Activation Of Sodium Current Research Articles

Modulation of sodium currents in rat CA1 neurons by carbamazepine and valproate after kindling epileptogenesis.

To determine the modulation of sodium currents in hippocampal CA1 neurons by carbamazepine (CBZ) and valproate (VPA), before and after kindling epileptogenesis. Voltage-dependent sodium current was measured in isolated hippocampal CA1 neurons, by using the whole-cell voltage-clamp technique. CBZ (15-100 microM) or VPA (0.5-5 mM) was applied by bath perfusion. Cells from fully kindled rats were compared with controls, 1 day and 5 weeks after the tenth generalized seizure. CBZ did not affect sodium current activation but selectively shifted the voltage dependence of steady-state inactivation to more hyperpolarized potentials. One day after the last kindled generalized seizure, the shift induced by 15 microM CBZ was 2.1+/-0.5 mV (mean +/- SEM; n = 20) compared with 4.3+/-0.3 mV (n = 16; p<0.001) in matched controls. The EC50 of the concentration-effect relation was 57+/-6 microM compared with 34+/-2 microM (p<0.01) in controls. Five weeks after kindling, these values had recovered to a level not different from control. VPA induces at a relatively high concentration a similar but smaller shift in voltage dependence of inactivation than does CBZ. After kindling, the shift induced by 2 mM VPA (2.8+/-0.6 mV; n = 19) was not different from controls (3.0+/-0.5 mV; n = 22). The EC50 for VPA was 2.6+/-0.3 mM compared with 2.5+/-0.4 mM in controls. Both CBZ and VPA selectively modulate the voltage dependence of sodium current steady-state inactivation and as a consequence reduce cellular excitability. The effect of CBZ was reduced immediately after kindling epileptogenesis, apparently by a reduced affinity of its receptor. In contrast, the shift induced by VPA was not different at any stage after kindling epileptogenesis. The change in CBZ sensitivity after kindling is related to epileptic activity rather than to the epileptic state, because it almost completely recovers in a period without seizures.

Effet des ciguatoxines sur le système cardio-circulatoire

The aim of the present review was to collect the main observations reported until now concerning the cardio-circulatory effects of polyether toxins, called ciguatoxins, which are involved in an endemic intoxication named ciguatera found in tropical and subtropical countries. Ciguatera is caused by the ingestion of fishes contaminated with the dinoflagellate Gamberdiscus toxicus. Due to both tropical fish exportation destined for food and tourism, the disease has now spread out to temperate areas. Several toxins have been isolated and purified from different fish species living in different geographical areas. They are classified into three main groups by the nature of certain cycles of their carbon skeleton. Clinical reports show evidence that ciguatera intoxication affect both electrocardiograms and blood pressure. In most cases, ciguateric intoxication mainly evoked bradycardia, hypotension, and the alteration of S-T segment in the electrocardiogram. Isolated and purified ciguatoxins strongly altered the morphology of cardiac tissue inducing swelling of the cells and alterations of cellular organelles. These toxins impair the conduction of cardiac nerves and increase the opening probability of Na+ channels in intracardiac ganglions. Depending on the concentration applied, the substances exerted either a fast positive inotropic effect or a negative inotropic effect on the contraction of mammalian atrial and ventricular cardiac muscle. These effects were attributed to a release of noradrenaline and acetylcholine from neural terminals of the autonomic nervous system present in cardiac tissue. They also exert a slow delayed inotropic effect on the contraction which has been attributed to a direct effect of the toxins on tetrodotoxin-sensitive voltage-dependent Na+ channels of cardiac membranes. Ciguatoxins depolarized the membrane of mammalian atrial and ventricular preparations and shifted the threshold of sodium current activation to more negative membrane potentials. In conclusion, the inotropic effects of ciguatoxins on cardiac tissues mainly depend on the toxin concentration sensitivity of autonomic nerve terminals, which released noradrenaline and/or acetylcholine, while the ciguatoxin-induced increase of the sodium influx could be involved in the cardiac cell swelling which coincides with reports in which ciguatoxins induced a mannitol-inhibited swelling of the Node of Ranvier.

