Spironolactone and its main metabolite canrenoic acid block hKv1.5, Kv4.3 and Kv7.1 + minK channels

Both spironolactone (SP) and its main metabolite, canrenoic acid (CA), prolong cardiac action potential duration and decrease the Kv11.1 (HERG) current. We examined the effects of SP and CA on cardiac hKv1.5, Kv4.3 and Kv7.1+minK channels that generate the human I(Kur), I(to1) and I(Ks), which contribute to the control of human cardiac action potential duration.hKv1.5 currents were recorded in stably transfected mouse fibroblasts and Kv4.3 and Kv7.1 + minK in transiently transfected Chinese hamster ovary cells using the whole-cell patch clamp. SP (1 microM) and CA (1 nM) inhibited hKv1.5 currents by 23.2 +/- 3.2 and 18.9 +/- 2.7%, respectively, shifted the midpoint of the activation curve to more negative potentials and delayed the time course of tail deactivation.SP (1 microM) and CA (1 nM) inhibited the total charge crossing the membrane through Kv4.3 channels at +50 mV by 27.1 +/- 6.4 and 27.4 +/- 5.7%, respectively, and accelerated the time course of current decay. CA, but not SP, shifted the inactivation curve to more hyperpolarised potentials (V(h)-37.0 +/- 1.8 vs -40.8 +/- 1.6 mV, n = 10, P < 0.05).SP (10 microM) and CA (1 nM) also inhibited Kv7.1 + minK currents by 38.6 +/- 2.3 and 22.1 +/- 1.4%, respectively, without modifying the voltage dependence of channel activation. SP, but not CA, slowed the time course of tail current decay.CA (1 nM) inhibited the I(Kur) (29.2 +/- 5.5%) and the I(to1) (16.1 +/- 3.9%) recorded in mouse ventricular myocytes and the I(K) (21.8 +/- 6.9%) recorded in guinea-pig ventricular myocytes.A mathematical model of human atrial action potentials demonstrated that K(+) blocking effects of CA resulted in a lengthening of action potential duration, both in normal and atrial fibrillation simulated conditions. The results demonstrated that both SP and CA directly block hKv1.5, Kv4.3 and Kv7.1 + minK channels, CA being more potent for these effects. Since peak free plasma concentrations of CA ranged between 3 and 16 nM, these results indicated that blockade of these human cardiac K(+) channels can be observed after administration of therapeutic doses of SP.Blockade of these cardiac K(+) currents, together with the antagonism of the aldosterone proarrhythmic effects produced by SP, might be highly desirable for the treatment of supraventricular arrhythmias.     

Cocaine blocks HERG, but not KvLQT1+minK, potassium channels

Cocaine causes cardiac arrhythmias, sudden death, and occasionally long QT syndrome in humans. We investigated the effect of cocaine on the human K(+) channels HERG and KvLQT1+minK that encode native rapidly (I(Kr)) and slowly (I(Ks)) activating delayed rectifier K(+) channels in the heart. HERG and KvLQT1+minK channels were heterologously expressed in human embryonic kidney 293 cells, and whole-cell currents were recorded. Cocaine had no effect on KvLQT1+minK current in concentrations up to 200 microM. In contrast, cocaine reversibly blocked HERG current with half-maximal block of peak tail current of 7.2 microM. By using a protocol to quickly activate HERG channels, we found that cocaine block developed rapidly after channel activation. At 0 mV, the time constants for the development of block were 38.2 +/- 2.1, 15.2 +/- 0.8, and 6.9 +/- 1.1 ms in 10, 50 and 200 microM cocaine, respectively. Cocaine-blocked channels also recovered rapidly from block after repolarization. At -100 mV, recovery from block followed a biphasic time course with fast and slow time constants of 3.5 +/- 0.7 and 100.3 +/- 15.4 ms, respectively. Using N-methyl-cocaine, a permanently charged, membrane-impermeable cocaine analog, block of HERG channels rapidly developed when the drug was applied intracellularly through the patch pipette, suggesting that the cocaine binding site on the HERG protein is located on a cytoplasmic accessible domain. These results indicate that cocaine suppresses HERG, but not KvLQT1+minK, channels by preferentially blocking activated channels, that it unblocks upon repolarization, and does so with unique ultrarapid kinetics. Because the cocaine concentration range we studied is achieved in humans, HERG block may provide an additional mechanism for cocaine-induced arrhythmias and sudden death.     

