Steady-state Inactivation (steady-state + inactivation)

Distribution by Scientific Domains

Selected Abstracts

Individual variation and hormonal modulation of a sodium channel , subunit in the electric organ correlate with variation in a social signal

He Liu
Abstract The sodium channel ,1 subunit affects sodium channel gating and surface density, but little is known about the factors that regulate ,1 expression or its participation in the fine control of cellular excitability. In this study we examined whether graded expression of the ,1 subunit contributes to the gradient in sodium current inactivation, which is tightly controlled and directly related to a social behavior, the electric organ discharge (EOD), in a weakly electric fish Sternopygus macrurus. We found the mRNA and protein levels of ,1 in the electric organ both correlate with EOD frequency. We identified a novel mRNA splice form of this gene and found the splicing preference for this novel splice form also correlates with EOD frequency. Androgen implants lowered EOD frequency and decreased the ,1 mRNA level but did not affect splicing. Coexpression of each splice form in Xenopus oocytes with either the human muscle sodium channel gene, hNav1.4, or a Sternopygus ortholog, smNav1.4b, sped the rate of inactivation of the sodium current and shifted the steady-state inactivation toward less negative membrane potentials. The translational product of the novel mRNA splice form lacks a previously identified important tyrosine residue but still functions normally. The properties of the fish , and coexpressed ,1 subunits in the oocyte replicate those of the electric organ's endogenous sodium current. These data highlight the role of ion channel , subunits in regulating cellular excitability. 2007 Wiley Periodicals, Inc. Develop Neurobiol, 2007. [source]

Inhibitory Effect of Lamotrigine on A-type Potassium Current in Hippocampal Neuron,Derived H19-7 Cells

EPILEPSIA, Issue 7 2004
Chin-Wei Huang
Summary:,Purpose: We investigated the effects of lamotrigine (LTG) on the rapidly inactivating A-type K+ current (IA) in embryonal hippocampal neurons. Methods: The whole-cell configuration of the patch-clamp technique was applied to investigate the ion currents in cultured hippocampal neuron,derived H19-7 cells in the presence of LTG. Effects of various related compounds on IA in H19-7 cells were compared. Results: LTG (30 ,M,3 mM) caused a reversible reduction in the amplitude of IA. The median inhibitory concentration (IC50) value required for the inhibition of IA by LTG was 160 ,M. 4-Aminopyridine (1 mM), quinidine (30 ,M), and capsaicin (30 ,M) were effective in suppressing the amplitude of IA, whereas tetraethylammonium chloride (1 mM) and gabapentin (100 ,M) had no effect on it. The time course for the inactivation of IA was changed to the biexponential process during cell exposure to LTG (100 ,M). LTG (300 ,M) could shift the steady-state inactivation of IA to a more negative membrane potential by approximately ,10 mV, although it had no effect on the slope of the inactivation curve. Moreover, LTG (100 ,M) produced a significant prolongation in the recovery of IA inactivation. Therefore in addition to the inhibition of voltage-dependent Na+ channels, LTG could interact with the A-type K+ channels to suppress the amplitude of IA. The blockade of IA by LTG does not simply reduce current magnitude, but alters current kinetics, suggesting a state-dependent blockade. LTG might have a higher affinity to the inactivated state than to the resting state of the IA channel. Conclusions: This study suggests that in hippocampal neurons, during exposure to LTG, the LTG-mediated inhibition of these K+ channels could be one of the ionic mechanisms underlying the increased neuronal excitability. [source]

Episodic ataxia type 1 mutations in the KCNA1 gene impair the fast inactivation properties of the human potassium channels Kv1.4-1.1/Kv,1.1 and Kv1.4-1.1/Kv,1.2

Paola Imbrici
Abstract Episodic ataxia type 1 (EA1) is an autosomal dominant neurological disorder characterized by constant muscle rippling movements (myokymia) and episodic attacks of ataxia. Several heterozygous point mutations have been found in the coding sequence of the voltage-gated potassium channel gene KCNA1 (hKv1.1), which alter the delayed-rectifier function of the channel. Shaker -like channels of different cell types may be formed by unique hetero-oligomeric complexes comprising Kv1.1, Kv1.4 and Kv,1.x subunits. Here we show that the human Kv,1.1 and Kv,1.2 subunits modulated the functional properties of tandemly linked Kv1.4-1.1 wild-type channels expressed in Xenopus laevis oocytes by (i) increasing the rate and amount of N-type inactivation, (ii) slowing the recovery rate from inactivation, (iii) accelerating the cumulative inactivation of the channel and (iv) negatively shifting the voltage dependence of inactivation. To date, the role of the human Kv1.4-1.1, Kv1.4-1.1/Kv,1.1 and Kv1.4-1.1/Kv,1.2 channels in the aetiopathogenesis of EA1 has not been investigated. Here we also show that the EA1 mutations E325D, V404I and V408A, which line the ion-conducting pore, and I177N, which resides within the S1 segment, alter the fast inactivation and repriming properties of the channels by decreasing both the rate and degree of N-type inactivation and by accelerating the recovery from fast inactivation. Furthermore, the E325D, V404I and I177N mutations shifted the voltage dependence of the steady-state inactivation to more positive potentials. The results demonstrate that the human Kv,1.1 and Kv,1.2 subunits regulate the proportion of wild-type Kv1.4-1.1 channels that are available to open. Furthermore, EA1 mutations alter heteromeric channel availability which probably modifies the integration properties and firing patterns of neurones controlling cognitive processes and body movements. [source]

