| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 Department of Anaesthesia, Harvard Medical School and Brigham and Women's Hospital, Boston, MA, USA2 Department of Biology, State University of New York at Albany, Albany, NY, USA
 |
Abstract |
|---|
 Top Abstract IntroductionMethodsResultsDiscussionReferences |
|---|
(Received 15 September 2003;
accepted after revision 7 November 2003;
first published online 7 November 2003)
Corresponding author G. K. Wang: Department of Anaesthesia, Harvard Medical School and Brigham and Women's Hospital, Boston, MA, USA. Email: wang{at}zeus.bwh.harvard.edu
|
Introduction |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
-subunit of the skeletal muscle Nav1.4 Na+ channel cause heritable muscle diseases such as paramyotonia congenita, hyperkalaemic periodic paralysis, and potassium-aggravated myotonia (Lehmann-Horn & Jurkat-Rott, 1999; Cannon, 2002). Fast inactivation is partially disrupted by most mutations, and some of mutations also cause a shift in activation and/or altered slow inactivation. A partial disruption of fast inactivation often results in persistent late Na+ currents during prolonged depolarization. Model simulations show that these functional defects are sufficient to cause repetitive discharges in myotonia or to induce paralysis.
The class 1b antiarrhythmic drug mexiletine has been taken orally for ventricular arrhythmias (Roden, 2001). It has also been used to reduce or prevent myotonia (Jackson et al. 1994; Lehmann-Horn & Jurkat-Rott, 1999), to treat long QT–3 syndrome (Schwartz et al. 1995), and to alleviate neuropathic pain (Chabal et al. 1992). Mexiletine is an analogue of the local anaesthetic (LA) lidocaine that, like most LAs, produces both tonic block when cells are stimulated infrequently and additional use-dependent block of Na+ current during repetitive pulses (Hille, 2001; Catterall & Mackie, 2001). Mexiletine displays different efficacies for different Na+ channel states. The resting-state affinity (KR= 650 µM) for mexiletine is low, whereas the inactivated-state affinity (KI= 28.3 µM) is 
22 times higher (Takahashi & Cannon, 2001). The therapeutic plasma concentration (2.8–11 µM), however, is much lower than these estimated values.
Mexiletine block of mutant cardiac Na+ channels that cause long QT syndromes was previously examined by Wang et al. (1997), who found that the affinity of the mutant channels in the inactivated state was similar to the wild-type, with a 50% inhibitory concentration (IC50) of 15–20 µM. Interestingly, block of the late-opening channels by mexiletine was achieved at significantly lower concentrations, with an IC50 of 2–3 µM at –20 mV. These authors suggested that this selective targeting of the persistent late Na+ currents by mexiletine is important for its therapeutic intervention in cardiac diseases.
We were interested in whether mexiletine can potently block the open state of skeletal muscle Na+ channels because of its efficacy in patients with myotonia. To address this question we expressed wild-type and inactivation-deficient hNav1.4 Na+ channels (hNav1.4-L443C/A444W) in HEK293t cells and compared their state-dependent block by mexiletine. We selected this mutant human Nav1.4 mutant muscle channel because it expressed well in HEK293t cells and displayed an inactivation-deficient phenotype similar to that in the rat rNav1.4-L437C/A438W muscle Na+ channel (Wang et al. 2003). We found that hNav1.4 Na+ channels exhibited three distinct affinities for mexiletine, with the open state of the inactivation-deficient mutant channels showing the highest affinity (IC50= 3.3 µM), the inactivated state showing an intermediate affinity (67.8 µM), and the resting state of the wild-type channel showing the weakest affinity (431.2 µM). The presence of a large persistent late Na+ current in this mutant during prolonged depolarization also allowed us to measure directly the time-dependent block at various mexiletine concentrations. Such measurements yielded an on-rate of 10.4 µM-1 s-1 and an off-rate of 54.4 s-1 for the open-channel block of mexiletine at +30 mV.
|
Methods |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
We used the QuikChange XL Site-Directed Mutagenesis Kit (Stratagene, La Jolla, CA, USA) to create a human skeletal muscle hNav1.4-L443C/A448W mutant clone (Wang et al. 2003). The hNav1.4 wild-type Na+ channel clone in the pRc/CMV vector was a generous gift from Dr T. R. Cummins (Yale University). We also created homologous clones of cardiac hNav1.5-L409C/A410W and an additional mutation F1760K in the hNav1.5-L409C/A410W backbone. The F1760 position at D4S6 is known to be critical for binding of antiarrhythmic agents and LAs (Ragsdale et al. 1994, 1996; Nau et al. 2000). These homologous clones displayed inactivation-deficient phenotypes comparable to those of their rNav1.4 counterparts (Wang et al. 2003).