Interactions of the narcotic l-alpha-acetylmethadol with human cardiac K+ channels

l-alpha-acetylmethadol is a long-acting narcotic analgesic that is used in the treatment of opiate addiction. However, the drug has been associated with cases of QT interval prolongation and ventricular arrhythmia. To understand the mechanism underlying these clinical findings, we examined the effects of l-alpha-acetylmethadol on the cloned human cardiac K(+) channels HERG (human ether-a-go-go-related gene), KvLQT1/minK and Kv4.3. Using patch clamp electrophysiology, we found that l-alpha-acetylmethadol inhibited HERG channel currents in a voltage-dependent manner displaying an IC(50) value of 3 microM. The major active metabolite of l-alpha-acetylmethadol, noracetylmethadol, inhibited HERG with an estimated IC(50) values of 12 microM. l-alpha-acetylmethadol had little or no effect on Kv4.3 or KvLQT1/minK K(+) channel currents at concentration up to 10 microM. We conclude that the proarrhythmic effects of l-alpha-acetylmethadol are due to specific blockade of the HERG cardiac K(+) channel and that its active metabolite noracetylmethadol may provide a safer alternative in the treatment of opiate addiction.     

High affinity blockade of the HERG cardiac K(+) channel by the neuroleptic pimozide

Pimozide is an antipsychotic agent also used to treat facial tics. Pimozide can cause acquired long QT syndrome and ventricular arrhythmias. To elucidate the mechanism behind these clinical findings, we examined the effects of pimozide on the cloned human cardiac K(+) channels HERG (human ether-a-go-go-related gene; rapid component of delayed rectifier), Kv1.5 (ultra-rapid delayed rectifier) and KvLQT1/minK (slow component of delayed rectifier). Using patch clamp electrophysiology, we found that pimozide was a potent inhibitor of HERG displaying an IC(50) value of 18 nM. In contrast, pimozide (10 microM) was a weak inhibitor of KvLQT1/minK and Kv1.5. We conclude that pimozide is a specific, high affinity antagonist of HERG, and that this interaction leads to prolongation of cardiac repolarization.     

Acute effects of dronedarone on both components of the cardiac delayed rectifier K+ current, HERG and KvLQT1/minK potassium channels

Dronedarone is a noniodinated benzofuran derivative that has been synthesized to overcome the limiting iodine-associated adverse effects of the potent antiarrhythmic drug amiodarone. In this study, the acute electrophysiological effects of dronedarone on repolarizing potassium channels were investigated to determine the class III antiarrhythmic action of this compound. HERG and KvLQT1/minK potassium channels conduct the delayed rectifier potassium current IK in human heart, being a primary target for class III antiarrhythmic therapy. HERG and KvLQT1/minK were expressed heterologously in Xenopus laevis oocytes, and the respective potassium currents were recorded using the two-microelectrode voltage-clamp technique. Dronedarone blocked HERG channels with an IC50 value of 9.2 microM and a maximum tail current reduction of 85.2%. HERG channels were blocked in the closed, open, and inactivated states. The half-maximal activation voltage was shifted by -6.1 mV, and HERG current block by dronedarone was voltage-dependent, but not use-dependent. Dronedarone exhibited a weaker block of KvLQT1/minK currents (33.2% at 100 microM drug concentration), without causing significant changes in the corresponding current-voltage relationships. In conclusion, these data demonstrate that dronedarone is an antagonist of cloned HERG potassium channels, with additional inhibitory effects on KvLQT1/minK currents at higher drug concentrations, providing a molecular mechanism for the class III antiarrhythmic action of the drug.     

Inhibitory effects of volatile anesthetics on currents produced on heterologous expression of KvLQT1 and minK in Xenopus oocytes

