Biophysical properties of zebrafish ether-à-go-go related gene potassium channels.

The zebrafish is increasingly recognized as an animal model for the analysis of hERG-related diseases. However, functional properties of the zebrafish orthologue of hERG have not been analyzed yet. We heterologously expressed cloned ERG channels in Xenopus oocytes and analyzed biophysical properties using the voltage clamp technique. zERG channels conduct rapidly activating and inactivating potassium currents. However, compared to hERG, the half-maximal activation voltage of zERG current is shifted towards more positive potentials and the half maximal steady-state inactivation voltage is shifted towards more negative potentials. zERG channel activation is delayed and channel deactivation is accelerated significantly. However, time course of zERG conducted current under action potential clamp is highly similar to the human orthologue. In summary, we show that ERG channels in zebrafish exhibit biophysical properties similar to the human orthologue. Considering the conserved channel function, the zebrafish represents a valuable model to investigate human ERG channel related diseases.

Inhibition of cardiac hERG potassium channels by tetracyclic antidepressant mianserin.

The antidepressant mianserin exhibits a tetracyclic structure that is different from typical tricyclic antidepressants (TCA) and that of selective serotonin reuptake inhibitors. In comparison to the older TCA, mianserin has been shown to have a superior risk profile regarding proarrhythmic effects, both in vitro and in vivo. However, the underlying molecular electrophysiological basis has not been elucidated to date. Therefore, we studied the effects of mianserin on cardiac hERG potassium channels, the predominant target of drug-induced proarrhythmia. HERG channels were expressed in the Xenopus oocyte expression system and in human embryonic kidney (HEK) cells and currents were measured with two-microelectrode voltage-clamp and whole-cell patch-clamp, respectively. Mianserin inhibited hERG currents in a dose-dependent manner with an IC(50) of 3.2 micromol/l in HEK cells. Onset of blockade was slow and the inhibitory effect was not reversible upon wash-out of the drug. In hERG channel mutants, Y652A and F656A, lacking aromatic residues in the S6 domain, the effect of mianserin was significantly reduced in comparison to the wild type. Mianserin inhibited hERG currents in the open and inactivated state, but not in the closed states. HERG inactivation kinetics were significantly altered by mianserin without marked effects on channel activation kinetics. The inhibitory effect was not frequency dependent. In conclusion, mianserin is a low-affinity hERG-blocking agent. However, taken together with the lack of APD-prolongation shown in other studies, mianserin seems to have a good safety profile. Lack of consistent QT prolonging effects of mianserin in previous studies may therefore be linked to additional effects such as inhibition of other cardiac ion channels. However, as demonstrated by clinical case reports, mianserin can induce proarrhythmic effects in susceptible patients. Therefore, in patients with complex co-medication (i.e., additional hERG-blocking agents) and in patients with risk factors for acquired long QT syndrome as well as in cases of overdose, adequate monitoring should be recommended.

Doxazosin induces apoptosis of cells expressing hERG K+ channels.

The antihypertensive drug doxazosin has been associated with an increased risk for congestive heart failure and cardiomyocyte apoptosis. Human ether-a-go-go-related gene (hERG) K(+) channels, previously shown to be blocked by doxazosin at therapeutically relevant concentrations, represent plasma membrane receptors for the antihypertensive drug. To elucidate the molecular basis for doxazosin-associated pro-apoptotic effects, cell death was studied in human embryonic kidney cells using three independent apoptosis assays. Doxazosin specifically induced apoptosis in hERG-expressing HEK cells, while untransfected control groups were insensitive to treatment with the antihypertensive agent. An unexpected biological mechanism has emerged: binding of doxazosin to its novel membrane receptor, hERG, triggers apoptosis, possibly representing a broader pathophysiological mechanism in drug-induced heart failure.

Kir2.x inward rectifier potassium channels are differentially regulated by adrenergic alpha1A receptors.

