tetrodotoxin and Tachycardia--Ventricular

Topics: Action Potentials; Animals; Calcium Channel Blockers; Death, Sudden, Cardiac; Electrocardiography; Gallopamil; Heart Conduction System; Humans; Models, Biological; Myocardial Ischemia; Rabbits; Sodium Channel Blockers; Tachycardia, Ventricular; Tetrodotoxin; Ventricular Fibrillation

The effects of antiarrhythmic agents, including Classes I and IV and 3-10 mM Mg2+ on aconitine-induced arrhythmias were examined using a conventional microelectrode and patch clamp method in Langendorff-perfused rabbit hearts and isolated guinea-pig ventricular myocytes. Intracoronary application of 0.1 microM aconitine induced polymorphic ventricular tachycardia (PVT) which continued for more than 60 minutes. Application of aconitine to ventricular myocytes caused a prolonged action potential duration (APD) and the appearance of early afterdepolarization (EAD) together with the occurrence of an inward hump of the I-V curve around -60 to -40 mV and increased outward current at positive voltages. Application of 10 microM TTX and 5 mM or higher Mg2+ restored aconitine-induced PVT to sinus rhythm in Langendorff-perfused preparations and also shortened the prolonged APD, demonstrating the abolishment of EAD by aconitine in ventricular myocytes. However, antiarrhythmic agents did not exert such effects. In conclusion, the antiarrhythmic actions of Mg2+ and TTX in aconitine-induced arrhythmia are to abolish EAD and shorten the prolonged APD by suppression of the inward Na+ current around -60 to -40 mV.

Topics: Aconitine; Action Potentials; Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Guinea Pigs; In Vitro Techniques; Magnesium; Membrane Potentials; Myocardium; Patch-Clamp Techniques; Rabbits; Tachycardia, Ventricular; Tetrodotoxin

Late Na(+) current (I(NaL)) and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) are both increased in the diseased heart. Recently, CaMKII was found to phosphorylate the Na(+) channel 1.5 (Na(v)1.5), resulting in enhanced I(NaL). Conversely, an increase of I(NaL) would be expected to cause elevation of intracellular Ca(2+) and activation of CaMKII. However, a relationship between enhancement of I(NaL) and activation of CaMKII has yet to be demonstrated. We investigated whether Na(+) influx via Na(v)1.5 leads to CaMKII activation and explored the functional significance of this pathway. In neonatal rat ventricular myocytes (NRVM), treatment with the I(NaL) activators anemone toxin II (ATX-II) or veratridine increased CaMKII autophosphorylation and increased phosphorylation of CaMKII substrates phospholamban and ryanodine receptor 2. Knockdown of Na(v)1.5 (but not Na(v)1.1 or Na(v)1.2) prevented ATX-II-induced CaMKII phosphorylation, providing evidence for a specific role of Na(v)1.5 in CaMKII activation. In support of this view, CaMKII activity was also increased in hearts of transgenic mice overexpressing a gain-of-function Na(v)1.5 mutant (N(1325)S). The effects of both ATX-II and the N(1325)S mutation were reversed by either I(NaL) inhibition (with ranolazine or tetrodotoxin) or CaMKII inhibition (with KN93 or autocamtide 2-related inhibitory peptide). Furthermore, ATX-II treatment also induced CaMKII-Na(v)1.5 coimmunoprecipitation. The same association between CaMKII and Na(v)1.5 was also found in N(1325)S mice, suggesting a direct protein-protein interaction. Pharmacological inhibitions of either CaMKII or I(NaL) also prevented ATX-II-induced cell death in NRVM and reduced the incidence of polymorphic ventricular tachycardia induced by ATX-II in rat perfused hearts. Taken together, these results suggest that a Na(v)1.5-dependent increase in Na(+) influx leads to activation of CaMKII, which in turn phosphorylates Na(v)1.5, further promoting Na(+) influx. Pharmacological inhibition of either CaMKII or Na(v)1.5 can ameliorate cardiac dysfunction caused by excessive Na(+) influx.

Topics: Acetanilides; Amino Acid Substitution; Animals; Animals, Newborn; Calcium; Calcium Signaling; Calcium-Binding Proteins; Calcium-Calmodulin-Dependent Protein Kinase Type 2; Caspase 3; Cell Death; Cell Survival; Cnidarian Venoms; Dose-Response Relationship, Drug; Electrophysiological Phenomena; Female; Gene Expression; Heart Ventricles; Humans; Mice; Mice, Inbred Strains; Mice, Transgenic; Myocytes, Cardiac; NAV1.5 Voltage-Gated Sodium Channel; Peptides; Perfusion; Phosphorylation; Piperazines; Protein Binding; Rabbits; Ranolazine; Rats; Rats, Sprague-Dawley; RNA, Small Interfering; Ryanodine Receptor Calcium Release Channel; Sodium; Sodium Channels; Sodium-Calcium Exchanger; Tachycardia, Ventricular; Tetrodotoxin; Veratridine

