Use-dependent block of Na+ currents by mexiletine at the single channel level in guinea-pig ventricular myocytes

Protein 14-3-3 Influences the Response of the Cardiac Sodium Channel Nav1.5 to Antiarrhythmic Drugs

The cardiac sodium channel Nav1.5 is a key contributor to the cardiac action potential, and dysregulations in Nav1.5 can lead to cardiac arrhythmias. Nav1.5 is a target of numerous antiarrhythmic drugs (AADs). Previous studies identified the protein 14-3-3 as a regulator of Nav1.5 biophysical coupling. Inhibition of 14-3-3 can remove the Nav1.5 functional coupling and has been shown to inhibit the dominant-negative effect of Brugada syndrome mutations. However, it is unknown whether the coupling regulation is involved with AADs' modulation of Nav1.5. Indeed, AADs could reveal important structural and functional information about Nav1.5 coupling. Here, we investigated the modulation of Nav1.5 by four classic AADs, quinidine, lidocaine, mexiletine, and flecainide, in the presence of 14-3-3 inhibition. The experiments were carried out by high-throughput patch-clamp experiments in an HEK293 Nav1.5 stable cell line. We found that 14-3-3 inhibition can enhance acute block by quinidine, whereas the block by other drugs was not affected. We also saw changes in the use- and dose-dependency of quinidine, lidocaine, and mexiletine when inhibiting 14-3-3. Inhibiting 14-3-3 also shifted the channel activation toward hyperpolarized voltages in the presence of the four drugs studied and slowed the recovery of inactivation in the presence of quinidine. Our results demonstrated that the protein 14-3-3 and Nav1.5 coupling could impact the effects of AADs. Therefore, 14-3-3 and Nav1.5 coupling are new mechanisms to consider in the development of drugs targeting Nav1.5. SIGNIFICANCE STATEMENT: The cardiac sodium channel Nav1.5 is a target of commonly used antiarrhythmic drugs, and Nav1.5 function is regulated by the protein 14-3-3. The present study demonstrated that the regulation of Nav1.5 by 14-3-3 influences Nav1.5's response to antiarrhythmic drugs. This study provides detailed information about how 14-3-3 differentially regulated Nav1.5 functions under the influence of different drug subtypes. These findings will guide future molecular studies investigating Nav1.5 and antiarrhythmic drugs outcomes.

Deletion of Trpm4 Alters the Function of the Nav1.5 Channel in Murine Cardiac Myocytes

Transient receptor potential melastatin member 4 (TRPM4) encodes a Ca²⁺-activated, non-selective cation channel that is functionally expressed in several tissues, including the heart. Pathogenic mutants in TRPM4 have been reported in patients with inherited cardiac diseases, including conduction blockage and Brugada syndrome. Heterologous expression of mutant channels in cell lines indicates that these mutations can lead to an increase or decrease in TRPM4 expression and function at the cell surface. While the expression and clinical variant studies further stress the importance of TRPM4 in cardiac function, the cardiac electrophysiological phenotypes in Trpm4 knockdown mouse models remain incompletely characterized. To study the functional consequences of Trpm4 deletion on cardiac electrical activity in mice, we performed perforated-patch clamp and immunoblotting studies on isolated atrial and ventricular cardiac myocytes and surfaces, as well as on pseudo- and intracardiac ECGs, either in vivo or in Langendorff-perfused explanted mouse hearts. We observed that TRPM4 is expressed in atrial and ventricular cardiac myocytes and that deletion of Trpm4 unexpectedly reduces the peak Na⁺ currents in myocytes. Hearts from Trpm4^−/− mice presented increased sensitivity towards mexiletine, a Na⁺ channel blocker, and slower intraventricular conduction, consistent with the reduction of the peak Na⁺ current observed in the isolated cardiac myocytes. This study suggests that TRPM4 expression impacts the Na⁺ current in murine cardiac myocytes and points towards a novel function of TRPM4 regulating the Na_v1.5 function in murine cardiac myocytes.