Inhibition of I(K1) currents by adrenergic alpha(1) receptors has been observed in cardiomyocytes and has been linked to arrhythmogenesis in an animal model. Both PKC-dependent and PKC-independent pathways have been implied in this regulation. The underlying molecular mechanisms, however, have not been elucidated to date. The molecular basis of native I(K1) current is mainly formed by Kir2.1 (KCNJ2), Kir2.2 (KCNJ12) and Kir2.3 (KCNJ4) channels that are differentially regulated by protein kinases. We therefore sought to investigate the role of those different Kir2.x channel subunits in this regulation and to identify the major signalling pathways involved. Adrenergic alpha(1A) receptors (the predominant cardiac isoform) were co-expressed with cloned Kir2.1, Kir2.2 and Kir2.3 channels in Xenopus oocytes and electrophysiological experiments were performed using two-microelectrode voltage clamp. Native I(K1) currents were measured with the whole-cell patch clamp technique in isolated rat ventricular cardiomyocytes. Activation of co-expressed adrenergic alpha(1A) receptors by phenylephrine induced differential effects in Kir2.x channels. No effect was noticed in Kir2.1 channels. However, a marked inhibitory effect was observed in Kir2.2 channels. This regulation was not attenuated by inhibitors of PKC, CamKII and PKA (chelerythrine, KN-93, KT-5720), and mutated Kir2.2 channels lacking functional phosphorylation sites for PKC and PKA exhibited the same effect as Kir2.2 wild-type channels. By contrast, the regulation could be suppressed by the general tyrosine kinase inhibitor genistein and by the src tyrosine kinase inhibitor PP2 indicating an essential role of src kinases. This finding was validated in rat ventricular cardiomyocytes where co-application of PP2 strongly attenuated the inhibitory regulation of I(K1) current by adrenergic alpha(1) receptors. The inactive analogue PP3 was tested as negative control for PP2 and did not reproduce the effects of PP2. In Kir2.3 channels, a marked inhibitory effect of alpha(1A) receptor activation was observed. This regulation could be attenuated by inhibition of PKC with chelerythrine or with Ro-32-0432, but not by tyrosine kinase inhibition with genistein. In summary, on the molecular level the inhibitory regulation of I(K1) currents by adrenergic alpha(1A) receptors is probably based on effects on Kir2.2 and Kir2.3 channels. Kir2.2 is regulated via src tyrosine kinase pathways independent of protein kinase C, whereas Kir2.3 is inhibited by protein kinase C-dependent pathways. Src tyrosine kinase pathways are essential for the inhibition of native I(K1) current by adrenergic alpha(1) receptors. This regulation may contribute to arrhythmogenesis under adrenergic stimulation.

Anticholinergic antiparkinson drug orphenadrine inhibits HERG channels: block attenuation by mutations of the pore residues Y652 or F656.

The anticholinergic antiparkinson drug orphenadrine is an antagonist at central and peripheral muscarinic receptors. Orphenadrine intake has recently been linked to QT prolongation and Torsade-de-Pointes tachycardia. So far, inhibitory effects on I (Kr) or cloned HERG channels have not been examined. HERG channels were heterologously expressed in a HEK 293 cell line and in Xenopus oocytes and HERG current was measured using the whole cell patch clamp and the double electrode voltage clamp technique. Orphenadrine inhibits cloned HERG channels in a concentration dependent manner, yielding an IC(50) of 0.85 microM in HEK cells. Onset of block is fast and reversible upon washout. Orphenadrine does not alter the half-maximal activation voltage of HERG channels. There is no shift of the half-maximal steady-state-inactivation voltage. Time constants of direct channel inactivation are not altered significantly and there is no use-dependence of block. HERG blockade is attenuated significantly in mutant channels lacking either of the aromatic pore residues Y652 and F656. In conclusion, we show that the anticholinergic agent orphenadrine is an antagonist at HERG channels. These results provide a novel molecular basis for the reported proarrhythmic side effects of orphenadrine.

