Physiological Roles of the Rapidly Activated Delayed Rectifier K+ Current in Adult Mouse Heart Primary Pacemaker Activity

Robust, spontaneous pacemaker activity originating in the sinoatrial node (SAN) of the heart is essential for cardiovascular function. Anatomical, electrophysiological, and molecular methods as well as mathematical modeling approaches have quite thoroughly characterized the transmembrane fluxes of Na+, K+ and Ca2+ that produce SAN action potentials (AP) and ‘pacemaker depolarizations’ in a number of different in vitro adult mammalian heart preparations. Possible ionic mechanisms that are responsible for SAN primary pacemaker activity are described in terms of: (i) a Ca2+-regulated mechanism based on a requirement for phasic release of Ca2+ from intracellular stores and activation of an inward current-mediated by Na+/Ca2+ exchange; (ii) time- and voltage-dependent activation of Na+ or Ca2+ currents, as well as a cyclic nucleotide-activated current, If; and/or (iii) a combination of (i) and (ii). Electrophysiological studies of single spontaneously active SAN myocytes in both adult mouse and rabbit hearts consistently reveal significant expression of a rapidly activating time- and voltage-dependent K+ current, often denoted IKr, that is selectively expressed in the leading or primary pacemaker region of the adult mouse SAN. The main goal of the present study was to examine by combined experimental and simulation approaches the functional or physiological roles of this K+ current in the pacemaker activity. Our patch clamp data of mouse SAN myocytes on the effects of a pharmacological blocker, E4031, revealed that a rapidly activating K+ current is essential for action potential (AP) repolarization, and its deactivation during the pacemaker potential contributes a small but significant component to the pacemaker depolarization. Mathematical simulations using a murine SAN AP model confirm that well known biophysical properties of a delayed rectifier K+ current can contribute to its role in generating spontaneous myogenic activity.

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Charybdotoxin selectively blocks small Ca-activated K channels in Aplysia neurons.

The Journal of General Physiology ◽

10.1085/jgp.90.1.27 ◽

1987 ◽

Vol 90 (1) ◽

pp. 27-47 ◽

Cited By ~ 85

Author(s):

A Hermann ◽

C Erxleben

Keyword(s):

Single Channel ◽

Delayed Rectifier ◽

Pacemaker Activity ◽

Dependent Manner ◽

Open Probability ◽

Apparent Dissociation Constant ◽

External Application ◽

K Channels ◽

Voltage Dependent ◽

K Current

The action of charybdotoxin (ChTX), a peptide component isolated from the venom of the scorpion Leiurus quinquestriatus, was investigated on membrane currents of identified neurons from the marine mollusk, Aplysia californica. Macroscopic current recordings showed that the external application of ChTX blocks the Ca-activated K current in a dose- and voltage-dependent manner. The apparent dissociation constant is 30 nM at V = -30 mV and increases e-fold for a +50- to +70-mV change in membrane potential, which indicates that the toxin molecule is sensitive to approximately 35% of the transmembrane electric field. The toxin is bound to the receptor with a 1:1 stoichiometry and its effect is reversible after washout. The toxin also suppresses the membrane leakage conductance and a resting K conductance activated by internal Ca ions. The toxin has no significant effect on the inward Na or Ca currents, the transient K current, or the delayed rectifier K current. Records from Ca-activated K channels revealed a single channel conductance of 35 +/- 5 pS at V = 0 mV in asymmetrical K solution. The channel open probability increased with the internal Ca concentration and with membrane voltage. The K channels were blocked by submillimolar concentrations of tetraethylammonium ions and by nanomolar concentrations of ChTX, but were not blocked by 4-aminopyridine if applied externally on outside-out patches. From the effects of ChTX on K current and on bursting pacemaker activity, it is concluded that the termination of bursts is in part controlled by a Ca-activated K conductance.

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A rapidly activating delayed rectifier K+ current regulates pacemaker activity in adult mouse sinoatrial node cells

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.00753.2003 ◽

2004 ◽

Vol 286 (5) ◽

pp. H1757-H1766 ◽

Cited By ~ 47

Author(s):

Robert B. Clark ◽

Matteo E. Mangoni ◽

Andreas Lueger ◽

Brigitte Couette ◽

Joel Nargeot ◽

...