Reduction of repolarization reserve increases the risk of arrhythmia. We hypothesized that inhibition of K(+) current (I(K)) to decrease repolarization reserve would unmask the proarrhythmic role of endogenous, physiological late Na(+) current (late I(Na)). Monophasic action potentials (MAP) and 12-lead electrocardiogram were recorded from female rabbit isolated hearts. To block I(K) and reduce repolarization reserve, E-4031, 4-aminopyridine, and BaCl(2) were used; to block endogenous late I(Na), tetrodotoxin (TTX) and ranolazine were used. E-4031 (1-60 nM) concentration-dependently prolonged MAP duration (MAPD(90)) and increased duration of the T wave from T(peak) to T(end) (T(peak)-T(end)), transmural dispersion of repolarization (TDR), and beat-to-beat variability (BVR) of MAPD(90). E-4031 caused spontaneous and pause-triggered polymorphic ventricular tachycardia [torsade de pointes (TdP)]. In the presence of 60 nM E-4031, TTX (0.6-3 muM) and ranolazine (5-10 muM) shortened MAPD(90), decreased TDR, BVR, and T(peak)-T(end) (n = 9-20, P < 0.01), and abolished episodes of TdP. In hearts treated with BaCl(2) or 4-aminopyridine plus E-4031, TTX (0.6-3 muM) shortened MAPD(90) and decreased T(peak)-T(end). Ranolazine could not reverse the effect of E-4031 to inhibit human ether-a-go-go-related gene (HERG) K(+) current; thus, the reversal by ranolazine of effects of E-4031 was likely due to inhibition of late I(Na) and not to antagonism of the HERG-blocking action of E-4031. We conclude that endogenous, physiological late I(Na) contributes to arrhythmogenesis in hearts with reduced repolarization reserve. Inhibition of this current partially reverses MAPD prolongation and abolishes arrhythmic activity caused by I(K) inhibitors.

Topics: 4-Aminopyridine; Acetanilides; Action Potentials; Animals; Anti-Arrhythmia Agents; Barium Compounds; Cell Line; Chlorides; Enzyme Inhibitors; ERG1 Potassium Channel; Ether-A-Go-Go Potassium Channels; Female; Heart Conduction System; Humans; Kidney; Patch-Clamp Techniques; Piperazines; Piperidines; Potassium Channel Blockers; Pyridines; Rabbits; Ranolazine; Sodium; Sodium Channel Blockers; Sodium Channels; Tachycardia, Ventricular; Tetrodotoxin; Torsades de Pointes

Congenital long QT syndrome (LQTS) with in utero onset of the rhythm disturbances is associated with a poor prognosis. In this study we investigated a newborn patient with fetal bradycardia, 2:1 atrioventricular block and ventricular tachycardia soon after birth.. Mutational analysis and DNA sequencing were conducted in a newborn. The 2:1 atrioventricular block improved to 1:1 conduction only after intravenous lidocaine infusion or a high dose of mexiletine, which also controlled the ventricular tachycardia.. A novel, spontaneous LQTS-3 mutation was identified in the transmembrane segment 6 of domain IV of the Na(v)1.5 cardiac sodium channel, with a G-->A substitution at codon 1763, which changed a valine (GTG) to a methionine (ATG). The proband was heterozygous but the mutation was absent in the parents and the sister. Expression of this mutant channel in tsA201 mammalian cells by site-directed mutagenesis revealed a persistent tetrodotoxin-sensitive but lidocaine-resistant current that was associated with a positive shift of the steady-state inactivation curve, steeper activation curve and faster recovery from inactivation. We also found a similar electrophysiological profile for the neighboring V1764M mutant. But, the other neighboring I1762A mutant had no persistent current and was still associated with a positive shift of inactivation.. These findings suggest that the Na(v)1.5/V1763M channel dysfunction and possible neighboring mutants contribute to a persistent inward current due to altered inactivation kinetics and clinically congenital LQTS with perinatal onset of arrhythmias that responded to lidocaine and mexiletine.

Topics: Bradycardia; Cell Line; DNA Mutational Analysis; Female; Humans; Infant, Newborn; Lidocaine; Long QT Syndrome; Male; Mutation; Myocardium; NAV1.5 Voltage-Gated Sodium Channel; Patch-Clamp Techniques; Sequence Analysis, DNA; Sodium Channels; Tachycardia, Ventricular; Tetrodotoxin; Transfection

Mutations in the gene (SCN5A) encoding the alpha-subunit of the cardiac Na+ channel cause congenital long QT syndrome (LQT-3). Here we describe a novel LQT-3 mutation I1768V (I1768V) located in the sixth transmembrane spanning segment of domain IV. This mutation is unusual in that it is located within a transmembrane spanning domain and does not promote the typically observed sustained inward current corresponding to a gain of channel function (bursting). Rather, I1768V increases the rate of recovery from inactivation and increases the channel availability, observed as a positive shift of the steady-state inactivation curve (+7.6 mV). Using a Markovian model of the cardiac Na+ channel, we simulated these changes in gating behavior and demonstrated that a small increase in the rate of recovery from inactivation is sufficient to explain all of the experimentally observed current changes. The effect of these alterations in channel gating results in an increase in window current that may act to disrupt cardiac repolarization.

