Selective open-channel block of Shaker (Kv1) potassium channels by s-nitrosodithiothreitol (SNDTT)

The Reducing Agent Dithiothreitol Modulates the Ventilatory Responses That Occur in Freely Moving Rats during and following a Hypoxic-Hypercapnic Challenge

The present study examined the hypothesis that changes in the oxidation-reduction state of thiol residues in functional proteins play a major role in the expression of the ventilatory responses in conscious rats that occur during a hypoxic-hypercapnic (HH) gas challenge and upon return to room air. A HH gas challenge in vehicle-treated rats elicited robust and sustained increases in minute volume (via increases in frequency of breathing and tidal volume), peak inspiratory and expiratory flows, and inspiratory and expiratory drives while minimally affecting the non-eupneic breathing index (NEBI). The HH-induced increases in these parameters, except for frequency of breathing, were substantially diminished in rats pre-treated with the potent and lipophilic disulfide-reducing agent, L,D-dithiothreitol (100 µmol/kg, IV). The ventilatory responses that occurred upon return to room air were also substantially different in dithiothreitol-treated rats. In contrast, pre-treatment with a substantially higher dose (500 µmol/kg, IV) of the lipophilic congener of the monosulfide, N-acetyl-L-cysteine methyl ester (L-NACme), only minimally affected the expression of the above-mentioned ventilatory responses that occurred during the HH gas challenge or upon return to room air. The effectiveness of dithiothreitol suggests that the oxidation of thiol residues occurs during exposure to a HH gas challenge and that this process plays an essential role in allowing for the expression of the post-HH excitatory phase in breathing. However, this interpretation is contradicted by the lack of effects of L-NACme. This apparent conundrum may be explained by the disulfide structure affording unique functional properties to dithiothreitol in comparison to monosulfides. More specifically, the disulfide structure may give dithiothreitol the ability to alter the conformational state of functional proteins while transferring electrons. It is also possible that dithiothreitol is simply a more efficient reducing agent following systemic injection, although one interpretation of the data is that the effects of dithiothreitol are not due to its reducing ability.

Oxidative stress and ion channels in neurodegenerative diseases

Numerous neurodegenerative diseases result from altered ion channel function and mutations. The intracellular redox status can significantly alter the gating characteristics of ion channels. Abundant neurodegenerative diseases associated with oxidative stress have been documented, including Parkinson's, Alzheimer's, spinocerebellar ataxia, amyotrophic lateral sclerosis, and Huntington's disease. Reactive oxygen and nitrogen species compounds trigger posttranslational alterations that target specific sites within the subunits responsible for channel assembly. These alterations include the adjustment of cysteine residues through redox reactions induced by reactive oxygen species (ROS), nitration, and S-nitrosylation assisted by nitric oxide of tyrosine residues through peroxynitrite. Several ion channels have been directly investigated for their functional responses to oxidizing agents and oxidative stress. This review primarily explores the relationship and potential links between oxidative stress and ion channels in neurodegenerative conditions, such as cerebellar ataxias and Parkinson's disease. The potential correlation between oxidative stress and ion channels could hold promise for developing innovative therapies for common neurodegenerative diseases.

Nitrergic modulation of ion channel function in regulating neuronal excitability

Nitric oxide (NO) signaling in the brain provides a wide range of functional properties in response to neuronal activity. NO exerts its effects through different signaling pathways, namely, through the canonical soluble guanylyl cyclase-mediated cGMP production route and via post-translational protein modifications. The latter pathways comprise cysteine S-nitrosylation and 3-nitrotyrosination of distinct tyrosine residues. Many ion channels are targeted by one or more of these signaling routes, which leads to their functional regulation under physiological conditions or facilities their dysfunction leading to channelopathies in many pathologies. The resulting alterations in ion channel function changes neuronal excitability, synaptic transmission, and action potential propagation. Transient and activity-dependent NO production mediates reversible ion channel modifications via cGMP and S-nitrosylation signaling, whereas more pronounced and longer-term NO production during conditions of elevated oxidative stress leads to increasingly cumulative and irreversible protein 3-nitrotyrosination. The complexity of this regulation and vast variety of target ion channels and their associated functional alterations presents a challenging task in assessing and understanding the role of NO signaling in physiology and disease.