The slowly activating component of delayed rectifier K+ current (IKs) in the heart modulates the repolarization of cardiac action potential. We investigated the effects of the volatile anesthetics isoflurane and sevoflurane on cloned IKs coexpressed by KvLQT1 and minK. Currents were induced following injection into oocytes of KvLQT1 mRNA (10 ng) with or without minK mRNA (1 ng), which were transcribed in vitro from cDNAs of normal rats hearts. A two-electrode voltage-clamp recording technique was used to investigate the effects of isoflurane (0-1.5 minimum alveolar concentration, MAC) and sevoflurane (0-1.5 MAC) on IKs (KvLQT1 with minK) and KvLQT1 alone currents. Currents were activated by step depolarizations to a series of potentials from a holding potential of -80 mV and measured as the deactivating tail current on repolarization to -60 mV. Following a 2-s depolarization to 40 mV, isoflurane and sevoflurane caused potency-dependent reductions in IKs and KvLQT1 currents. Both of the volatile anesthetics tested accelerated the deactivation of IKs and KvLQT1 currents. We conclude that the significant inhibitory effect of volatile anesthetics on the cloned IKs may partly contribute to the clinical observations of the prolongation of the ventricular repolarization (Q-T interval) by the anesthetics.     

Mechanisms of block of a human cloned potassium channel by the enantiomers of a new bradycardic agent: S-16257-2 and S-16260-2.

1. The effects of S-16257-2 (S57) and S-16260-2 (R60), the two enantiomers of a new bradycardic agent, were studied on human cloned K+ channels (hKv1.5) stably expressed in a mouse L cell line using the whole-cell configuration of the patch-clamp technique. 2. S57 and R60 did not modify the sigmoidal activation time course of the current but reduced the amplitude and increased the rate of the decay of the current during the application of depolarizing pulses. Both, S57 and R60 produced a concentration-dependent block of hKv1.5 channels with apparent KD values of 29.0 +/- 1.9 microM and 40.9 +/- 4.0 microM, respectively. Thus, S57 was 1.4 fold more potent than R60 in blocking hKv1.5 channels. 3. The blockade produced by S57 and R60 was voltage-dependent and increased steeply between -30 and 0 mV, which corresponded with the voltage range for channel opening. This result indicated that both enantiomers block the hKv1.5 channels, preferentially, when they are in the open state. Between 0 and +60 mV the blockade exhibited a shallow voltage-dependence which was described by an electrical distance of 0.18 +/- 0.002 and 0.19 +/- 0.004 for S57 and R60, respectively. 4. S57 and R60 also increased the rate of decline of the current during the application of depolarizing pulses. The time constant of such decline (tau Block) was faster in the presence of R60 than in the presence of S57 (16.2 +/- 1.5 ms vs. 24.0 +/- 2.6 ms; P < 0.01). The apparent association rate constants (k) were similar for S57 and R60 ((0.52 +/- 0.13) x 10(6) M-1 s-1 and (0.66 +/- 0.13) x 10(6) M-1 s-1, respectively), whereas the dissociation rate constant (l) was faster for R60 than for S57 (25.8 +/- 1.8 s-1 and 13.0 +/- 2.4 s-1, respectively). 5. Both enantiomers slowed the deactivation of the tail currents elicited upon repolarization to -40 mV, thus inducing a 'crossover' phenomenon. These results suggested that drug unbinding is required before hKv1.5 channels can close. 6. It is concluded that R60 and S57 produced a similar time- voltage- and state-dependent block of hKv1.5 channels that can be interpreted as open channel block by the charged form of each enantiomer. The main difference between R60 and S57 were linked to the apparent dissociation rate constants.     

Effects of phrixotoxins on the Kv4 family of potassium channels and implications for the role of Ito1 in cardiac electrogenesis

1. In the present study, two new peptides, phrixotoxins PaTx1 and PaTx2 (29-31 amino acids), which potently block A-type potassium currents, have been purified from the venom of the tarantula Phrixotrichus auratus. 2. Phrixotoxins specifically block Kv4.3 and Kv4.2 currents that underlie I(to1), with an 5 < IC50 < 70 nM, by altering the gating properties of these channels. 3. Neither are the Shaker (Kv1), Shab (Kv2) and Shaw (Kv3) subfamilies of currents, nor HERG, KvLQT1/IsK, inhibited by phrixotoxins which appear specific of the Shal (Kv4) subfamily of currents and also block I(to1) in isolated murine cardiomyocytes. 4. In order to evaluate the physiological consequences of the Ito1 inhibition, mice were injected intravenously with PaTx1, which resulted in numerous transient cardiac adverse reactions including the occurrence of premature ventricular beats, ventricular tachycardia and different degrees of atrioventricular block. 5. The analysis of the mouse electrocardiogram showed a dose-dependent prolongation of the QT interval, chosen as a surrogate marker for their ventricular repolarization, from 249 +/- 11 to 265 +/- 8 ms (P < 0.05). 6. It was concluded that phrixotoxins, are new and specific blockers of Kv4.3 and Kv4.2 potassium currents, and hence of I(to1) that will enable further studies of Kv4.2 and Kv4.3 channel and/or I(to1) expression.     

