Dr. Eberhard P. Scholz
Research interests
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InterestsElectrophysiology, Molecular Electrophysiology, Ion Channels, Arrhythmia
Other
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LanguagesGerman, English, French
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Scientific MembershipsEuropean Society of Cardiology (ESC), Deutsche Gesellschaft für Kardiologie (DGK)
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Other InterestsNew England Journal of Medicine
Circulation
Publications
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4.97Impact points
Central role of PKCα in isoenzyme-selective regulation of cardiac transient outward current Ito and Kv4.3 channels.
Journal of molecular and cellular cardiology. 07/2011; 51(5):722-9.
The transient outward current I(to) is an important determinant of the early repolarization phase. I(to) and its molecular basis Kv4.3 are regulated by adrenergic pathways including protein kinase C. However, the exact regulatory mechanisms have not been analyzed yet. We here analyzed isoenzyme spec... [more] The transient outward current I(to) is an important determinant of the early repolarization phase. I(to) and its molecular basis Kv4.3 are regulated by adrenergic pathways including protein kinase C. However, the exact regulatory mechanisms have not been analyzed yet. We here analyzed isoenzyme specific regulation of Kv4.3 and I(to) by PKC. Kv4.3 channels were expressed in Xenopus oocytes and currents were measured with double electrode voltage clamp technique. Patch clamp experiments were performed in isolated rat cardiomyocytes. Unspecific PKC stimulation with PMA resulted in a reduction of Kv4.3 current. Similar effects could be observed after activation of conventional PKC isoforms by TMX. Both effects were reversible by pharmacological inhibition of the conventional PKC isoenzymes (Gö6976). In contrast, activation of the novel PKC isoforms (ingenol) did not significantly affect Kv4.3 current. Whereas TMX-induced PKC activation was not attenuated inhibition of PKCβ, inhibition of PKCα with HBDDE prevented inhibitory effects of both PMA and TMX. Accordingly, stimulatory effects of PMA and TMX could be mimicked by the α-isoenzyme selective PKC activator iripallidal. Further evidence for the central role of PKCα was provided with the use of siRNAs. We found that PKCα siRNA but not PKCβ siRNA abolished the TMX induced effect. In isolated rat cardiomyocytes, PMA dependent I(to) reduction could be completely abolished by pharmacologic inhibition of PKCα. In summary we show that PKCα plays a central role in protein kinase C dependent regulation of Kv4.3 current and native I(to). These results add to the current understanding of isoenzyme selective ion channel regulation by protein kinases.
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1.08Impact points
Initial experience with robotic navigation for catheter ablation of paroxysmal and persistent atrial fibrillation.
Journal of electrocardiology. 06/2011; 45(2):95-101.
Remote robotic navigation (RRN) technology has been developed to facilitate catheter ablation of symptomatic atrial fibrillation (AF). Here, we assess procedural parameters of AF ablation obtained during initial use of RRN compared with a control group treated with a manual ablation approach. Consec... [more] Remote robotic navigation (RRN) technology has been developed to facilitate catheter ablation of symptomatic atrial fibrillation (AF). Here, we assess procedural parameters of AF ablation obtained during initial use of RRN compared with a control group treated with a manual ablation approach. Consecutive patients with symptomatic paroxysmal or persistent AF were subjected to radiofrequency catheter ablation with RRN (Sensei X [Hansen Medical, Mountain View, CA]; n = 25; mean age, 60 ± 2.3 years) or using the standard manual technique (n = 61; mean age, 62 ± 1.4 years). A circumferential pulmonary vein isolation approach guided by 3-dimensional electroanatomical mapping was followed. Remote robotic navigation was associated with reduction of overall fluoroscopy time by 26%. In a case-control subgroup analysis comparing 25 patients with similar clinical characteristics from each group, mean fluoroscopy time was reduced by 22%. Acute isolation of pulmonary veins was achieved in 97% (RRN) and 96% (conventional ablation), respectively. Ablation times and frequency of adverse events were not significantly different among study groups. The early use of RRN resulted in a significant reduction of overall fluoroscopy time and was equally effective and safe compared with manual catheter ablation.
