Effects of channel cytoplasmic regions on the activation mechanisms of cardiac versus skeletal muscle Na(+) channels

Suxiao Jiuxin Pills Prevent Ventricular Fibrillation from Inhibiting L-type Calcium Currents CaV1.2 in vivo and in vitro

OBJECTIVE

To investigate whether Suxiao Jiuxin Pills (SJP), a Chinese herbal remedy, is an anti-ventricular fibrillation (VF) agent.

METHODS

VF was induced by isoproterenolol (ISO) intraperitoneal injection followed by electrical pacing in mice and rabbits. The effects of SJP on the L-type calcium channel current (CaV1.2), voltage-dependent sodium channel current (I_Na), rapid and slow delayed rectifier potassium channel current (I_Kr and I_Ks, respectively) were studied by whole-cell patch-clamp method. Computer simulation was implemented to incorporate the experimental data of SJP effects on the CaV1.2 current into the action potential (AP) and pseudo-electrocardiography (pseudo-ECG) models.

RESULTS

SJP prevented VF induction and reduced VF durations significantly in mice and rabbits. Patch-clamp experiments revealed that SJP decreased the peak amplitude of the CaV1.2 current with a half maximal concentration (IC₅₀) value of 16.9 mg/L (SJP-30 mg/L, -32.8 ± 6.1 pA; Verapamil, -16.2 ±1.8 pA; vs. control, -234.5 ±16.7 pA, P<0.01, respectively). The steady-state activation curve, inactivation curve, and the recovery from inactivation of the CaV1.2 current were not shifted significantly. Specifically, SJP did not altered I_Na, I_Kr, and I_Ks currents significantly (SJP vs. control, P>0.05). Computer simulation showed that SJP-reduced CaV1.2 current shortened the AP duration, transiting VF into sinus rhythm in pseudo-ECG.

CONCLUSION

SJP reduced VF via inhibiting the CaV1.2 current with in vivo, in vitro, and in silico studies, which provide experimental basis for SJP anti-VF clinical application.

Differential thermosensitivity in mixed syndrome cardiac sodium channel mutants

Cardiac arrhythmias are often associated with mutations in SCN5A the gene that encodes the cardiac paralogue of the voltage-gated sodium channel, NaV 1.5. The NaV 1.5 mutants R1193Q and E1784K give rise to both long QT and Brugada syndromes. Various environmental factors, including temperature, may unmask arrhythmia. We sought to determine whether temperature might be an arrhythmogenic trigger in these two mixed syndrome mutants. Whole-cell patch clamp was used to measure the biophysical properties of NaV 1.5 WT, E1784K and R1193Q mutants. Recordings were performed using Chinese hamster ovary (CHOk1) cells transiently transfected with the NaV 1.5 α subunit (WT, E1784K, or R1193Q), β1 subunit, and eGFP. The channels' voltage-dependent and kinetic properties were measured at three different temperatures: 10ºC, 22ºC, and 34ºC. The E1784K mutant is more thermosensitive than either WT or R1193Q channels. When temperature is elevated from 22°C to 34°C, there is a greater increase in late INa and use-dependent inactivation in E1784K than in WT or R1193Q. However, when temperature is lowered to 10°C, the two mutants show a decrease in channel availability. Action potential modelling using Q10 fit values, extrapolated to physiological and febrile temperatures, show a larger transmural voltage gradient in E1784K compared to R1193Q and WT with hyperthermia. The E1784K mutant is more thermosensitive than WT or R1193Q channels. This enhanced thermosensitivity may be a mechanism for arrhythmogenesis in patients with E1784K sodium channels.

Sialic acids attached to N- and O-glycans within the Nav1.4 D1S5-S6 linker contribute to channel gating

BACKGROUND

Voltage-gated Na+ channels (Nav) are responsible for the initiation and conduction of neuronal and muscle action potentials. Nav gating can be altered by sialic acids attached to channel N-glycans, typically through isoform-specific electrostatic mechanisms.

METHODS

Using two sets of Chinese Hamster Ovary cell lines with varying abilities to glycosylate glycoproteins, we show for the first time that sialic acids attached to O-glycans and N-glycans within the Nav1.4 D1S5-S6 linker modulate Nav gating.

