Peak Sodium Current Research Articles

Blockade of sodium channels by bistramide A in voltage-clamped frog skeletal muscle fibres

The effect of Bistramide A, a toxin isolated from Bistratum lissoclinum Sluiter (Urochordata), on the peak sodium current ( I Na) of frog skeletal muscle fibres was studied with the double sucrose gap voltage clamp technique. External or internal application of Bistramide A inhibited I Na without alteration of the kinetic parameters of the current nor of the apparent reversal potential for Na. The steady-state activation curve of I Na was unchanged while the steady-state inactivation curve of I Na was shifted towards more negative membrane potentials. Dose-response curves indicated an apparent dissociation constant for Bistramide A of 3.3 μM and a Hill coefficient of 1.2 which suggested a one to one relation between the toxin and Na channel. The inhibition of I Na occurred at rest, and was more important at more positive holding potentials. Bistramide A exhibited only a weak frequency-dependent effect. The toxin did not interact with the use-dependent effect of lidocaine. It mainly blocked Na channels at more depolarized holding potentials. The toxin blocked Na channels when it was internally applyed and when the inactivation gating system has been previously destroyed by internal diffusion of iodate. The data suggest that Bistramide A inhibited the Na channel both at rest and in the inactivated state and occupied a site which was not located on the inactivation gate.

Voltage-sensitive and solvent-sensitive processes in ion channel gating. Kinetic effects of hyperosmolar media on activation and deactivation of sodium channels

Kinetic effects of osmotic stress on sodium ionic and gating currents have been studied in crayfish giant axons after removal of fast inactivation with chloramine-T. Internal perfusion with media made hyperosmolar by addition of formamide or sucrose, reduces peak sodium current (before and after removal of fast inactivation with chloramine-T), increases the half-time for activation, but has no effect on tail current deactivation rate(s). Kinetics of ON and OFF gating currents are not affected by osmotic stress. These results confirm (and extend to sodium channels) the separation of channel gating mechanisms into voltage-sensitive and solvent-sensitive processes recently proposed by Zimmerberg J., F. Bezanilla, and V. A. Parsegian. (1990. Biophys. J. 57:1049-1064) for potassium delayed rectifier channels. Additionally, the kinetic effects produced by hyperosmolar media seem qualitatively similar to the kinetic effects of heavy water substitution in crayfish axons (Alicata, D. A., M. D. Rayner, and J. G. Starkus. 1990. Biophys. J. 57:745-758). However, our observations are incompatible with models in which voltage-sensitive and solvent-sensitive gating processes are presumed to be either (a) strictly sequential or, (b) parallel and independent. We introduce a variant of the parallel model which includes explicit coupling between voltage-sensitive and solvent-sensitive processes. Simulations of this model, in which the total coupling energy is as small as 1/10th of kT, demonstrate the characteristic kinetic changes noted in our data.

A Phosphorylation Site in the Na + Channel Required for Modulation by Protein Kinase C

Voltage-gated sodium channels are responsible for generation of action potentials in excitable cells. Activation of protein kinase C slows inactivation of sodium channels and reduces peak sodium currents. Phosphorylation of a single residue, serine 1506, that is located in the conserved intracellular loop between domains III and IV and is involved in inactivation of the sodium channel, is required for both modulatory effects. Mutant sodium channels lacking this phosphorylation site have normal functional properties in unstimulated cells but do not respond to activation of protein kinase C. Phosphorylation of this conserved site in sodium channel alpha subunits may regulate electrical activity in a wide range of excitable cells.

Functional modulation of brain sodium channels by protein kinase C phosphorylation.

Voltage-gated sodium channels, which are responsible for the generation of action potentials in the brain, are phosphorylated by protein kinase C (PKC) in purified form. Activation of PKC decreases peak sodium current up to 80 percent and slows its inactivation for sodium channels in rat brain neurons and for rat brain type IIA sodium channel alpha subunits heterologously expressed in Chinese hamster ovary cells. These effects are specific for PKC because they can be blocked by specific peptide inhibitors of PKC and can be reproduced by direct application of PKC to the cytoplasmic surface of sodium channels in excised inside-out membrane patches. Modulation of brain sodium channels by PKC is likely to have important effects on signal transduction and synaptic transmission in the central nervous system.

