3-OST-7 regulates BMP-dependent cardiac contraction.

The 3-O-sulfotransferase (3-OST) family catalyzes rare modifications of glycosaminoglycan chains on heparan sulfate proteoglycans, yet their biological functions are largely unknown. Knockdown of 3-OST-7 in zebrafish uncouples cardiac ventricular contraction from normal calcium cycling and electrophysiology by reducing tropomyosin4 (tpm4) expression. Normal 3-OST-7 activity prevents the expansion of BMP signaling into ventricular myocytes, and ectopic activation of BMP mimics the ventricular noncontraction phenotype seen in 3-OST-7 depleted embryos. In 3-OST-7 morphants, ventricular contraction can be rescued by overexpression of tropomyosin tpm4 but not by troponin tnnt2, indicating that tpm4 serves as a lynchpin for ventricular sarcomere organization downstream of 3-OST-7. Contraction can be rescued by expression of 3-OST-7 in endocardium, or by genetic loss of bmp4. Strikingly, BMP misregulation seen in 3-OST-7 morphants also occurs in multiple cardiac noncontraction models, including potassium voltage-gated channel gene, kcnh2, affected in Romano-Ward syndrome and long-QT syndrome, and cardiac troponin T gene, tnnt2, affected in human cardiomyopathies. Together these results reveal 3-OST-7 as a key component of a novel pathway that constrains BMP signaling from ventricular myocytes, coordinates sarcomere assembly, and promotes cardiac contractile function.

Molecular mechanisms of chloroquine inhibition of heterologously expressed Kir6.2/SUR2A channels.

Chloroquine and related compounds can inhibit inwardly rectifying potassium channels by multiple potential mechanisms, including pore block and allosteric effects on channel gating. Motivated by reports that chloroquine inhibition of cardiac ATP-sensitive inward rectifier K(+) current (I(KATP)) is antifibrillatory in rabbit ventricle, we investigated the mechanism of chloroquine inhibition of ATP-sensitive potassium (K(ATP)) channels (Kir6.2/SUR2A) expressed in human embryonic kidney 293 cells, using inside-out patch-clamp recordings. We found that chloroquine inhibits the Kir6.2/SUR2A channel by interacting with at least two different sites and by two mechanisms of action. A fast-onset effect is observed at depolarized membrane voltages and enhanced by the N160D mutation in the central cavity, probably reflecting direct channel block resulting from the drug entering the channel pore from the cytoplasmic side. Conversely, a slow-onset, voltage-independent inhibition of I(KATP) is regulated by chloroquine interaction with a different site and probably involves disruption of interactions between Kir6.2/SUR2A and phosphatidylinositol 4,5-bisphosphate. Our findings reveal multiple mechanisms of K(ATP) channel inhibition by chloroquine, highlighting the numerous convergent regulatory mechanisms of these ligand-dependent ion channels.

Molecular coupling in the human ether-a-go-go-related gene-1 (hERG1) K+ channel inactivation pathway.

Emerging evidence suggests that K(+) channel inactivation involves coupling between residues in adjacent regions of the channel. Human ether-a-go-go-related gene-1 (hERG1) K(+) channels undergo a fast inactivation gating process that is crucial for maintaining electrical stability in the heart. The molecular mechanisms that drive inactivation in hERG1 channels are unknown. Using alanine scanning mutagenesis, we show that a pore helix residue (Thr-618) that points toward the S5 segment is critical for normal inactivation gating. Amino acid substitutions at position 618 modulate the free energy of inactivation gating, causing enhanced or reduced inactivation. Mutation of an S5 residue that is predicted to be adjacent to Thr-618 (W568L) abolishes inactivation and alters ion selectivity. The introduction of the Thr-618-equivalent residue in Kv1.5 enhances inactivation. Molecular dynamic simulations of the Kv1.2 tetramer reveal van der Waals coupling between hERG1 618- and 568-equivalent residues and a significant increase in interaction energies when threonine is introduced at the 618-equivalent position. We propose that coupling between the S5 segment and pore helix may participate in the inactivation process in hERG1 channels.

Differential voltage-dependent modulation of the ACh-gated K+ current by adenosine and acetylcholine.