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2.55Impact points
Reconstitution of defective protein trafficking rescues Long-QT syndrome in zebrafish.
Biochemical and biophysical research communications. 03/2011; 408(2):218-24.
Inherited cardiac arrhythmias are caused by genetic defects in ion channels and associated proteins. Mutations in these channels often do not affect their biophysical properties, but rather interfere with their trafficking to the cell membrane. Accordingly, strategies that could reroute the mutated ... [more] Inherited cardiac arrhythmias are caused by genetic defects in ion channels and associated proteins. Mutations in these channels often do not affect their biophysical properties, but rather interfere with their trafficking to the cell membrane. Accordingly, strategies that could reroute the mutated channels to the membrane should be sufficient to restore the electrical properties of the affected cells, thereby suppressing the underlying arrhythmia. We identified here both, embryonic and adult zebrafish breakdance (bre) as a valuable model for human Long-QT syndrome. Electrocardiograms of adult homozygous bre mutants exhibit significant QT prolongation caused by delayed repolarization of the ventricle. We further show that the bre mutation (zERG(I59S)) disrupts ERG protein trafficking, thereby reducing the amount of active potassium channels on the cell membrane. Interestingly, improvement of channel trafficking by cisapride or dimethylsulfoxid is sufficient to reconstitute ERG channels on the cell membrane in a manner that suffices to suppress the Long-QT induced arrhythmia in breakdance mutant zebrafish. In summary, we show for the first time that therapeutic intervention can cure protein trafficking defects and the associated cardiac arrhythmia in vivo.
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Atrial fibrillation-based electrical remodeling in a computer model of the human atrium
Computing in Cardiology, 2010; 10/2010
Atrial fibrillation (AF) is a common pathology. AF modifies the electrophysiological properties of cells (remodeling) promoting the occurrence and maintenance of AF. Electrical remodeling includes changes in I<sub>Ca,L</sub>, I<sub>to</sub>, I<sub>K1</sub> and I&l... [more] Atrial fibrillation (AF) is a common pathology. AF modifies the electrophysiological properties of cells (remodeling) promoting the occurrence and maintenance of AF. Electrical remodeling includes changes in I<sub>Ca,L</sub>, I<sub>to</sub>, I<sub>K1</sub> and I<sub>K,ACh</sub>.These effects were integrated in a human atrial computer model. Gap junction remodeling was considered in the conductivity of the monodomain equation calculating excitation. Specific features were calculated to determine the risk of AF initiation and perpetuation. ERP was reduced from 330ms to 103ms. CV was low ered from 755mm/s to 608mm/s. The WL reduction was even higher (from 249mm to 63mm) leading to a higher probability of occurrence and maintenance of AF. A maximum of 7 spirals waves were initiated leading to apeak in the power spectrum at 10.32Hz. The computer model underlines the relevance of remodeling in AF chronification. The results add to the knowledge of AF maintenance. Our model might prove to be a tool for the development of novel therapeutic strategies.
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2.74Impact points
Cardiovascular ion channels as a molecular target of flavonoids.
Cardiovascular therapeutics. 08/2010; 28(4):e46-52.
Flavonoids are a class of naturally occurring polyphenols abundant in edibles and beverages of plant origin. Epidemiological studies consistently associate high flavonoid intake with a reduced risk for the development of cardiovascular diseases. So far these beneficial effects have been mainly attri... [more] Flavonoids are a class of naturally occurring polyphenols abundant in edibles and beverages of plant origin. Epidemiological studies consistently associate high flavonoid intake with a reduced risk for the development of cardiovascular diseases. So far these beneficial effects have been mainly attributed to nonspecific antioxidant and antiinflammatory properties. However, there is an increasing body of evidence that flavonoids specifically target molecular structures including cardiovascular ion channels. Playing a pivotal role in the regulation of vascular tone and cardiac electric activity, ion channels represent a major target for the induction of antihypertensive and cardioprotective effects. Thus, pharmacological properties of flavonoids on cardiovascular ion channels, ion currents and tissue preparations are being increasingly addressed in experimental studies. Whereas it has become clear that cardiovascular ion channels represent an important molecular target of flavonoids, the published data have not yet been systematically reviewed.