Effect of valproic acid on sodium currents in cortical neurons from patients with pharmaco-resistant temporal lobe epilepsy

In a selected group of temporal lobe epilepsy patients with seizures refractory to pharmacological treatment, pharmacological seizure control can be attained by surgical resection of the epileptic zone. We investigated to what extent pharmaco-resistance is reflected in a reduced response at the cellular level, in neurons acutely isolated from the temporal cortex resected in 20 patients. We studied the effect of valproic acid (VPA) on the transient sodium current, measured under whole-cell voltage-clamp conditions. We compared neurons from patients with temporal lobe sclerosis (S) with neurons from patients without hippocampal sclerosis (nS) and compared hippocampal CA1 neurons (CA) with neocortical neurons (NC). We could not detect differences in the voltage dependence and kinetics of sodium current activation and inactivation in any of the group comparisons. VPA shifted the voltage dependence of steady-state inactivation (expressed as V(h,i) in a Boltzmann fit) to more hyperpolarized levels. The shift induced by 2 mM VPA was -5.1 +/- 0.7 mV in CA-S (n = 13), -5.1 +/- 0.7 mV in CA-nS (n = 25), -4.3 +/- 0.5 mV in NC-S (n = 17) and -4.9 +/- 0.5 mV in NC-nS (n = 16) The relation between concentration and voltage shift had an EC50 of 1.4 +/- 0.2 mM VPA (n = 16) and a maximal shift of 9.6 +/- 0.9 mV. We conclude that pharmaco-resistance in these patients is not associated with a changed modulation of the sodium current by VPA. Results are discussed in the light of a reduced sodium current modulation by carbamazepine in CA1 neurons of patients with hippocampal sclerosis and of similar observations in the kindling model of epileptogenesis.

Reduction of presynaptic action potentials by PAD: model and experimental study.

A compartmental model of myelinated nerve fiber was used to show that primary afferent depolarization (PAD), as elicited by axo-axonic synapses, reduces the amplitude of propagating action potentials primarily by interfering with ionic current responsible for the spike regeneration. This reduction adds to the effect of the synaptic shunt, increases with the PAD amplitude, and occurs at significant distances from the synaptic zone. PAD transiently enhances the sodium current activation, which partly accounts for the PAD-induced fiber hyperexcitability, and enhances sodium inactivation on a slower time course, thus reducing the amplitude of action potentials. In vivo, intraaxonal recordings from the intraspinal portion of group I afferent fibers were carried out to verify that depolarizations reduced the amplitude of propagating action potentials as predicted by the model. This article suggests PAD might play a major role in presynaptic inhibition.

Functional analysis of a rat sodium channel carrying a mutation for insect knock-down resistance ( kdr) to pyrethroids

Pyrethroid insensitivity in resistant ( kdr) insects has been correlated with a leucine to phenylalanine replacement in the S6 transmembrane segment of domain II of the axonal sodium channel α( para)-subunit. An α-subunit of rat brain type II sodium channel containing this mutation has been expressed and its sensitivity to permethrin compared with that of the wild-type channel. The steady-state activation curve of the mutant was shifted 14 mV in the depolarizing direction. We propose that an equivalent shift of the sodium current activation curve in kdr insects could account for their low sensitivity to permethrin toxicity.

Effects of extracellular pH on voltage-gated Na+, K+ and Ca2+ currents in isolated rat CA1 neurons.