RESULTS

All measured steady-state and kinetic parameters were shifted to more depolarized potentials under conditions of essentially no sialylation. When sialylation of only N-glycans or of only O-glycans was prevented, the observed voltage-dependent parameter values were intermediate between those observed under full versus no sialylation. Immunoblot gel shift analyses support the biophysical data.

CONCLUSIONS

The data indicate that sialic acids attached to both N- and O-glycans residing within the Nav1.4 D1S5-S6 linker modulate channel gating through electrostatic mechanisms, with the relative contribution of sialic acids attached to N- versus O-glycans on channel gating being similar.

GENERAL SIGNIFICANCE

Protein N- and O-glycosylation can modulate ion channel gating simultaneously. These data also suggest that environmental, metabolic, and/or congenital changes in glycosylation that impact sugar substrate levels, could lead, potentially, to changes in Nav sialylation and gating that would modulate AP waveforms and conduction.

Expression of the sialyltransferase, ST3Gal4, impacts cardiac voltage-gated sodium channel activity, refractory period and ventricular conduction

The sequential glycosylation process typically ends with sialic acid residues added through trans-Golgi sialyltransferase activity. Individuals afflicted with congenital disorders of glycosylation often have reduced glycoprotein sialylation and present with multi-system symptoms including hypotonia, seizures, arrhythmia and cardiomyopathy. Cardiac voltage-gated Na(+) channel (Nav) activity can be influenced by sialic acids likely contributing to an external surface potential causing channels to gate at less depolarized voltages. Here, a possible pathophysiological role for reduced sialylation is investigated by questioning the impact of gene deletion of the uniformly expressed beta-galactoside alpha-2,3-sialyltransferase 4 (ST3Gal4) on cardiac Nav activity, cellular refractory period and ventricular conduction. Whole-cell patch-clamp experiments showed that ventricular Nav from ST3Gal4 deficient mice (ST3Gal4(-/-)) gated at more depolarized potentials, inactivated more slowly and recovered from fast inactivation more rapidly than WT controls. Current-clamp recordings indicated a 20% increase in time to action potential peak and a 30ms decrease in ST3Gal4(-/-) myocyte refractory period, concurrent with increased Nav recovery rate. Nav expression, distribution and maximal Na(+) current levels were unaffected by ST3Gal4 expression, indicating that reduced sialylation does not impact Nav surface expression and distribution. However, enzymatic desialylation suggested that ST3Gal4(-/-) ventricular Nav are less sialylated. Consistent with the shortened myocyte refractory period, epicardial conduction experiments using optical mapping techniques demonstrated a 27% reduction in minimum ventricular refractory period and increased susceptibility to arrhythmias in ST3Gal4(-/-) ventricles. Thus, deletion of a single sialyltransferase significantly impacts ventricular myocyte electrical signaling. These studies offer insight into diseases of glycosylation that are often associated with pathological changes in excitability and highlight the importance of glycosylation in cardiac physiology.

Involvement of tetrodotoxin-resistant Na+ current and protein kinase C in the action of growth hormone (GH)-releasing hormone on primary cultured somatotropes from GH-green fluorescent protein transgenic mice

GHRH depolarizes the membrane of somatotropes, leading to an increase in intracellular free Ca2+ concentration and GH secretion. Na+ channels mediate the rapid depolarization during the initial phase of the action potential, and this regulates Ca2+ influx and GH secretion. GHRH increases a tetrodotoxin-sensitive somatotrope Na+ current that is mediated by cAMP. TTX-resistant (TTX-R) Na+ channels are abundant in sensory neurons and cardiac myocytes, but their occurrence and/or function in somatotropes has not been investigated. Here we demonstrate expression of TTX-R Na+ channels and a TTX-R Na+ current, using patch-clamp method, in green fluorescent protein-GH transgenic mouse somatotropes. GHRH (100 nm) increased the TTX-R Na+ current in a reversible manner. The GHRH-induced increase in TTX-R Na+ current was not affected by the cAMP antagonist Rp-cAMP or protein kinase A inhibitors KT5720 or H89. The TTX-R current was increased by 8-bromoadenosine-cAMP (cAMP analog), forskolin (adenylyl-cyclase activator), and 3-isobutyl-1-methylxanthine (phosphodiesterase inhibitor), but the additional, GHRH-induced increase in TTX-R Na+ currents was not affected. U-73122 (phospholipase C inhibitor) and protein kinase C (PKC) inhibitors, Gö-6983 and chelerythrine, blocked the effect of GHRH. PKC activators, phorbol dibutyrate and phorbol myristate acetate, increased the TTX-R Na+ current, but GHRH had no further effect on the current. Na+-free extracellular medium significantly reduced GHRH-stimulated GH secretion. We conclude that GHRH-induced increase in the TTX-R Na+ current in mouse somatotropes is mediated by the PKC system. An increase in the TTX-R Na+ current may contribute to the GHRH-induced exocytosis of GH granules from mouse somatotropes.