Pb 2+ blocks calcium currents of cultured dorsal root ganglion cells

The divalent cation lead (Pb 2+) blocks sustained and transient voltage sensitive calcium channel currents of cultured rat dorsal root ganglion cells. The IC 50 for inhibition of the total peak current evoked by a step depolarization from −80 to 0 mV was 0.6 μM, compared to an IC 50 of 2.2 μM for Cd 2+. The current activated by a depolarization from −40 to 0 mV was inhibited by 50% by 1.0 μM Pb 2+. Low threshold currents activated by a step from −100 to −30 mV were blocked by Pb 2+ at higher concentrations (IC 50=6 μM). The block progressed in the absence of channel activation and showed little voltage dependence. Peak sodium current was reduced by 6.6% at 1 μM Pb 2+ while at 20 μM the peak current was reduced by 40% with marked slowing of the time course of activation. The potassium rectifier current was reduced by 4.1% at 1 μM Pb 2+. Thus, Pb 2+ selectively blocks calcium currents at concentrations in the range of those causing toxicity in man.

Lead and zinc block a voltage-activated calcium channel of Aplysia neurons.

1. The effects of Pb2+ and Zn2+ on the peak of the voltage-activated calcium current of Aplysia neurons were examined. Calcium currents were reversibly blocked by Pb2+ at concentrations that did not significantly affect potassium and sodium currents and by Zn2+ at concentrations associated with a delay and reduction of peak sodium and potassium currents. 2. The block by both was concentration dependent, and percentage blockade was reduced in elevated Ca2+. The threshold Pb2+ concentration for blockade in 20 mM Ca artificial sea water (ASW) was approximately 1 microM, whereas for Zn2+ it was 2 mM. The Hill coefficient for Pb2+ action was near 1.0 under all conditions, whereas for Zn2+ it was 1.4-1.6. 3. With addition of Pb2+, the voltage at which peak calcium current was generated shifted to hyperpolarized voltages, an effect similar to that caused by reduction of Ca2+ concentration in the absence of Pb2+. Zn2+ shifted the voltage at which peak current was generated in a depolarizing direction. 4. Pb2+ did not significantly change inactivation but shifted the voltage dependence of activation to hyperpolarized voltages in a dose-dependent manner. Zn2+ shifted both activation and inactivation in a depolarizing direction in a dose-dependent fashion. 5. The blockade of calcium currents by Pb2+ but not Zn2+ was highly voltage dependent and increased with depolarization. 6. Our results suggest that Pb2+ is a specific, potent, competitive, and reversible blocker of calcium currents. These observations are consistent with a competition by Pb2+ with Ca2+ at a binding site within the calcium channel. In contrast, the blockade of calcium currents by Zn2+ is probably through actions at fixed charge sites external to the channel.

Kinetic analysis of phasic inhibition of neuronal sodium currents by lidocaine and bupivacaine

