Human Ether-a-go-go-related Gene Current Research Articles

Derek Nexus predicts human skin sensitisation accurately. What is the rationale behind its predictive performance?

Domiphen bromide and didecyl dimethylammonium bromide were widely used environmental chemicals with potent activity on blockade of human ether-a-go-go related gene (HERG) channels. But the mechanism of their action is not clear. The kinetics of block of HERG channels by domiphen bromide and didecyl dimethylammonium bromide was studied in order to characterize the inhibition of HERG currents by these quaternary ammonium compounds (QACs). Domiphen bromide and didecyl dimethylammonium bromide inhibited HERG channel currents in a dose-dependent manner with IC50 values of 9 nM and 5 nM, respectively. Block of HERG channel by domiphen bromide and didecyl dimethylammonium bromide was voltage-dependent and use-dependent. Domiphen bromide and didecyl dimethylammonium bromide caused substantial negative shift of the activation curves, accelerated activated process, but had no effects on the deactivation and reactivation processes. The docking models implied that these two compounds bound to PAS domain of HERG channels and inhibited its function. Our data demonstrated that domiphen bromide and didecyl dimethylammonium bromide blocked the HERG channel with a preference for the activated channel state.

Inhibition of HERG potassium channels by domiphen bromide and didecyl dimethylammonium bromide

Domiphen bromide and didecyl dimethylammonium bromide were widely used environmental chemicals with potent activity on blockade of human ether-a-go-go related gene (HERG) channels. But the mechanism of their action is not clear. The kinetics of block of HERG channels by domiphen bromide and didecyl dimethylammonium bromide was studied in order to characterize the inhibition of HERG currents by these quaternary ammonium compounds (QACs). Domiphen bromide and didecyl dimethylammonium bromide inhibited HERG channel currents in a dose-dependent manner with IC50 values of 9nM and 5nM, respectively. Block of HERG channel by domiphen bromide and didecyl dimethylammonium bromide was voltage-dependent and use-dependent. Domiphen bromide and didecyl dimethylammonium bromide caused substantial negative shift of the activation curves, accelerated activated process, but had no effects on the deactivation and reactivation processes. The docking models implied that these two compounds bound to PAS domain of HERG channels and inhibited its function. Our data demonstrated that domiphen bromide and didecyl dimethylammonium bromide blocked the HERG channel with a preference for the activated channel state.

Cholesterol regulates HERG K+ channel activation by increasing phospholipase C β1 expression

Human ether-a-go-go-related gene (HERG) K+ channel underlies the rapidly activating delayed rectifier K+ conductance (IKr) during normal cardiac repolarization. Also, it may regulate excitability in many neuronal cells. Recently, we showed that enrichment of cell membrane with cholesterol inhibits HERG channels by reducing the levels of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] due to the activation of phospholipase C (PLC). In this study, we further explored the effect of cholesterol enrichment on HERG channel kinetics. When membrane cholesterol level was mildly increased in human embryonic kidney (HEK) 293 cells expressing HERG channel, the inactivation and deactivation kinetics of HERG current were not affected, but the activation rate was significantly decelerated at all voltages tested. The application of PtdIns(4,5)P2 or inhibitor for PLC prevented the effect of cholesterol enrichment, while the presence of antibody against PtdIns(4,5)P2 in pipette solution mimicked the effect of cholesterol enrichment. These results indicate that the effect of cholesterol enrichment on HERG channel is due to the depletion of PtdIns(4,5)P2. We also found that cholesterol enrichment significantly increases the expression of β1 and β3 isoforms of PLC (PLCβ1, PLCβ3) in the membrane. Since the effects of cholesterol enrichment on HERG channel were prevented by inhibiting transcription or by inhibiting PLCβ1 expression, we conclude that increased PLCβ1 expression leads to the deceleration of HERG channel activation rate via downregulation of PtdIns(4,5)P2. These results confirm a crosstalk between two plasma membrane-enriched lipids, cholesterol and PtdIns(4,5)P2, in the regulation of HERG channels.