Inhibitory regulation of the heart is determined by both cholinergic M2 receptors (M2R) and adenosine A1 receptors (A1R) that activate the same signaling pathway, the ACh-gated inward rectifier K+ (KACh) channels via Gi/o proteins. Previously, we have shown that the agonist-specific voltage sensitivity of M2R underlies several voltage-dependent features of IKACh, including the 'relaxation' property, which is characterized by a gradual increase or decrease of the current when cardiomyocytes are stepped to hyperpolarized or depolarized voltages, respectively. However, it is unknown whether membrane potential also affects A1R and how this could impact IKACh. Upon recording whole-cell currents of guinea-pig cardiomyocytes, we found that stimulation of the A1R-Gi/o-IKACh pathway with adenosine only caused a very slight voltage dependence in concentration-response relationships (~1.2-fold EC50 increase with depolarization) that was not manifested in the relative affinity, as estimated by the current deactivation kinetics (τ = 4074 ± 214 ms at -100 mV and τ = 4331 ± 341 ms at +30 mV; P = 0.31). Moreover, IKACh did not exhibit relaxation. Contrarily, activation of the M2R-Gi/o-IKACh pathway with acetylcholine induced the typical relaxation of the current, which correlated with the clear voltage-dependent effect observed in the concentration-response curves (~2.8-fold EC50 increase with depolarization) and in the IKACh deactivation kinetics (τ = 1762 ± 119 ms at -100 mV and τ = 1503 ± 160 ms at +30 mV; P = 0.01). Our findings further substantiate the hypothesis of the agonist-specific voltage dependence of GPCRs and that the IKACh relaxation is consequence of this property.

Genetic and physiologic dissection of the vertebrate cardiac conduction system.

Vertebrate hearts depend on highly specialized cardiomyocytes that form the cardiac conduction system (CCS) to coordinate chamber contraction and drive blood efficiently and unidirectionally throughout the organism. Defects in this specialized wiring system can lead to syncope and sudden cardiac death. Thus, a greater understanding of cardiac conduction development may help to prevent these devastating clinical outcomes. Utilizing a cardiac-specific fluorescent calcium indicator zebrafish transgenic line, Tg(cmlc2:gCaMP)(s878), that allows for in vivo optical mapping analysis in intact animals, we identified and analyzed four distinct stages of cardiac conduction development that correspond to cellular and anatomical changes of the developing heart. Additionally, we observed that epigenetic factors, such as hemodynamic flow and contraction, regulate the fast conduction network of this specialized electrical system. To identify novel regulators of the CCS, we designed and performed a new, physiology-based, forward genetic screen and identified for the first time, to our knowledge, 17 conduction-specific mutations. Positional cloning of hobgoblin(s634) revealed that tcf2, a homeobox transcription factor gene involved in mature onset diabetes of the young and familial glomerulocystic kidney disease, also regulates conduction between the atrium and the ventricle. The combination of the Tg(cmlc2:gCaMP)(s878) line/in vivo optical mapping technique and characterization of cardiac conduction mutants provides a novel multidisciplinary approach to further understand the molecular determinants of the vertebrate CCS.

The antimalarial drug mefloquine inhibits cardiac inward rectifier K+ channels: evidence for interference in PIP2-channel interaction.

The antimalarial drug mefloquine was found to inhibit the KATP channel by an unknown mechanism. Because mefloquine is a Cationic amphiphilic drug and is known to insert into lipid bilayers, we postulate that mefloquine interferes with the interaction between PIP2 and Kir channels resulting in channel inhibition. We studied the inhibitory effects of mefloquine on Kir2.1, Kir2.3, Kir2.3(I213L), and Kir6.2/SUR2A channels expressed in HEK-293 cells, and on IK1 and IKATP from feline cardiac myocytes. The order of mefloquine inhibition was Kir6.2/SUR2A ≈ Kir2.3 (IC50 ≈ 2 µM) > Kir2.1 (IC50 > 30 µM). Similar results were obtained in cardiac myocytes. The Kir2.3(I213L) mutant, which enhances the strength of interaction with PIP2 (compared to WT), was significantly less sensitive (IC50 = 9 µM). In inside-out patches, continuous application of PIP2 strikingly prevented the mefloquine inhibition. Our results support the idea that mefloquine interferes with PIP2-Kir channels interactions.

