While studying the adult rat skeletal muscle Na+ channel outer vestibule, we found that certain mutations of the lysine residue in the domain III P region at amino acid position 1237 of the alpha subunit, which is essential for the Na+ selectivity of the channel, produced substantial changes in the inactivation process. When skeletal muscle alpha subunits (micro1) with K1237 mutated to either serine (K1237S) or glutamic acid (K1237E) were expressed in Xenopus oocytes and depolarized for several minutes, the channels entered a state of inactivation from which recovery was very slow, i.e., the time constants of entry into and exit from this state were in the order of approximately 100 s. We refer to this process as "ultra-slow inactivation". By contrast, wild-type channels and channels with the charge-preserving mutation K1237R largely recovered within approximately 60 s, with only 20-30% of the current showing ultra-slow recovery. Coexpression of the rat brain beta1 subunit along with the K1237E alpha subunit tended to accelerate the faster components of recovery from inactivation, as has been reported previously of native channels, but had no effect on the mutation-induced ultra-slow inactivation. This implied that ultra-slow inactivation was a distinct process different from normal inactivation. Binding to the pore of a partially blocking peptide reduced the number of channels entering the ultra-slow inactivation state, possibly by interference with a structural rearrangement of the outer vestibule. Thus, ultra-slow inactivation, favored by charge-altering mutations at site 1237 in micro1 Na+ channels, may be analogous to C-type inactivation in Shaker K+ channels.

By means of a prospective study, relapse after discontinuation of antiepileptic drug treatment in 433 children with epilepsy and 40 patients who were treated after the first seizure was investigated. Independent of the electroencephalographic findings, the age of the patients, and other factors, the antiepileptic drugs were reduced during 1, 3, 6, and 12 months after 2, 3, and 4 seizure-free years; and in children with absences after 1, 2, 3, and 4 seizure-free years. The observation period after stopping therapy was at least 3 (on average, 5-6) years. In 157 of 433 children (36.3%) and 5 of 40 patients (12.5%), relapses occurred during this time. More than half (61.8%) of the relapses occurred during the withdrawal period or within 3 months: altogether, 86% within 1 year after discontinuation of therapy. Eighty-six percent of these patients again became free from seizures on administration of the original therapy. The dependence on predictive factors of the rate of relapse was tested by multivariate statistical analysis. There was a pronounced significant dependence on the duration of the seizure-free period, the duration of the withdrawal period, the length of illness, the frequency and duration of seizures, and the presence of paroxysmal activity in the EEG at the start of the discontinuation of antiepileptic drug treatment. The stopping of therapy during the pubertal period did not present a higher risk.

We describe a mutation in the outer vestibule region of the adult rat skeletal muscle voltage-gated Na+ channel (microliter) that dramatically alters binding of mu-conotoxin GIIIA (mu-CTX). Mutating the glutamate at position 758 to glutamine (E758Q) decreased mu-CTX binding affinity by 48-fold. Because the mutant channel showed both low tetrodotoxin (TTX) and mu-CTX affinities, these results suggested that mu-CTX bound to the outer vestibule and implied that the TTX- and mu-CTX-binding sites partially overlapped in this region. The mutation decreased the association rate of the toxin with little effect on the dissociation rate, suggesting that Glu-758 could be involved in electrostatic guidance of mu-CTX to its binding site. We propose a mechanism for mu-CTX block of the Na+ channel based on the analogy with saxitoxin (STX) and TTX, on the requirement of mu-CTX to have an arginine in position 13 to occlude the channel, and on this experimental result suggesting that mu-CTX binds in the outer vestibule. In this model, the guanidinium group of Arg-13 of the toxin interacts with two carboxyls known to be important for selectivity (Asp-400 and Glu-755), with the association rate of the toxin increased by interaction with Glu-758 of the channel.

BackgroundDuchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, is associated with severe cardiac complications including cardiomyopathy and cardiac arrhythmias. Recent research suggests that impaired voltage-gated ion channels in dystrophic cardiomyocytes accompany cardiac pathology. It is, however, unknown if the ion channel defects are primary effects of dystrophic gene mutations, or secondary effects of the developing cardiac pathology.Methodology/Principal FindingsTo address this question, we first investigated sodium channel impairments in cardiomyocytes derived from dystrophic neonatal mice prior to cardiomyopahty development, by using the whole cell patch clamp technique. Besides the most common model for DMD, the dystrophin-deficient mdx mouse, we also used mice additionally carrying an utrophin mutation. In neonatal cardiomyocytes, dystrophin-deficiency generated a 25% reduction in sodium current density. In addition, extra utrophin-deficiency significantly altered sodium channel gating parameters. Moreover, also calcium channel inactivation was considerably reduced in dystrophic neonatal cardiomyocytes, suggesting that ion channel abnormalities are universal primary effects of dystrophic gene mutations. To assess developmental changes, we also studied sodium channel impairments in cardiomyocytes derived from dystrophic adult mice, and compared them with the respective abnormalities in dystrophic neonatal cells. Here, we found a much stronger sodium current reduction in adult cardiomyocytes. The described sodium channel impairments slowed the upstroke of the action potential in adult cardiomyocytes, and only in dystrophic adult mice, the QRS interval of the electrocardiogram was prolonged.Conclusions/SignificanceIon channel impairments precede pathology development in the dystrophic heart, and may thus be considered potential cardiomyopathy triggers.

The plant alkaloid ibogaine has promising anti-addictive properties. Albeit not licenced as a therapeutic drug, and despite hints that ibogaine may perturb the heart rhythm, this alkaloid is used to treat drug addicts. We have recently reported that ibogaine inhibits human ERG (hERG) potassium channels at concentrations similar to the drugs affinity for several of its known brain targets. Thereby the drug may disturb the heart's electrophysiology.Here, to assess the drug's cardiac ion channel profile in more detail, we studied the effects of ibogaine and its congener 18-Methoxycoronaridine (18-MC) on various cardiac voltage-gated ion channels. We confirmed that heterologously expressed hERG currents are reduced by ibogaine in low micromolar concentrations. Moreover, at higher concentrations, the drug also reduced human Nav1.5 sodium and Cav1.2 calcium currents. Ion currents were as well reduced by 18-MC, yet with diminished potency. Unexpectedly, although blocking hERG channels, ibogaine did not prolong the action potential (AP) in guinea pig cardiomyocytes at low micromolar concentrations. Higher concentrations (≥ 10 μM) even shortened the AP. These findings can be explained by the drug's calcium channel inhibition, which counteracts the AP-prolonging effect generated by hERG blockade. Implementation of ibogaine's inhibitory effects on human ion channels in a computer model of a ventricular cardiomyocyte, on the other hand, suggested that ibogaine does prolong the AP in the human heart. We conclude that therapeutic concentrations of ibogaine have the propensity to prolong the QT interval of the electrocardiogram in humans. In some cases this may lead to cardiac arrhythmias.