Beyond the muscular involvement in non-dystrophic myotonias: The emerging role of neuromodulation

BACKGROUND

The central nervous system involvement, in terms of a maladaptive sensory-motor plasticity, is well known in patients with dystrophic myotonias (DMs). To date, there are no data suggesting a central nervous system involvement in non-dystrophic myotonias (NDMs).

OBJECTIVE

To investigate sensory-motor plasticity in patients with Myotonia Congenita (MC) and Paramyotonia Congenita (PMC) with or without mexiletine.

METHODS

Twelve patients with a clinical, genetic, and electromyographic evidence of MC, fifteen with PMC, and 25 healthy controls (HC) were included in the study. TMS on both primary motor cortices (M1) and a rapid paired associative stimulation (rPAS) paradigm were carried out to assess M1 excitability and sensory-motor plasticity.

RESULTS

patients showed a higher cortical excitability and a deterioration of the topographic specificity of rPAS aftereffects, as compared to HCs. There was no correlation among neurophysiological and clinical-demographic characteristics. Noteworthy, the patients who were under mexiletine showed a minor impairment of the topographic specificity of rPAS aftereffects as compared to those who did not take the drug.

CONCLUSION

our findings could suggest the deterioration of cortical sensory-motor plasticity in patients with NDMs as a trait of the disease.

Effects of Na+ channel blockers on the restitution of refractory period, conduction time, and excitation wavelength in perfused guinea-pig heart

Na⁺ channel blockers flecainide and quinidine can increase propensity to ventricular tachyarrhythmia, whereas lidocaine and mexiletine are recognized as safe antiarrhythmics. Clinically, ventricular fibrillation is often precipitated by transient tachycardia that reduces action potential duration, suggesting that a critical shortening of the excitation wavelength (EW) may contribute to the arrhythmic substrate. This study examined whether different I_Na blockers can produce contrasting effects on the rate adaptation of the EW, which would explain the difference in their safety profile. In perfused guinea-pig hearts, effective refractory periods (ERP), conduction times, and EW values were determined over a wide range of cardiac pacing intervals. All I_Na blockers tested were found to flatten the slope of ERP restitution, indicating antiarrhythmic tendency. However, with flecainide and quinidine, the beneficial changes in ERP were reversed owing to the use-dependent conduction slowing, thereby leading to significantly steepened restitution of the EW. In contrast, lidocaine and mexiletine had no effect on ventricular conduction, and therefore reduced the slope of the EW restitution, as expected from their effect on ERP. These findings suggest that the slope of the EW restitution is an important electrophysiological determinant which can discriminate I_Na blockers with proarrhythmic and antiarrhythmic profile.

Pharmacology and Toxicology of Nav1.5-Class 1 anti-arrhythmic drugs

Although cardiac sodium channel blocking drugs can exert antiarrhythmic actions, they can also provoke life-threatening arrhythmias through a variety of mechanisms. This review addresses the way in which drugs interact with the channel, and how these effects translate to clinical beneficial or detrimental effects. A further understanding of the details of channel function and of drug-channel interactions may lead to the development of safer and more effective antiarrhythmic therapies.

The role of late I Na in development of cardiac arrhythmias

Late I Na is an integral part of the sodium current, which persists long after the fast-inactivating component. The magnitude of the late I Na is relatively small in all species and in all types of cardiomyocytes as compared with the amplitude of the fast sodium current, but it contributes significantly to the shape and duration of the action potential. This late component had been shown to increase in several acquired or congenital conditions, including hypoxia, oxidative stress, and heart failure, or due to mutations in SCN5A, which encodes the α-subunit of the sodium channel, as well as in channel-interacting proteins, including multiple β subunits and anchoring proteins. Patients with enhanced late I Na exhibit the type-3 long QT syndrome (LQT3) characterized by high propensity for the life-threatening ventricular arrhythmias, such as Torsade de Pointes (TdP), as well as for atrial fibrillation. There are several distinct mechanisms of arrhythmogenesis due to abnormal late I Na, including abnormal automaticity, early and delayed after depolarization-induced triggered activity, and dramatic increase of ventricular dispersion of repolarization. Many local anesthetic and antiarrhythmic agents have a higher potency to block late I Na as compared with fast I Na. Several novel compounds, including ranolazine, GS-458967, and F15845, appear to be the most selective inhibitors of cardiac late I Na reported to date. Selective inhibition of late I Na is expected to be an effective strategy for correcting these acquired and congenital channelopathies.