Inhibitory effect of terfenadine on Kir2.1 and Kir2.3 channels.

Terfenadine is a second-generation H1-antihistamine that despite potentially can produce severe side effects it has recently gained attention due to its anticancer properties. Lately, the subfamily 2 of inward rectifier potassium channels (Kir2) has been implicated in the progression of some tumoral processes. Hence, we characterized the effects of terfenadine on Kir2.x channels expressed in HEK-293 cells. Terfenadine inhibited Kir2.3 channels with a strikingly greater potency (IC50 = 1.06 ± 0.11 µmol L-1) compared to Kir2.1 channels (IC50 = 27.8 ± 4.8 µmol L-1). The Kir2.3(I213L) mutant, possessing a larger affinity for phosphatidylinositol 4,5-bisphosphate (PIP2) than the wild-type Kir2.3, was less sensitive to terfenadine inhibition (IC50 = 13.0 ± 2.9 µmol L-1). Additionally, the PIP2 intracellular application had largely reduced the inhibition of Kir2.1 channels by terfenadine. Our data support that Kir2.x channels are targets of terfena-dine by affecting their interaction with PIP2, which could be regarded as a mechanism of the antitumor properties of terfenadine.

Kir4.1/Kir5.1 channels possess strong intrinsic inward rectification determined by a voltage-dependent K+-flux gating mechanism.

Inwardly rectifying potassium (Kir) channels are broadly expressed in both excitable and nonexcitable tissues, where they contribute to a wide variety of cellular functions. Numerous studies have established that rectification of Kir channels is not an inherent property of the channel protein itself, but rather reflects strong voltage dependence of channel block by intracellular cations, such as polyamines and Mg2+. Here, we identify a previously unknown mechanism of inward rectification in Kir4.1/Kir5.1 channels in the absence of these endogenous blockers. This novel intrinsic rectification originates from the voltage-dependent behavior of Kir4.1/Kir5.1, which is generated by the flux of potassium ions through the channel pore; the inward K+-flux induces the opening of the gate, whereas the outward flux is unable to maintain the gate open. This gating mechanism powered by the K+-flux is convergent with the gating of PIP2 because, at a saturating concentration, PIP2 greatly reduces the inward rectification. Our findings provide evidence of the coexistence of two rectification mechanisms in Kir4.1/Kir5.1 channels: the classical inward rectification induced by blocking cations and an intrinsic voltage-dependent mechanism generated by the K+-flux gating.

Riluzole inhibits Kv4.2 channels acting on the closed and closed inactivated states.

Riluzole is an anticonvulsant drug also used to treat the amyotrophic lateral sclerosis and major depressive disorder. This compound has antiglutamatergic activity and is an important multichannel blocker. However, little is known about its actions on the Kv4.2 channels, the molecular correlate of the A-type K+ current (IA) and the fast transient outward current (Itof). Here, we investigated the effects of riluzole on Kv4.2 channels transiently expressed in HEK-293 cells. Riluzole inhibited Kv4.2 channels with an IC50 of 190 ± 14 µM and the effect was voltage- and frequency-independent. The activation rate of the current (at +50 mV) was not affected by the drug, nor the voltage dependence of channel activation, but the inactivation rate was accelerated by 100 and 300 µM riluzole. When Kv4.2 channels were maintained at the closed state, riluzole incubation induced a tonic current inhibition. In addition, riluzole significantly shifted the voltage dependence of inactivation to hyperpolarized potentials without affecting the recovery from inactivation. In the presence of the drug, the closed-state inactivation was significantly accelerated, and the percentage of inactivated channels was increased. Altogether, our findings indicate that riluzole inhibits Kv4.2 channels mainly affecting the closed and closed-inactivated states.

Tamoxifen inhibits cardiac ATP-sensitive and acetylcholine-activated K+ currents in part by interfering with phosphatidylinositol 4,5-bisphosphate-channel interaction.

