Spontaneous opening at zero membrane potential of sodium channels from eel electroplax reconstituted into lipid vesicles

Lidocaine has different effects and potencies on muscle and brain sodium channels

Lidocaine effects were studied at the single channel level on batrachotoxin-activated eel electroplax (muscle-derived) and on rat brain sodium channels in planar lipid bilayers to investigate whether these effects were the same on structurally different sodium channels. Lidocaine blocked the open state of brain channels with the same voltage dependence, but with 15-times as high a potency as muscle-derived channels. In brain channels, but not muscle-derived ones, the level of the open channel block showed periods of relief. Lidocaine at microM concentrations stabilized the highest conductance state in both channel types and at mM concentrations stabilized subconductance-like states in electroplax, but not in brain channels. In both channel types, lidocaine increased the lifetime and rate of entry to a long-nonconducting state. Since both channel types were studied under identical lipid and ionic conditions, the observed functional differences in the lidocaine action (effects, potency) must reflect channel structural differences.

Alkaloid-modified sodium channels from lobster walking leg nerves in planar lipid bilayers

Alkaloid-modified, voltage-dependent sodium channels from lobster walking leg nerves were studied in planar neutral lipid bilayers. In symmetrical 0.5 M NaCl the single channel conductance of veratridine (VTD) (10 pS) was less than that of batrachotoxin (BTX) (16 pS) modified channels. At positive potentials, VTD- but not BTX-modified channels remained open at a flickery substate. VTD-modified channels underwent closures on the order of milliseconds (fast process), seconds (slow process), and minutes. The channel fractional open time (f(o)) due to the fast process, the slow process, and all channel closures (overall f(o)) increased with depolarization. The fast process had a midpoint potential (V(a)) of -122 mV and an apparent gating charge (z(a)) of 2.9, and the slow process had a V(a) of -95 mV and a z(a) of 1.6. The overall f(o) was predominantly determined by closures on the order of minutes, and had a V(a) of about -24 mV and a shallow voltage dependence (z(a) approximately 0.7). Augmenting the VTD concentration increased the overall f(o) without changing the number of detectable channels. However, the occurrence of closures on the order of minutes persisted even at super-saturating concentrations of VTD. The occurrence of these long closures was nonrandom and the level of nonrandomness was usually unaffected by the number of channels, suggesting that channel behavior was nonindependent. BTX-modified channels also underwent closures on the order of milliseconds, seconds, and minutes. Their characterization, however, was complicated by the apparent low BTX binding affinity and by an apparent high binding reversibility (channel disappearance) of BTX to these channels. VTD- but not BTX-modified channels inactivated slowly at high positive potentials (greater than +30 mV). Single channel conductance versus NaCl concentrations saturated at high NaCl concentrations and was non-Langmuirian at low NaCl concentrations. At all NaCl concentrations the conductance of VTD-modified channels was lower than that of BTX-modified channels. However, this difference in conductance decreased as NaCl concentrations neared zero, approaching the same limiting value. The permeability ratio of sodium over potassium obtained under mixed ionic conditions was similar for VTD (2.46)- and BTX (2.48)-modified channels, whereas that obtained under bi-ionic conditions was lower for VTD (1.83)- than for BTX (2.70)-modified channels. Tetrodotoxin blocked these alkaloid-modified channels with an apparent binding affinity in the nanomolar range.

Ethanol decreases the velocity of spike propagation along a fast motor axon

Intracellular recordings were made from the fast bender excitor motor axon in autotomized crab limbs bathed in normal saline, and in salines made with up to 240 mM of ethanol. The presence of ethanol reduced the amplitude, the rise time and the decay time of the evoked action potential, and decreased the velocity at which the spike was conducted down the axon. There was a linear relationship between each of these four parameters and the concentration of ethanol in the saline. The close relationship between spike rise time and conduction velocity suggests that ethanol slows the rate of membrane depolarization by the spike and thus decreases the velocity at which action potentials are propagated along the axon.

Monoclonal antibodies raised against post-translational domains of the electroplax sodium channel

Eleven monoclonal antibodies were identified that recognized eel electroplax sodium channels. All the monoclonal antibodies specifically immunostained the mature TTX-sensitive sodium channel (Mr 265,000) on immunoblots. None of the monoclonal antibodies would precipitate the in vitro translated channel core polypeptide in solution. One monoclonal antibody, 3G4, was found to bind to an epitope involving terminal polysialic acids. Extensive digestion of the channel by the exosialidase, neuraminidase, or partial polysialic acid removal by the endosialidase, endo-N-acetylneuraminidase, destroy the 3G4 epitope. 3G4 is, therefore, a highly selective probe for the post-translationally attached polysialic acids. Except for this monoclonal antibody, the epitopes recognized by the remaining antibodies were highly resistant to extensive N-linked deglycosylation. Thus, the monoclonal antibodies may be directed against unique post-translationally produced domains of the electroplax sodium channel, presumably sugar groups that are abundant on this protein (Miller, J.A., Agnew, W.S., Levinson, S.R. 1983, Biochemistry 22:462-470). These monoclonal antibodies should prove useful as tools to study discrete post-translational processing events in sodium channel biosynthesis.

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)

Reconstituted voltage-sensitive sodium channels from eel electroplax: activation of permeability by quaternary lidocaine, N-bromoacetamide, and N-bromosuccinimide

We have investigated the ion permeability properties of sodium channels purified from eel electroplax and reconstituted into liposomes. Under the influence of a depolarizing diffusion potential, these channels appear capable of occasional spontaneous openings. Fluxes which result from these openings are sodium selective and blocked (from opposite sides of the membrane) by tetrodotoxin (TTX) and moderate concentrations of the lidocaine analogue QX-314. Low concentrations of QX-314 paradoxically enhance this channel-mediated flux. N-bromoacetamide (NBA) and N-bromosuccinimide (NBS), reagents which remove inactivation gating in physiological preparations, transiently stimulate the sodium permeability of inside-out facing channels to high levels. The rise and subsequent fall of permeability appear to result from consecutive covalent modifications of the protein. Titration of the protein with the more reactive NBS can be used to produce stable, chronically active forms of the protein. Low concentrations of QX-314 produce a net facilitation of channel activation by NBA, while higher concentrations produce block of conductance. This suggests that rates of modifications by NBA which lead to the activation of permeability are influenced by conformational changes induced by QX-314 binding.

Neurotoxin-modulated uptake of sodium by highly purified preparations of the electroplax tetrodotoxin-binding glycopeptide reconstituted into lipid vesicles

Using the dialysable detergent CHAPS (3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate), the tetrodotoxin-binding protein from the electroplax of the electric eel has been purified to a high degree of both chemical homogeneity and toxin-binding activity. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the best preparations showed only a single microheterogeneous band at Mr approximately 260,000, despite attempts to visualize smaller bands by sample overloading. Upon dialysis, this material became incorporated into the membranes of small unilamellar vesicles, and in this form the purified protein exhibited tetrodotoxin-binding properties similar to the component in the original electroplax membrane. Furthermore, in the presence of activator neurotoxins the vesicles were able to accumulate isotopic sodium in a manner similar to that previously described for less active or less pure preparations of vesicles containing either mammalian or eel electroplax toxin-binding proteins. Quantitative consideration of the isotopic transport activity of this pure material, along with the high degree of purity of the protein, strongly suggests that the 260-kDa glycopeptide from electroplax is necessary and sufficient to account for the sodium channel function seen in these studies, and eliminates the possible involvement of smaller peptides in the channel phenomena observed.