Transient transfection
Human embryonic kidney cells (HEK293t) were grown to 50% confluence in DMEM (Life Technologies, Inc., Rockville, MD, USA) containing 10% fetal bovine serum (HyClone, Logan, UT, USA), 1% penicillin and streptomycin solution (Sigma, St Louis, MO, USA), 3 mM taurine, and 25 mM Hepes (Life Technologies, Inc.) and then transfected by a calcium phosphate precipitation method (Cannon & Strittmatter, 1993). Transfection of hNav1.4-pRc/CMV or other mutant clones (5–10 µg) along with rat ß1-pcDNA1/Amp (10–20 µg) and reporter CD8-pih3m (1 µg) was adequate for current recording. Cells were replated 15 h after transfection in 35 mm dishes, maintained at 37°C in a 5% CO2 incubator, and used after 1–4 days. Transfection-positive cells were identified with immunobeads (CD8-Dynabeads, Lake Success, NY, USA).
Whole-cell voltage clamp
The whole-cell configuration was used to record Na+ currents (Hamill et al. 1981). Borosilicate micropipettes (Drummond Scientific Company, Broomall, PA, USA) were pulled with a puller (P-87, Sutter Instrument Company, Novato, CA, USA) and heat polished. Pipette electrodes contained 100 mM NaF, 30 mM NaCl, 10 mM EGTA, and 10 mM Hepes, adjusted to pH 7.2 with CsOH. The pipette electrodes had a tip resistance of 0.5–1.0 M
. Access resistance was 1–2 M and was further reduced by series resistance compensation. All experiments were performed at room temperature (22–24°C) under a Na+-containing bath solution with 65 mM NaCl, 85 mM choline chloride, 2 mM CaCl2, and 10 mM Hepes, adjusted to pH 7.4 with tetramethyl-ammonium hydroxide. Mexiletine hydrochloride was purchased from Sigma and dissolved in DMSO solution at 100 mM as stock solution. Final mexiletine concentrations were prepared from stock by serial dilution with bath solution. Chloramine-T was obtained from Fisher Chemicals (Fairlawn, NJ, USA) and freshly prepared at a final concentration of 0.5 mM. Tetrodotoxin (TTX) was purchased from Calbiochem-Navabiochem (San Diego, CA, USA). Whole-cell currents were measured by an Axopatch 200B amplifier (Axon Instruments, Foster City, CA, USA) or an EPC-7 amplifier (List Electronics, Darmstadt/Eberstadt, Germany), filtered at 3 kHz, collected, and analysed with pCLAMP8 software (Axon Instruments). The holding potential was set at –140 mV. Leak and capacitance were subtracted by the patch clamp device and further by the leak subtraction protocol (P/–4). Voltage error was <4 mV after series resistance compensation. A Student's unpaired t test was used to evaluate estimated parameters (mean ±S.E.M. or fitted value ±S.E.M. of the fit); P values of <0.05 were considered statistically significant.
|
Results |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Superimposed current traces of wild-type hNav1.4 Na+ channels at various voltages were recorded before and after 100 µM mexiletine application (Fig. 1A and B, respectively). At this concentration, the peak currents were reduced only slightly (10%) with minimal effects on current kinetics. The peak currents were measured, converted to Na+ conductance (gm), normalized, plotted against the corresponding voltage, and fitted with a Boltzmann equation (Fig. 1C). The midpoint voltage (V0.5) was shifted leftward by 5.1 mV and the slope became steeper after mexiletine treatment (P < 0.05).
|
We measured the steady-state inactivation before and after 100 µM mexiletine using a conventional two-pulse protocol. Figure 2A and B show the superimposed current traces elicited by a test pulse of +30 mV with various conditioning pulses. Peak currents were measured, normalized with respect to the maximal amplitude at the conditioning voltage of –160 mV, plotted against conditioning voltages, and fitted with a Bolzmann equation (Fig. 2C). The midpoint voltage (V0.5) was shifted leftward by 4.7 mV (P < 0.05) and the slope factor was not significantly changed.
