Inhibitory Effect of Cibenzoline on Na+/Ca2+ Exchange Current in Guinea-Pig Cardiac Ventricular Myocytes

Ion Channels in Genetic Epilepsy: From Genes and Mechanisms to Disease-Targeted Therapies

Epilepsy is a common and serious neurologic disease with a strong genetic component. Genetic studies have identified an increasing collection of disease-causing genes. The impact of these genetic discoveries is wide reaching-from precise diagnosis and classification of syndromes to the discovery and validation of new drug targets and the development of disease-targeted therapeutic strategies. About 25% of genes identified in epilepsy encode ion channels. Much of our understanding of disease mechanisms comes from work focused on this class of protein. In this study, we review the genetic, molecular, and physiologic evidence supporting the pathogenic role of a number of different voltage- and ligand-activated ion channels in genetic epilepsy. We also review proposed disease mechanisms for each ion channel and highlight targeted therapeutic strategies.

Flecainide ameliorates arrhythmogenicity through NCX flux in Andersen-Tawil syndrome-iPS cell-derived cardiomyocytes

Andersen-Tawil syndrome (ATS) is a rare inherited channelopathy. The cardiac phenotype in ATS is typified by a prominent U wave and ventricular arrhythmia. An effective treatment for this disease remains to be established. We reprogrammed somatic cells from three ATS patients to generate induced pluripotent stem cells (iPSCs). Multi-electrode arrays (MEAs) were used to record extracellular electrograms of iPSC-derived cardiomyocytes, revealing strong arrhythmic events in the ATS-iPSC-derived cardiomyocytes. Ca²⁺ imaging of cells loaded with the Ca²⁺ indicator Fluo-4 enabled us to examine intracellular Ca²⁺ handling properties, and we found a significantly higher incidence of irregular Ca²⁺ release in the ATS-iPSC-derived cardiomyocytes than in control-iPSC-derived cardiomyocytes. Drug testing using ATS-iPSC-derived cardiomyocytes further revealed that antiarrhythmic agent, flecainide, but not the sodium channel blocker, pilsicainide, significantly suppressed these irregular Ca²⁺ release and arrhythmic events, suggesting that flecainide's effect in these cardiac cells was not via sodium channels blocking. A reverse-mode Na^+/Ca²⁺exchanger (NCX) inhibitor, KB-R7943, was also found to suppress the irregular Ca²⁺ release, and whole-cell voltage clamping of isolated guinea-pig cardiac ventricular myocytes confirmed that flecainide could directly affect the NCX current (I_NCX). ATS-iPSC-derived cardiomyocytes recapitulate abnormal electrophysiological phenotypes and flecainide suppresses the arrhythmic events through the modulation of I_NCX.

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iPS cells are generated from three patients with ATS.

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ATS-iPS cell-derived cardiomyocytes show abnormal electrophysiological phenotypes.

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Flecainide suppresses abnormal electrophysiological phenotypes in ATS-iPS cell-derived cardiomyocytes.

Na(+) /Ca(2+) exchanger-heterozygote knockout mice display increased relaxation in gastric fundus and accelerated gastric transit in vivo

BACKGROUND

For the contraction and relaxation of gastric smooth muscles to occur, the intracellular Ca(2+) concentration must be increased and decreased, respectively. The Na(+) /Ca(2+) exchanger (NCX) is a plasma membrane transporter that is involved in regulating intracellular Ca(2+) concentrations.

METHODS

To determine the role of NCX in gastrointestinal tissues, we examined electric field stimulation (EFS)-induced relaxations in the circular muscles of the gastric fundus in NCX1 and NCX2 heterozygote knockout mice (HET).

KEY RESULTS

The myenteric plexus layers and the longitudinal and circular muscle layers in the gastric fundus of wild-type mice (WT) were strongly immunoreactive to NCX1 and NCX2. EFS induced a transient relaxation that was apparent during the stimulus and a sustained relaxation that persisted after the end of the stimulus. The amplitudes of EFS-induced transient relaxation and sustained relaxation were greater in NCX1 HET and NCX2 HET than in WT. When an inhibitor of nitric oxide synthase was added following the EFS, neither NCX1 HET nor NCX2 HET exhibited transient relaxation, similar to WT. Furthermore, when a PACAP antagonist was added following the EFS, sustained relaxation in NCX1 HET and NCX2 HET was not observed, similar to WT. Next, we examined the effect of NCX heterozygous deficiency on relaxation in response to NO and PACAP in smooth muscles. The magnitude of NOR-1- and PACAP-induced relaxations in NCX1 HET and NCX2 HET was similar to that of WT.

CONCLUSIONS & INFERENCES

In this study, we demonstrate that NCX1 and NCX2 expressed in neurons regulate the motility in the gastric fundus.