Bases of Bacterial Sodium Channel Selectivity Among Organic Cations

Hille’s (1971) seminal study of organic cation selectivity of eukaryotic voltage-gated sodium channels showed a sharp size cut-off for ion permeation, such that no ion possessing a methyl group was permeant. Using the prokaryotic channel, NaChBac, we found some similarity and two peculiar differences in the selectivity profiles for small polyatomic cations. First, we identified a diverse group of minimally permeant cations for wildtype NaChBac, ranging in sizes from ammonium to guanidinium and tetramethylammonium; and second, for both ammonium and hydrazinium, the charge-conserving selectivity filter mutation (E191D) yielded substantial increases in relative permeability (P_X/P_Na). The relative permeabilities varied inversely with relative K_d calculated from 1D Potential of Mean Force profiles (PMFs) for the single cations traversing the channel. Several of the cations bound more strongly than Na⁺, and hence appear to act as blockers, as well as charge carriers. Consistent with experimental observations, the E191D mutation had little impact on Na⁺ binding to the selectivity filter, but disrupted the binding of ammonium and hydrazinium, consequently facilitating ion permeation across the NaChBac-like filter. We concluded that for prokaryotic sodium channels, a fine balance among filter size, binding affinity, occupancy, and flexibility seems to contribute to observed functional differences.

Choline and NMDG directly reduce outward currents: reduced outward current when these substances replace Na+ is alone not evidence of Na+-activated K+ currents

Choline chloride is often, and N-methyl-d-glucamine (NMDG) sometimes, used to replace sodium chloride in studies of sodium-activated potassium channels. Given the high concentrations used in sodium replacement protocols, it is essential to test that it is not the replacement substances themselves, as opposed to the lack of sodium, that cause any observed effects. We therefore compared, in lobster stomatogastric neurons and leech Retzius cells, the effects of applying salines in which choline chloride replaced sodium chloride, and in which choline hydroxide or sucrose was added to normal saline. We also tested, in stomatogastric neurons, the effect of adding NMDG to normal saline. These protocols allowed us to measure the direct effects (i.e., effects not due to changes in sodium concentration or saline osmolarity or ionic strength) of choline on stomatogastric and leech currents, and of NMDG on stomatogastric currents. Choline directly reduced transient and sustained depolarization-activated outward currents in both species, and NMDG directly reduced transient depolarization-activated outward currents in stomatogastric neurons. Experiments with lower choline concentrations showed that adding as little as 150 mM (stomatogastric) or 5 mM (leech) choline reduced at least some depolarization-activated outward currents. Reductions in outward current with choline chloride or NMDG replacement alone are thus not evidence of sodium-activated potassium currents. NEW & NOTEWORTHY We show that choline or N-methyl-d-glucamine (NMDG) directly (i.e., not due to changes in extracellular sodium) decrease outward currents. Prior work studying sodium-activated potassium channels in which sodium was replaced with choline or NMDG without an addition control may therefore be artifactual.

A biophysically detailed computational model of urinary bladder small DRG neuron soma

Bladder small DRG neurons, which are putative nociceptors pivotal to urinary bladder function, express more than a dozen different ionic membrane mechanisms: ion channels, pumps and exchangers. Small-conductance Ca2+-activated K+ (SKCa) channels which were earlier thought to be gated solely by intracellular Ca2+ concentration ([Ca]i) have recently been shown to exhibit inward rectification with respect to membrane potential. The effect of SKCa inward rectification on the excitability of these neurons is unknown. Furthermore, studies on the role of KCa channels in repetitive firing and their contributions to different types of afterhyperpolarization (AHP) in these neurons are lacking. In order to study these phenomena, we first constructed and validated a biophysically detailed single compartment model of bladder small DRG neuron soma constrained by physiological data. The model includes twenty-two major known membrane mechanisms along with intracellular Ca2+ dynamics comprising Ca2+ diffusion, cytoplasmic buffering, and endoplasmic reticulum (ER) and mitochondrial mechanisms. Using modelling studies, we show that inward rectification of SKCa is an important parameter regulating neuronal repetitive firing and that its absence reduces action potential (AP) firing frequency. We also show that SKCa is more potent in reducing AP spiking than the large-conductance KCa channel (BKCa) in these neurons. Moreover, BKCa was found to contribute to the fast AHP (fAHP) and SKCa to the medium-duration (mAHP) and slow AHP (sAHP). We also report that the slow inactivating A-type K+ channel (slow KA) current in these neurons is composed of 2 components: an initial fast inactivating (time constant ∼ 25-100 ms) and a slow inactivating (time constant ∼ 200-800 ms) current. We discuss the implications of our findings, and how our detailed model can help further our understanding of the role of C-fibre afferents in the physiology of urinary bladder as well as in certain disorders.

