase 1 and peak plateau amplitudes by ,6 and ,3 mV, respectively (P < 0.02). and prolonged action potential duration at 90% repolarization from 235 ± 8 msec to 278 ± 8 msec (1 Hz; P = 0.02). Rapid pacing-induced changes in transmembrane ionic currents included (1) a more pronounced cumulative inactivation of the 4-aminopyridine-sensitive transient outward K+ current, I to over the range of physiologic frequencies, resulting from a ,30% decrease in the population of quickly reactivating channels; (2) increases in peak density of L-type Ca2+ currents, Ica.I.' by 15% to 35% between +10 and +60 mV; and (3) increases in peak density of the Ca2+ -activated chloride current, ICl.Ca' by 30% to 120% between +30 and +50 mV. Conclusion: Frequency-dependent reduction in Ito combined with enhanced ICa.I. causes an increase in net inward current that may he responsible for the observed changes in ventricular repolarization. This augmentation of net cation influx is partially antagonized by an increase in outward ICa.Cl. [source] The Kv4.2 mediates excitatory activity-dependent regulation of neuronal excitability in rat cortical neuronsJOURNAL OF NEUROCHEMISTRY, Issue 3 2008Bin Shen Abstract Neuronal excitability can cooperate with synaptic transmission to control the information storage. This regulation of neuronal plasticity can be affected by alterations in neuronal inputs and accomplished by modulation of voltage-dependent ion channels. In this study, we report that enhanced excitatory input negatively regulated neuronal excitability. Enhanced excitatory input by glutamate, electric field stimulation or high K+ increased transient outward K+ current, whereas did not affect the delayed rectifier K+ current in rat cultured cortical neurons. Both the voltage-dependent K+ channel 4.2 and 4.3 subunits contributed to the increase. The increase in the K+ current density by Kv4.2 was ascribed to its cytoplasmic membrane translocation, which was mediated by NMDA type of glutamate receptor. Furthermore, enhanced excitatory input inhibited neuronal excitability. Taken together, our results suggest that excitatory neurotransmission affects neuronal excitability via the regulation of the K+ channel membrane translocation. [source] Electrophysiological Remodeling in Human Atrial FibrillationPACING AND CLINICAL ELECTROPHYSIOLOGY, Issue 7p2 2003DAVID R. VAN WAGONER Atrial fibrillation (AF) is a progressive disease characterized by cumulative electrophysiological and structural remodeling of the atria. Cellular electrophysiological studies have revealed marked reductions in the densities of the L-type voltage-gated Ca2+ current, ICa,L, the transient outward K+ current, ITO, and the ultra-rapid delayed rectifier K+ current, IKur, in atrial myocytes from patients in persistent or permanent AF. The density of the muscarinic K+ current (IKACh) is also reduced, however the inward rectifier K+ current (IK1) density is increased. The net shortening or lengthening of the action potential is dependent on the balance between changes in inward and outward currents. The prominent reduction in ICa,L appears to be sufficient to explain the observed decreases in action potential duration and effective refractory period that are characteristic of the fibrillating atria. Earlier studies have shown that calcium overload and perturbations in calcium handling play prominent roles in AF induced atrial remodeling. More recently, we have shown that AF is associated with evidence of oxidative injury to atrial tissue, and suggested that oxidative stress may directly contribute to the pathophysiology of AF. It is anticipated that insights gleaned from mechanistic studies will facilitate the development of improved pharmacological approaches to treat AF and to prevent the progression of arrhythmia. (PACE 2003; 26[Pt. II]:1572,1575) [source] Depletion of membrane cholesterol eliminates the Ca2+ -activated component of outward potassium current and decreases membrane capacitance in rat uterine myocytesTHE JOURNAL OF PHYSIOLOGY, Issue 2 2007A. Shmygol Changes in membrane cholesterol content have potent effects on cell signalling and contractility in rat myometrium and other smooth muscles. We have previously shown that depletion of cholesterol with methyl-,-cyclodextrin (MCD) disrupts caveolar microdomains. The aim of this work was to determine the mechanism underlying the increase in Ca2+ signalling and contractility occurring in the myometrium with MCD. Patch clamp data obtained on freshly isolated myocytes from the uterus of day 19,21 rats showed that outward K+ current was significantly reduced by MCD. Membrane capacitance was also reduced. Cholesterol-saturated MCD had no effect on the amplitude of outward current suggesting that the