Activation of inwardly rectifying Kir2.x potassium channels by beta 3-adrenoceptors is mediated via different signaling pathways with a predominant role of PKC for Kir2.1 and of PKA for Kir2.2.

beta(3)-adrenoceptors have recently been shown to induce a complex modulation of intracellular signaling pathways including cyclic guanine monophosphate, cyclic adenosine monophosphate, nitric oxide, and protein kinases A and C. They are expressed in a broad variety of tissues including the myocardium, vascular smooth muscle, and endothelium. In those tissues, resting membrane potential is controlled mainly by inwardly rectifying potassium channels of the Kir2 family namely, Kir2.1 in the vascular smooth muscle, Kir2.1-2.3 in the myocardium, and Kir2.1-2.2 in the endothelium. In the present study, we investigated the possible modulation of Kir2 channel function by beta(3)-adrenoceptors in an expression system. Human-cloned beta(3)-adrenoceptors and Kir2.1 (KCNJ2), Kir2.2 (KCNJ12), and Kir2.3 (KCNJ4) channels were coexpressed in Xenopus oocytes, and currents were measured with double-microelectrode voltage clamp. Activation of beta(3)-adrenoceptors with isoproterenol resulted in markedly increased currents in Kir2.1 and in Kir2.2 potassium channels with EC50 values of 27 and 18 nM, respectively. In contrast, Kir2.3 currents were not modulated. Coapplication of specific inhibitors of protein kinase A (KT-5720) and calmodulin kinase II (KN-93) had no effects on the observed regulation in Kir2.1. However, coapplication of protein kinase C (PKC) inhibitors staurosporine and chelerythrine suppressed the observed effect. In Kir2.2, coapplication of KT-5720 reduced the effect of beta(3)-adrenoceptor activation. No differences in current increase after application of isoproterenol were observed between mutant Kir2.2 potassium channels lacking all functional PKC phosphorylation sites and Kir2.2 wild-type channels. In heteromeric Kir2.x channels, all types of heteromers were activated. The effect was most pronounced in Kir2.1/Kir2.2 and in Kir2.2/Kir2.3 channels. In summary, homomeric and heteromeric Kir2.x channels are activated by beta(3)-adrenoceptors via different protein kinase-dependent pathways: Kir2.1 subunits are modulated by PKC, whereas Kir2.2 is modulated by protein kinase A. In heteromeric composition, a marked activation of currents can be observed particularly with involvement of Kir2.2 subunits. This regulation may contribute to the hyperpolarizing effects of beta(3)-adrenoceptors in tissues that exhibit modulation by Kir2 channel function.

Orange flavonoid hesperetin modulates cardiac hERG potassium channel via binding to amino acid F656.

BACKGROUND AND AIMS: Hesperetin belongs to the flavonoid subgroup classified as citrus flavonoids and is the main flavonoid in oranges. A high dietary intake of flavonoids has been associated with a significant reduction in cardiovascular mortality. HERG potassium channels play a major role in cardiac repolarisation and represent the most important pharmacologic target of both antiarrhythmic and proarrhythmic drugs. METHODS AND RESULTS: We used the two-microelectrode voltage-clamp technique to analyse inhibitory effects of hesperetin on hERG potassium channels heterologously expressed in Xenopus oocytes. Hesperetin blocked hERG potassium channels in a concentration dependent manner. Onset of block was fast and completely reversible upon wash-out. There was no significant effect of hesperetin on channel kinetics. Affinity of hesperetin to mutant F656A hERG channel was significantly decreased compared to WT hERG, indicating a binding site in the channel pore cavity. In contrast, affinity of hesperetin to Y652A hERG was not different from the affinity to WT hERG. CONCLUSION: We found an antagonist of cardiac hERG channels that modulates hERG currents by accessing the aromatic pore binding site, particularly amino acid phe-656. Regarding high hesperetin concentrations found in oranges and the increasing consumption of oranges and orange juice in Europe, potential effects of hesperetin on cardiac electrophysiology in vivo deserve further investigation.

Atypical tetracyclic antidepressant maprotiline is an antagonist at cardiac hERG potassium channels.