Cardiovascular Effects of the Selective α1A-Adrenoceptor Antagonist Silodosin (KMD-3213), a Drug for the Treatment of Voiding Dysfunction

The aim of this study was to assess the cardiovascular effects of silodosin (CAS 160970-54-7, (-)-1-(3-hydroxypropyl)-5-[(2R)-2-({2-[2-(2,2,2-trifluoroethoxy) phenoxy]ethyllamino)propyll-2,3-dihy-dro-1H-indole-7-carboxamide, KMD-3213), a potent selective alpha1A-adrenoceptor (AR) antagonist used for the treatment of dysuria. In conscious dogs, orally administered silodosin, at doses (0.2, 2 and 20 mg/kg) considerably higher than the pharmacologically effective dose, decreased blood pressure. These doses, however, had no effects on heart rate or on the electrocardiogram (PR interval, QRS interval, QT interval or QTc). In addition, the cardiac effects of silodosin were evaluated in an in vitro electrophysiological study. Silodosin inhibited the human ether-a-go-go-related gene (HERG) tail current, leaving a residual tail current of 45 % control at 10 micromol/L. However, the concentration that inhibited the HERG tail current was considerably higher than its affinity for alphal-ARs. These results suggest that while the alpha1B-AR subtype is mainly involved in the regulation of blood pressure, the alphalA-AR subtype is not. There seems to exist no evidence of a correlation between the function of alphal-AR and ECG changes reflecting inhibition of the HERG current. The cardiovascular profile of silodosin suggests therefore that it is a safe and well-tolerated drug.

Inhibitory effects of a bisbenzylisoquinline alkaloid dauricine on HERG potassium channels

Ethnopharmacological relevanceThe roots of Menispermum dauricum have been widely used for the treatment of inflammation, allergy and arrhythmia in China for a long time. Dauricine (Dau), a bisbenzylisoquinline alkaloid from Menispermum dauricum, mainly contributes to the anti-arrhythmic effect and has received pharmacological attention. Dau can prolong the action potential duration (APD), which has been attributed to its ability to modulate Ca2+ and several K+ channels. However, its effects on human-ether-a-go-go-related gene (HERG) channels are unknown. Aim of the studyThe effects of Dau on HERG channels were investigated. Materials and methodsWhole-cell patch-clamp technique was used to record HERG current (IHERG) carried by recombinant HERG channels expressed in HEK293 cells. ResultsDau inhibited IHERG in a concentration-dependent manner with an IC50 of 3.5μM. Development of block and washout were fast. The inhibitory action of Dau was contingent on channel gating, showing significant voltage and time dependence. Dau inhibited IHERG in the open and inactivated states, but not in the closed states. The activation curve was shifted in a negative direction. ConclusionsDau inhibits HERG encoded potassium channels and this action might be a molecular mechanism for the previously reported APD prolongation with this drug.

Prolongation of Cardiac Ventricular Repolarization Under Paliperidone: How and How Much?

Paliperidone (9-hydroxyrisperidone) is a second-generation antipsychotic. As observed with risperidone, QT interval prolongation was reported with paliperidone. The aim was to evaluate the effects of paliperidone on cardiac ventricular repolarization. (1) Patch-clamp experiments: Human ether-a-go-go-related gene (HERG)- or KCNQ1 + KCNE1-transfected cells were exposed to 0.1-100 μmol/L paliperidone (N = 39 cells, total) to assess the drug effect on HERG and KCNQ1 + KCNE1 currents. (2) Langendorff perfusion experiments: Hearts isolated from male Hartley guinea pigs (N = 9) were exposed to 0.1 μmol/L paliperidone to assess drug-induced prolongation of monophasic action potential duration measured at 90% repolarization. (3) In vivo cardiac telemetry experiments: Guinea pigs (N = 8) implanted with transmitters were injected a single intraperitoneal dose of 1 mg/kg of paliperidone, and 24-hour electrocardiogram recordings were made. (1) The estimated concentration at which 50% of the maximal inhibitory effect is observed (IC(50)) for paliperidone on HERG current was 0.5276 μmol/L. In contrast, 1 μmol/L paliperidone had hardly any effect on KCNQ1 + KCNE1 current (4.0 ± 1.6% inhibition, N = 5 cells). (2) While pacing the hearts at cycle lengths of 150, 200, or 250 milliseconds, 0.1 μmol/L paliperidone prolonged monophasic action potential duration measured at 90% repolarization by, respectively, 6.1 ± 3.1, 9.8 ± 2.7, and 12.8 ± 2.7 milliseconds. (3) Paliperidone (1 mg/kg) intraperitoneal caused a maximal 15.7 ± 5.3-millisecond prolongation of QTc. Paliperidone prolongs the QT interval by blocking HERG current at clinically relevant concentrations and is potentially unsafe.