|
To measure the resting and inactivated affinities for mexiletine, we first determined whether saturable low and high affinities for this drug exist in hNav1.4 Na+ channels using a voltage-scanning protocol. A 10 s conditioning pulse at various voltages ranging from –180 to –40 mV was applied to allow mexiletine (100 µM) to bind with its receptor. Following a 100 ms interval at –140 mV, a test pulse at +30 mV was used to measure the remaining currents. Peak currents were measured, normalized, and plotted against voltages (Fig. 3A). We found that there are two types of mexiletine block: one low-affinity block at voltages from –180 to –120 mV and one higher-affinity block at voltages from –70 to –40 mV. The dose–response curves were subsequently constructed at –160 and –70 mV and fitted with a Hill equation to obtain the resting- and inactivated-state block by mexiletine (Fig. 3B). The IC50 values were 67.8 ± 7.0 µM[Hill coefficient, 0.85 ± 0.07] at –70 mV for the inactivated-state affinity and 431.2 ± 9.4 µM[1.3 ± 0.1] (n= 5) for the resting-state affinity. These values are comparable to the values reported by Takahashi & Cannon (2001) (28.3 µM and 650 µM for the inactivated- and resting-state affinities, respectively) under different ionic conditions and pulse protocols.
|
The development of inactivated-state block by mexiletine was measured using a pulse protocol shown in Fig. 4A (top). We found that the inactivated-state block reached its steady-state condition after a 1 s conditioning pulse at –50 mV with a time constant of 203 ± 11 ms (n= 5).
|
30% (Fig. 4B, 
), and the time constant of recovery was 0.74 ± 0.06 s (n= 5), while 70% of normal Na+ channels recovered as rapidly from their inactivated state as those found in the control (
). With a 500 ms conditioning pulse at –50 mV, the level of block was increased to 60% (Fig. 4B, •), but recovery had the same slow time constant of 0.70 ± 0.03 s (n= 5) as that with a 100 ms conditioning pulse. Families of inactivation-deficient Na+ currents before and after mexiletine treatment
Superimposed Na+ current traces of inactivation-deficient hNav1.4-L443C/A444W mutant channels at various voltages were recorded before and after 100 µM mexiletine application (Fig. 5A and B). The mutant current decayed more slowly and incompletely than the wild-type current (Fig. 5A versus Fig. 1A). A significant amount of Na+ current was maintained at the end of test pulses. With 100 µM mexiletine, the currents decayed rapidly (Fig. 5B) as if the maintained currents were potently blocked by mexiletine. The peak currents were measured, converted to conductance (gm), normalized, plotted against the membrane voltage, and fitted with a Boltzmann equation (Fig. 5C). The midpoint voltage (V0.5) was shifted leftward by 17 mV (P < 0.05) and the slope factor was smaller (14.8 mV versus 17.0 mV; P < 0.05). It is unclear why mexiletine elicits these changes in Na+ channels, but related smaller changes also exist in wild-type channel (Fig. 1C).
|
Since residual fast inactivation was evident in current traces, we measured the steady-state fast inactivation using a conventional two-pulse protocol as described in Fig. 2. Figure 6A and B shows the current traces with conditioning pulses ranging from –160 to –15 mV before and after 100 µM mexiletine application. With mexiletine, the decaying phase of the Na+ currents was rapid (Fig. 6B), suggesting a rapid time-dependent block of the open Na+ channels. The peak currents were measured, normalized with respect to the maximal value, plotted against the conditioning voltage, and fitted with a Bolzmann equation. Without mexiletine, about 60% of peak currents were non-inactivating for a 100 ms pulse at –20 mV (Fig. 6C; ). Two Bolzmann equations were used to fit the curve obtained with mexiletine, possibly due to the presence of both inactivated-channel block and open-channel block. Mexiletine blocked more than 95% of peak currents at –20 mV under these conditions (Fig. 6C; •).