4-Chloro-3-nitro-N-butylbenzenesulfonamide acts on KV3.1 channels by an open-channel blocker mechanism

The effects of 4-chloro-3-nitro-N-butylbenzenesulfonamide (SMD2) on K_V3.1 channels, heterologous expressed in L-929 cells, were studied with the whole cell patch-clamp technique. SMD2 blocks K_V3.1 in a reversible and use-dependent manner, with IC₅₀ around 10 µM, and a Hill coefficient around 2. Although the conductance vs. voltage relationship in control condition can be described by a single Boltzmann function, two terms are necessary to describe the data in the presence of SMD2. The activation and deactivation time constants are weakly voltage dependent both for control and in the presence of SMD2. SMD2 does not change the channel selectivity and tail currents show a typical crossover phenomenon. The time course of inactivation has a fast and a slow component, and SMD2 significantly decreased their values. Steady-state inactivation is best described by a Boltzmann equation with V _1/2 (the voltage where the probability to find the channels in the inactivated state is 50%) and K (slope factor) equals to -22.9 ± 1.5 mV and 5.3 ± 0.9 mV for control, and -30.3 ± 1.3 mV and 6 ± 0.8 mV for SMD2, respectively. The action of SMD2 is enhanced by high frequency stimulation, and by the time the channel stays open. Taken together, our results suggest that SMD2 blocks the open conformation of K_V3.1. From a pharmacological and therapeutic point of view, N-alkylsulfonamides may constitute a new class of pharmacological modulators of K_V3.1.

In silico identification of PAP-1 binding sites in the Kv1.2 potassium channel

Voltage-gated potassium channels of the Kv1 family play a crucial role in the generation and transmission of electrical signals in excitable cells affecting neuronal and cardiac activities. Small-molecule blockage of these channels has been proposed to occur via a cooperative mechanism involving two main blocking sites: the inner-pore site located below the selectivity filter, and a side-pocket cavity located between the pore and the voltage sensor. Using 0.5 μs molecular dynamics simulation trajectories complemented by docking calculations, the potential binding sites of the PAP-1 (5-(4-phenoxybutoxy)psoralen) blocker to the crystal structure of Kv1.2 channel have been studied. The presence of both mentioned blocking sites at Kv1.2 is confirmed, adding evidence in favor of a cooperative channel blockage mechanism. These observations provide insight into drug modulation that will guide further developments of Kv inhibitors.

K(+) channels: function-structural overview

Potassium channels are particularly important in determining the shape and duration of the action potential, controlling the membrane potential, modulating hormone secretion, epithelial function and, in the case of those K(+) channels activated by Ca(2+), damping excitatory signals. The multiplicity of roles played by K(+) channels is only possible to their mammoth diversity that includes at present 70 K(+) channels encoding genes in mammals. Today, thanks to the use of cloning, mutagenesis, and the more recent structural studies using x-ray crystallography, we are in a unique position to understand the origins of the enormous diversity of this superfamily of ion channels, the roles they play in different cell types, and the relations that exist between structure and function. With the exception of two-pore K(+) channels that are dimers, voltage-dependent K(+) channels are tetrameric assemblies and share an extremely well conserved pore region, in which the ion-selectivity filter resides. In the present overview, we discuss in the function, localization, and the relations between function and structure of the five different subfamilies of K(+) channels: (a) inward rectifiers, Kir; (b) four transmembrane segments-2 pores, K2P; (c) voltage-gated, Kv; (d) the Slo family; and (e) Ca(2+)-activated SK family, SKCa.

Homology modeling of Kv1.5 channel block by cationic and electroneutral ligands

The inner pore of potassium channels is targeted by many ligands of intriguingly different chemical structures. Previous studies revealed common and diverse characteristics of action of ligands including cooperativity of ligand binding, voltage- and use-dependencies, and patterns of ligand-sensing residues. Not all these data are rationalized in published models of ligand-channel complexes. Here we have used energy calculations with experimentally defined constraints to dock flecainide, ICAGEN-4, benzocaine, vernakalant, and AVE0118 into the inner pore of Kv1.5 channel. We arrived at ligand-binding models that suggest possible explanations for different values of the Hill coefficient, different voltage dependencies of ligands action, and effects of mutations of residues in subunit interfaces. Two concepts were crucial to build the models. First, the inner-pore block of a potassium channel requires a cationic "blocking particle". A ligand, which lacks a positively charged group, blocks the channel in a complex with a permeant ion. Second, hydrophobic moieties of a flexible ligand have a tendency to bind in hydrophobic subunit interfaces.