Effect of Cl, channel blockers on aconitine-induced arrhythmias in rat heart

Shi-Sheng Zhou
The effects of Cl, channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and niflumic acid (NFA) on aconitine-induced arrhythmias were investigated. Left ventricular pressure and electrocardiogram were monitored in Langendorff-perfused rat hearts. Whole-cell patch-clamp and current-clamp techniques were used to measure sodium current (INa) and action potential (AP), respectively, in single rat cardiac ventricular myocytes. Addition of the Na+ channel agonist aconitine (0.1 ,m) to the perfusion solution produced polymorphic ventricular arrhythmias with a latent period of 25.5 6.3 s. NPPB could reverse aconitine-induced arrhythmias. A similar effect was observed by using NFA. NPPB and NFA reversibly depressed the upstroke of the AP in a dose-dependent manner with IC50 values of ,12.3 and ,73.1 ,m, respectively, without significantly affecting the resting potential of rat ventricular myocytes. Both Cl, channel blockers inhibited INa and induced a leftward shift of the steady-state inactivation of INa. In conclusion, the results of this study demonstrate that NPPB as well as NFA can suppress aconitine-induced arrhythmias in rat hearts mainly by inhibiting cardiac INa. [source]

Reduction in the Sodium Currents in Isolated Ventricular Myocytes of Guinea Pigs Treated by Chronic L-Thyroxin Medication

Yu-Ping Ma
Objective: Cardiac remodeling induced by chronic medication of L-thyroxin is manifested by a much more severe cardiac arrhythmias on the occlusion/reperfusion of the coronary artery in rats. A pattern of changes in ion currents in a diseased heart (L-thyroxin induced cardiac remodeling) is possibly provided as a basis of promoting malignant cardiac arrhythmias. An enhanced delayed outward rectifier potassium currents the rapid (IKr) and slow (IKS) component was found in the remodeled heart by L-thyroxin chronic medication. It is interested to investigate the changes in the sodium currents in the L-thyroxin remodeled guinea pig ventricle. Method: The remodeling model in guinea pig was developed by L-thyroxin 4 mg po for 10 days. On d 11, the heart was removed and perfused to isolate ventricular myocytes with medium of Ca2+ free medium containing collagen. The whole cell holding technique was applied. Results: The INa density in the L-thyroxin caused hypertrophied myocytes was reduced significantly at holding potential ,30 mV, ,53.20 +/,10.78pA/pF against ,73.78+/,14.66pA/pF in the normal. (n = 45, p < 0.001). No difference in the steady-state inactivation and recovery kinetics between the remodeled and the normal was found. The recovery constant 37.54+/,3.63 ms in the remodeled vs 36.57+/,2.81 ms in the normal (n = 18, p > 0.05). The accelerated deactivation time constant 3.67+/,0.14 of the remodeled (n = 39) against the normal 4.14+/,0.15 ms (n = 43) (p < 0.05). Conclusion: There is a reduced INa in the L-thyroxin remodeled ventricular myocytes and the deactivation of the current is accelerated. A changed depolarization of the affected myocardium is likely involved in the mechanism of arrhythmogenesis of the remodeled ventricle. [source]

The action of Lambert,Eaton myasthenic syndrome immunoglobulin G on cloned human voltage-gated calcium channels

