The action of hydrocarbons and carbon tetrachloride on the sodium current of the squid giant axon

Divergent effects of anesthetics on lipid bilayer properties and sodium channel function

General anesthetics revolutionized medicine by allowing surgeons to perform more complex and much longer procedures. This widely used class of drugs is essential to patient care, yet their exact molecular mechanism(s) are incompletely understood. One early hypothesis over a century ago proposed that nonspecific interactions of anesthetics with the lipid bilayer lead to changes in neuronal function via effects on membrane properties. This model was supported by the Meyer-Overton correlation between anesthetic potency and lipid solubility and despite more recent evidence for specific protein targets, in particular ion-channels, lipid bilayer-mediated effects of anesthetics is still under debate. We therefore tested a wide range of chemically diverse general anesthetics on lipid bilayer properties using a sensitive and functional gramicidin-based assay. None of the tested anesthetics altered lipid bilayer properties at clinically relevant concentrations. Some anesthetics did affect the bilayer, though only at high supratherapeutic concentrations, which are unlikely relevant for clinical anesthesia. These results suggest that anesthetics directly interact with membrane proteins without altering lipid bilayer properties at clinically relevant concentrations. Voltage-gated Na⁺ channels are potential anesthetic targets and various isoforms are inhibited by a wide range of volatile anesthetics. They inhibit channel function by reducing peak Na⁺ current and shifting steady-state inactivation toward more hyperpolarized potentials. Recent advances in crystallography of prokaryotic Na⁺ channels, which are sensitive to volatile anesthetics, together with molecular dynamics simulations and electrophysiological studies will help identify potential anesthetic interaction sites within the channel protein itself.

Organic solvent-induced changes in membrane geometry in human SH-SY5Y neuroblastoma cells - a common narcotic effect?

Exposure to organic solvents may cause narcotic effects. At the cellular level, these narcotic effects have been associated with a reduction in neuronal excitability caused by changes in membrane structure and function. In order to critically test whether changes in membrane geometry contribute to these narcotic effects, cultured human SH-SY5Y neuroblastoma cells have been exposed to selected organic solvents. The solvent-induced changes in cell membrane capacitance were investigated using the whole-cell patch clamp technique for real-time capacitance measurements. Exposure of SH-SY5Y cells to the cyclic hydrocarbons m-xylene, toluene, and cyclohexane caused a rapid and reversible increase of membrane capacitance. The aliphatic, nonpolar n-hexane did not cause a detectable change of whole-cell membrane capacitance, whereas the amphiphiles n-hexanol and n-hexylamine caused an increase of membrane capacitance and a concomitant reduction in membrane resistance. Despite a large difference in dielectric properties, the chlorinated hydrocarbons 1,1,2,2-tetrachoroethane and tetrachloroethylene caused a similar magnitude increase in membrane capacitance. The theory on membrane capacitance has been applied to deduce changes in membrane geometry caused by solvent partitioning. Although classical observations have shown that solvents increase the membrane capacitance per unit area of membrane, i.e., increase membrane thickness, the present results demonstrate that solvent partitioning predominantly leads to an increase in membrane surface area and to a lesser degree to an increase in membrane thickness. Moreover, the present results indicate that the physicochemical properties of each solvent are important determinants for its specific effects on membrane geometry. This implies that the hypothesis that solvent partitioning is associated with a common perturbation of membrane structure needs to be revisited and cannot account for the commonly observed narcotic effects of different organic solvents.

Lipid Regulation of Sodium Channels

The lipid landscapes of cellular membranes are complex and dynamic, are tissue dependent, and can change with the age and the development of a variety of diseases. Researchers are now gaining new appreciation for the regulation of ion channel proteins by the membrane lipids in which they are embedded. Thus, as membrane lipids change, for example, during the development of disease, it is likely that the ionic currents that conduct through the ion channels embedded in these membranes will also be altered. This chapter provides an overview of the complex regulation of prokaryotic and eukaryotic voltage-dependent sodium (Nav) channels by fatty acids, sterols, glycerophospholipids, sphingolipids, and cannabinoids. The impact of lipid regulation on channel gating kinetics, voltage-dependence, trafficking, toxin binding, and structure are explored for Nav channels that have been examined in heterologous expression systems, native tissue, and reconstituted into artificial membranes. Putative mechanisms for Nav regulation by lipids are also discussed.

Toxicity of seven priority hazardous and noxious substances (HNSs) to marine organisms: Current status, knowledge gaps and recommendations for future research

Shipping industry and seaborne trade have rapidly increased over the last fifty years, mainly due to the continuous increasing demand for chemicals and fuels. Consequently, despite current regulations, the occurrence of accidental spills poses an important risk. Hazardous and noxious substances (HNSs) have been raising major concern among environmental managers and scientific community for their heterogeneity, hazardous potential towards aquatic organisms and associated social-economic impacts. A literature review on ecotoxicological hazards to aquatic organisms was conducted for seven HNSs: acrylonitrile, n-butyl acrylate, cyclohexylbenzene, hexane, isononanol, trichloroethylene and xylene. Information on the mechanisms of action of the selected HNS was also reviewed. The main purpose was to identify: i) knowledge gaps in need of being addressed in future research; and ii) a set of possible biomarkers suitable for ecotoxicological assessment and monitoring in both estuarine and marine systems. Main gaps found concern the scarcity of information available on ecotoxicological effects of HNS towards marine species and their poorly understood mode of action in wildlife. Differences were found between the sensitivity of freshwater and seawater organisms, so endpoints produced in the former may not be straightforwardly employed in evaluations for the marine environment. The relationship between sub-individual effects and higher level detrimental alterations (e.g. behavioural, morphological, reproductive effects and mortality) are not fully understood. In this context, a set of biomarkers associated to neurotoxicity, detoxification and anti-oxidant defences is suggested as potential indicators of toxic exposure/effects of HNS in marine organisms. Overall, to support the development of contingency plans and the establishment of environmental safety thresholds, it will be necessary to undertake targeted research on HNS ecotoxicity in the marine environment. Research should address these issues under more realistic exposure scenarios reflecting the prevailing spatial and temporal variability in ecological and environmental conditions.

Effects of intramuscular administration of lidocaine or bupivacaine on induction and maintenance doses of propofol evaluated by bispectral index

BACKGROUND

Interest in combining local and general anaesthesia has lead to studies investigating possible interactions. In a prospective, randomized, double-blind study, we tested whether local anaesthetics administered i.m. potentiate the hypnotic effect of propofol.

