Tissue and species distribution of mRNA for the IKr-like K+ channel, erg

Piceatannol, a derivative of resveratrol, moderately slows I(Na) inactivation and exerts antiarrhythmic action in ischaemia-reperfused rat hearts

BACKGROUND AND PURPOSE

Piceatannol is more potent than resveratrol in free radical scavenging in association with antiarrhythmic and cardioprotective activities in ischaemic-reperfused rat hearts. The present study aimed to investigate the antiarrhythmic efficacy and the underlying ionic mechanisms of piceatannol in rat hearts.

EXPERIMENTAL APPROACH

Action potentials and membrane currents were recorded by the whole-cell patch clamp techniques. Fluo-3 fluorimetry was used to measure cellular Ca2+ transients. Antiarrhythmic activity was examined from isolated Langendorff-perfused rat hearts.

KEY RESULTS

In rat ventricular cells, piceatannol (3-30 micromol.L(-1)) prolonged the action potential durations (APDs) and decreased the maximal rate of upstroke (V(max)) without altering Ca2+ transients. Piceatannol decreased peak I(Na) and slowed I(Na) inactivation, rather than induced a persistent non-inactivating current, which could be reverted by lidocaine. Resveratrol (100 micromol.L(-1)) decreased peak I(Na) without slowing I(Na) inactivation. The inhibition of peak I(Na) or V(max) was associated with a negative shift of the voltage-dependent steady-state I(Na) inactivation curve without altering the activation threshold. At the concentrations more than 30 micromol.L(-1), piceatannol could inhibit I(Ca,L), I(to), I(Kr), Ca2+ transients and Na+-Ca2+ exchange except I(K1). Piceatannol (1-10 micromol.L(-1)) exerted antiarrhythmic activity in isolated rat hearts subjected to ischaemia-reperfusion injury.

CONCLUSIONS AND IMPLICATIONS

The additional hydroxyl group on resveratrol makes piceatannol possessing more potent in I(Na) inhibition and uniquely slowing I(Na) inactivation, which may contribute to its antiarrhythmic actions at low concentrations less than 10 micromol.L(-1).

Genetic screening in C. elegans identifies rho-GTPase activating protein 6 as novel HERG regulator

The human ether-a-go-go related gene (HERG) constitutes the pore forming subunit of I(Kr), a K(+) current involved in repolarization of the cardiac action potential. While mutations in HERG predispose patients to cardiac arrhythmias (Long QT syndrome; LQTS), altered function of HERG regulators are undoubtedly LQTS risk factors. We have combined RNA interference with behavioral screening in Caenorhabditis elegans to detect genes that influence function of the HERG homolog, UNC-103. One such gene encodes the worm ortholog of the rho-GTPase activating protein 6 (ARHGAP6). In addition to its GAP function, ARHGAP6 induces cytoskeletal rearrangements and activates phospholipase C (PLC). Here we show that I(Kr) recorded in cells co-expressing HERG and ARHGAP6 was decreased by 43% compared to HERG alone. Biochemical measurements of cell-surface associated HERG revealed that ARHGAP6 reduced membrane expression of HERG by 35%, which correlates well with the reduction in current. In an atrial myocyte cell line, suppression of endogenous ARHGAP6 by virally transduced shRNA led to a 53% enhancement of I(Kr). ARHGAP6 effects were maintained when we introduced a dominant negative rho-GTPase, or ARHGAP6 devoid of rhoGAP function, indicating ARHGAP6 regulation of HERG is independent of rho activation. However, ARHGAP6 lost effectiveness when PLC was inhibited. We further determined that ARHGAP6 effects are mediated by a consensus SH3 binding domain within the C-terminus of HERG, although stable ARHGAP6-HERG complexes were not observed. These data link a rhoGAP-activated PLC pathway to HERG membrane expression and implicate this family of proteins as candidate genes in disorders involving HERG.

Evolution of ventricular myocyte electrophysiology

The relative importance of regulatory versus structural evolution for the evolution of different biological systems is a subject of controversy. The primacy of regulatory evolution in the diversification of morphological traits has been promoted by many evolutionary developmental biologists. For physiological traits, however, the role of regulatory evolution has received less attention or has been considered to be relatively unimportant. To address this issue for electrophysiological systems, we examined the importance of regulatory and structural evolution in the evolution of the electrophysiological function of cardiac myocytes in mammals. In particular, two related phenomena were studied: the change in action potential morphology in small mammals and the scaling of action potential duration across mammalian phylogeny. In general, the functional properties of the ion channels involved in ventricular action potential repolarization were found to be relatively invariant. In contrast, there were large changes in the expression levels of multiple ion channel and transporter genes. For the Kv2.1 and Kv4.2 potassium channel genes, which are primary determinants of the action potential morphology in small mammals, the functional properties of the proximal promoter regions were found to vary in concordance with species-dependent differences in mRNA expression, suggesting that evolution of cis-regulatory elements is the primary determinant of this trait. Scaling of action potential duration was found to be a complex phenomenon, involving changes in the expression of a large number of channels and transporters. In this case, it is concluded that regulatory evolution is the predominant mechanism by which the scaling is achieved.

Identification of a posttranslational mechanism for the regulation of hERG1 K+ channel expression and hERG1 current density in tumor cells

A common feature of tumor cells is the aberrant expression of ion channels on their plasma membrane. The molecular mechanisms regulating ion channel expression in cancer cells are still poorly known. K(+) channels that belong to the human ether-a-go-go-related gene 1 (herg1) family are frequently misexpressed in cancer cells compared to their healthy counterparts. We describe here a posttranslational mechanism for the regulation of hERG1 channel surface expression in cancer cells. This mechanism is based on the activity of hERG1 isoforms containing the USO exon. These isoforms (i) are frequently overexpressed in human cancers, (ii) are retained in the endoplasmic reticulum, and (iii) form heterotetramers with different proteins of the hERG family. (iv) The USO-containing heterotetramers are retained intracellularly and undergo ubiquitin-dependent degradation. This process results in decreased hERG1 current (I(hERG1)) density. We detailed such a mechanism in heterologous systems and confirmed its functioning in tumor cells that endogenously express hERG1 proteins. The silencing of USO-containing hERG1 isoforms induces a higher I(hERG1) density in tumors, an effect that apparently regulates neurite outgrowth in neuroblastoma cells and apoptosis in leukemia cells.

Ether-a-go-go-related gene potassium channels: what's all the buzz about?

Antipsychotic drugs are thought to exert their therapeutic action by antagonizing dopamine receptors but are also known to produce side effects in the heart by inhibiting cardiac ether-a-go-go-related gene (ERG) K(+) channels. Recently, it has been discovered that the same channels are present in the brain, including midbrain dopamine neurons. ERG channels are most active after the cessation of intense electrical activity, and blockade of these channels prolongs plateau potentials in bursting dopamine neurons. This change in excitability would be expected to alter dopamine release. Therefore, the therapeutic action of antipsychotic drugs may depend on inhibition of both postsynaptic dopamine receptors and presynaptic ERG K(+) channels.

Phosphatidylinositol 4,5-bisphosphate interactions with the HERG K(+) channel

Regulation of ion channel activity plays a central role in controlling heart rate, rhythm, and contractility responses to cardiovascular demands. Dynamic beat-to-beat regulation of ion channels is precisely adjusted by autonomic stimulation of cardiac G protein-coupled receptors. The rapidly activating delayed rectifier K(+) current (I (Kr)) is produced by the channel that is encoded by human ether-a-gogo-related gene (HERG) and is essential for the proper repolarization of the cardiac myocyte at the end of each action potential. Reduction of I (Kr) via HERG mutations or drug block can lead to lethal cardiac tachyarrhythmias. Autonomic regulation of HERG channels is an area of active investigation with the emerging picture of a complex interplay of signal transduction events, including kinases, second messengers, and protein-protein interactions. A recently described pathway for regulation of HERG is through channel interaction with the phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2). Changes in cellular PIP2 concentrations may occur with Gq-coupled receptor activation. Here, we review the evidence for PIP2-HERG interactions, its potential biological significance, and unfilled gaps in our understanding of this regulatory mechanism.

