External pH modifies sodium channel block by mexiletine in guinea pig ventricular myocytes

Mexiletine Block of Voltage-Gated Sodium Channels: Isoform- and State-Dependent Drug-Pore Interactions

Mexiletine is a class Ib antiarrhythmic drug and is also used clinically to reduce or prevent myotonia. In addition, mexiletine has neuroprotective effects in models of brain ischemia. We compared state-dependent affinities of mexiletine for Na_v1.2, Na_v1.4, and Na_v1.5 and examined the effects of mutations of transmembrane segment S6 residues on mexiletine block of Na_v1.5. Three channel isoforms had similar affinities of mexiletine for the rested state, and Na_v1.4 and Na_v1.5 had similar affinities for the open and inactivated states, while Na_v1.2 had lower affinity for these states than Na_v1.4 and Na_v1.5. Mutational studies revealed that the largest affinity change was observed for an Ala substitution of Phe in domain IV S6. In our homology modeling based on the bacterial Na⁺ channel, mexiletine changed its location and orientation in the pore depending on the state of the channel, irrespective of the channel isoform. Mexiletine occurred in the upper part in the pore in the open state and lower in the closed state. High-affinity binding of mexiletine in the open states of Na_v1.4 and Na_v1.5 was caused by a π-π interaction with Phe, whereas mexiletine was located away from Phe in the open state of Na_v1.2. These results provide crucial information on the mechanism of isoform differences in state-dependent block by local anesthetics and related drugs. Mexiletine at upper locations in the open state may effectively cause an electrostatic mechanism of block.

Probing kinetic drug binding mechanism in voltage-gated sodium ion channel: open state versus inactive state blockers

The kinetics and nonequilibrium thermodynamics of open state and inactive state drug binding mechanisms have been studied here using different voltage protocols in sodium ion channel. We have found that for constant voltage protocol, open state block is more efficient in blocking ionic current than inactive state block. Kinetic effect comes through peak current for mexiletine as an open state blocker and in the tail part for lidocaine as an inactive state blocker. Although the inactivation of sodium channel is a free energy driven process, however, the two different kinds of drug affect the inactivation process in a different way as seen from thermodynamic analysis. In presence of open state drug block, the process initially for a long time remains entropy driven and then becomes free energy driven. However in presence of inactive state block, the process remains entirely entropy driven until the equilibrium is attained. For oscillating voltage protocol, the inactive state blocking is more efficient in damping the oscillation of ionic current. From the pulse train analysis it is found that inactive state blocking is less effective in restoring normal repolarisation and blocks peak ionic current. Pulse train protocol also shows that all the inactive states behave differently as one inactive state responds instantly to the test pulse in an opposite manner from the other two states.

Preferred mexiletine block of human sodium channels with IVS4 mutations and its pH-dependence

The effects of extracellular pH (6.2, 7.4 and 8.2) and 0.1 mM mexiletine, a channel blocker of the lidocaine type, are studied on two mutations of the fourth voltage sensor of the Nav1.4 sodium channel, R1448H/C. The fast inactivated channel state to which mexiletine preferentially binds is destabilized by the mutations. By contrast to the expected low response of R1448H/C carriers, mexiletine is particularly effective in preventing exercise-induced stiffness and paralysis from which these patients suffer. Our measurements performed in the whole-cell mode on stably transfected HEK cells show for the first time that the mutations strikingly accelerate closed-state inactivation and, as steady-state fast inactivation is shifted to more negative potentials, stabilize the fast inactivated channel state in the potential range around the resting potential. At pH 7.4 and 8.2, the phasic mexiletine block is larger for R1448C (55%) and R1448H (47%) than for wild-type channels (31%) due to slowed recovery from block (tau is approximately 520 ms for R1448C versus 270 ms for wild-type at pH 7.4) although the recovery from inactivation is slightly faster for the mutants (tau is approximately 1.9 ms for R1448C versus 3.8 ms for wild-type at pH 7.4). At pH 6.2, recovery from block is relatively fast (tau is approximately 35 ms for R1448H/C and 14 ms for wild-type) and thus shows no use-dependence. We conclude that enhanced closed-state inactivation expands the concept of a mutation-induced uncoupling of channel inactivation from activation to a new potential range and that the higher mexiletine efficacy in R1448H/C carriers compared to other myotonic patients offers a pharmacogenetic strategy for mutation-specific treatment.

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.

Molecular mechanisms of the reversal of imipramine-induced sodium channel blockade by alkalinization in human cardiac myocytes

BACKGROUND

Alkalinizing agents reverse cardiotoxicity of a variety of sodium channel blockers, including tricyclic antidepressants, but their mechanisms of action are poorly understood.

