The blockage of the electrical conductance in a pore-containing membrane by the n-alkanes

The Effect of Cholesterol on the Dielectric Structure of Lipid Bilayers

Cholesterol plays an important role in regulating the properties of phospholipid bilayers and many mechanisms have been proposed to explain why cholesterol is so ubiquitous within biological membranes of animals. Here we present the results of studies on the effect of cholesterol on the electrical/dielectric properties of lipid membranes tethered to a solid substrate. These tethered bilayer lipid membranes tBLM were formed on a commercially available chemically modified gold substrate. These lipid bilayers are very robust. Very high-resolution electrical impedance spectroscopy (EIS) was used to determine the dielectric structure of the lipid bilayers and associated interfaces. The EIS data allowed the dielectric substructure of the lipid bilayers to be determined. The results showed that when cholesterol was present in the tethered membranes at a concentration of 10% (mol/mol); the thickness of the tBLMs increased and the membrane conductance decreased. However, when cholesterol was present in the tethered membrane at more than 30% (mol/mol) the effect of cholesterol was dramatically different; the membranes then became thinner and possessed a much larger electrical conductance. The EIS allowed a distinction to be made between a hydrophobic region in the center of the bilayer and another hydrophobic region further out towards the polar head region, in addition to the polar head region itself. Cholesterol was found to have the largest effect on the inner, hydrophobic region, although the outer hydrophobic region was also affected.

Effect of Osmotic Pressure on the Stability of Whole Inactivated Influenza Vaccine for Coating on Microneedles

Enveloped virus vaccines can be damaged by high osmotic strength solutions, such as those used to protect the vaccine antigen during drying, which contain high concentrations of sugars. We therefore studied shrinkage and activity loss of whole inactivated influenza virus in hyperosmotic solutions and used those findings to improve vaccine coating of microneedle patches for influenza vaccination. Using stopped-flow light scattering analysis, we found that the virus underwent an initial shrinkage on the order of 10% by volume within 5 s upon exposure to a hyperosmotic stress difference of 217 milliosmolarity. During this shrinkage, the virus envelope had very low osmotic water permeability (1 - 6×10-4 cm s-1) and high Arrhenius activation energy (Ea = 15.0 kcal mol-1), indicating that the water molecules diffused through the viral lipid membranes. After a quasi-stable state of approximately 20 s to 2 min, depending on the species and hypertonic osmotic strength difference of disaccharides, there was a second phase of viral shrinkage. At the highest osmotic strengths, this led to an undulating light scattering profile that appeared to be related to perturbation of the viral envelope resulting in loss of virus activity, as determined by in vitro hemagglutination measurements and in vivo immunogenicity studies in mice. Addition of carboxymethyl cellulose effectively prevented vaccine activity loss in vitro and in vivo, believed to be due to increasing the viscosity of concentrated sugar solution and thereby reducing osmotic stress during coating of microneedles. These results suggest that hyperosmotic solutions can cause biphasic shrinkage of whole inactivated influenza virus which can damage vaccine activity at high osmotic strength and that addition of a viscosity enhancer to the vaccine coating solution can prevent osmotically driven damage and thereby enable preparation of stable microneedle coating formulations for vaccination.

Regulation of membrane protein function by lipid bilayer elasticity-a single molecule technology to measure the bilayer properties experienced by an embedded protein

Membrane protein function is generally regulated by the molecular composition of the host lipid bilayer. The underlying mechanisms have long remained enigmatic. Some cases involve specific molecular interactions, but very often lipids and other amphiphiles, which are adsorbed to lipid bilayers, regulate a number of structurally unrelated proteins in an apparently non-specific manner. It is well known that changes in the physical properties of a lipid bilayer (e.g., thickness or monolayer spontaneous curvature) can affect the function of an embedded protein. However, the role of such changes, in the general regulation of membrane protein function, is unclear. This is to a large extent due to lack of a generally accepted framework in which to understand the many observations. The present review summarizes studies which have demonstrated that the hydrophobic interactions between a membrane protein and the host lipid bilayer provide an energetic coupling, whereby protein function can be regulated by the bilayer elasticity. The feasibility of this 'hydrophobic coupling mechanism' has been demonstrated using the gramicidin channel, a model membrane protein, in planar lipid bilayers. Using voltage-dependent sodium channels, N-type calcium channels and GABA(A) receptors, it has been shown that membrane protein function in living cells can be regulated by amphiphile induced changes in bilayer elasticity. Using the gramicidin channel as a molecular force transducer, a nanotechnology to measure the elastic properties experienced by an embedded protein has been developed. A theoretical and technological framework, to study the regulation of membrane protein function by lipid bilayer elasticity, has been established.