Modulation of ERG Channels by XE991

In neuronal tissue, KCNQ2-5 channels conduct the physiologically important M-current. In some neurones, the M-current may in addition be conducted partly by ERG potassium channels, which have widely overlapping expression with the KCNQ channel subunits. XE991 and linopiridine are known to be standard KCNQ potassium channel blockers. These compounds have been used in many different tissues as specific pharmacological tools to discern native currents conducted by KCNQ channels from other potassium currents. In this article, we demonstrate that ERG1-2 channels are also reversibly inhibited by XE991 in the micromolar range (EC(50) 107 microM for ERG1). The effect has been characterized in Xenopus laevis oocytes expressing ERG1-2 and in the mammalian HEK293 cell line stably expressing ERG1 channels. The IC(50) values for block of KCNQ channels by XE991 range 1-65 microM. In conclusion, great care should be taken when choosing the concentration of XE991 to use for experiments on native potassium channels or animal studies in order to be able to conclude on selective KCNQ channel-mediated effects.

Monophasic action potential in anaesthetized guinea pigs as a biomarker for prediction of liability for drug-induced delayed ventricular repolarization

INTRODUCTION

Drug-induced QT interval prolongation has been one of the critical issues for developing new chemical entities and pharmaceutical companies need to evaluate the risk early in the development stage. At such stage, guinea pigs are appropriate due to their small size requiring only small amounts of test drugs. The purpose of this study was to determine the utility of guinea pig monophasic action potential (MAP) using 12 reference drugs in order to clarify prediction of the QT interval prolonging risk.

METHODS

Male guinea pigs were anaesthetized with pentobarbital (40 mg/kg, i.p.). Parameters analyzed were epicardial MAP duration (MAP(90)) at sinus rhythm (MAP(90(sinus))) and MAP(90) during atrial pacing (MAP(90(pacing))). Test drugs were administered to animals intravenously and cumulatively.

RESULTS

Vehicle control did not affect the parameters tested. All 8 QT-prolonging drugs prolonged MAP(90(sinus)) and MAP(90(pacing)) dose-dependently, whereas all 4 non-QT-prolonging drugs showed no or very slight prolongations of these MAP(90) parameters. Rank order potency of MAP(90(pacing)) prolongations by the QT-prolonging drugs tended to correspond to clinical plasma concentrations associated with QT interval prolongations or Torsades de Pointes but showed less of a link with hERG inhibition activities.

CONCLUSION

The present study demonstrates that the MAP model using anaesthetized guinea pigs could predict the liability of drugs for QT interval prolongation with high accuracy. QT assessment using the combination of the hERG assay with high sensitivity and the current in vivo assay would be desirable for early risk assessment within drug development.

σ2-Receptor Ligand-Mediated Inhibition of Inwardly Rectifying K+ Channels in the Heart

The sigma(2)-receptor agonist, ifenprodil, was suggested as an inhibitor of G protein-coupled inwardly rectifying potassium channels. Nevertheless, an analysis of the role of sigma(2) receptors in cardiac electrophysiology has never been done. This work aims i) to identify the roles of cardiac sigma(2) receptors in the regulation of cardiac K(+) channel conductances and ii) to check whether sigma(2)-receptor agonists exhibit class III antiarrhythmic properties. The sigma(2)-receptor agonists ifenprodil, threo-ifenprodil, LNP250A [threo-8-[1-(4-hydroxyphenyl)-1-hydroxy-propan-2-yl]-1-phenyl-1,3,8-triazaspiro[4,5]decane-4-one] (a derivative of ifenprodil devoid of alpha(1)-adrenergic and N-methyl-d-aspartate glutamate receptor-blocking properties), and 1,3-di(2-tolyl)guanidine were used to discriminate the effects linked to sigma(2) receptors from those of the sigma(1) subtype, induced by (+/-)-N-allylnormetazocine (SKF-10,047). The sigma(2)-receptor antagonist 3-alpha-tropanyl-2(pCl-phenoxy)butyrate (SM-21) was employed to characterize sigma(2)-mediated effects in patch-clamp experiments. In rabbits, all sigma(2)-receptor agonists reduced phenylephrine-induced cardiac arrhythmias. They prolonged action potential duration in rabbit Purkinje fibers and reduced human ether-a-go-go-related gene (HERG) K(+) currents. (+)-SKF-10,047 was completely inactive in the last two tests. The effects of threo-ifenprodil were not antagonized by SM-21. In HERG-transfected COS-7 cells, SM-21 potentiated the ifenprodil-induced blockade of the HERG current. These data suggest that sigma(2)-receptor ligands block I(Kr) and that this effect could explain part of the antiarrhythmic properties of this ligands family. Nevertheless, an interaction with HERG channels not involving sigma(2) receptors seems to share this pharmacological property. This work shows for the first time that particular caution has to be taken toward ligands with affinity for sigma(2) receptors. The repolarization prolongation and the early-afterdepolarization can be responsible for "torsades de pointe" and sudden cardiac death.

Risk factors in sudden death in epilepsy (SUDEP): the quest for mechanisms

People with epilepsy may die suddenly and unexpectedly without a structural pathological cause. Most SUDEP cases are likely to be related to seizures. SUDEP incidence varies and is <1:1,000 person-years among prevalent cases in the community and approximately 1:250 person years in specialist centres. Case-control studies identified certain risk factors, some potentially amenable to manipulation, including uncontrolled convulsive seizures and factors relating to treatment and supervision. Both respiratory and cardiac mechanisms are important. The apparent protective effect of lay supervision supports an important role for respiratory factors, in part amenable to intervention by simple measures. Whereas malignant tachyarrhythmias are rare during seizures, sinus bradycardia/arrest, although infrequent, is well documented. Both types of arrhythmias can have a genetic basis. This article reviews SUDEP and explores the potential of coexisting liability to cardiac arrhythmias as a contributory factor, while acknowledging that at present, bridging evidence between cardiac inherited gene determinants and SUDEP is lacking.

Identification of IKr and Its Trafficking Disruption Induced by Probucol in Cultured Neonatal Rat Cardiomyocytes

The human ether-a-go-go-related gene (hERG) encodes a channel that conducts the rapidly activating delayed rectifier K(+) current (I(Kr)), which is important for cardiac repolarization. Mutations in hERG reduce I(Kr) and cause congenital long QT syndrome (LQTS). More frequently, common medications can reduce I(Kr) and cause LQTS as a side effect. Protein trafficking abnormalities are responsible for most hERG mutation-related LQTS and are recently recognized as a mechanism for drug-induced LQTS. Whereas hERG trafficking has been studied in recombinant expression systems, there has been no reported study on cardiac I(Kr) trafficking at the protein level. In the present study, we identified that I(Kr) is present in cultured neonatal rat ventricular myocytes and can be robustly recorded using Cs(+) as the charge carrier. We further discovered that 4,4'-(isopropylidenedithio)-bis-(2,6-di-t-butylphenol) (probucol), a cholesterol-lowering drug that induces LQTS, disrupted I(Kr) trafficking and prolonged the cardiac action potential duration. Probucol did not directly block I(Kr). Probucol also disrupted hERG trafficking and did not block hERG channels expressed in human embryonic kidney 293 cells. We conclude that probucol induces LQTS by disrupting ether-a-go-go-related gene trafficking, and that primary culture of neonatal rat cardiomyocytes represents a useful system for studying native I(Kr) trafficking.

Evaluation of functional and binding assays in cells expressing either recombinant or endogenous hERG channel

INTRODUCTION

The hERG (human ether-a-go-go related gene) potassium channel is required for normal cardiac repolarization, is susceptible to inhibition by a wide variety of compounds, and its blockage can lead to cardiac QT interval prolongation and life threatening arrhythmias. The present report examines the ability of hERG binding and functional assays to identify compounds with potential cardiovascular liabilities at the earliest stages of drug discovery.