METHODS

Sixty patients (three groups, n=20) undergoing lower abdominal surgery with total i.v. propofol anaesthesia were investigated. Patients in Group B received i.m. bupivacaine (5 mg ml(-1)) 1 mg kg(-1), patients in Group L received i.m. lidocaine (100 mg ml(-1)) 2 mg kg(-1) and patients in Group C received i.m. saline 5 ml before operation. Hypnosis was measured with bispectral index (BIS).

RESULTS

The induction (BIS <45), and the maintenance doses of propofol (BIS between 40 and 50) were significantly less in Group B and Group L compared with the control group. Induction doses were 1.58 (SD 0.39), 1.56 (0.24) and 2.03 (0.33) mg kg(-1) respectively; P<0.0001. Maintenance doses were 6.33 (2.06), 7.08 (1.23) and 9.95 (2.02) mg kg(-1) respectively in the first hour; P<0.0001. Groups B and L were associated with an attenuated haemodynamic response to both induction and intubation.

CONCLUSION

I.M. administered local anaesthetics are associated with a decrease in both the induction and maintenance doses of propofol during total i.v. anaesthesia and a reduction in haemodynamic responses.

Conducting gramicidin channel activity in phospholipid monolayers

Potential step amperometry (chronoamperometry) of the Tl(I)/Tl(Hg) electrochemical reduction process has been used to investigate the underlying mechanisms of gramicidin activity in phospholipid monolayers. The experiments were carried out at gramicidin-modified dioleoyl phosphatidylcholine (DOPC)-coated electrodes. Application of a potential step to the coated electrode system results in a current transient that can be divided into two regions. An initial exponential decay of current corresponds to the inactivation of monomer channel conductance and a longer time scale quasi-steady-state represents the diffusion of ions to a bimolecular surface reaction. Concentrations of monomer conducting channels are relatively low, and the results indicate that two or more forms of gramicidin are in equilibrium with each other in the layer. Aromatic/conjugated compounds incorporated into the monolayer increase the reduction current by decreasing the rate of channel inactivation and increasing the stability of the conducting channel. This effect is positively correlated with the degree of the compound's aromaticity. The anomalous influence of alkali metal ions on the reduction current is consistent with the model of gramicidin being speciated in the monolayer in more than one form. The results have implications on the lability of the peptide conformation in biological membranes and its dependence on lipid environment, solution composition, and applied potential.

Volatile anesthetics significantly suppress central and peripheral mammalian sodium channels

1. Voltage-dependent sodium channels are important for neuronal signal propagation and integration. 2. Non-mammalian preparations, such as squid giant axon, have sodium channels which have been found to be insensitive to clinical anesthetic concentrations. 3. On the other hand, sodium channels from mammalian neurons are much more sensitive to block by volatile anesthetics. 4. Due to a significant hyperpolarizing shift in steady-state inactivation, IC50s for sodium channel block at potentials close to the resting membrane potential overlapped with clinical anesthetic concentrations. 5. Hence, sodium channels in mammalian neurons may be sensitive molecular targets of volatile anesthetics.

The actions of some narcotic aromatic hydrocarbons on the ionic currents of the squid giant axon

The actions of the aromatic hydrocarbons benzene, toluene, ethyl benzene and n -propyl benzene on the ionic currents of the voltage-clamped giant axon of Loligo forbesi have been studied. All these substances produced a reversible inhibition of both sodium and potassium currents, the sodium currents being the more sensitive. Hydrocarbon solutions which suppressed the sodium current by 50% reduced the potassium current by 25%. A 0.15 (15% by volume) saturated benzene solution had effects similar to those of a 0.3 (30% by volume) saturated solution of n -propyl benzene. The most prominent effect of these substances on the sodium channel was a hyperpolarizing shift in the voltage dependence of the steady-state inactivation parameter h ∞ . Benzene also produced a reversible decrease in the electrical capacity of the axonal membrane measured at 100 kHz. The results indicate that in this preparation the aromatic hydrocarbons are active at lower fractional saturations than their aliphatic counterparts. Similar conclusions have been reached from other studies of hydrocarbons in marine systems. The molecular basis for the actions reported here is discussed and it is suggested that alterations in membrane thickness may be important.

Some effects of short-chain phospholipids and n-alkanes on a transient potassium current (IA) in identified Helix neurons

Many effects of short-chain phospholipids and n-alkanes on the squid axon sodium current (INa) are consistent with mechanisms involving changes in membrane thickness. Here, we suggest that the actions of short-chain phospholipids on an A-type potassium current (IA) in two-microelectrode voltage clamped Helix D1 and F77 neurons are incompatible with such simple mechanisms. Diheptanoyl phosphatidylcholine (diC7PC, 0.2 and 0.3 mM) caused substantial (58 and 79%), and in some cases partially reversible, increases in IA amplitude. These were correlated with hyperpolarizing shifts of up to -7 mV in the voltage dependence of current activation. The voltage dependence of steady-state inactivation was also moved in the hyperpolarizing direction. These effects are the opposite of those described for squid INa. 0.5 Saturated n-pentane and saturated n-hexane caused significant (-3 and -6 mV) hyperpolarizing shifts in the voltage dependence of IA inactivation, qualitatively consistent with their effects on squid INa, while the voltage dependence of activation was moved slightly to the left or unchanged. Hydrocarbons had variable effects on peak current amplitude, although saturated n-pentane produced a clear suppression. DiC7PC caused a 25% increase in the time constant of macroscopic IA inactivation (tau b) but 0.5 saturated n-pentane and saturated n-hexane reduced tau b by 40%. The effects of these agents on current-clamped cells were broadly consistent with their opposing actions on tau b--phospholipids tended to reduce excitability and n-alkanes tended to increase it. Possible mechanisms of IA perturbation are discussed.

Effects of general anaesthetics on neuronal sodium and potassium channels

1. The effects of clinical inhalation anaesthetics, such as halothane and methoxyflurane, and "model" anaesthetics, such as hydrocarbons and n-alkanols, on neuronal sodium and potassium channels are reviewed. 2. Lipid-based mechanisms for the actions of anaesthetics on the gating parameters of squid axon sodium and delayed rectifier potassium currents are considered in conjunction with evidence of more specific effects in other preparations, notably a fast inactivating potassium current in Helix neurones and a voltage-gated sodium current in rat dorsal root ganglion neurones. 3. The proconvulsant actions of some inhalation anaesthetics are discussed in relation to the induction of spontaneous firing of action potentials in the squid giant axon.