Keyword(s):

Membrane Potential ◽

Action Potential ◽

Adult Mouse ◽

Sinoatrial Node ◽

Physiological Role ◽

Inward Rectification ◽

Delayed Rectifier ◽

Pacemaker Activity ◽

Class Iii ◽

K Current

We have investigated the physiological role of the “rapidly activating” delayed rectifier K+ current ( IKr) in pacemaker activity in isolated sinoatrial node (SAN) myocytes and the expression of mouse ether-a-go-go (mERG) genes in the adult mouse SAN. In isolated, voltage-clamped SAN cells, outward currents evoked by depolarizing steps (greater than –40 mV) were strongly inhibited by the class III methanesulfonanilide compound E-4031 (1–2.5 μM), and the deactivation “tail” currents that occurred during repolarization to a membrane potential of –45 mV were completely blocked. E-4031-sensitive currents ( IKr) reached a maximum at a membrane potential of –10 mV and showed pronounced inward rectification at more-positive membrane potentials. Activation of IKr occurred at –40 to 0 mV, with half-activation at about –24 mV. The contribution of IKr to action potential repolarization and diastolic depolarization was estimated by determining the E-4031-sensitive current evoked during voltage clamp with a simulated mouse SAN action potential. IKr reached its peak value (∼0.6 pA/pF) near –25 mV, close to the midpoint of the repolarization phase of the simulated action potential, and deactivated almost completely during the diastolic interval. E-4031 (1 μM) slowed the spontaneous pacing rate of Langendorff-perfused, isolated adult mouse hearts by an average of 36.5% ( n = 5). Expression of mRNA corresponding to three isoforms coded by the mouse ERG1 gene (mERG1), mERG1a, mERG1a′, and mERG1b, was consistently found in the SAN. Our data provide the first detailed characterization of IKr in adult mouse SAN cells, demonstrate that this current plays an important role in pacemaker activity, and indicate that multiple isoforms of mERG1 can contribute to native SAN IKr.

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Calcium-independent depolarization-activated potassium currents in superior colliculus-projecting rat visual cortical neurons

Journal of Neurophysiology ◽

10.1152/jn.1995.73.6.2163 ◽

1995 ◽

Vol 73 (6) ◽

pp. 2163-2178 ◽

Cited By ~ 17

Author(s):

J. L. Albert ◽

J. M. Nerbonne

Keyword(s):

Superior Colliculus ◽

Cortical Neurons ◽

Outward Current ◽

Delayed Rectifier ◽

Voltage Dependent ◽

K Current ◽

Current Densities ◽

Visual Cortical ◽

K Currents

1. K+ conductances were characterized in isolated, identified superior colliculus-projecting (SCP) rat visual cortical neurons. SCP neurons were identified in vitro under epifluorescence illumination after in vivo retrograde labeling with rhodamine-labeled microspheres or "beads." For experiments, SCP neurons were isolated from the primary visual cortex of postnatal day 7 to 16 (P7-P16) Long Evans rat pups after bead injections into the ipsilateral superior colliculus at p5. 2. Recording conditions were optimized to allow the characterization of Ca2+ -independent K+ conductances. SCP cells that were largely devoid of processes were selected for recording, and experiments were completed 2-30 h after cell isolation. Ca2+ -independent, depolarization-activated K+ currents were routinely recorded during 200-ms voltage steps to potentials positive to -50 mV from a holding potential of -70 mV. 3. Peak outward current densities and the relative amplitudes of the peak and plateau outward currents evoked during 200-ms voltage steps varied among SCP cells. Although cells were isolated from animals at different ages (P7-P16) and maintained for varying times in vitro (2-30 h), no correlations were found between the variations in peak current densities or peak to plateau current ratios and the age of the animal from which the cell was isolated or the length of time the cell was maintained in vitro before recording. 4. Pharmacological experiments revealed the coexpression of three K+ current components in SCP cells that could be separated on the basis of differing sensitivities to the K+ channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). Varying the concentration of 4-AP, for example, facilitated the separation of two rapidly activating K+ currents similar to A (IA) and D(ID) type currents in other cells. ID in SCP neurons is blocked by micromolar concentrations of 4-AP, whereas micromolar concentrations of 4-AP are required to effect complete block of IA in these cells. The current component remaining in the presence of high concentrations (5-10 mM) of 4-AP is slowly activating outward K+ current, similar to delayed rectifier (IK) currents in other cells. IK in SCP neurons is blocked by micromolar concentrations of TEA. 5. Activation of IA, ID, and IK in SCP neurons is voltage dependent, although the three current components display distinct time- and voltage-dependent properties. For example, although both IA and ID begin to activate at approximately -50 mV, IA activates two to three times faster than ID. In addition, the threshold for activation of IK (-30 mV) is approximately 20 mV depolarized from that of IA (or ID), and the voltage dependence of IK activation is steeper than that of IA and ID.(ABSTRACT TRUNCATED AT 400 WORDS)

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Canine Myocytes Represent a Good Model for Human Ventricular Cells Regarding Their Electrophysiological Properties

Pharmaceuticals ◽

10.3390/ph14080748 ◽

2021 ◽

Vol 14 (8) ◽

pp. 748

Author(s):

Péter P. Nánási ◽

Balázs Horváth ◽

Fábián Tar ◽

János Almássy ◽

Norbert Szentandrássy ◽

...