1. The effects of extracellular H+ (pHo) in the pathophysiological range (pH 6-8) on voltage-gated sodium, potassium, and calcium currents were examined in acutely dissociated rat hippocampal CA1 neurons using the whole-cell patch clamp technique. All experiments were conducted in Hepes-buffered solutions and were performed at room temperature (21-23 degrees C). 2. TTX-sensitive sodium currents, evoked by both step and ramp depolarization, were reversibly depressed by moderate acidosis and enhanced slightly by alkaline exposure. Changes in current amplitude were coincident with small reversible shifts (+/- 3 mV) in the voltage dependence of activation. In contrast, sodium current activation and decay kinetics as well as steady-state inactivation were unaffected by acidosis. 3. Outward potassium currents could be separated into a transient, rapidly inactivating current (IA) and a sustained, slowly inactivating component (IK). Steady-state activation of both currents was unaffected by an increase or decrease in pHo. Similarly, IK activation and IA decay kinetics remained stable during pHo exchange. In contrast, the steady-state inactivation (h infinity) of both potassium currents was reversibly shifted by approximately +10 mV during acid exposure, but remained unchanged during alkaline treatment. 4. Calcium currents were found to be predominantly of the high-voltage-activated (HVA) type, which could be carried by Ba2+ and inhibited completely by cadmium. Moderate acidosis (pH 6.9-6.0) reversibly depressed HVA Ca2+ current amplitude and caused a positive shift in its voltage dependence. For both of these parameters, alkaline treatment (pH 8.0) had the opposite effect. The depression of HVA Ca2+ currents by low pHo was unaffected by raising the internal Hepes concentration from 10 to 50 mM in the patch pipette. A Hill plot of the effect of pH on Ca2+ current amplitude revealed a pK value (defined as the mid-point of the titration curve) of 7.1 and a slope of 0.6. 5. The rate of Ca2+ current activation was unaffected by pHo at positive potentials, but below 0 mV the activation rate increased at low pH and decreased at high pH, becoming significant at -20 mV. At this membrane voltage, a second HVA current was revealed during acid exposure as the whole-cell HVA current was depressed. Ca2+ current decay was described by two time constants, both of which were significantly reduced at pH 6.4 and slightly enhanced at pH 8.0. Steady-state Ca2+ current inactivation reached 50% near -50 mV and was not affected at either pH extreme. 6. These results demonstrate that extracellular pH shifts within the pathophysiological range are capable of modulating both the conductance and gating properties of voltage-gated ion channels in hippocampal CA1 neurons. The effects we describe are consistent with the wellknown effects of pHo on neuronal excitability and strengthen the idea that endogenous pHo shifts may help regulate cell activity in situ.

Calcium transients which accompany the activation of sodium current in rat ventricular myocytes at 37 degrees C: a trigger role for reverse Na-Ca exchange activated by membrane potential?

We investigated the role of the fast sodium current (INa) in triggering Ca release from the sarcoplasmic reticulum (SR), using adult rat left ventricular myocytes, loaded with Fura-2 to measure intracellular Ca (Cai), which were whole-cell patch-clamped at 35-37 degrees C. Before each test pulse, a series of 400-ms conditioning pulses to +10 mV were applied to establish a constant level of SR Ca load. Pulses were applied every 15 s. A test pulse from -80 mV to -50 mV elicited a rapid INa and a phasic Cai transient. When the solution perfusing a myocyte was rapidly switched for 15 s before a test pulse to one containing the L-type Ca channel blocker nifedipine (20 microM), the test pulse still activated INa and a phasic Cai transient, the amplitude of which was not significantly different from control (P > 0.05; t-test). When a rapid switch to 20 microM nifedipine plus 30 microM tetrodotoxin (TTX) was made 15 s before a test pulse, both INa and the Cai transient were completely abolished (n = 6). When a switch was made to Na-free (Li) solution, which contained 20 microM nifedipine to block L-type Ca current, ICa,L, there was no significant difference in the Cai transient amplitude from that of control (P > 0.05; n = 6). Brief depolarising test pulses (-80 mV to +20 mV, 10 ms duration) to simulate membrane potential escape also elicited a Cai transient which attained 90.0% (+/-2.8%; n = 7) of the Cai transient activated by a conditioning pulse to +10 mV. The Cai transient with a brief pulse was not significantly affected by application of 20 microM nifedipine (P > 0.05), but adding TTX with nifedipine reduced the Cai transient amplitude to 76.9% (+/-6.8%; P < 0.02; n = 8). In four cells, the Cai transient remaining in the presence of nifedipine plus TTX was abolished by adding 5 mM Ni. These data are consistent with voltage escape during activation of INa leading to a trigger Ca entry via a mechanism other than L-type Ca channels or subsarcolemmal Na accumulation with reverse Na-Ca exchange. The block by Ni of the Cai transient suggests that a brief membrane potential escape might directly activate reverse mode Na-Ca exchange to trigger SR release, and this mechanism would seem to account largely for the Cai transient which accompanies INa in rat myocytes, under these experimental recording conditions.

