The key characteristics of cardiotoxicity for the pervasive pollutant phenanthrene.

The key characteristic (KCs) framework has been used previously to assess the carcinogenicity and cardiotoxicity of various chemical and pharmacological agents. Here, the 12 KCs of cardiotoxicity are used to evaluate the previously reported cardiotoxicity of phenanthrene (Phe), a tricyclic polycyclic aromatic hydrocarbon (PAH), and major component of fossil fuel-derived air pollution. Phe is a semi-volatile pollutant existing in both the gas phase and particle phase through adsorption onto or into particulate matter (PM). Phe can translocate across the airways and gastrointestinal tract into the systemic circulation, enabling body-wide effects. Our evaluation based on a comprehensive literature review, indicates Phe exhibits 11 of the 12 KCs for cardiotoxicity. These include adverse effects on cardiac electromechanical performance, the vasculature and endothelium, immunomodulation and oxidative stress, and neuronal and endocrine control. Environmental agents that have similarly damaging effects on the cardiovascular system are heavily regulated and monitored, yet globally there is no air quality regulation specific for PAHs like Phe. Environmental monitoring of Phe is not the international standard with benzo[a]pyrene being frequently used as a proxy despite the two PAH species exhibiting significant differences in sources, concentration variations and toxic effects. The evidence summarised in this evaluation highlights the need to move away from proxied PAH measurements and develop a monitoring network capable of measuring Phe concentration. It also stresses the need to raise awareness amongst the medical community of the potential cardiovascular impact of PAH exposure. This will allow the production of mitigation strategies and possibly the development of new policies for the protection of the societal groups most vulnerable to cardiovascular disease.

Pro-arrhythmic effects of gain-of-function potassium channel mutations in the short QT syndrome.

The congenital short QT syndrome (SQTS) is a rare condition characterized by abbreviated rate-corrected QT (QTc) intervals on the electrocardiogram and by increased susceptibility to both atrial and ventricular arrhythmias and sudden death. Although mutations to multiple genes have been implicated in the SQTS, evidence of causality is particularly strong for the first three (SQT1-3) variants: these result from gain-of-function mutations in genes that encode K+ channel subunits responsible, respectively, for the IKr, IKs and IK1 cardiac potassium currents. This article reviews evidence for the impact of SQT1-3 missense potassium channel gene mutations on the electrophysiological properties of IKr, IKs and IK1 and of the links between these changes and arrhythmia susceptibility. Data from experimental and simulation studies and future directions for research in this field are considered. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Polyaromatic hydrocarbons in pollution: a heart-breaking matter.

Air pollution is associated with detrimental effects on human health, including decreased cardiovascular function. However, the causative mechanisms behind these effects have yet to be fully elucidated. Here we review the current epidemiological, clinical and experimental evidence linking pollution with cardiovascular dysfunction. Our focus is on particulate matter (PM) and the associated low molecular weight polycyclic aromatic hydrocarbons (PAHs) as key mediators of cardiotoxicity. We begin by reviewing the growing epidemiological evidence linking air pollution to cardiovascular dysfunction in humans. We next address the pollution-based cardiotoxic mechanisms first identified in fish following the release of large quantities of PAHs into the marine environment from point oil spills (e.g. Deepwater Horizon). We finish by discussing the current state of mechanistic knowledge linking PM and PAH exposure to mammalian cardiovascular patho-physiologies such as atherosclerosis, cardiac hypertrophy, arrhythmias, contractile dysfunction and the underlying alterations in gene regulation. Our aim is to show conservation of toxicant pathways and cellular targets across vertebrate hearts to allow a broad framework of the global problem of cardiotoxic pollution to be established. AhR; Aryl hydrocarbon receptor. Dark lines indicate topics discussed in this review. Grey lines indicate topics reviewed elsewhere.

The human ether-a'-go-go related gene (hERG) K+ channel blockade by the investigative selective-serotonin reuptake inhibitor CONA-437: limited dependence on S6 aromatic residues.