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2.63Impact points
The human cardiac K(2P)3.1 (TASK-1) potassium leak channel is a molecular target for the class III antiarrhythmic drug amiodarone.
Naunyn-Schmiedeberg's archives of pharmacology. 09/2009;
Two-pore-domain (K(2P)) potassium channels mediate background potassium currents, stabilizing resting membrane potential and expediting action potential repolarization. In the heart, K(2P)3.1 (TASK-1) channels are implicated in the cardiac plateau current, I ( KP ). Class III antiarrhythmic drugs ta... [more] Two-pore-domain (K(2P)) potassium channels mediate background potassium currents, stabilizing resting membrane potential and expediting action potential repolarization. In the heart, K(2P)3.1 (TASK-1) channels are implicated in the cardiac plateau current, I ( KP ). Class III antiarrhythmic drugs target cardiac K(+) currents, resulting in action potential prolongation and suppression of atrial and ventricular arrhythmias. The objective of this study was to investigate acute effects of the class III antiarrhythmic drug amiodarone on human K(2P)3.1 channels. Potassium currents were recorded from Xenopus oocytes using the two-microelectrode voltage clamp technique. Amiodarone produced concentration-dependent inhibition of hK(2P)3.1 currents (IC(50) = 0.40 microM) with maximum current reduction of 58.1%. Open rectification properties that are characteristic to hK(2P)3.1 currents were not altered by amiodarone. Channels were blocked in open and closed states in reverse frequency-dependent manner. hK(2P)3.1 channel inhibition was voltage-independent at voltages between -40 and +60 mV. Modulation of protein kinase C activity by amiodarone does not contribute to hK(2P)3.1 current reduction, as pre-treatment with the protein kinase C inhibitor, staurosporine, did not affect amiodarone block. Amiodarone is an inhibitor of cardiac hK(2P)3.1 background channels. Amiodarone blockade of hK(2P)3.1 may cause prolongation of cardiac repolarization and action potential duration in patients with high individual plasma concentrations, possibly contributing to the antiarrhythmic efficacy of the class III drug.
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2.55Impact points
Biophysical properties of zebrafish ether-à-go-go related gene potassium channels.
Biochemical and biophysical research communications. 03/2009;
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... [more] 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.
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5.20Impact points
Selective noradrenaline reuptake inhibitor atomoxetine directly blocks hERG currents.
British journal of pharmacology. 02/2009;
Background and purpose: Atomoxetine is a selective noradrenaline reuptake inhibitor, recently approved for the treatment of attention-deficit/hyperactivity disorder. So far, atomoxetine has been shown to be well tolerated, and cardiovascular effects were found to be negligible. However, two independ... [more] Background and purpose: Atomoxetine is a selective noradrenaline reuptake inhibitor, recently approved for the treatment of attention-deficit/hyperactivity disorder. So far, atomoxetine has been shown to be well tolerated, and cardiovascular effects were found to be negligible. However, two independent cases of QT interval prolongation, associated with atomoxetine overdose, have been reported recently. We therefore analysed acute and subacute effects of atomoxetine on cloned human Ether-à-Go-Go-Related Gene (hERG) channels. Experimental approach: hERG channels were heterologously expressed in Xenopus oocytes and in a human embryonic kidney cell line and hERG currents were measured using voltage clamp and patch clamp techniques. Action potential recordings were made in isolated guinea-pig cardiomyocytes. Gene expression and channel surface expression were analysed using quantitative reverse transcriptase polymerase chain reaction, Western blot and the patch clamp techniques. Key results: In human embryonic kidney cells, atomoxetine inhibited hERG current with an IC(50) of 6.3 micromol.L(-1). Development of block and washout were fast. Channel activation and inactivation were not affected. Inhibition was state-dependent, suggesting an open channel block. No use-dependence was observed. Inhibitory effects of atomoxetine were attenuated in the pore mutants Y652A and F656A. In guinea-pig cardiomyocytes, atomoxetine lengthened action potential duration without inducing action potential triangulation. Overnight incubation with high atomoxetine concentrations resulted in a decrease of channel surface expression. Conclusions and implications: Whereas subacute effects of atomoxetine seem negligible under therapeutically relevant concentrations, hERG channel block should be considered in cases of atomoxetine overdose and when administering atomoxetine to patients at increased risk for the development of acquired long-QT syndrome.
