Background potassium current active during the plateau of the action potential in guinea pig ventricular myocytes

Contribution of K2P Potassium Channels to Cardiac Physiology and Pathophysiology

Years before the first two-pore domain potassium channel (K2P) was cloned, certain ion channels had already been demonstrated to be present in the heart with characteristics and properties usually attributed to the TREK channels (a subfamily of K2P channels). K2P channels were later detected in cardiac tissue by RT-PCR, although the distribution of the different K2P subfamilies in the heart seems to depend on the species analyzed. In order to collect relevant information in this regard, we focus here on the TWIK, TASK and TREK cardiac channels, their putative roles in cardiac physiology and their implication in coronary pathologies. Most of the RNA expression data and electrophysiological recordings available to date support the presence of these different K2P subfamilies in distinct cardiac cells. Likewise, we show how these channels may be involved in certain pathologies, such as atrial fibrillation, long QT syndrome and Brugada syndrome.

K2p 3.1 protein is expressed as a transmural gradient across the rat left ventricular free wall

Introduction

K_2p3.1, also known as TASK‐1, is a twin‐pore acid‐sensitive repolarizing K⁺ channel, responsible for a background potassium current that significantly contributes to setting the resting membrane potential of cardiac myocytes. Inhibition of I_K2p3.1 alters cardiac repolarization and is proarrhythmogenic. In this study, we have examined the expression of K_2p3.1 and function of this channel in tissue and myocytes from across the left ventricular free wall.

Methods and Results

Using fluorescence immunocytochemistry, the expression of K_2p3.1 protein in myocytes from the subendocardial region was found to be twice (205% ± 13.5%) that found in myocytes from the subepicardial region of the left ventricle (100% ± 5.3%). The left ventricular free wall exhibited a marked transmural gradient of K_2p3.1 protein expression. Western blot analysis confirmed significantly higher K_2p3.1 protein expression in subendocardial tissue (156% ± 2.5%) than subepicardial tissue (100% ± 5.0%). However, there was no difference in K_2p3.1 messenger RNA expression. Whole‐cell patch clamp identified I_K2p3.1 current density to be significantly greater in myocytes isolated from the subendocardium (7.66 ± 0.53 pA/pF) compared with those from the subepicardium (3.47 ± 0.74 pA/pF).

Conclusions

This is the first study to identify a transmural gradient of K_2p3.1 in the left ventricle. This gradient has implications for understanding ventricular arrhythmogenesis under conditions of ischemia but also in response to other modulatory factors, such as adrenergic stimulation and the presence of anesthetics that inhibits or activates this channel.

Contribution of two-pore K+ channels to cardiac ventricular action potential revealed using human iPSC-derived cardiomyocytes

Two-pore K⁺ (K_2p) channels have been described in modulating background conductance as leak channels in different physiological systems. In the heart, the expression of K_2p channels is heterogeneous with equivocation regarding their functional role. Our objective was to determine the K_2p expression profile and their physiological and pathophysiological contribution to cardiac electrophysiology. Induced pluripotent stem cells (iPSCs) generated from humans were differentiated into cardiomyocytes (iPSC-CMs). mRNA was isolated from these cells, commercial iPSC-CM (iCells), control human heart ventricular tissue (cHVT), and ischemic (iHF) and nonischemic heart failure tissues (niHF). We detected 10 K_2p channels in the heart. Comparing quantitative PCR expression of K_2p channels between human heart tissue and iPSC-CMs revealed K_2p1.1, K_2p2.1, K_2p5.1, and K_2p17.1 to be higher expressed in cHVT, whereas K_2p3.1 and K_2p13.1 were higher in iPSC-CMs. Notably, K_2p17.1 was significantly lower in niHF tissues compared with cHVT. Action potential recordings in iCells after K_2p small interfering RNA knockdown revealed prolongations in action potential depolarization at 90% repolarization for K_2p2.1, K_2p3.1, K_2p6.1, and K_2p17.1. Here, we report the expression level of 10 human K_2p channels in iPSC-CMs and how they compared with cHVT. Importantly, our functional electrophysiological data in human iPSC-CMs revealed a prominent role in cardiac ventricular repolarization for four of these channels. Finally, we also identified K_2p17.1 as significantly reduced in niHF tissues and K_2p4.1 as reduced in niHF compared with iHF. Thus, we advance the notion that K_2p channels are emerging as novel players in cardiac ventricular electrophysiology that could also be remodeled in cardiac pathology and therefore contribute to arrhythmias.NEW & NOTEWORTHY Two-pore K⁺ (K_2p) channels are traditionally regarded as merely background leak channels in myriad physiological systems. Here, we describe the expression profile of K_2p channels in human-induced pluripotent stem cell-derived cardiomyocytes and outline a salient role in cardiac repolarization and pathology for multiple K_2p channels.

Species-Dependent Mechanisms of Cardiac Arrhythmia: A Cellular Focus

Although ventricular arrhythmia remains a leading cause of morbidity and mortality, available antiarrhythmic drugs have limited efficacy. Disappointing progress in the development of novel, clinically relevant antiarrhythmic agents may partly be attributed to discrepancies between humans and animal models used in preclinical testing. However, such differences are at present difficult to predict, requiring improved understanding of arrhythmia mechanisms across species. To this end, we presently review interspecies similarities and differences in fundamental cardiomyocyte electrophysiology and current understanding of the mechanisms underlying the generation of afterdepolarizations and reentry. We specifically highlight patent shortcomings in small rodents to reproduce cellular and tissue-level arrhythmia substrate believed to be critical in human ventricle. Despite greater ease of translation from larger animal models, discrepancies remain and interpretation can be complicated by incomplete knowledge of human ventricular physiology due to low availability of explanted tissue. We therefore point to the benefits of mathematical modeling as a translational bridge to understanding and treating human arrhythmia.

