Current-dependent inactivation induced by sodium depletion in normal and batrachotoxin-treated frog node of Ranvier

Post stimulus effects of high frequency biphasic electrical current on a fibre's conductibility in isolated frog nerves

OBJECTIVE

High frequency biphasic (HFB) electrical currents are widely used in nerve blocking studies. Their safety margins largely remain unknown and need to be investigated.

APPROACH

This study, exploring the post stimulus effects of HFB electrical currents on a nerve's conductibility, was performed on bullfrog sciatic nerves. Both compound action potentials (CAPs) and differential CAPs (DCAPs, i.e. control CAPs subtracted by CAPs following HFB currents) were obtained, and N1 and N2 components, which were the first and second upward components of DCAPs, were used for analyses of the effects introduced by HFB electrical stimulation.

MAIN RESULTS

First, HFB currents of 10 kHz at a completely blocking threshold were applied for 5 s. The maximum amplitudes and conducting velocities of the CAPs were significantly (P < 0.02) decreased within the observed period (60 s) following HFB currents. The DCAPs displayed clear N1 and N2 components, demonstrating respectively the losses of the fibres' normal conductibility and the appearances of new delayed conductions. Decreases of N1 amplitudes along time, regarded as the recovery of the nerve's conductibility, exhibited two distinct phases: a fast one lasting several seconds and a slow one lasting longer than 5 min. Further tests showed a linear relationship between the HFB stimulation durations and recovering periods of N1 amplitudes. Supra-threshold blocking did not cause higher N1 amplitudes.

SIGNIFICANCE

This study indicates that HFB electrical currents lead to long lasting post stimulus reduction of a nerve's conductibility, which might relate to potential nerve injuries. A possible mechanism, focusing on changes in intracellular and periaxonal ionic concentrations, was proposed to underlie the reduction of the nerve's conductibility and potential nerve injuries. Greater caution and stimulation protocols with greater safety margins should be explored when utilizing HFB electrical current to block nerve conductions.

The Boltzmann equation in molecular biology

In the 1870's, Ludwig Boltzmann proposed a simple equation that was based on the notion of atoms and molecules and that defined the probability of finding a molecule in a given state. Several years later, the Boltzmann equation was developed and used to calculate the equilibrium potential of an ion species that is permeant through membrane channels and to describe conformational changes of biological molecules involved in different mechanisms including: open probability of ion channels, effect of molecular crowding on protein conformation, biochemical reactions and cell proliferation. The aim of this review is to trace the history of the developments of the Boltzmann equation that account for the behaviour of proteins involved in molecular biology and physiology.

Point mutations in segment I-S6 render voltage-gated Na+ channels resistant to batrachotoxin

Batrachotoxin (BTX) is a steroidal alkaloid that causes Na+ channels to open persistently. This toxin has been used widely as a tool for studying Na+ channel gating processes and for estimating Na+ channel density. In this report we used point mutations to identify critical residues involved in BTX binding and to examine if such mutations affect channel gating. We show that a single asparagine --> lysine substitution of the rat muscle Na+ channel alpha-subunit, mu1-N434K, renders the channel completely insensitive to 5 microM BTX when expressed in mammalian cells. This mutant channel nonetheless displays normal current kinetics with minimal changes in gating properties. Another substitution, mu1-N434A, yields a partial BTX-sensitive mutant. Unlike wild-type currents, the BTX-modified mu1-N434A currents continue to undergo fast and slow inactivation as if the inactivation processes remain functional. This finding implies that the mu1-N434 residue upon binding with BTX is critical for subsequent changes on gating; alanine at the mu1-434 position apparently diminishes the efficacy of BTX on eliminating Na+ channel inactivation. Mutants of two adjacent residues, mu1-I433K and mu1-L437K, also were found to exhibit the identical BTX-resistant phenotype. We propose that the mu1-I433, mu1-N434, and mu1-L437 residues in transmembrane segment I-S6 probably form a part of the BTX receptor.