MUSCLE AND NERVE, Issue 5 2002
Ashwin Pinto MRCP, DPhil
Abstract In the Lambert,Eaton myasthenic syndrome (LEMS), immunoglobulin G (IgG) autoantibodies to presynaptic voltage-gated calcium channels (VGCCs) at the neuromuscular junction lead to a reduction in nerve-evoked release of neurotransmitter and muscle weakness. We have examined the action of LEMS IgGs on cloned human VGCCs stably expressed in transfected human embryonic kidney (HEK293) cell lines: 10,13 (,1A-2, ,2b,, ,4a) and C2D7 (,1B-1 , ,2b,, ,1b). All LEMS IgGs studied showed surface binding to [125I]-,-CTx-MVIIC-labeled VGCCs in the ,1A cell line and two of six IgGs showed surface binding to [125I]-,-CTx-GVIA-labeled VGCCs in the ,1B cell line. We next studied the effect of LEMS IgGs (2 mg/ml) on whole-cell calcium currents in the ,1A and ,1B cell lines. Overnight treatment of ,1A (10,13) cells with LEMS IgGs led to a significant reduction in peak current density without alteration of the current,voltage relationship or the voltage dependence of steady-state inactivation. In contrast, LEMS IgGs did not reduce peak current density in the ,1B cell line. Overall these data demonstrate the specificity of LEMS IgGs for the ,1A cell line and suggest that LEMS IgGs bind to and downregulate VGCCs in this cell line. Although several LEMS IgGs can be shown to bind to the ,1B (C2D7) cell line, no functional effects were seen on this channel. 2002 Wiley Periodicals, Inc. Muscle Nerve 25: 000,000, 2002 [source]

The molecular interactions of pyrethroid insecticides with insect and mammalian sodium channels,

Horia Vais
Abstract Recent progress in the cloning of , (para) and , (TipE) Na channel sub-units from Drosophila melanogaster (fruit fly) and Musca domestica (housefly) have facilitated functional expression studies of insect Na channels in Xenopus laevis oocytes, assayed by voltage clamp techniques. The effects of Type I and Type II pyrethroids on the biophysical properties of these channels are critically reviewed. Pyrethroid resistance mutations (termed kdr and super-kdr) that reduce the sensitivity of the insect Na channel to pyrethroids have been identified in a range of insect species. Some of these mutations (eg L1014F, M918T and T929I) have been incorporated into the para Na channel of Drosophila, either individually or in combination, to investigate their effects on the sensitivity of this channel to pyrethroids. The kdr mutation (L1014F) shifts the voltage dependence of both activation and steady-state inactivation by ,5,mV towards more positive potentials and facilitates Na channel inactivation. Incorporation of the super-kdr mutation (M918T) into the Drosophila Na channel also increases channel inactivation and causes a >100-fold reduction in deltamethrin sensitivity. These effects are shared by T929I, an alternative mutation that confers super-kdr -like resistance. Parallel studies have been undertaken using the rat IIA Na channel to investigate the molecular basis for the low sensitivity of mammalian brain Na channels to pyrethroids. Rat IIA channels containing the mutation L1014F exhibit a shift in their mid-point potential for Na activation, but their overall sensitivity to permethrin remains similar to that of the wild-type rat channel (ie both are 1000-fold less sensitive than the wild-type insect channel). Mammalian neuronal Na channels have an isoleucine rather than a methionine at the position (874) corresponding to the super-kdr (M918) residue of the insect channel. Replacement of the isoleucine of the wild-type rat IIA Na channel with a methionine (I874M) increases deltamethrin sensitivity 100-fold. In this way, studies of wild-type and mutant Na channels of insects and mammals are providing a molecular understanding of kdr and super-kdr resistance in insects, and of the low pyrethroid sensitivity of most mammalian Na channels. They are also giving valuable insights into the binding sites for pyrethroids on these channels. 2001 Society of Chemical Industry [source]

Inhibition of the formation or action of angiotensin II reverses attenuated K+ currents in type 1 and type 2 diabetes

Yakhin Shimoni
1Transient and sustained calcium-independent outward K+ currents (It and ISS) as well as action potentials were recorded in cardiac ventricular myocytes isolated from two models of diabetes mellitus. 2Rats injected (i.v.) with streptozotocin (STZ, 100 mg kg,1) 6,10 days before cell isolation developed insulin-dependent (type 1) diabetes. It and ISS were attenuated and the action potential prolonged. Incubation of myocytes (6-9 h) with the angiotensin II (ATII) receptor blockers saralasin or valsartan (1 ,m) significantly augmented these currents. Inclusion of valsartan (1 g l,1) in the drinking water for 5,10 days prior to and following STZ injection partially prevented current attenuation. 3Incubation of myocytes from STZ-treated rats (6-9 h) with 1 ,m quinapril, an angiotensin-converting enzyme (ACE) inhibitor, significantly augmented It and ISS and shortened the ventricular action potential. It augmentation was not due to changes in steady-state inactivation or in recovery from inactivation. No acute effects of quinapril were observed. 4The effects of quinapril and valsartan were abolished by 2 ,m cycloheximide. 5Myocytes were isolated from the db/db mouse, a leptin receptor mutant that develops symptoms of non-insulin-dependent (type 2) diabetes. K+ currents in these cells were also attenuated, and the action potentials prolonged. Incubation of these cells (> 6 h) with valsartan (1 ,m) significantly enhanced the transient and sustained outward currents. 6These results confirm recent suggestions that cardiac myocytes contain a renin-angiotensin system, which is activated in diabetes. It is proposed that chronic release of ATII leads to changes in ionic currents and action potentials, which can be reversed by blocking the formation or action of ATII. This may underlie the proven benefits of ATII receptor blockade or ACE inhibition in diabetes, by providing protection against cardiac arrhythmias. [source]