Blocking Scn10a channels in heart reduces late sodium current and is antiarrhythmic

RATIONALE

Although the sodium channel locus SCN10A has been implicated by genome-wide association studies as a modulator of cardiac electrophysiology, the role of its gene product Nav1.8 as a modulator of cardiac ion currents is unknown.

OBJECTIVE

We determined the electrophysiological and pharmacological properties of Nav1.8 in heterologous cell systems and assessed the antiarrhythmic effect of Nav1.8 block on isolated mouse and rabbit ventricular cardiomyocytes.

METHODS AND RESULTS

We first demonstrated that Scn10a transcripts are identified in mouse heart and that the blocker A-803467 is highly specific for Nav1.8 current over that of Nav1.5, the canonical cardiac sodium channel encoded by SCN5A. We then showed that low concentrations of A-803467 selectively block "late" sodium current and shorten action potentials in mouse and rabbit cardiomyocytes. Exaggerated late sodium current is known to mediate arrhythmogenic early afterdepolarizations in heart, and these were similarly suppressed by low concentrations of A-803467.

CONCLUSIONS

Scn10a expression contributes to late sodium current in heart and represents a new target for antiarrhythmic intervention.

Stable Expression and Characterization of Human PN1 and PN3 Sodium Channels

Nociceptive transduction in inflammatory and neuropathic pain involves peripherally expressed voltage-gated sodium channels, such as tetrodotoxin (TTX)-sensitive PN1 and TTX-resistant PN3. We generated recombinant cell lines stably expressing the human PN1 and PN3 sodium channels in Chinese hamster ovary (CHO) cells using inducible expression vectors. The PN1 and PN3 cDNAs were isolated from human adrenal gland and heart poly(A)+ RNAs, respectively. The recombinant human PN1 currents exhibited rapid activation and inactivation kinetics and were blocked by TTX with a half-maximal inhibitory concentration (IC50) of 32.6 nM. The human PN3 channel expressed in stable transfectants showed TTX-resistant inward currents with slow inactivation kinetics. The IC50 value for TTX was 73.3 microM. The voltage-dependence of activation of the PN3 channel was shifted to the depolarizing direction, compared to that of the PN1 channel. Lidocaine and mexiletine exhibited tonic and use-dependent block of PN1 and PN3 channels. The PN1 channel was more susceptible to inhibition by mexiletine than PN3. These results suggest that stable transfectants expressing the human PN1 and PN3 sodium channels will be useful tools to define subtype selectivity for sodium channel blockers.

Pharmacologic investigation of the mechanism underlying cold allodynia using a new cold plate procedure in rats with chronic constriction injuries

Cold allodynia is a frequent clinical symptom of patients with neuropathic pain. Despite numerous studies of cold allodynia, using animal models of neuropathic pain, little is known about its underlying mechanisms. This study was performed to establish a method for the pharmacologic evaluation of cold allodynia using several analgesics in a chronic constriction injury (CCI) rat model of neuropathic pain. Compared with the results obtained before the CCI operation, the CCI rats placed on a cork plate at 20 degrees C exhibited a slight change in the paw withdrawal latency because of the mechanical stimulus mediated by the injured paw touching the plate. By contrast, there was a significant reduction in the paw withdrawal latency on a cold metal plate compared with that on the cork plate after the CCI surgery, with the maximum decrease occurring on postoperative day 7. This reduction is thought to specifically reflect cold-induced pain behavior. In addition, both naïve and CCI rats showed behavioral changes at 5 and 0 degrees C, but not at 10 degrees C or higher. Interestingly, a subcutaneous morphine dose of 6 mg/kg completely inhibited cold allodynia induced at 10 degrees C on postoperative day 7. Under this condition, both the sodium channel blocker mexiletine (10 and 30 mg/kg, subcutaneously) and the calcium channel alpha2delta subunit blocker pregabalin (30 and 100 mg/kg, orally) significantly suppressed cold allodynia. Additionally, both resiniferatoxin (0.3 mg/kg, subcutaneously), an ultrapotent analog of capsaicin that desensitizes C fibers, and the VR1 channel antagonist N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl) tetrahydropyrazine-1(2H)-carboxamide (10 and 30 mg/kg, orally) significantly prolonged the paw withdrawal latency. In conclusion, our data suggest that the activation of C fibers mediates cold allodynia.

