Interaction of spin-labeled local anesthetics with the sodium channel of squid axon membranes

Summary The effects of spin-labeled local anesthetics on sodium currents of internally perfused squid axons were studied using the voltage-clamp technique. Internal application (10 m) of the most potent spin-labeled local anesthetic used in this study produced a small initial block of sodium currents. However, after sixty repetitive pulses (to +80 mV) given at 1 Hz, the sodium currents were drastically reduced. In addition to this frequency-dependent phenomenon, the anesthetic effect on the sodium currents was also sensitive to the voltage of the pulses. Both the frequency- and voltage-dependent properties remained intact after removal of sodium inactivation with pronase. The recovery of sodium currents from this frequency-dependent anesthetic effect followed a single exponential curve with a surprisingly long time constant of about 10 min. Such a long recovery time, which is longer than any known sodium inactivation process, led us to suggest that the recovery process represents the dissociation of drug molecules from their binding sites. We have also found that increasing hydrophobic character of the homologues series of spin-labeled local anesthetics enhances the frequency- and voltage-dependent block of sodium currents. This effect strongly suggests that hydrophobic interaction is an integral component of the binding site. These probes with their selective effects on the sodium currents, are expected to be highly useful in studying the molecular structure of the sodium channels.     

Interaction of deoxycholate with the sodium channel of squid axon membranes

Deoxycholate can react with sodium channels with a high potency. The apparent dissociation constant for the saturable binding reaction is 2 microM at 8 degrees C, and the heat of reaction is approximately -7 kcal/mol. Four independent test with Na-free media, K-free media, tetrodotoxin, and pancuronium unequivocally indicate that it is the sodium channel that is affected by deoxycholate. Upon depolarization of the membrane, the drug modified channel exhibits a slowly activating and noninactivating sodium conductance. The kinetic pattern of the modified channel was studied by increasing deoxycholate concentration, lowering the temperature, chemical elimination of sodium inactivation, or conditioning depolarization. The slow activation of the modified channel can be represented by a single exponential function with the time constant of 1--5 ms. The modified channel is inactivated only partially with a time constant of 1 S. The reversal potential is unchanged by the drug. Observations in tail currents and the voltage dependence of activation suggest that the activation gate is actually unaffected. The apparently slow activation may reflect an interaction betweem deoxycholate and the sodium channel in resting state.     

Ionic mechanism of action of a spin-labeled local anesthetic on squid axon membranes.

The ionic mechanism of action of a spin-labeled local anesthetic (SLA), 2-[N-methyl-N-(2,2,6,6-tetramethylpiperidonooxyl)]-ethyl 4-ethoxylbenzoate, was studied by means of voltage clamp technique with squid giant axons in comparison with the parent compound without spin label moiety, 2-(N,N-dimethyl)ethyl 4-ethoxylbenzoate (GS-01). Like other local anesthetics, they suppressed both sodium and potassium conductance increases. However, three remarkable differences have been noted between SLA and GS-01: (1) SLA is more effective than GS-01 in suppressing the sodium and potassium conductance increases; (2) SLA induces a potassium inactivation, whereas GS-01 is lacking this ability; (3) SLA has no effect on the time to peak sodium current, whereas GS-01 prolongs it. GS-01 resembles procaine with respect to (2) and (3) above. SLA will become a useful probe for the study of the molecular mechanism of local anesthetic aciton and of ionic channel function.     

Permeability of axon membranes to local anesthetics

The permeability of the neutral form of tertiary amine local anesthetics across squid axon membranes was studied by utilizing three different experimental methods: (1) narcotic action of axon excitability was measured by monitoring the time derivative of action potential and the results were analyzed in terms of a diffusion reaction equation of local anesthetics to obtain their permeabilities; (2) the influx of local anesthetic into the axon was measured by use of the radioisotope tracer technique; and (3) the desorption rates of the neutral form of local anesthetics from lipid monolayers were measured and the desorption rate was correlated with permeability.The relative permeabilities obtained for procaine, lidocaine and tetracaine by the above three methods were comparable. The order of relative permeabilities was $\text{procaine}\text{>}\text{lidocaine}\text{>}\text{tetracaine}$, and had an inverse correlation with the partition coefficients of anesthetics at oil/water phases. Some discussion concerning the concept of permeability is made when the partition coefficient of a permeant molecule is high.     

Interaction of local anesthetics with bilayer phospholipid membranes

Local anesthetics--cocaine, lidocaine, novocaine were tested for conductivity of bilayer lipid membranes containing anion-selective channels formed by polyene antibiotic amphotericin B. It has been shown that 5 X 10(-4) M cocaine doubles the membrane conductance. The line of efficiency of the tested anesthetics is: cocaine greater than lidocaine greater than novocaine. Possible molecular mechanism of the discovered effect is discussed.     