Diverse non-cardiac drugs adversely influence cardiac electrophysiology by inhibiting repolarising K(+) currents mediated by channels encoded by the human ether-a-go-go-related gene (hERG). In this study, pharmacological blockade of hERG K(+) channel current (I(hERG)) by a novel investigative serotonin-selective reuptake inhibitor (SSRI), CONA-437, was investigated. Whole-cell patch-clamp measurements of I(hERG) were made from human embryonic kidney (HEK 293) cells expressing wild-type (WT) or mutant forms of the hERG channel. With a step-ramp voltage-command, peak I(hERG) was inhibited with an IC(50) of 1.34 µM at 35 ±1°C; the IC(50) with the same protocol was not significantly different at room temperature. Voltage-command waveform selection had only a modest effect on the potency of I(hERG) block: the IC50 with a ventricular action potential command was 0.72 µM. I(hERG) blockade developed rapidly with time following membrane depolarisation and showed a weak dependence on voltage, accompanied by a shift of ≈ -5 mV in voltage-dependence of activation. There was no significant effect of CONA-437 on voltage-dependence of I(hERG) inactivation, though at some voltages an apparent acceleration of the time-course of inactivation was observed. Significantly, mutation of the S6 aromatic amino acid residues Y652 and F656 had only a modest effect on I(hERG) blockade by CONA-437 (a 3-4 fold shift in affinity). CONA-437 at up to 30 µM had no significant effect on either Nav1.5 sodium channels or L-type calcium channels. In conclusion, the novel SSRI CONA-437 is particularly notable as a gating-dependent hERG channel inhibitor for which neither S6 aromatic amino-acid constituent of the canonical drug binding site on the hERG channel appears obligatory for I(hERG) inhibition to occur.

In silico investigation of a KCNQ1 mutation associated with familial atrial fibrillation.

Mutations in transmembrane domains of the KCNQ1 subunit of the I(Ks) potassium channel have been associated with familial atrial fibrillation. We have investigated mechanisms by which the S1 domain S140G KCNQ1 mutation influences atrial arrhythmia risk and, additionally, whether it can affect ventricular electrophysiology. In perforated-patch recordings, S140G-KCNQ1+KCNE1 exhibited leftward-shifted activation, slowed deactivation and marked residual current. In human atrial action potential (AP) simulations, AP duration and refractoriness were shortened and rate-dependence flattened. Simulated I(Ks) but not I(Kr) block offset AP shortening produced by the mutation. In atrial tissue simulations, temporal vulnerability to re-entry was little affected by the S140G mutation. Spatial vulnerability was markedly increased, leading to more stable and stationary spiral wave re-entry in 2D stimulations, which was offset by I(Ks) block, and to scroll waves in 3D simulations. These changes account for vulnerability to AF with this mutation. Ventricular AP clamp experiments indicate a propensity for increased ventricular I(Ks) with the S140G KCNQ1 mutation and ventricular AP simulations showed model-dependent ventricular AP abbreviation.

ORM-10103: a significant advance in sodium-calcium exchanger pharmacology?

The sodium-calcium exchanger (NCX) is an electrogenic transporter that is widely expressed in different tissues. In the heart, the NCX plays important roles in calcium ion homeostasis, excitation-contraction coupling and the electrophysiological properties of cardiac myocytes. Precise determination of the roles of the NCX has somewhat been hampered by a lack of selective small molecule inhibitors. In this issue of the BJP, Jost and colleagues present data on a new NCX inhibitor, ORM-10103, which has submicromolar EC50 values against cardiac forward and reverse exchange activity. The compound exhibits improved selectivity over existing small molecule NCX inhibitors and, in particular, appears to be without effect on L-type calcium channels at high concentrations. ORM-10103 could therefore have significant value for studies of the (patho)physiological roles of the NCX in the heart. Further pharmacological studies are required to investigate the actions of ORM-10103 on cardiac cells and tissues and to determine its effects on non-cardiac NCX isoforms.