Inhibitory Actions of HERG Currents by the Immunosuppressant Drug Cyclosporin A

The effect of cyclosporin A (CsA), an immunosuppressant, on human ether-a-go-go-related gene (HERG) channel as it is expressed in human embryonic kidney cells was studied using a whole-cell, patch-clamp technique. CsA inhibited the HERG channel in a concentration-dependent manner, with an IC(50) value and a Hill coefficient of 3.17 µM and 0.89, respectively. Pretreatment with cypermethrine, a calcineurin inhibitor, had no effect on the CsA-induced inhibition of the HERG channel. The CsA-induced inhibition of HERG channels was voltage-dependent, with a steep increase over the voltage range of the channel opening. However, the inhibition exhibited voltage independence over the voltage range of fully activated channels. CsA blocked the HERG channels predominantly in the open and inactivated states rather than in the closed state. Results of the present study suggest that CsA acts directly on the HERG channel as an open-channel blocker, and it acts independently of its effect on calcineurin activity.

Cholesterol modulates ion channels via down‐regulation of phosphatidylinositol 4,5‐bisphosphate

Ubiquitously expressed Mg(2+)-inhibitory cation (MIC) channels are permeable to Ca2+ and Mg2+ and are essential for cell viability. When membrane cholesterol level was increased by pre-incubating cells with a water-soluble form of cholesterol, the endogenous MIC current in HEK293 cells was negatively regulated. The application of phosphatidylinositol 4,5-bisphosphate (PIP2) recovered MIC current from cholesterol effect. As PIP2 is the direct modulator for MIC channels, high cholesterol content may cause down-regulation of PIP2. To test this possibility, we examined the effect of cholesterol on two exogenously expressed PIP2-sensitive K+ channels: human Ether-a-go-go related gene (HERG) and KCNQ. Enrichment with cholesterol inhibited HERG currents, while inclusion of PIP2 in the pipette solution blocked the cholesterol effect. KCNQ channel was also inhibited by cholesterol. The effects of cholesterol on these channels were blocked by pre-incubating cells with inhibitors for phospholipase C, which may indicate that cholesterol enrichment induces the depletion of PIP2 via phospholipase C activation. Lipid analysis showed that cholesterol enrichment reduced gamma-(32)P incorporation into PIP2 by approximately 35%. Our results suggest that cholesterol may modulate ion channels by changing the levels of PIP2. Thus, an important cross-talk exists among two plasma membrane-enriched lipids, cholesterol and PIP2.

HERG biosynthesis: the positive influence of negative charge

HERG biosynthesis: the positive influence of negative charge

Blockade of HERG K+ channel by an antihistamine drug brompheniramine requires the channel binding within the S6 residue Y652 and F656

A number of clinically used drugs block delayed rectifier K+ channels and prolong the duration of cardiac action potentials associated with long QT syndrome. This study investigated the molecular mechanisms of voltage-dependent inhibition of human ether-a-go-go-related gene (HERG) delayed rectifier K+ channels expressed in HEK-293 cells by brompheniramine, an antihistamine. Brompheniramine inhibited HERG current in a concentration-dependent manner with the half-maximal inhibitory concentration (IC50) value of 1.7 microm at 0 mV. A block of HERG current by brompheniramine was enhanced by progressive membrane depolarization and showed significantly negative shift in voltage-dependence of channel activation. Inhibition of HERG current by brompheniramine showed time-dependence. The S6 residue HERG mutant Y652A and F656C largely reduced the blocking potency of HERG current. These results indicate that brompheniramine mainly inhibited the HERG potassium channel through the residue Y652 and F656 and these residues may be an obligatory determinant in inhibition of HERG current for brompheniramine.

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.