|
|
To measure directly the open-channel affinity for mexiletine in inactivation-deficient mutant channels, we applied a test pulse of +30 mV with a duration of 50 ms. Figure 8A shows current traces before and after mexiletine applications. Without the drug, the inactivation-deficient mutant hNav1.4-L443C/A444W current decayed with biphasic kinetics. The fast decaying phase represented the residual fast inactivation and the slower phase represented the slow inactivation (Wang et al. 2003). The persistent late currents were blocked by mexiletine in a dose-dependent manner; higher concentrations elicited a faster time-dependent block and blocked a larger percentage of persistent currents (Fig. 8A). This time-dependent block by mexiletine was fitted by an exponential function and the time constant plotted against the concentration (Fig. 8B). The on-rate corresponded to the slope of the fitted line and the off-rate corresponded to the y-intercept (O'Leary & Chahine, 2002). The on- and off-rates (n= 5) of mexiletine block at +30 mV were estimated to be 10.4 ± 1.8 µM-1 s-1 and 54.4 ± 6.0 s-1, respectively, slightly larger than estimated values for a typical LA, bupivacaine (6–7 µM-1 s-1 and 20–30 s-1; Valenzuela et al. 1995). The calculated equilibrium dissociation constant (KD= off-rate/on-rate) for mexiletine was 5.2 µM. The dose–response curves for resting-channel and open-channel block were constructed directly from data for peak current block and steady-state block of the persistent currents at the end of the test pulse, respectively (Fig. 8C, for the resting block, for the open-channel block). The IC50 values [Hill coefficients] for resting- and open-channel block were 336.4 ± 20.0 µM[1.2 ± 0.1] and 3.3 ± 0.1 (M[0.92 ± 0.02], respectively (n= 5).
|
|
Repetitive pulses elicited an additional mexiletine block in wild-type Na+ channels, which was termed the use-dependent block. We measured this use-dependent block of wild-type and inactivation-deficient mutant currents with a test pulse of +30 mV for 24 ms at a frequency of 5 Hz. Peak currents were measured, normalized with respect to the amplitude of the first pulse, and plotted against the pulse number. Figure 10A shows that the use-dependent block reached a steady-state level within a few pulses that was 70% of wild-type currents, whereas in inactivation-deficient hNav1.4-L443C/A444W mutant channels, the block achieved a steady-state level rapidly, by the second pulse, that was 55% of currents remaining (Fig. 10B). This result demonstrated that the inactivation-deficient Na+ channels were more susceptible to mexiletine block during repetitive pulses.
|
Since mexiletine is an antiarrhythmic agent targeting human heart Na+ channels, we measured the potency of open-channel block by mexiletine in human cardiac inactivation-deficient hNav1.5-L409C/A410W mutant Na+ channels (Fig. 11A). Dose–response data show that mexiletine blocked the open-channel with an IC50 of 11.1 µM and blocked the resting-channel with an IC50 of 157.4 µM (Fig. 11C; 
and , respectively). The ratio of the resting-channel block versus open-channel block was 14.2 in this inactivation-deficient cardiac mutant Na+ channel. This ratio is less than that found in homologous hNav1.4 mutant channel (or resting/open block = 336 µM/3.3 µM ( 100). The reason for this difference is unclear. Previously, Weiser et al. (1999) reported that the mexiletine receptor in different isoforms may have different affinities for this drug. We also noticed that the residual fast inactivation in hNav1.5-L409C/A410W mutant channels is minimal, unlike that in hNav1.4-L443C/A444W counterparts (Fig. 11A versus Fig. 8A).
|
). The ratio of resting-channel versus open-channel block was only 1.39 in this F1760K inactivation-deficient mutant channel. Open-channel block by mexiletine in wild-type hNav1.4 Na+ channels treated with chloramine-T
Chloramine-T is known to impair fast inactivation and generates a significant amount of persistent late Na+ current (Wang et al. 1987). Using this alternative method for fast-inactivation removal, we investigated whether the high-affinity open-channel block by mexiletine also occurs in wild-type hNav1.4 Na+ channels. Figure 12A shows that persistent late Na+ currents were evident after a brief chloramine-T treatment. These persistent late Na+ currents were again sensitive to mexiletine in a concentration-dependent manner. The time-dependent block by mexiletine was also apparent but with a slower time course than that shown in inactivation-deficient mutant channels (Fig. 8A). Most of these persistent currents originated from hNav1.4 Na+ channels since they were blocked by 1 µM TTX. Using this type of data set, we constructed the dose–response curve (Fig. 12B), which yielded an IC50 of 7.5 ± 0.8 µM with a Hill coefficent of 1.0 ± 0.1 (n= 5). It is noteworthy that, like site-directed mutagenesis, chemical and/or enzymatic treatment of Na+ channels may alter the channel structure globally, which in turn may affect the potency of LAs directly or indirectly. Previous investigations on the open-channel block of wild-type hNav1.4 Na+ channels yielded inconsistent results, often dependent on the method of inactivation removal and/or on the type of local anaesthetic used (e.g. Cahalan, 1978; Yeh, 1978; Wang et al. 1987).