Maprotiline is an antidepressant compound with an atypical tetracyclic structure that is widely used in elderly patients due to its favourable side-effect profile. However, there have been reports of proarrhythmia associated with maprotiline and in vitro studies of its electrophysiological properties have been lacking. Therefore, we characterised the effects of maprotiline on cardiac hERG channels. hERG channels were expressed in HEK cells and in the Xenopus oocyte expression system. Currents were measured using a whole-cell patch clamp and a two-microelectrode voltage-clamp. Maprotiline inhibited hERG currents with an IC(50) of 8.2 micromol/l in HEK cells and 29.2 micromol/l in Xenopus oocytes. Onset of the effect was rather slow and took several minutes. No wash-out of effect was observed. Maprotiline blocked hERG channels in the open and inactivated states, but not in the closed states. In mutant hERG channels Y652A and F656A, the effect was markedly attenuated (hERG-F656A) or completely abolished (hERG-Y652A). Voltage dependence of hERG current activation and inactivation was not affected by maprotiline. hERG inactivation was accelerated at positive potentials. The effect of maprotiline on hERG currents was voltage-dependent with a marked reduction at a more positive potential. hERG blockade by maprotiline was not frequency-dependent. Maprotiline is an antagonist of cardiac hERG potassium channels that preferably accesses the putative pore binding site Y652/F656. Although the affinity of maprotiline to hERG channels is low, its use in patients with risk factors for acquired long QT syndrome should be monitored appropriately.

In vitro modulation of HERG channels by organochlorine solvent trichlormethane as potential explanation for proarrhythmic effects of chloroform.

Acute chloroform intoxication can cause depression of the central nervous system and may lead to death from lethal arrhythmias or respiratory arrest. Thus, the organic solvent is no longer in clinical use as an anaesthetic, but still plays a role in cases of suicide, homicide or inhalation for psychotropic effects. Several cases of lethal arrhythmia after intoxication with chloroform have been described. Pharmacological inhibition of cardiac "human ether-à-go-go-related gene" (HERG) potassium channels is linked to proarrhythmic effects of cardiac and noncardiac drugs. To further investigate the electrophysiological basis of the arrhythmogenic potential of chloroform, we analysed inhibitory effects of chloroform on cloned HERG potassium channels, heterologously expressed in Xenopus oocytes and in Human Embryonic Kidney (HEK 293) cells using the double-electrode voltage-clamp technique and the whole-cell patch-clamp technique, respectively. In HEK cells, chloroform blocked HERG tail currents with an IC(50) of 4.97mM. Biophysical properties were further investigated in the Xenopus oocyte expression system. Onset and wash-out of block was fast and inhibition was completely reversible. Chloroform did not alter channel activation, however, direct channel inactivation was accelerated significantly. Steady-state-inactivation of HERG was not affected. Chloroform dependent block of HERG channels was voltage dependent with a decrease of inhibition at more positive membrane potentials. No frequency-dependence of block could be observed. In summary, chloroform blocked HERG potassium channels probably in a toxicologically relevant concentration. These findings contribute to the pathophysiology of proarrhythmic effects in acute chloroform intoxication.

Regulation of cardiac inwardly rectifying potassium current IK1 and Kir2.x channels by endothelin-1.

To elucidate the ionic mechanism of endothelin-1 (ET-1)-induced focal ventricular tachyarrhythmias, the regulation of I(K1) and its main molecular correlates, Kir2.1, Kir2.2 and Kir2.3 channels, by ET-1 was investigated. Native I(K1) in human atrial cardiomyocytes was studied with whole-cell patch clamp. Human endothelin receptors were coexpressed with human Kir2.1, Kir2.2 and Kir2.3 channels in Xenopus oocytes. Currents were measured with a two-microelectrode voltage clamp. In human cardiomyocytes, ET-1 induced a marked inhibition of I(K1) that could be suppressed by the protein kinase C (PKC) inhibitor staurosporine. To investigate the molecular mechanisms underlying this regulation, we studied the coupling of ET(A) receptors to homomeric and heteromeric Kir2.1, Kir2.2 and Kir2.3 channels in the Xenopus oocyte expression system. ET(A) receptors coupled functionally to Kir2.2 and Kir2.3 channels but not to Kir2.1 channels. In Kir2.2 channels lacking functional PKC phosphorylation sites, the inhibitory effect was abolished. The inhibition of Kir2.3 currents could be suppressed by the PKC inhibitors staurosporine and chelerythrine. The coupling of ET(A) receptors to heteromeric Kir2.1/Kir2.2 and Kir2.2/Kir2.3 channels resulted in a strong inhibition of currents comparable with the effect observed in Kir2.2 homomers. Surprisingly, in heteromeric Kir2.1/Kir2.3 channels, no effect was observed. ET-1 inhibits human cardiac I(K1) current via a PKC-mediated phosphorylation of Kir2.2 channel subunits and additional regulatory effects on Kir2.3 channels. This mechanism may contribute to the intrinsic arrhythmogenic potential of ET-1.