Effects of rupatadine, a new dual antagonist of histamine and platelet-activating factor receptors, on human cardiac kv1.5 channels

1. The effects of rupatadine, a new dual antagonist of both histamine H1 and platelet-activating factor receptors, were studied on human cloned hKv1.5 channels expressed in Ltk- cells using the whole-cell patch-clamp technique. 2. Rupatadine produced a use- and concentration-dependent block of hKv1.5 channels (KD=2.4+/-0.7 micronM) and slowed the deactivation of the tail currents, thus inducing the 'crossover' phenomenon. 3. Rupatadine-induced block was voltage-dependent increasing in the voltage range for channel opening suggesting an open channel interaction. At potentials positive to +10 mV the blockade decreased with a shallow voltage-dependence. Moreover, rupatadine also modified the voltage-dependence of hKv1.5 channel activation, which exhibited two components, the midpoint of the steeper component averaging -25. 2+/-2.7 mV. 4. When the intracellular K+ concentration ([K+]i) was lowered to 25% the voltage-dependent unblock observed at positive potentials was suppressed and the activation curve in the presence of rupatadine did not exhibit two components even when the midpoint of the activation curve was shifted to more negative potentials (-30. 3+/-1.3 mV). 5. On channels mutated on the residue R485 (R485Y) which is located on the external entryway of the pore the rupatadine-induced block did not decrease at potentials positive to +10 mV. In contrast, on V512M channels rupatadine reproduced all the features of the blockade observed on wild type channels. 6. All these results suggest that rupatadine blocks hKv1.5 channels binding to an external and to an internal binding site but only at concentrations much higher than therapeutic plasma levels in man. Efflux of K+ promotes the unbinding from the external site. Furthermore, rupatadine binds to an internal site and dramatically modifies the voltage-dependence of channel opening.     

The membrane permeable calcium chelator BAPTA-AM directly blocks human ether a-go-go-related gene potassium channels stably expressed in HEK 293 cells

BAPTA-AM is a well-known membrane permeable Ca(2+) chelator. The present study found that BAPTA-AM rapidly and reversibly suppressed human ether a-go-go-related gene (hERG or Kv11.1) K(+) current, human Kv1.3 and human Kv1.5 channel currents stably expressed in HEK 293 cells, and the effects were not related to Ca(2+) chelation. The externally applied BAPTA-AM inhibited hERG channels in a concentration-dependent manner (IC(50): 1.3 microM). Blockade of hERG channels was dependent on channel opening, and tonic block was minimal. Steady-state activation V(0.5) of hERG channels was negatively shifted by 8.5 mV (from -3.7+/-2.8 of control to -12.2+/-3.1 mV, P<0.01), while inactivation V(0.5) was negatively shifted by 6.1 mV (from -37.9+/-2.0 mV of control to -44.0+/-1.6 mV, P<0.05) with application of 3 microM BAPTA-AM. The S6 mutant Y652A and the pore helix mutant S631A significantly attenuated blockade by BAPTA-AM at 10 microM causing profound blockade of wild-type hERG channels. In addition, BAPTA-AM inhibited hKv1.3 and hKv1.5 channels in a concentration-dependent manner (IC(50): 1.45 and 1.23 microM, respectively), and the blockade of these two types of channels was also dependent on channel opening. Moreover, EGTA-AM was found to be an open channel blocker of hERG, hKv1.3, hKv1.5 channels, though its efficacy is weaker than that of BAPTA-AM. These results indicate that the membrane permeable Ca(2+) chelator BAPTA-AM (also EGTA-AM) exerts an open channel blocking effect on hERG, hKv1.3 and hKv1.5 channels.     