Maprotiline block of the human ether-a-go-go-related gene (HERG) K+ channel

Maprotiline, an atypical antidepressant, can induce prolonged QT and torsades de pointes. We studied the effects of maprotiline on human ether-a-go-go-related gene (HERG) channels expressed in Xenopus oocytes and HEK293 cells. Maprotiline induced a concentration-dependent decrease in current amplitudes at the end of the voltage steps and tail currents of HERG. The V1/2 values in the absence and presence of 1-20 microM maprotiline were not significantly different, while the values decreased according to the concentrations of the drug at 50-300 microM. The IC50 for a maprotiline block of HERG current in Xenopus oocytes did not change according to depolarization; 39.5 +/- 3.2 microM at -40 mV and 43.6 +/- 2.8 microM at +40 mV. The block of HERG by maprotiline was examined after treatment of trinitrobenzene sulfonic acid (TNBS), an amino-group reagent that neutralizes the positively charged amino-groups of peptide N-terminal and lysine residues. TNBS inhibited the change of V1/2 values induced by 50-300 mM maprotiline, and aggravated the drug-induced gmax decrease. The IC50 for the maprotiline-induced blockade of HERG currents in HEK293 cells at 36 degrees C was 0.13 microM at +20 mV. Our findings suggest that the arrhythmogenic side effects of maprotiline are caused by a blockade of HERG and possibly by a blockade of delayed rectifier K+ channel.

Effects of fluoroquinolones on HERG channels and on pancreatic β-cell ATP-sensitive K + channels

An inhibition of the cardiac rapid delayed rectifier K + current ( I Kr) and of the ATP-sensitive K + (K ATP) current seems to be involved in the mechanisms of the cardiotoxic effects and the alterations in glucose homeostasis, respectively, induced by some fluoroquinolones. The aim of the present study was to compare the effects of fluoroquinolone derivatives on the pore-forming subunit of the cardiac I Kr, which is encoded by human ether-a-go-go-related gene (HERG), and on the ATP-sensitive K + (K ATP) channel from the clonal insulinoma cell line RINm5F. Sparfloxacin blocked HERG currents half-maximally (IC 50 value) at a concentration of 33.2 μM, whereas norfloxacin and lomefloxacin each tested at a concentration of 300 μM inhibited HERG currents only by 2.8 ± 3.6% and 12.3 ± 4.7%, respectively. Four newly synthesized fluoroquinolone derivatives with either a p-fluoro-phenyl (compound C3) or an o-fluoro-phenyl (compound C4) substituent at position N 1 and an additional dimethylated piperazine ring (compounds C1 and C2) inhibited HERG currents by 7.3–14.7% at test concentrations of 100 μM. The rank order of potency for the inhibition of K ATP currents was C2 > C1, C4, sparfloxacin > C3. In conclusion, the structural requirements for fluoroquinolones to inhibit I Kr currents and K ATP currents appear to differ. The amino group at position C 5 seems to be primarily responsible for the strong HERG current blocking property of sparfloxacin. In contrast, for the block of pancreatic β-cell K ATP currents by fluoroquinolones the substituents at positions N 1, C 7 and C 8 all might play a role.

Blockade of HERG human K+ channels and IKr of guinea-pig cardiomyocytes by the antipsychotic drug clozapine.

Clozapine, a commonly used antipsychotic drug, can induce QT prolongation, which may lead to torsades de pointes and sudden death. To investigate the arrhythmogenic side effects of clozapine, we studied the impact of clozapine on human ether-a-go-go-related gene (HERG) channels expressed in Xenopus oocytes and HEK293 cells, and on the delayed rectifier K(+) currents of guinea-pig cardiomyocytes. Clozapine dose-dependently decreased the amplitudes of the currents at the end of voltage steps, and the tail currents of HERG. The IC(50) for the clozapine blockade of HERG currents in Xenopus oocytes progressively decreased relative to depolarization (39.9 microM at -40 mV, 28.3 microM at 0 mV and 22.9 microM at +40 mV), whereas the IC(50) for the clozapine-induced blockade of HERG currents in HEK293 cells at 36 degrees C was 2.5 microM at +20 mV. The clozapine-induced blockade of HERG currents was time dependent: the fractional current was 0.903 of the control at the beginning of the pulse, but declined to 0.412 after 4 s at a test potential of 0 mV. The clozapine-induced blockade of HERG currents was use-dependent, exhibiting more rapid onset and greater steady state blockade at higher frequencies of activation, with a partial relief of blockade observed when the frequency of activation was decreased. In guinea-pig ventricular myocytes held at 36 degrees C, treatment with 1 and 5 microM clozapine blocked the rapidly activating delayed rectifier K(+) current (I(Kr)) by 24.7 and 79.6%, respectively, but did not significantly block the slowly activating delayed rectifier K(+) current (I(Ks)). Our findings collectively suggest that blockade of HERG currents and I(Kr), but not I(Ks), may contribute to the arrhythmogenic side effects of clozapine.