METHODS

Competitive binding assays were developed using (3)H-dofetilide and membranes from HEK293EBNA cells stably expressing recombinant hERG (HEK293-hERG) and IMR-32 cells expressing hERG endogenously. hERG functional assays were also developed using membrane potential indicator dye and rubidium efflux. The ability of these assays to identify compounds with potential adverse cardiac effects was examined using drugs with known cardiac effects ranging from those with no known adverse effects to drugs that were withdrawn from the market due to increased risk of sudden death associated with Torsades de Points.

RESULTS

Binding assays using HEK293-hERG membranes and (3)H-dofetilide were robust (Z'=0.69+/-0.015, mean+/-S.E.M.), highly reproducible (test-retest slope=1.04, r(2)=0.98), and correlated well with IC(50) values obtained by patch clamp (slope=0.98, r(2)=0.89). Binding assays using IMR-32 membranes were less sensitive (Z'=0.4+/-0.03, mean+/-S.E.M., false negative rate=0.4) but still correlated well with patch clamp data (slope=1.06, r(2)=0.83). The hERG membrane potential assay could detect potent hERG inhibitors (defined by hERG patch clamp IC(50)<0.1 muM) using HEK293-hERG cells, but were prone to generate false-negative results with less potent inhibitors (false negative rate=0.5). Finally, the rubidium efflux assay gave highly reproducible results (Z'=0.80+/-0.02, mean+/-S.E.M.) that correlated with patch clamp IC(50) values (slope=0.87, r(2)=0.73).

DISCUSSION

The hERG binding and rubidium efflux assays are robust, predictive of patch clamp results, and can be used at the earliest stages of drug discovery.

A novel hypothesis for the binding mode of HERG channel blockers

We present a new docking model for HERG channel blockade. Our new model suggests three key interactions such that (1) a protonated nitrogen of the channel blocker forms a hydrogen bond with the carbonyl oxygen of HERG residue T623; (2) an aromatic moiety of the channel blocker makes a pi-pi interaction with the aromatic ring of HERG residue Y652; and (3) a hydrophobic group of the channel blocker forms a hydrophobic interaction with the benzene ring of HERG residue F656. The previous model assumes two interactions such that (1) a protonated nitrogen of the channel blocker forms a cation-pi interaction with the aromatic ring of HERG residue Y652; and (2) a hydrophobic group of the channel blocker forms a hydrophobic interaction with the benzene ring of HERG residue F656. To test these models, we classified 69 known HERG channel blockers into eight binding types based on their plausible binding modes, and further categorized them into two groups based on the number of interactions our model would predict with the HERG channel (two or three). We then compared the pIC(50) value distributions between these two groups. If the old hypothesis is correct, the distributions should not differ between the two groups (i.e., both groups show only two binding interactions). If our novel hypothesis is correct, the distributions should differ between Groups 1 and 2. Consistent with our hypothesis, the two groups differed with regard to pIC(50), and the group having more predicted interactions with the HERG channel had a higher mean pIC(50) value. Although additional work will be required to further validate our hypothesis, this improved understanding of the HERG channel blocker binding mode may help promote the development of in silico predictions methods for identifying potential HERG channel blockers.

Restoring depressed HERG K+ channel function as a mechanism for insulin treatment of abnormal QT prolongation and associated arrhythmias in diabetic rabbits

Abnormal QT prolongation (QT-P) in diabetic patients has become a nonnegligible clinical problem and has attracted increasing attention from basic scientists, because it increases the risk of lethal ventricular arrhythmias. Correction of QT-P may be an important measure in minimizing sudden cardiac death in diabetic patients. Here we report the efficacy of insulin in preventing QT-P and the associated arrhythmias and the mechanisms underlying the effects in a rabbit model of type 1 insulin-dependent diabetes mellitus (IDDM). The heart rate-corrected QT (QTc) interval and action potential duration were considerably prolonged, with frequent ventricular tachycardias. The rapid delayed rectifier K+ current (IKr) was markedly reduced in IDDM hearts, and hyperglycemia depressed the function of the human ether-a-go-go-related gene (HERG), which conducts IKr. The impairment was primarily ascribed to the enhanced oxidative damage to the myocardium, as indicated by the increased intracellular level of reactive oxygen species and simultaneously decreased endogenous antioxidant reserve and by the increased lipid peroxidation and protein oxidation. Moreover, IDDM or hyperglycemia resulted in downregulation of HERG protein level. Insulin restored the depressed IKr/HERG and prevented QTc/action potential duration prolongation and the associated arrhythmias, and the beneficial actions of insulin are partially due to its antioxidant ability. Our study represents the first documentation of oxidative stress as the major metabolic mechanism for HERG K+ dysfunction, which causes diabetic QT-P, and suggests IKr/HERG as a potential therapeutic target for treatment of the disorder.

The functional HERG variant 897T is associated with Conn's adenoma

OBJECTIVE

Aldosterone secreting adenomas (aldosteronomas) have an unknown molecular origin. Ion channel currents are involved in signal transduction leading to aldosterone synthesis and secretion. HERG (human-ether-a-go-go-related gene) encodes for a potassium channel responsible for the outward rectifying delayed current and it is mutation prone. When mutated it causes most of the familial forms of both long QT and short QT syndromes. Abnormal repolarization in glomerulosa cells might increase aldosterone secretion or induce a proliferative advantage. The aims of this study were to: (1) evaluate HERG expression in aldosteronomas; (2) search for HERG somatic mutations; and (3) determine whether there is any relationship between the common HERG functional variant (A2690C, leading from lysine 897 to threonine, K897T) and aldosteronoma.

DESIGN AND METHODS

Aldosteronoma and blood samples from 17 patients were studied to evaluate HERG expression, full-length HERG complementary DNA sequencing, and genotyping for K897T alleles. The prevalence of HERG 897 alleles was also tested in a control population and a population consisting entirely of hypertensive individuals.

RESULTS

HERG was expressed in all aldosteronomas analysed. HERG somatic mutations were not detected. The 897T variant of HERG was significantly more common among patients with aldosteronoma (897T allele 41%) than in patients with moderate-severe essential hypertension (897T allele 20%, P = 0.007) or in the control population (897T allele 12%, P < 0.0001). The 897T/T genotype was present in 24% of the aldosteronoma patients versus 7% (P = 0.040) and 3% (P = 0.001) in essential hypertension and in the control population, respectively. When the chi test was performed considering the three groups together, the significance was similar (for alleles P < 0.0001 and for genotypes P = 0.004).

CONCLUSION

The common functional HERG variant 897T may predispose to the development of aldosteronoma.

Expression pattern of the ether-a-go-go-related (ERG) family proteins in the adult mouse central nervous system: evidence for coassembly of different subunits

Voltage-dependent K+ channels are the main determinants in controlling cellular excitability within the central nervous system. Among voltage-dependent K+ channels, the ERG subfamily is deeply involved in the control of cellular excitability, both in mammals and in invertebrates. ERG channels are encoded by different genes: the erg1 gene, which can generate two alternative transcripts (erg1a and erg1b), erg2 and erg3. The aim of the present study was to determine the expression pattern and cellular localization of ERG proteins (ERG1, ERG2, and ERG3) in the mouse CNS, differentiating, for the first time, the ERG1A and ERG1B isoforms. To this purpose, novel specific antibodies were raised against the various channel proteins and their specificity and immunoreactivity tested. It emerged that: 1) all the erg genes were indeed translated in neuronal tissue; 2) ERG proteins distribution in the mouse CNS often overlapped, and only in specific areas each ERG protein showed a distinct pattern of expression; and 3) ERG proteins were generally expressed in neuronal soma, but dendritic and/or white matter labeling could be detected in specific areas. The finding that ERG proteins often have an overlapping expression suggests that neuronal ERG currents could be determined, at least in part, by heterotetrameric ERG channels. This suggestion is demonstrated to occur for ERG1A/ERG1B by showing that the two isoforms coassemble in mouse brain.