Kinetic analysis of pancuronium interaction with sodium channels in squid axon membranes

The interaction of pancuronium with sodium channels was investigated in squid axons. Sodium current turns on normally but turns off more quickly than the control with pancuronium 0.1-1mM present internally; The sodium tail current associated with repolarization exhibits an initial hook and then decays more slowly than the control. Pancuronium induces inactivation after the sodium inactivation has been removed by internal perfusion of pronase. Such pancuronium-induced sodium inactivation follows a single exponential time course, suggesting first order kinetics which represents the interaction of the pancuronium molecule with the open sodium channel. The rate constant of association k with the binding site is independent of the membrane potential ranging from 0 to 80 mV, but increases with increasing internal concentration of pancuronium. However, the rate constant of dissociation l is independent of internal concentration of pancuronium but decreases with increasing the membrane potential. The voltage dependence of l is not affected by changine external sodium concentration, suggesting a current-independent conductance block, The steady-state block depends on the membrane potential, being more pronounced with increasing depolarization, and is accounted for in terms of the voltage dependence of l. A kinetic model, based on the experimental observations and the assumption on binding kinetics of pancuronium with the open sodium channel, successfully simulates many features of sodium current in the presence of pancuronium.     

Purification and reconstitution of potassium channel proteins from squid axon membranes

Voltage-dependent K⁺ channels are responsible for repolarization of the cell membrane during the late phase of the action potential. Here we report the purification of proteins from squid axon membranes which bind the K⁺-channel blocker noxiustoxin (NTX), and their subsequent functional reconstitution in planar bilayers. The NXT-affinity purified proteins had M_r values of 60000 ± 6000, 160000 ± 15000 and 220000 ± 20000. Their incorporation into bilayers resulted in single-channel currents with three conductances, the most frequent one of 11 pS in 300/100 mM KCl (cis/trans). The voltage dependence, reversal potential and bursting behavior suggest that these are the K⁺ channels involved in the squid axon action potential.     

Removal of sodium channel inactivation in squid axon by the oxidant chloramine-T

We have investigated the effects of a mild oxidant, chloramine-T(CT), on the sodium and potassium currents of squid axons under voltage-clamp conditions. Sodium channel inactivation of squid giant axons can be completely removed by CT at neutral pH. Internal and external CT treatment are both effective. CT apparently removes inactivation in an irreversible, all-or-none manner. The activation process of sodium channels is little affected, as judged from the voltage dependence of peak sodium currents, the rising phase of sodium currents, and the time course of tail currents following the repolarization. The removal of inactivation by CT is pH-dependent; higher pH decreases the removal rate, whereas lower pH increases it. Internal metabisulfite, a strong reductant, does not protect inactivation from the action of external CT, nor does external metabisulfite protect from internal CT application. CT slightly depresses the peak potassium currents at comparable concentrations but has no apparent effects on their kinetics. Our results suggest that the neutral form of CT modifies an embedded methionine residue that is involved in sodium channel inactivation.     

Inhibition of sodium currents by local anesthetics in chloramine-T- treated squid axons. The role of channel activation

In order to test the requirement of Na channel inactivation for the action of local anesthetics, we investigated the inhibitory effects of quaternary and tertiary amine anesthetics on normally inactivating and noninactivating Na currents in squid axons under voltage clamp. Either the enzymatic mixture pronase, or chloramine-T (CT), a noncleaving, oxidizing reagent, was used to abolish Na channel inactivation. We found that both the local anesthetics QX-314 and etidocaine, when perfused internally at 1 mM, elicited a "tonic" (resting) block of Na currents, a "time-dependent" block that increased during single depolarizations, and a "use-dependent" (phasic) block that accumulated as a result of repetitive depolarizations. All three effects occurred in both control and CT-treated axons. As in previous reports, little time-dependent or phasic block by QX-314 appeared in pronase-treated axons, although tonic block remained. Time-dependent block was greatest and fastest at large depolarizations (Em greater than +60 mV) for both the control and CT-treated axons. The recovery kinetics from phasic block were the same in control and CT-modified axons. The voltage dependence of the steady state phasic block in CT-treated axons differed from that in the controls; an 8-10% reduction of the maximum phasic block and a steepening and shift of the voltage dependence in the hyperpolarizing direction resulted from CT treatment. The results show that these anesthetics can bind rapidly to open Na channels in a voltage-dependent manner, with no requirement for fast inactivation. We propose that the rapid phasic blocking reactions in nerve are consequences primarily of channel activation, mediated by binding of anesthetics to open channels, and that the voltage dependence of phasic block arises directly from that of channel activation.     