Phasic ("use-dependent") inhibition of sodium currents by the tertiary amine local anesthetics, lidocaine and bupivacaine, was observed in voltage-clamped node of Ranvier of the toad, Bufo marinus. Local anesthetics were assumed to inhibit sodium channels through occupation of a binding site with 1:1 stoichiometry. A three-parameter empirical model for state-dependent anesthetic binding to the Na channel is presented: this model includes two discrete parameters that represent the time integrals of binding and unbinding reactions during a depolarizing pulse, and one continuous parameter that represents the rate of unbinding of drug between pulses. The change in magnitude of peak sodium current during a train of depolarizing pulses to 0 mV was used as an assay of the extent of anesthetic binding at discrete intervals; estimates of model parameters were made by applying a nonlinear least-squares algorithm to the inhibition of currents obtained at two or more depolarizing pulse rates. Increasing the concentration of drug increased the rate of binding but had little or no effect on unbinding, as expected for a simple bimolecular reaction. The dependence of the model parameters on pulse duration was assessed for both drugs: as the duration of depolarizing pulses was increased, the fraction of channels binding drug during each pulse became significantly larger, whereas the fraction of occupied channels unbinding drug remained relatively constant. The rate of recovery from block between pulses was unaffected by pulse duration or magnitude. The separate contributions of open (O) and inactivated (I) channel binding of drug to the net increase in block per pulse were assessed at 0 mV: for lidocaine, the forward binding rate ko was 1.3 x 10(5) M-1 s-1, kl was 2.4 x 10(4) M-1 s-1; for bupivacaine, ko was 2.5 x 10(5) M-1 s-1, kl was 4.4 x 10(4) M-1 s-1. These binding rates were similar to those derived from time-dependent block of maintained Na currents in nodes where inactivation was incomplete due to treatment with chloramine-T. The dependence of model parameters on the potential between pulses (holding potential) was examined. All three parameters were found to be nearly independent of holding potential from -70 to -100 mV. These results are discussed with respect to established models of dynamic local anesthetic-Na channel interactions.

Sodium currents in Schwann cells from myelinated and non‐myelinated nerves of neonatal and adult rabbits.

1. Patch-clamp methods were used to study sodium channels in Schwann cells obtained from four different tissue sources. Primary cultures of Schwann cells were prepared from the sciatic nerve and from the vagus nerve of neonatal and of adult rabbits. In the adult, the sciatic is predominantly myelinated whereas the vagus is predominantly non-myelinated. Whole-cell currents, and single-channel currents in outside-out membrane patches, were analysed. 2. No substantial differences were noted in the passive electrical properties (input resistance, cell capacitance, resting membrane potential) of the four groups of cells. Similarly, no substantial differences were found in the average properties of sodium currents (maximum current, maximum conductance, time-to-peak current, current-voltage relation, h infinity relation) recorded from each type of cell in cultures less than 8 days old. At 10-17 days a fall in the size of the sodium currents recorded from cells in the vagal cultures was found. 3. Exposure of the cells to proteolytic enzymes or collagenase, under conditions similar to those used when the cells were put in culture initially, substantially reduced the size of the peak sodium currents recorded from the cells 24 h later. 4. The results of experiments on Schwann cells with retracted processes indicated that sodium channels are present in the processes extending from each pole of the cell soma and that the plasmalemmal density of these channels in the processes is about the same as it is at the soma. 5. Recordings from outside-out patches revealed no apparent differences in the properties of single-channel sodium currents in patches from cells obtained from the four different sources. The single-channel conductance was about 20 pS for each of the four groups. Ensemble currents from single-channel records were similar in time course to those of whole-cell currents. 6. Saxitoxin reduced the maximum sodium conductance in Schwann cells and bound to the cells with equally high affinity. The equilibrium dissociation constant was about 2 nM at 20-22 degrees C. 7. It is argued that the expression of sodium channels in myelinating Schwann cells does not differ substantially from that of non-myelinating Schwann cells.

Block of deltamethrin-modified sodium current in cultured mouse neuroblastoma cells: local anesthetics as potential antidotes

Effects of local anesthetics and anticonvulsants on the pyrethroid-modified sodium current in cultured mouse neuroblastoma cells have been investigated using the suction pipette voltage clamp technique. In the presence of 10 μM of the pyrethroid deltamethrin the sodium current consists of an enhanced peak current during membrane depolarization and a slowly decaying, deltamethrin-induced tail current remaining after repolarization. At the onset of block the local anesthetics tetracaine, lidocaine and QX 314 reduced the deltamethrin-induced tail current more effectively than the peak current. Lidocaine, but not phenytoin, caused a time-dependent block of tail currents evoked by membrane depolarizations lasting 10–1000 ms. Both lidocaine- and phenytoin-induced blocks were independent of the membrane potential during the tail current. The anticonvulsants phenytoin, phenobarbital and valproate blocked the tail and the peak sodium current to the same extent, but diazepam, mephenesin and urethane blocked the peak current more effectively. Vitamin E, which suppresses pyrethroid-induced paresthesia of the skin, had no effect on the voltage-dependent sodium current. It is concluded that indirect effects of anticonvulsants on pyrethroid-induced toxic symptoms predominate, whereas local anesthetics preferentially block the pyrethroid-induced tail current. Therefore, local anesthetics are potentially useful pyrethroid antidotes.