Effect of mexiletine on vincristine-induced painful neuropathy in mice

In the present study, we examined the effect of mexiletine on vincristine-induced thermal hyperalgesia in mice. Mice were intraperitoneally treated with vincristine at a dose of 0.05 mg/kg one day after the measurement of the pre-drug latency in the tail-flick test, and then treated with a dose of 0.125 mg/kg twice a week for 6 weeks. In vincristine-treated mice, a significant decrease in tail-flick latency developed at 6 weeks after treatment. Pretreatment with mexiletine, at doses of 3, 10 and 30 mg/kg, i.p., dose-dependently increased the tail-flick latency in vincristine-treated mice. A significant reduction of the tail-flick latency was observed when the tail-flick latency was examined 60 min after i.t. administration of NG-nitro-L-arginine methyl ester (L-NAME, 30 nmol), a nitric oxide synthase (NOS) inhibitor, in naive mice. This L-NAME-induced thermal hyperalgesia was dose-dependently attenuated by pretreatment with mexiletine (10 and 30 mg/kg, i.p.), 10 min before the injection of L-NAME. The duration of nociceptive behavioral response induced by fenvalerate, at a dose of 0.1 microg, i.t., was significantly increased by pretreatment with L-NAME (30 nmol, i.t.). Intrathecal pretreatment with L-arginine (300 pmol) significantly reversed the L-NAME-induced enhancement of fenvalerate-induced nociceptive responses. The present study demonstrates that systemic mexiletine can effectively attenuate vincristine-induced thermal hyperalgesia. Furthermore, these results suggest that blockade of nitric oxide-induced enhancement of nociceptive transmission, in which tetrodotoxin-resistant sodium channels play an important role, may participate in the antinociceptive effect of mexiletine on vincristine-induced thermal hyperalgesia.

Effect of sodium channel blocker (mexiletine) on pathological ectopic firing pattern in a rat chronic constriction nerve injury model

We studied the efficacy of mexiletine as a sodium channel blocker for neuropathic pain by investigating the effect of mexiletine on the pathological ectopic firing pattern in a chronic constriction nerve injury (CCI) model. The experiment was conducted with 60 male Wistar rats. The CCI model was created by loosely ligating the sciatic nerve. After breeding 7 days, the frequency and pattern of ectopic firing antidromically recorded from the sural nerve and the amplitude of antidromic sensory nerve-evoked potential were analyzed. The CCI rats were given an intravenous injection of normal saline and mexiletine (5 or 15 mg/kg). Mexiletine significantly suppressed spontaneous firing frequency, an on-off firing pattern that consisted of cyclic bursting spikes and ectopic firing generation under the hypoxic condition. Mexiletine did not influence the amplitude of A-delta component in the antidromic sensory nerve-evoked potential. Mexiletine suppressed ectopic firing by blocking activity of the abnormal sodium channel at the nerve-injured site and dorsal root ganglion without blocking nerve conduction. This study suggests that mexiletine is useful for treating neuropathic pain in peripheral neuropathy.