Two-Pore K+ Channel TREK-1 Regulates Sinoatrial Node Membrane Excitability

Background

Two‐pore K⁺ channels have emerged as potential targets to selectively regulate cardiac cell membrane excitability; however, lack of specific inhibitors and relevant animal models has impeded the effort to understand the role of 2‐pore K⁺ channels in the heart and their potential as a therapeutic target. The objective of this study was to determine the role of mechanosensitive 2‐pore K⁺ channel family member TREK‐1 in control of cardiac excitability.

Methods and Results

Cardiac‐specific TREK‐1–deficient mice (αMHC‐Kcnk^f/f) were generated and found to have a prevalent sinoatrial phenotype characterized by bradycardia with frequent episodes of sinus pause following stress. Action potential measurements from isolated αMHC‐Kcnk2^f/f sinoatrial node cells demonstrated decreased background K⁺ current and abnormal sinoatrial cell membrane excitability. To identify novel pathways for regulating TREK‐1 activity and sinoatrial node excitability, mice expressing a truncated allele of the TREK‐1–associated cytoskeletal protein β_IV‐spectrin (qv^4J mice) were analyzed and found to display defects in cell electrophysiology as well as loss of normal TREK‐1 membrane localization. Finally, the β_IV‐spectrin/TREK‐1 complex was found to be downregulated in the right atrium from a canine model of sinoatrial node dysfunction and in human cardiac disease.

Conclusions

These findings identify a TREK‐1–dependent pathway essential for normal sinoatrial node cell excitability that serves as a potential target for selectively regulating sinoatrial node cell function.

Gain-of-function mutation in TASK-4 channels and severe cardiac conduction disorder

Analyzing a patient with progressive and severe cardiac conduction disorder combined with idiopathic ventricular fibrillation (IVF), we identified a splice site mutation in the sodium channel gene SCN5A. Due to the severe phenotype, we performed whole-exome sequencing (WES) and identified an additional mutation in the KCNK17 gene encoding the K_2P potassium channel TASK-4. The heterozygous change (c.262G>A) resulted in the p.Gly88Arg mutation in the first extracellular pore loop. Mutant TASK-4 channels generated threefold increased currents, while surface expression was unchanged, indicating enhanced conductivity. When co-expressed with wild-type channels, the gain-of-function by G88R was conferred in a dominant-active manner. We demonstrate that KCNK17 is strongly expressed in human Purkinje cells and that overexpression of G88R leads to a hyperpolarization and strong slowing of the upstroke velocity of spontaneously beating HL-1 cells. Thus, we propose that a gain-of-function by TASK-4 in the conduction system might aggravate slowed conductivity by the loss of sodium channel function. Moreover, WES supports a second hit-hypothesis in severe arrhythmia cases and identified KCNK17 as a novel arrhythmia gene.

The role of acid-sensitive two-pore domain potassium channels in cardiac electrophysiology: focus on arrhythmias

The current kinetics of two-pore domain potassium (K2P) channels resemble those of the steady-state K(+) currents being active during the plateau phase of cardiac action potentials. Recent studies support that K2P channels contribute to these cardiac currents and thereby influence action potential duration in the heart. Ten of the 15 K2P channels present in the human genome are sensitive to variations of the extracellular and/or intracellular pH value. This review focuses on a set of K2P channels which are inhibited by extracellular protons, including the subgroup of tandem of P domains in a weak inward-rectifying K(+) (TWIK)-related acid-sensitive potassium (TASK) and TWIK-related alkaline-activated K(+) (TALK) channels. The role of TWIK-1 in the heart is also discussed since, after successful expression, an extracellular pH dependence, similar to that of TASK-1, was described as a hallmark of TWIK-1. The expression profile in cardiac tissue of different species and the functional data in the heart are summarized. The distinct role of the different acid-sensitive K2P channels in cardiac electrophysiology, inherited forms of arrhythmias and pharmacology, and their role as drug targets is currently emerging and is the subject of this review.

Cardiac ion channels and mechanisms for protection against atrial fibrillation

Atrial fibrillation (AF) is recognised as the most common sustained cardiac arrhythmia in clinical practice. Ongoing drug development is aiming at obtaining atrial specific effects in order to prevent pro-arrhythmic, devastating ventricular effects. In principle, this is possible due to a different ion channel composition in the atria and ventricles. The present text will review the aetiology of arrhythmias with focus on AF and include a description of cardiac ion channels. Channels that constitute potentially atria-selective targets will be described in details. Specific focus is addressed to the recent discovery that Ca(2+)-activated small conductance K(+) channels (SK channels) are important for the repolarisation of atrial action potentials. Finally, an overview of current pharmacological treatment of AF is included.

TREK-1 K(+) channels in the cardiovascular system: their significance and potential as a therapeutic target

Potassium (K(+) ) channels are important in cardiovascular disease both as drug targets and as a cause of underlying pathology. Voltage-dependent K(+) (K(V) ) channels are inhibited by the class III antiarrhythmic agents. Certain vasodilators work by opening K(+) channels in vascular smooth muscle cells (VSMCs), and K(+) channel activation may also be a route to improving endothelial function. The two-pore domain K(+) (K(2P) ) channels form a group of 15 known channels with an expanding list of functions in the cardiovascular system. One of these K(2P) channels, TREK-1, is the focus of this review. TREK-1 channel activity is tightly regulated by intracellular and extracellular pH, membrane stretch, polyunsaturated fatty acids (PUFAs), temperature, and receptor-coupled second messenger systems. TREK-1 channels are also activated by volatile anesthetics and some neuroprotectant agents, and they are inhibited by selective serotonin reuptake inhibitors (SSRIs) as well as amide local anesthetics. Some of the clinical cardiovascular effects and side effects of these drugs may be through their actions on TREK-1 channels. It has recently been suggested that TREK-1 channels have a role in mechano-electrical coupling in the heart. They also seem important in the vascular responses to PUFAs, and this may underlie some of the beneficial cardiovascular effects of the essential dietary fatty acids. Development of selective TREK-1 openers and inhibitors may provide promising routes for intervention in cardiovascular diseases.