Cardiac ion channel modulation by the hypoglycaemic agent rosiglitazone.

The hypoglycaemic thiazolidinedione rosiglitazone is used clinically in the treatment of type 2 diabetes. However, in 2010, information relating to rosiglitazone-associated increased cardiovascular risk led the European Medicines Agency to recommend suspension of marketing authorizations for rosiglitazone-containing anti-diabetes drugs, while the US Food and Drug Administration recommended significant restriction on the agent's use. Two timely studies in this issue of the British Journal of Phrarmacology provide new information regarding modification of cardiac cellular electrophysiology by rosiglitazone. Szentandrássy et al. demonstrate canine ventricular action potential modification and concentration-dependent suppression of L-type Ca current and of transient outward and rapid delayed rectifier K currents. Jeong et al. demonstrate concentration-dependent inhibition of recombinant K(v) 4.3 channels, providing mechanistic insight into the likely molecular basis of transient outward K current inhibition by the compound. Further studies using diabetic models would be of value to determine whether, in a diabetic setting, rosiglitazone modification of these channels could affect the risk of arrhythmia at clinically relevant drug concentrations.

Pharmacological inhibition of the hERG potassium channel is modulated by extracellular but not intracellular acidosis.

INTRODUCTION: Human ether-à-go-go related gene (hERG) is responsible for channels that mediate the rapid delayed rectifier K(+) channel current (I(Kr) ), which participates in repolarization of the ventricles and is a target for some antiarrhythmic drugs. Acidosis occurs in the heart in some pathological situations and can modify the function and responses to drugs of ion channels. The aim of this study was to determine the effects of extracellular and intracellular acidosis on the potency of hERG channel current (I(hERG)) inhibition by the antiarrhythmic agents dofetilide, flecainide, and amiodarone at 37 °C. METHODS AND RESULTS: Whole-cell patch-clamp recordings of I(hERG) were made at 37 °C from hERG-expressing Human Embryonic Kidney (HEK293) cells. Half-maximal inhibitory concentration (IC(50)) values for I(hERG) tail inhibition at -40 mV following depolarizing commands to +20 mV were significantly higher at external pH 6.3 than at pH 7.4 for both flecainide and dofetilide, but not for amiodarone. Lowering pipette pH from 7.2 to 6.3 altered neither I(hERG) kinetics nor the extent of observed I(hERG) blockade by any of these drugs. CONCLUSION: Conditions leading to localized extracellular acidosis may facilitate heterogeneity of action of dofetilide and flecainide, but not amiodarone via modification of hERG channel blockade. Such effects depend on the external pH change rather than intracellular acidification.

Enhanced inhibitory effect of acidosis on hERG potassium channels that incorporate the hERG1b isoform.

Extracellular acidosis occurs in the heart during myocardial ischemia and can lead to dangerous arrhythmias. Potassium channels encoded by hERG (human ether-à-go-go-related gene) mediate the cardiac rapid delayed rectifier K+ current (IKr), and impaired hERG function can exacerbate arrhythmia risk. Nearly all electrophysiological investigations of hERG have centred on the hERG1a isoform, although native IKr channels may be comprised of hERG1a and hERG1b, which has a unique shorter N-terminus. This study has characterised for the first time the effects of extracellular acidosis (an extracellular pH decrease from 7.4 to 6.3) on hERG channels incorporating the hERG1b isoform. Acidosis inhibited hERG1b current amplitude to a significantly greater extent than that of hERG1a, with intermediate effects on coexpressed hERG1a/1b. IhERG tail deactivation was accelerated by acidosis for both isoforms. hERG1a/1b activation was positively voltage-shifted by acidosis, and the fully-activated current-voltage relation was reduced in amplitude and right-shifted (by ∼10 mV). Peak IhERG1a/1b during both ventricular and atrial action potentials was both suppressed and positively voltage-shifted by acidosis. Differential expression of hERG isoforms may contribute to regional differences in IKr in the heart. Therefore inhibitory effects of acidosis on IKr could also differ regionally, depending on the relative expression levels of hERG1a and 1b, thereby increasing dispersion of repolarization and arrhythmia risk.

