Isoform-specific modulation of voltage-gated Na(+) channels by calmodulin

Regulation of sodium channel function by bilayer elasticity: the importance of hydrophobic coupling. Effects of Micelle-forming amphiphiles and cholesterol

Membrane proteins are regulated by the lipid bilayer composition. Specific lipid–protein interactions rarely are involved, which suggests that the regulation is due to changes in some general bilayer property (or properties). The hydrophobic coupling between a membrane-spanning protein and the surrounding bilayer means that protein conformational changes may be associated with a reversible, local bilayer deformation. Lipid bilayers are elastic bodies, and the energetic cost of the bilayer deformation contributes to the total energetic cost of the protein conformational change. The energetics and kinetics of the protein conformational changes therefore will be regulated by the bilayer elasticity, which is determined by the lipid composition. This hydrophobic coupling mechanism has been studied extensively in gramicidin channels, where the channel–bilayer hydrophobic interactions link a “conformational” change (the monomer↔dimer transition) to an elastic bilayer deformation. Gramicidin channels thus are regulated by the lipid bilayer elastic properties (thickness, monolayer equilibrium curvature, and compression and bending moduli). To investigate whether this hydrophobic coupling mechanism could be a general mechanism regulating membrane protein function, we examined whether voltage-dependent skeletal-muscle sodium channels, expressed in HEK293 cells, are regulated by bilayer elasticity, as monitored using gramicidin A (gA) channels. Nonphysiological amphiphiles (β-octyl-glucoside, Genapol X-100, Triton X-100, and reduced Triton X-100) that make lipid bilayers less “stiff”, as measured using gA channels, shift the voltage dependence of sodium channel inactivation toward more hyperpolarized potentials. At low amphiphile concentration, the magnitude of the shift is linearly correlated to the change in gA channel lifetime. Cholesterol-depletion, which also reduces bilayer stiffness, causes a similar shift in sodium channel inactivation. These results provide strong support for the notion that bilayer–protein hydrophobic coupling allows the bilayer elastic properties to regulate membrane protein function.

Channel activation voltage alone is directly altered in an isoform-specific manner by Na(v1.4) and Na(v1.5) cytoplasmic linkers

The isoform-specific direct role of cytoplasmic loops in the gating of two voltage-gated sodium channel isoforms, the human cardiac channel (Na(v1.5); hH1) and the human adult skeletal muscle channel (Na(v1.4); hSkM1), was investigated. Comparison of biophysical characteristics was made among hSkM1, hH1, and several hSkM1/hH1 chimeras in which the putative cytoplasmic loops that join domain I to II (loop A) and domain II to III (loop B) from one isoform replaced one or both of the analogous loops from the other isoform. For all parameters measured, hSkM1 and hH1 behavior were significantly different. Comparison of hSkM1 and hH1 biophysical characteristics with the function of their respective chimeras indicate that only the half-activation voltage ( V(a)) is directly and differently altered by the species of cytoplasmic loop such that a channel consisting of one or both hSkM1 loops activates at smaller depolarizations, while a larger depolarization is required for activation of a channel containing one or both of the analogous hH1 loops. When either cardiac channel loop A or B is attached to hSkM1, a 6-7 mV depolarizing shift in V(a) is measured, increasing to a nearly 20 mV depolarization when both cardiac-channel loops are attached. The addition of either skeletal muscle-channel loop to hH1 causes a 7 mV hyperpolarization in V(a), which increases to about 10 mV for the double loop chimera. There is no significant difference in either steady-state inactivation or in the recovery from inactivation data between hSkM1 and its chimeras and between hH1 and its chimeras. Data indicate that the cytoplasmic loops contribute directly to the magnitude of the window current, suggesting that channels containing skeletal muscle loops have three times the peak persistent channel activity compared to channels containing the cardiac loops. An electrostatic mechanism, in which surface charge differences among these loops might alter differently the voltage sensed by the gating mechanism of the channel, can not account for the observed isoform-specific effects of these loops only on channel activation voltage. In summary, although the DI-DII and DII-DIII loop structures among isoforms are not well conserved, these data indicate that only one gating parameter, V(a) is affected directly and in an isoform-specific manner by these divergent loop structures, creating loop-specific window currents and percentages of persistently active channels at physiological voltages that will likely impact the excitability of the cell.