Tamoxifen inhibits transmembrane currents of the Kir2.x inward rectifier potassium channels by interfering with the interaction of the channels with membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)). We tested the hypothesis that Kir channels with low affinity for PIP(2), like the adenosine triphosphate (ATP)-sensitive K(+) channel (K(ATP)) and acetylcholine (ACh)-activated K(+) channel (K(ACh)), have at least the same sensitivity to tamoxifen as Kir2.3. We investigated the effects of tamoxifen (0.1 - 10 microM) on Kir6.2/SUR2A (K(ATP)) and Kir3.1/3.4 (K(ACh)) channels expressed in HEK-293 cells and ATP-sensitive K(+) current (I(KATP)) and ACh-activated K(+) current (I(KACh)) in feline atrial myocytes. The onset of tamoxifen inhibition of both I(KATP) and I(KACh) was slow (T(1/2) approximately 3.5 min) and concentration-dependent but voltage-independent. The time course and degree of inhibition was independent of external or internal drug application. Tamoxifen interacts with the pore forming subunit, Kir6.2, rather than with the SUR subunit. The inhibitory potency of tamoxifen on the Kir6.2/SUR2A channel was decreased by the mutation (C166S) on Kir6.2 and in the continuous presence of PIP(2). In atrial myocytes, the mechanism and potency of the effects of tamoxifen on K(ATP) and K(ACh) channels were comparable to those in HEK-293 cells. These data suggest that, similar to its effects on Kir2.x currents, tamoxifen inhibits K(ATP) and K(ACh) currents by interfering with the interaction between the channel and PIP(2).

Voltage-induced structural modifications on M2 muscarinic receptor and their functional implications when interacting with the superagonist iperoxo.

It has been reported that muscarinic type-2 receptors (M2R) are voltage sensitive in an agonist-specific manner. In this work, we studied the effects of membrane potential on the interaction of M2R with the superagonist iperoxo (IXO), both functionally (using the activation of the ACh-gated K+ current (IKACh) in cardiomyocytes) and by molecular dynamics (MD) simulations. We found that IXO activated IKACh with remarkable high potency and clear voltage dependence, displaying a larger effect at the hyperpolarized potential. This result is consistent with a greater affinity, as validated by a slower (τ = 14.8 ± 2.3 s) deactivation kinetics of the IXO-evoked IKACh than that at the positive voltage (τ = 6.7 ± 1.2 s). The voltage-dependent M2R-IXO interaction induced IKACh to exhibit voltage-dependent features of this current, such as the 'relaxation gating' and the modulation of rectification. MD simulations revealed that membrane potential evoked specific conformational changes both at the external access and orthosteric site of M2R that underlie the agonist affinity change provoked by voltage on M2R. Moreover, our experimental data suggest that the 'tyrosine lid' (Y104, Y403, and Y426) is not the previously proposed voltage sensor of M2R. These findings provide an insight into the structural and functional framework of the biased signaling induced by voltage on GPCRs.

Chloroquine blocks a mutant Kir2.1 channel responsible for short QT syndrome and normalizes repolarization properties in silico.

Short QT Syndrome (SQTS) is a novel clinical entity characterized by markedly rapid cardiac repolarization and lethal arrhythmias. A mutation in the Kir2.1 inward rectifier K+ channel (D172N) causes one form of SQTS (SQT3). Pharmacologic block of Kir2.1 channels may hold promise as potential therapy for SQT3. We recently reported that the anti-malarial drug chloroquine blocks Kir2.1 channels by plugging the cytoplasmic pore domain. In this study, we tested whether chloroquine blocks D172N Kir2.1 channels in a heterologous expression system and if chloroquine normalizes repolarization properties using a mathematical model of a human ventricular myocyte. Chloroquine caused a dose- and voltage-dependent reduction in wild-type (WT), D172N and WT-D172N heteromeric Kir2.1 current. The potency and kinetics of chloroquine block of D172N and WT-D172N Kir2.1 current were similar to WT. In silico modeling of the heterozygous WT-D172N Kir2.1 condition predicted that 3 microM chloroquine normalized inward rectifier K+ current magnitude, action potential duration and effective refractory period. Our results suggest that therapeutic concentrations of chloroquine might lengthen cardiac repolarization in SQT3.

Tamoxifen inhibits inward rectifier K+ 2.x family of inward rectifier channels by interfering with phosphatidylinositol 4,5-bisphosphate-channel interactions.

