Class III antiarrhythmic effects of zatebradine. Time-, state-, use-, and voltage-dependent block of hKv1.5 channels

Screening effects of HCN channel blockers on sleep/wake behavior in zebrafish

Hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels generate electrical rhythmicity in various tissues although primarily heart, retina and brain. The HCN channel blocker compound, Ivabradine (Corlanor), is approved by the US Food and Drug Administration (FDA) as a medication to lower heart rate by blocking hyperpolarization activated inward current in the sinoatrial node. In addition, a growing body of evidence suggests a role for HCN channels in regulation of sleep/wake behavior. Zebrafish larvae are ideal model organisms for high throughput drug screening, drug repurposing and behavioral phenotyping studies. We leveraged this model system to investigate effects of three HCN channel blockers (Ivabradine, Zatebradine Hydrochloride and ZD7288) at multiple doses on sleep/wake behavior in wild type zebrafish. Results of interest included shorter latency to daytime sleep at 0.1 μM dose of Ivabradine (ANOVA, p: 0.02), moderate reduction in average activity at 30 μM dose of Zatebradine Hydrochloride (ANOVA, p: 0.024) in daytime, and increased nighttime sleep at 4.5 μM dose of ZD7288 (ANOVA, p: 0.036). Taken together, shorter latency to daytime sleep, decrease in daytime activity and increased nighttime sleep indicate that different HCN channel antagonists affected different parameters of sleep and activity.

Ih current stabilizes excitability in rodent DRG neurons and reverses hyperexcitability in a nociceptive neuron model of inherited neuropathic pain

We show here that hyperpolarization-activated current (Ih ) unexpectedly acts to inhibit the activity of dorsal root ganglion (DRG) neurons expressing WT Nav1.7, the largest inward current and primary driver of DRG neuronal firing, and hyperexcitable DRG neurons expressing a gain-of-function Nav1.7 mutation that causes inherited erythromelalgia (IEM), a human genetic model of neuropathic pain. In this study we created a kinetic model of Ih and used it, in combination with dynamic-clamp, to study Ih function in DRG neurons. We show, for the first time, that Ih increases rheobase and reduces the firing probability in small DRG neurons, and demonstrate that the amplitude of subthreshold oscillations is reduced by Ih . Our results show that Ih , due to slow gating, is not deactivated during action potentials (APs) and has a striking damping action, which reverses from depolarizing to hyperpolarizing, close to the threshold for AP generation. Moreover, we show that Ih reverses the hyperexcitability of DRG neurons expressing a gain-of-function Nav1.7 mutation that causes IEM. In the aggregate, our results show that Ih unexpectedly has strikingly different effects in DRG neurons as compared to previously- and well-studied cardiac cells. Within DRG neurons where Nav1.7 is present, Ih reduces depolarizing sodium current inflow due to enhancement of Nav1.7 channel fast inactivation and creates additional damping action by reversal of Ih direction from depolarizing to hyperpolarizing close to the threshold for AP generation. These actions of Ih limit the firing of DRG neurons expressing WT Nav1.7 and reverse the hyperexcitability of DRG neurons expressing a gain-of-function Nav1.7 mutation that causes IEM. KEY POINTS: Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, the molecular determinants of hyperpolarization-activated current (Ih ) have been characterized as a 'pain pacemaker', and thus considered to be a potential molecular target for pain therapeutics. Dorsal root ganglion (DRG) neurons express Nav1.7, a channel that is not present in central neurons or cardiac tissue. Gain-of-function mutations (GOF) of Nav1.7 identified in inherited erythromelalgia (IEM), a human genetic model of neuropathic pain, produce DRG neuron hyperexcitability, which in turn produces severe pain. We found that Ih increases rheobase and reduces firing probability in small DRG neurons expressing WT Nav1.7, and demonstrate that the amplitude of subthreshold oscillations is reduced by Ih . We also demonstrate that Ih reverses the hyperexcitability of DRG neurons expressing a GOF Nav1.7 mutation (L858H) that causes IEM. Our results show that, in contrast to cardiac cells and CNS neurons, Ih acts to stabilize DRG neuron excitability and prevents excessive firing.

Effects of rosiglitazone, an antidiabetic drug, on Kv3.1 channels

Rosiglitazone is a thiazolidinedione-class antidiabetic drug that reduces blood glucose and glycated hemoglobin levels. We here investigated the interaction of rosiglitazone with Kv3.1 expressed in Chinese hamster ovary cells using the whole-cell patch-clamp technique. Rosiglitazone rapidly and reversibly inhibited Kv3.1 currents in a concentration-dependent manner (IC₅₀ = 29.8 μM) and accelerated the decay of Kv3.1 currents without modifying the activation kinetics. The rosiglitazone-mediated inhibition of Kv3.1 channels increased steeply in a sigmoidal pattern over the voltage range of -20 to +30 mV, whereas it was voltage-independent in the voltage range above +30 mV, where the channels were fully activated. The deactivation of Kv3.1 current, measured along with tail currents, was also slowed by the drug. In addition, the steady-state inactivation curve of Kv3.1 by rosiglitazone shifts to a negative potential without significant change in the slope value. All the results with the use dependence of the rosiglitazone-mediated blockade suggest that rosiglitazone acts on Kv3.1 channels as an open channel blocker.

The antidiabetic drug rosiglitazone blocks Kv1.5 potassium channels in an open state

An antidiabetic drug, rosiglitazone is a member of the drug class of thiazolidinedione. Although restrictions on use due to the possibility of heart toxicity have been removed, it is still a drug that is concerned about side effects on the heart. We here examined, using Chinese hamster ovary cells, the action of rosiglitazone on Kv1.5 channels, which is a major determinant of the duration of cardiac action potential. Rosiglitazone rapidly and reversibly inhibited Kv1.5 currents in a concentration-dependent manner (IC₅₀ = 18.9 µM) and accelerated the decay of Kv1.5 currents without modifying the activation kinetics. In addition, the deactivation of Kv1.5 current, assayed with tail current, was slowed by the drug. All of the results as well as the use-dependence of the rosiglitazone-mediated blockade indicate that rosiglitazone acts on Kv1.5 channels as an open channel blocker. This study suggests that the cardiac side effects of rosiglitazone might be mediated in part by suppression of Kv1.5 channels, and therefore, raises a concern of using the drug for diabetic therapeutics.

Bidirectional flow of the funny current (If) during the pacemaking cycle in murine sinoatrial node myocytes

Sinoatrial node myocytes (SAMs) act as cardiac pacemaker cells by firing spontaneous action potentials (APs) that initiate each heartbeat. The funny current (If) is critical for the generation of these spontaneous APs; however, its precise role during the pacemaking cycle remains unresolved. Here, we used the AP-clamp technique to quantify If during the cardiac cycle in mouse SAMs. We found that If is persistently active throughout the sinoatrial AP, with surprisingly little voltage-dependent gating. As a consequence, it carries both inward and outward current around its reversal potential of -30 mV. Despite operating at only 2 to 5% of its maximal conductance, If carries a substantial fraction of both depolarizing and repolarizing net charge movement during the firing cycle. We also show that β-adrenergic receptor stimulation increases the percentage of net depolarizing charge moved by If, consistent with a contribution of If to the fight-or-flight increase in heart rate. These properties were confirmed by heterologously expressed HCN4 channels and by mathematical models of If Modeling further suggested that the slow rates of activation and deactivation of the HCN4 isoform underlie the persistent activity of If during the sinoatrial AP. These results establish a new conceptual framework for the role of If in pacemaking, in which it operates at a very small fraction of maximal activation but nevertheless drives membrane potential oscillations in SAMs by providing substantial driving force in both inward and outward directions.