Voltage-gated sodium channels expressed in the human cerebellar medulloblastoma cell line TE671

A characterization of the properties of voltage-gated sodium channels expressed in the human cerebellar medulloblastoma cell line TE671 is presented. Membrane currents were recorded under voltage clamp conditions using the patch clamp technique in both the whole-cell and the excised-patch configurations. Macroscopic sodium currents display a typical transient time course with a sigmoidal rise to a peak followed by an exponential decay. The rates of early activation and subsequent inactivation accelerate and approach a maximum in response to test potentials, V, of greater depolarization. The magnitude of peak sodium current increased from negligible values below V = −50 mV and reached a maximum at V = −3.6 mV ± 2.7 mV (mean ± S.E.M., n = 12). Sodium currents reversed at V = + 70 mV, near the predicted Nernst equilibrium potential for a Na + selective channel. The peak sodium conductance, g peak increased with depolarizing voltages to a maximum at V = ∼ 0 mV, exhibiting half-activation voltage at V ≽ −36.8 mV and an e-fold change in g peak/9.5 mV. The Hodgkin-Huxley inactivation parameter h ∞ indicates that at V = −73.6 mV half of the sodium currents were inactivated. Single channel current recordings demonstrated the occurrence of discrete events: the latency for first opening was shorter as the depolarizing pulse became more positive. The single-channel current amplitude was ohmic with a slope conductance, γ = 17.13 pS ± 0.66 pS. Sodium channel currents were reversibly blocked by tetrodotoxin (TTX). Thus, the TE671 human neurons in culture express a sodium channel which exhibits the voltage-gating properties, cation selectivity and neurotoxin sensitivity characteristic of ‘classical’ sodium channels of excitable cells.

Effect of phoratoxin B, a toxin isolated from mistletoe, on frog skeletal muscle fibres

M.-P. Sauviat. Effect of phoratoxin B, a toxin isolated from mistletoe, on frog skeletal muscle fibres. Toxicon 28, 83–89, 1990.—The effect of phoratoxin B on transmembrane potentials and currents of frog skeletal muscle was studied using intracellular microelectrode recording and double sucrose gap voltage clamp techniques. Phoratoxin B irreversibly depolarized the membrane. The depolarization was insensitive to tetrodotoxin, tetraethylammonium, 4-aminopyridine, cesium, cadmium and D-600. In voltage clamp experiments, phoratoxin B induced an inward resting current ( I rest ) which did not inactivate. I rest was blocked by increasing the external calcium concentration in the Ringer solution; it was not blocked when Sr 2+ replaced Ca 2+. Analysis of the peak sodium current indicated that while both Ca 2+ and Sr 2+ screen membrane surface charges, Ca 2+ bound and reversed the phoratoxin B induced I rest whereas Sr 2+ did not. The inward I rest induced by phoratoxin B developed in the presence of external chloride and remained unchanged in the presence of bicarbonate. The data suggest that the depolarizing action of the toxin might be attributed to an increase in the non-selective leak current and that it might act as a detergent.

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.