The novel antiarrhythmic drug dronedarone: comparison with amiodarone.

Dronedarone is a noniodinated benzofuran derivative that has been developed to overcome the limiting iodine-associated adverse effects of the commonly used antiarrhythmic drug, amiodarone. It displays a wide cellular electrophysiological spectrum largely similar to amiodarone, inhibiting the potassium currents I(Kr), I(Ks), I(KI), I(KACh), and I(sus), as well as sodium currents and L-type calcium currents in isolated cardiomyocytes. In addition, dronedarone exhibits antiadrenergic properties. In vivo, dronedarone has been shown to be more effective than amiodarone in several arrhythmia models, particularly in preventing ischemia- and reperfusion-induced ventricular fibrillation and in reducing mortality. However, an increased incidence of torsades de pointes with dronedarone in dogs shows that possible proarrhythmic effects of dronedarone require further evaluation. The clinical trails DAFNE, EURIDIS, and ADONIS indicated safety, antiarrhythmic efficacy and low proarrhythmic potential of the drug in low-risk patients. In contrast, the increased incidence of death in the dronedarone group of the discontinued ANDROMEDA trial raises safety concerns for patients with congestive heart failure and moderate to severe left ventricular dysfunction. Dronedarone appears to be effective in preventing relapses of atrial fibrillation and atrial flutter. Torsades de pointes, the most severe adverse effect associated with amiodarone, has not yet been reported in humans with dronedarone. Unlike amiodarone, dronedarone had little effect on thyroid function and hormone levels in animal models and had no significant effects on human thyroid function in clinical trials. In conclusion, dronedarone could be a useful drug for prevention of atrial fibrillation and atrial flutter relapses in low-risk patients. However, further experimental studies and long-term clinical trials are required to provide additional evidence of efficacy and safety of dronedarone.

Inhibition of cardiac HERG channels by grapefruit flavonoid naringenin: implications for the influence of dietary compounds on cardiac repolarisation.

Flavonoids are naturally occurring food ingredients that have been associated with reduced cardiovascular mortality in epidemiological studies. In a previous study, we demonstrated for the first time that flavonoids are inhibitors of cardiac human ether-à-go-go-related gene (HERG) channels. Furthermore, we observed that grapefruit juice induced mild QTc prolongation in healthy subjects. HERG blockade by grapefruit flavonoid naringenin is most likely to be the mechanism underlying this effect. Therefore, the electrophysiological properties of HERG blockade by naringenin were analysed in detail. HERG potassium currents expressed in Xenopus oocytes were measured with a two-microelectrode voltage clamp. Naringenin blocked HERG potassium channels with an IC50 value of 102.6 microM in Xenopus oocytes. The onset of blockade was fast. The effect was completely reversible upon wash-out. Naringenin binding to HERG required aromatic residue F656 in the putative pore binding site. Channels were blocked in the open and inactivated states but not in the closed states. Naringenin did not affect HERG current activation. However, the half maximal inactivation voltage was shifted by 14.9 mV towards more negative potentials and current inactivation at negative potentials was accelerated. No frequency dependence of blockade was observed. Naringenin inhibits HERG channels with pharmacological characteristics similar to those of well-known HERG antagonists. From a clinical point of view, this effect could have both proarrhythmic and antiarrhythmic consequences. This may have important implications for phytotherapy and for dietary recommendations for cardiologic patients. Therefore, electrophysiological effects of flavonoids deserve further investigation.

Dominant-negative I(Ks) suppression by KCNQ1-deltaF339 potassium channels linked to Romano-Ward syndrome.