Functional expression of "cardiac-type" Nav1.5 sodium channel in canine intracardiac ganglia

BACKGROUND

The autonomic nervous system has been implicated in several arrhythmogenic diseases, including long QT syndrome type 3 (LQT3) and Brugada syndrome. Scarce information on the cellular components of the intrinsic cardiac ganglia from higher mammals has limited our understanding of the role of the autonomic nervous system in such diseases.

OBJECTIVES

The purpose of this study was to isolate and characterize the electrophysiologic properties of canine intracardiac neurons.

METHODS

Action potentials (APs) and ionic currents were studied in enzymatically dissociated canine intracardiac neurons under current and voltage clamp conditions. Immunohistochemical and reverse transcription-polymerase chain reaction analysis was performed using freshly isolated intracardiac ganglia.

RESULTS

APs recorded from intracardiac neurons displayed a tetrodotoxin-resistant (TTX-R) component. TTX-R APs were abolished in the absence of sodium but persisted in the absence of external calcium. Immunohistochemical studies showed the presence of TTX-R sodium channels in these ganglia. Sodium currents were characterized by two components with different affinities for TTX: a tetrodotoxin-sensitive (TTX-S) component and a TTX-R component. TTX-S current inactivation was characteristic of neuronal sodium currents, whereas TTX-R current inactivation time constants were similar to those previously reported for Na(v)1.5 channels. TTX sensitivity (IC(50) = 1.17 microM) of the TTX-R component was in the range reported for Na(v)1.5 channels. Expression of Na(v)1.5 channels in intracardiac ganglia was confirmed by PCR analysis and sequencing.

CONCLUSION

Our results suggest that canine intracardiac neurons functionally express Na(v)1.5 channels. These findings open an exciting new door to our understanding of autonomically modulated arrhythmogenic diseases linked to mutations in Na(v)1.5 channels, including Brugada syndrome and LQT3.

Differential sialylation modulates voltage-gated Na+ channel gating throughout the developing myocardium

Voltage-gated sodium channel function from neonatal and adult rat cardiomyocytes was measured and compared. Channels from neonatal ventricles required an ∼10 mV greater depolarization for voltage-dependent gating events than did channels from neonatal atria and adult atria and ventricles. We questioned whether such gating shifts were due to developmental and/or chamber-dependent changes in channel-associated functional sialic acids. Thus, all gating characteristics for channels from neonatal atria and adult atria and ventricles shifted significantly to more depolarized potentials after removal of surface sialic acids. Desialylation of channels from neonatal ventricles did not affect channel gating. After removal of the complete surface N-glycosylation structures, gating of channels from neonatal atria and adult atria and ventricles shifted to depolarized potentials nearly identical to those measured for channels from neonatal ventricles. Gating of channels from neonatal ventricles were unaffected by such deglycosylation. Immunoblot gel shift analyses indicated that voltage-gated sodium channel α subunits from neonatal atria and adult atria and ventricles are more heavily sialylated than α subunits from neonatal ventricles. The data are consistent with approximately 15 more sialic acid residues attached to each α subunit from neonatal atria and adult atria and ventricles. The data indicate that differential sialylation of myocyte voltage-gated sodium channel α subunits is responsible for much of the developmental and chamber-specific remodeling of channel gating observed here. Further, cardiac excitability is likely impacted by these sialic acid–dependent gating effects, such as modulation of the rate of recovery from inactivation. A novel mechanism is described by which cardiac voltage-gated sodium channel gating and subsequently cardiac rhythms are modulated by changes in channel-associated sialic acids.

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.