Recovery from charge immobilization in sodium channels of the frog node of Ranvier

The gating current off-response of the frog node of Ranvier shows a fast and a slow phase, reflecting presumably charges moving back into the resting position and charges returning from immobilization. The paper describes measurements of the time constant of the slow component, tau off2, at different potentials at 20 or 17 degrees C. The time constant tau off2 decreased markedly when the potential to which the fibre was repolarized at the end of the test pulses was decreased from -100 to -145 mV. tau off2 was compared with tau re gat and tau re Na, the time constants of the recovery of gating current and peak sodium current from the effect of a depolarizing conditioning pulse. The three time constants were equal at -145 mV, but somewhat different at -100 to -130 mV, the sequence being tau re gat greater than tau off2 greater than tau re Na. The inequality tau re gat greater than tau off2 was small and statistically not significant. It does not seem to be due to contamination of the charge movement with ionic currents because a) the Qoff/Qon ratio was near 1.0 at -100 to -120 mV, b) partial replacement of internal CsCl by KCl did not significantly alter tau off2 or Qoff/Qon, c) tau off2 was independent of pulse size. The small inequality tau re gat greater than tau off2 suggests that charges which have returned from immobilization are not immediately available for displacement. The inequality tau re gat greater than tau re Na was larger and statistically significant at -100 and -110 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Asymmetric charge movement and calcium currents in ventricular myocytes of neonatal rat.

1. Calcium and sodium currents and non-linear capacitive currents were recorded from isolated ventricular cells from neonatal rats, using the whole-cell patch-clamp technique, usually with a holding potential of -100 mV. 2. When recording with internal and external solutions designed to suppress virtually all ionic currents except the calcium current, careful subtraction of all linear capacitive and ionic currents revealed that depolarizations elicited a small transient outward current which preceded the inward calcium current. This outward current was discernible just below the threshold potential for the calcium current and increased with larger depolarizations to a maximum for potentials of about +30 mV and above. 3. Elimination of the calcium current revealed that at each potential the transient outward current was accompanied by a roughly equal transient inward current upon repolarization. The properties of these currents indicate that they are non-linear capacitive currents. Best-fit Boltzmann curves of the 'on' charge (integral of the transient outward current) gave values for qmax, V and k of 3.9 nC/microF, -29.3 mV and 15.5 mV with internal Cs+. The maximum 'on' charge is similar to that found with calcium currents (4.3 nC/microF). Similar values were obtained with internal TEA+. 4. Boltzmann fits of conductance vs. voltage for the calcium channel gave mean values of -15.5 and 13.3 mV for V and k (with internal Cs+); the corresponding values for the sodium channel were -49.9 and 5.4 mV. 5. Pre-pulses (20 ms) to -60 mV inactivated 77% of the peak sodium current, but only inactivated about 10% of the peak calcium current and reduced the maximum 'on' charge (moved at potentials positive to -60 mV) by 19%. 6. With a holding potential of -100 mV, 10 microM-nifedipine blocked 89% of the calcium current, but had little effect on the amount of 'on' charge. The 'off' charge appeared to be slower in the presence of nifedipine. 7. These results and consideration of the number of calcium channels and high-affinity binding sites for dihydropyridines (DHP), suggest that a large part of the charge movement may be related to DHP binding sites and involved with gating calcium channels. Comparison with skeletal muscle suggests similarities in the mechanisms involved in excitation-contraction coupling.

Nonlinear relation between Vmax and INa in canine cardiac Purkinje cells.

We studied the relation of the maximal upstroke velocity (Vmax) of action potentials to the peak sodium current (INa) under voltage clamp in single, internally perfused, canine cardiac Purkinje cells under conditions that ensured membrane action potentials due only to INa. Three different methods of altering sodium channel availability were investigated: voltage-dependent inactivation, tetrodotoxin (TTX) block, and use-dependent block by quinidine. Under all three conditions, the relation of Vmax to INa was nonlinear, and no relation was found that would allow prediction of INa results from Vmax measurements. With voltage-dependent inactivation or TTX block, sodium channel availability measured by Vmax was reduced less than availability measured by peak INa, so that Vmax overestimated sodium channel availability. This overestimation of sodium channel availability by Vmax could be attributed to greater sodium channel mobilization during the slowed action potential upstrokes. The overestimation varied with experimental temperature as a consequence of changes in sodium channel kinetics. Vmax also overestimated sodium channel availability during TTX exposure so that the Kd for TTX block was 4.5 micron from Vmax measurements but only 1.6 microM from INa measurements. Use-dependent block of INa by quinidine had a striking voltage-dependent component under voltage clamp that could not be appreciated from action potentials. Consequently, block could be underestimated or overestimated by Vmax measurements. We conclude that Vmax measurements represent a convenient index for INa, but Vmax is not a reliable method for quantitative studies of sodium channel behavior.