OBJECTIVE: Hereditary long QT syndrome (LQTS) is a genetically heterogeneous disease characterized by prolonged QT intervals and an increased risk for ventricular arrhythmias and sudden cardiac death. Mutations in the voltage-gated potassium channel subunit KCNQ1 induce the most common form of LQTS. KCNQ1 is associated with two different entities of LQTS, the autosomal-dominant Romano-Ward syndrome (RWS), and the autosomal-recessive Jervell and Lange-Nielsen syndrome (JLNS) characterized by bilateral deafness in addition to cardiac arrhythmias. In this study, we investigate and discuss dominant-negative I(Ks) current reduction by a KCNQ1 deletion mutation identified in a RWS family. METHODS: Single-strand conformation polymorphism analysis and direct sequencing were used to screen LQTS genes for mutations. Mutant KCNQ1 channels were heterologously expressed in Xenopus oocytes, and potassium currents were recorded using the two-microelectrode voltage clamp technique. RESULTS: A heterozygous deletion of three nucleotides (CTT) identified in the KCNQ1 gene caused the loss of a single phenylalanine residue at position 339 (KCNQ1-deltaF339). Electrophysiological measurements in the presence and absence of the regulatory beta-subunit KCNE1 revealed that mutant and wild type forms of an N-terminal truncated KCNQ1 subunit (isoform 2) caused much stronger dominant-negative current reduction than the mutant form of the full-length KCNQ1 subunit (isoform 1). CONCLUSION: This study highlights the functional relevance of the truncated KCNQ1 splice variant (isoform 2) in establishment and mode of inheritance in long QT syndrome. In the RWS family presented here, the autosomal-dominant trait is caused by multiple dominant-negative effects provoked by heteromultimeric channels formed by wild type and mutant KCNQ1-isoforms in combination with KCNE1.

QTc prolongation by grapefruit juice and its potential pharmacological basis: HERG channel blockade by flavonoids.

BACKGROUND: A high intake of dietary flavonoids, which are abundant in fruits, vegetables, tea, and wine, is known to reduce cardiovascular mortality. The effects of flavonoids on cardiac electrophysiology, which theoretically may have both antiarrhythmic and proarrhythmic consequences, have not been studied systematically to date. METHODS AND RESULTS: We screened a broad spectrum of flavonoids for their inhibitory activity on HERG channels by using heterologous expression in Xenopus oocytes. At a concentration of 1 mmol/L, 10 compounds caused a significant inhibition of HERG currents, whereas 11 other flavonoids had no effect. The IC50 value for HERG block by naringenin, the most potent inhibitor, was 102.3 micromol/L in Xenopus oocytes and 36.5 micromol/L in HEK cells. To demonstrate the physiological relevance of these findings, we studied the effects of pink grapefruit juice, which contains large amounts of naringenin glycosides (>1000 micromol/L), in human volunteers. In 10 persons, we observed a peak QTc prolongation of 12.5+/-4.2 ms 5 hours after oral ingestion of 1 L of grapefruit juice. This effect was significant (P=0.02). CONCLUSIONS: We found a significant QTc prolongation by grapefruit juice in healthy volunteers, probably caused by block of HERG channels by flavonoids. These findings reveal new perspectives on the potential for dietary modification of cardiac electrophysiology.

Class Ia anti-arrhythmic drug ajmaline blocks HERG potassium channels: mode of action.

Ajmaline is a class Ia anti-arrhythmic drug used in several European countries and Japan as first-line treatment for ventricular tachyarrhythmia. Ajmaline has been reported to induce cardiac output (QT) prolongation and to inhibit cardiac potassium currents in guinea pig cardiomyocytes. In order to elucidate the molecular basis of these effects, we examined effects of ajmaline on human ether a-go-go related gene HERG potassium channels. Electrophysiological experiments were performed with human embryonic kidney (HEK) cells (whole-cell patch clamp) and Xenopus oocytes (double-electrode voltage clamp) expressing wild-type and mutant HERG channels. Ajmaline blocked HERG currents with an IC(50) of 1.0 micromol/l in HEK cells and 42.3 micromol/l in Xenopus oocytes. The onset of block was fast and reached steady-state conditions after 180 s. The inhibitory effect was completely reversible upon wash-out. In HERG mutant channels Y652A and F656A lacking aromatic residues in the S6 domain, the inhibitory effect of ajmaline was completely abolished. Ajmaline induced a small shift in HERG current half-maximal activation voltage towards more negative potentials. Ajmaline did not markedly affect HERG inactivation. Inhibitory effects were not voltage-dependent. Ajmaline block exhibited positive frequency dependence. Ajmaline blocked HERG channels in the open, but not in the closed states. Binding of ajmaline to inactivated HERG channels may also be possible. In inactivation-deficient HERG S620T channels, the sensitivity to ajmaline was markedly reduced. The IC(50) of HERG channel blockade in HEK cells lies within the range of unbound therapeutic plasma concentrations of ajmaline. Therefore, inhibitory effects on HERG channels may contribute to both the high anti-arrhythmic efficacy of ajmaline and to its pro-arrhythmic potential.