ATPo but not cAMPi activates a chloride conductance in mouse ventricular myocytes

The aim was to test for the presence of cAMPi activated and ATPo activated chloride conductances (ICl(cAMP) and ICl(ATP)) in mouse ventricular myocytes. Membrane currents were measured in enzymatically dissociated ventricular myocytes using the whole cell configuration of the patch clamp technique. After inhibiting the activation of sodium, calcium, and potassium currents, increasing [cAMP]i with isoprenaline (1 microM), forskolin (10 microM), or isobutylmethylxanthine (500 microM) had no effect on residual membrane current but extracellular ATP (100 microM) elicited a time independent background conductance. The ATP-activated conductance was observed during voltage steps ranging from +40 to -110 mV from a holding potential of -30 mV within 1 min after adding ATP to the bath. The reversal potential for the ATP activated conductance varied with changes in the chloride gradient and was within several mV of the predicted Nernst potential for chloride. Partially substituting external chloride with aspartate attenuated the ATP activated currents and shifted the reversal potential to values close to those predicted for ECl, suggesting the chloride selective nature of the conductance. The poorly hydrolysable ATP analogue ATP gamma S (100 microM) also elicited the chloride conductance, suggesting purinoceptor linked activation rather than a mechanism dependent on hydrolysis of ATP by ectoATPases. The conductance may be activated following P2 and not P1 purinoceptor stimulation, since the P1 purinoceptor agonists adenosine (500 microM) and adenosine monophosphate (500 microM) had no effect in seven and four cells, respectively. Activation of ICl(ATP) is not likely to be dependent on increased [Ca2+]i since ATP activated the current in cells dialysed with 10 mM EGTA or 20 mM BAPTA. (1) cAMPi does not activate a chloride conductance in mouse ventricular myocytes; (2) ATPo does activate a chloride conductance, through stimulation of a P2 purinoceptor; and (3) Ca2+i and cAMPi are not involved in activation of ICl(ATP). Also, macroscopic ICl(ATP) from mouse ventricle and ICl(cAMP) from other species appear to be indistinguishable in terms of time independence and rectification properties that depend on the concentration and permeability of intracellular anions. ICl(ATP) could be of physiological and pathophysiological significance considering the ability of chloride channels to modulate cardiac electrical activity and the fact that ATP may be an important neuromediator in the heart.

Block of human voltage‐sensitive Na+ currents in differentiated SH‐SY5Y cells by lifarizine

1. The ability of lifarizine (RS-87476) to block human voltage-sensitive Na+ channel currents was studied by use of whole cell patch clamp recording from differentiated neuroblastoma cells (SH-SY5Y). 2. The Na+ conductance in differentiated SH-SY5Y cells (24.0 +/- 2.4 nS, n = 11) was half-maximally activated by 10 ms depolarizations to -37 +/- 2 mV and was half-maximally inactivated by predepolarizing pulses of 200 ms duration to -86 +/- 3 mV (n = 11). 3. At low stimulus frequencies (0.1 to 0.33 Hz) voltage-dependent sodium currents were completely blocked, in a concentration-dependent manner, by extracellular application of either tetrodotoxin (EC50 = 4 +/- 1 nM, n = 12) or by lifarizine (EC50 = 783 +/- 67 nM, n = 9). The onset of block by lifarizine (tau = 91 +/- 14 s at 10 microM) was considerably slower than that of tetrodotoxin (tau = 16 +/- 3 s at 100 nM). 4. Lifarizine (1 microM) reduced the peak sodium conductance in each cell (from 26.4 +/- 2.0 nS to 15.1 +/- 2.7 nS, n = 4) without changing the macroscopic kinetics of sodium current activation or inactivation (V1/2 = -35 1 mV and -87 +/- 4 mV respectively, n = 4). Similarly, lifarizine (1 microM) did not affect the reversal potential of the macroscopic sodium current (+14 +/- 5 mV in control and +16 +/- 2 mV in 1 microM lifarizine; n = 4) or reactivation time-constant (tau = 14.0 +/- 4.4 ms). 5. Block of the sodium channel open state by tetrodotoxin (30 nM) did not prevent the inhibition caused by a subsequent application of lifarizine (3 micro M). In contrast the depression caused by lifarizinewas readily reversible after pretreatment of cells with the local anaesthetic, lignocaine (1O mM).6. These data demonstrate that lifarizine is a use- and voltage-dependent antagonist of human voltage sensitive sodium currents. The slow kinetics and pharmacology of the block by lifarizine indicate that access of this drug to the channel is more restricted than that of tetrodotoxin and may involve an allosteric site or state of the channel that is also regulated by local anaesthetics.