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2.55Impact points
Dual regulation of renal Kir7.1 potassium channels by protein Kinase A and protein Kinase C.
Biochemical and biophysical research communications. 01/2009; 377(3):981-6.
The renal inward rectifier potassium channel Kir7.1 has been proposed to be functionally important for tubular K(+) recycling and secretion. This study investigated the regulation of Kir7.1 by PKA and PKC. Cloned human Kir7.1 channels were expressed heterologously in Xenopus oocytes. After pharmacol... [more] The renal inward rectifier potassium channel Kir7.1 has been proposed to be functionally important for tubular K(+) recycling and secretion. This study investigated the regulation of Kir7.1 by PKA and PKC. Cloned human Kir7.1 channels were expressed heterologously in Xenopus oocytes. After pharmacological PKC activation, Kir7.1 currents were strongly inhibited. Co-application of PKC inhibitors attenuated this effect. Inactivation of PKC consensus sites also strongly attenuated the effect with a single site ((201)S) being essential for almost the total PKC sensitivity. In contrast, PKA activation induced an increase of Kir7.1 currents. This effect was absent in mutant Kir7.1 channels lacking PKA consensus site (287)S. In summary, this study demonstrates the dual regulation of Kir7.1 channel function by PKA and PKC. Structurally, these regulations depend on two key residues in the C-terminal channel domain ((Ser)201 for PKC and (Ser)287 for PKA).
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Dual regulation of renal Kir7.1 potassium channels by protein Kinase A and protein Kinase C.
Biochemical and biophysical research communications. 11/2008;
The renal inward rectifier potassium channel Kir7.1 has been proposed to be functionally important for tubular K(+) recycling and secretion. This study investigated the regulation of Kir7.1 by PKA and PKC. Cloned human Kir7.1 channels were expressed heterologously in Xenopus oocytes. After pharmacol... [more] The renal inward rectifier potassium channel Kir7.1 has been proposed to be functionally important for tubular K(+) recycling and secretion. This study investigated the regulation of Kir7.1 by PKA and PKC. Cloned human Kir7.1 channels were expressed heterologously in Xenopus oocytes. After pharmacological PKC activation, Kir7.1 currents were strongly inhibited. Co-application of PKC inhibitors attenuated this effect. Inactivation of PKC consensus sites also strongly attenuated the effect with a single site ((201)S) being essential for almost the total PKC sensitivity. In contrast, PKA activation induced an increase of Kir7.1 currents. This effect was absent in mutant Kir7.1 channels lacking PKA consensus site (287)S. In summary, this study demonstrates the dual regulation of Kir7.1 channel function by PKA and PKC. Structurally, these regulations depend on two key residues in the C-terminal channel domain ((Ser)201 for PKC and (Ser)287 for PKA).