Action potential clamp and mefloquine sensitivity of recombinant 'I KS' channels incorporating the V307L KCNQ1 mutation.

The slow delayed rectifier potassium current, 'I(Ks)', contributes to repolarisation of cardiac ventricular action potentials and thereby to the duration of the QT interval of the electrocardiogram. Mutations to I(Ks) channel subunits occur in clinically significant cardiac repolarisation disorders. The short QT syndrome (SQTS) is associated with accelerated ventricular repolarisation and with an increased risk of arrhythmia and sudden death. The SQT2 variant of the SQTS has been linked to a gain-of-function amino-acid substitution (V307L) in the KCNQ1-encoded I(Ks) channel alpha-subunit. This study reports the first action potential (AP) voltage-clamp comparison between wild-type (WT) and V307L KCNQ1 (co-expressed with KCNE1 to recapitulate I(Ks)) and identifies an effective pharmacological inhibitor of recombinant 'I(Ks)' channels incorporating the V307L KCNQ1 mutation. Perforated-patch voltage-clamp recordings at 37 degrees C of whole-cell current carried by co-expressed KCNQ1 and KCNE1 showed a marked (-36 mV) shift in half-maximal activation for V307L compared to WT KCNQ1; a significant slowing of current deactivation was also observed. Under AP clamp, peak repolarising current was significantly augmented for V307L KCNQ1 compared to WT KCNQ1 for both ventricular and atrial AP commands, consistent with an ability of the V307L mutation to increase repolarising I(Ks) in both regions. The quinoline agent mefloquine inhibited WT KCNQ1+KCNE1 with an IC(50) of 3.4 muM compared to 3.3 muM for V307L KCNQ1+KCNE1 (P >0.05). This establishes mefloquine as an effective inhibitor of recombinant 'I(Ks)' channels incorporating this SQT2 KCNQ1 mutation.

The S140G KCNQ1 atrial fibrillation mutation affects 'I(KS)' profile during both atrial and ventricular action potentials.

KCNQ1 is responsible for the pore-forming subunit of channels that mediate the cardiac 'IKs' potassium channel current. The S140G KCNQ1 gain-of-function mutation is responsible for a form of heritable atrial fibrillation. Here the action potential (AP) voltage clamp technique was used to elucidate the effect of S140G KCNQ1 on the profile of recombinant I(Ks) during atrial and ventricular APs applied to KCNQ1+KCNE1 expressing CHO cells, at 37°C. Under conventional voltage clamp the S140G KCNQ1 mutation shifted voltage-dependent activation by ≈-62 mV, with a marked instantaneous current component evident on membrane depolarisation. Under atrial AP clamp, cells expressing wild-type (WT) KCNQ1 exhibited modest outward currents during atrial repolarisation, whilst those expressing S140G KCNQ1 exhibited a marked instantaneous outward current and peak repolarising current >4-fold that for WT KCNQ1. Under ventricular AP clamp, both WT and mutant KCNQ1 conditions showed greater peak repolarising current than when an atrial AP command was used and the S140G mutation resulted in peak repolarising current that was >3-fold that for WT KCNQ1. We conclude that the S140G KCNQ1 mutation would be predicted to augment substantially repolarising current both early and throughout atrial APs and, in principle, also to influence markedly ventricular AP repolarisation.

One-dimensional mathematical model of the atrioventricular node including atrio-nodal, nodal, and nodal-his cells.