|
|
Discussion |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Distinct mexiletine affinities for resting, inactivated and open Na+ channels
We identified three distinct binding affinities of human skeletal muscle Na+ channels for mexiletine. The resting-state affinity for mexiletine is weak, with an IC50 of 431 µM. The inactivated-state affinity for mexiletine is intermediate, with an IC50 of 67.8 µM. In comparison, the open state of hNav1.4 Na+ channels has the highest affinity for mexiletine, with an IC50 of 3.3 µM at +30 mV. This affinity is about 20-fold and 130-fold higher than the inactivated- and resting-state affinities, respectively. The on-rate of the open-channel block by mexiletine was 10.4 µM-1 s-1 and the off-rate 54 s-1 at +30 mV (Fig. 9B). The KD was calculated to be 5.2 µM, which is in fair agreement with the direct IC50 measurement.
These kinetic data strongly support the hypothesis that selective targeting of the persistent late Na+ currents by mexiletine is the basis for its therapeutic intervention, as previously suggested by Wang et al. (1997). Their simulations of an ideal antiarrhythmic drug at 10 µM using the on-rate (10 µM-1 s-1) and off-rate (30 s-1) for the open-channel block demonstrate that no suppression of peak Na+ current occurs but marked suppression of the late openings is achieved. Our on- and off-rates for open-channel block by mexiletine (10.4 µM-1 s-1 and 54 s-1, respectively) are very close to the values of an ideal drug. Simulations with a small off-rate (e.g. 10 s-1) caused suppression of the peak Na+ currents as well as the late current.
The fact that the open-state block by mexiletine is more effective than the inactivated-state block raises the possibility that mutations at hNav1.4-L443C/A444W residues increase the open-channel affinity preferentially. We think that this is unlikely for three reasons. First, Wang et al. (1997) reported that the open-channel block by mexiletine (IC50= 2–3 µM) was about 5–10 times higher than the inactivated-state block (IC50= 15–20 µM) in all three LQT hNav1.5 Na+ channels with mutations at three different loci (hNav1.5-(KPQ, N1325S, and R1644H). Unfortunately, it was not possible to measure directly the on-and off-rate kinetics in these mutants because of the small size of persistent currents (5% or less of the peak currents). Second, persistent late Na+ currents generated by the oxidant chloramine-T also remained sensitive to mexiletine block (Fig. 12), suggesting again that the open channel has a higher affinity for the drug than the resting and the inactivated channels. Third, the high-affinity open-channel block was nearly abolished by a single mutation at D4S6 (Fig. 11B and C; F1760K in the hNav1.5-L409C/A410W backbone). This phenotype indicates that the LA receptor for mexiletine in this inactivation-deficient mutant channel is comparable to the wild-type and can be further altered by a specific F1760K mutation. Additional investigations on the structure and activity of mexiletine (pKa= 9.1) are needed in order to understand why this specific drug binds preferentially with the open state of the Na+ channel. The inactivation-deficient muscle hNav1.4-L443C/A444W and cardiac hNav1.5-L409C/A410W mutant Na+ channels will be suitable for these studies due to their high level of expression in HEK293t cells. These inactivation-deficient mutant channels should also be applicable for the screening of novel open-channel blockers in the future (e.g. De Luca et al. 2000).
Use-dependent block and the role of the inactivated block
Use-dependent block of Na+ currents by antiarrhythmic agents and by LAs arises presumably because these blockers have higher affinities for the open and the inactivated states of the Na+ channel (Hille, 1977; Hondeghem & Katzung, 1977). In this study we established that the open-channel block by mexiletine played a dominant role in this use-dependent phenomenon (Fig. 10). The development of mexiletine block of the open channel is relatively fast (Fig. 8A). This result suggests that the open-channel block by mexiletine can account for a major portion of the use-dependent block during repetitive pulses found in wild-type or inactivation-deficient mutant Na+ channels (Fig. 10A and B).