Tamoxifen, an estrogen receptor antagonist used in the treatment of breast cancer, inhibits the inward rectifier potassium current (I(K1)) in cardiac myocytes by an unknown mechanism. We characterized the inhibitory effects of tamoxifen on Kir2.1, Kir2.2, and Kir2.3 potassium channels that underlie cardiac I(K1). We also studied the effects of 4-hydroxytamoxifen and raloxifene. All three drugs inhibited inward rectifier K(+) 2.x (Kir2.x) family members. The order of inhibition for all three drugs was Kir2.3 > Kir2.1 approximately Kir2.2. The onset of inhibition of Kir2.x current by these compounds was slow (T(1/2) approximately 6 min) and only partially recovered after washout ( approximately 30%). Kir2.x inhibition was concentration-dependent but voltage-independent. The time course and degree of inhibition was independent of external or internal drug application. We tested the hypothesis that tamoxifen interferes with the interaction between the channel and the membrane-delimited channel activator, phosphatidylinositol 4,5-bisphosphate (PIP(2)). Inhibition of Kir2.3 currents was significantly reduced by a single point mutation of I213L, which enhances Kir2.3 interaction with membrane PIP(2). Pretreatment with PIP(2) significantly decreased the inhibition induced by tamoxifen, 4-hydroxytamoxifen, and raloxifene on Kir2.3 channels. Pretreatment with spermine (100 microM) decreased the inhibitory effect of tamoxifen on Kir2.1, probably by strengthening the channel's interaction with PIP(2). In cat atrial and ventricular myocytes, 3 microM tamoxifen inhibited I(K1), but the effect was greater in the former than the latter. The data strongly suggest that tamoxifen, its metabolite, and the estrogen receptor inhibitor raloxifene inhibit Kir2.x channels indirectly by interfering with the interaction between the channel and PIP(2).

Molecular determinants of Kv7.1/KCNE1 channel inhibition by amitriptyline.

Amitriptyline (AMIT) is a compound widely prescribed for psychiatric and non-psychiatric conditions including depression, migraine, chronic pain, and anorexia. However, AMIT has been associated with risks of cardiac arrhythmia and sudden death since it can induce prolongation of the QT interval on the surface electrocardiogram and torsade de pointes ventricular arrhythmia. These complications have been attributed to the inhibition of the rapid delayed rectifier potassium current (IKr). The slow delayed rectifier potassium current (IKs) is the main repolarizing cardiac current when IKr is compromised and it has an important role in cardiac repolarization at fast heart rates induced by an elevated sympathetic tone. Therefore, we sought to characterize the effects of AMIT on Kv7.1/KCNE1 and homomeric Kv7.1 channels expressed in HEK-293H cells. Homomeric Kv7.1 and Kv7.1/KCNE1 channels were inhibited by AMIT in a concentration-dependent manner with IC50 values of 8.8 ±â€¯2.1 µM and 2.5 ±â€¯0.8 µM, respectively. This effect was voltage-independent for both homomeric Kv7.1 and Kv7.1/KCNE1 channels. Moreover, mutation of residues located on the P-loop and S6 domain along with molecular docking, suggest that T312, I337 and F340 are the most important molecular determinants for AMIT-Kv7.1 channel interaction. Our experimental findings and modeling suggest that AMIT preferentially blocks the open state of Kv7.1/KCNE1 channels by interacting with specific residues that were previously reported to be important for binding of other compounds, such as chromanol 293B and the benzodiazepine L7.

Chloroquine blocks the Kir4.1 channels by an open-pore blocking mechanism.

Kir4.1 channels have been implicated in various physiological processes, mainly in the K+ homeostasis of the central nervous system and in the control of glial function and neuronal excitability. Even though, pharmacological research of these channels is very limited. Chloroquine (CQ) is an amino quinolone derivative known to inhibit Kir2.1 and Kir6.2 channels with different action mechanism and binding site. Here, we employed patch-clamp methods, mutagenesis analysis, and molecular modeling to characterize the molecular pharmacology of Kir4.1 inhibition by CQ. We found that this drug inhibits Kir4.1 channels heterologously expressed in HEK-293 cells. CQ produced a fast-onset voltage-dependent pore-blocking effect on these channels. In inside-out patches, CQ showed notable higher potency (IC50 ≈0.5µM at +50mV) and faster onset of block when compared to whole-cell configuration (IC50 ≈7µM at +60mV). Also, CQ showed a voltage-dependent unblock with repolarization. These results suggest that the drug directly blocks Kir4.1 channels by a pore-plugging mechanism. Moreover, we found that two residues (Thr128 and Glu158), facing the central cavity and located within the transmembrane pore, are particularly important structural determinants of CQ block. This evidence was similar to what was previously reported with Kir6.2, but distinct from the interaction site (cytoplasmic pore) CQ-Kir2.1. Thus, our findings highlight the diversity of interaction sites and mechanisms that underlie amino quinolone inhibition of Kir channels.