Effects of Carbon Tetrachloride on Perfused Cultures of Hepatic and Neuronal Cells

Cultured hepatocytes and hemisphere neurons from chick embryos and mouse neuroblastoma cells were exposed to carbon tetrachloride (CC14; 0, 1, 2, 3 and 4mM) for 1 hour, using a perfusion system developed for studying the effects of volatile substances. In the perfused cultures, three parameters were compared: lipid peroxidation, membrane integrity and cellular respiration. In addition, cytochrome C oxidase activity was determined after incubation of cell homogenates with CC14. A concentration-dependent increase in lipid peroxidation and membrane permeability was found in the neuroblastoma cells. The hepatocytes responded to a lesser extent with respect to membrane permeability and their lipid peroxidation did not differ from that of controls. The hepatocytes responded with a 35% decrease in respiration when exposed to 3mM CC14, and a 20% decrease in cytochrome C oxidase activity after treatment with 1.5mM CCl4. In the neuronal cells, much smaller decreases in respiration were found and their cytochrome C oxidase activity remained unaffected. These results are very similar to those obtained after incubation in a closed chamber system. However, the perfused cells were found to be less sensitive to CCl4than cells exposed under static conditions.

Differential effect of carbon tetrachloride on the cell membranes of neurons and astrocytes

Primary cultures of neurons and astrocytes from chick embryos were used to study the effect of CCl4 on the plasma membranes. The cultures were exposed to 0, 1, 2, 3 or 4 mM CCl4 in a closed chamber system for 30 or 60 minutes. The effects of the exposure were examined by means of scanning electron microscopy and by measuring the degree of lipid peroxidation. In neuron cultures the presence of 1 mM CCl4 for 60 min caused holes in the plasma membranes and led to a swelling of the cell bodies. At 4 mM CCl4 the membranes were totally destroyed, leaving cytoskeletal elements visible. In astrocyte cultures, on the other hand, no effects up to 2 mM were observed. At 4 mM some cells had rounded up, but the membranes were still intact. These data correspond very well with the results that neurons, in contrast to astrocytes, show a concentration-dependent increase in lipid peroxidation. The results from this study may indicate that the mechanism of action of CCl4 is different in neurons and astrocytes.

The role of inactivation in the effects of n-alkanols on the sodium current of cultured rat sensory neurones

1. The whole-cell patch-clamp technique has been used to investigate the actions of n-butanol, n-pentanol, n-hexanol and n-octanol on the sodium current of cells isolated from the dorsal root ganglia (DRGs) of neonatal rats and maintained in short-term tissue culture. 2. The influence of n-alkanols on the level of steady-state inactivation of the sodium current was investigated by a standard two-pulse protocol. All alkanols increased the level of resting inactivation and this was manifested as a hyperpolarizing shift of the relationship between the steady-state inactivation parameter (h infinity) and membrane potential. The mid-point of the h infinity curve was moved by up to -30 mV. 3. The relationship between the shift in the mid-point of the inactivation curve (delta Vh) and aqueous n-alkanol concentration has been derived for each n-alkanol. These are complex in shape and do not appear consistent with a hypothesis that the increase in inactivation results from 1:1 binding of an alkanol molecule to a single site on the channel protein. 4. The aqueous concentrations used ranged from 70 mM-n-butanol to 0.05 mM-n-octanol. However, equal fractional saturations of n-alkanols produced approximately equal shifts in the h infinity curve, particularly in the range 0.01-0.07 saturated. This implies a hydrophobic site of action, with a standard free energy per methylene group for adsorption to the site from the aqueous phase of ca -3.2 kJ/mol. 5. The increase in resting inactivation was not the sole means by which n-alkanols reduced the sodium current. The current was still reduced in cells pre-pulsed to sufficiently negative potentials to remove steady-state inactivation even in the presence of alkanols. The concentration required to reduce the current by 50% (ED50) has been interpolated for each n-alkanol. From these data it was estimated that the standard free energy per methylene group for adsorption to the site of action was ca -3.1 kJ/mol, similar to that calculated for the effect on inactivation. The concentration dependence of this residual block indicated the involvement of more than one n-alkanol molecule. 6. The n-alkanols increase the level of inactivation of rat DRG cell sodium channels at potentials around the resting membrane potential and this effect contributes to their local anaesthetic action.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of arachidonic acid and the other long-chain fatty acids on the membrane currents in the squid giant axon

The effects of arachidonic acid and some other long-chain fatty acids on the ionic currents of the voltage-clamped squid giant axon were investigated using intracellular application of the test substances. The effects of these acids, which are usually insoluble in solution, were examined by using alpha-cyclodextrin as a solvent, alpha-cyclodextrin itself had no effect on the excitable membrane. Arachidonic acid mainly suppresses the Na current but has little effect on the K current. These effects are completely reversed after washing with control solution. The concentration required to suppress the peak inward current by 50% (ED50) was 0.18 mM, which was 10 times larger than that of medium-chain fatty acids like 2-decenoic acid. The Hill number was 1.5 for arachidonic acid, which is almost the same value as for medium-chain fatty acids. This means that the mechanisms of the inhibition are similar in both long- and medium-chain fatty acids. When the long-chain fatty acids were compared, the efficacy of suppression of Na current was about the same value for arachidonic acid, docosatetraenoic acid and docosahexaenoic acid. The suppression effects of linoleic acid and linolenic acid on Na currents were one-third of that of arachidonic acid. Oleic acid had a small suppression effect and stearic acid had almost no effect on the Na current. The currents were fitted to equations similar to those proposed by Hodgkin and Huxley (Hodgkin, A.L., Huxley, A.F. (1952) J. Physiol (London) 117:500-544) and the change in the parameters of these equations in the presence of fatty acids were calculated.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitation of the squid giant axon by general anaesthetics