N,N′,N′-triacetylglucosamine, an inhibitor of lysozyme, prevents myocardial depression in Escherichia coli sepsis in dogs*

OBJECTIVE

Reversible myocardial depression in sepsis has been ascribed to the release of inflammatory mediators. We recently found that lysozyme c (Lzm-S), consistent with that originating from the spleen, was a mediator of myocardial depression in an Escherichia coli model of septic shock in dogs. We further showed in a right ventricular trabecular (RVT) preparation that Lzm-S's depressant activity could be blocked by N,N',N" triacetylglucosamine (TAC), a competitive inhibitor of Lzm-S. We hypothesized that Lzm-S binds to or cleaves a cardiac membrane glycoprotein, thereby interfering with myocardial contraction in sepsis. In the present study, we examined whether TAC could prevent myocardial depression in an in vivo preparation and whether other related N-acetylglucosamine (NAG) structures could also inhibit Lzm-S's effect in RVT.

DESIGN

Randomized experimental study.

SETTING

University laboratory.

SUBJECTS

Anesthetized, mechanically ventilated dogs.

INTERVENTIONS

We produced sepsis by infusion of E. coli over an approximately 6-hr period.

MEASUREMENTS AND MAIN RESULTS

We examined the effect of TAC on stroke work, our primary index of myocardial function, when treatment was administered before sepsis (pretreatment) and after 1.5 hrs (early treatment study) and 3.5 hrs of sepsis (late treatment study; LTS). In the pretreatment study and early treatment study, myocardial depression would have not yet occurred but would have already been present in the late treatment study. In RVT, we assessed the effect of other NAG oligosaccharides and variants to the NAG structure on Lzm-S's depressant activity. In pretreatment and the early treatment study, TAC prevented the reduction in stroke work observed in nontreated septic groups but did not reverse the reduction found in the late treatment study. In RVT, of the compounds tested, only N,N'-diacetylglucosamine showed an inhibitory effect.

CONCLUSIONS

We found that TAC, a competitive inhibitor of Lzm-S, prevented myocardial depression in experimental sepsis. Only specific NAG structures are inhibitory to Lzm-S's depressant activity. TAC may be useful in attenuating cardiovascular collapse in sepsis.

Voltage-gated Na+ channels confer invasive properties on human prostate cancer cells

Prostate cancer is the second leading cause of cancer deaths in American males, resulting in an estimated 37,000 deaths annually, typically the result of metastatic disease. A consequence of the unsuccessful androgen ablation therapy used initially to treat metastatic disease is the emergence of androgen-insensitive prostate cancer, for which there is currently no prescribed therapy. Here, three related human prostate cancer cell lines that serve as a model for this dominant form of prostate cancer metastasis were studied to determine the correlation between voltage-gated sodium channel expression/function and prostate cancer metastatic (invasive) potential: the non-metastatic, androgen-dependent LNCaP LC cell line and two increasingly tumorogenic, androgen-independent daughter cell lines, C4 and C4-2. Fluorometric in vitro invasion assays indicated that C4 and C4-2 cells are more invasive than LC cells. Immunoblot analysis showed that voltage-gated sodium channel expression increases with the invasive potential of the cell line, and this increased invasive potential can be blocked by treatment with the specific voltage-gated sodium channel inhibitor, tetrodotoxin (TTX). These data indicate that increased voltage-gated sodium channel expression and function are necessary for the increased invasive potential of these human prostate cancer cells. When the human adult skeletal muscle sodium channel Na(v1.4) was expressed transiently in each cell line, there was a highly significant increase in the numbers of invading LC, C4, and C4-2 cells. This increased invasive potential was reduced to control levels by treatment with TTX. These data are the first to indicate that the expression of voltage-gated sodium channels alone is sufficient to increase the invasive potential of non-metastatic (LC cells) as well as more aggressive cells (i.e., C4 and C4-2 cells). Together, the data suggest that increased voltage-gated sodium channel expression alone is necessary and sufficient to increase the invasive potential of a set of human prostate cancer cell lines that serve as a model for prostate cancer metastasis.