Ligand action on sodium, potassium, and calcium channels: role of permeant ions

Ion channels are targets for many naturally occurring toxins and small-molecule drugs. Despite great progress in the X-ray crystallography of ion channels, we still do not have a complete understanding of the atomistic mechanisms of channel modulation by ligands. In particular, the importance of the simultaneous interaction of permeant ions with the ligand and the channel protein has not been the focus of much attention. Considering these interactions often allows one to rationalize the highly diverse experimental data within the framework of relatively simple structural models. This has been illustrated in earlier studies on the action of local anesthetics, sodium channel activators, as well as blockers of potassium and calcium channels. Here, we discuss the available data with a view to understanding the use-, voltage-, and current carrying cation-dependence of the ligand action, paradoxes in structure--activity relationships, and effects of mutations in these ion channels.

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.

Mechanism of estradiol-induced block of voltage-gated K+ currents in rat medial preoptic neurons

The present study was conducted to characterize possible rapid effects of 17-β-estradiol on voltage-gated K⁺ channels in preoptic neurons and, in particular, to identify the mechanisms by which 17-β-estradiol affects the K⁺ channels. Whole-cell currents from dissociated rat preoptic neurons were studied by perforated-patch recording. 17-β-estradiol rapidly (within seconds) and reversibly reduced the K⁺ currents, showing an EC₅₀ value of 9.7 µM. The effect was slightly voltage dependent, but independent of external Ca²⁺, and not sensitive to an estrogen-receptor blocker. Although 17-α-estradiol also significantly reduced the K⁺ currents, membrane-impermeant forms of estradiol did not reduce the K⁺ currents and other estrogens, testosterone and cholesterol were considerably less effective. The reduction induced by estradiol was overlapping with that of the K_V-2-channel blocker r-stromatoxin-1. The time course of K⁺ current in 17-β-estradiol, with a time-dependent inhibition and a slight dependence on external K⁺, suggested an open-channel block mechanism. The properties of block were predicted from a computational model where 17-β-estradiol binds to open K⁺ channels. It was concluded that 17-β-estradiol rapidly reduces voltage-gated K⁺ currents in a way consistent with an open-channel block mechanism. This suggests a new mechanism for steroid action on ion channels.

Celecoxib blocks cardiac Kv1.5, Kv4.3 and Kv7.1 (KCNQ1) channels: effects on cardiac action potentials

Celecoxib is a COX-2 inhibitor that has been related to an increased cardiovascular risk and that exerts several actions on different targets. The aim of this study was to analyze the effects of this drug on human cardiac voltage-gated potassium channels (Kv) involved on cardiac repolarization Kv1.5 (I(Kur)), Kv4.3+KChIP2 (I(to1)) and Kv7.1+KCNE1 (I(Ks)) and to compare with another COX-2 inhibitor, rofecoxib. Currents were recorded in transfected mammalian cells by whole-cell patch-clamp. Celecoxib blocked all the Kv channels analyzed and rofecoxib was always less potent, except on Kv4.3+KChIP2 channels. Kv1.5 block increased in the voltage range of channel activation, decreasing at potentials positive to 0 mV. The drug modified the activation curve of the channels that became biphasic. Block was frequency-dependent, increasing at fastest frequencies. Celecoxib effects were not altered by TEA(out) in R487Y mutant Kv1.5 channels but the kinetics of block were slower and the degree of block was smaller with TEA(in), indicating that celecoxib acts from the cytosolic side. We confirmed the blocking properties of celecoxib on native Kv currents from rat vascular cells, where Kv1.5 are the main contributors (IC(50)≈ 7 μM). Finally, we demonstrate that celecoxib prolongs the action potential duration in mouse cardiac myocytes and shortens it in guinea pig cardiac myocytes, suggesting that Kv block induced by celecoxib may be of clinical relevance.