Bertosamil blocks HERG potassium channels in their open and inactivated states

1. Bertosamil is chemically related to the class-III anti-arrhythmic drug tedisamil and has been developed as a bradycardic, anti-ischemic and anti-arrhythmic drug. Its anti-arrhythmic properties might in part be attributed to its block of voltage-dependent potassium channels Kv(1.2), Kv(1.4). and Kv(1.5). However, HERG-potassium channel block as an important target for class-III drugs has not yet been investigated. 2. We investigated the effect of bertosamil on the HERG potassium channel heterologously expressed in Xenopus oocytes with the two-electrode voltage-clamp technique. 3. Bertosamil (70 microM) inhibited HERG tail currrent after a test pulse to 30 mV by 49.3+/-8.4% (n=5) and the IC(50) was 62.7 microM. Onset of block was fast, i.e. 90% of inhibition developed within 180+/-8.22 s (n=5), and block was totally reversible upon washout within 294+/-38.7 s (n=5). 4. Bertosamil-induced block of HERG potassium channels was state-dependent with block mainly to open- and inactivated channels. Half-maximal activation voltage was slightly shifted towards more negative potentials. 5. Steady-state inactivation of HERG was not influenced by bertosamil. Bertosamil block elicited voltage-but no frequency-dependent effects. 6. In summary, bertosamil blocked the HERG potassium channel. These blocking properties may contribute to the anti-arrhythmic effects of bertosamil in the treatment of atrial and particular ventricular arrhythmias.     

Characterization of a novel Kv1.5 channel blocker in Xenopus oocytes, CHO cells, human and rat cardiomyocytes

The inhibitory effects of the novel Kv1.5 channel blocker, S9947 (2'-(benzyloxycarbonylaminomethyl)biphenyl-2-carboxylic acid 2-(2-pyridyl)ethylamide), on cloned human Kv1.5 (hKv1.5), expressed in both Xenopus oocytes and Chinese hamster ovary (CHO) cells, and on native cardiac ultrarapid delayed rectifier potassium currents (IKur) in rat (ventricle myocytes) and human (atrial myocytes) were investigated. The influence of S9947 on the action potential was examined in rat ventricular myocytes. Using the two-electrode voltage-clamp technique in Xenopus oocytes and the patch-clamp technique (whole cell configuration) in CHO cells, hKv1.5 was inhibited by S9947 with IC50 values of 0.65 microM and 0.42 microM, respectively. In addition, inhibition of human Kv4.3 (hKv4.3) and HERG by 10 microM S9947 was low (approximately 20%) and absent, respectively. Using the patch-clamp technique in the whole cell configuration, IKur currents in rat ventricular (rIKur) cardiomyocytes and human atrial (hIKur) cardiomyocytes were inhibited by S9947 with IC50 values of 0.96 microM and 0.07 microM, respectively. In contrast, rat cardiac inward rectifier current (rIK1) and rat (rIto) and human (hIto) cardiac transient outward currents were only inhibited by approximately 20% with 10 microM S9947. In rat cardiomyocytes, using the patch-clamp technique, action potential duration was increased by S9947 in a concentration-dependent (0.3-10 microM) and rate-independent manner. The data show that S9947 suppresses both cloned (Kv1.5) and native (IKur) cardiac potassium currents. Furthermore, S9947 prolongs rat action potential in a rate-independent manner.     

Non-specific action of methoxamine on Ito, and the cloned channels hKv 1.5 and Kv 4.2

The alpha1-adrenoceptor agonist methoxamine acted independently of receptor activation to reduce Ito and the sustained outward current in rat ventricular myocytes, and hKv 1.5 and Kv 4.2 cloned K+ channel currents. Two hundred microM methoxamine reduced Ito by 36% in the presence of 2 microM prazosin, and by 37 and 38% after preincubation of myocytes with either N-ethylmaleimide or phenoxybenzamine (n=6). The EC50 values at +60 mV for direct reduction of Ito, hKv 1.5, and Kv 4.2 by methoxamine were 239, 276, and 363 microM, respectively, with Hill coefficients of 0.87-1.5. Methoxamine accelerated Ito and Kv 4.2 current inactivation in a concentration- and voltage-dependent manner. Apparent rate constants for methoxamine binding and unbinding gave Kd values in agreement with EC50 values measured from dose-response relations. The voltage-dependence of block supported charged methoxamine binding to a putative intracellular site that sensed approximately 20% of the transmembrane electrical field. In the presence of methoxamine, deactivating Kv 4.2 tail currents displayed a distinct rising phase, and were slowed relative to control, such that tail current crossover was observed. These observations support a dominant mechanism of open channel block, although closed channel block could not be ruled out. Single-channel data from hKv 1.5 patches revealed increased closed times with blank sweeps and decreased burst duration in the presence of drug, and a reduction of mean channel open time from 1.8 ms in control to 0.4 ms in 500 microM methoxamine. For this channel, therefore, both open and closed channel block appeared to be important mechanisms for the action of methoxamine.     