1. The effects of 'clinical' concentrations of some general anaesthetics on the minimum stimulus required to produce an action potential in the squid giant axon have been examined as a function of time from exposure to the anaesthetic. The resting potential in these experiments was also monitored. 2. The minimum stimulus varied with time in different ways for different anaesthetics. For chloroform, diethyl ether, n-pentanol, halothane and cyclopropane the stimulus initially declined, reached a minimum after about 3 min and then recovered to near-normal values at 10-15 min. For n-pentane, cyclopentane and, to a lesser extent methoxyflurane, the stimulus often declined to such low values that the axon exhibited spontaneous action potentials which persisted until the anaesthetic was removed. For one substance, the experimental local anaesthetic diheptanoyl phosphatidylcholine, the stimulus increased considerably over the 10-15 min required to reach the steady state. In all instances the axons reverted to normal behaviour after removal of the anaesthetic although the time course by which they did so was more variable than for the initial exposure. 3. For all anaesthetics the resting potential changed in the positive direction monotonically by ca. 1-5 mV and reached a steady state in approximately 3 min. On removal of the anaesthetic the resting potential returned to normal, also monotonically. 4. The voltage-gated Na+ and K+ currents were significantly affected even at the low anaesthetic concentrations used. Estimates of the changes in the Hodgkin-Huxley parameters were obtained partly by direct experiment and partly from results previously obtained for higher anaesthetic concentrations. 5. The time dependencies of the minimum stimuli have been accounted for semi-quantitatively in terms of the resting potential changes and the voltage shifts in the Na+ current steady-state activation, and the time dependencies respectively of these two parameters. 6. Quantitative calculations of the resting potential changes for comparison with experiment have been made based on the changes in K+ conductance determined in the preceding paper (Haydon, Requena & Simon, 1988) and changes in the Hodgkin-Huxley parameters of the Na+ and delayed-rectifier K+ currents. 7. Calculations of the minimum stimulus in the steady state have been made from the experimental resting potential changes and from the anaesthetic-affected Hodgkin-Huxley parameters. Good agreement with the experimental stimuli was found, especially in the prediction of high and low values.

The potassium conductance of the resting squid axon and its blockage by clinical concentrations of general anaesthetics

1. The effects of some neutral clinical and experimental general anaesthetics on the resting potential of normal squid axons and squid axons exposed to tetrodotoxin and 3,4-diaminopyridine have been studied. 2. Depolarizations of 1-4 mV were produced by all the anaesthetics at 'clinical' concentrations in the normal axon. Larger depolarizations (5-11 mV) were produced by the same anaesthetic concentrations in axons exposed to tetrodotoxin and 3,4-diaminopyridine. 3. The conductance of axons exposed to tetrodotoxin and either tetraethyl-ammonium or 3,4-diaminopyridine in zero Na+, 430 mM-K+ artificial sea water was examined by voltage clamp and AC bridge techniques. 4. The evidence that this conductance is due predominantly to K+ is discussed. 5. Pre-pulse protocols under voltage clamp have been used to show that part of this conductance arises from the incompletely blocked delayed rectifier. 6. Substantial reductions in this conductance are produced by anaesthetics at 'clinical' concentrations. 7. It is concluded that there is a component of the K+ conductance of the resting squid axon other than the Hodgkin-Huxley delayed rectifier which is extremely sensitive to anaesthetics and which to an appreciable extent determines the resting potential.

The effects of external and internal application of disopyramide on the ionic currents of the squid giant axon

1 The actions of the class I anti-arrythmic agent, disopyramide, on the ionic currents of the voltage-clamped squid axon have been investigated, by use of both extra-axonal and intra-axonal routes of application. 2 Extra-axonal application of 0.1 mM disopyramide produced no significant effects on the membrane currents. External disopyramide at 1.0 mM caused small, poorly reversible inhibition of both sodium and potassium currents. This block was use-dependent and was enhanced by use of test stimuli to more positive membrane potentials. 3 Intra-axonal application of 0.1 mM disopyramide caused a 40% reduction in the first-pulse sodium current (tonic block) and an additional use-dependent block. Analysis of first-pulse currents in terms of the Hodgkin-Huxley formalism indicated that the block resulted mainly from a reduction in the maximum available sodium conductance (gNa); there were no effects on the voltage dependence of the steady-state activation and inactivation parameters, m infinity and h infinity. 4 The use-dependent actions of disopyramide were investigated with a double voltage-clamp pulse protocol. The significant use-dependent effects of the drug were a further reduction in gNa and an increase in the time constant of inactivation (tau h). 5 Disopyramide appears to enter a blocking site in the sodium channel which is only readily accessible from the axoplasmic phase. Partition to the site depends on membrane voltage and on the state of the channel gates. Disopyramide binds at a significant rate to both open and inactivated forms of the sodium channel.

Local anaesthetic effects of benzene and structurally related molecules, including benzocaine, on the squid giant axon

(1). The effects of benzene and several of its derivatives on sodium currents in the voltage-clamped squid giant axon have been studied. Substances tested were benzene, aniline, benzyl alcohol, propiophenone, 4-amino-propiophenone, methyl benzoate, ethyl benzoate, and 4-amino ethyl benzoate (benzocaine). (2.) All substances tested reduced the sodium current in both intact axons and axons internally perfused with CsF. (3.) There were four major actions of benzene on the sodium current: (a) an increase in the resting level of inactivation, (b) an increase in the depolarization required to produce the maximum current, (c) a decrease in the maximum sodium conductance, and (d) an increase in the rate of inactivation. (4.) 4-amino ethyl benzoate (benzocaine) had actions on the sodium current which were very similar to those of benzene with the exception that the rate of inactivation was scarcely affected and, at comparable shifts, the slope of the steady state inactivation curve was slightly smaller. (5.) The results obtained with the substances structurally intermediate between benzene and 4-amino ethyl benzoate allow some conclusions to be drawn as to the role of each functional group.

The mechanisms of sodium current inhibition by benzocaine in the squid giant axon

(1) The effects of benzocaine on the ionic currents in the voltage-clamped squid giant axon have been examined under various conditions; intact axons, axons internally perfused with CsF and axons dialysed with tetraethylammonium ions were used. (2) Both the steady state outward (potassium) current and the early transient (sodium) current were reduced by ca. 50% by benzocaine (1 mM). (3) Plots of the changes produced by benzocaine (1 mM) in the Hodgkin-Huxley parameters for the steady state activation (m infinity), the steady state inactivation (h infinity) and the time constants (tau m and tau h) for activation and inactivation of the sodium current are shown. The m infinity and h infinity curves are shifted in positive and negative directions respectively on the voltage axis. The time constants are not greatly affected. (4) In axons in which the sodium current inactivation had been largely removed by treatment with chloramine T, the sodium current was still reduced by ca. 50% by 1 mM benzocaine and the positive shift in activation remained unchanged. (5) The dependence on benzocaine concentration (for less than or equal to 2 mM) of the peak sodium current reduction and the shift in steady state inactivation have been determined. (6) It is concluded that in the squid axon the effects on inactivation are not the main reason for the reduction of the sodium current by benzocaine and that, in common with many other neutral anaesthetics, there are at least two sites at which benzocaine acts.