Potassium channel block by a tripartite complex of two cationophilic ligands and a potassium ion

Voltage-gated potassium channels (Kv) are targets for drugs of large chemical diversity. Although hydrophobic cations block Kv channels with Hill coefficients of 1, uncharged electron-rich (cationophilic) molecules often display Hill coefficients of 2. The mechanism of the latter block is unknown. Using a combination of computational and experimental approaches, we mapped the receptor for the immunosuppressant PAP-1 (5-(4-phenoxybutoxy)psoralen), a high-affinity blocker of Kv1.3 channels in lymphocytes. Ligand-docking using Monte Carlo minimizations suggested a model in which two cationophilic PAP-1 molecules coordinate a K(+) ion in the pore with their coumarin moieties, whereas the hydrophobic phenoxyalkoxy side chains extend into the intrasubunit interfaces between helices S5 and S6. We tested the model by generating 58 point mutants involving residues in and around the predicted receptor and then determined their biophysical properties and sensitivity to PAP-1 by whole-cell patch-clamp. The model correctly predicted the key PAP-1-sensing residues in the outer helix, the P-loop, and the inner helix and explained the Hill coefficient of 2 by demonstrating that the Kv1.3 pore can accommodate two or even four PAP-1 molecules. The model further explained the voltage-dependence of block by PAP-1 and its thousand-fold selectivity for Kv1.3 over non-Kv1 channels. The 23- to 125-fold selectivity of PAP-1 for Kv1.3 over other Kv1 channels is probably due to its preferential affinity to the C-type inactivated state, in which cessation of K(+) flux stabilizes the tripartite PAP-1:K(+):PAP-1 complex in the pore. Our study provides a new concept for potassium channel block by cationophilic ligands.

Atomic determinants of state-dependent block of sodium channels by charged local anesthetics and benzocaine

Molecular modeling predicts that a local anesthetic (LA) lidocaine binds to the resting and open Na(v)1.5 in different modes, interacting with LA-sensing residues known from experiments. Besides the major pathway via the open activation gate, LAs can reach the inner pore via a "sidewalk" between D3S6, D4S6, and D3P. The ammonium group of a cationic LA binds in the focus of the pore-helices macrodipoles, which also stabilize a Na(+) ion chelated by two benzocaine molecules. The LA's cationic group and a Na(+) ion in the selectivity filter repel each other suggesting that the Na(+) depletion upon slow inactivation would stabilize a LA, while a LA would stabilize slow-inactivated states.

Cu2+ (1,10 phenanthroline)3 is an open-channel blocker of the human skeletal muscle sodium channel

The formation of disulfide bridges is a classical approach used to study the mobility, proximity and distances of residues in a variety of proteins, including ligand- and voltage-gated ion channels. We performed patch-clamp studies to investigate the interaction of a pair of cysteines introduced into the human skeletal muscle voltage-gated Na+ channel (hNa(v)1.4) using the oxidation catalyst, Cu2+ (1,10-phenanthroline)3 (CuPhen). Our experiments resulted in a surprising finding, a reversible current inhibition of the mutant I1160C/L1482C containing two cysteines in the D3/and D4/S4-S5 loops, subjected to oxidative cross-linking in the presence of CuPhen. We report here that CuPhen is an open channel blocker of both mutant and wild-type (WT) hNa(v)1.4 channels, however, for WT channels a more than 10-fold higher concentration was needed to induce the same effect. Moreover, 1,10-phenanthroline was capable of blocking Na+ channels in the absence of Cu2+ ions. Our results indicate a use- and voltage-dependent binding and unbinding of CuPhen, reminiscent of the lidocaine quaternary derivative QX-314 and the neurotoxin batrachotoxin. Care should be taken when using CuPhen as an oxidizing reagent in cross-linking experiments, since it may directly affect channel activity. Our results identify CuPhen (and phenantroline) as a novel use-dependent inhibitor of Na+ channels, a mechanism that is shared by drugs widely used in the treatment of epilepsy, neuropathic pain, cardiac arrhythmia and myotonia. We hypothesize that I1160C in D3/S4-S5 and the corresponding L1482C mutation in D4/S4-S5 could allosterically affect a binding site located in the inner pore region of the channel.