Calmodulin Mediates Ca2+ Sensitivity of Sodium Channels

Ca2+ has been proposed to regulate Na+ channels through the action of calmodulin (CaM) bound to an IQ motif or through direct binding to a paired EF hand motif in the Nav1 C terminus. Mutations within these sites cause cardiac arrhythmias or autism, but details about how Ca2+ confers sensitivity are poorly understood. Studies on the homologous Cav1.2 channel revealed non-canonical CaM interactions, providing a framework for exploring Na+ channels. In contrast to previous reports, we found that Ca2+ does not bind directly to Na+ channel C termini. Rather, Ca2+ sensitivity appears to be mediated by CaM bound to the C termini in a manner that differs significantly from CaM regulation of Cav1.2. In Nav1.2 or Nav1.5, CaM bound to a localized region containing the IQ motif and did not support the large Ca(2+)-dependent conformational change seen in the Cav1.2.CaM complex. Furthermore, CaM binding to Nav1 C termini lowered Ca2+ binding affinity and cooperativity among the CaM-binding sites compared with CaM alone. Nonetheless, we found suggestive evidence for Ca2+/CaM-dependent effects upon Nav1 channels. The R1902C autism mutation conferred a Ca(2+)-dependent conformational change in Nav1.2 C terminus.CaM complex that was absent in the wild-type complex. In Nav1.5, CaM modulates the Cterminal interaction with the III-IV linker, which has been suggested as necessary to stabilize the inactivation gate, to minimize sustained channel activity during depolarization, and to prevent cardiac arrhythmias that lead to sudden death. Together, these data offer new biochemical evidence for Ca2+/CaM modulation of Na+ channel function.

Functional role of the C-terminus of voltage-gated sodium channel Nav1.8

Sodium channel Na(v)1.8 requires stronger depolarization than other sodium channels for activation and inactivation. The contribution of Na(v)1.8 C-terminus to this property was investigated by producing Na(v)1.8 and Na(v)1.4 chimeras and expressing them in ND7/23 cells. Current densities of the chimeras were significantly different than in parental channels, and the voltage-dependence of activation was depolarized in Na(v)1.4/1.8C compared to Na(v)1.4. Analysis of steady-state inactivation showed that only Na(v)1.8 and Na(v)1.4/1.8C currents demonstrate a non-inactivated fraction. Thus, the C-terminus of Na(v)1.8 contributes to regulation of channel density at the cell surface, modulates channel gating, and regulates the generation of sustained current.

Hyperpolarized shifts in the voltage dependence of fast inactivation of Nav1.4 and Nav1.5 in a rat model of critical illness myopathy

Critical illness myopathy is a disorder in which skeletal muscle becomes electrically inexcitable. We previously demonstrated that a shift in the voltage dependence of fast inactivation of sodium currents contributes to inexcitability of affected fibres in an animal model of critical illness myopathy in which denervated rat skeletal muscle is treated with corticosteroids (steroid-denervated; SD). In the current study we examined whether expression of Nav1.5 contributes to the altered voltage dependence of sodium channel inactivation in SD muscle. We used TTX and mu-conotoxin GIIIB to selectively block Nav1.4 in SD muscle and found that the level of Nav1.5 did not correlate closely with the shift in fast inactivation. Surprisingly, we found that the voltage dependence of inactivation of Nav1.4 was similar to that of Nav1.5 in skeletal muscle in vivo. In severely affected fibres, inactivation of both Nav1.4 and Nav1.5 was shifted towards hyperpolarized potentials. We examined the role of denervation and steroid treatment in the shift of the voltage dependence of inactivation and found that both denervation and steroid treatment contribute to the shift in inactivation. Our results suggest that modulation of the voltage dependence of inactivation of both Nav1.4 and Nav1.5 in vivo contributes to loss of electrical excitability in SD muscle.