What then is the role of inactivated-state block by mexiletine in vivo? The IC50 for inactivated-state block is 67.8 µM, which is 20 times higher than the IC50 for the open-channel block (3.3 µM). The development of inactivated-state block by mexiletine at –50 mV has a time constant of 0.203 s for the wild-type hNav1.4 channels. The time constant for the recovery from the inactivated-state block is 0.70 s at –140 mV (Fig. 4B). On the basis of these measurements, we concluded that the inactivated block by 100 µM mexiletine has a much slower on-rate than the open block (
< 2 ms at 100 µM; Fig. 8A) and has a slightly faster off-rate than the open-channel block upon repolarization (= 0.91 s; Fig. 9). Our dose–response results show that about 20% of inactivated channels are blocked by 10 µM mexiletine (Fig. 3), a concentration that is at the upper end of the therapeutic range (2.8–11 µM). Thus, the inactivated-channel block by mexiletine is also likely to play an important role in modulating the availability of Na+ channels for action potentials, albeit not as crucial as the open-channel block. Mexiletine at 10 µM will block 75% of late open channels during a 50 ms depolarization. Accordingly, it is likely that mexiletine at a therapeutic concentration will silence persistent late Na+ channel openings in vivo efficiently.
Care should be taken in applying results from heterologous expression studies to the clinical management of patients (Takahashi & Cannon, 2001). Temperature, lipid environment, subunit composition, and additional post-translational modifications of Na+ channels in myocytes may also affect the degree of inactivated-channel block by mexiletine. Nonetheless, these factors seem unlikely to alter the fact that the open-channel block by mexiletine in human skeletal muscle hNav1.4 Na+ channels is far more effective than the resting- and inactivated-channel block.
|
References |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Cannon SC (2002). An expanding view of the molecular basis of familial periodic paralysis. Neuromuscular Disorders 12, 533–543.[CrossRef][Medline]
Cannon SC & Strittmatter SM (1993). Functional expression of sodium channel mutations identified in families with periodic paralysis. Neuron 10, 317–326.[CrossRef][Medline]
Catterall WA & Mackie K (2001). Local anesthetics. In Goodman and Gilman's the Pharmacological Basis of Therapeutics, ed. Hardman, JG, Limbird, LE, Molinoff, PB, Ruddon, RW & Gilman, AG, pp. 367–384. Macmillan Publishing Co., New York.
Chabal C, Jacobson L, Mariano A, Chaney E & Britell CW (1992). The use of oral mexiletine for the treatment of pain after peripheral nerve injury. Anesthesiology 76, 513–517.[Medline]
De Luca A, Natuzzi F, Desaphy JF, Loni G, Lentini G, Franchini C, Tortorella V & Camerino DC (2000). Molecular determinants of mexiletine structure for potent and use-dependent block of skeletal muscle sodium channels. Mol Pharmacol 57, 268–277.
Hamill OP, Marty E, Neher ME, Sakmann B & Sigworth FJ (1981). Improved patch-clamp techniques for high-resolution current recording from cells and cell-free membrane patches. Pflugers Arch 391, 85–100.[CrossRef][Medline]
Hille B (1977). Local anesthetics: hydrophilic and hydrophobic pathways for the drug receptor reaction. J General Physiol 69, 497–515.
Hille B (2001). Classical mechanisms of block. In Ion Channels of Excitable Membranes, pp. 503–536. Sinauer Associate Inc., Sunderland, MA, USA.
Hondeghem LM & Katzung BG (1977). Time- and voltage-dependent interactions of antiarrhythmic drugs with cardiac sodium channels. Biochim Biophys Acta 472, 373–398.[Medline]
Jackson CE, Barohn RJ & Ptacek LJ (1994). Paramyotonia congenita: abnormal short exercise test, and improvement after mexiletine therapy. Muscle Nerve 17, 763–768.[CrossRef][Medline]
Ju YK, Saint DA & Gage PW (1996). Hypoxia increases persistent sodium current in rat ventricular myocytes. J Physiol 497, 337–347.
Lehmann-Horn F & Jurkat-Rott K (1999). Voltage-gated ion channels and hereditary disease. Physiol Rev 79, 1317–1372.