Na(v)1.5 underlies the 'third TTX-R sodium current' in rat small DRG neurons

In addition to slow-inactivating and persistent TTX-R Na(+) currents produced by Na(v)1.8 and Na(v)1.9 Na(+) channels, respectively, a third TTX-R Na(+) current with fast activation and inactivation can be recorded in 80% of small neurons of dorsal root ganglia (DRG) from E15 rats, but in only 3% of adult small DRG neurons. The half-time for activation, the time constant for inactivation, and the midpoints of activation and inactivation of the third TTX-R Na(+) currents are significantly different from those of Na(v)1.8 and Na(v)1.9 Na(+) currents. The estimated TTX K(i) (2.11+/-0.34 microM) of the third TTX-R Na(+) current is significantly lower than those of Na(v)1.8 and Na(v)1.9 Na(+) currents. The Cd(2+) sensitivity of third TTX-R Na(+) current is closer to cardiac Na(+) currents. A concentration of 1 mM Cd(2+) is required to completely block this current, which is significantly lower than the 5 mM required to block Na(v)1.8 and Na(v)1.9 currents. The third TTX-R Na(+) channel is not co-expressed with Na(v)1.8 and Na(v)1.9 Na(+) channels in DRG neurons of E18 rats, at a time when all three currents show comparable densities. The physiological and pharmacological profiles of the third TTX-R Na(+) current are similar to those of the cardiac Na(+) channel Na(v)1.5 and RT-PCR and restriction enzyme polymorphism analysis, show a parallel pattern of expression of Na(v)1.5 in DRG during development. Taken together, these results demonstrate that Na(v)1.5 is expressed in a developmentally regulated manner in DRG neurons and suggest that Na(v)1.5 Na(+) channel produces the third TTX-R current.

Heterogeneity of sodium current in atrial vs epicardial ventricular myocytes of adult guinea pig hearts

The different sodium channel currents (I(Na)) were reported in myocardium, neuron, and skeletal muscles. To study whether I(Na) is homogeneous within the heart, we applied whole-cell voltage clamp technique to evaluate fast voltage-gated I(Na) in atrial and ventricular myocytes isolated from guinea pig heart. It was found that the density of inward I(Na) was 50% greater at -35 mV in atrial (-42.6+/-2.9 pA/pF) than in ventricular (-27.5+/-1.8 pA/pF, P<0.01) myocytes. The half activation and inactivation voltages (V(0.5)) of I(Na) in atrial myocytes were shifted 4.5+/-0.2 and 9.6+/-0.3 mV negative to those of ventricular myocytes. Time constants for I(Na) activation (tau(m)) and inactivation (tau(h)) were twice as rapid in atrial as in ventricular myocytes. The tau(m) and tau(h) were 0.34+/-0.03 and 1.36+/-0.07 ms for atrial myocytes, and 0.69+/-0.05 and 3.27+/-0.23 ms for ventricular myocytes, respectively. Recovery of I(Na) from inactivation was slower in atrial than in ventricular myocytes, whereas the development of resting state inactivation was more rapid in atrial (tau=67.5+/-4.3 ms) than in ventricular (152.8+/-7.5 ms, P<0.01) myocytes. The results reveal marked heterogeneity of I(Na) in the density and biophysical properties in atrial and ventricular myocytes, and the study suggests the potential possibility of tissue specific cardiac sodium channel isoforms.

Isoform-specific effects of sialic acid on voltage-dependent Na+ channel gating: functional sialic acids are localized to the S5-S6 loop of domain I

The isoform specific role of sialic acid in human voltage-gated sodium channel gating was investigated through expression and chimeric analysis of two human isoforms, Na(v1.4) (hSkM1), and Na(v1.5) (hH1) in Chinese hamster ovary (CHO) cell lines. Immunoblot analyses indicate that both hSkM1 and hH1 are glycosylated and that hSkM1 is more glycosylated than hH1. Four sets of voltage-dependent parameters, the voltage of half-activation (V(a)), the voltage of half-inactivation (V(i)), the time constants for fast inactivation (tau(h)), and the time constants for recovery from inactivation (tau(rec)), were measured for hSkM1 and hH1 expressed in two CHO cell lines, Pro5 and Lec2, to determine the effect of changing sialylation on channel gating under conditions of full (Pro5) or reduced (Lec2) sialylation. For all parameters measured, hSkM1 gating showed a consistent 11-15 mV depolarizing shift under conditions of reduced sialylation, while hH1 showed no significant change in any gating parameter. Shifts in channel V(a) with changing external [Ca2+] indicated that sialylation of hSkM1, but not hH1, directly contributes to a negative surface potential. Functional analysis of two chimeras, hSkM1P1 and hH1P1, indicated that the responsible sialic acids are localized to the hSkM1 S5-S6 loop of domain I. When hSkM1 IS5-S6 was replaced by the analogous hH1 loop (hSkM1P1), changing sialylation had no significant effect on any voltage-dependent parameter. Conversely, when hSkM1 IS5-S6 was added to hH1 (hH1P1), all four parameters shifted by 6-7 mV in the depolarized direction under conditions of reduced sialylation. In summary, the gating of two human sodium channel isoforms show very different dependencies on sialic acid, with hSkM1 gating uniformly altered by sialic acid levels through an apparent electrostatic mechanism, while hH1 gating is unaffected by changing sialylation. Sialic acid-dependent gating can be removed or created by replacing or inserting hSkM1 IS5-S6, respectively, indicating that the functionally relevant sialic acid residues are localized to the first domain of the channel.