A consistent model for thermal fluctuations and protein-induced deformations in lipid bilayers

We present an elastic Hamiltonian for membrane energetics that captures bilayer undulation and peristaltic deformations over all wavelengths, including the short wavelength protrusion regime. The model implies continuous functional forms for thermal undulation and peristaltic amplitudes as a function of wavelength and predicts previously overlooked relationships between these curves. Undulation and peristaltic spectra display excellent agreement with data from both atomistic and coarse-grained models over all simulated length scales. Additionally, the model accurately predicts the bilayer's response to a cylindrical protein inclusion as observed in coarse-grained simulation. This elastic response provides an explanation for gramicidin ion channel lifetime versus membrane thickness data that requires no fit constants. The physical parameters inherent to this picture may be expressed in terms of familiar material properties associated with lipid monolayers. Inclusion of a finite monolayer spontaneous curvature is essential to obtain fully consistent agreement between theory and the full range of available simulation/experimental data.

Proton transfer in gramicidin channels is modulated by the thickness of monoglyceride bilayers

The thickness of monoglyceride planar bilayers has significant effects on the transfer of protons in both native gramicidin A (gA) and in covalently linked SS- and RR-dioxolane-linked gA proteins. Planar bilayers with various thicknesses were formed from an appropriate combination of monoglyceride with various fatty acid lengths and solvent. Bilayer thicknesses ranged from 25 A (monoolein in squalene) to 54 A (monoeicosenoin in decane). Single-channel conductances to protons (g(H)) were measured in the concentration range of 10-5000 mM HCl. In native gA as well as in RR channels, the shape of the log(g(H))-log([H(+)]) relationships was nonlinear and remained basically unaltered in monoglyceride bilayers with various thicknesses. For both native gA and RR channels, g(H) values were systematically and significantly larger in thin than in thick bilayers. By contrast, the shape of the log(g(H))-log([H(+)]) relationships in the SS channel was linear (with a slope considerably smaller than 1) in thick (>37 A) bilayers. However, in thin (<37 A) bilayers these plots became nonlinear and g(H) values approached those obtained in native gA channels. The linearization of the log-log plots in the SS channel in thick bilayers is a consequence of a dramatic increase (instead of a decrease as in native gA and RR channels) of g(H) in these bilayers in [H(+)] <1 M. The gating characteristics of the various gA channels as a function of bilayer thickness followed the same pattern as described previously. It was noticed, however, that in the thickest monoglyceride bilayer used in this study, both the SS- and RR-dioxolane-linked channels opened in a mode of bursting activity instead of remaining in the open state as in thin bilayers. It is proposed that the thickness of monoglyceride bilayers modulates proton transfer in native gA channels by a combination of factors including the access resistances of channels to H(+), and fluctuations in both the structure of the lipid bilayer and in the distance between gA monomers. The differential effects of relatively thick monoglyceride bilayers on proton transfer in both dioxolane-linked gA channels must relate to distinct interactions between the bilayers and the SS and RR dioxolanes.