The early phase of sodium channel gating current in the squid giant axon

A fast component of displacement current which accompanies the sodium channel gating current has been recorded from the membrane of the giant axon of the squid Loligo forbesii. This component is characterized by relaxation time constants typically shorter than 25 µs. The charge displaced accounts for about 10% (or 2 nC/cm²) of the total displacement charge attributed to voltage-dependent sodium channels. Using a low noise, wide-band voltage clamp system and specially designed voltage step protocols we could demonstrate that this component: (i) is not a recording artifact; (ii) is kinetically independent from the sodium channel activation and inactivation processes; (iii) can account for a significant fraction of the initial amplitude of recorded displacement current and (iv) has a steady state charge transfer which saturates for membrane potentials above + 20 mV and below – 100 mV This component can be modelled as a single step transition using the Eyring-Boltzmann formalism with a quantal charge of 1 e^– and an asymmetrical energy barrier. Furthermore, if it were associated with the squid sodium channel, our data would suggest one fast transition per channel. A possible role as a sodium channel activation trigger, which would still be consistent with kinetic independence, is discussed. Despite uncertainties about its origin, the property of kinetic independence allows subtraction of this component from the total displacement current to reveal a rising phase in the early time course of the remaining current. This will have to be taken into account when modelling the voltage-dependent sodium channel.     

Slowing of sodium channel opening kinetics in squid axon by extracellular zinc

The interaction of Zn ion on Na channels was studied in squid giant axons. At a concentration of 30 mM Zn2+ slows opening kinetics of Na channels with almost no alteration of closing kinetics. The effects of Zn2+ can be expressed as a "shift" of the gating parameters along the voltage axis, i.e., the amount of additional depolarization required to overcome the Zn2+ effect. In these terms the mean shifts caused by 30 mM Zn2+ were +29.5 mV for Na channel opening (on) kinetics (t1/2 on), +2 mV for closing (off) kinetics (tau off), and +8.4 mV for the gNa-V curve. Zn2+ does not change the shape of the instantaneous I-V curve for inward current, but reduces it in amplitude by a factor of or approximately 0.67. Outward current is unaffected. Effects of Zn2+ on gating current (measured in the absence of TTX) closely parallel its actions on gNa. On gating current kinetics are shifted by +27.5 mV, off kinetics by +6 mV, and the Q-V distribution by +6.5 mV. Kinetic modeling shows that Zn2+ slows the forward rate constants in activation without affecting backward rate constants. More than one of the several steps in activation must be affected. The results are not compatible with the usual simple theory of uniform fixed surface charge. They suggest instead that Zn2+ is attracted by a negatively charged element of the gating apparatus that is present at the outer membrane surface at rest, and migrates inward on activation.     

Binding of spin-labeled local anesthetics to lobster nerves

Summary The hyperfine coupling constant of spin-labeled local anesthetics, 2-(N-methyl N-(2,2,6,6-tetramethylpiperidinooxyl)) ethyl 4-alkoxybenzoates, showed these compounds to partition betwen the aqueous exterior and the hydrocarbon phase of the membrane. Increased partitioning into the hydrocarbon phase of the membrane was in the order: hexyloxy>butoxy>ethoxy. Since these compounds are known to have different durations of anesthesia in the same order, this suggests that durations of activity and ability to partition into the hydrocarbon region of the membrane are related.     

Interaction of local anesthetics with calmodulin

A crude folic acid antagonist, previously designated as X-methyl folate was studied. Five components were found to inhibit the growth of Streptococcus faecalis. 9-Methyl folic acid was the major bioactive constituent by weight (5%), but because of its low specific activity, contributed only 1% of the biological activity of the crude reaction product. The most active compound accounted for 40% of the activity and constituted 0.04% of the crude product by weight. Spectroscopic data suggest this to be a novel folic acid analogue.     

Single sodium channels from the squid giant axon

Since the work of A. L. Hodgkin and A. F. Huxley (1952. J. Physiol. [Lond.].117:500-544) the squid giant axon has been considered the classical preparation for the study of voltage-dependent sodium and potassium channels. In this preparation much data have been gathered on macroscopic and gating currents but no single sodium channel data have been available. This paper reports patch clamp recording of single sodium channel events from the cut-open squid axon. It is shown that the single channel conductance in the absence of external divalent ions is approximately 14 pS, similar to sodium channels recorded from other preparations, and that their kinetic properties are consistent with previous results on gating and macroscopic currents obtained from the perfused squid axon preparation.     

On the voltage dependence of inactivation in the sodium channel of the squid giant axon.

Measurements of the macroscopic sodium current in the squid giant axon show that the inactivation gate carries around 1.3 units of electronic charge. The contrary evidence from single-channel studies is considered, and a modified series-parallel model of the sodium channel is proposed that might help to resolve the disagreement.