Repolarization of the cardiac action potential. Does an increase in repolarization capacity constitute a new anti-arrhythmic principle?

The cardiac action potential can be divided into five distinct phases designated phases 0-4. The exact shape of the action potential comes about primarily as an orchestrated function of ion channels. The present review will give an overview of ion channels involved in generating the cardiac action potential with special emphasis on potassium channels involved in phase 3 repolarization. In humans, these channels are primarily K(v)11.1 (hERG1), K(v)7.1 (KCNQ1) and K(ir)2.1 (KCNJ2) being the responsible alpha-subunits for conducting I(Kr), I(Ks) and I(K1). An account will be given about molecular components, biophysical properties, regulation, interaction with other proteins and involvement in diseases. Both loss and gain of function of these currents are associated with different arrhythmogenic diseases. The second part of this review will therefore elucidate arrhythmias and subsequently focus on newly developed chemical entities having the ability to increase the activity of I(Kr), I(Ks) and I(K1). An evaluation will be given addressing the possibility that this novel class of compounds have the ability to constitute a new anti-arrhythmic principle. Experimental evidence from in vitro, ex vivo and in vivo settings will be included. Furthermore, conceptual differences between the short QT syndrome and I(Kr) activation will be accounted for.

Targeting TASK-1 channels as a therapeutic approach

The voltage-independent background two-pore domain K(+) channel TASK-1 sets the resting membrane potential in excitable cells and renders these cells sensitive to a variety of vasoactive factors. There is clear evidence for TASK-1 in human pulmonary artery smooth muscle cells and TASK-1 channels are likely to regulate the pulmonary vascular tone through their regulation by hypoxia, pH, inhaled anesthetics, and G protein-coupled pathways. Furthermore, TASK-1 is a strong candidate to play a role in hypoxic pulmonary vasoconstriction. On the other hand, consistent with the activation of TASK-1 channels by volatile anesthetics, TASK-1 contributes to the anesthetic-induced pulmonary vasodilation. TASK-1 channels are unique among K(+) channels because they are regulated by both, increases and decreases from physiological pH, thus contributing to their protective effect on the pulmonary arteries. Moreover, TASK-1 may also have a critical role in mediating the vasoactive response of G protein-coupled pathways in resistance arteries which can offer promising therapeutic solutions to target diseases of the pulmonary circulation.

Molecular mechanisms underlying membrane-potential-mediated regulation of neuronal K2P2.1 channels

The activity of background K(2P) channels adjusts the resting membrane potential to enable plasticity of excitable cells. Here we have studied the regulation of neuronal K(2P)2.1 (KCNK2, TREK-1) channel activity by resting membrane potential. When heterologously expressed, K(2P)2.1 currents gradually increased at hyperpolarizing potentials and declined at depolarizing potentials, with a midpoint potential of -60 mV. As K(2P) channels are not equipped with an integral voltage sensor, we sought extrinsic cellular components that could convert changes in the membrane electrical field to cellular activity that would indirectly modify K(2P)2.1 currents. We propose that membrane depolarization activated the Gq protein-coupled receptor pathway, in the apparent absence of ligand, resulting in phosphatidylinositol-4,5-bisphosphate (PIP(2)) depletion through the action of phospholipase C. Our results suggest a novel mechanism in which an indirect pathway confers membrane potential regulation onto channels that are not intrinsically voltage sensitive to enhance regulation of neuronal excitability levels.

Gating the pore of potassium leak channels

A key feature of potassium channel function is the ability to switch between conducting and non-conducting states by undergoing conformational changes in response to cellular or extracellular signals. Such switching is facilitated by the mechanical coupling of gating domain movements to pore opening and closing. Two-pore domain potassium channels (K(2P)) conduct leak or background potassium-selective currents that are mostly time- and voltage-independent. These channels play a significant role in setting the cell resting membrane potential and, therefore modulate cell responsiveness and excitability. Thus, K(2P) channels are key players in numerous physiological processes and were recently shown to also be involved in human pathologies. It is well established that K(2P) channel conductance, open probability and cell surface expression are significantly modulated by various physical and chemical stimuli. However, in understanding how such signals are translated into conformational changes that open or close the channels gate, there remain more open questions than answers. A growing line of evidence suggests that the outer pore area assumes a critical role in gating K(2P) channels, in a manner reminiscent of C-type inactivation of voltage-gated potassium channels. In some K(2P) channels, this gating mechanism is facilitated in response to external pH levels. Recently, it was suggested that K(2P) channels also possess a lower activation gate that is positively coupled to the outer pore gate. The purpose of this review is to present an up-to-date summary of research describing the conformational changes and gating events that take place at the K(2P) channel ion-conducting pathway during the channel regulation.

Endothelin-1 inhibits background two-pore domain channel TASK-1 in primary human pulmonary artery smooth muscle cells

Endothelin (ET)-1 causes long-lasting vasoconstriction and vascular remodeling by interacting with specific G-protein-coupled receptors in pulmonary artery smooth muscle cells (PASMCs), and thus plays an important role in the pathophysiology of pulmonary arterial hypertension. The two-pore domain K(+) channel, TASK-1, controls the resting membrane potential in human PASMCs (hPASMCs), and renders these cells sensitive to a variety of vasoactive factors, as previously shown. ET-1 may exert its vasoconstrictive effects in part by targeting TASK-1. To clarify this, we analyzed the ET-1 signaling pathway related to TASK-1 in primary hPASMCs. We employed the whole-cell patch-clamp technique combined with TASK-1 small interfering RNA (siRNA) in hPASMC and the isolated, perfused, and ventilated mouse lung model. We found that ET-1 depolarized primary hPASMCs by phosphorylating TASK-1 at clinically relevant concentrations. The ET sensitivity of TASK-1 required ET(A) receptors, phospholipase C, phosphatidylinositol 4,5-biphosphate, diacylglycerol, and protein kinase C in primary hPASMCs. The ET-1 effect on membrane potential and TASK-1 was abrogated using TASK-1 siRNA. This is the first time that the background K(+) channel, TASK-1, has been identified in the ET-1-mediated depolarization in native hPASMC, and might represent a novel pathologic mechanism related to pulmonary arterial hypertension.