Membrane and calcium clock mechanisms contribute variably as a function of temperature to setting cardiac pacemaker rate in zebrafish Danio rerio

Here, we show that heart rate in zebrafish Danio rerio is dependent upon two pacemaking mechanisms and it possesses a limited ability to reset the cardiac pacemaker with temperature acclimation. Electrocardiogram recordings, taken from individual, anaesthetised zebrafish that had been acclimated to 18, 23 or 28°C were used to follow the response of maximum heart rate (f_Hmax ) to acute warming from 18°C until signs of cardiac failure appeared (up to c. 40°C). Because f_Hmax was similar across the acclimation groups at almost all equivalent test temperatures, warm acclimation was limited to one significant effect, the 23°C acclimated zebrafish had a significantly higher (21%) peak f_Hmax and reached a higher (3°C) test temperature than the 18°C acclimated zebrafish. Using zatebradine to block the membrane hyperpolarisation-activated cyclic nucleotide-gated channels (HCN) and examine the contribution of the membrane clock mechanisms to cardiac pacemaking, f _Hmax was significantly reduced (by at least 40%) at all acute test temperatures and significantly more so at most test temperatures for zebrafish acclimated to 28°C vs. 23°C. Thus, HCN channels and the membrane clock were not only important, but could be modified by thermal acclimation. Using a combination of ryanodine (to block sarcoplasmic calcium release) and thapsigargin (to block sarcoplasmic calcium reuptake) to examine the contribution of sarcoplasmic reticular handling of calcium and the calcium clock, f _Hmax was again consistently reduced independent of the test temperature and acclimation temperature, but to a significantly lesser degree than zatebradine for zebrafish acclimated to both 28 and 18°C. Thus, the calcium clock mechanism plays an additional role in setting pacemaker activity that was independent of temperature. In conclusion, the zebrafish cardiac pacemaker has a limited temperature acclimation ability compared with known effects for other fishes and involves two pacemaking mechanisms, one of which was independent of temperature.

Antidepressant drug paroxetine blocks the open pore of Kv3.1 potassium channel

In patients with epilepsy, depression is a common comorbidity but difficult to be treated because many antidepressants cause pro-convulsive effects. Thus, it is important to identify the risk of seizures associated with antidepressants. To determine whether paroxetine, a very potent selective serotonin reuptake inhibitor (SSRI), interacts with ion channels that modulate neuronal excitability, we examined the effects of paroxetine on Kv3.1 potassium channels, which contribute to highfrequency firing of interneurons, using the whole-cell patch-clamp technique. Kv3.1 channels were cloned from rat neurons and expressed in Chinese hamster ovary cells. Paroxetine reversibly reduced the amplitude of Kv3.1 current, with an IC₅₀ value of 9.43 µM and a Hill coefficient of 1.43, and also accelerated the decay of Kv3.1 current. The paroxetine-induced inhibition of Kv3.1 channels was voltage-dependent even when the channels were fully open. The binding (k₊₁) and unbinding (k₋₁) rate constants for the paroxetine effect were 4.5 µM⁻¹s⁻¹ and 35.8 s⁻¹, respectively, yielding a calculated K_D value of 7.9 µM. The analyses of Kv3.1 tail current indicated that paroxetine did not affect ion selectivity and slowed its deactivation time course, resulting in a tail crossover phenomenon. Paroxetine inhibited Kv3.1 channels in a usedependent manner. Taken together, these results suggest that paroxetine blocks the open state of Kv3.1 channels. Given the role of Kv3.1 in fast spiking of interneurons, our data imply that the blockade of Kv3.1 by paroxetine might elevate epileptic activity of neural networks by interfering with repetitive firing of inhibitory neurons.

The Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels: from Biophysics to Pharmacology of a Unique Family of Ion Channels

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels are important members of the voltage-gated pore loop channels family. They show unique features: they open at hyperpolarizing potential, carry a mixed Na/K current, and are regulated by cyclic nucleotides. Four different isoforms have been cloned (HCN1-4) that can assemble to form homo- or heterotetramers, characterized by different biophysical properties. These proteins are widely distributed throughout the body and involved in different physiologic processes, the most important being the generation of spontaneous electrical activity in the heart and the regulation of synaptic transmission in the brain. Their role in heart rate, neuronal pacemaking, dendritic integration, learning and memory, and visual and pain perceptions has been extensively studied; these channels have been found also in some peripheral tissues, where their functions still need to be fully elucidated. Genetic defects and altered expression of HCN channels are linked to several pathologies, which makes these proteins attractive targets for translational research; at the moment only one drug (ivabradine), which specifically blocks the hyperpolarization-activated current, is clinically available. This review discusses current knowledge about HCN channels, starting from their biophysical properties, origin, and developmental features, to (patho)physiologic role in different tissues and pharmacological modulation, ending with their present and future relevance as drug targets.

Development of correction formula for field potential duration of human induced pluripotent stem cell-derived cardiomyocytes sheets

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have been used in many studies to assess proarrhythmic risks of chemical compounds. In those studies, field potential durations (FPD) of hiPSC-CMs have been corrected by clinically used Fridericia's and/or Bazett's formulae, however, the rationale for the use of these formulae has not been well established. In the present study, we developed a correction formula for experiments using hiPSC-CMs. First, we analyzed the effect of beating rate on FPD in the hiPSC-CMs sheets with electrical stimuli and a HCN channel inhibitor zatebradine. Next, we examined the relationship between the electrophysiological properties and the expression levels of ion channel genes in the cell sheets. Zatebradine slowed the beating rate and allowed to analyze FPD changes at various pacing cycle lengths. Rate-dependent change in the repolarization period was smaller in the cell sheets than that reported on the human hearts, which can be partly explained by lower gene expression level of hKCNJ2 and hKCNE1. Thus, non-linear equation for correcting FPD in the cell sheet; FPDc = FPD/RR^0.22 with RR given in second was obtained, which may make it feasible to assess net repolarization delay by various chemical compounds with a chronotropic action.

Rate-Dependent Role of IKur in Human Atrial Repolarization and Atrial Fibrillation Maintenance

The atrial-specific ultrarapid delayed rectifier K⁺ current (I_Kur) inactivates slowly but completely at depolarized voltages. The consequences for I_Kur rate-dependence have not been analyzed in detail and currently available mathematical action-potential (AP) models do not take into account experimentally observed I_Kur inactivation dynamics. Here, we developed an updated formulation of I_Kur inactivation that accurately reproduces time-, voltage-, and frequency-dependent inactivation. We then modified the human atrial cardiomyocyte Courtemanche AP model to incorporate realistic I_Kur inactivation properties. Despite markedly different inactivation dynamics, there was no difference in AP parameters across a wide range of stimulation frequencies between the original and updated models. Using the updated model, we showed that, under physiological stimulation conditions, I_Kur does not inactivate significantly even at high atrial rates because the transmembrane potential spends little time at voltages associated with inactivation. Thus, channel dynamics are determined principally by activation kinetics. I_Kur magnitude decreases at higher rates because of AP changes that reduce I_Kur activation. Nevertheless, the relative contribution of I_Kur to AP repolarization increases at higher frequencies because of reduced activation of the rapid delayed-rectifier current I_Kr. Consequently, I_Kur block produces dose-dependent termination of simulated atrial fibrillation (AF) in the absence of AF-induced electrical remodeling. The inclusion of AF-related ionic remodeling stabilizes simulated AF and greatly reduces the predicted antiarrhythmic efficacy of I_Kur block. Our results explain a range of experimental observations, including recently reported positive rate-dependent I_Kur-blocking effects on human atrial APs, and provide insights relevant to the potential value of I_Kur as an antiarrhythmic target for the treatment of AF.