Functional expression of Kir2.x in human aortic endothelial cells: the dominant role of Kir2.2

Inward rectifier K+channels (Kir) are a significant determinant of endothelial cell (EC) membrane potential, which plays an important role in endothelium-dependent vasodilatation. In the present study, several complementary strategies were applied to determine the Kir2 subunit composition of human aortic endothelial cells (HAECs). Expression levels of Kir2.1, Kir2.2, and Kir2.4 mRNA were similar, whereas Kir2.3 mRNA expression was significantly weaker. Western blot analysis showed clear Kir2.1 and Kir2.2 protein expression, but Kir2.3 protein was undetectable. Functional analysis of endothelial inward rectifier K+current ( IK) demonstrated that 1) IKcurrent sensitivity to Ba2+and pH were consistent with currents determined using Kir2.1 and Kir2.2 but not Kir2.3 and Kir2.4, and 2) unitary conductance distributions showed two prominent peaks corresponding to known unitary conductances of Kir2.1 and Kir2.2 channels with a ratio of ∼4:6. When HAECs were transfected with dominant-negative (dn)Kir2.x mutants, endogenous current was reduced ∼50% by dnKir2.1 and ∼85% by dnKir2.2, whereas no significant effect was observed with dnKir2.3 or dnKir2.4. These studies suggest that Kir2.2 and Kir2.1 are primary determinants of endogenous K+conductance in HAECs under resting conditions and that Kir2.2 provides the dominant conductance in these cells.

Isolation and characterization of I(Kr) in cardiac myocytes by Cs+ permeation

Isolation of the rapidly activating delayed rectifier potassium current (I(Kr)) from other cardiac currents has been a difficult task for quantitative study of this current. The present study was designed to separate I(Kr) using Cs+ in cardiac myocytes. Cs+ have been known to block a variety of K+ channels, including many of those involved in the cardiac action potential such as inward rectifier potassium current I(K1) and the transient outward potassium current I(to). However, under isotonic Cs+ conditions (135 mM Cs+), a significant membrane current was recorded in isolated rabbit ventricular myocytes. This current displayed the voltage-dependent onset of and recovery from inactivation that are characteristic to I(Kr). Consistently, the current was selectively inhibited by the specific I(Kr) blockers. The biophysical and pharmacological properties of the Cs+-carried human ether-a-go-go-related gene (hERG) current were very similar to those of the Cs+-carried I(Kr) in ventricular myocytes. The primary sequence of the selectivity filter in hERG was in part responsible for the Cs+ permeability, which was lost when the sequence was changed from GFG to GYG, characteristic of other, Cs+-impermeable K+ channels. Thus the unique high Cs+ permeability in I(Kr) channels provides an effective way to isolate I(Kr) current. Although the biophysical and pharmacological properties of the Cs+-carried I(Kr) are different from those of the K+-carried I(Kr), such an assay enables I(Kr) current to be recorded at a level that is large enough and sufficiently robust to evaluate any I(Kr) alterations in native tissues in response to physiological or pathological changes. It is particularly useful for exploring the role of reduction of I(Kr) in arrhythmias associated with heart failure and long QT syndrome due to the reduced hERG channel membrane expression.

Novel Method to Assess Cardiac Electrophysiology in the Rat

The rat continues to be an important tool to assess cardiac electrophysiologic (EP) effects of test agents and to study the distribution/role of ion channels in cardiovascular diseases. However, no data have been described that accurately measure discrete cardiac EP parameters in rats in vivo. Therefore, we developed a method to assess cardiac EP in rats and then profiled several ion channel agents. Briefly, rats were instrumented with endocardially placed electrodes to assess cardiac refractoriness and conduction. Administration of class I agents resulted in a dose-dependent slowing of ventricular conduction. The potassium channel blocker 4-aminopyridine caused significant increases in atrial and ventricular refractoriness. An IKr blocker had little or no effect on atrial and ventricular refractoriness but significantly increased AV nodal refractoriness. Additionally, an IKs blocker had little effect on rat cardiac EP. The L-type blocker diltiazem caused a dose-dependent delay in AV node conduction and an increase in AV node refractoriness. Overall, this study provides normative data that describe the roles of Na, Ca, and K channels in rat cardiac electrophysiology, in vivo. Furthermore, the model provides a method to assess changes in cardiac electrophysiology in the setting of disease by using well-established rat models of induced or genetic cardiovascular disease.

Modulating effect of ginseng saponins on heterologously expressed HERG currents in Xenopus oocytes

AIM

To examine the effects of ginseng saponins on the heterologously expressed human ether-a-go-go related gene (HERG) that encodes the rapid component of the delayed rectifier K+ channel.

METHODS

A two-electrode voltage clamp technique was used. HERG currents were recorded in Xenopus oocytes injected with HERG cRNA.

RESULTS

Crude saponins of Korean red ginseng (GS) induced a minimal increase of the maximal HERG conductance without changes in the voltage-dependent HERG current activation and inactivation curves. GS, however, decelerated HERG current deactivation in a concentration-dependent manner, which was more noticeable with panaxitriol (PT) than panaxidiol (PD). Consistently, ginseng saponins increased the HERG deactivation time constants with the order of potency of Rg1 (a major component of PT)>Rf1>Rb1 (a major component of PD). Re had little effect on HERG deactivation. During a cardiac action potential, GS increased the outward HERG current.

CONCLUSION

Ginseng saponins enhance HERG currents, which could be in part a possible mechanism of the shortening cardiac action potential of ginseng saponins.

Electrical activity alterations induced by chronic absorption of lindane (γ-hexachlorocyclohexane) trace concentrations in adult rat heart

The heart of adult rat offspring, born to mothers treated with trace concentrations of lindane (0.5 to 2 ppb) through a beverage and to mothers chronically treated with lindane (CL-T) with the same trace concentration, also through a beverage, during lactation and growth has a round shape and accumulates lindane. The left ventricle (LV) presents a hypertrophied area, atrophied papillary muscles, and unorganized collagen bundles and layers. These observations led us to study the electrical activity of their left ventricle papillary muscles (LVPM) by recording action potential using intracellular microelectrodes. CL-T shortened LVPM action potential duration (APD): 1 ppb shortened the plateau; 2 ppb shortened the plateau and the slow repolarizing phase. In CL-T (2 ppb) and untreated groups, low temperature (22 °C) decreased the resting potential and prolonged APD. TEA (tetraethylammonium; 1-2 mmol/L) partially lengthened CL-T (2 ppb lindane) APD. Quinidine (0.2 mmol/L) and E-4031 (10 nmol/L) prolonged CL-T APD, suggesting that the rapid delayed outward K+ current (IKr) was increased. Our results indicate the silent effects of chronic exposure to trace concentrations of lindane on the morphological and electrical activity of heart muscle. They demonstrate that chronic lindane treatment of female rats alters the tissue integrity and electrical activity in the LV of their offspring.Key words: heart muscle, membrane potential, lindane, K+ channel.

Expression of ether à go-go potassium channels in human gliomas

Ether à go-go (EAG) K(+) channels have been shown to be involved in tumor generation and malignant growth. Gliomas have not been investigated thus far. Using RT-PCR we investigated healthy human brain and human gliomas of different subtypes and malignancy grades for the expression of human EAG1 and eag-related gene (ERG) 1 channels. mRNA of both channels was detected in all tissues. Expression was strong in normal brain, moderate in high-grade and high in low-grade gliomas. Our findings suggest a differential expression of hEAG1 and hERG1 in gliomas depending on the malignancy grade and nature of the tumor cells. However, the hypothesis that EAG channels are related to the oncogenic process itself is only partly supported by this study.

Acquired QT interval prolongation and HERG: implications for drug discovery and development

Putative interactions between the Human Ether-a-go-go Related Gene (HERG), QT interval prolongation and Torsades de Pointes (TdP) are now integral components of any discussion on drug safety. HERG encodes for the inwardly rectifying potassium channel (I(Kr)), which is essential to the maintenance of normal cardiac function. HERG channel mutations are responsible for one form of familial long QT syndrome, a potentially deadly inherited cardiac disorder associated with TdP. Moreover, drug-induced (acquired) QT interval prolongation has been associated with an increase in the incidence of sudden unexplained deaths, with HERG inhibition implicated as the underlying cause. Subsequently, a number of non-cardiovascular drugs which induce QT interval prolongation and/or TdP have been withdrawn. However, a definitive link between HERG, QT interval prolongation and arrhythmogenesis has not been established. Nevertheless, this area is subject to ever increasing regulatory scrutiny. Here we review the relationship between HERG, long QT syndrome and TdP, together with a summary of the associated regulatory issues, and developments in pre-clinical screening.