Fiber type conversion alters inactivation of voltage-dependent sodium currents in murine C2C12 skeletal muscle cells

Each skeletal muscle of the body contains a unique composition of "fast" and "slow" muscle fibers, each of which is specialized for certain challenges. This composition is not static, and the muscle fibers are capable of adapting their molecular composition by altered gene expression (i.e., fiber type conversion). Whereas changes in the expression of contractile proteins and metabolic enzymes in the course of fiber type conversion are well described, little is known about possible adaptations in the electrophysiological properties of skeletal muscle cells. Such adaptations may involve changes in the expression and/or function of ion channels. In this study, we investigated the effects of fast-to-slow fiber type conversion on currents via voltage-gated Na+ channels in the C(2)C(12) murine skeletal muscle cell line. Prolonged treatment of cells with 25 nM of the Ca2+ ionophore A-23187 caused a significant shift in myosin heavy chain isoform expression from the fast toward the slow isoform, indicating fast-to-slow fiber type conversion. Moreover, Na+ current inactivation was significantly altered. Slow inactivation less strongly inhibited the Na+ currents of fast-to-slow fiber type-converted cells. Compared with control cells, the Na+ currents of converted cells were more resistant to block by tetrodotoxin, suggesting enhanced relative expression of the cardiac Na+ channel isoform Na(v)1.5 compared with the skeletal muscle isoform Na(v)1.4. These results imply that fast-to-slow fiber type conversion of skeletal muscle cells involves functional adaptation of their electrophysiological properties.

An EF-hand in the sodium channel couples intracellular calcium to cardiac excitability

Sodium channels initiate the electrical cascade responsible for cardiac rhythm, and certain life-threatening arrhythmias arise from Na(+) channel dysfunction. We propose a novel mechanism for modulation of Na(+) channel function whereby calcium ions bind directly to the human cardiac Na(+) channel (hH1) via an EF-hand motif in the C-terminal domain. A functional role for Ca(2+) binding was identified electrophysiologically, by measuring Ca(2+)-induced modulation of hH1. A small hH1 fragment containing the EF-hand motif was shown to form a structured domain and to bind Ca(2+) with affinity characteristic of calcium sensor proteins. Mutations in this domain reduce Ca(2+) affinity in vitro and the inactivation gating effects of Ca(2+) in electrophysiology experiments. These studies reveal the molecular basis for certain forms of long QT syndrome and other arrhythmia-producing syndromes, and suggest a potential pharmacological target for antiarrhythmic drug design.

Voltage-gated sodium channels and pain pathways

Acute, inflammatory, and neuropathic pain can all be attenuated or abolished by local treatment with sodium channel blockers such as lidocaine. The peripheral input that drives pain perception thus depends on the presence of functional voltage-gated sodium channels. Remarkably, two voltage-gated sodium channel genes (Nav1.8 and Nav1.9) are expressed selectively in damage-sensing peripheral neurons, while a third channel (Nav1.7) is found predominantly in sensory and sympathetic neurons. An embryonic channel (Nav1.3) is also upregulated in damaged peripheral nerves and associated with increased electrical excitability in neuropathic pain states. A combination of antisense and knock-out studies support a specialized role for these sodium channels in pain pathways, and pharmacological studies with conotoxins suggest that isotype-specific antagonists should be feasible. Taken together, these data suggest that isotype-specific sodium channel blockers could be useful analgesics.

Calmodulin binds to the C terminus of sodium channels Nav1.4 and Nav1.6 and differentially modulates their functional properties