Functional properties of rat and human neocortical voltage-sensitive sodium currents

1. The functional properties of sodium currents in acutely dissociated adult human, neonatal rat [postnatal day (P) 3 and P10], and mature rat (P21-23) neocortical pyramidal neurons were studied using whole-cell patch-clamp techniques. 2. The voltage dependence of activation and steady-state inactivation of neonatal rat sodium currents was shifted in the positive direction when compared with mature rat sodium currents. In contrast, no difference was detected between the voltage dependence of activation and steady-state inactivation of mature rat and adult human sodium currents. 3. The fast inactivation of rat (neonatal and mature) and human neocortical sodium currents were best fit with three components; a fast decay component, a slow decay component, and a persistent component. The magnitude of the persistent current in neocortical neurons averaged 1-3% of the peak current. Inactivation was faster for sodium currents in neonatal rat neocortical neurons than in mature neurons. No difference was detected in the kinetics of inactivation between mature rat and adult human sodium currents. 4. Saxitoxin (STX) inhibited neuronal sodium currents at nanomolar concentrations in neonatal and mature rat and adult human neocortical neurons. STX-insensitive channels were not detected. 5. STX affinity was also assayed using 3H-STX. A single high-affinity binding site was found in neonatal rat, mature rat, and adult human neocortical tissue. A developmental increase in STX binding site density in the rat neocortex was tightly correlated with the increase in the sodium current density (normalized to cell capacitance). Human neocortical tissue and mature rat neocortical tissue did not differ in STX binding site density or sodium current density. 6. From these electrophysiological and autoradiographic studies we conclude that 1) the increase in the normalized sodium current density and STX binding density with age postnatally reflects an increase in binding sites of sodium channels functionally expressed on neuronal membranes, 2) the functional differences in channel behavior with maturation can explain the higher threshold for excitation in neonatal neocortical neurons and the increase in accommodation or adaptation in firing in the mature neuron, and 3) mature rat neocortical neurons represent a valid model for the study of adult human pyramidal neocortical neurons in terms of Na+ channel expression and function.

Serotonin activation of a cyclic AMP-dependent sodium current in an identified neuron from Helisoma trivolvis

The mechanisms by which neurotransmitters regulate neurite extension and growth cone motility have been extensively studied using identified Helisoma neurons regenerating in cell culture. Specific neurons, such as buccal neuron B19, display a complex response to the addition of 5-HT involving the generation of action potentials, influx of extracellular calcium, and cessation of neurite extension and growth cone motility. While several studies have addressed the role of calcium in this cascade, little is known about the mechanism underlying the 5-HT-induced excitation of neuron B19. Therefore, we have begun to characterize the ion currents, receptors, and second messengers involved in the 5-HT-dependent depolarization of B19. Exposure of B19 to 5-HT resulted in the activation of a maintained inward current. Ion substitution experiments revealed that this current was carried mainly by sodium ions. The use of 8-bromo-cAMP, forskolin, or the phosphodiesterase inhibitor isobutyl methylxanthine (IBMX) to increase intracellular cAMP levels all resulted in inward current activation in the absence of 5-HT. Moreover, preloading the neuron with 8-bromo-cAMP was sufficient to prevent further current activation by 5-HT. In addition, the IBMX-activated current was greatly enhanced when induced in the presence of 5-HT. Protein kinase inhibitors failed to prevent 5-HT activation of sodium current, suggesting that cAMP may directly activate the current, independent of phosphorylation. Pharmacological experiments showed the B19 5-HT receptor has an EC50 of approximately 10(-7) M and can be activated by various indole analogs of 5-HT. Furthermore, methysergide displayed partial agonist activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential sensitivity of tetrodotoxin-sensitive and tetrodotoxin-resistant sodium channels to the insecticide allethrin in rat dorsal root ganglion neurons

The pyrethroid insecticides are known to modify neuronal sodium channels to cause a prolongation of whole cell current. The sodium channels expressed in the dorsal root ganglion neurons of the rat are of two types, one highly sensitive to tetrodotoxin and the other highly resistant to tetrodotoxin. The pyrethroid allethrin exerted profound effects on tetrodotoxin-resistant sodium channels while causing minimal effects on tetrodotoxin-sensitive sodium channels. Currents derived from tetrodotoxin-resistant sodium channels were greatly prolonged during a step depolarization; the tail currents upon repolarization were also augmented and prolonged. In the tetrodotoxin-sensitive sodium channel currents, these changes caused by allethrin were much smaller or negligible. The activation and inactivation voltages of tetrodotoxin-resistant peak sodium currents were not significantly altered by allethrin. The differential action of allethrin on the two types of sodium channels would be important not only in identifying the target molecular structure but also in interpreting the symptoms of poisoning in mammals.