Developmentally regulated expression of the mouse homologues of the potassium channel encoding genes m-erg1, m-erg2 and m-erg3

Deciphering the expression pattern of K+ channel encoding genes during development can help in the understanding of the establishment of cellular excitability and unravel the molecular mechanisms of neuromuscular diseases. We focused our attention on genes belonging to the erg family, which is deeply involved in the control of neuromuscular excitability in Drosophila flies and possibly other organisms. Both in situ hybridisation and RNase Protection Assay experiments were used to study the expression pattern of mouse (m)erg1, m-erg2 and m-erg3 genes during mouse embryo development, to allow the pattern to be compared with their expression in the adult. M-erg1 is first expressed in the heart and in the central nervous system (CNS) of embryonic day 9.5 (E9.5) embryos; the gene appears in ganglia of the peripheral nervous system (PNS) (dorsal root (DRG) and sympathetic (SCG) ganglia, mioenteric plexus), in the neural layer of retina, skeletal muscles, gonads and gut at E13.5. In the adult m-erg1 is expressed in the heart, various structures of the CNS, DRG and retina. M-erg2 is first expressed at E9.5 in the CNS, thereafter (E13.5) in the neural layer of retina, DRG, SCG, and in the atrium. In the adult the gene is present in some restricted areas of the CNS, retina and DRG. M-erg3 displayed an expression pattern partially overlapping that of m-erg1, with a transitory expression in the developing heart as well. A detailed study of the mouse adult brain showed a peculiar expression pattern of the three genes, sometimes overlapping in different encephalic areas.

Sino-atrial nodal cells of mammalian hearts: ionic currents and gene expression of pacemaker ionic channels

The cardiac pacemaker is a sino-atrial (SA) nodal cell. The signal induced by this pacemaker is distributed over the heart surface by a specialised conduction system and is clinically recorded as the ECG. The SA nodal cells are highly resistant to cardiac failure and ischemia. Under calcium overload conditions, some dysrythmias of SA nodal cells occur easily. Morphological analysis under these conditions shows swelling of the cisternae of the Golgi apparatus, with little or no other histological change or damage being observed. The rate of sinus rhythm is quite different between various species. The investigations of SA nodal cells have so far clarified the pacemaker mechanisms involved. A number of ionic channel currents or pacemaker currents, contribute to the depolarization of the pacemaker potential (phase 4). This will not occur with a single current. Recent experiments have identified several novel pacemaker currents and have also revealed several differences in the pacemaker currents between species. The marked hyperpolarization-activated inward current (I(f)) appears in SA nodal cells of most species, while the inwardly rectifying K+ current (I(K1)) with masked I(f) current is found in those of the rat and monkey. In addition, the rapidly activated current (I(Kr)) and slowly activated current (I(Ks)) of the delayed rectifier K+ current (I(K)) contribute to the pacemaker potential in guinea pig SA nodal cells, with only the I(Ks) current in porcine SA nodal cells and only the I(Kr) current in the rat and rabbit. These differences in ionic channels presumably result from differences in gene expression. Some smooth muscle cells also possess the capacity to beat spontaneously. Uterine smooth muscle cells also exhibit an I(f) current. The basal mechanism for spontaneous activity in both SA nodal cells and smooth muscle cells is almost the same, but some differences in the ionic channels and their genetic expression may contribute to their respective pacemaker currents.

Muscarinic modulation of erg potassium current

We studied modulation of current in human embryonic kidney tsA-201 cells coexpressing rat erg1 channels with M(1) muscarinic receptors. Maximal current was inhibited 30% during muscarinic receptor stimulation, with a small positive shift of the midpoint of activation. Inhibition was attenuated by coexpression of the regulator of G-protein signalling RGS2 or of a dominant-negative protein, G(q), but not by N-ethylmaleimide or C3 toxin. Overexpression of a constitutively active form of G(q) (but not of G(13) or of G(s)) abolished the erg current. Hence it is likely that G(q/11), and not G(i/o) or G(13), mediates muscarinic inhibition. Muscarinic suppression of erg was attenuated by chelating intracellular Ca(2+) to < 1 nm free Ca(2+) with 20 mm BAPTA in the pipette, but suppression was normal if internal Ca(2+) was strongly clamped to a 129 nm free Ca(2+) level with a BAPTA buffer and this was combined with numerous other measures to prevent intracellular Ca(2+) transients (pentosan polysulphate, preincubation with thapsigargin, and removal of extracellular Ca(2+)). Hence a minimum amount of Ca(2+) was necessary for the inhibition, but a Ca(2+) elevation was not. The ATP analogue AMP-PCP did not prevent inhibition. The protein kinase C (PKC) blockers staurosporine and bisindolylmaleimide I did not prevent inhibition, and the PKC-activating phorbol ester PMA did not mimic it. Neither the tyrosine kinase inhibitor genistein nor the tyrosine phosphatase inhibitor dephostatin prevented inhibition by oxotremorine-M. Hence protein kinases are not needed. Experiments with a high concentration of wortmannin were consistent with recovery being partially dependent on PIP(2) resynthesis. Wortmannin did not prevent muscarinic inhibition. Our studies of muscarinic inhibition of erg current suggest a role for phospholipase C, but not the classical downstream messengers, such as PKC or a calcium transient.

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

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.

Subcellular localization of the delayed rectifier K+channels KCNQ1 and ERG1 in the rat heart

In the heart, several K+channels are responsible for the repolarization of the cardiac action potential, including transient outward and delayed rectifier K+currents. In the present study, the cellular and subcellular localization of the two delayed rectifier K+channels, KCNQ1 and ether- a- go- go-related gene-1 (ERG1), was investigated in the adult rat heart. Confocal immunofluorescence microscopy of atrial and ventricular cells revealed that whereas KCNQ1 labeling was detected in both the peripheral sarcolemma and a structure transversing the myocytes, ERG1 immunoreactivity was confined to the latter. Immunoelectron microscopy of atrial and ventricular myocytes showed that the ERG1 channel was primarily expressed in the transverse tubular system and its entrance, whereas KCNQ1 was detected in both the peripheral sarcolemma and in the T tubules. Thus, whereas ERG1 displays a very restricted subcellular localization pattern, KCNQ1 is more widely distributed within the cardiac cells. The localization of these K+channels to the transverse tubular system close to the Ca2+channels renders them with maximal repolarizing effect.

Sensitivity and specificity of isolated perfused guinea pig heart to test for drug-induced lengthening of QTc

INTRODUCTION

The purpose of this study was to determine the sensitivity and specificity for predicting the liability of a compound to lengthen QTc using isolated, perfused guinea pig hearts (Langendorff preparation).

METHODS

QTc (Fridericia correction) was calculated from bipolar transventricular electrograms. Hearts were exposed to escalating concentrations of 26 compounds thought to lengthen, and 13 compounds thought not to lengthen, QTc in humans.

RESULTS

In this preparation, QTc was found to lengthen in 26 of 26 compounds thought to be positive (sensitivity 1.00) and not to lengthen or to lengthen insignificantly in 13 of 13 compounds thought to be negative (specificity 1.0) in man. Probucol and ontazolast could not be studied because of limited solubility. Successful experiments were conducted on over 98% of guinea pigs anesthetized.

DISCUSSION

We believe that the isolated perfused guinea pig heart is an in vitro preparation that could be utilized early in preclinical testing for identifying a liability to lengthen QTc in humans, but we do not believe--as is true also for other in vitro methods--that the concentration at which the liability is demonstrated in vitro necessarily predicts the concentration at which a liability exists in man.