Maltsev VA, Sabbah HN & Undrovinas AI (2001). Late sodium current is a novel target for amiodarone: studies in failing human myocardium. J Mol Cell Cardiol 33, 923–932.[CrossRef][Medline]
Nau C, Wang S-Y, Strichartz GR & Wang GK (2000). Block of human heart hH1 sodium channels by the enantiomers of bupivacaine. Anesthesiology 93, 1022–1033.[CrossRef][Medline]
O'Leary ME & Chahine M (2002). Cocaine binds to a common site on open and inactivated human heart (Nav1.5) sodium channel. J Physiol 541, 701–716.
Ragsdale DS, McPhee JC, Scheuer T & Catterall WA (1994). Molecular determinants of state-dependent block of Na+ channels by local anesthetics. Science 265, 1724–1728.
Ragsdale DS, McPhee JC, Scheuer T & Catterall WA (1996). Common molecular determinants of local anesthetic, antiarrhythmic, and anticonvulsant block of voltage-gated Na+ channels. Proc Natl Acad Sci U S A 93, 9270–9275.
Roden DM (2001). Antiarrhythmic drugs. In Goodman and Gilman's the Pharmacological Basis of Therapeutics, ed. Hardman, JG, Limbird, LE, Molinoff, PB, Ruddon, RW & Gilman, AG, pp. 933–970. Macmillan Publishing Co., New York.
Schwartz PJ, Priori SG, Locati EH, Napolitano C, Cantu F, Towbin JA, Keating MT, Hammoude H, Brown AM & Chen LS (1995). Long QT syndrome patients with mutations of the SCN5A and HERG genes have differential responses to Na+ channel blockade and to increases in heart rate. Implications for gene-specific therapy. Circulation 92, 3381–3386.
Takahashi MP & Cannon SC (2001). Mexiletine block of disease-associated mutations in S6 segments of the human skeletal muscle Na+ channel. J Physiol 537, 701–714.
Valenzuela C, Snyders DJ, Bennett PB, Tamargo J & Hondeghem LM (1995). Stereoselective block of cardiac sodium channels by bupivacaine in guinea pig ventricular myocytes. Circulation 92, 3014–3024.
Wang DW, Yazawa K, Maita N, George ALJ & Bennett PB (1997). Pharmacological targeting of long QT mutant sodium channels. J Clin Invest 99, 1714–1720.[Medline]
Wang GK, Brodwick MS, Eaton DC & Strichartz GR (1987). Inhibition of sodium currents by local anesthetics in chloramine-T treated squid axons. J General Physiol 89, 645–667.
Wang S-Y, Bonner K, Russell C & Wang GK (2003). Tryptophan scanning of D1S6 and D4S6 C-termini in voltage-gated sodium channels. Biophys J 85, 911–920.
Weiser T, Qu Y, Catterall WA & Scheuer T (1999). Differential interaction of R-mexiletine with the local anesthetic receptor site on brain and heart sodium channel alpha-subunits. Mol Pharmacol 56, 1238–1244.
Yeh JZ (1978). Sodium inactivation mechanism modulates QA-314 block of sodium channels in squid axons. Biophys J 24, 569–574.
|
Acknowledgements |
|---|
This article has been cited by other articles:
|
|
 |
|
  K. Liu, T. Yang, P. C. Viswanathan, and D. M. Roden New Mechanism Contributing to Drug-Induced Arrhythmia: Rescue of a Misprocessed LQT3 Mutant Circulation, November 22, 2005; 112(21): 3239 - 3246. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S.-Y. Wang, J. Mitchell, E. Moczydlowski, and G. K. Wang Block of Inactivation-deficient Na+ Channels by Local Anesthetics in Stably Transfected Mammalian Cells: Evidence for Drug Binding Along the Activation Pathway J. Gen. Physiol., November 29, 2004; 124(6): 691 - 701. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K. Sasaki, N. Makita, A. Sunami, H. Sakurada, N. Shirai, H. Yokoi, A. Kimura, N. Tohse, M. Hiraoka, and A. Kitabatake Unexpected Mexiletine Responses of a Mutant Cardiac Na+ Channel Implicate the Selectivity Filter as a Structural Determinant of Antiarrhythmic Drug Access Mol. Pharmacol., August 1, 2004; 66(2): 330 - 336. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
) of hNav1.4 was obtained from data as shown in A and B and plotted against the conditioning voltage. Data were fitted with the Boltzmann function, y= 1/{1 + exp[(V–V0.5)/k]}. The average midpoint (V0.5) and slope factor (k) for the wild-type (