Modulation of voltage-gated sodium channels (VGSC) can have a major impact on cell excitability. Analysis of calmodulin (CaM) binding to GST-fusion proteins containing the C-terminal domains of Nav1.1-Nav1.9 indicates that some of the tetrodotoxin-sensitive VGSC isoforms, including NaV1.4 and NaV1.6, are able to bind CaM in a calcium-independent manner. Here we demonstrate that association with CaM is important for functional expression of NaV1.4 and NaV1.6 VGSCs. Disrupting the interaction between CaM and the C terminus of NaV1.4 and NaV1.6 channels reduced current amplitude by 99 and 62%, respectively. Overexpression of CaM increased the current generated by Nav1.4 and Nav1.6 C-terminal mutant constructs that exhibited intermediate current densities and intermediate binding affinities for CaM, demonstrating that this effect on current density was directly dependent on the ability of the C terminus to bind CaM. In addition to the effects on current density, calmodulin also was able to modulate the inactivation kinetics of Nav1.6, but not Nav1.4, currents in a calcium-dependent manner. Our data demonstrate that CaM can regulate the properties of VGSCs via calcium-dependent and calcium-independent mechanisms and suggest that modulation of neuronal sodium channels may play a role in calcium-dependent neuronal plasticity.

Phenotypic differences in transient outward K+ current of human and canine ventricular myocytes: insights into molecular composition of ventricular Ito

The Ca(2+)-independent transient outward K(+) current (I(to)) plays an important electrophysiological role in normal and diseased hearts. However, its contribution to ventricular repolarization remains controversial because of differences in its phenotypic expression and function across species. The dog, a frequently used model of human cardiac disease, exhibits altered functional expression of I(to). To better understand the relevance of electrical remodeling in dogs to humans, we studied the phenotypic differences in ventricular I(to) of both species with electrophysiological, pharmacological, and protein-chemical techniques. Several notable distinctions were elucidated, including slower current decay, more rapid recovery from inactivation, and a depolarizing shift of steady-state inactivation in human vs. canine I(to). Whereas recovery from inactivation of human I(to) followed a monoexponential time course, canine I(to) recovered with biexponential kinetics. Pharmacological sensitivity to flecainide was markedly greater in human than canine I(to), and exposure to oxidative stress did not alter the inactivation kinetics of I(to) in either species. Western blot analysis revealed immunoreactive bands specific for Kv4.3, Kv1.4, and Kv channel-interacting protein (KChIP)2 in dog and human, but with notable differences in band sizes across species. We report for the first time major variations in phenotypic properties of human and canine ventricular I(to) despite the presence of the same subunit proteins in both species. These data suggest that differences in electrophysiological and pharmacological properties of I(to) between humans and dogs are not caused by differential expression of the K channel subunit genes thought to encode I(to), but rather may arise from differences in molecular structure and/or posttranslational modification of these subunits.

Small conductance Ca2+-activated K+ channels and calmodulin: cell surface expression and gating

Small conductance Ca2+-activated K+ channels (SK channels) are heteromeric complexes of pore-forming alpha subunits and constitutively bound calmodulin (CaM). The binding of CaM is mediated in part by the electrostatic interaction between residues Arg-464 and Lys-467 of SK2 and Glu-84 and Glu-87 of CaM. Heterologous expression of the double charge reversal in SK2, SK2 R464E/K467E (SK2:64/67), did not yield detectable surface expression or channel activity in whole cell or inside-out patch recordings. Coexpression of SK2:64/67 with wild type CaM or CaM1,2,3,4, a mutant lacking the ability to bind Ca2+, rescued surface expression. In patches from cells coexpressing SK2:64/67 and wild type CaM, currents were recorded immediately following excision into Ca2+-containing solution but disappeared within minutes after excision or immediately upon exposure to Ca2+-free solution and were not reactivated upon reapplication of Ca2+-containing solution. Channel activity was restored by application of purified recombinant Ca2+-CaM or exposure to Ca2+-free CaM followed by application of Ca2+-containing solution. Coexpression of the double charge reversal E84R/E87K in CaM (CaM:84/87) with SK2:64/67 reconstituted stable Ca2+-dependent channel activity that was not lost with exposure to Ca2+-free solution. Therefore, Ca2+-independent interactions with CaM are required for surface expression of SK channels, whereas the constitutive association between the two channel subunits is not an essential requirement for gating.