Molecular basis for exaggerated sensitivity to mexiletine in the cardiac isoform of the fast Na channel

Cardiac sodium channels have been shown to have a higher sensitivity to local anesthetic agents, such as lidocaine, than the sodium channels of other tissues. To examine if this is also true for mexiletine, we have systematically measured mexiletine sensitivity of the Na channel isoforms, rH1, (mu)1, and rBII, which were transiently expressed in human embryonic kidney (HEK) 293 cells. We confirmed that the cardiac isoform rH1 exhibited the highest sensitivity among the three tested channel isoforms. In rH1, (mu)1, and rBII, the respective IC(50) values were 62, 294, and 308 microM mexiletine, in regard to tonic block, and 18, 54, and 268 microM mexiletine, in relation to use (8 Hz)-dependent block. The relatively high drug sensitivity of rH1 was an invariant finding, irrespective of channel state or whether channels were subjected to infrequent or frequent depolarizing stimuli. Mutating specific amino acids in the skeletal muscle isoform (mu)1 (namely, (mu)1-I433V and (mu)1-S251A) to those of the cardiac isoform at putative binding sites for local anesthetic agents revealed that only one of the point mutations ((mu)1-S251A) has relevance to the high cardiac drug sensitivity, because mexiletine produced significantly more use-dependent and tonic block in (mu)1-S251A than wild-type (mu)1.

Homologation of mexiletine alkyl chain and stereoselective blockade of skeletal muscle sodium channels

The optical isomers (-)-(S)- and (+)-(R)-3-(2, 6-dimethylphenoxy)-2-methyl-1-propanamine (Me2), homologues of the antiarrhythmic and antimyotonic drug mexiletine (Mex), were synthesized and assayed as new potential antimyotonic agents. As observed with Mex, Me2 exhibits an enantioselective behaviour. Tests carried out on sodium currents of single muscle fibres of Rana esculenta demonstrated that (-)-(S)- and (+)-(R)-Me2 were less potent than Mex in producing tonic block, but showed a higher use-dependent block. (-)-(S)-Me2 and (-)-(R)-Mex were also used to study the excitability of muscle fibres of myotonic ADR mice, a phenotype of a recessive form of low G(Cl) myotonia. (-)-(S)-Me2 reduced spontaneous discharges and after discharges better than (-)-(R)-Mex in agreement with the use-dependent block of sodium currents.

Increased hindrance on the chiral carbon atom of mexiletine enhances the block of rat skeletal muscle Na+ channels in a model of myotonia induced by ATX

1 The antiarrhythmic drug mexiletine (Mex) is also used against myotonia. Searching for a more efficient drug, a new compound (Me5) was synthesized substituting the methyl group on the chiral carbon atom of Mex by an isopropyl group. Effects of Me5 on Na+ channels were compared to those of Mex in rat skeletal muscle fibres using the cell-attached patch clamp method. 2 Me5 (10 microM) reduced the maximal sodium current (INa) by 29.7+/-4.4 % (n=6) at a frequency of stimulation of 0.3 Hz and 65.7+/-4.4 % (n=6) at 1 Hz. At same concentration (10 microM), Mex was incapable of producing any effect (n=3). Me5 also shifted the steady-state inactivation curves by -7. 9+/-0.9 mV (n=6) at 0.3 Hz and -12.2+/-1.0 mV (n=6) at 1 Hz. 3 In the presence of sea anemone toxin II (ATX; 5 microM), INa decayed more slowly and no longer to zero, providing a model of sodium channel myotonia. The effects of Me5 on peak INa were similar whatever ATX was present or not. Interestingly, Me5 did not modify the INa decay time constant nor the steady-state INa to peak INa ratio. 4 Analysis of ATX-induced late Na+ channel activity shows that Me5 did not affect mean open times and single-channel conductance, thus excluding open channel block property. 5 These results indicate that increasing hindrance on the chiral atom of Mex increases drug potency on wild-type and ATX-induced noninactivating INa and that Me5 might improve the prophylaxis of myotonia.