The actions of halogenated ethers on the ionic currents of the squid giant axon

The effects of fourteen halogenated ethers on the sodium and potassium currents of voltage-clamped squid giant axons have been examined. Effects under open-circuit were also studied. In voltage-clamped axons, the ethers tended to reduce potassium currents at least as much, if not more, than sodium currents. This finding distinguishes the halogenated ethers from many other general anaesthetics. Certain, but not all, halogenated ethers induced a pronounced maximum in potassium current traces as a function of time. This property can be formally described if an inactivation term is added to the Hodgkin-Huxley equation for potassium currents. Large shifts in the sodium-current inactivation parameter h infinity were produced in some instances. Two fully halogenated methyl ethyl ethers, known to produce convulsions in mice, depressed both sodium and potassium currents, but with a very slow time course of action. The electrophysiological effects of the halogenated ethers investigated appear to depend on the position and number of hydrogen bonds that can be formed.

Local anesthetic action of carboxylic esters: evidence for the significance of molecular volume and for the number of sites involved

The effects of the homologous series of carboxylic esters, methyl propionate to methyl decanoate, on the steady-state inactivation of the sodium current in squid axons have been studied. The esters moved the relationship between the inactivation parameter, h infinity, and the membrane potential in the hyperpolarizing direction, thus reducing the number of sodium channels available at the resting potential. The concentration dependence of the shift at the mid-point of the curve of h infinity against potential has been measured for all esters except decanoate, which was almost inactive. Two aspects of these concentration dependences suggest that molecular volume is an important determinant of the effectiveness of each ester. Firstly, there is a sharp decline in activity above methyl hexanoate. This cut-off in activity resembles that for hydrocarbons where it has been suggested [e.g., Haydon, D.A., Urban, B.W. 1983, J. Physiol. (London) 341:411-427] to a result from a decrease in uptake with increasing molecular volume. (Further data for the hydrocarbons n-butane to n-heptane are reported here.) Secondly, the smallest compounds, methyl propionate and methyl butyrate, are less effective than would be predicted if equal membrane concentrations of each ester produced the same shift. The aqueous concentration dependences for these esters indicate that below methyl hexanoate, as the series is descended, progressively higher membrane concentrations are required to produce a given shift. This would be expected if the volume of ester in the membrane, rather than the number of molecules, is important.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of fatty acids on membrane currents in the squid giant axon

The effects of fatty acids on the ionic currents of the voltage-clamped squid giant axon were investigated using intracellular and extracellular application of the test substances. Fatty acids mainly suppress the Na current but have little effect on the K current. These effects are completely reversed after washing with control solution. The concentrations required to suppress the peak inward current by 50% and Hill number were determined for each fatty acid. ED50 decreased about 1/3 for each increase of one carbon atom. The standard free energy was -3.05 kJ mole-1 for CH2. The Hill number was 1.58 for 2-decenoic acid. The suppression effect of the fatty acids depends on the number of carbon atoms in the compounds and their chemical structure. Suppression of the Na current was clearly observed when the number of carbon atoms exceeded eight. When fatty acids of the same chain length were compared, 2-decenoic acid had strong inhibitory activity, but sebacic acid had no effect at all on the Na channel. The currents were fitted to equations similar to those proposed by Hodgkin and Huxley (J. Physiol. (London) 117:500-544, 1952) and the changes in the parameters of these equations in the presence of fatty acids were calculated. The curve of the steady-state activation parameter (m infinity) for the Na current against membrane potential and the time constant of activation (tau m) were shifted 20 mV in a depolarizing direction by the application of fatty acids. The time constant for inactivation (tau h) was almost no change by application of the fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of N-alcohols on potassium conductance in squid giant axons

The effect of bath application of several short chain N-alcohols on voltage-dependent potassium conductance has been studied in intact giant axons of Loligo forbesi under voltage-clamp conditions. All tested alcohols (methanol, ethanol, propanol, butanol, heptanol and octanol) were found to depress potassium conductance only at concentrations much larger than those necessary to reduce sodium conductance. The efficacy of the different molecules was correlated with the carbon-chain length. In all cases the effects were found to be at least partly reversible. Low concentrations of propanol (100 mM) or heptanol (1 mM) were found to increase potassium conductance whereas higher concentrations had the usual depressing effect. The two alcohols were found to induce a slow inactivation of the potassium conductance. A detailed analysis of the time course of the turning-on of the potassium current for various pulse potentials in the presence of TTX revealed that, for membrane potential values more positive than -20 mV, the time constant of activation was reduced in the presence of propanol or heptanol. The delay which separates the change in potential and the turning-on of the potassium current, which was systematically analysed for different pulse and prepulse potential values, was increased by the two alcohols, the curve relating this delay to prepulse potential being shifted towards larger (positive) delays. This high degree of complexity in the effects on potassium conductance suggests that the alcohol molecules modify several more or less independent mechanisms associated with the turning-on of the potassium current.

Increases in internal Ca2+ and decreases in internal H+ are induced by general anesthetics in squid axons

Squid axons were injected with arsenazo III and treated with sea water containing compounds usually classified as general anesthetics, (pentanol-decanol and a variety of hydrocarbons and their derivatives). Such treatment led to an increase in absorbance by arsenazo III at wavelengths sensitive to [Ca]i. The effect was independent of the presence or absence of Ca++ in sea water and it was not modified by substances that release Ca from internal stores. The effect was easily reversible. In axons injected with phenol red or impaled with a glass electrode sensitive to H+, a similar treatment led to an alkalinization that was also readily reversible. Both Ca release and the change to an alkaline pH had identical time courses. The dose required for action by all of the chemical agents studied could be predicted from a knowledge of their fractional saturation in sea water, i.e. from their thermodynamic activity. For compounds with 8-10 carbon atoms, Ca-release effects can occur at concentration less than those necessary to block either conduction or Na/Ca exchange. A special chemical agent was octylamine, which induced a marked rise in pHi and in addition its nonionic form produced the typical Ca release associated with general anesthetics.