Intracellular calcium release resulting from mGluR1 receptor activation modulates GABAA currents in wide-field retinal amacrine cells: a study with caffeine

The modulatory action of calcium (Ca2+) released from intracellular stores on GABAA receptor-mediated current was investigated in wide-field amacrine cells isolated from the teleost, Morone chrysops, retina. Caffeine, ryanodine or inositol 1,4,5-triphosphate (IP3) markedly inhibited the GABAA current by elevating [Ca2+]i. The inhibition resulted from the activation of a Ca2+--> Ca2+/calmodulin --> calcineurin cascade. Long (>12 s) exposure to glutamate mimicked the caffeine effect, i.e. it inhibited the GABAA current by elevating [Ca2+]i through mGluR1 receptor activation and consequent IP3 generation. This pathway provides a 'timed' disinhibitory mechanism to potentiate excitatory postsynaptic potentials in wide-field amacrine cells. It occurs as a result of the suppression of GABA-mediated conductances as a function of the duration of presynaptic excitatory input activity. This is much like some forms of long-term potentiation in the central nervous system. In a local retinal circuit this will selectively accentuate particular excitatory inputs to the wide-field amacrine cell. Similar to other neural systems, we suggest that activity-dependent postsynaptic disinhibition is an important feature of the signal processing in the inner retina.

Mechanism linking NMDA receptor activation to modulation of voltage-gated sodium current in distal retina

In this study, we investigated the mechanism that links activation of N-methyl-D-aspartate (NMDA) receptors to inhibition of voltage-gated sodium channels in isolated catfish cone horizontal cells. NMDA channels were activated in voltage-clamped cells incubated in low-calcium saline or dialyzed with the calcium chelator BAPTA to determine that calcium influx through NMDA channels is required for sodium channel modulation. To determine whether calcium influx through NMDA channels triggers calcium-induced calcium release (CICR), cells were loaded with the calcium-sensitive dye calcium green 2 and changes in relative fluorescence were measured in response to NMDA. Responses were compared with measurements obtained when caffeine depleted stores. Voltage-clamp studies demonstrated that CICR modulated sodium channels in a manner similar to that of NMDA. Blocking NMDA receptors with AP-7, blocking CICR with ruthenium red, depleting stores with caffeine, or dialyzing cells with calmodulin antagonists W-5 or peptide 290-309 all prevented sodium channel modulation. These results support the hypothesis that NMDA modulation of voltage-gated sodium channels in horizontal cells requires CICR and activation of a calmodulin-dependent signaling pathway.

A modeling study predicts the presence of voltage gated Ca2+ channels on myelinated central axons

The objective of this current study was to investigate whether voltage gated Ca(2+) channels are present on axons of the adult rat optic nerve (RON). Simulations of axonal excitability using a Hodgkin-Huxley based one-compartment model incorporating I(Na), I(K) and leak currents were used to predict conditions under which the potential contribution of a Ca(2+) current to an evoked action potential could be measured. Under control conditions the inclusion of a high threshold Ca(2+) current (I(Ca)) in the model had a negligible effect on the action potential. Reducing I(K), by decreasing the value of g(K), elongated the repolarizing phase of the action potential, increasing its duration. Subsequent incorporation of I(Ca) in the model revealed a significant I(Ca) contribution to the repolarizing phase of the action potential. The simulation thus suggests that Ca(2+) channels may be present on RON axons, but that pharmacological intervention is required to unmask their presence. Experiments based on the simulations revealed that there was no significant contribution of I(Ca) to the control action potential. However, as predicted by the simulation, reducing the repolarizing effect of I(K) by adding the K(+) channel blocker 4-AP revealed a Ca(2+) component on the repolarizing phase of the action potential that was blocked by the Ca(2+) channel inhibitor nifedipine.