Keyword(s):

Action Potential ◽

Time Course ◽

Laboratory Animals ◽

Delayed Rectifier ◽

Ion Currents ◽

Human Cardiomyocytes ◽

Ventricular Cells ◽

Repolarization Reserve ◽

Electrophysiological Studies ◽

K Current

Due to the limited availability of healthy human ventricular tissues, the most suitable animal model has to be applied for electrophysiological and pharmacological studies. This can be best identified by studying the properties of ion currents shaping the action potential in the frequently used laboratory animals, such as dogs, rabbits, guinea pigs, or rats, and comparing them to those of human cardiomyocytes. The authors of this article with the experience of three decades of electrophysiological studies, performed in mammalian and human ventricular tissues and isolated cardiomyocytes, summarize their results obtained regarding the major canine and human cardiac ion currents. Accordingly, L-type Ca2+ current (ICa), late Na+ current (INa-late), rapid and slow components of the delayed rectifier K+ current (IKr and IKs, respectively), inward rectifier K+ current (IK1), transient outward K+ current (Ito1), and Na+/Ca2+ exchange current (INCX) were characterized and compared. Importantly, many of these measurements were performed using the action potential voltage clamp technique allowing for visualization of the actual current profiles flowing during the ventricular action potential. Densities and shapes of these ion currents, as well as the action potential configuration, were similar in human and canine ventricular cells, except for the density of IK1 and the recovery kinetics of Ito. IK1 displayed a largely four-fold larger density in canine than human myocytes, and Ito recovery from inactivation displayed a somewhat different time course in the two species. On the basis of these results, it is concluded that canine ventricular cells represent a reasonably good model for human myocytes for electrophysiological studies, however, it must be borne in mind that due to their stronger IK1, the repolarization reserve is more pronounced in canine cells, and moderate differences in the frequency-dependent repolarization patterns can also be anticipated.

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Molecular mechanisms of regulation of fast-inactivating voltage-dependent transient outward K+current in mouse heart by cell volume changes

The Journal of Physiology ◽

10.1113/jphysiol.2005.091264 ◽

2005 ◽

Vol 568 (2) ◽

pp. 423-443 ◽

Cited By ~ 16

Author(s):

Guan-Lei Wang ◽

Ge-Xin Wang ◽

Shintaro Yamamoto ◽

Linda Ye ◽

Heather Baxter ◽

...

Keyword(s):

Cell Volume ◽

Molecular Mechanisms ◽

Mouse Heart ◽

Volume Changes ◽

Voltage Dependent ◽

K Current

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Complexity of the outward K+ current of the rat megakaryocyte

AJP Cell Physiology ◽

10.1152/ajpcell.1997.272.5.c1525 ◽

1997 ◽

Vol 272 (5) ◽

pp. C1525-C1531 ◽

Cited By ~ 5

Author(s):

E. Romero ◽

R. Sullivan

Keyword(s):

K Channel ◽

Delayed Rectifier ◽

Channel Blockers ◽

Excitable Cells ◽

Electrophysiological Properties ◽

Relative Contribution ◽

Delayed Rectifier Current ◽

Dependent Inactivation ◽

Voltage Dependent ◽

K Current

Megakaryocytes isolated from rat bone marrow express a voltage-dependent, outward K+ current with complex kinetics of activation and inactivation. We found that this current could be separated into at least two components based on differential responses to K+ channel blockers. One component, which exhibited features of the "transient" or "A-type" K+ current of excitable cells, was more strongly blocked by 4-aminopyridine (4-AP) than by tetrabutylammonium (TBA). This current, which we designated as "4-AP-sensitive" current, activated rapidly at potentials more positive than -40 mV and subsequently underwent rapid voltage-dependent inactivation. A separate current that activated slowly was blocked much more effectively by TBA than by 4-AP. This "TBA-sensitive" component, which resembled a typical delayed rectifier current, was much more resistant to voltage-dependent inactivation. The relative contribution of each of these components varied from cell to cell. The effect of charybdotoxin was similar to that of 4-AP. Our data indicate that the voltage-dependent K+ current of resting megakaryocytes is more complex than heretofore believed and support the emerging concept that megakaryocytes possess intricate electrophysiological properties.