Structural basis of differences in isoform-specific gating and lidocaine block between cardiac and skeletal muscle sodium channels

Voltage-gated Na(+) channels underlie rapid conduction in heart and skeletal muscle. Cardiac sodium channels open and close over more negative potentials than do skeletal muscle sodium channels; heart channels are also more sensitive to lidocaine block. The structural basis of these differences is poorly understood. We mutated nine isoform-specific micro1 (rat skeletal muscle) channel residues in domain IV to those at equivalent locations in hH1 (human cardiac) channels. Channel constructs were expressed in tsA-201 cells and screened for changes in gating and lidocaine sensitivity. Only L1373E, located in the linker between the S1 and S2 transmembrane segments, shifted activation gating and use-dependent block by lidocaine toward that seen in hH1. The converse mutation, hH1-E1555L, shifted the phenotype of hH1 to resemble that of micro1. Therefore, we identified a previously unsuspected glutamate-to-leucine isoform-specific variant site (i.e., 1555 in hH1 and 1373 in micro1) that significantly influences gating and drug block in sodium channels. The identification of the residue at this position plays a major role in shaping the responses of sodium channels to voltage and to lidocaine, helping to rationalize the distinctive behavior of cardiac sodium channels.

Channel cytoplasmic loops alter voltage-dependent sodium channel activation in an isoform-specific manner

1. The isoform-specific functional role of cytoplasmic structures of two voltage-gated sodium channel isoforms, the human cardiac channel (hH1) and the adult human skeletal muscle channel (hSkM1) was investigated through functional comparison of chimeras. 2. The voltage of half-activation (V(a)) for hH1 was shifted by > 20 mV in the hyperpolarised direction following internal papain treatment ('papain sensitive'), while V(a) for hSkM1 was unaffected ('papain insensitive'). 3. The hH1 region(s) responsible for this papain sensitivity was localised by testing a series of hH1/hSkM1 chimeras in which combinations of the large hH1 cytoplasmic loops joining the four transmembrane domains replaced analogous hSkM1 loops. Various chimeras were used to determine the smallest subset of loops that converted fully the papain-insensitive hSkM1 into a papain-sensitive channel. Then three converse chimeras were tested in which hSkM1 loops replaced hH1 loops to determine the smallest subset of loops necessary and sufficient to convert the papain-sensitive hH1 into a papain-insensitive channel. 4. Functional studies of this inclusive set of chimeras indicate that the first two cytoplasmic loops of the cardiac sodium channel that join domain I to II (loop A), and domain II to III (loop B), are both necessary, and together are sufficient to produce a papain-induced hyperpolarising shift in the voltage at which channels activate. When both loops are present (wild-type hH1 and the chimera hSkM1AB), V(a) for the channel shifts in the hyperpolarised direction by > 20 mV with papain treatment. When the analogous hSkM1 loops are present (wild-type hSkM1 and the chimera hH1AB), V(a) for the channel is not sensitive to treatment with papain. For channels that contain only one of the two hH1 loops, the effect of papain on V(a) is intermediary. 5. Experiments performed in the absence of papain showed that the activation voltages of the double loop chimeras, hSkM1AB and hH1AB, were shifted significantly from V(a) for hSkM1 and V(a) for hH1, respectively, indicating that these loops directly alter channel activation voltage. The resulting shifts in V(a) were in opposing directions, suggesting that cytoplasmic control of activation voltage is isoform specific. V(a) for hSkM1AB was about 20 mV more depolarised than V(a) for hSkM1, and V(a) for hH1AB was about 9 mV more negative than V(a) for hH1. 6. These data are the first to indicate isoform-specific cytoplasmic regions of the voltage-gated sodium channel that directly and differently alter the voltage of channel activation.