Computational biology in the study of cardiac ion channels and cell electrophysiology

The cardiac cell is a complex biological system where various processes interact to generate electrical excitation (the action potential, AP) and contraction. During AP generation, membrane ion channels interact nonlinearly with dynamically changing ionic concentrations and varying transmembrane voltage, and are subject to regulatory processes. In recent years, a large body of knowledge has accumulated on the molecular structure of cardiac ion channels, their function, and their modification by genetic mutations that are associated with cardiac arrhythmias and sudden death. However, ion channels are typically studied in isolation (in expression systems or isolated membrane patches), away from the physiological environment of the cell where they interact to generate the AP. A major challenge remains the integration of ion-channel properties into the functioning, complex and highly interactive cell system, with the objective to relate molecular-level processes and their modification by disease to whole-cell function and clinical phenotype. In this article we describe how computational biology can be used to achieve such integration. We explain how mathematical (Markov) models of ion-channel kinetics are incorporated into integrated models of cardiac cells to compute the AP. We provide examples of mathematical (computer) simulations of physiological and pathological phenomena, including AP adaptation to changes in heart rate, genetic mutations in SCN5A and HERG genes that are associated with fatal cardiac arrhythmias, and effects of the CaMKII regulatory pathway and beta-adrenergic cascade on the cell electrophysiological function.

Two-pore-domain potassium channels in smooth muscles: new components of myogenic regulation

Gastrointestinal (GI) smooth muscles are influenced by many levels of regulation, including those provided by enteric motor neurones, hormones and paracrine substances. The integrated contractile responses to these regulatory mechanisms depend heavily on the state of excitability of smooth muscle cells. Resting ionic conductances and myogenic responses to agonists and physical parameters, such as stretch, are important in establishing basal excitability. This review discusses the role of 2-pore-domain K+ channels in contributing to background conductances and in mediating responses of GI muscles to enteric inhibitory nerve stimulation and stretch. Murine GI muscles express TREK-1 channels and display a stretch-dependent K+ (SDK) conductance that is also activated by nitric oxide via a cGMP-dependent mechanism. Cloning and expression of mTREK-1 produced an SDK conductance that was activated by cGMP-dependent phosphorylation at serine-351. GI muscle cells also express TASK-1 and TASK-2 channels that are inhibited by lidocaine and external acidification. These conductances appear to provide significant background K+ permeability that contributes to the negative resting potentials of GI muscles.

Molecular physiology of cardiac repolarization

The heart is a rhythmic electromechanical pump, the functioning of which depends on action potential generation and propagation, followed by relaxation and a period of refractoriness until the next impulse is generated. Myocardial action potentials reflect the sequential activation and inactivation of inward (Na(+) and Ca(2+)) and outward (K(+)) current carrying ion channels. In different regions of the heart, action potential waveforms are distinct, owing to differences in Na(+), Ca(2+), and K(+) channel expression, and these differences contribute to the normal, unidirectional propagation of activity and to the generation of normal cardiac rhythms. Changes in channel functioning, resulting from inherited or acquired disease, affect action potential repolarization and can lead to the generation of life-threatening arrhythmias. There is, therefore, considerable interest in understanding the mechanisms that control cardiac repolarization and rhythm generation. Electrophysiological studies have detailed the properties of the Na(+), Ca(2+), and K(+) currents that generate cardiac action potentials, and molecular cloning has revealed a large number of pore forming (alpha) and accessory (beta, delta, and gamma) subunits thought to contribute to the formation of these channels. Considerable progress has been made in defining the functional roles of the various channels and in identifying the alpha-subunits encoding these channels. Much less is known, however, about the functioning of channel accessory subunits and/or posttranslational processing of the channel proteins. It has also become clear that cardiac ion channels function as components of macromolecular complexes, comprising the alpha-subunits, one or more accessory subunit, and a variety of other regulatory proteins. In addition, these macromolecular channel protein complexes appear to interact with the actin cytoskeleton and/or the extracellular matrix, suggesting important functional links between channel complexes, as well as between cardiac structure and electrical functioning. Important areas of future research will be the identification of (all of) the molecular components of functional cardiac ion channels and delineation of the molecular mechanisms involved in regulating the expression and the functioning of these channels in the normal and the diseased myocardium.

Ionic currents involved in shock-induced nonlinear changes in transmembrane potential responses of single cardiac cells

An exhaustive characterization of how an isolated cardiac cell responds to applied electric fields could serve as an important groundwork for understanding responses of more complex higher order systems. Field stimulation of single cardiac cells during the early plateau of the action potential results in a nonuniform change in transmembrane potential (Vm) across the cell length that is more heavily weighted in the negative direction. These negatively shifted Vm responses are not replicated theoretically using present day membrane models. The goal of this study was to explore the membrane currents involved in the field responses during the plateau by selectively blocking various ion channels. Enzymatically isolated single guinea pig cells were stimulated with uniform field S1-S2 pulses, and the transmembrane potential responses were optically recorded from several sites along the cell length to assess the drug effect. We used nine different pharmacological agents to manipulate the conductance of major cardiac ion channels of which only barium (Ba2+) altered the transmembrane potential responses. At 50 microM Ba2+, which specifically blocks inwardly rectifying current I(K1), the negative shift in Vm responses was accentuated. At 1 mM Ba2+ , which blocks both I(K1) and sustained plateau current I(Kp), the negative shift diminished. However, 1 mM Ba2+ also depolarized the cells, and depressed or completely eliminated the action potential. Based on these results we conclude that I(K1) contributes to field-induced responses during the plateau stimulation by passing a net inward current, which when blocked accentuates the negative shift in the Vm responses. A conclusive role of I(Kp) could not be demonstrated because of confounding changes in membrane potential. However, from our results it remains as the most viable candidate for the elusive current that contributes a net outward current to produce negatively weighted Vm responses during plateau stimulation and warrants further investigation.