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The development of voltege-dependent ionic conductances In murine spinal cord neurones in culture

Canadian Journal of Physiology and Pharmacology ◽

10.1139/y88-217 ◽

1988 ◽

Vol 66 (10) ◽

pp. 1328-1336 ◽

Cited By ~ 14

Author(s):

C. Krieger ◽

T. A. Sears

Keyword(s):

Spinal Cord ◽

Sodium Current ◽

Potassium Conductance ◽

Calcium Currents ◽

Delayed Rectifier ◽

Patch Clamp Technique ◽

Calcium Dependent ◽

Ionic Conductances ◽

Voltage Dependent

The development of voltage-dependent ionic conductances of foetal mouse spinal cord neurones was examined using the whole-cell patch-clamp technique on neurones cultured from embryos aged 10–12 days (E10–E12) which were studied between the first day in vitro (V1) to V10. A delayed rectifier potassium conductance (IK) and a leak conductance were observed in neurones of E10.V1, E11, V1, and E12, V1 as well as in neurones cultured for longer periods. A rapidly activating and inactivating potassium conductance (IA) was seen in neurones from E11, V2 and E12, V1 and at longer times in vitro. A tetrodotoxin (TTX) sensitive sodium-dependent inward current was observed in neurones of E11 and E12 from V1 onwards. Calcium-dependent conductances were not detectable in these neurones unless the external calcium concentration was raised 10- to 20-foid and potassium conductances were blocked. Under these conditions calcium currents could be observed as early as E11, V3 and E12, V2 and at subsequent times in vitro. The pattern of development of voltage-dependent ionic conductances in murine spinal neurones is such that initially leak and potassium currents are present followed by sodium current and subsequently calcium current.

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A time- and voltage-dependent K+ current in single cardiac cells from bullfrog atrium.

The Journal of General Physiology ◽

10.1085/jgp.88.6.777 ◽

1986 ◽

Vol 88 (6) ◽

pp. 777-798 ◽

Cited By ~ 27

Author(s):

J R Hume ◽

W Giles ◽

K Robinson ◽

E F Shibata ◽

R D Nathan ◽

...

Keyword(s):

Time Course ◽

Voltage Dependence ◽

Outward Current ◽

Cardiac Cells ◽

Delayed Rectifier ◽

Mechanical Dispersion ◽

Atrial Cells ◽

Voltage Dependent ◽

K Current ◽

Kinetics Of

Individual myocytes were isolated from bullfrog atrium by enzymatic and mechanical dispersion, and a one-microelectrode voltage clamp was used to record the slow outward K+ currents. In normal [K+]o (2.5 mM), the slow outward current tails reverse between -95 and -100 mV. This finding, and the observed 51-mV shift of Erev/10-fold change in [K+]o, strongly suggest that the "delayed rectifier" in bullfrog atrial cells is a K+ current. This current, IK, plays an important role in initiating repolarization, and it is distinct from the quasi-instantaneous, inwardly rectifying background current, IK. In atrial cells, IK does not exhibit inactivation, and very long depolarizing clamp steps (20 s) can be applied without producing extracellular K+ accumulation. The possibility of [K+]o accumulation contributing to these slow outward current changes was assessed by (a) comparing reversal potentials measured after short (2 s) and very long (15 s) activating prepulses, and (b) studying the kinetics of IK at various holding potentials and after systematically altering [K+]o. In the absence of [K+]o accumulation, the steady state activation curve (n infinity) and fully activated current-voltage (I-V) relation can be obtained directly. The threshold of the n infinity curve is near -50 mV, and it approaches a maximum at +20 mV; the half-activation point is approximately -16 mV. The fully activated I-V curve of IK is approximately linear in the range -40 to +30 mV. Semilog plots of the current tails show that each tail is a single-exponential function, which suggests that only one Hodgkin-Huxley conductance underlies this slow outward current. Quantitative analysis of the time course of onset of IK and of the corresponding envelope of tails demonstrate that the activation variable, n, must be raised to the second power to fit the sigmoid onset accurately. The voltage dependence of the kinetics of IK was studied by recording and curve-fitting activating and deactivating (tail) currents. The resulting 1/tau n curve is U-shaped and somewhat asymmetric; IK exhibits strong voltage dependence in the diastolic range of potentials. Changes in the [Ca2+]o in the superfusing Ringer's, and/or addition of La3+ to block the transmembrane Ca2+ current, show that the time course and magnitude of IK are not significantly modulated by transmembrane Ca2+ movements, i.e., by ICa. These experimentally measured voltage- and time-dependent descriptors of IK strongly suggest an important functional role for IK in atrial tissue: it initiates repolarization and can be an important determinant of rate-induced changes in action potential duration.

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