Quantitative structure-activity studies of pyrethroids: Physicochemical properties and the rate of development of the residual and tail sodium currents in crayfish giant axon

The effects of 28 pyrethroids including variously substituted benzyl chrysanthemates and pyrethrates and their related compounds on sodium currents were examined in internally perfused giant axons of crayfish under voltage clamp conditions. The compounds increased the steady-state sodium current and induced a residual current during step depolarization of the axonal membrane. They also induced a tail current upon step repolarization. Each of the ratios of the residual and tail sodium currents to the transient peak sodium current at each measurement increased with time after the start of the internal application of the compounds until a steady level was reached. The rate constants of the development of the two kinds of sodium currents, residual and tail, in terms of the ratios were estimated by use of first-order kinetics and had a linear relationship for this series of compounds. Variations in each of these rate constants among compounds were parabolically related with the hydrophobicity of the molecule, giving a common optimum log P ( P, the partition coefficient in the 1-octanol/water system) at about 4.6. The rate of development of the residual and tail currents is probably decided by the penetration rate of the compounds to the target site(s).

Sodium and potassium currents in acutely demyelinated internodes of rabbit sciatic nerves.

1. Voltage-clamp experiments were performed on single internodes isolated from rabbit sciatic nerve fibres acutely demyelinated with the detergent lysolecithin or a synthetic analogue, lysophosphatidyl choline palmitoyl. 2. The extent of demyelination was monitored by a gradual increase in the internodal leak conductance and capacitance. Voltage- and time-dependent inward and outward currents, absent during the early phase (30-40 min) of detergent treatment, appeared during the final phase (40-60 min) of treatment. 3. The internodal ionic currents elicited by depolarizations consisted of three components pharmacologically identified as (a) a transient sodium current which was inhibited by tetrodotoxin, (b) a delayed rectifying potassium current which was inhibited by internal caesium and (c) a time-dependent current that was abolished by replacement of external chloride with ascorbate. 4. The current-voltage relations and h infinity curves for the internodal sodium current were similar in shapes to those of the nodal sodium current. 5. The amplitudes of the three internodal currents increased with the increase in the measured internodal capacity during the final phase of demyelination. 6. At high degrees of demyelination a peak sodium current of about 90 nA could be observed in an internodal segment of 100 micron length. 7. Interestingly, the membrane capacity measured at the time of such a large sodium current was about 10 times larger than could be accounted for by the axonal membrane in the recording pool alone. A suggestion is made that this represents lysolecithin-induced membrane fusion between the Schwann cell and the internodal axon.

Sodium currents in axon‐associated Schwann cells from adult rabbits.