On the origin of closing flickers in gramicidin channels: a new hypothesis

The submillisecond closing events (flickers) and the single channel conductances to protons (g(H)) were studied in native gramicidin A (gA) and in the SS and RR diastereoisomers of dioxolane-linked gA channels in planar bilayers. Bilayers were formed from glycerylmonooleate (GMO) in various solvents. In GMO/decane (thick) bilayers, the largest flicker frequency occurred in the SS channel (39 s(-1)), followed by the RR (4 s(-1)) and native gA channels (3 s(-1)). These frequencies were attenuated in GMO/squalene (thin) bilayers by 100-, 30-, and 70-fold in the SS, RR, and native gA channels, respectively. In thin bilayers, the average burst duration of native gA channels was 30-fold longer than in thick bilayers. The RR dioxolane-linked gA dimer "inactivated" in GMO/decane but not in squalene-containing bilayers. The mean closed time of flickers (approximately 0.12 ms) was essentially the same in various gA channels. In thin bilayers, g(H) values were larger by approximately 10% (SS), 30% (RR), and 20% (native gA) in relation to thick bilayers. It is concluded that flickers are not related to pre-dissociation or dissociation states of gA monomers, and do not seem to be caused by intrinsic conformational changes of channel proteins. It is proposed that flickers are caused by undulations of the bilayer that obliterate the openings of gA channels. Differences between flicker frequencies in various gA channels are likely to result from variations in channel geometries at the bilayer/channel interface. The smaller g(H) in thick bilayers suggests that the deformation of these bilayers around the gA channel creates a diffusional pathway next to the mouths of the channel that is longer and more restrictive than in thin GMO bilayers. A possible molecular interpretation for these effects is attempted.

Small-angle neutron scattering study of the n-decane effect on the bilayer thickness in extruded unilamellar dioleoylphosphatidylcholine liposomes

Dioleoylphosphatidylcholine (DOPC) and n-decane were mixed and hydrated afterwards in an excess of heavy water at 1 wt.% of DOPC. From this dispersion, unilamellar liposomes were prepared by extrusion through polycarbonate filter with 500-A pores. Small-angle neutron scattering (SANS) was conducted on these liposomes. From the Kratky-Porod plot ln[I(Q)Q2] vs. Q2 of SANS intensity I(Q) in the range of scattering vectors Q corresponding to the interval 0.001 A(-2) < or = Q2 < or = 0.006 A(-2), the liposome bilayer radius of gyration Rg and the bilayer thickness parameter d(g) = 12(0.5)Rg were obtained. The values of d(g) indicated that the bilayer thickness is within the experimental error constant up to n-decane/DOPC approximately 0.5 molar ratio, and then increases by 2.4 +/- 1.3 A up to n-decane/DOPC = 1.2 molar ratio.

Anesthetic mechanisms in-vitro and in general anesthesia

1. There is no generally accepted and quantifiable definition of general anesthesia. 2. Ion channels are discussed as examples of in-vitro systems used to investigate anesthetic mechanisms. Experimental evidence has accumulated demonstrating that voltage-gated ion channels and ligand-gated ion channels may be affected by clinically relevant concentrations of general anesthetics. 3. There are many factors modulating the anesthetic sensitivity of ion channels, including membrane lipid, electrolyte environment, ion channel subtype, and the functional state of the network. 4. Neuronal networks relevant to human general anesthesia have to be identified before criteria can be formulated as to which component of any particular ion channel function has to be altered to what extent in order to make a contribution to clinical anesthesia.

Energetics of inclusion-induced bilayer deformations

The material properties of lipid bilayers can affect membrane protein function whenever conformational changes in the membrane-spanning proteins perturb the structure of the surrounding bilayer. This coupling between the protein and the bilayer arises from hydrophobic interactions between the protein and the bilayer. We analyze the free energy cost associated with a hydrophobic mismatch, i.e., a difference between the length of the protein's hydrophobic exterior surface and the average thickness of the bilayer's hydrophobic core, using a (liquid-crystal) elastic model of bilayer deformations. The free energy of the deformation is described as the sum of three contributions: compression-expansion, splay-distortion, and surface tension. When evaluating the interdependence among the energy components, one modulus renormalizes the other: e.g., a change in the compression-expansion modulus affects not only the compression-expansion energy but also the splay-distortion energy. The surface tension contribution always is negligible in thin solvent-free bilayers. When evaluating the energy per unit distance (away from the inclusion), the splay-distortion component dominates close to the bilayer/inclusion boundary, whereas the compression-expansion component is more prominent further away from the boundary. Despite this complexity, the bilayer deformation energy in many cases can be described by a linear spring formalism. The results show that, for a protein embedded in a membrane with an initial hydrophobic mismatch of only 1 A, an increase in hydrophobic mismatch to 1.3 A can increase the Boltzmann factor (the equilibrium distribution for protein conformation) 10-fold due to the elastic properties of the bilayer.