Discovery of glycine hydrazide pore-occluding CFTR inhibitors: mechanism, structure-activity analysis, and in vivo efficacy

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is a cAMP-regulated epithelial Cl⁻ channel that, when defective, causes cystic fibrosis. Screening of a collection of 100,000 diverse small molecules revealed four novel chemical classes of CFTR inhibitors with K_i < 10 μM, one of which (glycine hydrazides) had many active structural analogues. Analysis of a series of synthesized glycine hydrazide analogues revealed maximal inhibitory potency for N-(2-naphthalenyl) and 3,5-dibromo-2,4-dihydroxyphenyl substituents. The compound N-(2-naphthalenyl)-[(3,5-dibromo-2,4-dihydroxyphenyl)methylene]glycine hydrazide (GlyH-101) reversibly inhibited CFTR Cl⁻ conductance in <1 min. Whole-cell current measurements revealed voltage-dependent CFTR block by GlyH-101 with strong inward rectification, producing an increase in apparent inhibitory constant K_i from 1.4 μM at +60 mV to 5.6 μM at −60 mV. Apparent potency was reduced by lowering extracellular Cl⁻ concentration. Patch-clamp experiments indicated fast channel closures within bursts of channel openings, reducing mean channel open time from 264 to 13 ms (−60 mV holding potential, 5 μM GlyH-101). GlyH-101 inhibitory potency was independent of pH from 6.5–8.0, where it exists predominantly as a monovalent anion with solubility ∼1 mM in water. Topical GlyH-101 (10 μM) in mice rapidly and reversibly inhibited forskolin-induced hyperpolarization in nasal potential differences. In a closed-loop model of cholera, intraluminal GlyH-101 (2.5 μg) reduced by ∼80% cholera toxin–induced intestinal fluid secretion. Compared with the thiazolidinone CFTR inhibitor CFTR_inh-172, GlyH-101 has substantially greater water solubility and rapidity of action, and a novel inhibition mechanism involving occlusion near the external pore entrance. Glycine hydrazides may be useful as probes of CFTR pore structure, in creating animal models of CF, and as antidiarrheals in enterotoxic-mediated secretory diarrheas.

Mechanism of block of hEag1 K+ channels by imipramine and astemizole

Ether à go-go (Eag; KV10.1) voltage-gated K+ channels have been detected in cancer cell lines of diverse origin and shown to influence their rate of proliferation. The tricyclic antidepressant imipramine and the antihistamine astemizole inhibit the current through Eag1 channels and reduce the proliferation of cancer cells. Here we describe the mechanism by which both drugs block human Eag1 (hEag1) channels. Even if both drugs differ in their affinity for hEag1 channels (IC50s are approximately 2 microM for imipramine and approximately 200 nM for astemizole) and in their blocking kinetics, both drugs permeate the membrane and inhibit the hEag1 current by selectively binding to open channels. Furthermore, both drugs are weak bases and the IC50s depend on both internal an external pH, suggesting that both substances cross the membrane in their uncharged form and act from inside the cell in their charged forms. Accordingly, the block by imipramine is voltage dependent and antagonized by intracellular TEA, consistent with imipramine binding in its charged form to a site located close to the inner end of the selectivity filter. Using inside- and outside-out patch recordings, we found that a permanently charged, quaternary derivative of imipramine (N-methyl-imipramine) only blocks channels from the intracellular side of the membrane. In contrast, the block by astemizole is voltage independent. However, as astemizole competes with imipramine and intracellular TEA for binding to the channel, it is proposed to interact with an overlapping intracellular binding site. The significance of these findings, in the context of structure-function of channels of the eag family is discussed.

Nitration and functional loss of voltage-gated K+ channels in rat coronary microvessels exposed to high glucose

Coronary microvessels generate reactive oxygen species in response to high glucose (HG), resulting in vasodilator defects involving an impaired function of vascular K(+) channels. Inhibition of voltage-gated K(+) (K(v)) channels by peroxynitrite (ONOO(-)), formed by the interaction of superoxide and nitric oxide, may contribute to impaired dilation. The present study investigated whether HG induces ONOO(-) formation to mediate nitration and impairment of K(v) channels in rat small coronary arteries (RSCAs). Exposure to ONOO(-) reduced the dilator influence of K(v) channels in RSCAs. Patch-clamp studies revealed that ONOO(-) diminished whole-cell and unitary K(v) currents attributable to the K(v)1 gene family in smooth muscle cells. Subsequently, immunohistochemically detected enhancement of nitrotyrosine residues in RSCAs that were cultured in HG (23 mmol/l) compared with normal glucose (5.5 mmol/l) for 24 h correlated with the nitration of K(v)1.2 channel alpha-subunits. HG-induced nitrotyrosine formation was partially reversed by scavenging ONOO(-). Finally, RSCAs that were exposed to HG for 24 h showed a loss of K(v) channel dilator influence that also was partially restored by the ONOO(-) scavengers urate and ebselen. We conclude that ONOO(-) generated by HG impairs K(v) channel function in coronary microvessels, possibly by nitrating tyrosine residues in the pore-forming region of the K(v) channel protein.