Calmodulin regulation of excitation-contraction coupling in cardiac myocytes

Calmodulin (CaM) as a ubiquitous Ca2+ sensor interacts with multiple key molecules involved in excitation-contraction (EC) coupling. In the present study, we report that adenoviral expression of a mutant CaM lacking all of its four Ca2+-binding sites, CaM(1-4), at a level 6.5-fold over endogenous CaM markedly increases the amplitude and abbreviates the decay time of Ca2+ transients and contraction in cultured rat ventricular myocytes. To determine the underlying mechanisms, we examined the properties of L-type Ca2+ channels, Ca2+/CaM-dependent protein kinase II (CaMKII), and phospholamban (PLB) in the sarcoplasmic reticulum (SR). We found that CaM(1-4) expression markedly augmented L-type Ca2+ current amplitude and slowed its inactivation. Surprisingly, overexpression of CaM(1-4) increased CaMKII activity and phosphorylation of PLB-Thr-17. Moreover, CaM(1-4) elevated diastolic Ca2+ and caffeine-labile Ca2+ content of the SR. Inhibition of CaMKII by KN-93 or a myristoylated autocamtide-2 related inhibitory peptide prevented the aforementioned PLB phosphorylation and reversed the positive inotropic and relaxant effects, indicating that CaMKII is essential to CaM(1-4) actions. These results demonstrate that CaM modulates Ca2+ influx, SR Ca2+ release, and Ca2+ recycling during cardiac EC coupling. A novel finding of this study is that expression of a Ca2+-insensitive CaM mutant can lead to activation of CaMKII in cardiac myocytes.

Sensory neuron proteins interact with the intracellular domains of sodium channel NaV1.8

Voltage-gated sodium channels initiate and propagate action potentials in excitable cells. The tetrodotoxin-resistant Na(+) channel (Na(V)1.8/SNS) is expressed in damage-sensing neurons (nociceptors) and plays an important role in pain pathways. Expression of high levels of functional Na(V)1.8 in heterologous cells has proved problematic, even in the presence of known sodium channel accessory beta-subunits. This suggests that other regulatory proteins are required for normal levels of Na(V)1.8 expression. Here we report the use of a yeast two-hybrid system and a rat dorsal root ganglion cDNA library to identify 28 different clones encoding proteins which interact with intracellular domains of Na(V)1.8. Many clones are expressed at high levels in small diameter DRG neurons as judged by in situ hybridization. Interacting proteins include cytoplasmic elements and linker proteins (e.g. beta-actin and moesin), enzymes (e.g. inositol polyphosphate 5-phosphatase and TAO2 thousand and one protein kinase), channels and membrane-associated proteins (voltage-dependent anion channel VDAC3V and tetraspanin), as well as motor proteins (dynein intermediate and light chain) and transcripts encoding previously undescribed proteins. Immunoprecipitation (pull-down) assays confirm that some of the proteins interact with, and may hence regulate, Na(V)1.8 in vivo.

Modulation of the cardiac sodium channel Nav1.5 by fibroblast growth factor homologous factor 1B

We have previously shown that fibroblast growth factor homologous factor 1B (FHF1B), a cytosolic member of the fibroblast growth factor family, associates with the sensory neuron-specific channel Na(v)1.9 but not with the other sodium channels present in adult rat dorsal root ganglia neurons. We show in this study that FHF1B binds to the C terminus of the cardiac voltage-gated sodium channel Na(v)1.5 and modulates the properties of the channel. The N-terminal 41 amino acid residues of FHF1B are essential for binding to Na(v)1.5, and the conserved acidic rich domain (amino acids 1773-1832) in the C terminus of Na(v)1.5 is sufficient for association with this factor. Binding of the growth factor to recombinant wild type human Na(v)1.5 in human embryonic kidney 293 cells produces a significant hyperpolarizing shift in the voltage dependence of channel inactivation. An aspartic acid to glycine substitution at position 1790 of the channel, which underlies one of the LQT-3 phenotypes of cardiac arrythmias, abolishes the interaction of the Na(v)1.5 channel with FHF1B. This is the first report showing that interaction with a growth factor can modulate properties of a voltage-gated sodium channel.