Regulation of human cardiac Kv1.5 channels by extracellular acidification

Human Kv1.5 channels (hKv1.5) conduct the ultra-rapid delayed rectifier potassium current (I _Kur), which plays an important role in action potential repolarization of atrial myocytes. The present study was undertaken to examine the effects of acidic pH on hKv1.5 wild-type (WT) and its pore mutant channels heterologously expressed in Chinese hamster ovary (CHO) cells using site-directed mutagenesis combined with whole-cell patch-clamp technique. Both extracellular and intracellular acidifications equally and reversely reduced the amplitude of hKv1.5 currents. The extracellular acidification significantly shifted the voltage dependence of current activation to more depolarized potentials and accelerated deactivation kinetics of the current. The ancillary β subunits Kvβ1.3 and Kvβ1.2, known to modify the pharmacological sensitivities of hKv1.5, enhanced the extracellular proton-induced inhibitory effect on hKv1.5 current. In addition, several mutants (T462C, T479A, T480A, and I508A) exhibited significantly higher sensitivity to acidic pH-induced inhibition compared with WT channel, whereas the inhibitory effect of acidic pH was markedly reduced in H463G mutant. These observations indicate that (1) extracellular acidification modifies hKv1.5 gating and activity, (2) β subunits and several residues (T462, T479, T480, and I508) play critical roles in determining the sensitivity of the channel to acidic exposure, and (3) H463 may be a critical sensor for the channel inhibition by extracellular protons.

Blockade of Kv1.5 channels by the antidepressant drug sertraline

Sertraline, a selective serotonin reuptake inhibitor (SSRI), has been reported to lead to cardiac toxicity even at therapeutic doses including sudden cardiac death and ventricular arrhythmia. And in a SSRI-independent manner, sertraline has been known to inhibit various voltage-dependent channels, which play an important role in regulation of cardiovascular system. In the present study, we investigated the action of sertraline on Kv1.5, which is one of cardiac ion channels. The eff ect of sertraline on the cloned neuronal rat Kv1.5 channels stably expressed in Chinese hamster ovary cells was investigated using the whole-cell patch-clamp technique. Sertraline reduced Kv1.5 whole-cell currents in a reversible concentration-dependent manner, with an IC 50 value and a Hill coefficient of 0.71 µM and 1.29, respectively. Sertraline accelerated the decay rate of inactivation of Kv1.5 currents without modifying the kinetics of current activation. The inhibition increased steeply between -20 and 0 mV, which corresponded with the voltage range for channel opening. In the voltage range positive to +10 mV, inhibition displayed a weak voltage dependence, consistent with an electrical distance δ of 0.16. Sertraline slowed the deactivation time course, resulting in a tail crossover phenomenon when the tail currents, recorded in the presence and absence of sertraline, were superimposed. Inhibition of Kv1.5 by sertraline was use-dependent. The present results suggest that sertraline acts on Kv1.5 currents as an open-channel blocker.

Blockade of Kv1.5 by paroxetine, an antidepressant drug

Paroxetine, a selective serotonin reuptake inhibitor (SSRI), has been reported to have an effect on several ion channels including human ether-a-go-go-related gene in a SSRI-independent manner. These results suggest that paroxetine may cause side effects on cardiac system. In this study, we investigated the effect of paroxetine on Kv1.5, which is one of cardiac ion channels. The action of paroxetine on the cloned neuronal rat Kv1.5 channels stably expressed in Chinese hamster ovary cells was investigated using the whole-cell patch-clamp technique. Paroxetine reduced Kv1.5 whole-cell currents in a reversible concentration-dependent manner, with an IC 50 value and a Hill coefficient of 4.11 µM and 0.98, respectively. Paroxetine accelerated the decay rate of inactivation of Kv1.5 currents without modifying the kinetics of current activation. The inhibition increased steeply between -30 and 0 mV, which corresponded with the voltage range for channel opening. In the voltage range positive to 0 mV, inhibition displayed a weak voltage dependence, consistent with an electrical distance δ of 0.32. The binding (k+1) and unbinding (k-1) rate constants for paroxetine-induced block of Kv1.5 were 4.9 µM(-1)s(-1) and 16.1 s(-1), respectively. The theoretical K D value derived by k-1/k+1 yielded 3.3 µM. Paroxetine slowed the deactivation time course, resulting in a tail crossover phenomenon when the tail currents, recorded in the presence and absence of paroxetine, were superimposed. Inhibition of Kv1.5 by paroxetine was use-dependent. The present results suggest that paroxetine acts on Kv1.5 currents as an open-channel blocker.

Putative binding sites for arachidonic acid on the human cardiac Kv 1.5 channel

BACKGROUND AND PURPOSE

In human heart, the Kv 1.5 channel contributes to repolarization of atrial action potentials. This study examined the electrophysiological and molecular mechanisms underlying arachidonic acid (AA)-induced inhibition of the human Kv 1.5 (hKv 1.5) channel.

EXPERIMENTAL APPROACH

Site-directed mutagenesis was conducted to mutate amino acids that reside within the pore domain of the hKv 1.5 channel. Whole-cell patch-clamp method was used to record membrane currents through wild type and mutant hKv 1.5 channels heterologously expressed in CHO cells. Computer docking simulation was conducted to predict the putative binding site(s) of AA in an open-state model of the Kv 1.5 channel.

KEY RESULTS

The hKv 1.5 current was minimally affected at the onset of depolarization but was progressively reduced during depolarization by the presence of AA, suggesting that AA acts as an open-channel blocker. AA itself affected the channel at extracellular sites independently of its metabolites and signalling pathways. The blocking effect of AA was attenuated at pH 8.0 but not at pH 6.4. The blocking action of AA developed rather rapidly by co-expression of Kv β1.3. The AA-induced block was significantly attenuated in H463C, T480A, R487V, I502A, I508A, V512A and V516A, but not in T462C, A501V and L510A mutants of the hKv 1.5 channel. Docking simulation predicted that H463, T480, R487, I508, V512 and V516 are potentially accessible for interaction with AA.

CONCLUSIONS AND IMPLICATIONS

AA itself interacts with multiple amino acids located in the pore domain of the hKv 1.5 channel. These findings may provide useful information for future development of selective blockers of hKv 1.5 channels.

Elisidepsin Interacts Directly with Glycosylceramides in the Plasma Membrane of Tumor Cells to Induce Necrotic Cell Death

Plasma membrane integrity is essential for cell life. Any major break on it immediately induces the death of the affected cell. Different molecules were described as disrupting this cell structure and thus showing antitumor activity. We have previously defined that elisidepsin (Irvalec^®, PM02734) inserts and self-organizes in the plasma membrane of tumor cells, inducing a rapid loss of membrane integrity, cell permeabilization and necrotic death. Here we show that, in sensitive HCT-116 colorectal cells, all these effects are consequence of the interaction of elisidepsin with glycosylceramides in the cell membrane. Of note, an elisidepsin-resistant subline (HCT-116-Irv) presented reduced levels of glycosylceramides and no accumulation of elisidepsin in the plasma membrane. Consequently, drug treatment did not induce the characteristic necrotic cell death. Furthermore, GM95, a mutant derivative from B16 mouse melanoma cells lacking ceramide glucosyltransferase (UGCG) activity and thus the synthesis of glycosylceramides, was also resistant to elisidepsin. Over-expression of UGCG gene in these deficient cells restored glycosylceramides synthesis, rendering them sensitive to elisidepsin, at a similar level than parental B16 cells. These results indicate that glycosylceramides act as membrane targets of elisidepsin, facilitating its insertion in the plasma membrane and the subsequent membrane permeabilization that leads to drug-induced cell death. They also indicate that cell membrane lipids are a plausible target for antineoplastic therapy.