Modification of cloned brain Na+ channels by batrachotoxin

The effects of batrachotoxin (BTX) on cloned alpha-subunit Na+ channels were examined in CHO-K1 cells (a chinese hamster ovary cell line) transfected with rat brain NaIIA cDNA. Under whole-cell patch clamp conditions, BTX shifted the voltage dependence of the activation process by about 45 mV towards the hyperpolarizing direction and eliminated the inactivating phase of Na+ currents. Repetitive depolarizations greatly facilitated the binding of BTX with NaIIA channels while the membrane was held at -100 mV. In chloramine-T-pretreated cells, the association rate of BTX binding with the NaIIA channel was 6.5-fold faster than that in untreated cells. The estimated association rate constant for BTX binding with the open form of NaIIA channel was 1.11 x 10(6) mol-1.s-1 at room temperature. BTX-modified NaIIA channels were blocked by tetrodotoxin (TTX) in a complicated manner. First, the TTX binding to the closed state of BTX-modified NaIIA channels was not voltage dependent. The KD value of TTX was measured at 8.9 nM, which was similar to that of unmodified channels (KD = 14.2 nM). Second, the block of the open state of BTX-modified NaIIA channels by TTX was voltage dependent; depolarization reduced the potency of TTX block between -20 mV to +50 mV. Below -30 mV, the TTX affinity began to level off, probably because of the increased presence of the closed state. Unexpectedly, steady-state inactivation of BTX-modified NaIIA channels was minimal as measured by the two-pulse protocol, a phenomenon distinctly different from that found in GH3 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Inactivation of batrachotoxin-modified Na+ channels in GH3 cells. Characterization and pharmacological modification

Batrachotoxin (BTX)-modified Na+ currents were characterized in GH3 cells with a reversed Na+ gradient under whole-cell voltage clamp conditions. BTX shifts the threshold of Na+ channel activation by approximately 40 mV in the hyperpolarizing direction and nearly eliminates the declining phase of Na+ currents at all voltages, suggesting that Na+ channel inactivation is removed. Paradoxically, the steady-state inactivation (h infinity) of BTX-modified Na+ channels as determined by a two-pulse protocol shows that inactivation is still present and occurs maximally near -70 mV. About 45% of BTX-modified Na+ channels are inactivated at this voltage. The development of inactivation follows a sum of two exponential functions with tau d(fast) = 10 ms and tau d(slow) = 125 ms at -70 mV. Recovery from inactivation can be achieved after hyperpolarizing the membrane to voltages more negative than -120 mV. The time course of recovery is best described by a sum of two exponentials with tau r(fast) = 6.0 ms and tau r(slow) = 240 ms at -170 mV. After reaching a minimum at -70 mV, the h infinity curve of BTX-modified Na+ channels turns upward to reach a constant plateau value of approximately 0.9 at voltages above 0 mV. Evidently, the inactivated, BTX-modified Na+ channels can be forced open at more positive potentials. The reopening kinetics of the inactivated channels follows a single exponential with a time constant of 160 ms at +50 mV. Both chloramine-T (at 0.5 mM) and alpha-scorpion toxin (at 200 nM) diminish the inactivation of BTX-modified Na+ channels. In contrast, benzocaine at 1 mM drastically enhances the inactivation of BTX-modified Na+ channels. The h infinity curve reaches minimum of less than 0.1 at -70 mV, indicating that benzocaine binds preferentially with inactivated, BTX-modified Na+ channels. Together, these results imply that BTX-modified Na+ channels are governed by an inactivation process.

The inactivation of sodium channels in the node of Ranvier and its chemical modification

The many experimental studies reported demonstrate the complexity of what is termed inactivation, the decrease of current flow through sodium channels at maintained depolarization. Even at the normal resting potential of, say, -70 mV for a frog node of Ranvier, ca. 20% of the channels are closed and inactivated, i.e., incapable of passing current on a sudden depolarization, in contrast to the remaining 80% of closed but resting channels. The term inactivation has thus evolved from bulk current ("macroscopic") phenomena and is applied to channels although its single-channel ("microscopic") basis is not entirely clear and may even vary among preparations. It is conceivable that the macroscopic phenomenon may have more than a single microscopic cause; this point will probably not be settled until a physical description of the conformational states of the channel macromolecule becomes available. At any rate, channel transition into an inactivated closed state can be easily affected by numerous reagents of highly diverse chemical nature and, most likely, different primary sites of action as already suggested by the sidedness of effective application, e.g., iodate and endopeptidases to the inside, polypeptide toxins to the outside. But also the search for a common denominator, a secondary target of all these treatments, has not been very successful as demonstrated by the experiments with group-specific reagents. Since modification of inactivation is often accompanied by shifts in the voltage dependence of gating parameters, a target could be the "voltage sensor" of the channel, charged and/or dipolar components of the channel macromolecule that, by being moved in the electric field, somehow induce gating and whose movement is measured as gating current (e.g, Hille, 1984). The fraction of open channels as a function of membrane potential, F(E), may serve as an indicator. It may be simply shifted (to more negative potentials) as by veratridine (Leibowitz et al., 1987) or flattened (reduction of gating charge?) and shifted (in the positive direction) as by Anemonia sulcata toxin II (Ulbricht and Schmidtmayer, 1981) or chloramine-T (Drews, 1987). On the other hand, the steady-state inactivation curve is shifted to more negative potentials by the toxin (Ulbricht and Schmidtmayer, 1981), but to more positive potentials by chloramine-T (Wang, 1984a; Schmidtmayer, 1985). Obviously, modifiers may affect activation and inactivation quite differently, a result that touches on the question as to what extent inactivation derives its potential dependence from activation.(ABSTRACT TRUNCATED AT 400 WORDS)