Amino acids located in the outer vestibule of the voltage-gated Na؉ channel determine the permeation properties of the channel. Recently, residues lining the outer pore have also been implicated in channel gating. The domain (D) IV P-loop residue alanine 1529 forms a part of the putative selectivity filter of the adult rat skeletal muscle (1) Na ؉ channel. Here we report that replacement of alanine 1529 by aspartic acid enhances entry to an ultraslow inactivated state. Ultra-slow inactivation is characterized by recovery time constants on the order of ϳ100 s from prolonged depolarizations and by the fact that entry to this state can be reduced by binding to the pore of a mutant -conotoxin GIIIA, suggesting that ultra-slow inactivation may reflect a structural rearrangement of the outer vestibule. The voltage dependence of ultra-slow inactivation in DIV-A1529D is U-shaped, with a local maximum near ؊60 mV, whereas activation is maximal only above ؊20 mV. Furthermore, a train of brief depolarizations produces more ultra-slow inactivation than a single maintained depolarization of the same duration. These data suggest that ultra-slow inactivation emanates from "partially activated" closed states and that the P-loop in DIV may undergo a conformational change during channel activation, which is accentuated by DIV-A1529D. Upon depolarization voltage-gated Naϩ channels first open and then enter one or more inactivated states. These inactivated states can be separated by their contribution to the time course of recovery. Upon repolarization from brief (millisecond time scale) depolarizations recovery is characterized by a single kinetic phase with a time constant of a few milliseconds ("fast inactivation"). If adult rat skeletal muscle (1) Na ϩ channels, heterologously expressed in Xenopus oocytes, are inactivated for Ն20 ms and then repolarized, the channels recover from inactivation with three distinct time constants, implying that there are at least three distinct inactivated states. These time constants are in the order of several ms (fast inactivation

Duchenne muscular dystrophy (DMD), induced by mutations in the gene encoding for the cytoskeletal protein dystrophin, is an inherited disease characterized by progressive muscle weakness. Besides the relatively well characterized skeletal muscle degenerative processes, DMD is also associated with cardiac complications. These include cardiomyopathy development and cardiac arrhythmias. The current understanding of the pathomechanisms in the heart is very limited, but recent research indicates that dysfunctional ion channels in dystrophic cardiomyocytes play a role. The aim of the present study was to characterize abnormalities in L-type calcium channel function in adult dystrophic ventricular cardiomyocytes. By using the whole cell patch-clamp technique, the properties of currents through calcium channels in ventricular cardiomyocytes isolated from the hearts of normal and dystrophic adult mice were compared. Besides the commonly used dystrophin-deficient mdx mouse model for human DMD, we also used mdx-utr mice, which are both dystrophin- and utrophin-deficient. We found that calcium channel currents were significantly increased, and channel inactivation was reduced in dystrophic cardiomyocytes. Both effects enhance the calcium influx during an action potential (AP). Whereas the AP in dystrophic mouse cardiomyocytes was nearly normal, implementation of the enhanced dystrophic calcium conductance in a computer model of a human ventricular cardiomyocyte considerably prolonged the AP. Finally, the described dystrophic calcium channel abnormalities entailed alterations in the electrocardiograms of dystrophic mice. We conclude that gain of function in cardiac L-type calcium channels may disturb the electrophysiology of the dystrophic heart and thereby cause arrhythmias.

Recently, we reported that mutation A1529D in the domain (D) IV P-loop of the rat skeletal muscle Na ؉ channel 1 (DIV-A1529D) enhanced entry to an inactivated state from which the channels recovered with an abnormally slow time constant on the order of ϳ100 s. Transition to this "ultra-slow" inactivated state (USI) was substantially reduced by binding to the outer pore of a mutant -conotoxin GIIIA. This indicated that USI reflected a structural rearrangement of the outer channel vestibule and that binding to the pore of a peptide could stabilize the pore structure (Hilber, K., Sandtner, W., Kudlacek, O., Glaaser, I. W., Weisz, E., Kyle, J. W., Voltage-gated Na ϩ channels mediate the Na ϩ conductance responsible for the rapidly rising phase of the action potential in nerve and muscle cells. Upon repolarization from brief depolarizations (Ͻ50 ms), Na ϩ channels recover from inactivation with a single kinetic phase whose time constant is on the order of a few milliseconds. After long depolarizations (seconds to minutes), recovery from inactivation proceeds through multiple kinetic phases whose time constants range over several orders of magnitude up to tens of seconds (1, 2). The most rapid phase of recovery is thought to correspond to the process of exit from the fast inactivated state, which occurs via intracellular occlusion of the ion-conducting pore by a cluster of hydrophobic amino acids in the loop that links DIII and DIV, the so-called