Interaction of propofol with voltage-gated human Kv1.5 channel through specific amino acids within the pore region

The intravenous anesthetic propofol affects the function of a diversity of ligand-gated and voltage-gated ion channels. However, there is little information as to whether propofol directly interacts with voltage-gated ion channel proteins to modulate their functions. The Kv1.5 channel is activated by membrane depolarization during action potentials and contributes to atrial repolarization in the human heart. This study was undertaken to examine the effect of propofol on voltage-gated human Kv1.5 (hKv1.5) channel and to elucidate the underlying molecular determinants. Site-directed mutagenesis was carried out through six amino acids that reside within the pore domain of hKv1.5 channel. Whole-cell patch-clamp technique was used to record membrane currents through the wild type and mutant hKv1.5 channels heterologously expressed in Chinese hamster ovary cells. Propofol (≥5 μM) reversibly and concentration-dependently (IC50 of 49.3±9.4 μM; n=6) blocked hKv1.5 current. Propofol-induced block of hKv1.5 current gradually progressed during depolarizing voltage-clamp steps and was enhanced by higher frequency of activation, consistent with a preferential block of the channels in their open state. The degree of current block by propofol was significantly attenuated in T480A, I502A, I508A and V516A, but not in H463C and L510A mutants of hKv1.5 channel. Thus, several amino acids near the selectivity filter (Thr480) or within S6 (Ile502, Ile508 and Val516) are found to be critically involved in the blocking action of propofol. This study provides the first evidence suggesting that direct interaction with specific amino acids underlies the blocking action of propofol on voltage-gated hKv1.5 channel.

Polyunsaturated Fatty acids modify the gating of kv channels

Polyunsaturated fatty acids (PUFAs) have been reported to exhibit antiarrhythmic properties, which are attributed to their capability to modulate ion channels. This PUFAs ability has been reported to be due to their effects on the gating properties of ion channels. In the present review, we will focus on the role of PUFAs on the gating of two Kv channels, Kv1.5 and Kv11.1. Kv1.5 channels are blocked by n-3 PUFAs of marine [docosahexaenoic acid (DHA) and eicosapentaenoic acid] and plant origin (alpha-linolenic acid, ALA) at physiological concentrations. The blockade of Kv1.5 channels by PUFAs steeply increased in the range of membrane potentials coinciding with those of Kv1.5 channel activation, suggesting that PUFAs-channel binding may derive a significant fraction of its voltage sensitivity through the coupling to channel gating. A similar shift in the activation voltage was noted for the effects of n-6 arachidonic acid (AA) and DHA on Kv1.1, Kv1.2, and Kv11.1 channels. PUFAs-Kv1.5 channel interaction is time-dependent, producing a fast decay of the current upon depolarization. Thus, Kv1.5 channel opening is a prerequisite for the PUFA-channel interaction. Similar to the Kv1.5 channels, the blockade of Kv11.1 channels by AA and DHA steeply increased in the range of membrane potentials that coincided with the range of Kv11.1 channel activation, suggesting that the PUFAs-Kv channel interactions are also coupled to channel gating. Furthermore, AA regulates the inactivation process in other Kv channels, introducing a fast voltage-dependent inactivation in non-inactivating Kv channels. These results have been explained within the framework that AA closes voltage-dependent potassium channels by inducing conformational changes in the selectivity filter, suggesting that Kv channel gating is lipid dependent.

Novel blockers of hyperpolarization-activated current with isoform selectivity in recombinant cells and native tissue

BACKGROUND AND PURPOSE Selective hyperpolarization activated, cyclic nucleotide-gated channel (HCN) blockers represent an important therapeutic goal due to the wide distribution and multiple functions of these proteins, representing the molecular correlate of f- and h-current (I(f) or I(h) ). Recently, new compounds able to block differentially the homomeric HCN isoforms expressed in HEK293 have been synthesized. In the present work, the electrophysiological and pharmacological properties of these new HCN blockers were characterized and their activities evaluated on native channels. EXPERIMENTAL APPROACH HEK293 cells expressing mHCN1, mHCN2 and hHCN4 isoforms were used to verify channel blockade. Selected compounds were tested on native guinea pig sinoatrial node cells and neurons from mouse dorsal root ganglion (DRG) by patch-clamp recordings and on dog Purkinje fibres by intracellular recordings. KEY RESULTS In HEK293 cells, EC18 was found to be significantly selective for HCN4 and MEL57A for HCN1 at physiological membrane potential. When tested on guinea pig sinoatrial node cells, EC18 (10 µM) maintained its activity, reducing I(f) by 67% at -120 mV, while MEL57A (3 µM) reduced I(f) by 18%. In contrast, in mouse DRG neurons, only MEL57A (30 and 100 µM) significantly reduced I(h) by 60% at -80 mV. In dog cardiac Purkinje fibres, EC18, but not MEL57A, reduced the amplitude and slowed the slope of the spontaneous diastolic depolarization. CONCLUSIONS Our results have identified novel and highly selective HCN isoform blockers, EC18 and MEL57A; the selectivity found in recombinant system was maintained in various tissues expressing different HCN isoforms.

Molecular identity, ontogeny, and cAMP modulation of the hyperpolarization-activated current in vestibular ganglion neurons

Properties, developmental regulation, and cAMP modulation of the hyperpolarization-activated current (I(h)) were investigated by the whole cell patch-clamp technique in vestibular ganglion neurons of the rat at two postnatal stages (P7-10 and P25-28). In addition, by RT-PCR and immunohistochemistry the identity and distribution of hyperpolarization-activated and cyclic nucleotide-gated channel (HCN) isoforms that generate I(h) were investigated. I(h) current density was larger in P25-28 than P7-10 rats, increasing 410% for small cells (<30 pF) and 200% for larger cells (>30 pF). The half-maximum activation voltage (V(1/2)) of I(h) was -102 mV in P7-10 rats and in P25-28 rats shifted 7 mV toward positive voltages. At both ages, intracellular cAMP increased I(h) current density, decreased its activation time constant (τ), and resulted in a rightward shift of V(1/2) by 9 mV. Perfusion of 8-BrcAMP increased I(h) amplitude and speed up its activation kinetics. I(h) was blocked by Cs(+), zatebradine, and ZD7288. As expected, these drugs also reduced the voltage sag caused with hyperpolarizing pulses and prevented the postpulse action potential generation without changes in the resting potential. RT-PCR analysis showed that HCN1 and HCN2 subunits were predominantly amplified in vestibular ganglia and end organs and HCN3 and HCN4 to a lesser extent. Immunohistochemistry showed that the four HCN subunits were differentially expressed (HCN1 > HCN2 > HCN3 ≥ HCN4) in ganglion slices and in cultured neurons at both P7-10 and P25-28 stages. Developmental changes shifted V(1/2) of I(h) closer to the resting membrane potential, increasing its functional role. Modulation of I(h) by cAMP-mediated signaling pathway constitutes a potentially relevant control mechanism for the modulation of afferent neuron discharge.

Carvedilol blocks the cloned cardiac Kv1.5 channels in a β-adrenergic receptor-independent manner

Carvedilol, a non-selective β-adrenergic blocker, is widely used for the treatment of angina pectoris and hypertension. We examined the action of carvedilol on cloned Kv1.5 expressed in CHO cells, using the whole-cell patch clamp technique. Carvedilol reduced the peak amplitude of Kv1.5 and accelerated the inactivation rate in a concentration-dependent manner with an IC50 of 2.56 μM. Using a first-order kinetics analysis, we calculated k(+1) = 19.68 μM(-1)s(-1) for the association rate constant, and k(-1) = 44.89 s(-1) for the dissociation rate constant. The apparent K(D) (k(-1)/k(+1)) was 2.28 μM, which is similar to the IC50 value. Other β-adrenergic blockers (alprenolol, oxprenolol and carteolol) had little or no effect on Kv1.5 currents. Carvedilol slowed the deactivation time course, resulting in a tail crossover phenomenon. Carvedilol-induced block was voltage-dependent in the voltage range for channel activation, but voltage-independent in the voltage range for full activation. The voltage dependences for both steady-state activation and inactivation were unchanged by carvedilol. Carvedilol affected Kv1.5 in a use-dependent manner. When stimulation frequencies were increased to quantify a use-dependent block, however, the block by carvedilol was slightly increased with IC50 values of 2.56 μM at 0.1 Hz, 2.38 μM at 1 Hz and 2.03 μM at 2 Hz. Carvedilol also slowed the time course of recovery from inactivation of Kv1.5. These results indicate that carvedilol blocks Kv1.5 in a reversible, concentration-, voltage-, time-, and use-dependent manner, but only at concentrations slightly higher than therapeutic plasma concentrations in humans. These effects are probably relevant to an understanding of the ionic mechanism underlying the antiarrhythmic property of carvedilol.