Evidence for a Novel K+Channel Modulated by α1A-Adrenoceptors in Cardiac Myocytes

Accumulating evidence suggests that steady-state K(+) currents modulate excitability and action potential duration, particularly in cardiac cell types with relatively abbreviated action potential plateau phases. Despite representing potential drug targets, at present these currents and their modulation are comparatively poorly characterized. Therefore, we investigated the effects of phenylephrine [PE; an alpha(1)-adrenoceptor (alpha(1)-AR) agonist] on a sustained outward K(+) current in rat ventricular myocytes. Under K(+) current-selective conditions at 35 degrees C and whole-cell patch clamp, membrane depolarization elicited transient (I(t)) and steady-state (I(ss)) outward current components. PE (10 microM) significantly decreased I(ss) amplitude, without significant effect on I(t). Preferential modulation of I(ss) by PE was confirmed by intracellular application of the voltage-gated K(+) channel blocker tetraethylammonium, which largely inhibited I(t) without affecting the PE-sensitive current (I(ss,PE)). I(ss,PE) had the properties of an outwardly rectifying steady-state K(+)-selective conductance. Acidification of the external solution or externally applied BaCl(2) or quinidine strongly inhibited I(ss,PE). However, I(ss,PE) was not abolished by anandamide, ruthenium red, or zinc, inhibitors of TASK acid-sensitive background K(+) channels. Furthermore, the PE-sensitive current was partially inhibited by external administration of high concentrations of tetraethylammonium and 4-aminopyridine, which are voltage-gated K(+) channel-blockers. Power spectrum analysis of I(ss,PE) yielded a large unitary conductance of 78 pS. I(ss,PE) resulted from PE activation of the alpha(1A)-AR subtype, involved a pertussis toxin-insensitive G-protein, and was independent of cytosolic Ca(2+). These results collectively demonstrate that alpha(1A)-AR activation results in the inhibition of an outwardly rectifying steady-state K(+) current with properties distinct from previously characterized cardiac K(+) channels.

Mutual functional destruction of HIV-1 Vpu and host TASK-1 channel

Sequence analysis predicted significant structural homology between the HIV-1 accessory protein Vpu and the N-terminal region of TASK-1, a mammalian background K(+) channel. If the homology resulted from molecular piracy during HIV-1 evolution, these two proteins may have important functional interactions. Here we demonstrate that TASK and Vpu physically interact in cultured cells and in AIDS lymphoid tissues. The functional consequences were potentially destructive for both components: Vpu abolished TASK-1 current, while overexpressing TASK led to a marked impairment of Vpu's ability to enhance viral particle release. Further, the first 40 amino acids of TASK-1 (part of the homology to Vpu) were capable of enhancing HIV-1 particle release. This virus-host interaction may influence HIV-1/AIDS progression, as well as electrical signaling in infected host tissues.

Actions of the anti-oestrogen agent clomiphene on outward K+ currents in rat ventricular myocytes

1. The effects of clomiphene (CLM) on cardiac outward K+ current components from rat isolated ventricular myocytes were investigated using the whole-cell patch-clamp technique. Clomiphene (10 micromol/L) significantly inhibited both peak (Ipeak) and end-pulse (Ilate) outward currents (elicited by a 500 msec voltage step from -40 to +50 mV in the presence of K+-containing intracellular and extracellular solutions) by approximately 37% (n = 6; P < 0.01) and 49% (n = 6; P < 0.01), respectively. In contrast, CLM had no effect on outward currents when K+-free solutions were used. 2. A double-pulse protocol and Boltzmann fitting were used to separate individual K+ current components on the basis of their voltage-dependent inactivation properties. At potentials positive to -80 mV, two inactivating transient outward components (Ito) and (IKx) and a non-inactivating steady state component (Iss) could be distinguished. 3. Clomiphene inhibited both Ito and Iss. The maximal block of Ito and Iss induced by CLM (100 micromol/L) was approximately 61% (n = 5) and 43% (n = 5) with IC50 values of 1.54 +/- 0.39 and 2.2 +/- 0.4 micromol/L, respectively. In contrast, the peak magnitude of IKx was unaltered by CLM, although its time-course of inactivation was accelerated. 4. Further experiments whereby myocytes were superfused with the vasoactive peptide endothelin (ET)-1 (20 nmol/L) revealed that CLM (10 micro mol/L) completely abolished the ET-1-sensitive component of Iss. 5. Our findings demonstrate, for the first time, the effects of CLM on distinct cardiac K+ current components and show that CLM modulates the voltage-gated K+ current components Ito and IKx and inhibits the steady state outward current Iss in rat ventricular myocytes.

Structural and functional basis for the long QT syndrome: relevance to veterinary patients

Long QT syndrome (LQTS) is a condition characterized by prolongation of ventricular repolarization and is manifested clinically by lengthening of the QT interval on the surface ECG. Whereas inherited forms of LQTS associated with mutations in the genes that encode ion channel proteins are identified only in humans, the acquired form of LQTS occurs in humans and companion animal species. Often, acquired LQTS is associated with drug-induced block of the cardiac K+ current designated I(Kr). However, not all drugs that induce potentially fatal ventricular arrhythmias antagonize I(Kr), and not all drugs that block I(Kr), are associated with ventricular arrhythmias. In clinical practice, the extent of QT interval prolongation and risk of ventricular arrhythmia associated with antagonism of I(Kr) are modulated by pharmacokinetic and pharmacodynamic variables. Veterinarians can influence some of the potential risk factors (eg, drug dosage, route of drug administration, presence or absence of concurrent drug therapy, and patient electrolyte status) but not all (eg, patient gender/genetic background). Veterinarians need to be aware of the potential for acquired LQTS during therapy with drugs identified as blockers of HERG channels and I(Kr).