1. Patch-clamp and electron-microscopic studies were carried out on individual axon-Schwann-cell complexes 2-6 h after they were isolated from the sciatic nerves of rabbits 5, 10 and 20 weeks old. 2. Under Hoffman modulation contrast optics Schwann cells associated with both myelinated and non-myelinated axons could be seen. Frequently, fine cable-like structures about 1 micron in diameter, which are presumably axons, could be seen in isolation from a Schwann cell. 3. Cross-sectional electron-microscopic studies directly demonstrated the presence of axons engulfed by Schwann cells. For Schwann cells associated with non-myelinated axons, multiple fine axons (approximately 1 micron) could be seen enclosed by one or few turns of spiralling tongues of Schwann cells. Schwann cells associated with a single large myelinated axon showed characteristic compact myelin wrappings. No membrane fusion between Schwann cells and the axons could be detected. 4. Giga-seals could readily be formed when a patch pipette was pressed against the body region of a Schwann cell associated with either non-myelinated or myelinated axons. In contrast, giga-seals were only infrequently obtained on fine cable-like structures (1 micron) visually identified to be separated from the Schwann cell body. 5. Whole-cell recordings made from the body region of a Schwann cell revealed a TTX-sensitive fast inward current. Intriguingly, the expression of this current appeared to be dependent on the type of associated axon; this current was detectable in virtually all recordings made at the body region of Schwann cells associated with small non-myelinated axons, but not from those associated with large myelinated axons. 6. The inward current was like a neuronal sodium current; it had voltage-gated kinetics similar to the Hodgkin-Huxley sodium current, and exhibited a reversal potential close to the expected Nernstian potential for sodium ions. 7. From the observed size of the whole-cell membrane capacity and the electron-microscopic observations that the surface area of the Schwann cell at the body region was much larger than that of a 1 micron non-myelinated axon, it was argued that the whole-cell recordings were from Schwann cells rather than from single axons. Furthermore, the peak sodium current density was similar to that of Schwann cells cultured from new-born rabbits in which axons were presumed to be absent. 8. The results suggested that Schwann cells normally associated with non-myelinated axons in the rabbit sciatic nerves maintain an active synthesis of neuronal-like sodium channels throughout normal development.

Modification of sodium channel currents by lanthanum and lanthanide ions in human heart cells

I examined the effects of 100 microM extracellular lanthanum and lanthanide ions on the fast transmembrane sodium channel currents of human heart cell segments. The experiments were conducted under control of the transmembrane electrical and chemical gradients. Lanthanum and lanthanide ion exposure decreased the amplitude and increased the inactivation time constant of the sodium current. Only a transient increase occurred for the activation time constant of the sodium current. The dependence of peak sodium current on excitatory and holding potentials (steady-state activation and inactivation curves, respectively) was transiently shifted to less negative potentials during the first 3 min of exposure, as if these cations were momentarily neutralizing the effective negative charges at the extracellular side of the membrane. The curves then returned to their original position and only the inactivation curves continued shifting progressively towards a limit at more negative membrane potentials. Membrane capacitance was always reduced and this may explain these late effects in terms of changes in membrane dielectric properties and free and bound charges, in addition to traditional screening and binding concepts. These effects were related to the electronic structure of these ions.

A comparison of sodium currents in rat and frog myelinated nerve: normal and modified sodium inactivation.

1. Sodium currents were measured under voltage-clamp conditions in Ranvier nodes of rat and frog nerve fibres at 20 degrees C. Voltage errors due to the resistance in series with the nodal membrane were minimized by reducing sodium currents with tetrodotoxin in the extracellular solutions. 2. The stationary and kinetic properties of sodium activation and inactivation were determined for a wide range of potentials (V) from -40 to 160 mV with respect to the initial holding level (V = 0 mV). 3. The curves m infinity(V) and h infinity(V) of stationary sodium activation and inactivation were not different in rat and frog fibres. 4. The time constants tau m, tau h of sodium activation and inactivation were normally larger in the rat than in the frog. At moderate depolarizations (0 less than or equal to V less than or equal to 80 mV) tau m in the rat was 15-50% larger; the ratio of the rat to the frog tau h values was usually smaller. Thus tau m/tau h = 0.116 for the rat and 0.0965 for the frog at V = 60 mV (potential with maximum peak sodium inward current). 5. Sodium inactivation in rat nerve was slowed and became incomplete by application of intra-axonal iodate or by treatment with external Anemonia toxin II (ATX II), chloramine-T or Ruthenium Red. Peak sodium currents were not increased by these substances. 6. Wash-out of ATX II from frog nerve was rapid and complete but partly irreversible in rat nerve. This suggests different properties or accessibilities of sodium channels in frog and rat nodes.