Inhibition of Kir4.1 potassium channels by quinacrine.

Inwardly rectifying potassium (Kir) channels are expressed in many cell types and contribute to a wide range of physiological processes. Particularly, Kir4.1 channels are involved in the astroglial spatial potassium buffering. In this work, we examined the effects of the cationic amphiphilic drug quinacrine on Kir4.1 channels heterologously expressed in HEK293 cells, employing the patch clamp technique. Quinacrine inhibited the currents of Kir4.1 channels in a concentration and voltage dependent manner. In inside-out patches, quinacrine inhibited Kir4.1 channels with an IC50 value of 1.8±0.3µM and with extremely slow blocking and unblocking kinetics. Molecular modeling combined with mutagenesis studies suggested that quinacrine blocks Kir4.1 by plugging the central cavity of the channels, stabilized by the residues E158 and T128. Overall, this study shows that quinacrine blocks Kir4.1 channels, which would be expected to impact the potassium transport in several tissues.

DITPA restores the repolarizing potassium currents Itof and Iss in cardiac ventricular myocytes of diabetic rats.

Diabetes Mellitus (DM) can produce an increase in the cardiac action potential duration and QT interval that can be associated with sudden death. These cardiac effects are due to a region-specific decrease in repolarizing outward K(+) currents. Some authors have suggested that the proarrhythmic effects of diabetes can be due to diabetes-induced hypothyroidism. Thus, we have examined the effect of the thyroid hormone analog diiodothyropropionic acid (DITPA) on calcium-independent outward potassium currents in ventricular myocytes from diabetic rats. Sustained (I(ss)) and fast transient outward (I(tof)) K(+) currents were recorded using the whole-cell configuration of the patch-clamp technique. Myocytes were enzymatically isolated from the free wall of the right ventricle, and the epicardial and endocardial layers of the left ventricle of healthy, diabetic and DITPA-treated diabetic rats. Circulating thyroid hormones were measured by electrochemiluminescence. DITPA-treatment of diabetic rats restored I(tof) and I(ss) current densities in cardiac myocytes from the three regions studied, but did not alter current densities in myocytes of control rats. T(3) and T(4) levels were reduced by diabetes, and DITPA-treatment increased circulating T(3) levels. T(3)-treatment of diabetic rats also restored current densities to control values. However, direct incubation of diabetic myocytes with DITPA did not restore current densities. In summary, DITPA-treatment of diabetic rats restored the potassium current (I(tof) and I(ss)) densities in myocytes from all ventricular regions.

Occurrence of a tetrodotoxin-sensitive calcium current in rat ventricular myocytes after long-term myocardial infarction.

OBJECTIVE: To determine the characteristics of a TTX-sensitive Ca(2+) current that occurred only following remodelling after myocardial infarction in Wistar rat. METHODS: Using the whole-cell patch-clamp technique, we studied ionic inward current in myocytes isolated from four different ventricular regions of control Wistar rat hearts, or from hearts 4 to 6 months after ligation of the left coronary artery. Inward current characteristics were also analysed in Xenopus laevis oocytes that heterologously expressed the human sodium channel alpha-subunit Nav1.5. The effects of oxidative stress by hydrogen peroxide or tert-butyl-hydroxyperoxide as well as those of PKA-dependent phosphorylation, which partly mimic the pathological conditions, were investigated on control cardiomyocytes and Nav1.5-expressing oocytes. RESULTS: In Na-free solution, a low-threshold, tetrodotoxin-sensitive inward current was found in 20 out of 78 cells isolated from 16 post-myocardial infarcted (PMI) cardiomyocytes but not in cardiomyocytes from young and sham rat hearts. This current exhibited kinetics and pharmacological properties similar to the I(Ca(TTX)) current previously reported. I(Ca(TTX))-like current was critically dependent on extracellular Na(+) and was reduced by micromolar Na(+) concentrations. Neither in normal rat cardiomyocytes nor in Nav1.5-expressing oocytes could a I(Ca(TTX))-like current be elicited in Na(+)-free extracellular solution, even after oxidative stress or PKA-dependent phosphorylation. CONCLUSIONS: Our data suggest that I(Ca(TTX))-like current in PMI myocytes does not arise from classical Na(+) channels modified by oxidative stress or PKA phosphorylation and most probably represents a different Na(+) channel type re-expressed in some cells after remodelling.