Activation of cardiac human ether-a-go-go related gene potassium currents is regulated by alpha(1A)-adrenoceptors.

Patients with cardiac disease typically develop life-threatening ventricular arrhythmias during physical or emotional stress, suggesting a link between adrenergic stimulation and regulation of the cardiac action potential. Human ether-a-go-go related gene (hERG) potassium channels conduct the rapid component of the repolarizing delayed rectifier potassium current, I(Kr). Previous studies have revealed that hERG channel activation is modulated by activation of the beta-adrenergic system. In contrast, the influence of the alpha-adrenergic signal transduction cascade on hERG currents is less well understood. The present study examined the regulation of hERG currents by alpha(1A)-adrenoceptors. hERG channels and human alpha(1A)-adrenoceptors were heterologously coexpressed in Xenopus laevis oocytes, and currents were measured using the two-microelectrode voltage clamp technique. Stimulation of alpha(1A)-receptors by applying 20 microM phenylephrine caused hERG current reduction due to a 9.6-mV shift of the activation curve towards more positive potentials. Simultaneous application of the alpha(1)-adrenoceptor antagonist prazosin (20 microM) prevented the activation shift. Inhibition of PKC (3 microM Ro-32-0432) or PKA (2.5 microM KT 5720) abolished the alpha-adrenergic activation shift, suggesting that PKC and PKA are required within the regulatory mechanism. The effect was still present when the PKA- and PKC-dependent phosphorylation sites in hERG were deleted by mutagenesis. In summary, cardiac repolarizing hERG/I(Kr) potassium currents are modulated by alpha(1A)-adrenoceptors via PKC and PKA independently of direct channel phosphorylation. This novel regulatory pathway of alpha1-adrenergic hERG current regulation provides a link between stress and ventricular arrhythmias, in particular in patients with heart disease.

Inhibition of cardiac HERG potassium channels by the atypical antidepressant trazodone.

Trazodone is an atypical antidepressant that is commonly used in the treatment of affective disorders. There have repeatedly been reports of cardiac arrhythmia associated with this drug and concerns have been raised regarding the cardiac safety of trazodone. However, interaction with HERG channels as a main factor of cardiac side effects has not been studied to date. Therefore, we investigated the effect of trazodone on HERG potassium channels expressed in human embryonic kidney (HEK) cells and in Xenopus oocytes. Trazodone inhibited HERG currents in a dose-dependent manner with an IC50 of 2.9 microM in HEK cells and 13.2 microM in Xenopus oocytes. The electrophysiological properties of HERG blockade were analysed in detail. In HERG channel mutants Y652A and F656A lacking aromatic residues in the S6 domain, the affinity of trazodone was reduced profoundly. Trazodone accelerated inactivation of HERG currents without markedly affecting activation. Blockade was voltage dependent with a small reduction of block at positive membrane potentials. Frequency dependence of block was not observed. Trazodone block of HERG channels was state dependent. Channels were affected in the activated and inactivated states, but not in the closed states. In summary, the atypical antidepressant trazodone blocks cardiac HERG channels at concentrations that are probably relevant in vivo, particularly in overdosage.

Human cardiac inwardly rectifying current IKir2.2 is upregulated by activation of protein kinase A.