Gramicidin channel-induced lipid membrane deformation energy: influence of chain length and boundary conditions

The influence of boundary conditions on the deformation energy of a lipid membrane containing a gramicidin A channel was evaluated numerically. A liquid crystal model was used to calculate the relative contributions of compression, splay and surface tension. It is proposed that the nearest neighbor lipid molecules are displaced from the channel end in a direction perpendicular to the bilayer and it is concluded that surface tension is the major component of the deformation free energy for monoolein (gmo)/n-alkane membranes. This unexpected result supports the validity of the liquid crystal models of membrane deformation since gramicidin lifetime has been shown to correlate with surface tension for gmo membranes. The theory accurately predicts the experimentally measured relative lifetimes without the use of adjustable parameters. For conditions where splay may be neglected surface tension is always the major component of the deformation energy, irrespective of the magnitude of the compression coefficient. The deformation may extend for hundreds of angstroms from the peptide. The results obtained here are expected to be important for the characterization of protein-membrane interactions in general.

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.

Lysophospholipids modulate channel function by altering the mechanical properties of lipid bilayers

Lipid metabolites, free fatty acids and lysophospholipids, modify the function of membrane proteins including ion channels. Such alterations can occur through signal transduction pathways, but may also result from "direct" effects of the metabolite on the protein. To investigate possible mechanisms for such direct effects, we examined the alterations of gramicidin channel function by lysophospholipids (LPLs): lysophosphatidylcholine (LPC), lysophosphatidylethanolamine (LPE), lysophosphatidylserine (LPS), and lysophosphatidylinositol (LPI). The experiments were done on planar bilayers formed by diphytanoylphosphatidylcholine in n-decane a system where receptor-mediated effects can be excluded. At aqueous concentrations below the critical micelle concentration (CMC), LPLs can increase the dimerization constant for membrane-bound gramicidin up to 500-fold (at 2 microM). The relative potency increases as a function of the size of the polar head group, but does not seem to vary as a function of head group charge. The increased dimerization constant results primarily from an increase in the rate constant for channel formation, which can increase more than 100-fold (in the presence of LPC and LPI), whereas the channel dissociation rate constant decreases only about fivefold. The LPL effect cannot be ascribed to an increased membrane fluidity, which would give rise to an increased channel dissociation rate constant. The ability of LPC to decrease the channel dissociation rate constant varies as a function of channel length (which is always less than the membrane's equilibrium thickness): as the channel length is decreased, the potency of LPC is increased. LPC has no effect on membrane thickness or the surface tension of monolayers at the air/electrolyte interface. The bilayer-forming glycerolmonooleate does not decrease the channel dissociation rate constant. These results show that LPLs alter gramicidin channel function by altering the membrane deformation energy, and that the changes in deformation energy can be related to the molecular "shape" of the membrane-modifying compounds. Similar alterations in the mechanical properties of biological membranes may form a general mechanism by which one can alter membrane protein function.

Monitoring the surface tension of lipid membranes by a bubble method

A new method for the determination of lipid membrane surface tension was developed. Advantages of this method are that it allows for multiple measurements on a single membrane, is fast and direct not requiring empirical corrections, may be applied for dynamical surface-tension measurements and may be used with thin films and asymmetrical electrolytes. The pressure and radius of a bubble are measured. A piezoresistive sensor is used to minimize the transducer compliance. By moulding the sensor to a brass plate a resolution of 0.025 mm H2O (0.25 Pa) is obtained. The bubble is filmed using a videocamera and the radius of the bubble determined with the aid of a microcomputer. Data for monoolein/hexadecane in potassium chloride solutions and a cooling curve are presented and compared with previous results.