reduction in the outward current was due to cholesterol depletion induced by MCD rather than a direct inhibitory action of MCD on the K+ channels. Confocal visualization of the membrane bound indicator Calcium Green C18, revealed internalization of the surface membrane with MCD treatment. Large conductance, Ca2+ -sensitive K+ channel proteins have been shown to localize to caveolae. When these channels were blocked by iberiotoxin outward current was significantly reduced in the uterine myocytes; MCD treatment reduced the density of outward current. Following reduction of outward current by MCD pretreatment, iberiotoxin was unable to produce any additional decrease in the current, suggesting a common target. MCD treatment also increased the amplitude and frequency of spontaneous rises in cytosolic Ca2+ level ([Ca2+]i transients) in isolated myocytes. In intact rat myometrium, MCD treatment increased Ca2+ signalling and contractility, consistent with previous findings, and this effect was also found to be reduced by BK channel inhibition. These data suggest that (1) disruption of cholesterol-rich microdomains and caveolae by MCD leads to a decrease in the BK channel current thus increasing cell excitability, and (2) the changes in membrane excitability produced by MCD underlie the changes found in Ca2+ signalling and uterine contractility. [source] Oscillatory transient inward currents in ventricular myocytes of healthy versus myopathic Syrian hamsterCLINICAL AND EXPERIMENTAL PHARMACOLOGY AND PHYSIOLOGY, Issue 10 2004Sze-Hsueh Wu Summary 1.,The present experiments were performed in order to study abnormal action potential configuration and ion channel activity in ventricular myocytes obtained from 23 male myopathic Syrian hamsters (Biobreeders strain 14.6, 32,52 weeks old) compared with 10 age-matched healthy control hamsters (Biobreeders F1B) by means of whole-cell patch-clamp techniques. 2.,The results show that the myopathic myocytes had a longer action potential duration, a reduced transient outward K+ current on depolarization and a smaller transient inward current on repolarization after prolonged depolarizing pulses (> 500 msec). However, the L-type Ca2+ current and the inwardly rectifing K+ current were not significantly different from those of healthy myocytes. 3.,The oscillatory transient inward currents could be diminished by treatment with ryanodine (0.01,1 µmol/L), a sarcoplasmic reticulum (SR) Ca2+ release channel blocker, or with Na+ -free superfusate. 4.,We conclude that the hereditary myopathic hamsters are less likely to develop delayed afterdepolarization-related transient inward currents and triggered arrhythmias owing to a smaller SR Ca2+ content. [source] Development of ionic currents of zebrafish slow and fast skeletal muscle fibersDEVELOPMENTAL NEUROBIOLOGY, Issue 3 2006Christopher A. Coutts Abstract Voltage-gated Na+ and K+ channels play key roles in the excitability of skeletal muscle fibers. In this study we investigated the steady-state and kinetic properties of voltage-gated Na+ and K+ currents of slow and fast skeletal muscle fibers in zebrafish ranging in age from 1 day postfertilization (dpf) to 4,6 dpf. The inner white (fast) fibers possess an A-type inactivating K+ current that increases in peak current density and accelerates its rise and decay times during development. As the muscle matured, the V50s of activation and inactivation of the A-type current became more depolarized, and then hyperpolarized again in older animals. The activation kinetics of the delayed outward K+ current in red (slow) fibers accelerated within the first week of development. The tail currents of the outward K+ currents were too small to allow an accurate determination of the V50s of activation. Red fibers did not show any evidence of inward Na+ currents; however, white fibers expressed Na+ currents that increased their peak current density, accelerated their inactivation kinetics, and hyperpolarized their V50 of inactivation during development. The action potentials of white fibers exhibited significant changes in the threshold voltage and the half width. These findings indicate that there are significant differences in the ionic current profiles between the red and white fibers and that a number of changes occur in the steady-state and kinetic properties of Na+ and K+ currents of developing zebrafish skeletal muscle fibers, with the most dramatic changes occurring around the end of the first day following egg fertilization. © 2005 Wiley Periodicals, Inc. J Neurobiol, 2006 [source] Electrophysiological and morphological characterization of dentate astrocytes in the hippocampusDEVELOPMENTAL NEUROBIOLOGY, Issue 2 2005Masako Isokawa Abstract We studied electrophysiological