The action of pyrethroids on sodium channels in myelinated nerve fibres and spinal ganglion cells of the frog

The interaction of pyrethroids with the voltage-dependent sodium channel was studied in voltage-clamped nodes of Ranvier and isolated spinal ganglion neurons of the clawed frog, Xenopus laevis. In the node, pyrethroids prolonged the sodium tail current associated with a step repolarization of the membrane. It was found that the amplitude of the slow, pyrethroid-induced, sodium tail current (PIT) first increased and then decreased as a function of the duration of membrane depolarization (to -5 mV). This decrease of the PIT amplitude was absent when depolarizations to the sodium equilibrium potential (+40 mV) were used. Measurements of changes in sodium reversal potential indicated that sodium ion depletion in the perinodal space is largely responsible for the inactivation of the pyrethroid-modified sodium current. Inactivation is not completely abolished by pyrethroid treatment since the probability of channel opening, measured in membrane patches excised from spinal ganglion cells, decreased slowly during prolonged depolarization. Analysis of unitary currents indicated that both activation and inactivation are retarded by pyrethroids. The arrival of sodium channels in the pyrethroid-modified open state followed a time course that was slower than both activation and inactivation of unmodified sodium channels. Our findings indicate that sodium channels are modified when in the closed resting state and that both opening and closing kinetics are delayed by pyrethroids.

Properties of maintained sodium current induced by a toxin from Androctonus scorpion in frog node of Ranvier

1. The effects of toxin II from scorpion Androctonus australis Hector (AaH II) on the Na current of frog myelinated nerve fibres were analysed under voltage-clamp conditions. 2. Like other alpha-scorpion toxins and Anemonia toxin II, AaH II both increased the inactivation time constants of peak Na current and induced a non-inactivatable Na current (maintained current). 3. In the presence of AaH II, the slope of the maintained conductance-voltage curve was less steep than that corresponding to the peak conductance and the maintained current reversed at a voltage about 20 mV more negative than the peak current. 4. When the peak current was inactivated by pre-depolarizations, 'on' and 'off' relaxation kinetics of the maintained current were an exponential function whose time constant changed with voltage in a bell-shaped manner. At 0 mV, the time constant was about 10 ms. 5. The effects of AaH II could be decomposed into fast effects (increase in inactivation time constants of the peak current) which developed within about 5 s and slow effects (increase in maintained current and changes in initial amplitudes of fast and slow phases of peak current inactivation) which developed within about 30 s. 6. These two types of AaH II effects could be completely removed by conditioning depolarizations giving rise to outward currents. 7. A model is proposed in which the binding of the toxin with its receptor is modulated by membrane potential and internal cations, the appearance of the maintained current is modulated by the environment of channels and the change in inactivation time constants is modulated by membrane potential. The maintained current would correspond to the transformation of a fraction of channels into a non-inactivable (late) form.