Engineered calmodulins reveal the unexpected eminence of Ca2+ channel inactivation in controlling heart excitation

Engineered calmodulins (CaMs), rendered Ca2+-insensitive by mutations, function as dominant negatives in heterologous systems, and have revealed mechanisms of ion channel modulation by Ca2+/CaM. The use of these CaMs in native mammalian cells now emerges as a strategy to unmask the biology of such Ca2+ feedback. Here, we developed recombinant adenoviruses bearing engineered CaMs to facilitate their expression in adult heart cells, where Ca2+ regulation may be essential for moment-to-moment control of the heartbeat. Engineered CaMs not only eliminated the Ca2+-dependent inactivation of native calcium channels, but exposed an unexpectedly large impact of removing such feedback: the unprecedented (4- to 5-fold) prolongation of action potentials. This striking result recasts the basic paradigm for action-potential control and illustrates the promise of virally delivered engineered CaM to investigate the biology of numerous other CaM-signaling pathways.

Gating of the late Na+ channel in normal and failing human myocardium

We previously reported an ultraslow inactivating late Na+ current (INaL) in left ventricular cardiomyocytes (VC) isolated from normal (NVC) and failing (FVC) human hearts. This current could play a role in heart failure-induced repolarization abnormalities. To identify properties of NaCh contributing to INaL, we examined early and late openings in cell-attached patches of HEK293 cells expressing human cardiac NaCh alpha-subunit (alpha-HEK) and in VC of one normal and three failing human hearts. Two types of the late NaCh openings underlay INaL in all three preparations: scattered late (SLO) and bursts (BO). Amplitude analysis revealed that slope conductance for both SLO and BO was the same compared to the main level of early openings (EO) in both VC (21 vs 22.7pS, NVC; 22.7 vs 22.6pS, FVC) and alpha-HEK (23.2 vs 23pS), respectively. Analysis of SLO latencies revealed voltage-independent ultraslow inactivation in all preparations with tendency to be slower in FVC compared to NCV. EO and SLO render one open voltage-independent state (tau approximately 0.4ms) for NVC and FVC. One open (voltage-dependent) and two closed states (one voltage-dependent and another voltage-independent) were found in BO of both specimens. Burst duration tend to be longer in FVC ( approximately 50ms) than in NVC ( approximately 30ms). In FVC we found both modes SLO and BO at membrane potential of -10mV that is attribute for take-off voltages (from -18 to -2mV) for early afterdepolarizations (EAD's) in FVC. In conclusions, we found a novel gating mode SLO that manifest slow (hundreds of ms), voltage-independent inactivation in both NVC and FVC. We were unable to reliably demonstrate any differences in the properties of the late NaCh in failing vs a normal human heart. Accordingly, the late current appears to be generated by a single population of channels in normal and failing human ventricular myocardium. Both SLO and BO could be implicated in EADs in HF.

Identification of gating modes in single native Na+ channels from human atrium and ventricle

The aim of the present study was to investigate the single-channel properties of different gating modes in the native human cardiac Na+ channel. Patch-clamp experiments were performed at low noise using ultrathick-walled pipettes. In 17 cell-attached patches containing only one channel, fast back and forth switching between five different Na+-channel gating modes (F-mode, M1-mode, M2-mode, S-mode, and P-mode) was identified, but no difference in the gating properties was found between normal and diseased cardiomyocytes from atrium or ventricle, respectively. Hodgkin-Huxley fits to the ensemble-averaged currents yielded the activation-time (tau(m)) and inactivation-time (tau(h)) constants. tau(m) was comparably fast in the F-mode, M1-mode, M2-mode, and S-mode (0.15 ms) and slow in the P-mode (0.3 ms). tau(h) ranged from 0.35 ms (F-mode) to 4.5 ms (S-mode and P-mode). The mean open-channel lifetime (tau(o)) was shortest in the F-mode and P-mode (0.15 ms) and longest in the S-mode (1.25 ms). The time before which half of the first channel openings occurred (t(0.5)) was comparably short in the F-mode, M1-mode, M2-mode, and S-mode (0.3 ms) and long in the P-mode (0.9 ms). It is concluded that (1) a single native human cardiac Na+ channel can be recorded at low noise, (2) this channel can change its gating properties at a time scale of milliseconds, (3) lifetimes of the observed gating modes are short ranging from milliseconds to seconds only, and (4) the gating modes are characterized by specific activation and inactivation kinetics and differ at least in their mean open time and first latency.