OBJECTIVE: The cardiac inwardly rectifying potassium current IK1 and its molecular correlates Kir2.1 and Kir2.2 play an important role in cardiac repolarisation and in the pathogenesis of hereditary long-QT syndrome (LQTS-7). Protein kinases A (PKA) and C (PKC) are key enzymes in adrenergic signal transduction, inducing arrhythmias in heart disease. This study investigated the regulation of Kir2.2 (KCNJ12) by PKA. METHODS: Cloned Kir2.2 channels were expressed heterologously in Xenopus oocytes and currents were measured with the double-electrode voltage-clamp technique. RESULTS: After activation of PKA by forskolin (100 micromol/l) or Ro-20-1724 (100 micromol/l), wild type currents at -120 mV were increased by 93.7% and 79.0%, respectively. Coapplication of the PKA inhibitor KT-5720 (2.5 micromol/l) attenuated this effect. No significant changes were apparent after mutation of the single PKA consensus site S430. In addition, removal of all four PKC consensus sites in Kir2.2 induced a phorbolester-mediated current increase which could be suppressed by PKA inhibitors H-89 (50 micromol/l) and KT-5720 (2.5 micromol/l). CONCLUSIONS: This study demonstrates antagonistic effects of PKA and PKC in the regulation of Kir2.2. Phosphorylation by PKC has been shown to cause an inhibition of Kir2.2 currents, whereas activation of PKA leads to current upregulation.

Inhibition of cardiac HERG currents by the DNA topoisomerase II inhibitor amsacrine: mode of action.

1 The topoisomerase II inhibitor amsacrine is used in the treatment of acute myelogenous leukemia. Although most anticancer drugs are believed not to cause acquired long QT syndrome (LQTS), concerns have been raised by reports of QT interval prolongation, ventricular fibrillation and death associated with amsacrine treatment. Since blockade of cardiac human ether-a-go-go-related gene (HERG) potassium currents is an important cause of acquired LQTS, we investigated the acute effects of amsacrine on cloned HERG channels to determine the electrophysiological basis for its proarrhythmic potential. 2 HERG channels were heterologously expressed in human HEK 293 cells and Xenopus laevis oocytes, and the respective potassium currents were recorded using patch-clamp and two-microelectrode voltage-clamp electrophysiology. 3 Amsacrine blocked HERG currents in HEK 293 cells and Xenopus oocytes in a concentration-dependent manner, with IC50 values of 209.4 nm and 2.0 microm, respectively. 4 HERG channels were primarily blocked in the open and inactivated states, and no additional voltage dependence was observed. Amsacrine caused a negative shift in the voltage dependence of both activation (-7.6 mV) and inactivation (-7.6 mV). HERG current block by amsacrine was not frequency dependent. 5 The S6 domain mutations Y652A and F656A attenuated (Y652A) or abolished (F656A, Y652A/F656A) HERG current blockade, indicating that amsacrine binding requires a common drug receptor within the pore-S6 region. 6 In conclusion, these data demonstrate that the anticancer drug amsacrine is an antagonist of cloned HERG potassium channels, providing a molecular mechanism for the previously reported QTc interval prolongation during clinical administration of amsacrine.

Drug binding to aromatic residues in the HERG channel pore cavity as possible explanation for acquired Long QT syndrome by antiparkinsonian drug budipine.

Budipine is a non-dopaminergic antiparkinsonian drug causing acquired forms of Long QT syndrome (aLQTS). As a consequence, the manufacturer has restricted the use of budipine in patients who exhibit additional risk factors for the development of "Torsades-de-Pointes" tachycardias (TdP). The molecular basis of this serious side effect has not been elucidated yet. Human ether-a-go-go related gene (HERG) channel block being the main cause of drug induced QT prolongation, we investigated the effect of budipine on the rapid component of the delayed-rectifier potassium current (I(K(r))) in guinea pig cardiomyocytes and on HERG potassium channels heterologously expressed in Xenopus oocytes. In guinea pig cardiomyocytes, budipine (10 microM) inhibited I(K(r)) by 86% but was without any effect on calcium currents. In Xenopus oocytes, HERG potassium channels were blocked by budipine with an IC(50) of 10.2 microM. Onset of block was fast and block was only slowly and incompletely reversible upon washout. Budipine blocked HERG channels in the open and inactivated state, but not in the closed states. The half-maximal activation voltage was slightly shifted towards more negative potentials. Steady-state inactivation of HERG was also influenced by budipine. Budipine block was neither voltage- nor frequency-dependent. In HERG channel mutants Y652A and F656A, drug affinity was reduced dramatically. Therefore, these two aromatic residues in the channel pore are likely to form a main part of the binding site for budipine. In summary, this is the first study that provides a molecular basis for the budipine-associated aLQTS observed in clinical practice. Furthermore, these findings underline the importance of the aromatic residues Y652 and F656 in the binding of lipophilic drugs to HERG channels.