Sodium depletion in the periaxonal space of the squid axon treated with pyrethroids

Pyrethroids are known to increase the steady-state sodium current during a step depolarization and to increase and prolong the tail sodium current associated with a step repolarization of the membrane. The pyrethroid-induced tail sodium current of squid axons developed as a function of the duration of the conditioning depolarizing pulse. However, with further lengthening the conditioning pulse duration, it decreased, further increased, or remained constant depending on the direction of sodium current during the depolarization, irrespective of the membrane potential per se. The depletion or accumulation of sodium in the periaxonal space during a 200-ms conditioning depolarizing pulse in the axon internally treated with pronase, pyrethroids, or both, was demonstrated by measurements of the changes in sodium reversal potential. Thus the observed changes in tail current amplitude as a function of the conditioning pulse duration are explicable in terms of changes in sodium concentration in the periaxonal space without assuming inactivation of the pyrethroid-modified channel.

Toxin gamma of the scorpion Tityus serrulatus modifies both activation and inactivation of sodium permeability of nerve membrane

The effects of the major neurotoxic fraction isolated from scorpion venom of Tityus serrulatus, TiTx gamma, on peripheral nerve membrane of Xenopus laevis were studied under current- and voltage-clamp conditions. 700 nmol/l TiTx gamma depolarized the membrane and induced spontaneous activity (150 s-1, maximum value), which ceased within a few minutes. It reduced the amplitude of the action potentials from 109 mV to 52 mV and increased their duration from 1.25 ms to 4.5 ms. 440 nmol/l TiTx gamma induced inward Na current flow at resting potential. The descending branch of the Na current-voltage curve was flattened and shifted approximately 10 mV to more negative potentials. Maximum Na permeability was reduced to about 20%. Both development of and recovery from inactivation of Na permeability were slowed. The steepness of the steady-state inactivation curve was decreased, but the mid-potential changed only insignificantly. No prepulse was necessary to elicit either a shift of activation or an inward current at resting potential. Expressing the toxin effect either in terms of the decrease of Na peak current or of the slowing of inactivation, half-maximum effects were found with 0.3 +/- 0.1 and 3.7 +/- 0.7 mumol/l TiTx gamma, respectively.

Cooperativity of tetrodotoxin action in the frog node of Ranvier

The steady state effects and rates of action of tetrodotoxin (TTX) on sodium current were studied in the voltage clamped frog node of Ranvier. Inactivation of the sodium current was separated into fast and slow phases. Both phases were assumed to correspond to two different currents (fast and slow) flowing through fast and slow channels (Benoit et al. 1985). The dose-response curve of the steady state effect of tetrodotoxin on the fast current was sigmoid. An analysis of this effect in double logarithmic coordinates gave a Hill coefficient of 1.74. The rates of tetrodotoxin action on the fast current were determined by the reversible reduction of the peak current recorded at a potential (+20 mV) at which the slow current was relatively small. After an initial delay, the onset of TTX effect followed an exponential function of time whose constant decreased with increasing tetrodotoxin concentrations. Expressed as the time corresponding to a reduction of 2% of the current, the delay (delta t2%) increased from about 100 ms with 300 nM-TTX to about 30 s with 1 nM-TTX. When tetrodotoxin was removed, the offset developed quasi-instantaneously and followed an exponential function of time whose constant was independent of the toxin concentration. Both steady state and rates of tetrodotoxin effects could be fitted well if one assumed that the block of one fast channel occurred after binding of two TTX molecules to two cooperative sites.

Evidence for two transient sodium currents in the frog node of Ranvier

Na current (INa) was monitored in isolated voltage-clamped frog nodes of Ranvier in order to analyse the pharmacological and kinetic properties of fast and slow phases of inactivation. Niflumic acid (0.1-10 mM) and tetrodotoxin (0.3-30 nM) did not alter fast and slow inactivation time courses but preferentially reduced the amplitude of the fast phase of inactivation. The block of both phases of inactivation by niflumic acid and tetrodotoxin was well described if one assumed that more than one molecule of drug reacted with one channel. Fast and slow currents, corresponding respectively to fast and slow phases of inactivation, reversed at different potentials, had different threshold voltages of activation and the slopes of their steady-state inactivation curves were different. The recovery from inactivation of the compound INa could be described by the sum of two exponentials (plus a delay) corresponding respectively to fast and slow currents. When calculated from INa recorded without and with niflumic acid or tetrodotoxin, the slow current activated about three times more slowly than the fast current. Large prehyperpolarizations delayed both the activation and the inactivation of the fast current but only the activation of the slow current. Lowering the temperature decreased the fast current but increased the slow current. We conclude that the inactivatable Na current of the nodal membrane is made up of two components (INa,f and INa,s) corresponding to two different and interconvertible forms of the Na channel.