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Effects of the reggae mutation on sinus node function: A simulation study
Computers in Cardiology, 2008; 10/2008
The sinus node is the primary pacemaker of the heart. A complex interplay of heterogeneities is assumed to be the basic mechanism that the sinus node can drive the heart. This interplay can be disturbed by e.g. diseases, drugs or mutations. In this work the effect of a mutation on the sinus node fun... [more] The sinus node is the primary pacemaker of the heart. A complex interplay of heterogeneities is assumed to be the basic mechanism that the sinus node can drive the heart. This interplay can be disturbed by e.g. diseases, drugs or mutations. In this work the effect of a mutation on the sinus node function were investigated. Therefore, measurement data of wild-type and mutant I<sub>Kr</sub> channels were integrated with aid of optimization procedures into the heterogeneous sinus node model of Zhang et al. The measurement data shows a shift of the steady-state inactivation to more positive potentials. Simulated central sinus node cells lose their ability to depolarize spontaneously. Peripheral cell are also effected by the mutation. The main changes are the shortening of the action potential duration from 108 ms to 84 ms and the increase of auto-rhythmic frequency from 6.37 Hz to 7.62 Hz due to an increased mean I<sub>Kr</sub> current. In a future study the bradycardial effect of this mutation will be shown in a tissue model.
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2.63Impact points
Inhibition of cardiac hERG potassium channels by tetracyclic antidepressant mianserin.
Naunyn-Schmiedeberg's archives of pharmacology. 08/2008; 378(1):73-83.
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, ... [more] 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 mumol/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.
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5.20Impact points
Regulation of two-pore-domain (K(2P)) potassium leak channels by the tyrosine kinase inhibitor genistein.
British journal of pharmacology. 07/2008;
Background and purpose:Two-pore-domain potassium (K(2P)) channels mediate potassium background (or 'leak') currents, controlling excitability by stabilizing membrane potential below firing threshold and expediting repolarization. Inhibition of K(2P) currents permits membrane potential depola... [more] Background and purpose:Two-pore-domain potassium (K(2P)) channels mediate potassium background (or 'leak') currents, controlling excitability by stabilizing membrane potential below firing threshold and expediting repolarization. Inhibition of K(2P) currents permits membrane potential depolarization and excitation. As expected for key regulators of excitability, leak channels are under tight control from a plethora of stimuli. Recently, signalling via protein tyrosine kinases (TKs) has been implicated in ion channel modulation. The objective of this study was to investigate TK regulation of K(2P) channels.Experimental approach:The two-electrode voltage clamp technique was used to record K(2P) currents in Xenopus oocytes. In addition, K(2P) channels were studied in Chinese hamster ovary (CHO) cells using the whole-cell patch clamp technique.Key results:Here, we report inhibition of human K(2P)3.1 (TASK-1) currents by the TK antagonist, genistein, in Xenopus oocytes (IC(50)=10.7 muM) and in CHO cells (IC(50)=12.3 muM). The underlying molecular mechanism was studied in detail. hK(2P)3.1 was not affected by genistin, an inactive analogue of genistein. Perorthovanadate, an inhibitor of tyrosine phosphatase activity, reduced the inhibitory effect of genistein. Current reduction was voltage independent and did not require channel protonation at position H98 or phosphorylation at the single TK phosphorylation site, Y323. Among functional hK(2P) family members, genistein also reduced K(2P)6.1 (TWIK-2), K(2P)9.1 (TASK-3) and K(2P)13.1 (THIK-1) currents, respectively.Conclusions and implications:Modulation of K(2P) channels by the TK inhibitor, genistein, represents a novel molecular mechanism to alter background K(+) currents.British Journal of Pharmacology advance online publication, 2 June 2008; doi:10.1038/bjp.2008.213.
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14.82Impact points
Deficient zebrafish ether-à-go-go-related gene channel gating causes short-QT syndrome in zebrafish reggae mutants.
Circulation. 03/2008; 117(7):866-75.