Mathematical models are a repository of knowledge as well as research and teaching tools. Although action potential models have been developed for most regions of the heart, there is no model for the atrioventricular node (AVN). We have developed action potential models for single atrio-nodal, nodal, and nodal-His cells. The models have the same action potential shapes and refractoriness as observed in experiments. Using these models, together with models for the sinoatrial node (SAN) and atrial muscle, we have developed a one-dimensional (1D) multicellular model including the SAN and AVN. The multicellular model has slow and fast pathways into the AVN and using it we have analyzed the rich behavior of the AVN. Under normal conditions, action potentials were initiated in the SAN center and then propagated through the atrium and AVN. The relationship between the AVN conduction time and the timing of a premature stimulus (conduction curve) is consistent with experimental data. After premature stimulation, atrioventricular nodal reentry could occur. After slow pathway ablation or block of the L-type Ca(2+) current, atrioventricular nodal reentry was abolished. During atrial fibrillation, the AVN limited the number of action potentials transmitted to the ventricle. In the absence of SAN pacemaking, the inferior nodal extension acted as the pacemaker. In conclusion, we have developed what we believe is the first detailed mathematical model of the AVN and it shows the typical physiological and pathophysiological characteristics of the tissue. The model can be used as a tool to analyze the complex structure and behavior of the AVN.

hERG1a/1b heteromeric currents exhibit amplified attenuation of inactivation in variant 1 short QT syndrome.

Potassium channels encoded by hERG (human ether-à-go-go-related gene) underlie the cardiac rapid delayed rectifier K+ current (IKr) and hERG mutations underpin clinically important repolarization disorders. Virtually all electrophysiological investigations of hERG mutations have studied exclusively the hERG1a isoform; however, recent evidence indicates that native IKr channels may be comprised of hERG1a together with the hERG1b variant, which has a shorter N-terminus. Here, for the first time, electrophysiological effects were studied of a gain-of-function hERG mutation (N588K; responsible for the 'SQT1' variant of the short QT syndrome) on current (I(hERG1a/1b)) carried by co-expressed hERG1a/1b channels. There were no significant effects of N588K on I(hERG1a/1b) activation or deactivation, but N588K I(hERG1a/1b) showed little inactivation up to highly positive voltages (< or = +80 mV), a more marked effect than seen for hERG1a expressed alone. I(hERG1a/1b) under action potential voltage-clamp, and the effects on this of the N588K mutation, also showed differences from those previously reported for hERG1a. The amplified attenuation of I(hERG) inactivation for the N588K mutation reported here indicates that the study of co-expressed hERG1a/1b channels should be considered when investigating clinically relevant hERG channel mutations, even if these reside outside of the N-terminus region.

Comparative effects of the short QT N588K mutation at 37 degrees C on hERG K+ channel current during ventricular, Purkinje fibre and atrial action potentials: an action potential clamp study.

The short QT syndrome (SQTS) is a cardiac repolarisation disorder characterised by abbreviated QT intervals on the electrocardiogram and by an increased risk of atrial and ventricular arrhythmias and sudden death. The SQT1 variant involves a gain-of-function mutation (N588K) that impairs inactivation of the hERG (human ether-a-go-go-related gene) potassium channel and, thereby, increases current mediated by the rapid delayed rectifier potassium current (I(Kr)) in the heart. Here, the action potential voltage clamp (AP clamp) technique was applied to Chinese Hamster Ovary cells expressing wild-type or N588K-hERG at 37 degrees C, to compare effects of the N588K mutation on hERG current (I(hERG)) during ventricular, atrial and Purkinje fibre APs. The N588K mutation altered the I(hERG) profile during each AP type; increased maximal repolarising current occurred earlier during AP repolarisation (with shifts of +60 mV, +30 mV and +15 mV respectively for ventricular, Purkinje fibre and atrial APs). Thus SQT1 may influence repolarising I(hERG) for each cell type, with AP clamp experiments and simulation data indicating the greatest effect during ventricular APs. Changes in the timing of outward I(hERG) transients elicited by premature stimuli following AP commands indicate that SQT1 may alter the protection that hERG provides cardiac tissue against premature arrhythmogenic stimuli.