Effect of Membrane Incorporation of 1-Palmitoylcarnitine on Surface Charge of Human Erythrocytes

1-Palmitoylcarnitine (1-PC) reduced the electrophoretic mobility of human erythrocytes bathed in low ionic strength solution. Unlike divalent cations which appear to reduce electrophoretic mobility by screening surface negative charge, cationic 1-PC does so by being incorporated into the plasma membrane. Incorporation of 1-PG was assessed directly by measurement of the partition coefficients which are 1.18 × 10 5 and 1.38 × 10 5 in sarcoplasmic reticulum vesicles and erythrocyte ghosts, respectively. The hydrophobic nature of the amphiphile interaction with erythrocyte membrane was also evident as an antihemolytic effect at 1-PC concentrations that also reduced electrophoretic mobility. Moreover, the potency and efficacy of acylcarnitines to reduce electrophoretic mobility increased as the acyl chain length increased. 1-PC also reduced the electrophoretic mobility of guinea-pig ventricular myocytes in low ionic strength solution. Estimation of the zeta potential yielded positive shifts of 4mV in myocytes and 5mV on crythrocytes at 1 μM 1-PC. These voltage shifts are essentially the same as those reported for activation and inactivation of sodium and calcium currents in myocytes. Therefore, the surface charge effect of 1-PC depends upon membrane incorporation and underlies the electrophysiological actions of the amphiphile including arrhythmias.

Alteration of sodium currents by new peptide toxins from the venom of a molluscivorous Conus snail.

TxIA and TxIB, peptides with 27-amino acid residues recently isolated from the molluscivorous marine snail Conus textile neovicarius, exhibit strong paralytic activity in molluscs, with no paralytic effects on athropods and vertebrates. At concentrations of 0.25-0.5 microM the toxins cause spontaneous repetitive firing and dramatic broadening of the action potential of cultured Aplysia neurons. The action potential duration partially recovers within 30 min in the presence of the toxins. Under these conditions a second toxin application does not change the spike duration. TxI-induced spike broadening occurs when potassium and calcium conductances are blocked. Voltage-clamp experiments revealed that the toxins alter the kinetics of the sodium current either by slowing down the rate of sodium current inactivation or by recruiting silent sodium channels with slower activation and inactivation kinetics. The toxins shift the voltage-dependent steady-state Na+ current inactivation curve to more positive values by 6 mV. These changes are not associated with alteration in the rate of sodium current activation, in the peak sodium current, or the sodium current reversal potential. TxI apparently represents a new class of conotoxins with an unusual phylogenic specificity and may therefore be useful as a probe for the study of molluscan neuronal sodium channels.

Sodium currents during differentiation in a human neuroblastoma cell line.

The electrophysiological properties of a human neuroblastoma cell line, LA-N-5, were studied with the whole-cell configuration of the patch clamp technique before and after the induction of differentiation by retinoic acid, a vitamin A metabolite. Action potentials could be elicited from current clamped cells before the induction of differentiation, suggesting that some neuroblasts of the developing sympathetic nervous system are excitable. The action potential upstroke was carried by a sodium conductance, which was composed of two types of sodium currents, described by their sensitivity to tetrodotoxin (TTX) as TTX sensitive and TTX resistant. TTX-sensitive and TTX-resistant sodium currents were blocked by nanomolar and micromolar concentrations of TTX, respectively. The voltage sensitivity of activation and inactivation of TTX-resistant sodium current is shifted -10 to -30 mV relative to TTX-sensitive sodium current, suggesting that TTX-resistant sodium current could play a role in the initiation of action potentials. TTX-sensitive current comprised greater than 80% of the total sodium current in undifferentiated LA-N-5 cells. The surface density of total sodium current increased from 24.9 to 57.8 microA/microF after cells were induced to differentiate. The increase in total sodium current density was significant (P less than 0.05). The surface density of TTX-resistant sodium current did not change significantly during differentiation, from which we conclude that an increase in TTX-sensitive sodium current underlies the increase in total current.