Two-pore domain K+ channels-molecular sensors

Two-pore domain K(+) (K2P) channels have been cloned from a variety of species and tissues. They have been characterised biophysically as a 'background' K(+)-selective conductance and are gated by pH, stretch, heat, coupling to G-proteins and anaesthetics. Whilst their precise physiological function is unknown, they are likely to represent an increasingly important family of membrane proteins.

Expression of TASK-1, a pH-sensitive twin-pore domain K(+) channel, in rat myocytes

We have investigated the expression of TASK-1, a pH-sensitive, twin-pore domain K(+) channel in the rat heart. A mammalian cell line of Chinese hamster ovary cells (CHO), transfected with a plasmid containing mouse TASK-1, demonstrated the specificity of the anti-TASK-1 antibody. TASK-1 expression in cardiac tissue was initially demonstrated by Western blot and then localized by immunofluorescence. In single rat ventricular myocytes, strong staining of the TASK-1 protein was located at the intercalated disks and across the cell in a striated pattern, corresponding to the transverse axial tubular network (T tubules). In contrast, single rat atrial myocytes were stained at the intercalated disks with a weak punctate, striated pattern corresponding to underdeveloped T tubules. Also, formamide was used to induce the detubulation of ventricular myocytes, which enabled confirmation that TASK-1 protein expression occurs in T tubules. Consistent with this, RT-PCR revealed the expression of TASK-1 mRNA in total RNA from both the ventricles and atria. In this study, we conclusively demonstrated that TASK-1 protein and mRNA were expressed in rat atrial and ventricular tissue. The extensive distribution of TASK-1 shown to exist within myocyte membranes may provide a potential future target for antiarrhythmic drugs.

Genistein inhibits the inward rectifying potassium current in guinea pig ventricular myocytes

Genistein is an isoflavone with potent inhibitory activity on protein tyrosine kinase. Previous studies have shown that genistein has additional effects, among which the direct blocking effects on various ionic channels have recently been disclosed. Using whole-cell voltage clamp and current clamp techniques, we demonstrate that micromolar concentrations of genistein dose-dependently and reversibly inhibit the inward rectifying K(+) current, and depolarize the resting membrane potential, resulting in abnormal automaticity in guinea pig ventricular myocytes. Interestingly, another potent tyrosine kinase inhibitor, tyrphostin 51, did not produce the same inhibitory effect, while the inactive analogue of genistein, daidzein, had a similar blocking effect. We suggest that genistein directly blocks the inward rectifying K(+) current in ventricular myocytes, and one should be cautious of its pro-arrhythmic effect in clinical use.

Cardiac ion channels

The normal electrophysiologic behavior of the heart is determined by ordered propagation of excitatory stimuli that result in rapid depolarization and slow repolarization, thereby generating action potentials in individual myocytes. Abnormalities of impulse generation, propagation, or the duration and configuration of individual cardiac action potentials form the basis of disorders of cardiac rhythm, a continuing major public health problem for which available drugs are incompletetly effective and often dangerous. The integrated activity of specific ionic currents generates action potentials, and the genes whose expression results in the molecular components underlying individual ion currents in heart have been cloned. This review discusses these new tools and how their application to the problem of arrhythmias is generating new mechanistic insights to identify patients at risk for this condition and developing improved antiarrhythmic therapies.

KCNKØ: opening and closing the 2-P-domain potassium leak channel entails "C-type" gating of the outer pore

Essential to nerve and muscle function, little is known about how potassium leak channels operate. KCNKØ opens and closes in a kinase-dependent fashion. Here, the transition is shown to correspond to changes in the outer aspect of the ion conduction pore. Voltage-gated potassium (VGK) channels open and close via an internal gate; however, they also have an outer pore gate that produces "C-type" inactivation. While KCNKØ does not inactivate, KCNKØ and VGK channels respond in like manner to outer pore blockers, potassium, mutations, and chemical modifiers. Structural relatedness is confirmed: VGK residues that come close during C-type gating predict KCNKØ sites that crosslink (after mutation to cysteine) to yield channels controlled by reduction and oxidization. We conclude that similar outer pore gates mediate KCNKØ opening and closing and VGK channel C-type inactivation despite their divergent structures and physiological roles.

Effects of endothelin-1 on K(+) currents from rat ventricular myocytes

It has been suggested that the positive inotropic effect of the vasoactive peptide hormone, endothelin-1 (ET-1), involves inhibition of cardiac K(+) currents. In order to identify the K(+) currents modulated by ET-1, the outward K(+) currents of isolated rat ventricular myocytes were investigated using whole-cell patch-clamp recording techniques. Outward currents were elicited by depolarisation to +40 mV for 200 ms from the holding potential of -60 mV. Currents activated rapidly, reaching a peak (I(pk)) of 1310 +/- 115 pA and subsequently inactivating to an outward current level of 1063 +/- 122 pA at the end of the voltage-pulse (I(late)) (n = 11). ET-1 (20 nM) reduced I(pk) by 247.6 +/- 60.7 pA (n = 11, P < 0.01) and reduced I(late) by 323.2 +/- 43.9 pA (P < 0.001). The effects of ET-1 were abolished in the presence of the nonselective ET receptor antagonist, PD 142893 (10 microM, n = 5). Outward currents were considerably reduced and the effects of ET-1 were not observed when K(+) was replaced with Cs(+) in the experimental solutions; this indicates that ET-1 modulated K(+)-selective currents. A double-pulse protocol was used to investigate the inactivation of the currents. The voltage-dependent inactivation of the currents from potentials positive to -80 mV was fitted by a Boltzmann equation revealing the existence of an inactivating transient outward component (I(to)) and a noninactivating steady-state component (I(ss)). ET-1 markedly inhibited I(ss) by 43.0 +/- 3.8% (P < 0.001, n = 7) and shifted the voltage-dependent inactivation of I(to) by +3.3 +/- 1.2 mV (P < 0.05). Although ET-1 had little effect on the onset of inactivation of the currents elicited from a conditioning potential of -70 mV, the time-independent noninactivating component of the currents was markedly inhibited. In conclusion, the predominant effect of ET-1 was to inhibit a noninactivating steady-state background K(+) current (I(ss)). These results are consistent with the hypothesis that I(ss) inhibition contributes to the inotropic effects of ET-1.