Effects of n-alkanols on the calcium current of intracellularly perfused neurons of Helix aspersa

The effects of n-alkanols on the calcium current (ICa) were studied in molluscan neurons perfused intracellularly and voltage clamped using a suction pipette technique. All n-alkanols employed in this experiment (methanol, ethanol and butanol) decreased the peak amplitude of ICa and caused acceleration of the decay of ICa in a dose-dependent manner at all membrane potentials. The concentrations of n-alkanols required for these actions decreased as the hydrocarbon chain increased in length. The results suggest that these effects on the ICa of molluscan neurons may be related to the lipophilic properties of n-alkanols.

Effects of dimethylsulfoxide on membrane currents of neuroblastoma x glioma hybrid cell

Giga-ohm seal whole cell recording technique was used to examine ionic currents changes induced by dimethylsulfoxide (DMSO) in neuroblastoma X glioma hybrid NG 108-15 cells. DMSO (0.5-1%) reversible blocks sodium, potassium and calcium currents and shifts by about 6 mV the sodium inactivation curve towards more negative voltages.

The effect of temperature on Na currents in rat myelinated nerve fibres

Voltage clamp experiments were performed in single myelinated nerve fibres of the rat and the effect of temperature on Na currents was investigated between 0 degrees C and 40 degrees C. The amplitude of the peak Na current changed with a Q10 = 1.1 between 40 degrees and 20 degrees C and with a Q10 = 1.3 between 20 degrees and 10 degrees C. Below 10 degrees C the peak Na current changed with a Q10 = 1.9. The temperature coefficient for time-to-peak (tp), the measure for Na activation, and tau h1 and tau h2, the time constants for Na inactivation changed throughout the temperature range. Q10 for all of these kinetic parameters increased from 1.8-2.1 between 40 degrees and 20 degrees C to 2.6-2.7 between 20 degrees and 10 degrees C. Below 10 degrees C Q10 increased to 3.7 for tau h1 and tp, and to 2.9 for tau h2. When the series resistance artifacts were minimized by addition of 6 nM TTX, the Q10's at T less than 10 degrees C were 2.9-3.0. When the temperature was decreased from 20 degrees to 0 degrees C, both the curve relating Na permeability to potential, PNa(V), and the steady state Na inactivation curve, h infinity (V), were reversibly shifted towards more negative potentials by 6 mV and 11 mV, respectively. When the temperature was increased from 20 degrees to 37 degrees C no shifts occurred. The Hodgkin-Huxley rate constants alpha h(V) and beta h(V) were calculated from h infinity (V) and tau h (or tau h1) at 20 degrees and 4 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

The actions of some general anaesthetics on the potassium current of the squid giant axon

A number of small organic molecules with general anaesthetic action have been examined for their effects on the voltage-dependent potassium current of the squid giant axon. They include representatives of the three classes of anaesthetics examined in previous studies on the sodium current (Haydon & Urban, 1983a, b, c), i.e. the non-polar molecules n-pentane, cyclopentane and CCl4, several n-alkanols and the inhalation anaesthetics chloroform, halothane, diethyl ether and methoxyflurane. Potassium currents under voltage clamp were recorded in intact and in intracellularly perfused axons before, during and after exposure to the test substances, and the records were fitted with equations similar to those proposed by Hodgkin & Huxley (1952). Shifts in the curves of the steady-state activation against membrane potential and reductions in the potassium conductance at 60 or 70 mV membrane potential have been tabulated. On the same intact axons, all the anaesthetics with the exception of methoxyflurane reduced potassium currents less than sodium currents by about a factor of two or more. For the n-alkanols, butanol to decanol, the concentrations required to reduce the potassium current at 60 mV membrane potential by 50% were determined. For n-butanol to n-heptanol, the standard free energy per CH2 for adsorption to the site of action was estimated to be -2.91 kJ mol-1 as compared with -3.04 kJ mol-1 for reduction of the sodium current. The magnitude of the free energy decreased for alkanols with longer chain lengths. At anaesthetic concentrations that reduce the sodium current by 50%, the hydrophobic substances n-pentane and cyclopentane reduced the maximal sodium conductance, gNa, and the potassium conductance at 70 mV, gK70, equally by about a third, while the n-alkanols reduced both parameters by less than 10%. By contrast, diethyl ether and methoxyflurane were more effective in reducing the maximal potassium conductance. All of the test substances examined, except n-pentane and n-hexane, shifted the voltage dependence of the potassium steady-state activation in the depolarizing direction. A broad qualitative correlation was found between the shifts in the activation curves for sodium and potassium currents but, quantitatively, the agreement between the two shifts was poor. In n-decanol and methoxyflurane solutions, the voltage-clamped potassium currents exhibited pronounced inactivation-like behaviour. These currents can be fitted by the Hodgkin-Huxley formalism if an inactivation term analogous to the sodium current inactivation is added.(ABSTRACT TRUNCATED AT 400 WORDS)

Blocking and modifying actions of octanol on Na channels in frog myelinated nerve

The actions of externally applied n-octanol on Na channels in myelinated frog nerve fibres were studied under voltage clamp conditions. Upon octanol application peak Na inward currents declined in two phases: 90% of the reduction occurred in less than 2 min but a steady-state was reached only after 15 min. During washout the currents came to a stable level within 10 min. The reduction of Na inward currents by octanol was dependent on the amplitude and duration of prepotentials. At the resting potential (VH = 0 mV) 0.4 mM octanol reduced peak Na inward currents at V = 60 mV by 50%. After a prepulse of -60 mV and 50 ms duration Na currents decreased only by 20%. At a hyperpolarizing holding potential of VH = -28 mV 0.7 mM octanol reduced peak inward Na currents to one half. Octanol depressed Na currents at all potentials by approximately the same factor. The Na reversal potential VNa remained unchanged. 0.7 mM external octanol shifted the Na activation curve m infinity (V) by 5 mV to more positive and the inactivation curve h infinity (V) by 14 mV to more negative potentials. The midpoint slopes of both curves were reduced. The time constants of Na activation and inactivation at small depolarizations were decreased. The conductance gamma of a single Na channel and the number No of conducting Na channels per node were determined from nonstationary Na current fluctuations. 0.7 mM octanol increased gamma by a factor of 1.6 and reduced No by a factor of 0.34. It is concluded that octanol blocks some Na channels and modifies the remaining unblocked channels.