and morphological properties of astrocytes in the dentate gyrus of the rat hippocampus in slices. Intracellular application of Lucifer yellow revealed two types of morphology: one with a long process extruding from the cell body, and the other with numerous short processes surrounding the cell body. Their electrophysiological properties were either passive, that is, no detectable voltage-dependent conductance, or complex, with Na+/K+ currents similar to those reported in the Ammon's horn astrocytes. We did not find any morphological correlate to the types of electrophysiological profile or dye coupling. Chelation of cytoplasmic calcium ([Ca2+]i) by BAPTA increased the incidence of detecting a low Na+ conductance and transient outward K+ currents. However, an inwardly rectifying K+ current (Kir), a hallmark of differentiated CA1/3 astrocytes, was not a representative K+ -current in the complex dentate astrocytes, suggesting that these astrocytes could retain an immature form of K-currents. Dentate astrocytes may possess a distinct current profile that is different from those in CA1/3 Ammon's horn. © 2005 Wiley Periodicals, Inc. J Neurobiol, 2005 [source] PACAP inhibits delayed rectifier potassium current via a cAMP/PKA transduction pathway: evidence for the involvement of IK in the anti-apoptotic action of PACAPEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 6 2004Y. A. Mei Abstract Activation of potassium (K+) currents plays a critical role in the control of programmed cell death. Because pituitary adenylate cyclase-activating polypeptide (PACAP) has been shown to inhibit the apoptotic cascade in the cerebellar cortex during development, we have investigated the effect of PACAP on K+ currents in cultured cerebellar granule cells using the patch-clamp technique in the whole-cell configuration. Two types of outward K+ currents, a transient K+ current (IA) and a delayed rectifier K+ current (IK) were characterized using two different voltage protocols and specific inhibitors of K+ channels. Application of PACAP induced a reversible reduction of the IK amplitude, but did not affect IA, while the PACAP-related peptide vasoactive intestinal polypeptide had no effect on either types of K+ currents. Repeated applications of PACAP induced gradual attenuation of the electrophysiological response. In the presence of guanosine 5,-[,thio]triphosphate (GTP,S), PACAP provoked a marked and irreversible IK depression, whereas cell dialysis with guanosine 5,-[,thio]diphosphate GDP,S totally abolished the effect of PACAP. Pre-treatment of the cells with pertussis toxin did not modify the effect of PACAP on IK. In contrast, cholera toxin suppressed the PACAP-induced inhibition of IK. Exposure of granule cells to dibutyryl cyclic adenosine monophosphate (dbcAMP) mimicked the inhibitory effect of PACAP on IK. Addition of the specific protein kinase A inhibitor H89 in the patch pipette solution prevented the reduction of IK induced by both PACAP and dbcAMP. PACAP provoked a sustained increase of the resting membrane potential in cerebellar granule cells cultured either in high or low KCl-containing medium, and this long-term depolarizing effect of PACAP was mimicked by the IK specific blocker tetraethylammonium chloride (TEA). In addition, pre-incubation of granule cells with TEA suppressed the effect of PACAP on resting membrane potential. TEA mimicked the neuroprotective effect of PACAP against ethanol-induced apoptotic cell death, and the increase of caspase-3 activity observed after exposure of granule cells to ethanol was also significantly inhibited by TEA. Taken together, the present results demonstrate that, in rat cerebellar granule cells, PACAP reduces the delayed outward rectifier K+ current by activating a type 1 PACAP (PAC1) receptor coupled to the adenylyl cyclase/protein kinase A pathway through a cholera toxin-sensitive Gs protein. Our data also show that PACAP and TEA induce long-term depolarization of the resting membrane potential, promote cell survival and inhibit caspase-3 activity, suggesting that PACAP-evoked inhibition of IK contributes to the anti-apoptotic effect of the peptide on cerebellar granule cells. [source] Voltage-gated ionic currents in an identified modulatory cell type controlling molluscan feedingEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 1 2002Kevin Staras Abstract An important modulatory cell type, found in all molluscan feeding networks, was investigated using two-electrode voltage- and current-clamp methods. In the cerebral giant cells of Lymnaea, a transient inward Na+ current was identified with activation at ,58 ± 2 mV. It was sensitive to tetrodotoxin only in high concentrations (, 50% block at 100 µm), a characteristic of Na+ channels