ERG K+ currents regulate pacemaker activity in ICC

Ether-à-go-go-related gene (ERG) K channels have been implicated in the generation of pacemaker activities in the heart. To study the presence and function of ERG K channels in the pacemaker cells of the small intestine [the interstitial cells of Cajal (ICC)], a combination of patch-clamp techniques, tissue and live cell immunohistochemistry, RT-PCR, and in vitro functional studies were performed. Nonenzymatically isolated ICC in culture were identified by vital staining and presence of rhythmic inward currents. RT-PCR showed the presence of ERG mRNA in the intestinal musculature, and immunohistochemistry on tissue and cultured cells demonstrated that protein similar to human ERG was concentrated on ICC in the Auerbach's plexus region. Whole cell ERG K+ currents were evoked on hyperpolarization from 0 mV (but not from -70 mV) up to -120 mV and showed strong inward rectification. The currents were inhibited by E-4031, cisapride, La3+, and Gd3+ but not by 50 microM Ba2+. The ERG K+ inward current had a typical transient component with fast activation and inactivation kinetics followed by significant steady-state current. E-4031 also inhibited tetraethylammonium (TEA)-insensitive outward current indicating that the ERG K+ current is operating at depolarizing potentials. In contrast to TEA, blockers of the ERG K+ currents caused marked increase in tissue excitability as reflected by an increase in slow-wave duration and an increase in superimposed action potential activity. In summary, ERG K channels in ICC contribute to the membrane potential and play a role in regulation of pacemaker activity of the small intestine.

Does QT Widening in the Langendorff-perfused Rat Heart Represent the Effect of Repolarization Delay or Conduction Slowing?

It has been suggested that class I antiarrhythmic drugs and ischemia can widen the QT interval in the Langendorff-perfused rat heart preparation as a consequence of slowed ventricular conduction. If this were so, it would undermine the clinical relevance of the preparation and its effectiveness as an antiarrhythmic bioassay. To test this, the authors determined whether three different class I drugs could prolong QT in the preparation and whether this effect was augmented by ischemia and elevation of the potassium (K+) content of the perfusion solution. Baseline drug-free QT intervals correlated inversely with the K+ content (3 microM vs. 5 mM). QT intervals widened during the first 3-5 minutes of ischemia (P < 0.05), then returned gradually to baseline. Lidocaine (3.88 microM and 12.93 microM) had no effect on the QT interval before or during ischemia, whereas quinidine (7.90 microM but not 0.79 microM) and flecainide (1.48 microM but not 0.74 microM) caused QT widening before and during ischemia (P < 0.05). Elevating perfusion solution K+ content from 3 microM to 5 mM reduced the QT-widening effects of quinidine and flecainide (P < 0.05). Because lidocaine, a relatively selective sodium (Na+) channel blocker, failed to widen QT interval whereas quinidine and flecainide (combined Na+ and K+ channel blockers) did so, and because K+ elevation reduced rather than potentiated the drug-induced QT widening, it is unlikely that Na+ channel blockade and conduction slowing play any role in ischemia- or class I drug-induced QT widening in this model.

Functional roles of an ERG current isolated in cerebellar Purkinje neurons

Transcripts encoding ERG potassium channels are expressed by most neurons of the CNS. By patch-clamp whole cell recording from Purkinje neurons in slices of young (5-9 days old) mouse cerebellum we have been able to isolate a tail current [IK(ERG)] with the same characteristics as previously described for ERG channels. In zero external Ca2+ and high K+ (40 mM) the V1/2 of activation was -50.7 mV, the V1/2 of inactivation was -70.6 mV, and the deactivation rate was double exponential and voltage dependent. IK(ERG) was 93.0% blocked by WAY-123,398 (1 microM) and 78.2% by haloperidol (2 microM). The role of IK(ERG) on evoked firing was studied in adult mice, where WAY-123,398 application decreased the first spike latency, increased the firing frequency, and suppressed the frequency adaptation. However, the shape of individual action potentials was not affected. Stimulation of presynaptic climbing fibers evoked the Purkinje neuron "complex spike," composed of an initial spike and several spikelets. IK(ERG) block caused an increase of the number of spikelets of the "complex spike." These data show, for the first time, an IK(ERG) in a neuron of the CNS, the cerebellar Purkinje neuron, and indicate that such a current is involved in the control of membrane excitability, firing frequency adaptation, and in determining the effects of the climbing fiber synapse.

Acceleration of human myoblast fusion by depolarization: graded Ca2+ signals involved

We have previously shown that human myoblasts do not fuse when their voltage fails to reach the domain of a window T-type Ca(2+) current. We demonstrate, by changing the voltage in the window domain, that the Ca(2+) signal initiating fusion is not of the all-or-none type, but can be graded and is interpreted as such by the differentiation program. This was carried out by exploiting the properties of human ether-à-go-go related gene K(+) channels that we found to be expressed in human myoblasts. Methanesulfonanilide class III antiarrhythmic agents or antisense-RNA vectors were used to suppress completely ether-à-go-go related gene current. Both procedures induced a reproducible depolarization from -74 to -64 mV, precisely in the window domain where the T-type Ca(2+) current increases with voltage. This 10 mV depolarization raised the cytoplasmic free Ca(2+) concentration, and triggered a tenfold acceleration of myoblast fusion. Our results suggest that any mechanism able to modulate intracellular Ca(2+) concentration could affect the rate of myoblast fusion.

Selective knockout of mouse ERG1 B potassium channel eliminates I(Kr) in adult ventricular myocytes and elicits episodes of abrupt sinus bradycardia

The ERG1 gene encodes a family of potassium channels. Mutations in human ERG1 lead to defects in cardiac repolarization, referred to as the long QT syndrome. Through homologous recombination in mouse embryonic stem cells the ERG1 B potassium channel transcript was eliminated while the ERG1 A transcript was maintained. Heterologous expression of ERG1 isoforms had previously indicated that the deactivation time course of ERG1 B is 10-fold more rapid than that of ERG1 A. In day-18 fetal +/+ myocytes, I(Kr) exhibited two time constants of deactivation (3,933 +/- 404 and 350 +/- 19 ms at -50 mV), whereas in age-matched ERG1 B(-/-) mice the rapid component was absent. Biexponential deactivation rates (2,039 +/- 268 and 163 +/- 43 ms at -50 mV) were also observed in adult +/+ myocytes. In adult ERG1 B(-/-) myocytes no I(Kr) was detected. Electrocardiogram intervals were similar in +/+ and -/- mice. However, adult -/- mice manifested abrupt spontaneous episodes of sinus bradycardia (>100 ms of slowing) in 6 out of 21 mice. This phenomenon was never observed in +/+ mice (0 out of 16). We conclude that ERG1 B is necessary for I(Kr) expression in the surface membrane of adult myocytes. Knockout of ERG1 B predisposes mice to episodic sinus bradycardia.

Cloning and functional characterization of the smooth muscle ether-a-go-go-related gene K+ channel. Potential role of a conserved amino acid substitution in the S4 region

The human ether-a-go-go-related gene (HERG) product forms the pore-forming subunit of the delayed rectifier K(+) channel in the heart. Unlike the cardiac isoform, the erg K(+) channels in native smooth muscle demonstrate gating properties consistent with a role in maintaining resting potential. We have cloned the smooth muscle isoform of HERG, denoted as erg1-sm, from human and rabbit colon. erg1-sm is truncated by 101 amino acids in the C terminus due to a single nucleotide deletion in the 14th exon. Sequence alignment against HERG showed a substitution of alanine for valine in the S4 domain. When expressed in Xenopus oocytes, erg1-sm currents had much faster activation and deactivation kinetics compared with HERG. Step depolarization positive to -20 mV consistently produced a transient outward component. The threshold for activation of erg1-sm was -60 mV and steady-state conductance was approximately 10-fold greater than HERG near the resting potential of smooth muscle. Site-directed mutagenesis of alanine to valine in the S4 region of erg1-sm converted many of the properties to that of the cardiac HERG, including shifts in the voltage dependence of activation and slowing of deactivation. These studies define the functional role of a novel isoform of the ether-a-go-go-related gene K(+) channel in smooth muscle.