Modulation by testosterone of an endogenous hERG potassium channel current.

hERG (human ether-a-go-go-related gene) potassium (K+) channels are expressed in a range of tissue types including neuroblastoma cells and the heart, in which hERG K+ current is important for action potential repolarization. Whilst gender differences in cardiac repolarization and the QT interval of the cardiac electrocardiogram are well-established, comparatively little is known about regulation of hERG channels by sex hormones. In this study, whole-cell patch-clamp recordings were made at 37 degrees C from SH-SY5Y human neuroblastoma cells to investigate modulation of endogenous hERG K+ channel current (I(hERG)) by testosterone. Acutely applied testosterone at a physiologically relevant concentration (10 nM) produced a modest (approximately 13-15 %) increase in I(hERG) amplitude, whilst a high concentration (1 microM) slightly decreased I(hERG). The stimulatory effect of testosterone was inhibited by the androgen receptor antagonist flutamide (10 microM) and the PI-3 kinase inhibitor wortmannin (1 microM). Chronic (24 h) application of testosterone also augmented IhERG via flutamide-sensitive receptor activation, without modulation of the current's voltage-dependence. These results demonstrate for the first time that testosterone can stimulate (hERG) K+ channels via activation of classical androgen receptors and implicate PI-3 kinase in the acute response.

Pharmacology of the short QT syndrome N588K-hERG K+ channel mutation: differential impact on selected class I and class III antiarrhythmic drugs.

BACKGROUND AND PURPOSE: The short QT syndrome (SQTS) is associated with cardiac arrhythmias and sudden death. The SQT1 form of SQTS results from an inactivation-attenuated, gain-of-function mutation (N588K) to the human ether-à-go-go-related gene (hERG) potassium channel. Pharmacological blockade of this mutated hERG channel may have therapeutic value. However, hERG-blocking potencies of canonical inhibitors such as E-4031 and D-sotalol are significantly reduced for N588K-hERG. Here, five hERG-blocking drugs were compared to determine their relative potencies for inhibiting N588K channels, and two other inactivation-attenuated mutant channels were tested to investigate the association between impaired inactivation and altered drug potency. EXPERIMENTAL APPROACH: Whole-cell patch clamp measurements of hERG current (I(hERG)) mediated by wild-type and mutant (N588K, S631A and N588K/S631A) channels were made at 37 degrees C CHO cells. KEY RESULTS: The N588K mutation attenuated I(hERG) inhibition in the following order: E-4031>amiodarone>quinidine>propafenone>disopyramide. Comparing the three inactivation mutants, the two single mutations, although occurring in different modules of the channel, attenuated inactivation to a nearly identical degree, whereas the double mutant caused considerably greater attenuation, permitting the titration of inactivation. Attenuation of channel inhibition was similar between the single mutants for each drug, and was significantly greater with the double mutant. CONCLUSIONS AND IMPLICATIONS: The degree of drug inhibition of hERG channels may vary based on the level of channel inactivation. Drugs previously identified as useful for treating SQT1 have the least dependence on hERG inactivation. In addition, our findings indicate that amiodarone may warrant further investigation as a potential treatment for SQT1.

Spontaneous frequency of rabbit atrioventricular node myocytes depends on SR function.

Spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) appears to play an important role in cardiac sinoatrial node pacemaking. However, comparatively little is known about the role of intracellular Ca(2+) in the atrioventricular node (AVN). Intracellular Ca(2+) was therefore monitored in cells isolated from the rabbit AVN, using fluo-3 in conjunction with confocal microscopy. These cells displayed spontaneous Ca(2+) transients and action potentials. Ca(2+) transients were normally preceded by a small, slow increase (ramp) of intracellular Ca(2+) which was sometimes, but not always, accompanied by Ca(2+) sparks. During the Ca(2+) transient, intracellular [Ca(2+)] increased initially at the cell periphery and propagated inhomogeneously to the cell centre. The rate of spontaneous activity was decreased by ryanodine (1muM) and increased by isoprenaline (500nM); these changes were accompanied by a decrease and increase, respectively, in the slope of the preceding Ca(2+) ramp, with no significant change in Ca(2+) spark characteristics. Rapidly reducing bathing [Na(+)] inhibited spontaneous activity. These findings provide the first information on Ca(2+) handling at the sub-cellular level and link cellular Ca(2+) cycling to the genesis of spontaneous activity in the AVN.