Calculation of deformation energies and conformations in lipid membranes containing gramicidin channels

In this paper we calculate surface conformation and deformation free energy associated with the incorporation of gramicidin channels into phospholipid bilayer membranes. Two types of membranes are considered. One is a relatively thin solvent-free membrane. The other is a thicker solvent-containing membrane. We follow the approach used for the thin membrane case by Huang (1986) in that we use smectic liquid crystal theory to evaluate the free energy associated with distorting the membrane to other than a flat configuration. Our approach is different from Huang, however, in two ways. One is that we include a term for surface tension, which Huang did not. The second is that one of our four boundary conditions for solving the fourth-order differential equation describing the free energy of the surface is different from Huang's. The details of the difference are described in the text. Our results confirm that for thin membranes Huang's neglect of surface tension is appropriate. However, the precise geometrical form that we calculate for the surface of the thin membrane in the region of the gramicidin channel is somewhat different from his. For thicker membranes that have to deform to a greater extent to accommodate the channel, we find that the contribution of surface tension to the total energy in the deformed surface is significant. Computed results for the shape of the deformed surface, the total energy in the deformed surface, and the contributions of different components to the total energy, are presented for the two types of membranes considered. These results may be significant for understanding the mechanisms of dimer formation and breakup, and the access resistance for ions entering gramicidin channels.

Deformation free energy of bilayer membrane and its effect on gramicidin channel lifetime

The deformation free energy of a lipid bilayer is presented based on the principle of a continuum theory. For small deformations, the free energy consists of a layer-compression term, a splay-distortion term, and a surface-tension term, equivalent to the elastic free energy of a two-layer smectic liquid crystal with surface tension. Minimization of the free energy leads to a differential equation that, with boundary conditions, determines the elastic deformation of a bilayer membrane. When a dimeric gramicidin channel is formed in a membrane of thickness greater than the length of the channel, the membrane deformation reduces the stability of the channel. Previously this effect was studied by comparing the variation of channel lifetime with the surface tension of bilayers (Elliott, J. R., D. Needham, J. P. Dilger, and D. A. Hayden, 1983, Biochim. Biophys. Acta, 735:95-103). The tension was assumed to pull a dimer for a distance z before the channel loses ion conductivity. To account for the data, z was found to be 18 A. With the deformation free energy, the data can be accounted for with z less than or approximately to 1 A, which is consistent with the breaking of hydrogen bonds in a dimer dissociation. Increasing the strength of lipid-protein interactions is not the only consequence of the complete free energy compared with the previous discussions. It also changes the shape of membrane deformation around an embedded channel from convex to concave, and increases the range of deformation from less than 10 A to greater than 20 A. Clearly these will be important factors in the general considerations of lipid-protein interactions and membrane-mediated interactions between proteins. In addition, thermal fluctuations of a membrane are calculated; in particular, we calculate the relations between the intrinsic thickness and the experimentally measured values. The experimental parameters of monoolein-squalene membranes are used for quantitative analyses.

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)

Optical and electrical properties of thin monoolein lipid bilayers

Monoolein lipid bilayers were formed using a monolayer transfer technique and from dispersions of monoolein in squalene, triolein, 1-chlorodecane and 1-bromodecane. Measurements of optical reflectance and electrical capacitance were used to determine the thickness and dielectric constant of the bilayers. The thickness of the hydrocarbon region of the five bilayer systems ranged from 2.5 to 3.0 nm. Two of the bilayer systems (made from 1-chlorodecane and 1-bromodecane solvents) had a high dielectric constant (2.8 to 2.9) whereas the other bilayer systems had dielectric constants close to that of pure hydrocarbons (2.2). The charge-pulse technique was used to study the transport kinetics of three lipophilic ions and two ion carrier complexes in the bilayers. For the low dielectric constant bilayers, the transport of the lipophilic ions tetraphenylborate, tetraphenylarsonium and dipicrylamine was governed mainly by the thickness of the hydrocarbon region of the bilayer whereas the transport of the ion-carrier complexes proline valinomycin-K+ and valinomycin-Rb+ was nearly independent of thickness. This is consistent with previous studies on thicker monoolein bilayers. The transport of lipophilic anions across bilayers with a high dielectric constant was 20 to 50 times greater than expected on the basis of thickness alone. This agrees qualitatively with predictions based on Born charging energy calculations. High dielectric constant bilayers were three times more permeable to the proline valinomycin-K+ complex than were low dielectric constant bilayers but were just as permeable as low dielectric constant bilayers to the valinomycin-Rb+ complex.