Effect of mexiletine on sea anemone toxin-induced non-inactivating sodium channels of rat skeletal muscle: a model of sodium channel myotonia

The sea anemone toxin ATX II impairs skeletal muscle sodium channel inactivation, mimicking the persistent inward current observed in patients suffering from sodium channel myotonia. Mexiletine has beneficial effects on myotonia. To verify the efficiency of the drug on persistent inward current, we investigated the effect of 50 microM R(-)-mexiletine on sodium channels in cell-attached patches of rat skeletal muscle fibres, in the absence or presence of 2 microM ATX II. With the toxin, a proportion of channels displayed remarkable abnormal activity lasting the entire depolarisation, which resulted in a persistent inward current that represented up to 2.0% of the peak current. Mexiletine reduced by 75% the peak current elicited by depolarisation from -100 to -20 mV. This was due to the reduction by 60% of the maximal available peak current Imax and to the negative shift by -7 mV of steady-state inactivation. Mexiletine also greatly decreased the late current, but the effect was limited to 60% of reduction, comparable to that on Imax. Therefore mexiletine was able to block the ATX II-modified sodium channels, inhibiting the myotonia-producing persistent inward current.

Antiarrhythmic drugs and cardiac ion channels: mechanisms of action

In this review a description and an analysis are given of the interaction of antiarrhythmic drugs with their molecular target, i.e. ion channels and receptors. Our approach is based on the concept of vulnerable parameter, i.e. the electrophysiological property which plays a crucial role in the genesis of arrhythmias. To prevent or stop the arrhythmia a drug should modify the vulnerable parameter by its action on channel or receptor targets. In the first part, general aspects of the interaction between drugs channel molecules are considered. Drug binding depends on the state of the channel: rested, activated pre-open, activated open, or inactivated state. The change in channel behaviour with state is presented in the framework of the modulated-receptor hypothesis. Not only inhibition but also stimulation can be the result of drug binding. In the second part a detailed and systematic description and an analysis are given of the interaction of drugs with specific channels (Na+, Ca2+, K+, "pacemaker") and non-channel receptors. Emphasis is given to the type of state-dependent block involved (rested, activated and inactivated state block) and the change in channel kinetics. These properties vary and determine the voltage- and frequency-dependence of the change in ionic current. Finally, the question is asked as to whether the available drugs by their action on channels and receptors modify the vulnerable parameter in the desired way to stop or prevent arrhythmias.

Voltage- and use-dependent inhibition of Na+ channels in rat sensory neurones by 4030W92, a new antihyperalgesic agent

1. Whole cell patch clamp techniques were used to study the effects of 4030W92 (2,4-diamino-5-(2,3-dichlorophenyl)-6-fluoromethylpyrimidine), a new antihyperalgesic agent, on rat dorsal root ganglion (DRG) neurones. 2. In small diameter, presumably nociceptive DRG neurones under voltage-clamp, 4030W92 (1-100 microM) produced a concentration-related inhibition of slow tetrodotoxin-resistant Na+ currents (TTXR). From a holding potential (Vh) of -90 mV, currents evoked by test pulses to 0 mV were inhibited by 4030W92 with a mean IC50 value of approximately 103 microM. 3. The inhibitory effect of 4030W92 on TTX(R) was both voltage- and use-dependent. Currents evoked from a Vh of -60 mV were inhibited by 4030W92 with a mean IC50 value of 22 microM, which was 5 fold less than the value obtained at -90 mV. Repeated activation of TTX(R) by a train of depolarizing pulses (5 Hz, 20 ms duration) enhanced the inhibitory effects of 4030W92. These data could be explained by a preferential interaction of the drug with inactivation states of the channel. In support of this hypothesis 4030W92 (30 microM) produced a significant hyperpolarizing shift of 10 mV in the slow inactivation curve for TTX(R) and markedly slowed the recovery from channel inactivation. 4. Fast TTX-sensitive Na+ currents (TTXs) were also inhibited by 4030W92 in a voltage-dependent manner. The IC50 values obtained from Vhs of -90 mV and -70 mV were 37 microM and 5 microM, respectively. 4030W92 (30 microM) produced a 13 mV hyperpolarizing shift in the steady-state inactivation curve of TTXs. 5. High threshold voltage-gated Ca2+ currents were only weakly inhibited by 4030W92. The reduction in peak Ca2+ current amplitude produced by 100 microM 4030W92 was 20+/-6% (n=6). Low threshold T-type Ca2+ currents were inhibited by 17+/-8% and 43+/-3% by concentrations of 4030W92 of 30 microM and 100 microM, respectively (n=6). 6. Under current clamp, some cells exhibited broad TTX-resistant action potentials whilst others showed fast TTX-sensitive action potentials in response to a depolarizing current injection. In most cells a long duration (800 ms) supramaximal current injection evoked a train of action potentials. 4030W92 (10-30 microM) had little effect on the first spike in the train but produced a concentration-related inhibition of the later spikes. The number of spikes per train was significantly reduced from 9.7+/-1.5 to 4.2+/-1.0 and 2.6+/-1.1 in the presence of 10 microM and 30 microM 4030W92, respectively (n=5). 7. Thus, 4030W92 is a potent voltage- and use-dependent inhibitor of Na+ channels in sensory neurones. This profile can be explained by a preferential action of the drug on a slow inactivation state of the channel that results in a delayed recovery to the resting state. This state-dependent modulation by 4030W92 of Na+ channels that are important in sensory neurone function may underlie or contribute to the antihyperalgesic profile of this compound observed in vivo.