Alternatively spliced alpha(1G) (Ca(V)3.1) intracellular loops promote specific T-type Ca(2+) channel gating properties

At least three genes encode T-type calcium channel alpha(1) subunits, and identification of cDNA transcripts provided evidence that molecular diversity of these channels can be further enhanced by alternative splicing mechanisms, especially for the alpha(1G) subunit (Ca(V)3.1). Using whole-cell patch-clamp procedures, we have investigated the electrophysiological properties of five isoforms of the human alpha(1G) subunit that display a distinct III-IV linker, namely, alpha(1G-a), alpha(1G-b), and alpha(1G-bc), as well as a distinct II-III linker, namely, alpha(1G-ae), alpha(1G-be), as expressed in HEK-293 cells. We report that insertion e within the II-III linker specifically modulates inactivation, steady-state kinetics, and modestly recovery from inactivation, whereas alternative splicing within the III-IV linker affects preferentially kinetics and voltage dependence of activation, as well as deactivation and inactivation. By using voltage-clamp protocols mimicking neuronal activities, such as cerebellar train of action potentials and thalamic low-threshold spike, we describe that inactivation properties of alpha(1G-a) and alpha(1G-ae) isoforms can support channel behaviors reminiscent to those described in native neurons. Altogether, these data demonstrate that expression of distinct variants for the T-type alpha(1G) subunit can account for specific low-voltage-activated currents observed in neuronal tissues.

Block of wild-type and inactivation-deficient cardiac sodium channels IFM/QQQ stably expressed in mammalian cells

The role of inactivation as a central mechanism in blockade of the cardiac Na(+) channel by antiarrhythmic drugs remains uncertain. We have used whole-cell and single channel recordings to examine the block of wild-type and inactivation-deficient mutant cardiac Na(+) channels, IFM/QQQ, stably expressed in HEK-293 cells. We studied the open-channel blockers disopyramide and flecainide, and the lidocaine derivative RAD-243. All three drugs blocked the wild-type Na(+) channel in a use-dependent manner. There was no use-dependent block of IFM/QQQ mutant channels with trains of 20 40-ms pulses at 150-ms interpulse intervals during disopyramide exposure. Flecainide and RAD-243 retained their use-dependent blocking action and accelerated macroscopic current relaxation. All three drugs reduced the mean open time of single channels and increased the probability of their failure to open. From the abbreviation of the mean open times, we estimated association rates of approximately 10(6)/M/s for the three drugs. Reducing the burst duration contributed to the acceleration of macroscopic current relaxation during exposure to flecainide and RAD-243. The qualitative differences in use-dependent block appear to be the result of differences in drug dissociation rate. The inactivation gate may play a trapping role during exposure to some sodium channel blocking drugs.

Residues in Na(+) channel D3-S6 segment modulate both batrachotoxin and local anesthetic affinities

Batrachotoxin (BTX) alters the gating of voltage-gated Na(+) channels and causes these channels to open persistently, whereas local anesthetics (LAs) block Na(+) conductance. The BTX and LA receptors have been mapped to several common residues in D1-S6 and D4-S6 segments of the Na(+) channel alpha-subunit. We substituted individual residues with lysine in homologous segment D3-S6 of the rat muscle mu1 Na(+) channel from F1274 to N1281 to determine whether additional residues are involved in BTX and LA binding. Two mutant channels, mu1-S1276K and mu1-L1280K, when expressed in mammalian cells, become completely resistant to 5 microM BTX during repetitive pulses. The activation and/or fast inactivation gating of these mutants is substantially different from that of wild type. These mutants also display approximately 10-20-fold reduction in bupivacaine affinity toward their inactivated state but show only approximately twofold affinity changes toward their resting state. These results demonstrate that residues mu1-S1276 and mu1-L1280 in D3-S6 are critical for both BTX and LA binding interactions. We propose that LAs interact readily with these residues from D3-S6 along with those from D1-S6 and D4-S6 in close proximity when the Na(+) channel is in its inactivated state. Implications of this state-dependent binding model for the S6 alignment are discussed.