Impact of the whole-cell patch-clamp configuration on the pharmacological assessment of the hERG channel: trazodone as a case example.

INTRODUCTION: Voltage- and state-dependent blocks are important mechanisms by which drugs affect voltage-gated ionic channels. However, spontaneous (i.e. drug-free) time-dependent changes in the activation and inactivation of hERG and Na(+) channels have been reported when using conventional whole-cell patch-clamp in HEK-293 cells. METHODS: hERG channels were heterologously expressed in HEK-293 cells and in Xenopus laevis oocytes. hERG current (IhERG) was recorded using both conventional and perforated whole-cell patch-clamp (HEK-293 cells), and two microelectrode voltage-clamp (Xenopus oocytes) in drug-free solution, and in the presence of the drug trazodone. RESULTS: In conventional whole-cell setup, we observed a spontaneous time-dependent hyperpolarizing shift in the activation curve of IhERG. Conversely, in perforated patch whole-cell (HEK-293 cells) or in two microelectrode voltage-clamp (Xenopus oocytes) activation curves of IhERG were very stable for periods ~50min. Voltage-dependent inactivation of IhERG was not significantly altered in the three voltage clamp configurations tested. When comparing voltage- and state-dependent effects of the antidepressant drug trazodone on IhERG, similar changes between the three voltage clamp configurations were observed as under drug-free conditions. DISCUSSION: The comparative analysis performed in this work showed that only under conventional whole-cell voltage-clamp conditions, a leftward shift in the activation curve of IhERG occurred, both in the presence and absence of drugs. These spontaneous time-dependent changes in the voltage activation gate of IhERG are a potential confounder in pharmacological studies on hERG channels expressed in HEK-293 cells.

Tamoxifen inhibition of kv7.2/kv7.3 channels.

KCNQ genes encode five Kv7 K(+) channel subunits (Kv7.1-Kv7.5). Four of these (Kv7.2-Kv7.5) are expressed in the nervous system. Kv7.2 and Kv7.3 are the principal molecular components of the slow voltage-gated M-channel, which regulates neuronal excitability. In this study, we demonstrate that tamoxifen, an estrogen receptor antagonist used in the treatment of breast cancer, inhibits Kv7.2/Kv7.3 currents heterologously expressed in human embryonic kidney HEK-293 cells. Current inhibition by tamoxifen was voltage independent but concentration-dependent. The IC50 for current inhibition was 1.68 ± 0.44 µM. The voltage-dependent activation of the channel was not modified. Tamoxifen inhibited Kv7.2 homomeric channels with a higher potency (IC50 = 0.74 ± 0.16 µM). The mutation Kv7.2 R463E increases phosphatidylinositol- 4,5-bisphosphate (PIP2) - channel interaction and diminished dramatically the inhibitory effect of tamoxifen compared with that for wild type Kv7.2. Conversely, the mutation Kv7.2 R463Q, which decreases PIP2 -channel interaction, increased tamoxifen potency. Similar results were obtained on the heteromeric Kv7.2 R463Q/Kv7.3 and Kv7.2 R463E/Kv7.3 channels, compared to Kv7.2/Kv7.3 WT. Overexpression of type 2A PI(4)P5-kinase (PIP5K 2A) significantly reduced tamoxifen inhibition of Kv7.2/Kv7.3 and Kv7.2 R463Q channels. Our results suggest that tamoxifen inhibited Kv7.2/Kv7.3 channels by interfering with PIP2-channel interaction because of its documented interaction with PIP2 and the similar effect of tamoxifen on various PIP2 sensitive channels.