A 1H-NMR investigation of the effects of ethanol and general anaesthetics on ion channels and membrane fusion using unilamellar phospholipid membranes

Using 1H-NMR of small unilamellar vesicles in the presence of the lanthanide probe ion Pr3+, the effects of ethanol, diethyl ether and chloroform on various mechanisms of channel-mediated transport were studied. The mechanisms include channel formation by the polypeptide Alamethicin 30 and vesicular lysis at the gel to liquid-crystal phase transition of the lipid. Channel stabilisation and membrane fusion induced by sub-critical micelle concentrations of Triton X-100 were also investigated. The observation that ethanol and diethyl ether increase membrane permeability and fusion while chloroform inhibits them suggests a common locus of action on the properties and structure of channel-associated water. This conclusion is discussed in terms of current theories of general anaesthesia.

The effects of bilayer thickness and tension on gramicidin single-channel lifetime

Measurements have been made of gramicidin single-channel lifetimes in monoacylglycerol bilayers chosen so that their thickness ranged from above to below the length of the gramicidin channel. Contact angles, electrical capacities and bulk-phase interfacial tensions have also been determined for these systems. The mean channel lifetime decreased with the hydrocarbon thickness of the membrane until the latter reached 2.2 nm, after which the lifetime was relatively constant. A theoretical model has been proposed which relates the mean channel lifetime (or dissociation constant) to both the thickness and the tension of the bilayers. The analysis of the present results and of those of previous studies has led to the idea that aggregates of water molecules may play an important rôle in the dissociation of the gramicidin channel.

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

The effects of the n-alkanes propane to hexane, cyclopropane, cyclopentane and cyclohexane and carbon tetrachloride on the ionic currents and electrical capacity of the squid giant axon membrane have been examined. Both the peak inward and steady-state outward currents were reduced reversibly by each substance, though propane at 1 atm had very little effect. The membrane capacity at 100 kHz was reduced by all substances except propane at 1 atm. Na currents were recorded in intracellularly perfused axons before and during exposure to the hydrocarbons 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. The effects of the various hydrocarbons and carbon tetrachloride on the parameters of the Hodgkin-Huxley equations suggest that the suppression of the Na current by these substances originates from several different phenomena. The underlying physico-chemical events are considered in the light of the observed capacity changes and of information on artificial pore-containing membranes.

Tensions and free energies of formation of "solventless" lipid bilayers. Measurement of high contact angles

A method is described for the accurate measurement of the interfacial tension of lipid bilayer membranes containing little or no solvent. The tensions were obtained from the interfacial tensions of the equilibrium film-forming solution in the Plateau-Gibbs border, measured by conventional techniques, and the contact angle between the border and the bilayer. The contact angles in these systems are large (greater than 10 degrees) and were estimated by a new method that involved the injection of small known volumes of lipid solution into the bilayer so as to form a lens. Results have been obtained for monoolein-triolein, monoolein-squalene, and monoolein-squalene-decane systems. Half bilayer tensions in these systems were up to approximately 1 mN m-1 less than the single interface tensions. Although bilayer tension tended to increase with bilayer thickness, the interdependence of these quantities varied with the alkane solvents present. In the monoolein-squalene-decane systems, small concentrations of decane have a larger effect on tension than on thickness. Free energies of formation of the near-solventless bilayers were much greater than estimated from the simple application of Lifshitz theory.

Ion movement through gramicidin A channels. Interfacial polarization effects on single-channel current measurements

Gramicidin A single-channel current-voltage characteristics were studied at low permeant ion concentrations and very high applied potentials. The purpose of these experiments was to elucidate the basis for the small, but definite, voltage dependence observed under these circumstances. It was found that this residual voltage dependence is a reflection of interfacial polarization effects, similar to those proposed by Walz et al. (Biophys. J. 9:1150-1159). It will be concluded that there exists an effectively voltage-independent step in the association reaction between a gramicidin A channel and the permeating ion. Some consequences of interfacial polarization effects for the analysis of conductance vs. activity relations will be discussed.