Open channel block of hKv1.5 by psoralen fromHeracleum moellendorffii hance

A furocoumarin derivative, psoralen (7H-furo[3,2-g][1]benzopyran-7-one), was isolated from the n-hexane fraction of Heracleum moellendorffii Hance. We examined the effects of psoralen on a human Kv1.5 potassium channel (hKv1.5) cloned from human heart and stably expressed in Ltk- cells. We found that psoralen inhibited the hKv1.5 current in a concentration-, use- and voltage-dependent manner with an IC50 value of 180 +/- 21 nM at +60 mV. Psoralen accelerated the inactivation kinetics of the hKv1.5 channel, and it slowed the deactivation kinetics of the hKv1.5 current resulting in a tail crossover phenomenon. These results indicate that psoralen acts on the hKv1.5 channel as an open channel blocker. Furthermore, psoralen prolonged the action potential duration of rat atrial muscles in a dose-dependent manner. Taken together, the present results strongly suggest that psoralen may be an ideal antiarrhythmic drug for atrial fibrillation.     

Characterization of human cardiac Kv1.5 inhibition by the novel atrial-selective antiarrhythmic compound AVE1231

OBJECTIVE: Atrial-selective drug therapy represents a novel therapeutic approach for atrial fibrillation management. The aim of the present study was to investigate the mechanism of hKv1.5 channel inhibition by the atrial-selective compound AVE1231. METHODS: Ionic currents were recorded from CHO cells transfected with KCNA5 cDNA with whole-cell patch-clamp technique. The effect of AVE1231 on human atrial cell action potentials was explored with a computer model. RESULTS: KCNA5 expression resulted in typical K currents that activated and inactivated voltage dependently. Ascending concentrations of AVE1231 (0.1-100 microM) led to concentration- and voltage-dependent current inhibition (IC50 at +40 mV: 2.0 +/- 0.5 microM, Hill coefficient 0.69 +/- 0.12). Acceleration of hKv1.5 current inactivation occurred with increasing AVE1231 concentrations, indicating channel inhibition in the open state (eg, taufast at +40 mV: 318 +/- 92 milliseconds under control; 14 +/- 1 milliseconds with 3 microM, P < 0.05). Using 1/taufast as an approximation of the time course of drug-channel interaction, association rate (K+1) and dissociation rate (K-1) constants were 8.18 x 10 M/s and 45.95 seconds, respectively (KD = 5.62 microM). The onset of current inhibition occurred more rapidly with higher concentrations along with a prominent tail current crossover phenomenon after AVE1231 application. Drug inhibition remained effective through a range of stimulation frequencies. Computer modeling suggested more pronounced prolongation of action potential duration under conditions of atrial remodeling. CONCLUSION: AVE1231 is an inhibitor of hKv1.5 currents with predominant action on channels in their open state; thus, it may be suitable for the treatment of AF.     

Effects of oxypeucedanin on hKv1.5 and action potential duration

A furocoumarin derivative, oxypeucedanin, was purified from Angelica dahurica, and its effects on the human Kv1.5 (hKv1.5) channel and on the cardiac action potential duration (APD), were examined using the patch-clamp technique and the conventional microelectrode technique. Oxypeucedanin inhibited the hKv1.5 current in a concentration-dependent manner, with an IC(50) value of 76 nM, while it had no effect on human eag-related gene (HERG) current. Oxypeucedanin induced an initial fast decline of hKv1.5 current during depolarizations. The inhibition of hKv1.5 channel by oxypeucedanin was voltage-dependent, especially at depolarizing pulses between -40 and 0 mV which corresponds to the voltage range of the channel's opening. Oxypeucedanin also slowed the deactivation time course, resulting in a tail crossover phenomenon. Additionally, oxypeucedanin prolonged the APD of rat atrial and ventricular muscles in a dose-dependent manner. These results suggest that oxypeucedanin is a kind of open-channel blocker of the hKv1.5 channel and it prolongs the APD; therefore, it is an excellent candidate as an antiarrhythmic drug for atrial fibrillation.     