Pharmacological targeting of long QT mutant sodium channels

The congenital long QT syndrome (LQTS) is an inherited disorder characterized by a delay in cardiac cellular repolarization leading to cardiac arrhythmias and sudden death often in young people. One form of the disease (LQT3) involves mutations in the voltage-gated cardiac sodium channel. The potential for targeted suppression of the LQT defect was explored by heterologous expression of mutant channels in cultured human cells. Kinetic and steady state analysis revealed an enhanced apparent affinity for the predominantly charged, primary amine compound, mexiletine. The affinity of the mutant channels in the inactivated state was similar to the wild type (WT) channels (IC50 approximately 15-20 microM), but the late-opening channels were inhibited at significantly lower concentrations (IC50 = 2-3 microM) causing a preferential suppression of the late openings. The targeting of the defective behavior of the mutant channels has important implications for therapeutic intervention in this disease. The results provide insights for the selective suppression of the mutant phenotype by very low concentrations of drug and indicate that mexiletine equally suppresses the defect in all three known LQT3 mutants.

Blockade of cardiac Na+ channels by a charged class I antiarrhythmic agent, bisaramil: possible interaction of the drug with a pre-open closed state

The mechanism of cardiac Na+ channel block by a charged class I antiarrhythmic agent, bisaramil, was studied in guinea-pig ventricular myocytes using patch-clamp techniques of whole-cell, cell-attached and inside-out configurations. Bath application of bisaramil caused the use-dependent block of whole cell Na+ current (INa) in a concentration-dependent manner and EC30 value was 2.0 microM. At 5 microM bisaramil, the degree of the use-dependent block of INa with a short (5 ms) pulse protocol (44.9 +/- 5.7% of the first pulse INa) was comparable to that with a long (200 ms) pulse protocol (42.8 +/- 5.9%). In cell-attached patches, bisaramil applied to the bath solution (external application) concentration dependently blocked macropatch Na+ currents (50.3 +/- 3.1% inhibition with 10 microM bisaramil). Internal application of bisaramil decreased the inside-out macropatch currents (82.6 +/- 1.3% inhibition with 10 microM bisaramil). Blocking effects of bisaramil applied to the bath solution were greater than those seen on the pipette application in all of the whole-cell, cell-attached and inside-out configurations. In cell-attached patches containing a single active channel, bath application of 10 microM bisaramil increased the null sweeps with a significant (P < 0.001) nonrandom clustering and decreased the total number of openings, whereas no changes in the number of openings per active sweep, unitary current amplitude, mean open time and mean closed time were observed. While the peak average current was decreased by 51.0 +/- 5.6% with 10 microM bisaramil, the number of active sweeps was decreased by 31.4 +/- 6.2%. In the presence of 10 microM bisaramil, the mean values of first latencies were significantly (P < 0.05) increased and the peak value of the first latency density function was decreased by 15.8 +/- 3.6%. From these results, we conclude that a charged tertiary amine, bisaramil interacts with cardiac Na+ channels preferentially in the activated state. Interactions with pre-open closed states might contribute to the activated channel block by the drug.