A novel Nav1.7 mutation producing carbamazepine-responsive erythromelalgia,

Tanya Z. Fischer MD
Objective Human and animal studies have shown that Nav1.7 sodium channels, which are preferentially expressed within nociceptors and sympathetic neurons, play a major role in inflammatory and neuropathic pain. Inherited erythromelalgia (IEM) has been linked to gain-of-function mutations of Nav1.7. We now report a novel mutation (V400M) in a three-generation Canadian family in which pain is relieved by carbamazepine (CBZ). Methods We extracted genomic DNA from blood samples of eight members of the family, and the sequence of SCN9A coding exons was compared with the reference Nav1.7 complementary DNA. Wild-type Nav1.7 and V400M cell lines were then analyzed using whole-cell patch-clamp recording for changes in activation, deactivation, steady-state inactivation, and ramp currents. Results Whole-cell patch-clamp studies of V400M demonstrate changes in activation, deactivation, steady-state inactivation, and ramp currents that can produce dorsal root ganglia neuron hyperexcitability that underlies pain in these patients. We show that CBZ, at concentrations in the human therapeutic range, normalizes the voltage dependence of activation and inactivation of this inherited erythromelalgia mutation in Nav1.7 but does not affect these parameters in wild-type Nav1.7. Interpretation Our results demonstrate a normalizing effect of CBZ on mutant Nav1.7 channels in this kindred with CBZ-responsive inherited erythromelalgia. The selective effect of CBZ on the mutant Nav1.7 channel appears to explain the ameliorative response to treatment in this kindred. Our results suggest that functional expression and pharmacological studies may provide mechanistic insights into hereditary painful disorders. Ann Neurol 2009;65:733,741 [source]

Quercetin as a novel activator of L-type Ca2+ channels in rat tail artery smooth muscle cells

Simona Saponara
The aim of this study was to investigate the effects of quercetin, a natural polyphenolic flavonoid, on voltage-dependent Ca2+ channels of smooth muscle cells freshly isolated from the rat tail artery, using either the conventional or the amphotericin B-perforated whole-cell patch-clamp method. Quercetin increased L-type Ca2+ current [ICa(L)] in a concentration- (pEC50=5.090.05) and voltage-dependent manner and shifted the maximum of the current-voltage relationship by 10 mV in the hyperpolarizing direction, without, however, modifying the threshold and the equilibrium potential for Ca2+. Quercetin-induced ICa(L) stimulation was reversible upon wash-out. T-type Ca2+ current was not affected by quercetin. Quercetin shifted the voltage dependence of the steady-state inactivation and activation curves to more negative potentials by about 5.5 and 7.5 mV respectively, in the mid-potential of the curves as well as increasing the slope of activation. Quercetin slowed both the activation and the deactivation kinetics of the ICa(L). The inactivation time course was also slowed but only at voltages higher than 10 mV. Moreover quercetin slowed the rate of recovery from inactivation. These results prove quercetin to be a naturally-occurring L-type Ca2+ channel activator. British Journal of Pharmacology (2002) 135, 1819,1827; doi:10.1038/sj.bjp.0704631 [source]

Inhibition of cardiac Na+ current by primaquine

Gerardo Orta-Salazar
The electrophysiological effects of the anti-malarial drug primaquine on cardiac Na+ channels were examined in isolated rat ventricular muscle and myocytes. In isolated ventricular muscle, primaquine produced a dose-dependent and reversible depression of dV/dt during the upstroke of the action potential. In ventricular myocytes, primaquine blocked INa+ in a dose-dependent manner, with a Kd of 8.2 ,M. Primaquine (i) increased the time to peak current, (ii) depressed the slow time constant of INa+ inactivation, and (iii) slowed the fast component for recovery of INa+ from inactivation. Primaquine had no effect on: (i) the shape of the I , V curve, (ii) the reversal potential for Na+, (iii) the steady-state inactivation and gNa+ curves, (iv) the fast time constant of inactivation of INa+, and (v) the slow component of recovery from inactivation. Block of INa+ by primaquine was use-dependent. Data obtained using a post-rest stimulation protocol suggested that there was no closed channel block of Na+ channels by primaquine. These results suggest that primaquine blocks cardiac Na+ channels by binding to open channels and unbinding either when channels move between inactivated states or from an inactivated state to a closed state. Cardiotoxicity observed in patients undergoing malaria therapy with aminoquinolines may therefore be due to block of Na+ channels, with subsequent disturbances of impulse conductance and contractility. British Journal of Pharmacology (2002) 135, 751,763; doi:10.1038/sj.bjp.0704460 [source]