Acacetin causes a frequency- and use-dependent blockade of hKv1.5 channels by binding to the S6 domain

We have demonstrated that the natural flavone acacetin selectively inhibits ultra-rapid delayed rectifier potassium current (I(Kur)) in human atria. However, molecular determinants of this ion channel blocker are unknown. The present study was designed to investigate the molecular determinants underlying the ability of acacetin to block hKv1.5 channels (coding I(Kur)) in human atrial myocytes using the whole-cell patch voltage-clamp technique to record membrane current in HEK 293 cells stably expressing the hKv1.5 gene or transiently expressing mutant hKv1.5 genes generated by site-directed mutagenesis. It was found that acacetin blocked hKv1.5 channels by binding to both closed and open channels. The blockade of hKv1.5 channels by acacetin was use- and frequency-dependent, and the IC(50) of acacetin for inhibiting hKv1.5 was 3.5, 3.1, 2.9, 2.1, and 1.7μM, respectively, at 0.2, 0.5, 1, 3, and 4Hz. The mutagenesis study showed that the hKv1.5 mutants V505A, I508A, and V512A in the S6-segment remarkably reduced the channel blocking properties by acacetin (IC(50), 29.5μM for V505A, 19.1μM for I508A, and 6.9μM for V512A). These results demonstrate the novel information that acacetin mainly blocks open hKv1.5 channels by binding to their S6 domain. The use- and rate-dependent blocking of hKv1.5 by acacetin is beneficial for anti-atrial fibrillation.

Ceramide inhibits Kv currents and contributes to TP-receptor-induced vasoconstriction in rat and human pulmonary arteries

Neutral sphingomyelinase (nSMase)-derived ceramide has been proposed as a mediator of hypoxic pulmonary vasoconstriction (HPV), a specific response of the pulmonary circulation. Voltage-gated K+ (Kv) channels are modulated by numerous vasoactive factors, including hypoxia, and their inhibition has been involved in HPV. Herein, we have analyzed the effects of ceramide on Kv currents and contractility in rat pulmonary arteries (PA) and in mesenteric arteries (MA). The ceramide analog C6-ceramide inhibited Kv currents in PA smooth muscle cells (PASMC). Similar effects were obtained after the addition of bacterial sphingomyelinase (SMase), indicating a role for endogenous ceramide in Kv channel regulation. Kv current was reduced by stromatoxin and diphenylphosphine oxide-1 (DPO-1), selective inhibitors of Kv2.1 and Kv1.5 channels, respectively. The inhibitory effect of ceramide was still present in the presence of stromatoxin or DPO-1, suggesting that this sphingolipid inhibited both components of the native Kv current. Accordingly, ceramide inhibited Kv1.5 and Kv2.1 channels expressed in Ltk− cells. Ceramide-induced effects were reduced in human embryonic kidney 293 cells expressing Kv1.5 channels but not the regulatory subunit Kvβ2.1. The nSMase inhibitor GW4869 reduced the thromboxane-endoperoxide receptor agonist U46619-induced, but not endothelin-1-induced pulmonary vasoconstriction that was partly restored after addition of exogenous ceramide. The PKC-ζ pseudosubstrate inhibitor (PKCζ-PI) inhibited the Kv inhibitory and contractile effects of ceramide. In MA ceramide had no effect on Kv currents and GW4869 did not affect U46619-induced contraction. The effects of SMase were also observed in human PA. These results suggest that ceramide represents a crucial signaling mediator in the pulmonary vasculature.

Irvalec inserts into the plasma membrane causing rapid loss of integrity and necrotic cell death in tumor cells

Irvalec is a marine-derived antitumor agent currently undergoing phase II clinical trials. In vitro, Irvalec induces a rapid loss of membrane integrity in tumor cells, accompanied of a significant Ca(2+) influx, perturbations of membrane conductivity, severe swelling and the formation of giant membranous vesicles. All these effects are not observed in Irvalec-resistant cells, or are significantly delayed by pretreating the cells with Zn(2+). Using fluorescent derivatives of Irvalec it was demonstrated that the compound rapidly interacts with the plasma membrane of tumor cells promoting lipid bilayer restructuration. Also, FRET experiments demonstrated that Irvalec molecules localize in the cell membrane close enough to each other as to suggest that the compound could self-organize, forming supramolecular structures that likely trigger cell death by necrosis through the disruption of membrane integrity.

Inhibitory action of fluvoxamine on Kv1.5 currents

The voltage-gated K(+) channel Kv1.5 was expressed in Chinese hamster ovary cells, and its interaction with fluvoxamine was studied using a whole-cell patch-clamp technique. Fluvoxamine reduced Kv1.5 whole-cell currents in a reversible, concentration-dependent manner, with an IC(50) value of 2.0 microM, and a Hill coefficient of 0.7. Fluvoxamine-induced inhibition of Kv1.5 caused a time-dependent blockade without modifying the kinetics of current activation. Fluvoxamine additionally inhibited Kv1.5 in a closed/resting or nonconducting state after deactivation. Inhibition increased steeply between -30 and 0 mV, corresponding to the voltage range for channel opening. Inhibition displayed an additional voltage dependence at voltages greater than 0 mV, consistent with an electrical distance of 0.16. Fluvoxamine slowed the deactivation time course and a tail crossover phenomenon was observed when the tail currents, recorded in the presence and absence of fluvoxamine, were superimposed. Inhibition of Kv1.5 by fluvoxamine was use-dependent. These results suggest that fluvoxamine strongly inhibits Kv1.5 currents and the inhibition of Kv1.5 by fluvoxamine is mixed channel-state dependent.

Open channel block of Kv1.5 currents by citalopram

AIM

To examine whether selective serotonin reuptake inhibitor citalopram interacts with Kv1.5, one of the cardiovascular-specific Kv channel isoforms.

METHODS

The interaction between citalopram and Kv1.5 expressed in Chinese hamster ovary cells was studied using the whole-cell patch-clamp technique.

RESULTS

Citalopram reduced Kv1.5 whole-cell currents in a reversible concentration-dependent manner, with an IC(50) value and a Hill coefficient of 2.8+/-1.1 micromol/L and 0.8+/-0.3, respectively. Citalopram-induced inhibition of Kv1.5 is associated with time-dependent development of block without modifying the kinetics of current activation. The inhibition increased steeply between -30 and 0 mV, which corresponded with the voltage range for channel opening. In the voltage range positive to 0 mV, inhibition displayed an additional voltage dependence, consistent with an electrical distance delta of 0.19. Citalopram slowed the deactivation time course, resulting in a tail crossover phenomenon when the tail currents, recorded in the presence and absence of citalopram, were superimposed. Inhibition of Kv1.5 by citalopram was use-dependent.

CONCLUSION

The present results suggest that citalopram acts on Kv1.5 currents as an open-channel blocker, and much caution about arrhythmogenic risk is required when using citalopram in the treatment with depressed patients.

I(f) channels as a therapeutic target in heart disease

In the normal heart, impulses are generated from the sinoatrial node. It is generally accepted that the pacemaker current, I(f), plays a major role in the spontaneous rhythmic activity. Recently, several electrophysiological and molecular data demonstrate that I(f) channels are present in embryonic and post-natal ventricular myocytes and undergo a downregulation during maturation. Interestingly, the I(f) current is re-expressed in some pathological conditions such as cardiac hypertrophy and heart failure. In these conditions, the overexpression of f-channels is a consequence of electrophysiological remodeling and may represent an arrhythmogenic mechanism in heart failure, a condition associated with high risk for sudden cardiac death. For its physiological and pathophysiological role and the availability of selective f-channel blockers, I(f) may be a suitable therapeutic target in heart failure.