KCNK2: reversible conversion of a hippocampal potassium leak into a voltage-dependent channel

Potassium leak channels are essential to neurophysiological function. Leaks suppress excitability through maintenance of resting membrane potential below the threshold for action potential firing. Conversely, voltage-dependent potassium channels permit excitation because they do not interfere with rise to threshold, and they actively promote recovery and rapid re-firing. Previously attributed to distinct transport pathways, we demonstrate here that phosphorylation of single, native hippocampal and cloned KCNK2 potassium channels produces reversible interconversion between leak and voltage-dependent phenotypes. The findings reveal a pathway for dynamic regulation of excitability.

Characterization of TASK-4, a novel member of the pH-sensitive, two-pore domain potassium channel family

We report the primary sequence of TASK-4, a novel member of the acid-sensitive subfamily of tandem pore K(+) channels. TASK-4 transcripts are widely expressed in humans, with highest levels in liver, lung, pancreas, placenta, aorta and heart. In Xenopus oocytes TASK-4 generated K(+) currents displaying a marked outward rectification which was lost by elevation of extracellular K(+). TASK-4 currents were efficiently blocked by barium (83% inhibition at 2 mM), only weakly inhibited by 1 mM concentrations of quinine, bupivacaine and lidocaine, but not blocked by tetraethylammonium, 4-aminopyridine and Cs(+). TASK-4 was sensitive to extracellular pH, but in contrast to other TASK channels, pH sensitivity was shifted to more alkaline pH. Thus, TASK-4 in concert with other TASK channels might regulate cellular membrane potential over a wide range of extracellular pH.

Potassium leak channels and the KCNK family of two-P-domain subunits

With a bang, a new family of potassium channels has exploded into view. Although KCNK channels were discovered only five years ago, they already outnumber other channel types. KCNK channels are easy to identify because of their unique structure--they possess two preforming domains in each subunit. The new channels function in a most remarkable fashion: they are highly regulated, potassium-selective leak channels. Although leak currents are fundamental to the function of nerves and muscles, the molecular basis for this type of conductance had been a mystery. Here we review the discovery of KCNK channels, what has been learned about them and what lies ahead. Even though two-P-domain channels are widespread and essential, they were hidden from sight in plain view--our most basic questions remain to be answered.

Kcnkø: single, cloned potassium leak channels are multi-ion pores

KCNKØ was the first clone to show attributes of a leak conductance: voltage-independent potassium currents that develop without delay. Its novel product is predicted to have two nonidentical P domains and four transmembrane segments and to assemble in pairs. Here, the mechanistic basis for leak is examined at the single-channel level. KCNKØ channels open at all voltages in bursts that last for minutes with open probability close to 1. The channels also enter a minutes-long closed state in a tightly regulated fashion. KCNKØ has a common relative permeability series (Eisenman type IV) but conducts only thallium and potassium readily. KCNKØ exhibits concentration-dependent unitary conductance, anomalous mole fraction behavior, and pore occlusion by barium. These observations argue for ion-channel and ion-ion interactions in a multi-ion pore and deny the operation of independence or constant-field current formulations. Despite their unique function and structure, leakage channels are observed to operate like classical potassium channels formed with one-P-domain subunits.

Opening and closing of KCNKO potassium leak channels is tightly regulated

Potassium-selective leak channels control neuromuscular function through effects on membrane excitability. Nonetheless, their existence as independent molecular entities was established only recently with the cloning of KCNKO from Drosophila melanogaster. Here, the operating mechanism of these 2 P domain leak channels is delineated. Single KCNKO channels switch between two long-lived states (one open and one closed) in a tenaciously regulated fashion. Activation can increase the open probability to approximately 1, and inhibition can reduce it to approximately 0.05. Gating is dictated by a 700-residue carboxy-terminal tail that controls the closed state dwell time but does not form a channel gate; its deletion (to produce a 300-residue subunit with two P domains and four transmembrane segments) yields unregulated leak channels that enter, but do not maintain, the closed state. The tail integrates simultaneous input from multiple regulatory pathways acting via protein kinases C, A, and G.

Molecular and functional properties of two-pore-domain potassium channels

The two-pore-domain K(+) channels, or K(2P) channels, constitute a novel class of K(+) channel subunits. They have four transmembrane segments and are active as dimers. The tissue distribution of these channels is widespread, and they are found in both excitable and nonexcitable cells. K(2P) channels produce currents with unusual characteristics. They are quasi-instantaneous and noninactivating, and they are active at all membrane potentials and insensitive to the classic K(+) channel blockers. These properties designate them as background K(+) channels. They are expected to play a major role in setting the resting membrane potential in many cell types. Another salient feature of K(2P) channels is the diversity of their regulatory mechanisms. The weak inward rectifiers TWIK-1 and TWIK-2 are stimulated by activators of protein kinase C and decreased by internal acidification, the baseline TWIK-related acid-sensitive K(+) (TASK)-1 and TASK-2 channels are sensitive to external pH changes in a narrow range near physiological pH, and the TWIK-related (TREK)-1 and TWIK-related arachidonic acid-stimulated K(+) (TRAAK) channels are the first cloned polyunsaturated fatty acids-activated and mechanogated K(+) channels. The recent demonstration that TASK-1 and TREK-1 channels are activated by inhalational general anesthetics, and that TRAAK is activated by the neuroprotective agent riluzole, indicates that this novel class of K(+) channels is an interesting target for new therapeutic developments.