Nicorandil stimulates a Na⁺/Ca²⁺ exchanger by activating guanylate cyclase in guinea pig cardiac myocytes

Nicorandil, a hybrid of an ATP-sensitive K(+) (KATP) channel opener and a nitrate generator, is used clinically for the treatment of angina pectoris. This agent has been reported to exert antiarrhythmic actions by abolishing both triggered activity and spontaneous automaticity in an in vitro study. It is well known that delayed afterdepolarizations (DADs) are caused by the Na(+)/Ca(2+) exchange current (I NCX). In this study, we investigated the effect of nicorandil on the cardiac Na(+)/Ca(2+) exchanger (NCX1). We used the whole-cell patch clamp technique and the Fura-2/AM (Ca(2+) indicator) method to investigate the effect of nicorandil on I NCX in isolated guinea pig ventricular myocytes and CCL39 fibroblast cells transfected with dog heart NCX1. Nicorandil enhanced I NCX in a concentration-dependent manner. The EC50 (half-maximum concentration for enhancement of the drug) values were 15.0 and 8.7 μM for the outward and inward components of I NCX, respectively. 8-Bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP), a membrane-permeable analog of guanosine 3',5'-cyclic monophosphate (cGMP), enhanced I NCX. 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a guanylate cyclase inhibitor (10 μM), completely abolished the nicorandil-induced I NCX increase. Nicorandil increased I NCX in CCL39 cells expressing wild-type NCX1 but did not affect mutant NCX1 without a long intracellular loop between transmembrane segments (TMSs) 5 and 6. Nicorandil at 100 μM abolished DADs induced by electrical stimulation with ouabain. Nicorandil enhanced the function of NCX1 via guanylate cyclase and thus may accelerate Ca(2+) exit via NCX1. This may partially contribute to the cardioprotection by nicorandil in addition to shortening action potential duration (APD) by activating KATP channels.

Regulation of the Cav1.2 cardiac channel by redox via modulation of CaM interaction with the channel

Although it has been well documented that redox can modulate Cav1.2 channel activity, the underlying mechanisms are not fully understood. In our study, we examined the effects of redox on Cav1.2 channel activity and on CaM interaction with the Cav1.2 α1 subunit. Dithiothreitol (DTT, 1 mM) in the cell-attached mode decreased, while hydrogen peroxide (H2O2, 1 mM) increased channel activity to 72 and 303%, respectively. The effects of redox were maintained in the inside-out mode where channel activity was induced by CaM + ATP: DTT (1 mM) decreased, while H2O2 (1 mM) increased the channel activity. These results were mimicked by the thioredoxin and oxidized glutathione system. To test whether the redox state might determine channel activity by affecting the CaM interaction with the channel, we examined the effects of DTT and H2O2 on CaM binding to the N- and C-terminal fragments of the channel. We found that DTT concentration-dependently inhibited CaM binding to the C-terminus (IC50 37 μM), but H2O2 had no effect. Neither DTT nor H2O2 had an effect on CaM interaction with the N-terminus. These results suggest that redox-mediated regulation of the Cav1.2 channel is governed, at least partially, by modulation of the CaM interaction with the channel.

Na/Ca(2+) exchanger 1 transgenic mice display increased relaxation in the distal colon

Na(+)/Ca(2+) exchanger 1 (NCX1) is a plasma membrane transporter involved in regulating intracellular Ca(2+) concentrations. NCX1 is critical for Ca(2+) regulation in cardiac muscle, vascular smooth muscle and nerve fibers. However, little is known about the physiological role of NCX1 in gastrointestinal motility. To determine the role of NCX1 in gastrointestinal tissues, we examined electric field stimulation (EFS)-induced responses in the longitudinal smooth muscle of the distal colon in smooth muscle-specific NCX1 transgenic mice (Tg). Tg show that NCX1 protein was overexpressed in the distal colon at a level twofold greater than that of endogenous NCX1. We found that the amplitudes of EFS-induced relaxation that persisted during EFS were greater in Tg than in wild-type mice (WT). Under the nonadrenergic, noncholinergic condition, the EFS-induced relaxation in Tg was also greater than that in WT. Inhibition of NO synthase, CO synthase, soluble guanylate cyclase (sGC), and protein kinase G (PKG) all attenuated the enhanced relaxation in Tg, demonstrating the importance of NCX1 in NO/sGC/PKG signaling. The action of NOR-1, an NO donor, induced enhanced relaxation in Tg compared with that in WT. Unlike NOR-1, pituitary adenylate cyclase-activating peptide and vasoactive intestinal peptide induced a similar relaxation in Tg compared with that in WT. In this study, we demonstrate that NCX1 plays an important role in smooth muscle motility in the mouse distal colon.

Na⁺/Ca²⁺ exchanger 1/2 double-heterozygote knockout mice display increased nitric oxide component and altered colonic motility

The Na⁺/Ca²⁺ exchanger (NCX) is a plasma membrane transporter involved in regulating intracellular Ca²⁺ concentrations. NCX is critical for Ca²⁺ regulation in cardiac muscle, vascular smooth muscle, and nerve fibers. To determine the role of NCX1 and NCX2 in gastrointestinal tissues, we examined electric field stimulation (EFS)-induced responses in the longitudinal smooth muscle of the distal colon in NCX1 and NCX2 double-heterozygote knockoutmice (Double HET). We found that the amplitudes of EFS-induced relaxation that persisted during EFS were greater in Double HET than in wild-type mice (WT). Under the non-adrenergic, non-cholinergic (NANC) condition, EFS-induced relaxation in Double HET was similar in amplitude to that of WT. In the experiments in which l-NNA was added under NANC conditions following the EFS, the magnitudes of EFS-induced relaxation were smaller in Double HET than those in WT. In addition, an NCX inhibitor, SN-6, enhanced EFS-induced relaxation but did not affect EFS-induced relaxation under NANC condition, as in Double HET. Moreover, the magnitudes of relaxation induced by NOR-1, which generates NO, were greater in Double HET compared with WT. Similarly, SN-6 potentiated the magnitudes of NOR-1-induced relaxation. In this study, we demonstrate that NCX regulate colonic motility by altering the sensitivity of the inhibitory component.