Chlorthalidone inhibits the KvLQT1 potassium current in guinea-pig ventricular myocytes and oocytes from Xenopus laevis

BACKGROUND AND PURPOSE: Chlorthalidone is used for the treatment of hypertension as it produces a lengthening of the cardiac action potential. However, there is no experimental evidence that chlorthalidone has electrophysiological effects on the potassium currents involved in cardiac repolarization. EXPERIMENTAL APPROACH: Ventricular myocytes and oocytes, transfected with human ionic channels that produce IK current, were exposed to different concentrations of chlorthalidone. Action potentials and potassium currents were recorded using a patch clamp technique. To determine which component of the current was affected by chlorthalidone, human channel proteins (hERG, minK and KvLQT1) were used. KEY RESULTS: Chlorthalidone prolonged the ventricular action potential at 50 and 90% by 13 and 14%, respectively. The cardiac potassium currents I(to) and IK(1) were not affected by chlorthalidone at any concentration, whereas the delayed rectifier potassium current, IK, was blocked in a dose-response, voltage-independent fashion. In our preparation, 100 microM chlorthalidone blocked the two components of the delayed rectifier potassium current with the same potency (50.1+/-5% for IK(r) and 54.6+/-6% for IK(s)) (n=7, P<0.05). The chlorthalidone-sensitive current was slow and saturated at potentials greater than +30 mV. In our conditions only the KvLQT1 potassium current was affected by the drug, by 14%. CONCLUSIONS AND IMPLICATIONS: Chlorthalidone was demonstrated to have a direct effect on cardiac ventricular myocytes; it blocked the delayed rectifier potassium current (IK), specifically the KvLQT1 component of the potassium current. These results indicate that it has potential for use as an antiarrhythmic but further studies are needed.     

Inhibitory effect of bepridil on hKv1.5 channel current: comparison with amiodarone and E-4031

Effects of bepridil on the depolarization-activated outward K⁺ currents (I_out) in rat atrial myocytes and the human cardiac K⁺ (hKv1.5) channel current stably expressed in human embryonic kidney (HEK) 293 cells were examined, and compared with those of amiodarone and N-[4-[[1-[2-(6-methyl-2-pyridinyl)ethyl]-4-piperidinyl]carbonyl]phenyl] methanesulphonamide dihydrochloride dihydrate (E-4031). Membrane currents were recorded using patch-clamp techniques in enzymatically isolated rat atrial myocytes and HEK 293 cells expressing hKv1.5 channels. Bepridil potently inhibited I_out elicited by depolarization pulses and prolonged the action potential in rat atrial cells. Bepridil also inhibited the hKv1.5 channel current with the IC₅₀ value of 6.6 μM. The inhibitory effects of bepridil on the currents in HEK 293 cells were voltage-dependent. Amiodarone weakly inhibited rat atrial I_out and hKv1.5 channel current. In contrast, E-4031 at a concentration of 10 μM had little influence on these currents. Thus, bepridil inhibits hKv1.5 channel current and the inhibitory effect may be useful for the treatment of atrial fibrillation.     

Synthesis of psoralen derivatives and their blocking effect of hKv1.5 channel

Previously, we found that a furocoumarin derivative, psoralen (7H-furo[3,2-g][1]benzopyran-7-one), blocked a human Kv1.5 potassium channel (hKv1.5) and has a potential antiarrhythmic effect. In the present study, to develop more potent hKv1.5 blockers or antiarrhythmic drugs, we synthesized ten psoralen derivatives and examined their blocking effects on hKv1.5 stably expressed in Ltk cells. Among the newly synthesized psoralen derivatives, three derivatives (Compounds 5, 9 and 10) showed the open channel-blocking effect. Compound 9 among them was the most potent in blocking hKv1.5. We found that compound 9, one of the psoralen derivatives, inhibited the hKv1.5 current in a concentration-, use- and voltage-dependent manner with an IC50 value of 27.4 +/- 5.1 nM at +60 mV. Compound 9 accelerated the inactivation kinetics of the hKv1.5 channel, slowed the deactivation kinetics of hKv1.5 current resulting in a tail crossover phenomenon. Compound 9 inhibited hKv1.5 current in a use-dependent manner. These results indicate that compound 9, one of psoralen derivatives, acts on hKv1.5 channel as an open channel blocker and is much more potent than psoralen in blocking hKv1.5 channel. If further studies were done, compound 9 might be an ideal antiarrhythmic drug for atrial fibrillation.