The dog's role in the preclinical assessment of QT interval prolongation

During the development of a new therapeutic, few pharmacodyamic outcomes currently receive as much scrutiny as the effect of a potential medication on the electrocardiographic QT interval. The recent withdrawal from marketing of several drugs due to potential drug-related cardiac arrhythmias have greatly increased concern about drug-related changes on the QT interval. In order to reduce the incidence of these idiosyncratic episodes, regulatory agencies have suggested that sponsors use more rigorous methodology during the safety evaluation of new pharmaceuticals. Along with enhanced electrocardiographic assessments during clinical trials, advanced preclinical examinations of effect on QT interval and ventricular repolarization have become de rigueur. In this arena, the beagle dog is the preclinical species often associated with the most reliable predictivity for human safety assessment. To this end, canine models of cardiovascular safety assessment are discussed along with the relevance of these assays to human electrocardiography.

Pacemaker Mechanism of Porcine Sino-atrial Node Cells

In cardiac sino-atrial node (SAN) cells, time- and voltage-dependent changes in the gating of various ionic currents provide spontaneous, stable and repetitive firing of action potentials. To address the ionic nature of the species-dependent heart rate, action potentials and membrane currents were recorded in single cells dissociated from the porcine SAN, and compared with those from SAN cells of rabbits, guinea-pigs and mice. The porcine SAN cells exhibited spontaneous activity with a frequency of 60-80 min(-1), which was much slower than that of rabbit SAN cells. Under voltage clamp conditions, depolarization activated the L-type Ca2+ current (I(CaL)) followed by a gradual activation of the delayed rectifier K+ current (I(K)) while hyperpolarization activated the hyperpolarization-activated cation current (I(h)). It was found that the major component of I(K) in porcine SAN is the slowly activating I(K) (I(Ks)), in contrast to SAN cells of the rabbit and other species in which the rapid I(K) (I(Kr)) plays an active role in repolarization and the subsequent pacemaker depolarization. Replacement of rabbit I(Kr) with porcine I(Ks) and a slight modification in the gating parameters and amplitudes of other current systems in the 'Kyoto Model' gave an adequate reconstruction of spontaneous action potentials as well as of the voltage clamp recordings. We conclude that the density and the kinetics of I(K) contribute, in part, to the different heart rates of various species.

The functional properties of the human ether-à-go-go-like (HELK2) K+ channel

The voltage-dependent K+ channels belonging to the ether-à-go-go family (eag, erg, elk) are widely expressed in the mammalian CNS. Their neuronal function, however, is poorly understood. Among the elk clones, elk2 is the most abundantly expressed in the brain. We have characterized the human ELK2 channel (HELK2) expressed in mammalian cell lines. Moreover, we have detected helk2 mRNA and ELK2-like currents in freshly dissociated human astrocytoma cells. HELK2 was inhibited by Cs+ in a voltage-dependent way (Kd was 0.7 mm, at -120 mV). It was not affected by Way 123398 (5 micro m), dofetilide (10 micro m), quinidine (10 micro m), verapamil (20 micro m), haloperidol (2 micro m), astemizole (1 micro m), terfenadine (1 micro m) and hydroxyzine (30 micro m), compounds known to inhibit the biophysically related HERG channel. The crossover of the activation and inactivation curves produced a steady state 'window' current with a peak around -20 mV and considerably broader than it usually is in voltage-dependent channels, including HERG. Similar features were observed in the ELK2 clone from rat, in the same experimental conditions. Thus, ELK2 channels are active within a wide range of membrane potentials, both sub- and suprathreshold. Moreover, the kinetics of channel deactivation and removal of inactivation was about one order of magnitude quicker in HELK2, compared to HERG. Overall, these properties suggest that ELK2 channels are very effective at dampening the neuronal excitability, but less so at producing adaptation of action potential firing frequency. In addition, we suggest experimental ways to recognize HELK2 currents in vivo and raise the issue of the possible function of these channels in astrocytoma.

Open channel block by KCB-328 [1-(2-amino-4-methanesulfonamidophenoxy)-2-[N-(3,4-dimethoxyphenethyl)-N-methylamino]ethane hydrochloride] of the heterologously expressed human ether-a-go-go-related gene K+ channels

KCB-328 [1-(2-amino-4-methanesulfonamidophenoxy)-2-[N-(3,4-dimethoxyphenethyl)-N-methylamino]ethane hydrochloride] is a newly synthesized class III antiarrhythmic drug and is known to be highly effective against various types of arrhythmias induced by coronary artery ligation, reperfusion, and programmed electrical stimulation. To understand the potential ionic mechanisms, we examined the effects of KCB-328, which encodes the rapidly activating delayed rectifier K(+) current in cardiac tissues, on human ether-a-go-go-related gene (HERG) channels expressed in Xenopus oocytes. The amplitudes of steady-state currents and tail currents of HERG were decreased by KCB-328 dose dependently. The decrease became more pronounced at more positive potential, suggesting that the block of HERG by KCB-328 is voltage-dependent. IC(50) values at -30, -20, -10, 0, +10, +20, +30, and +40 mV were 7.6 +/- 0.5, 4.8 +/- 0.4, 3.2 +/- 0.3, 2.1 +/- 0.3, 1.7 +/- 0.2, 1.4 +/- 0.2, 1.3 +/- 0.1, and 1.2 +/- 0.1 microM, respectively. Induction of block depended on depolarization beyond the threshold for channel opening. In addition, time-dependent block developed slowly, with tau = 1.7 +/- 0.3 s (100 microM) at 0 mV, and was delayed by a stronger depolarization to +80 mV, at which HERG channel is inactivated. We can conclude that KCB-328 preferentially blocks open (or activated) HERG channels. The block of HERG current might in part explain the underlying ionic mechanism for the antiarrhythmic and proarrhythmic effect of KCB-328.

Expression and coassociation of ERG1, KCNQ1, and KCNE1 potassium channel proteins in horse heart

In dogs and in humans, potassium channels formed by ether-a-go-go-related gene 1 protein ERG1 (KCNH2) and KCNQ1 alpha-subunits, in association with KCNE beta-subunits, play a role in normal repolarization and may contribute to abnormal repolarization associated with long QT syndrome (LQTS). The molecular basis of repolarization in horse heart is unknown, although horses exhibit common cardiac arrhythmias and may receive drugs that induce LQTS. In horse heart, we have used immunoblotting and immunostaining to demonstrate the expression of ERG1, KCNQ1, KCNE1, and KCNE3 proteins and RT-PCR to detect KCNE2 message. Peptide N-glycosidase F-sensitive forms of horse ERG1 (145 kDa) and KCNQ1 (75 kDa) were detected. Both ERG1 and KCNQ1 coimmunoprecipitated with KCNE1. Cardiac action potential duration was prolonged by antagonists of either ERG1 (MK-499, cisapride) or KCNQ1/KCNE1 (chromanol 293B). Patch-clamp analysis confirmed the presence of a slow delayed rectifier current. These data suggest that repolarizing currents in horses are similar to those of other species, and that horses are therefore at risk for acquired LQTS. The data also provide unique evidence for coassociation between ERG1 and KCNE1 in cardiac tissue.

Functional up-regulation of HERG K+ channels in neoplastic hematopoietic cells

Kv1.3 channels regulate proliferation of normal lymphocytes, but the role of voltage-gated potassium channels in transformed hematopoietic cells is not known. We examined transcripts for Kv1.3, h-erg, h-eag, and BEC1 genes in primary lymphocytes and leukemias and in several hematopoietic cell lines. Surprisingly, BEC1, formerly thought to be brain-specific, was present in all the primary leukemias examined, in resting peripheral blood lymphocytes, and in proliferating activated tonsillar cells, lymphocytes from Sjögren's patients, and Epstein-Barr virus-transformed B-cells. Only h-erg mRNA was up-regulated in the cancer cells, but this was not due to proliferation per se, because it was not elevated in any of the proliferating noncancerous lymphocyte types examined. Nor did h-erg transcript levels correlate with the B-cell subset, because it was elevated in immature neoplastic B-CLL cells (CD5(+)) and in a CD5(-) Burkitt's lymphoma cell line (Raji) but not in Sjögren's syndrome cells (enriched in CD5(+) B-cells) or Epstein-Barr virus-transformed B-cells, which are mature CD5(-) B-cells. The protein and whole cell current levels roughly corresponded with the amount of mRNA expressed in three hematopoietic cell lines: CEM (an acute lymphoblastic leukemic line), K562 (a chronic myelogenous leukemic line), and U937 (an acute promyelocytic leukemic line). The selective HERG channel blocker, E-4031, reduced proliferation of CEM, U937, and K562 cells, and this appears to be the first direct evidence of a functional role for the HERG current in cancer cells. Selective up-regulation of h-erg appears to occur in neoplastic hematopoietic cells, thus providing a marker and potential therapeutic target.