Inactivation of the sodium current in squid giant axons by hydrocarbons

The voltage dependence of the steady state inactivation parameter (h infinity) of the sodium current in the squid giant axon is known to be shifted in the hyperpolarizing direction by hydrocarbons and it has been suggested that the shifts arise from thickness changes in the axon membrane, analogous to those produced in lipid bilayers (Haydon, D. A., and J. E. Kimura, 1981, J. Physiol. [Lond.], 312:57-70; Haydon, D. A., and B. W. Urban, 1983, J. Physiol. [Lond.], 338:435-450; Haydon, D. A., J. R. Elliott, and B. M. Hendry, 1984, Curr. Top. Membr. Transp., 22:445-482). This hypothesis has been tested systematically by examining the effects of a range of concentrations of cyclopentane on the high-frequency capacitance per unit area both of the axonal membrane and of lipid bilayers formed from monoolein plus squalene. A similar comparison has been made for cyclopropane and n-butane, both at a pressure of 1 atm. The results are consistent with the notion that thickness increases in the axolemma produce the shifts in h infinity. Except at very high concentrations, however, the thickness changes in the lipid bilayer were too small to account for the h infinity shifts. A possible explanation of this finding is discussed.

Further evidence that membrane thickness influences voltage-gated sodium channels

The short-chain phospholipid, diheptanoyl phosphatidylcholine, at 520 microM, reduced the maximum inward sodium current in voltage-clamped squid giant axons by greater than 50%. Analysis of these currents by means of the Hodgkin-Huxley equations showed this reduction to be mainly the result of a large depolarizing shift in the voltage dependence of the steady state activation parameter, m infinity. The voltage dependence of the steady state inactivation parameter, h infinity, was also moved in the depolarizing direction and the axonal membrane capacitance per unit area measured at 100 kHz was increased. A longer chain length derivative, didecanoyl phosphatidylcholine, had no significant effect on the axonal sodium current at concentrations of 3.7 and 18.5 microM. Dioctanoyl phosphatidylcholine was intermediate in its effects, 200 microM producing approximately the same current suppression as 520 microM diheptanoyl phosphatidylcholine, together with depolarizing shifts in m infinity and h infinity. These effects may be contrasted with those of the normal and cyclic alkanes (1-3), which tend to move both m infinity and h infinity in the hyperpolarizing direction and to reduce the capacitance per unit area at 100 kHz. The above results are all consistent with the hypothesis that small hydrocarbons thicken, while short-chain phospholipids thin, the axonal membrane. Thus membrane thickness changes may be of considerable importance in determining the behavior of the voltage-gated sodium channel.

Dual effects of internal n-alkyltrimethylammonium ions on the sodium current of the squid giant axon

The actions of members of the homologous series of alkyl cations CH3 (CH2)n-1 N+ (CH3)3 (Cn TMA) on the sodium current in giant axons of Loligo forbesi have been investigated. The substances tested correspond to n = 6, 8, 10, 12, 14 and 16. These cations only produced significant sodium current suppression when applied inside the axon. Actions on first-pulse sodium currents and use-dependent effects were separately studied. The shorter members of the series (C6TMA and C8TMA) produced suppression of first-pulse sodium currents without causing significant use dependence. The first-pulse suppression arose partly from a positive shift along the voltage axis of the steady-state activation parameter (m infinity) and partly from a reduction in the maximum sodium conductance (gNa). C12TMA and C14TMA produced little first-pulse suppression but caused clear use dependence. C10TMA showed intermediate properties while C16TMA was inactive. The use-dependent actions have been quantitatively investigated using a double-pulse protocol. The results are consistent with a model in which the cations enter a blocking site on the ion-channel via the intra-axonal aqueous phase. The cations appear able to bind to inactivated sodium channels at significant rates. The possible molecular locations of the sites responsible for m infinity shifts and use dependence are discussed. It is argued that the existence of two separate sites may help to explain certain distinctions between the actions of neutral general anaesthetics and clinical local anaesthetics on the sodium channel.

The admittance of the squid giant axon at radio frequencies and its relation to membrane structure

The admittance of the squid giant axon membrane has been measured, using an intracellular electrode, at frequencies up to 40 MHz. The existence of a radio frequency dispersion, previously detected with extracellular electrodes (Cole, 1976) and attributed to the Schwann cell layer, has been confirmed and followed to higher frequencies. For a comparable method of analysis, membrane parameters similar to those given by Cole (1976) have been calculated. The radio frequency dispersion has a centre frequency at approximately 1.8 MHz, and the properties of a parallel combination of a 28 nF cm-2 capacity and a 3.3 omega cm2 resistance. When the axon membrane capacity is calculated, taking into account the radio frequency dispersion, as described above, the capacity remains frequency dependent throughout the range studied. If it is assumed that at high frequencies the axolemma capacity becomes constant at approximately the value for a lipid bilayer, a radio frequency dispersion is found which cannot be accounted for in terms of a simple equivalent circuit with two passive components, but appears to arise from a network with a distribution of relaxation times. This result could be consistent with the morphology of the Schwann cell layer. The radio frequency dispersion referred to in (4) can be described reasonably well by a circuit with two dispersions having centre frequencies of 250 kHz and 3.2 MHz respectively. The corresponding axolemma capacity (100-500 kHz) would be approximately 0.6 microF cm-2. It is argued that between 50 and 100 kHz the geometrical capacity arising from the non-polar regions of the membrane is a major contributor to the axon membrane capacity, and that capacity variations arising from compositional changes in the lipid bilayer are best monitored in this frequency range.

Interactions of hexachlorocyclohexanes with the (Ca2+ + Mg2+)-ATPase from sarcoplasmic reticulum

Hexachlorocyclohexanes have been shown to inhibit the (Ca2+ + Mg2+)-ATPase of muscle sarcoplasmic reticulum reconstituted into bilayers of dioleoylphosphatidylcholine. However, for the ATPase reconstituted into bilayers of dimyristoleoylphosphatidylcholine, a pattern of activation at low concentration followed by inhibition at higher concentration is seen for hexachlorocyclohexanes and alkanes such as decane and hexadecane. The ATPase in sarcoplasmic reticulum vesicles is also inhibited by the hexachlorocyclohexanes. The effects of hexachlorocyclohexanes on activity are largely independent of concentrations of Ca2+ and ATP. Inhibition is more marked at lower temperatures. The hexachlorocyclohexanes quench the tryptophan fluorescence of the ATPase, and the quenching can be used to obtain partition coefficients into the membrane system. As for simple lipid bilayers, partition exhibits a negative temperature coefficient. Binding is related to effects on ATPase activity.