I(Kur)/Kv1.5 channel blockers for the treatment of atrial fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia. Anti-arrhythmic drugs remain the mainstay of therapy, but the available class I and III anti-arrhythmic drugs are only moderately effective in long-term restoring/maintaining sinus rhythm (SR) and can produce potentially fatal ventricular pro-arrhythmia. In an attempt to identify safer and more effective anti-arrhythmic drugs, drug discovery efforts have focused on 'atrial selective drugs' that target cardiac ion channel(s) that are exclusively or predominantly expressed in the atria. The ultra-rapid activating delayed rectifier K(+) current (I(Kur)), carried by Kv1.5 channels, is a major repolarizing current in human atria, but seems to play no role in the ventricle. This finding offers the possibility of developing selective I(Kur) blockers to restore and maintain SR without a risk of ventricular pro-arrhythmia. Several I(Kur) blockers are now being developed but clinical data are still limited, so the precise role of these agents in the treatment of AF remains to be defined. In this review we analyze the possible advantages and disadvantages of the developmental I(Kur) blockers as they represent the first step for the development of potential atrial selective drugs for a more effective and safer treatment and prevention of AF.

Effect of psoralen on the cloned Kv3.1 currents

The psoralen, a furocoumarin derivative, on the cloned neuronal rat Kv3.1 channels stably expressed in Chinese hamster ovary cells was investigated using the whole-cell patch-clamp technique. Psoralen reduced Kv3.1 whole-cell currents in a reversible concentration-dependent manner, with an IC50 value and a Hill coefficient of 2.3 +/- 0.03 microM and 0.9 +/- 0.08, respectively. Psoralen accelerated the decay rate of inactivation of Kv3.1 currents without modifying the kinetics of current activation. The psoralen-induced inhibition of Kv3.1 channels was voltage-dependent, with a steep increase over the voltage range of channel opening. However, the inhibition exhibited voltage independence over the voltage range in which channels are fully activated. Psoralen slowed the deactivation time course, resulting in a tail crossover phenomenon when the tail currents, recorded in the presence and absence of psoralen, were superimposed. Inhibition of Kv3.1 by psoralen was use-dependent at a frequency of 1 Hz. The present results suggest that psoralen acts on Kv3.1 currents as an open-channel blocker.

Inhibitory actions of the phosphatidylinositol 3-kinase inhibitor LY294002 on the human Kv1.5 channel

BACKGROUND AND PURPOSE

Kv1.5 channels conduct the ultra-rapid delayed rectifier potassium current (I(Kur)), and in humans, Kv1.5 channels are highly expressed in cardiac atria but are scarce in ventricles. Pharmacological blockade of human Kv1.5 (hKv1.5) has been regarded as effective for prevention and treatment of re-entry-based atrial tachyarrhythmias. Here we examined blockade of hKv1.5 channels by LY294002, a well-known inhibitor of phosphatidylinositol 3-kinase (PI3K).

EXPERIMENTAL APPROACH

hKv1.5 channels were heterologously expressed in Chinese hamster ovary cells. Effects of LY294002 on wild-type and mutant (T462C, H463C, T480A, R487V, A501V, I502A, I508A, L510A and V516A) hKv1.5 channels were examined by using the whole-cell patch-clamp method.

KEY RESULTS

LY294002 rapidly and reversibly inhibited hKv1.5 current in a concentration-dependent manner (IC(50) of 7.9 micromol.L(-1)). In contrast, wortmannin, a structurally distinct inhibitor of PI3K, had little inhibitory effect on hKv1.5 current. LY294002 block of hKv1.5 current developed with time during depolarizing voltage-clamp steps, and this blockade was also voltage-dependent with a steep increase over the voltage range for channel openings. The apparent binding (k(+1)) and unbinding (k(-1)) rate constants were calculated to be 1.6 micromol.L(-1) (-1).s(-1) and 5.7 s(-1) respectively. Inhibition by LY294002 was significantly reduced in several hKv1.5 mutant channels: T480A, R487V, I502A, I508A, L510A and V516A.

CONCLUSIONS AND IMPLICATIONS

LY294002 acts directly on hKv1.5 currents as an open channel blocker, independently of its effects on PI3K activity. Amino acid residues located in the pore region (Thr480, Arg487) and the S6 segment (Ile502, Ile508, Leu510, Val516) appear to constitute potential binding sites for LY294002.

Open channel block of Kv3.1 currents by fluoxetine

The action of fluoxetine, a serotonin reuptake inhibitor, on the cloned neuronal rat Kv3.1 channels stably expressed in Chinese hamster ovary cells was investigated using the whole-cell patch-clamp technique. Fluoxetine reduced Kv3.1 whole-cell currents in a reversible, concentration-dependent manner, with an IC(50) value and a Hill coefficient of 13.4 muM and 1.4, respectively. Fluoxetine accelerated the decay rate of inactivation of Kv3.1 currents without modifying the kinetics of current activation. The inhibition increased steeply between 0 and +30 mV, which corresponded with the voltage range for channel opening. In the voltage range positive to +30 mV, inhibition displayed a weak voltage dependence, consistent with an electrical distance delta of 0.38. The binding (k(+1)) and dissociation (k(-1)) rate constants for fluoxetine-induced block of Kv3.1 were 5.7 microM(-1)s(-1) and 53.5 s(-1), respectively. The theoretical K(D) value derived by k(-1)/k(+1) yielded 9.3 microM. Fluoxetine did not affect the ion selectivity of Kv3.1. Fluoxetine slowed the deactivation time course, resulting in a tail crossover phenomenon when the tail currents, recorded in the presence and absence of fluoxetine, were superimposed. Inhibition of Kv3.1 by fluoxetine was use-dependent. The present results suggest that fluoxetine acts on Kv3.1 currents as an open-channel blocker.

The vicissitudes of the pacemaker current I Kdd of cardiac purkinje fibers

The mechanisms underlying the pacemaker current in cardiac tissues is not agreed upon. The pacemaker potential in Purkinje fibers has been attributed to the decay of the potassium current I (Kdd). An alternative proposal is that the hyperpolarization-activated current I (f) underlies the pacemaker potential in all cardiac pacemakers. The aim of this review is to retrace the experimental development related to the pacemaker mechanism in Purkinje fibers with reference to findings about the pacemaker mechanism in the SAN as warranted. Experimental data and their interpretation are critically reviewed. Major findings were attributed to K(+) depletion in narrow extracellular spaces which would result in a time dependent decay of the inward rectifier current I (K1). In turn, this decay would be responsible for a "fake" reversal of the pacemaker current. In order to avoid such a postulated depletion, Ba(2+) was used to block the decay of I (K1). In the presence of Ba(2+) the time-dependent current no longer reversed and instead increased with time and more so at potentials as negative as -120 mV. In this regard, the distinct possibility needs to be considered that Ba(2+) had blocked I (Kdd) (and not only I (K1)). That indeed this was the case was demonstrated by studying single Purkinje cells in the absence and in the presence of Ba(2+). In the absence of Ba(2+), I (Kdd) was present in the pacemaker potential range and reversed at E (K). In the presence of Ba(2+), I (Kdd) was blocked and I (f) appeared at potentials negative to the pacemaker range. The pacemaker potential behaves in a manner consistent with the underlying I (Kdd) but not with I (f). The fact that I (f) is activated on hyperpolarization at potential negative to the pacemaker range makes it suitable as a safety factor to prevent the inhibitory action of more negative potentials on pacemaker discharge. It is concluded that the large body of evidence reviewed proves the pacemaker role of I (Kdd) (but not of I (f)) in Purkinje fibers.