More types than one: multiple muscarinic receptor coupled K+ currents undergo remodelling in an experimental model of atrial fibrillation.

The common cardiac arrhythmia atrial fibrillation (AF) tends to show progression in its severity, which is associated with 'remodelling': structural and electrophysiological changes that facilitate arrhythmia induction and maintenance. In this issue of the BJP, Yeh and colleagues demonstrate for the first time, down-regulation of three types of muscarinic cholinergic receptor (mAChR) coupled K+ currents (IKM2, IKM3 and IKM4) and of M2, M3 and M4 mAChR subtype proteins, in a canine model of atrial tachycardia (AT) induced remodelling. The IKMs and their extent of AT-induced remodelling were similar in left-atrial and pulmonary vein (PV) myocytes, so remodelling of M2-M4 receptor-linked currents appears not to underlie the unique contribution of PVs to AF. Parasympathetic stimulation can increase susceptibility to AF; thus remodelling of M2-M4 receptors and K+ currents could be adaptive in AT. Further work is warranted to determine whether or not remodelling of multiple mAChRs and currents also contributes to human AF.

Inhibition of the HERG potassium channel by the tricyclic antidepressant doxepin.

HERG (human ether-à-go-go-related gene) encodes channels responsible for the cardiac rapid delayed rectifier potassium current, I(Kr). This study investigated the effects on HERG channels of doxepin, a tricyclic antidepressant linked to QT interval prolongation and cardiac arrhythmia. Whole-cell patch-clamp recordings were made at 37 degrees C of recombinant HERG channel current (I(HERG)), and of native I(Kr) 'tails' from rabbit ventricular myocytes. Doxepin inhibited I(HERG) with an IC(50) value of 6.5+/-1.4 microM and native I(Kr) with an IC(50) of 4.4+/-0.6 microM. The inhibitory effect on I(HERG) developed rapidly upon membrane depolarization, but with no significant dependence on voltage and with little alteration to the voltage-dependent kinetics of I(HERG). Neither the S631A nor N588K inactivation-attenuating mutations (of residues located in the channel pore and external S5-Pore linker, respectively) significantly reduced the potency of inhibition. The S6 point mutation Y652A increased the IC(50) for I(HERG) blockade by approximately 4.2-fold; the F656A mutant also attenuated doxepin's action at some concentrations. HERG channel blockade is likely to underpin reported cases of QT interval prolongation with doxepin. Notably, this study also establishes doxepin as an effective inhibitor of mutant (N588K) HERG channels responsible for variant 1 of the short QT syndrome.

An outwardly rectifying anionic background current in atrial myocytes from the human heart.

This report describes a hitherto unreported anionic background current from human atrial cardiomyocytes. Under whole-cell patch-clamp with anion-selective conditions, an outwardly rectifying anion current (I(ANION)) was observed, which was larger with iodide than nitrate, and with nitrate than chloride as charge carrier. In contrast with a previously identified background anionic current from small mammal cardiomyocytes, I(ANION) was not augmented by the pyrethroid tefluthrin (10 microM); neither was it inhibited by hyperosmolar external solution nor by DIDS (200 microM); thus I(ANION) was not due to basal activity of volume-sensitive anion channels. I(ANION) was partially inhibited by the Cl(-) channel blockers NPPB (50 microM) and Gly H-101 (30 microM). Incorporation of I(ANION) into a human atrial action potential (AP) simulation led to depression of the AP plateau, accompanied by alterations to plateau inward calcium current, and to AP shortening at 50% but not 90% of complete repolarization, demonstrating that I(ANION) can influence the human atrial AP profile.