Proton block and voltage gating are potassium-dependent in the cardiac leak channel Kcnk3

Potassium leak conductances were recently revealed to exist as independent molecular entities. Here, the genomic structure, cardiac localization, and biophysical properties of a murine example are considered. Kcnk3 subunits have two pore-forming P domains and unique functional attributes. At steady state, Kcnk3 channels behave like open, potassium-selective, transmembrane holes that are inhibited by physiological levels of proton. With voltage steps, Kcnk3 channels open and close in two phases, one appears to be immediate and one is time-dependent (tau = approximately 5 ms). Both proton block and gating are potassium-sensitive; this produces an anomalous increase in outward flux as external potassium levels rise because of decreased proton block. Single Kcnk3 channels open across the physiological voltage range; hence they are "leak" conductances; however, they open only briefly and rarely even after exposure to agents that activate other potassium channels.

Molecular basis of functional voltage-gated K+ channel diversity in the mammalian myocardium

In the mammalian heart, Ca2+-independent, depolarization-activated potassium (K+) currents contribute importantly to shaping the waveforms of action potentials, and several distinct types of voltage-gated K+ currents that subserve this role have been characterized. In most cardiac cells, transient outward currents, Ito,f and/or Ito,s, and several components of delayed reactivation, including IKr, IKs, IKur and IK,slow, are expressed. Nevertheless, there are species, as well as cell-type and regional, differences in the expression patterns of these currents, and these differences are manifested as variations in action potential waveforms. A large number of voltage-gated K+ channel pore-forming (alpha) and accessory (beta, minK, MiRP) subunits have been cloned from or shown to be expressed in heart, and a variety of experimental approaches are being exploited in vitro and in vivo to define the relationship(s) between these subunits and functional voltage-gated cardiac K+ channels. Considerable progress has been made in defining these relationships recently, and it is now clear that distinct molecular entities underlie the various electrophysiologically distinct repolarizing K+ currents (i.e. Ito,f, Ito,s, IKr, IKs, IKur, IK,slow, etc.) in myocyardial cells.

Four different components contribute to outward current in rat ventricular myocytes

In rat ventricle, two Ca(2+)-insensitive components of K(+) current have been distinguished kinetically and pharmacologically, the transient, 4-aminopyridine (4-AP)-sensitive I(to) and the sustained, tetraethylammonium (TEA)-sensitive I(K). However, a much greater diversity of depolarization-activated K(+) channels has been reported on the level of mRNA and protein. In the search for electrophysiological evidence of further current components, the whole cell voltage-clamp technique was used to analyze steady-state inactivation of outward currents by conditioning potentials in a wide voltage range. Peak (I(peak)) and late (I(late)) currents during the test pulse were analyzed by Boltzmann curve fitting, producing three fractions each. Fractions a and b had different potentials of half-maximum inactivation (V(0.5)); the third residual fraction, r, did not inactivate. Fractions a for I(peak) and I(late) had similar relative amplitudes and V(0.5) values, whereas size and V(0.5) of fractions b differed significantly between I(peak) and I(late). Only b of I(peak) was transient, suggesting a relation with I(to), whereas a, b, and r of I(late) appeared to be three different sustained currents. Therefore, four individual outward current components were distinguished: I(to) (b of I(peak)), I(K) (a), the steady-state current I(ss) (r), and the novel current I(Kx) (b of I(late)). This was further supported by differential sensitivity to TEA, 4-AP, clofilium, quinidine, dendrotoxin, heteropodatoxin, and hanatoxin. With the exception of I(to), none of the currents exhibited a marked transmural gradient. Availability of I(K) was low at resting potential; nevertheless, I(K) contributed to action potential shortening in hyperpolarized subendocardial myocytes. In conclusion, on the basis of electrophysiological and pharmacological evidence, at least four components contribute to outward current in rat ventricular myocytes.

Concomitant Block of the Rapid (I(Kr)) and Slow (I(Ks)) Components of the Delayed Rectifier Potassium Current is Associated With Additional Drug Effects on Lengthening of Cardiac Repolarization

BACKGROUND: The delayed rectifier potassium current, which comprises both a rapid (I(Kr)) and as slow (I(Ks)) component, is a major outward current involved in repolarization of cardiac myocytes. I(Kr) is the target of most drugs that prolong repolarization, whereas electrophysiological effects resulting from combined block of I(Kr) and I(Ks) still need to be characterized. METHODS AND RESULTS: Studies in isolated, buffer-perfused guinea pig hearts were undertaken to compare lengthening of cardiac repolarization under conditions of I(Kr) block alone, I(Ks) Block alone, or combined block of I(Kr) and I(Ks). In protocol A, isolated perfusion with N-acetylprocainamide (NAPA) (I(Kr) block), indapamide (I(Ks) block), or combined NAPA/indapamide was performed at a pacing cycle length of 250 msec. Increases in monophasic action potential duration measured at 90% polarization (MAPD(90)) from baseline after perfusion with NAPA 100 µmol/L (IC(50) for block of I(Kr)) was 19 +/- 6 msed (P <.05), after indapamide 100 µmol/L (EC(50) for block of I(Ks)) 13 +/- 2 msec (P <.05), but 42 +/- 5 msec after combined NAPA 100 µmol/L and indapamide 100 µmol/L (P <.05 vs. baseline and isolated administrations), suggesting the possibility of excessive lengthening of cardiac repolarization by blocking both I(Kr) and I(Ks). As well, in protocol B where sequential perfusions with dofetilide (I(Kr) blocker), dofetilide/indapamide, and indapamide in the same hearts were used, combined dofetilide/indapamide infusion showed a greater increase in MAPD(90) during all pacing cycles studied (250 to 150 msec). CONCLUSIONS: Combined I(Kr) and I(Ks) block may lead to excessive lengthening of cardiac repolarization. This may predispose patients to proarrhythmia during coadministration of drugs.