Anaesthetic action of esters and ketones: evidence for an interaction with the sodium channel protein in squid axons

The effects of methyl butyl ketone, methyl heptyl ketone and methyl pentanoate on the sodium current of the squid giant axon have been examined. The peak inward current in intact axons was reduced reversibly by each substance. Sodium currents were recorded in intracellularly perfused axons before and during exposure to the test substances and the records were fitted with equations similar to those proposed by Hodgkin & Huxley (1952). Shifts in the voltage dependence of the steady-state activation and inactivation parameters (m infinity and h infinity), reductions in the peak heights of the activation and inactivation time constants (tau m and tau h) and changes in the maximum sodium conductance (gNa) caused by these substances have been tabulated and compared with the effects of methyl octanoate (Haydon & Urban, 1983b). Each compound shifted the voltage dependence of the steady-state inactivation parameter in the hyperpolarizing direction and that of the steady-state activation parameter in the depolarizing direction. The shifts produced by the ketones are compared with those produced by methyl pentanoate and by methyl octanoate. The possible role of an interaction between the carbonyl oxygen of the test substance and the sodium channel protein in producing the h infinity shift is discussed. The peak time constants are reduced and the voltage dependences of tau m and tau h are shifted in a direction commensurate with the shifts in steady-state properties. The maximum sodium conductance is not much affected either by the ketones or by methyl pentanoate. Large reductions in peak inward current coupled with little effect on gNa have been reported for the n-alkanols and other surface-active compounds (Haydon & Urban, 1983b). This lack of a large effect on gNa indicates that whatever direct interaction does take place between the test substance and the channel protein, it does not result in a blockage of the channel.

The asymmetrical effects of some ionized n-octyl derivatives on the sodium current of the giant axon of Loligo forbesi

The effects of octyltrimethylammonium ions (OTMA+), octyl sulphate ions (OS-) and octanoic acid (OA) on the sodium current of the voltage-clamped squid giant axon have been investigated using intracellular and extracellular application of the test substances. OTMA+ applied externally at concentrations of 0.8-5.0 mM produces a small reversible increase in the peak inward sodium current in both intact and CsF-perfused axons. Intracellular application of OTMA+ at 0.8 mM to CsF-perfused axons causes a reversible 50% suppression of peak inward sodium current. The inhibition of peak inward current by internal OTMA+ arises largely from a shift of the steady-state activation parameter (m infinity) in the depolarizing direction along the voltage axis. There is little use dependence of the current suppression by OTMA+ OA applied either internally or externally is more effective at suppressing peak inward sodium current at pH 6.0 than at pH 7.4. At pH 6.0 external application of 5 mM-OA to perfused axons causes approximately 60% suppression. This is associated with a depolarizing shift of m infinity of about 13 mV and a hyperpolarizing shift of the steady-state inactivation (h infinity) curve of about 4 mV. The effects of internal and external OA are broadly similar except that the h infinity shift is not seen with internal application. OS- at concentrations above 2.0 mM produces complete irreversible loss of sodium current. At 2.0 mM, OS- produces 10% current suppression and a small depolarizing shift of the m infinity curve. Internal and external applications of OS- differ little except that external OS- causes a 25% increase in the time constant of activation (tau m). The possible origins of these effects are discussed. It is proposed that the shift of m infinity caused by internal OTMA+ is due to a diminution of the lipid dipole potential at the internal surface of the membrane caused by OTMA+ adsorption. This effect could also account for the m infinity shift caused by OA. The results showing that OA produces shifts of opposite sign in the voltage dependence of m infinity and h infinity are discussed with respect to their implications for models of sodium channel gating.

The action of alcohols and other non-ionic surface active substances on the sodium current of the squid giant axon

The effects of several n-alkanols and n-alkyl oxyethylene alcohols, methyl octanoate, glycerol 1-monooctanoate and dioctanoyl phosphatidylcholine on the ionic currents and electrical capacity of the squid giant axon membrane have been examined. The peak inward current in voltage-clamped axons was reduced reversibly by each substance. For n-pentanol to n-decanol the concentrations required to suppress the peak inward current by 50% were determined. From these data, it was estimated that the standard free energy per CH2 for adsorption to the site of action was -3.04 kJ mole-1, as compared with -3.11 kJ mole-1 for adsorption into phospholipid bilayers or an n-alkane/aqueous solution interface. The membrane capacity at 100 kHz was not greatly by any of the test substances at concentrations which reduced the inward current by 50%. Na currents under voltage clamp were recorded in intracellularly perfused axons before, during and sometimes after exposure to the test substances and the records were fitted with equations similar to those proposed by Hodgkin & Huxley (1952). Shifts in the curves of the steady-state activation and inactivation parameters (m infinity and h infinity) against membrane potential, changes in the peak heights of the activation and inactivation time constants (tau m and tau h) and reductions in the maximum Na conductance (gNa) have been tabulated. All of the test substances shifted the voltage dependence of the steady-state activation in the depolarizing direction and lowered the peak time constants for both activation and inactivation. The origins of these effects, and of the differences in the present results from those of the hydrocarbons (Haydon & Urban, 1983), have been discussed in terms of the physico-chemical properties of the two groups of substances and with reference to their effects on artificial membranes.

The effects of some inhalation anaesthetics on the sodium current of the squid giant axon

The effects of diethyl ether, methoxyflurane, halothane, dichloromethane and chloroform on the ionic currents and electrical capacity of the squid giant axon have been examined. The peak inward current in voltage-clamped axons was reduced reversibly by each substance. Sodium currents under voltage clamp were recorded in intracellularly perfused axons before, during, and sometimes after exposure to the test substances, and the records were fitted with equations similar to those proposed by Hodgkin & Huxley (1952). Shifts in the dependence of the steady-state activation and inactivation parameters (m infinity and h infinity) on membrane potential, reductions in the peak heights of the activation and inactivation time constants (tau m and tau h) and decreases in the maximum Na conductance (gNa) have been tabulated. For each of the anaesthetics the steady-state inactivation curve was shifted in the hyperpolarizing direction though less markedly than for the hydrocarbons. The steady-state activation curve was in each instance shifted in the depolarizing direction, as for the alcohols and other surface active substances. In common with both the hydrocarbons and the surface active substances the peak time constants were invariably reduced. The membrane capacity at 100 kHz was affected significantly only by methoxyflurane, where decreases of ca. 9% were observed for 3 mM solutions. The extent to which the results can be accounted for in terms of the perturbation of membrane lipid has been discussed.