Sodium ionic and gating currents in mammalian cells

Ionic and gating currents from voltage-gated sodium channels were recorded in mouse neuroblastoma cells using the path-clamp technique. Displacement currents were measured from whole-cell recordings. The gating charge displaced during step depolarizations increased with the applied membrane potential and reached saturating levels above 20 mV. Prolonged large depolarizations produced partial immobilization of the gating charge, and only about one third of the displaced charge was quickly reversed upon return to negative holding potentials. The activation and inactivation properties of macroscopic sodium currents were characterized by voltage-clamp analysis of large outside-out patches and the single-channel conductance was estimated from non-stationary noise analysis. The general properties of the sodium channels in mouse neuroblastoma cells are very similar to those previously reported for various preparations of invertebrate and vertebrate nerve cells.

Block of sodium channels by psychotropic drugs in single guinea‐pig cardiac myocytes

1. Effects of imipramine and haloperidol on voltage-gated sodium channels were investigated in guinea-pig isolated ventricular myocytes by the whole-cell patch clamp technique. Some additional experiments were also performed with chlorpromazine for the purpose of comparison. 2. All test drugs in micromolar concentrations suppressed the amplitude of peak sodium current associated with step depolarization from a holding potential of -140 mV in a reversible manner. The order of potency was chlorpromazine greater than imipramine greater than haloperidol. 3. Dose-response curves obtained with a holding potential of -140 mV were best fitted by 2:1 stoichiometry in all three drugs and were shifted in the direction of lower concentrations when a holding potential of -90 mV was used. 4. The drug-induced block was not associated with any change in the time courses of sodium current activation and inactivation. 5. Steady-state sodium channel inactivation curve was shifted in the direction of more negative potentials by the drugs. 6. All three drugs also produced marked use-dependent block as demonstrated by a cumulative increase in the block during a train of depolarizing pulses. 7. The use dependence was due to a higher affinity of the drugs for the inactivated state of sodium channels than the resting state and to a very slow repriming of the drug-bound sodium channels from inactivation. 8. The steady-state and use-dependent block of voltage-gated sodium channels by psychotropic drugs may contribute to their cardiotoxic and perhaps antiarrhythmic effect.

Etiology of the supernormal period

In many excitable cells, there is a time after the action potential when the threshold for eliciting a second action potential is lower than it is in the steady state. The Hodgkin and Huxley (1952) equations predict such a supernormal period. Using their model, it is shown that the supernormal period results from the slow kinetics of the potassium current and does not depend on sodium current activation or inactivation or on the after-depolarization.

Kinetics of intramembrane charge movement and conductance activation of batrachotoxin-modified sodium channels in frog node of Ranvier.

Sodium current and intramembrane gating charge movement (Q) were monitored in voltage-clamped frog node of Ranvier after modification of all sodium channels by batrachotoxin (BTX). Sodium current activation followed a single-exponential time course, provided a delay was interposed between the onset of the step ON depolarization and that of the current change. The delay decreased with increased ON depolarization and, for a constant ON depolarization, increased with prehyperpolarization. ON charge movement followed a single-exponential time course with time constants tau Q,ON slightly larger than tau Na, ON. For pulses between -70 and -50 mV, tau Q,ON/tau Na,ON = 1.14 +/- 0.08. The OFF charge movement and OFF sodium current tails after a depolarizing pulse followed single-exponential time courses, with tau Q, OFF larger than tau Na, OFF. tau Q,OFF/tau Na,OFF increased with OFF voltage from 1 near -100 mV to 2 near -160 mV. At a set OFF potential (-120 mV), both tau Q,OFF and tau Na,OFF increased with ON pulse duration. The delay in INa activation and the effect of ON pulse duration on tau Q,OFF and tau Na,OFF are inconsistent with a simple two-state, single-transition model for the gating of batrachotoxin-modified sodium channels.

Single cells and rapid inward sodium current.

This chapter presents a brief review of measurements of rapid inward sodium current in single cardiac cells. It is shown that the simplified morphology of the individual cells, with a lack of restricted extracellular space, has been exploited to provide improved spatial and temporal voltage control, resulting in the first recordings of both the activation and inactivation phases of rapid inward sodium current. It is to be expected that future research will produce much interesting data on this component of membrane current, which will have direct relevance to many processes concerned with cardiac function at the cellular level.