Chemical modification of the human ether-a-go-go-related gene (HERG) K+ current by the amino-group reagent trinitrobenzene sulfonic acid

We investigated the effects of trinitrobenzene sulfonic acid (TNBS), an amino-group reagent, on the human ether-a-go-go-related gene (HERG) K+ channels expressed in Xenopus oocytes. TNBS neutralizes the positively charged amino-groups of peptide N-terminal and lysine residues. External application of TNBS at 10 mM for 5 min irreversibly shifted the curves for currents at the end of the pulse and tail currents of HERG to a more negative potential and decreased the maximal amplitude of the I(tail) curve (I(tail,max)). TNBS had little effect on either the activated current-voltage relationship or the reversal potential of HERG current, indicating that TNBS did not change ion selectivity properties. TNBS shifted the time constant curves of both activation and deactivation of the HERG current to a more hyperpolarized potential; TNBS's effect was greater on channel opening than channel closing. External H+ is known to inhibit HERG current by shifting V(1/2) to the right and decreasing I(tail,max). TNBS enhanced the blockade of external H+ by exaggerating the effect of H+ on I(tail,max), not on V(1/2). Our data provide evidence for the presence of essential amino-groups that are associated with the normal functioning of the HERG channel and evidence that these groups modify the blocking effect of external H+ on the current.

The Effects of Sevoflurane and Propofol on QT Interval and Heterologously Expressed Human Ether-A-Go-Go Related Gene Currents in Xenopus Oocytes

Sevoflurane can induce prolongation of the cardiac QT interval by inhibiting the repolarization phase of the action potential. This may occur as a result of inhibition of the human ether-a-go-go related gene (HERG) channel. To clarify the mechanisms of anesthetics on HERG channels, we monitored the electrocardiogram and measured QT intervals in the guinea pig in the presence of sevoflurane and propofol. Sevoflurane (1%-4%) prolonged QTc dose-dependently (7.5%-21.2%), but propofol did not affect it. Furthermore, HERG channels were expressed in Xenopus oocytes and outward HERG currents were obtained on step depolarization from a holding potential of -70 mV. Repolarization to -70 mV from positive test potentials resulted in large outward tail currents. Sevoflurane (1%-4%), in a dose-dependent manner, inhibited the HERG outward tail currents (9.7%-26.6%), whereas steady-state currents were inhibited only at large concentrations. The time constant of the converging current was decreased in the presence of sevoflurane, but the inactivation and activation curves were not shifted. Propofol did not affect these currents within the clinically relevant concentration. In conclusion, compared with steady-state currents, sevoflurane was more potent in inhibiting the outward tail currents, suggesting that sevoflurane may modulate the HERG channel kinetics in its inactivated state.