Stereoselective effects of mexiletine enantiomers on sodium currents and excitability characteristics of adult skeletal muscle fibers

The effects of the enantiomers of mexiletine were tested on sodium currents of frog skeletal muscle fibers recorded by means of the three vaseline gap voltage clamp method and compared with the effects produced by tocainide enantiomers. The R-(-) mexiletine produced a tonic block of the sodium current, elicited by single depolarizing test pulses from the holding potential of -100 mV to -20 mV, with an IC50 of 43.9 +/- 1 microM, whereas the corresponding S-(+) enantiomer produced the same effects at about twofold higher concentrations. A similar steroselectivity was observed with tocainide enantiomers, but at about 5 fold higher concentrations. Both the R-(-) and S-(+) enantiomers of mexiletine and tocainide produced a further use-dependent block of sodium currents when the test pulse was applied repetitively at a frequency of 2 Hz. The use dependent behavior led to a significant lowering of the IC50 values with respect to the tonic block but the eudismic ratios ([IC50S-(+)]/[IC50R(-)]) and the relative potency between mexiletine and tocainide were maintained. All the tested compounds produced a left shift of the steady state inactivation curves (h infinity), suggesting a high-affinity interaction with the inactivated sodium channels. Again a stronger potency of R-(-) vs. S-(+) enantiomers and of mexiletine vs. tocainide was observed. The excitability characteristics recorded from the semitendinosus muscle by the two microelectrode technique were modified by the tested drugs in agreement with their ability to block sodium current. Thus a concentration-related increase in the threshold current required to elicit an action potential as observed along with a decrease in the amplitude and a shortening of the latency of action potential and a decrease in the firing capability of the membrane. Again the R-(-) isomers were more potent than the S-(+) ones and mexiletine was more effective than tocainide. These data corroborate the presence of a stereospecific site for these drugs on adult skeletal muscle sodium channels. The constant eudismic ratios between the enantiomers during both tonic and use-dependent block suggest that the increase in the apparent affinity of the receptor during state-dependent conformational changes of the channel does not enhance its stereospecificity. The decrease in effective concentration upon high frequency stimulation supports the potential usefulness of low doses of R-(-) mexiletine in the treatment of the abnormal hyperexcitability of the myotonic muscles, with a likely reduction of unwanted side effects.

Interaction between external Na+ and mexiletine on Na+ channel in guinea-pig ventricular myocytes

To assess the modulation of Na+ channel block with local anaesthetics by the change of external Na+ concentration ([Na+]o), we examined the block by mexiletine at different [Na+]o using the whole-cell and the cell-attached configurations of the patch-clamp technique. Lowering [Na+]o increased the degree of use-dependent block of the whole-cell Na+ current. The external Na+ dependence of the Na+ current block was caused by the interaction of mexiletine with the activated Na+ channel, but not with the inactivated channel. In single-Na+ channel current recordings at a reduced [Na+]o of 70 mM, mexiletine shortened the mean open time of the channels (1.32 +/- 0.06 ms in the control vs. 0.86 +/- 0.12 ms with the drug, P < 0.05) without changes in the unitary current amplitude, whereas the drug did not affect mean open time at a [Na+]o of 140 mM. Moreover, the open time distributions during drug exposure at the reduced [Na+]o were better fitted to a double exponential than to a single exponential in four out of six experiments. These data suggest that mexiletine induces two conductive states: the native open state and a state representing the first step of open channel block. The transition from the former to the latter is dependent on [Na+]o, suggesting an antagonistic interaction of external Na+ with mexiletine.