Topics: Adolescent; Cell Line; DNA; DNA Mutational Analysis; Genotype; Humans; Kinetics; Long QT Syndrome; Male; Membrane Potentials; Mutation; NAV1.5 Voltage-Gated Sodium Channel; Patch-Clamp Techniques; Plasmids; Sodium Channels; Syncope; Tachycardia, Ventricular; Tetrodotoxin; Transfection

The combined effects of excitability and action potential duration (APD) restitution on wavefront dynamics remain unclear.. We used optical mapping techniques to study Langendorff-perfused rabbit hearts. In protocol IA (n=10), D600 at increasing concentrations was infused during ventricular fibrillation (VF). With concentration increased to 0.5 mg/L, fast VF (dominant frequency, 19.1+/-1.8 Hz) was consistently converted to ventricular tachycardia (VT). However, increasing D600 further to 2.5 or 5.0 mg/L converted VT to slow VF (11.9+/-2.3 Hz, P=0.0011). In an additional 4 hearts (protocol IB), tetrodotoxin converted a preexisting VT to slow VF (11.0+/-1.4 Hz). Optical maps show wandering wavelets in fast VF, organized reentry in VT, and spatiotemporal periodicity in slow VF. In protocol II, we determined APD and conduction time(-1) (CT(-1)) restitutions during D600 infusion. CT(-1) was used as an estimate of excitability. At 0.1 mg/L, APD and CT(-1) restitutions were steep and flat, respectively. APD restitution became flattened when D600 increased to 0.5 mg/L, converting fast VF to VT. Further increasing D600 to 2.5 or 5.0 mg/L steepened CT(-1) restitution and widened the range of S(1) pacing cycle lengths over which CT(-1) decreased, converting VT to slow VF.. Two types of VF exist in isolated rabbit hearts. Fast (type I) VF is associated with a steep APD restitution, a flat CT(-1) restitution, and wandering wavelets. Slow (type II) VF is associated with a flat APD restitution, a steep CT(-1) restitution, and spatiotemporal periodicity. Both excitability and APD restitution are important in VF maintenance.

Topics: Action Potentials; Animals; Body Surface Potential Mapping; Calcium Channel Blockers; Cardiac Pacing, Artificial; Dose-Response Relationship, Drug; Electrocardiography; Electrodes, Implanted; Electrophysiologic Techniques, Cardiac; Fluorescent Dyes; Fourier Analysis; Gallopamil; Heart; In Vitro Techniques; Light; Optics and Photonics; Pyridinium Compounds; Rabbits; Sodium Channel Blockers; Tachycardia, Ventricular; Tetrodotoxin; Time Factors; Ventricular Fibrillation

We examined the effects of Mg2+ on aconitine-induced polymorphic ventricular tachycardias (PVT) in excised rabbit hearts under Langendorff perfusion and in Purkinje-muscle preparations. Local electrograms using bipolar electrodes and transmembrane potentials with the microelectrode technique were recorded from Langendorff hearts and Purkinje-muscle preparations, respectively. In Langendorff preparations, intracoronary application of 0.1 microM aconitine induced PVT 28.8 +/- 3.4 min after development of regular monomorphic ventricular tachycardias (MVT) in all 18 preparations. Application of 5 and 10 mM Mg2+ restored aconitine-induced PVT to sinus rhythm after 26.8 +/- 3.4 min (n = 9), but < 3 mM Mg2+ was not effective in restoring of sinus rhythm. Increased Mg2+ concentrations < or = 5 mM in the coronary perfusate prevented development of PVT by aconitine. Intracoronary application of 10 microM tetrodotoxin (TTX) also restored aconitine-induced PVT to sinus rhythm after 3.2 +/- 0.8 min (n = 4). Although applications of 50 microM lidocaine, 10 microM flecainide, or 1 microM verapamil could change PVT to MVT, they were not effective in restoring sinus rhythm. In Purkinje-muscle preparations, spontaneous action potentials (AP) from slow diastolic depolarization appeared after aconitine at the maximum diastolic potential of -75.0 +/- 3.7 mV in Purkinje fibers and were conducted to ventricular muscles (n = 5). Spontaneous activity gradually increased in rate and then developed triggered activity arising from early after depolarization (EAD). EAD induced by aconitine always appeared first in Purkinje fibers and later in muscle fibers. Once triggered activities started from EAD, rate, rhythm and amplitudes of APs became fast and variable.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Aconitine; Animals; Anti-Arrhythmia Agents; Electrophysiology; Heart; Heart Ventricles; In Vitro Techniques; Magnesium; Microelectrodes; Purkinje Fibers; Rabbits; Tachycardia, Ventricular; Tetrodotoxin