ω-3 and ω-6 polyunsaturated fatty acids block HERG channels

Dietary polyunsaturated fatty acids (PUFAs) have been reported to exhibit antiarrhythmic properties, which have been attributed to their availability to modulate Na(+), Ca(2+), and several K(+) channels. However, their effects on human ether-a-go-go-related gene (HERG) channels are unknown. In this study we have analyzed the effects of arachidonic acid (AA, omega-6) and docosahexaenoic acid (DHA, omega-3) on HERG channels stably expressed in Chinese hamster ovary cells by using the whole cell patch-clamp technique. At 10 microM, AA and DHA blocked HERG channels, at the end of 5-s pulses to -10 mV, to a similar extent (37.7 +/- 2.4% vs. 50.2 +/- 8.1%, n = 7-10, P > 0.05). 5,6,11,14-Eicosatetrayenoic acid, a nonmetabolizable AA analog, induced effects similar to those of AA on HERG current. Both PUFAs shifted the midpoint of activation curves of HERG channels by -5.1 +/- 1.8 mV (n = 10, P < 0.05) and -11.2 +/- 1.1 mV (n = 7, P < 0.01). Also, AA and DHA shifted the midpoint of inactivation curves by +12.0 +/- 3.9 mV (n = 4; P < 0.05) and +15.8 +/- 4.3 mV (n = 4; P < 0.05), respectively. DHA and AA accelerated the deactivation kinetics and slowed the inactivation kinetics at potentials positive to +40 mV. Block induced by DHA, but not that produced by AA, was higher when measured after applying a pulse to -120 mV (I-->O). Finally, both AA and DHA induced a use-dependent inhibition of HERG channels. In summary, block induced by AA and DHA was time, voltage, and use dependent. The results obtained suggest that both PUFAs bind preferentially to the open state of the channel, although an interaction with inactivated HERG channels cannot be ruled out for AA.

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

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. A two-electrode voltage clamp technique was used. HERG currents were recorded in Xenopus oocytes injected with HERG cRNA. 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. Ginseng saponins enhance HERG currents, which could be in part a possible mechanism of the shortening cardiac action potential of ginseng saponins.

Comparison of the Effects of Metoclopramide and Domperidone on HERG Channels

Torsades de pointes (TdP) is a potentially fatal form of ventricular arrhythmia that occurs under conditions where cardiac repolarization is delayed (as indicated by prolonged QT intervals from electrocardiographic recordings). A likely mechanism for QT prolongation and TdP is blockade of the rapid component of the cardiac delayed rectifier K⁺ current (I_Kr), which is encoded by HERG (human ether-a-go-go-related gene). The gastroprokinetic agent cisapride is a potent blocker of HERG currents and serious cardiac arrhythmias and deaths from TdP and ventricular fibrillation have been reported in patients taking cisapride. The aim of the present study was to compare the effects of the gastroprokinetic agents domperidone and metoclopramide on HERG channels transiently expressed in human embryonic kidney (HEK 293) cells using the whole-cell configuration of the patch-clamp technique. Both domperidone and metoclopramide concentration-dependently blocked HERG currents, and the following values were calculated for IC₅₀ (the concentrations causing half-maximal inhibition) and n (the Hill coefficient): 57.0 nmol/l and 0.99 for domperidone, 5.4 µmol/l and 0.95 for metoclopramide. The observation that the extent of block of HERG currents by domperidone increased at more positive membrane potentials whereas block of HERG currents by metoclopramide displayed a smaller degree of voltage dependency seems to indicate that domperidone and metoclopramide have distinct binding sites on HERG channels. In conclusion, the potency for block of HERG currents is about 100-fold lower for metoclopramide when compared to domperidone.

Modulation of inactivation by mutation N588K in KCNH2: A link to arrhythmogenesis in short QT syndrome

Short QT syndrome (SQTS) is characterized by ventricular arrhythmias and sudden death. One form of SQTS is caused by mutation N588K in human ether-a-go-go-related gene (HERG). In this study we sought to determine the potential role of N588K in arrhythmias. We measured the characteristics of HERG current generated by wild-type (WT) KCNH2 and the N588K mutant channel expressed in mammalian TSA201 cells. Whole-cell patch-clamp recordings of WT HERG currents showed the usual rapid onset of inactivation (rectification) at potentials more positive than +10 mV. In contrast, N588K currents rectified at potentials over +80 mV. Over the physiological range of potentials, N588K currents do not inactivate. During an action potential clamp, WT currents displayed a "hump" like waveform with slow activation kinetics and a rapid increase during phase 3 repolarization. In contrast, N588K currents were proportional to the amplitude of the action potential and displayed a dome-like configuration and a much larger current during the initial phases in the ventricle. Purkinje cell action potentials display a more negative phase 2 repolarization than the ventricle and elicited much smaller WT and N588K currents of similar amplitudes. Physiologically the N588K mutation abolishes rectification of HERG currents and specifically increases I(Kr) in the ventricle with minimal effects on the Purkinje fiber action potential duration. Such preferential prolongation may explain the separation of the T and U waves observed in the ECG of SQTS patients and lead to re-excitation of the ventricle endocardium.