in many molluscan neurons. A much smaller low-threshold persistent Na+ current (activation at <,,90 mV) was also identified. Two purely voltage-sensitive outward K+ currents were also found: (i) a transient A-current type which was activated at ,59 ± 4 mV and blocked by 4-aminopyridine; (ii) a sustained tetraethylammonium-sensitive delayed rectifier current which was activated at ,47 ± 2 mV. There was also evidence that a third, Ca2+ -activated, K+ channel made a contribution to the total outward current. No inwardly rectifying currents were found. Two Ca2+ currents were characterized: (i) a transient low-voltage (,65 ± 2 mV) activated T-type current, which was blocked in NiCl2 (2 mm) and was completely inactivated at ,,,50 mV; (ii) A sustained high voltage (,40 ± 1 mV) activated current, which was blocked in CdCl2 (100 µm) but not in ,-conotoxin GVIA (10 µm), ,-agatoxin IVA (500 nm) or nifedipine (10 µm). This current was enhanced in Ba2+ saline. Current-clamp experiments revealed how these different current types could define the membrane potential and firing properties of the cerebral giant cells, which are important in shaping the wide-acting modulatory influence of this neuron on the rest of the feeding network. [source] Inhibition of the formation or action of angiotensin II reverses attenuated K+ currents in type 1 and type 2 diabetesTHE JOURNAL OF PHYSIOLOGY, Issue 1 2001Yakhin Shimoni 1Transient and sustained calcium-independent outward K+ currents (It and ISS) as well as action potentials were recorded in cardiac ventricular myocytes isolated from two models of diabetes mellitus. 2Rats injected (i.v.) with streptozotocin (STZ, 100 mg kg,1) 6,10 days before cell isolation developed insulin-dependent (type 1) diabetes. It and ISS were attenuated and the action potential prolonged. Incubation of myocytes (6-9 h) with the angiotensin II (ATII) receptor blockers saralasin or valsartan (1 ,m) significantly augmented these currents. Inclusion of valsartan (1 g l,1) in the drinking water for 5,10 days prior to and following STZ injection partially prevented current attenuation. 3Incubation of myocytes from STZ-treated rats (6-9 h) with 1 ,m quinapril, an angiotensin-converting enzyme (ACE) inhibitor, significantly augmented It and ISS and shortened the ventricular action potential. It augmentation was not due to changes in steady-state inactivation or in recovery from inactivation. No acute effects of quinapril were observed. 4The effects of quinapril and valsartan were abolished by 2 ,m cycloheximide. 5Myocytes were isolated from the db/db mouse, a leptin receptor mutant that develops symptoms of non-insulin-dependent (type 2) diabetes. K+ currents in these cells were also attenuated, and the action potentials prolonged. Incubation of these cells (> 6 h) with valsartan (1 ,m) significantly enhanced the transient and sustained outward currents. 6These results confirm recent suggestions that cardiac myocytes contain a renin-angiotensin system, which is activated in diabetes. It is proposed that chronic release of ATII leads to changes in ionic currents and action potentials, which can be reversed by blocking the formation or action of ATII. This may underlie the proven benefits of ATII receptor blockade or ACE inhibition in diabetes, by providing protection against cardiac arrhythmias. [source] Inhibitory effects of glucosamine on lipopolysaccharide-induced activation in microglial cellsCLINICAL AND EXPERIMENTAL PHARMACOLOGY AND PHYSIOLOGY, Issue 12 2005Hyon-Ah Yi Summary 1.,The aim of the present study was to investigate the effects of glucosamine on lipopolysaccharide (LPS)-induced cellular activation in microglia and to evaluate the inhibitory mechanisms involved. 2.,Lipopolysaccharide (100 ng/mL) was used for the activation of primary cultured rat microglial or BV2 microglial cells. Changes in intracellular Ca2+ levels and outward K+ currents were measured using fura-2/AM and whole-cell patch-clamp methods, respectively. Lipopolysaccharide-induced expression of tumour necrosis factor (TNF)-, mRNA was analysed by reverse transcription,polymerase chain reaction. 3.,Lipopolysaccharide transformed cell morphology into an amoeboid shape in vitro and induced microglial activation in vivo, as measured by immunohistochemical staining, but glucosamine inhibited this activation. Glucosamine also inhibited LPS-induced Ca2+ influx, outward K+ currents and TNF-, mRNA expression, which are typically representative of microglial activation. 4.,The results suggest that the inhibitory mechanisms of glucosamine on LPS-induced microglial activation include inhibition of Ca2+ influx and outward K+ currents, as well as downregulation of the microglial activator gene TNF-,. [source] | |||||||||||||