σ2-Receptor Ligand-Mediated Inhibition of Inwardly Rectifying K+ Channels in the Heart

The sigma(2)-receptor agonist, ifenprodil, was suggested as an inhibitor of G protein-coupled inwardly rectifying potassium channels. Nevertheless, an analysis of the role of sigma(2) receptors in cardiac electrophysiology has never been done. This work aims i) to identify the roles of cardiac sigma(2) receptors in the regulation of cardiac K(+) channel conductances and ii) to check whether sigma(2)-receptor agonists exhibit class III antiarrhythmic properties. The sigma(2)-receptor agonists ifenprodil, threo-ifenprodil, LNP250A [threo-8-[1-(4-hydroxyphenyl)-1-hydroxy-propan-2-yl]-1-phenyl-1,3,8-triazaspiro[4,5]decane-4-one] (a derivative of ifenprodil devoid of alpha(1)-adrenergic and N-methyl-d-aspartate glutamate receptor-blocking properties), and 1,3-di(2-tolyl)guanidine were used to discriminate the effects linked to sigma(2) receptors from those of the sigma(1) subtype, induced by (+/-)-N-allylnormetazocine (SKF-10,047). The sigma(2)-receptor antagonist 3-alpha-tropanyl-2(pCl-phenoxy)butyrate (SM-21) was employed to characterize sigma(2)-mediated effects in patch-clamp experiments. In rabbits, all sigma(2)-receptor agonists reduced phenylephrine-induced cardiac arrhythmias. They prolonged action potential duration in rabbit Purkinje fibers and reduced human ether-a-go-go-related gene (HERG) K(+) currents. (+)-SKF-10,047 was completely inactive in the last two tests. The effects of threo-ifenprodil were not antagonized by SM-21. In HERG-transfected COS-7 cells, SM-21 potentiated the ifenprodil-induced blockade of the HERG current. These data suggest that sigma(2)-receptor ligands block I(Kr) and that this effect could explain part of the antiarrhythmic properties of this ligands family. Nevertheless, an interaction with HERG channels not involving sigma(2) receptors seems to share this pharmacological property. This work shows for the first time that particular caution has to be taken toward ligands with affinity for sigma(2) receptors. The repolarization prolongation and the early-afterdepolarization can be responsible for "torsades de pointe" and sudden cardiac death.

Open channel block of A-type, kv4.3, and delayed rectifier K+ channels, Kv1.3 and Kv3.1, by sibutramine

The effects of sibutramine on voltage-gated K+ channel (Kv)4.3, Kv1.3, and Kv3.1, stably expressed in Chinese hamster ovary cells, were investigated using the whole-cell patch-clamp technique. Sibutramine did not significantly decrease the peak Kv4.3 currents, but it accelerated the rate of decay of current inactivation in a concentration-dependent manner. This phenomenon was effectively characterized by integrating the total current over the duration of a depolarizing pulse to +40 mV. The IC50 value for the sibutramine block of Kv4.3 was 17.3 microM. Under control conditions, the inactivation of Kv4.3 currents could be fit to a biexponential function, and the time constants for the fast and slow components were significantly decreased after the application of sibutramine. The association (k+1) and dissociation (k-1) rate constants for the sibutramine block of Kv 4.3 were 1.51 microM-1s-1 and 27.35 s-1, respectively. The theoretical KD value, derived from k-1/k+1, yielded a value of 18.11 microM. The block of Kv4.3 by sibutramine displayed a weak voltage dependence, increasing at more positive potentials, and it was use-dependent at 2 Hz. Sibutramine did not affect the time course for the deactivating tail currents. Neither steady-state activation and inactivation nor the recovery from inactivation was affected by sibutramine. Sibutramine caused the concentration-dependent block of the Kv1.3 and Kv3.1 currents with an IC50 value of 3.7 and 32.7 microM, respectively. In addition, sibutramine reduced the tail current amplitude and slowed the deactivation of the tail currents of Kv1.3 and Kv3.1, resulting in a crossover phenomenon. These results indicate that sibutramine acts on Kv4.3, Kv1.3, and Kv3.1 as an open channel blocker.

Open channel block of Kv1.3 by rosiglitazone and troglitazone: Kv1.3 as the pharmacological target for rosiglitazone

The effects of rosiglitazone and troglitazone were examined on cloned Kv1.3 channels stably expressed in Chinese hamster ovary cells using the whole-cell configuration of the patch-clamp technique. Rosiglitazone decreased the Kv1.3 currents and accelerated the decay rate of current inactivation in a concentration-dependent manner with an IC(50) of 18.6 microM. These effects were reversible after washout of the drug. Troglitazone caused the block of Kv1.3 with a similar pattern but was five times more potent than rosiglitazone with an IC(50) of 3.5 microM. The block of Kv1.3 by rosiglitazone and troglitazone was voltage-dependent at a membrane potential coinciding with the activation of the channels. Both drugs decreased the tail current amplitude and slowed the deactivation process of Kv1.3, resulting in a tail crossover phenomenon. These results indicate that rosiglitazone and troglitazone block the open state of Kv1.3 channels, suggesting that it is an important pharmacological target for rosiglitazone as a potent blocker of Kv1.3 channels.

Novel, Potent Inhibitors of Human Kv1.5 K+ Channels and Ultrarapidly Activating Delayed Rectifier Potassium Current

We have identified a series of diphenyl phosphine oxide (DPO) compounds that are potent frequency-dependent inhibitors of cloned human Kv1.5 (hKv1.5) channels. DPO inhibited hKv1.5 expressed in Chinese hamster ovary cells in a concentration-dependent manner preferentially during channel activation and slowed the deactivating tail current, consistent with a predominant open-channel blocking mechanism. Varying kinetics of DPO interaction with Kv1.5 channels resulted in differing potencies and frequency dependencies of inhibition that were comparable for both expressed hKv1.5 current and native ultrarapidly activating delayed rectifier potassium current (IKur) in human atrial myocytes. Selectivity of DPO versus other cardiac K+ channels was demonstrated in human atrial myocytes (IKur versus transient outward potassium current) and guinea pig ventricular myocytes [IKur versus rapidly activating delayed rectifier potassium current (IKr), slowly activating delayed rectifier potassium current (IKs) and inward rectifier potassium current (IK1), and one compound (DPO-1) was shown to be 15-fold more selective for Kv1.5 versus Kv3.1 channels expressed in Xenopus oocytes. DPO-1 also prolonged action potentials of isolated human atrial but not ventricular myocytes, in contrast to the effect of a selective IKr blocker. The selectivity and kinetics of inhibition hKv1.5 and IKur by DPO and the resulting selective prolongation of atrial repolarization could provide an effective profile for treatment of supraventricular arrhythmias.

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.

Design, synthesis and preliminary biological evaluation of zatebradine analogues as potential blockers of the hyperpolarization-activated current

A series of zatebradine analogues, differing in the basic moiety and in the methylene spacer, have been synthesized; their negative chronotropic activity has been determined in guinea pig atria. The most active compounds have been studied for their blocking properties on the hyperpolarization-activated current If (which is one of the main currents underlying automatic activity in the sinus node) measured on ventricular myocytes of old spontaneously-hypertensive rats (SHR) by means of the patch-clamp technique. The majority of the substances were able to block If, with one of them (15) being slightly more potent than zatebradine. Surprisingly one analogue (6), while showing good negative chronotropic activity, was found to inhibit If only at high concentration and to markedly reduce outward currents, suggesting for this substance a different mechanism of action responsible for the negative chronotropic effect.