Regulation of an ERG K+ current by Src tyrosine kinase

The human "ether-a-go-go"-related gene (HERG) K(+) channel, and its homologues are present in heart, neuronal tissue, some cancer cells, and the MLS-9 rat microglia cell line (Zhou, W., Cayabyab, F. S., Pennefather, P. S., Schlichter, L. C., and DeCoursey, T. E. (1998) J. Gen. Physiol. 111, 781-794). Despite its importance, there are few studies of ERG modulation. In this first report of regulation by tyrosine phosphorylation we show that MLS-9 cells express transcripts for r-erg1 (rat homologue of HERG) and r-erg2, and an immunoreactive doublet was identified using an anti-HERG antibody. The constitutive tyrosine phosphorylation of the ERG1 protein, detected by co-immunoprecipitation, was reduced by the protein-tyrosine kinase inhibitors, lavendustin A, herbimycin A, or genistein (but not daidzein). The whole cell ERG current was reduced by protein-tyrosine kinase inhibitors or the Src-selective inhibitory peptide, src40-58, but not by a scrambled peptide. Conversely, the current was increased by the Src-activating peptide, srcpY, but not by an inactive analogue. Activating endogenous Src or transfecting constitutively active v-Src altered the voltage dependence and deactivation kinetics to produce more current at negative potentials. Co-immunoprecipitation identified an association between the channel protein and Src. Thus, r-ERG1 and Src tyrosine kinase appear to exist in a signaling complex that is well positioned to modulate this K(+) channel and affect its contribution to cellular functions.

Development of drugs that alter ventricular repolarization

Many drugs are found to alter ventricular repolarization, as manifest by T-wave and U-wave changes on the surface electrocardiogram. These changes have frequently been associated with malignant ventricular arrhythmias. There is no perfectly sensitive and specific way of anticipating such arrhythmias, but some clinical and preclinical screening methods are better than others. The author reviews some of these methods, commenting on some of the regulatory implications.

Molecular and functional characterization of ERG, KCNQ, and KCNE subtypes in rat stomach smooth muscle

Contribution of K(+) channels derived from the expression of ERG, KCNQ, and KCNE subtypes, which are responsible for rapidly and slowly activating delayed rectifier K(+) currents (I(Kr) and I(Ks), respectively) in cardiac myocytes, to membrane currents was examined in stomach circular smooth muscle cells (SMCs). The region-qualified multicell RT-PCR showed that ERG1/KCNE2 transcripts were expressed in rat stomach fundus and antrum SMCs and that KCNQ1/KCNE1 transcripts were expressed in antrum but not fundus. Western blotting and immunocytochemical analyses indicate that ERG1 proteins were substantially expressed in both regions, whereas KCNE1 proteins were faintly expressed in antrum and not in fundus. Both I(Kr)- and I(Ks)-like currents susceptible to E-4031 and indapamide, respectively, were identified in circular SMCs of antrum but only I(Kr)-like current was identified in fundus. It is strongly suggested that I(Kr)- and I(Ks)-like currents functionally identified in rat stomach SMCs are attributable to the expression of ERG1/KCNE2 and KCNQ1/KCNE1, respectively. The membrane depolarization by 1 microM E-4031 indicates the contribution of K(+) channels encoded by ERG1/KCNE2 to the resting membrane potential in stomach SMCs.

NaCl consumption is attenuated in female KCNE1 null mutant mice

The role of potassium channels in the regulation of NaCl intake has not been investigated previously. One potassium channel, KCNQ1, and its regulator, KCNE1, are expressed in salivary glands and kidneys, and KCNE1 null mutant mice are deficient in KCNQ1 potassium currents. To understand the role of the KCNQ1/KCNE1 channel complex in NaCl taste and intake, we compared the NaCl consumption of KCNE1 +/+ (129/Sv), KCNE1 +/-, and KCNE1 -/- mice using two-bottle intake tests and lick rate tests. Although KCNE1 +/+ and KCNE1 +/- mice exhibited consumption patterns for 75-150 mM NaCl solutions considered typical for 129/Sv mice, the KCNE1 -/- null mutant 129/Sv mice were indifferent to or rejected them. This effect was observed in female mice only, required prior exposure to NaCl solutions, and the extent of rejection was greater after prior exposure to 150 mM NaCl solution than 75 mM NaCl solution. No differences were observed in the avidity for KCl solutions or in lick rates of naive mice for 150 or 300 mM NaCl solutions. These results demonstrate that a single potassium channel gene can influence voluntary NaCl intake. We speculate that disruption of the KCNE1 gene impairs sodium metabolism in female mice drinking high levels of 150 mM NaCl, which causes malaise that becomes associated with NaCl taste, and as a consequence, reduced preference for NaCl.

I(Kr): the hERG channel

G.-N. Tseng. I(Kr): The hERG Channel. Journal of Molecular and Cellular Cardiology (2001) 33, 835-849. The rapid delayed rectifier (I(Kr)) channel is important for cardiac action potential repolarization. Suppressing I(Kr)function, due to either genetic defects in its pore-forming subunit (hERG) or adverse drug effects, can lead to long-QT (LQT) syndrome that carries increased risk of life-threatening arrhythmias. The implication of I(Kr)in cardiac arrhythmias and in anti-arrhythmic/pro-arrhythmic actions of drugs has driven intensive research interests in its structure-function relationship, the linkage between LQT-associated mutations and changes in channel function, and the mechanism of drug actions. This review will cover the following topics: (1) heterogeneous contribution of I(Kr)to action potential repolarization in the heart, (2) structure-function relationship of I(Kr)/hERG channels, (3) role of regulatory & bgr; subunits in I(Kr)/hERG channel function, (4) structural basis for the unique pharmacological properties of I(Kr)/hERG channels, and (5) I(Kr)/hERG channel modulation by changes in cellular milieu under physiological and pathological conditions of the heart. It is anticipated that further advances in our understanding of I(Kr)/hERG, particularly in the areas of roles of different (& agr; and & bgr;) subunits in native I(Kr)function, alterations in I(Kr)function in diseased hearts, and the 3-dimensional structure of the I(Kr)/hERG pore based on homology modeling using the KcsA model, will help us better define the role of I(Kr)in arrhythmias and design therapeutic agents that can increase I(Kr)and are useful for LQT syndrome.

Mice disrupted for the KvLQT1 potassium channel regulator IsK gene accumulate mature T cells

The IsK protein associates with KvLQT1 potassium channels to generate the slow component of the outward rectifying K(+) current involved in human cardiac repolarization. Mutations in either KCNE1 (encoding IsK) or KCNQ1 (encoding KvLQT1) genes have been associated with the long QT syndrome, a genetic disorder leading to prolonged cardiac repolarization and sudden death. We now report that the IsK protein is also involved in mature T cell homeostasis. In KCNE1 gene knockout mice, we observed a significant increase in the T cell compartment. Thymus and peripheral lymphoid organs of KCNE1-/- mice displayed a significant increase in mature T cells. The immunological phenotype of KCNE1-/- is age-dependent and only expressed in adult mice. Both IsK and KvLQT1 mRNA are expressed in murine thymus. Our data suggest that, in addition to its role in myocardial repolarization, the IsK-KvLQT1 tandem also plays a crucial role in T cell homeostasis.