A gastropod toxin selectively slows early transitions in the Shaker K channel's activation pathway

A toxin from a marine gastropod's defensive mucus, a disulfide-linked dimer of 6-bromo-2-mercaptotryptamine (BrMT), was found to inhibit voltage-gated potassium channels by a novel mechanism. Voltage-clamp experiments with Shaker K channels reveal that externally applied BrMT slows channel opening but not closing. BrMT slows K channel activation in a graded fashion: channels activate progressively slower as the concentration of BrMT is increased. Analysis of single-channel activity indicates that once a channel opens, the unitary conductance and bursting behavior are essentially normal in BrMT. Paralleling its effects against channel opening, BrMT greatly slows the kinetics of ON, but not OFF, gating currents. BrMT was found to slow early activation transitions but not the final opening transition of the Shaker ILT mutant, and can be used to pharmacologically distinguish early from late gating steps. This novel toxin thus inhibits activation of Shaker K channels by specifically slowing early movement of their voltage sensors, thereby hindering channel opening. A model of BrMT action is developed that suggests BrMT rapidly binds to and stabilizes resting channel conformations.

Characterization of a novel gastropod toxin (6-bromo-2-mercaptotryptamine) that inhibits shaker K channel activity

A novel potassium channel antagonist has been purified from the defensive mucus secreted by Calliostoma canaliculatum, a marine snail found in the temperate coastal waters of the western Pacific. The toxin is expelled from the hypobranchial gland as part of a defensive response and is contained within a viscous matrix that minimizes dilution and degradation. The active compound was isolated by multistage microbore HPLC separations followed by bioactivity assays. Nuclear magnetic resonance, combined with electrospray ionization Fourier-transform ion cyclotron resonance and electrospray ionization ion trap mass spectrometry indicate that the active component is a heretofore unknown indole-derivative, a disulfide-linked dimer of 6-bromo-2-mercaptotryptamine (BrMT). Exudates from the hypobranchial glands of various marine mollusks have been sources for dye compounds such as 6-6 dibromoindigo, the ancient dye Tyrian purple. BrMT represents the first correlation of a hypobranchial gland exudate with a molecular response. Voltage clamp experiments with a number of K channel subtypes indicate that BrMT inhibits certain voltage-gated K channels of the Kv1 subfamily.

Endogenous Kv channels in human embryonic kidney (HEK-293) cells

The human embryonic kidney cells (HEK-293) have been widely used as one mammalian expression system in the study of voltage-gated K+ (Kv) channels. Understanding the endogenous Kv channels in these cells is the prerequisite for the characterization of the heterogeneously expressed Kv channels in these cells. In the present study we screened the transcriptional expression of different Kv genes in HEK-293 cells using reverse transcribed DNApolymerase chain reaction (RT-PCR) method. Among 16 Kv genes examined in native HEK-293 cells 10 Kv genes were reproducibly amplified, including those Kv a subunits encoding for the delayed rectifier (IK) [Kv1.1, Kv1.2, Kv1.3, Kv1.6, and Kv3.1], and for the transient outward Kv channels (IA) [Kv1.4, Kv3.3, Kv3.4, and Kv4.1] as well as a Kvbeta2 subunit. The whole-cell outward rectifier IK currents in the native HEK-293 cells were recorded (203 +/- 13 pA at +30 mV, n = 82) with the patch-clamp technique. In about 42% of the examined cells, IA coexisted with IK currents. IK currents were inhibited by tetraethylammonium chloride (TEA) at 1 and 10 mM by 39.5 and 48.4%, respectively. A 39.6% inhibition of IK currents was also observed in the presence of4-aminopyridine (4-AP, 5 mM). Interestingly, both TEAand 4-AP also inhibited IA currents. 4-acelamido-4'-isothiocyanalostilbene-2, 2'-disulfonic acid (1 mM), a Cl- channel blocker, had no effect on the endogenous outward currents. We concluded that multiple endogenous Kv genes were expressed in native HEK-293 cells, which possessed significant endogenous IK and IA currents with unique pharmacological properties.