Direct block of cloned hKv1.5 channel by cytochalasins, actin-disrupting agents

The action of cytochalasins, actin-disrupting agents on human Kv1.5 channel (hKv1.5) stably expressed in Ltk−cells was investigated using the whole cell patch-clamp technique. Cytochalasin B inhibited hKv1.5 currents rapidly and reversibly at +60 mV in a concentration-dependent manner with an IC50of 4.2 μM. Cytochalasin A, which has a structure very similar to cytochalasin B, inhibited hKv1.5 (IC50of 1.4 μM at +60 mV). Pretreatment with other actin filament disruptors cytochalasin D and cytochalasin J, and an actin filament stabilizing agent phalloidin had no effect on the cytochalasin B-induced inhibition of hKv1.5 currents. Cytochalasin B accelerated the decay rate of inactivation for the hKv1.5 currents. Cytochalasin B-induced inhibition of the hKv1.5 channels was voltage dependent with a steep increase over the voltage range of the channel's opening. However, the inhibition exhibited voltage independence over the voltage range in which channels are fully activated. Cytochalasin B produced no significant effect on the steady-state activation or inactivation curves. The rate constants for association and dissociation of cytochalasin B were 3.7 μM/s and 7.5 s−1, respectively. Cytochalasin B produced a use-dependent inhibition of hKv1.5 current that was consistent with the slow recovery from inactivation in the presence of the drug. Cytochalasin B (10 μM) also inhibited an ultrarapid delayed rectifier K+current ( IK,ur) in human atrial myocytes. These results indicate that cytochalasin B primarily blocks activated hKv1.5 channels and endogenous IK,urin a cytoskeleton-independent manner as an open-channel blocker.

Characteristics of ginsenoside Rg3-mediated brain Na+ current inhibition

We demonstrated previously that ginsenoside Rg(3) (Rg(3)), an active ingredient of Panax ginseng, inhibits brain-type Na(+) channel activity. In this study, we sought to elucidate the molecular mechanisms underlying Rg(3)-induced Na(+) channel inhibition. We used the two-microelectrode voltage-clamp technique to investigate the effect of Rg(3) on Na(+) currents (I(Na)) in Xenopus laevis oocytes expressing wild-type rat brain Na(V)1.2 alpha and beta1 subunits, or mutants in the channel entrance, the pore region, the lidocaine/tetrodotoxin (TTX) binding sites, the S4 voltage sensor segments of domains I to IV, and the Ile-Phe-Met inactivation cluster. In oocytes expressing wild-type Na(+) channels, Rg(3) induced tonic and use-dependent inhibitions of peak I(Na). The Rg(3)-induced tonic inhibition of I(Na) was voltage-dependent, dose-dependent, and reversible, with an IC(50) value of 32 +/- 6 microM. Rg(3) treatment produced a 11.2 +/- 3.5 mV depolarizing shift in the activation voltage but did not alter the steady-state inactivation voltage. Mutations in the channel entrance, pore region, lidocaine/TTX binding sites, or voltage sensor segments did not affect Rg(3)-induced tonic blockade of peak I(Na). However, Rg(3) treatment inhibited the peak and plateau I(Na) in the IFMQ3 mutant, indicating that Rg(3) inhibits both the resting and open states of Na(+) channel. Neutralization of the positive charge at position 859 of voltage sensor segment domain II abolished the Rg(3)-induced activation voltage shift and use-dependent inhibition. These results reveal that Rg(3) is a novel Na(+) channel inhibitor capable of acting on the resting and open states of Na(+) channel via interactions with the S4 voltage-sensor segment of domain II.

Physiology and pharmacology of the cardiac pacemaker (“funny”) current

First described over a quarter of a century ago, the cardiac pacemaker "funny" (I(f)) current has been extensively characterized since, and its role in cardiac pacemaking has been thoroughly demonstrated. A similar current, termed I(h), was later described in different types of neurons, where it has a variety of functions and contributes to the control of cell excitability and plasticity. I(f) is an inward current activated by both voltage hyperpolarization and intracellular cAMP. In the heart, as well as generating spontaneous activity, f-channels mediate autonomic-dependent modulation of heart rate: beta-adrenergic stimulation accelerates, and vagal stimulation slows, cardiac rate by increasing and decreasing, respectively, the intracellular cAMP concentration and, consequently, the f-channel degree of activation. Four isoforms of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels have been cloned more recently and shown to be the molecular correlates of native f-channels in the heart and h-channels in the brain. Individual HCN isoforms have kinetic and modulatory properties which differ quantitatively. A comparison of their biophysical properties with those of native pacemaker channels provides insight into the molecular basis of the pacemaker current properties and, together with immunolabelling and other detection techniques, gives information on the pattern of HCN isoform distribution in different tissues. Because of their relevance to cardiac pacemaker activity, f-channels are a natural target of drugs aimed at the pharmacological control of heart rate. Several agents developed for their ability to selectively reduce heart rate act by a specific inhibition of f-channel function; these substances have a potential for the treatment of diseases such as angina and heart failure. In the near future, devices based on the delivery of f-channels in situ, or of a cellular source of f-channels (biological pacemakers), will likely be developed for use in therapies for diseases of heart rhythm with the aim of replacing electronic pacemakers.

Staurosporine inhibits voltage-dependent K+ current through a PKC-independent mechanism in isolated coronary arterial smooth muscle cells

We examined the effects of the protein kinase C (PKC) inhibitor staurosporine (ST) on voltage-dependent K (KV) channels in rabbit coronary arterial smooth muscle cells. ST inhibited the KV current in a dose-dependent manner with a Kd value of 1.3 microM. The inhibition of the KV current by ST was voltage-dependent between -30 and +10 mV. The additive inhibition of the KV current by ST was voltage-dependent throughout the activation voltage range. The rate constants of association and dissociation of ST were 0.63 microM s and 0.92 s, respectively. ST produced use-dependent inhibition of the KV current. ST shifted the activation curve to more positive potentials but did not have any significant effect on the voltage dependence of the inactivation curve. ST did not have any significant effects on other types of K channel. Another PKC inhibitor, chelerythrine, and PKA inhibitor peptide (PKA-IP) had little effect on the KV current. These results suggest that ST interacts with KV channels that are in the closed state and that ST inhibits KV channels in the open state in a manner that is phosphorylation-independent and voltage-, time-, and use-dependent.

The protein kinase C inhibitor, bisindolylmaleimide (I), inhibits voltage-dependent K+ channels in coronary arterial smooth muscle cells

We examined the effects of the protein kinase C (PKC) inhibitor, bisindolylmaleimide (BIM) (I), on voltage-dependent K+ (K(V)) channels in rabbit coronary arterial smooth muscle cells using whole-cell patch clamp technique. BIM (I) reversibly and dose-dependently inhibited the K(V) currents with an apparent Kd value of 0.27 microM. The inhibition of the K(V) current by BIM (I) was highly voltage-dependent between -30 and +10 mV (voltage range of channel activation), and the additive inhibition of the K(V) current by BIM (I) was voltage-dependence in the full activation voltage range. The rate constants of association and dissociation for BIM (I) were 18.4 microM(-1) s(-1) and 4.7 s(-1), respectively. BIM (I) had no effect on the steady-state activation and inactivation of K(V) channels. BIM (I) caused use-dependent inhibition of K(V) current, which was consistent with the slow recovery from inactivation in the presence of BIM (I) (recovery time constants were 856.95 +/- 282.6 ms for control, and 1806.38 +/- 110.0 ms for 300 nM BIM (I)). ATP-sensitive K+ (K(ATP)), inward rectifier K+ (K(IR)), Ca2+-activated K+ (BK(Ca)) channels, which regulate the membrane potential and arterial tone, were not affected by BIM (I). The PKC inhibitor, chelerythrine, and protein kinase A (PKA) inhibitor, PKA-IP, had little effect on the K(V) current and did not significantly alter the inhibitory effects of BIM (I) on the K(V) current. These results suggest that BIM (I) inhibits K(V) channels in a phosphorylation-independent, and voltage-, time- and use-dependent manner.