BACKGROUND: Genetic predisposition is believed to be responsible for most clinically significant arrhythmias; however, suitable genetic animal models to study disease mechanisms and evaluate new treatment strategies are largely lacking. METHODS AND RESULTS: In search of suitable arrhythmia models, w... [more] BACKGROUND: Genetic predisposition is believed to be responsible for most clinically significant arrhythmias; however, suitable genetic animal models to study disease mechanisms and evaluate new treatment strategies are largely lacking. METHODS AND RESULTS: In search of suitable arrhythmia models, we isolated the zebrafish mutation reggae (reg), which displays clinical features of the malignant human short-QT syndrome such as accelerated cardiac repolarization accompanied by cardiac fibrillation. By positional cloning, we identified the reg mutation that resides within the voltage sensor of the zebrafish ether-à-go-go-related gene (zERG) potassium channel. The mutation causes premature zERG channel activation and defective inactivation, which results in shortened action potential duration and accelerated cardiac repolarization. Genetic and pharmacological inhibition of zERG rescues recessive reg mutant embryos, which confirms the gain-of-function effect of the reg mutation on zERG channel function in vivo. Accordingly, QT intervals in ECGs from heterozygous and homozygous reg mutant adult zebrafish are considerably shorter than in wild-type zebrafish. CONCLUSIONS: With its molecular and pathophysiological concordance to the human arrhythmia syndrome, zebrafish reg represents the first animal model for human short-QT syndrome.
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2.59Impact points
Doxazosin induces apoptosis of cells expressing hERG K+ channels.
European journal of pharmacology. 02/2008; 579(1-3):98-103.
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 mem... [more] 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.
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4.97Impact points
Kir2.x inward rectifier potassium channels are differentially regulated by adrenergic alpha1A receptors.
Journal of molecular and cellular cardiology. 02/2008; 44(1):84-94.
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 b... [more] 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.
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2.63Impact points
Anticholinergic antiparkinson drug orphenadrine inhibits HERG channels: block attenuation by mutations of the pore residues Y652 or F656.
Naunyn-Schmiedeberg's archives of pharmacology. 01/2008; 376(4):275-84.
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... [more] 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.
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2.55Impact points
Green tea flavonoid epigallocatechin-3-gallate (EGCG) inhibits cardiac hERG potassium channels.
Biochemical and biophysical research communications. 01/2008; 364(3):429-35.
The catechin EGCG is the main flavonoid compound of green tea and has received enormous pharmacological attention because of its putative beneficial health effects. This study investigated for the first time the effect of EGCG on hERG channels, the main pharmacological target of drugs that cause acq... [more] The catechin EGCG is the main flavonoid compound of green tea and has received enormous pharmacological attention because of its putative beneficial health effects. This study investigated for the first time the effect of EGCG on hERG channels, the main pharmacological target of drugs that cause acquired long QT syndrome. Cloned hERG channels were expressed in Xenopus oocytes and in HEK293 cells. Heterologous hERG currents were inhibited by EGCG with an IC50 of 6.0 micromol/l in HEK293 cells and an IC50 of 20.5 micromol/l in Xenopus laevis oocytes. Onset of effect was slow and only little recovery from inhibition was observed upon washout. In X. laevis oocytes EGCG inhibited hERG channels in the open and inactivated states, but not in the closed states. The half-maximal activation voltage of hERG currents was shifted by EGCG towards more positive potentials. In conclusion, EGCG is a low-affinity inhibitor of hERG sharing major electrophysiological features with pharmaceutical hERG antagonists.
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3.52Impact points
Orange flavonoid hesperetin modulates cardiac hERG potassium channel via binding to amino acid F656.
Nutrition, metabolism, and cardiovascular diseases : NMCD. 12/2007; 17(9):666-75.
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 cardi... [more] 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.
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2.63Impact points
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.
Naunyn-Schmiedeberg's archives of pharmacology. 08/2007; 375(5):311-22.
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 myocardi... [more] 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.
Following (24)
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Hoda Javadi
Rajaei Cardiovascular Medical and Research Center -
Ijad Madisch
Massachusetts General Hospital -
Lennart Bongartz
Universiteit Utrecht -
Ekaterina Zakharchenko
Kuban State Medical University -
Soenke Bartling
German Cancer Research Center / UMC Mannheim
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