PURPOSE

To establish the mechanisms by which alkalinization diminishes the sodium channel blocking action of imipramine.

METHODS

The whole-cell voltage-clamp technique was used to measure INa during a variety of depolarizing pulse protocols in isolated human atrial myocytes, in the presence and absence of imipramine. A three-state model was used to analyze state-dependent INa block.

RESULTS

Imipramine (1 and 5 microM) strongly inhibited INa. Experimental data and piecewise exponential analysis suggested significant binding to both activated and inactivated states. Alkalosis antagonized imipramine-induced INa blockade by increasing the unbinding rate, with intracellular alkalosis being more effective than extracellular alkalosis. The dissociation constant (Kd) for the inactivated state was increased from 0.55 to 1.40 microM by extracellular alkalosis and to 2.51 microM by intracellular alkalosis. Along with the reversal of drug-induced shifts in the inactivation curve, these data indicate that alkalosis on either side of the membrane antagonized drug interactions with the inactivated state. On the other hand, only intracellular alkalosis antagonized activated state block, increasing the Kd from 0.67 microM to 2.18 microM, while extracellular alkalosis left the activated state Kd unaltered at 0.67 microM.

CONCLUSIONS

Alkalinization antagonizes the INa-blocking action of imipramine by promoting unbinding from the receptor. Intracellular alkalosis has a particularly important effect related to the activated-state interaction. The lipid-soluble, uncharged moiety appears to be a critical determinant of imipramine's ability to dissociate from the Na+ channel receptor.

Mechanism of lidocaine block of late current in long Q-T mutant Na+ channels

Inherited long Q-T syndrome is a ventricular arrhythmia associated with delayed repolarization and the risk of sudden death. The chromosome 3-linked form of the disease (LQT3) is associated with mutations in the cardiac Na+ channel (N1325S or R1644H; or deletion of residues 1,505-1,507, delta KPQ) that increase late inward currents and may cause delayed repolarization. Late currents arise from dispersed reopenings (N1325S and R1644H) or from reopenings combined with prolonged bursts (delta KPQ). Therefore, we tested whether lidocaine blockade of late current varied among the different LQT3 mutant channels. We found that lidocaine preferentially blocked late over peak current and that the blockade was equally effective in all three channels, expressed in Xenopus oocytes. Lidocaine inhibited both dispersed reopenings and bursting in single channels without affecting mean open times. In the absence of drug, inactivating prepulses inhibited bursting but not dispersed reopenings. We suggest that lidocaine block of late current in LQT3 channels acts via a common mechanism involving stabilization of inactivation. Therefore, blockers that target the inactivated state may be effective therapeutic agents in all three biophysical phenotypes of LQT3.

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.

Relative role of alkalosis and sodium ions in reversal of class I antiarrhythmic drug-induced sodium channel blockade by sodium bicarbonate

BACKGROUND

Hypertonic sodium salts are used to treat sodium channel-blocking drug cardiotoxicity. The relative roles of alkalinization and increased sodium concentration ([Na+]o) for various drugs are incompletely known.

METHODS AND RESULTS

The effects of four class I drugs on action potential characteristics of canine Purkinje fibers at equieffective concentrations (disopyramide 30 mumol/L, mexiletine 80 mumol/L, flecainide 7 mumol/L, imipramine 5 mumol/L) were studied in the presence of normal Tyrode solution and one altered solution (increased [Na+]o, increased bicarbonate concentration, or both) in each experiment. Combined increases in sodium and bicarbonate concentration significantly reduced the depressant effects of flecainide, imipramine, and mexiletine on phase 0 upstroke (Vmax) but did not alter the effects of disopyramide. The effects of sodium bicarbonate were entirely due to alkalinization in the case of imipramine, but both alkalinization and increased [Na+]o contributed to the interaction with flecainide and mexiletine. The reversal of Vmax depression by increased [Na+]o and pH was due in part to hyperpolarization. In addition, alkalosis directly reversed the hyperpolarizing shift in Vmax inactivation caused by flecainide and imipramine without altering the shift caused by disopyramide and mexiletine.

CONCLUSIONS

Increases in sodium bicarbonate concentration reverse the effects of class I antiarrhythmic drugs to a varying extent, with drug-specific contributions of the sodium and bicarbonate moiety. The molecular basis for this drug specificity remains to be elucidated, but it has important potential implications for the use of hypertonic sodium salts to treat cardiotoxicity caused by sodium channel-blocking drugs.

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.