The influence of n-alkanols and cholesterol on the duration and conductance of gramicidin single channels in monoolein bilayers

The mean lifetime of gramicidin A channels in bilayers formed from monoolein and squalane was sharply reduced by the absorption of a range of n-alkanols and cholesterol. Results are shown for n-hexanol, n-octanol, n-decanol, n-dodecanol, n-tetradecanol, n-hexadecanol, n-octadecanol and cholesterol. The longer chain n-alkanols were apparently more effective than the shorter members and cholesterol was the most effective of the substances examined. The single channel conductance was also affected, though to a much lesser extent than the mean channel lifetime, the n-alkanols producing increases and cholesterol a decrease. It is suggested that membrane fluidity changes are not likely to be primarily responsible for the reductions in channel lifetimes but that the bilayer tension, which is known to be increased by n-octanol, could be significant.

Neurochemical effects of extended exposure to white spirit vapour at three concentration levels

Male Wistar rats were exposed to 575 (100 ppm), 2875 (500 ppm) or 5750 mg/m3 (1000 ppm) white spirit vapour for 4-17 weeks 5 days a week, 6 h daily. Perirenal fat solvent concentration corresponded in composition and concentration to those of the vapour at all times. The neurochemical effects included a dose-dependent decrease in the cerebellar succinate dehydrogenase activity for 8 weeks while creatine kinase activity increased after 12 weeks. The specific creatine kinase activity in the glial cell fraction, a marker for astroglia, did not increase suggesting proliferation of astroglial cells in the homogenate. The serum creatine kinase activity originating mainly from striated muscle was below the control range at the two higher concentrations after 12 weeks. Simultaneous analyses for isolated muscle membrane sialic acid and uronic acid residues showed decreased concentrations in proportion to lipid phosphorus or total membrane protein. Thus, the white spirit mixture has neurochemical effects possibly caused by paraffins and the same components may have caused the muscle cell membrane effects. The lowest exposure concentration represents a virtual 'no effect' level for rats in the 17-week exposure.

Effects of n-alkanes on the morphology of lipid bilayers. A freeze-fracture and negative stain analysis

The effect of n-alkanes on the ultrastructure of lipid bilayers has been investigated using freeze-fracture and negative stain electron microscopy. It has been found that the morphology of bilayers containing the long alkane tetradecane is quite different from bilayers containing the short alkane hexane. The smooth fracture faces of gel and liquid crystalline state bilayers are unmodified by tetradecane. However, hexane dramatically alters the hydrophobic bilayer interior, producing large (20 to 50 nm) mounds and depressions in the fracture faces. The fracture steps in these multilayer preparations containing hexane are variable in thickness and often considerably wider than the corresponding fracture steps in multilayers which contain tetradecane or are solvent-free. Alkanes also modify the structure of the P beta' or 'banded' phase of phosphatidylcholine bilayers. The incorporation of tetradecane removes the banded structure from both the bilayer's hydrophilic surface, as viewed by negative staining, and the bilayer's hydrophobic interior, as viewed by the freeze-fracture technique. These results are consistent with X-ray diffraction data which imply that long alkanes are primarily located between adjacent lipid hydrocarbon chains in each monolayer of the bilayer, while short alkanes can partition into the geometric center of the bilayer between apposing monolayers.

The dependence of the conductance and lifetime of gramicidin channels on the thickness and tension of lipid bilayers

The lifetimes of channels formed by natural gramicidin and its dimeric analog in monoglyceride lipid bilayers of various compositions were investigated. The bilayer surface tension was altered by changing the length of the monoglycerides' fatty acid chain or the chain length of hydrocarbon solvent by isomerization or saturation of the lipid, by varying the amount of solvent in the bilayer, and by changing the salt composition of the aqueous solutions. The logarithms of mean channel lifetimes were found to be proportional to the surface tension of the membrane irrespective of how the surface tension was changed. In contrast, no simple relationship between channel conductance and surface tension or bilayer thickness was found.

Some effects of n-pentane on the sodium and potassium currents of the squid giant axon

1. Sodium and potassium currents have been recorded in intracellularly perfused squid giant axons before, during and after exposure to solutions of n-pentane in artificial sea water. 2. The currents were fitted with equations similar to those proposed by Hodgkin & Huxley (1952) and the changes in the parameters of these equations in the presence of pentane were calculated. 3. In the range of membrane potential -40 to 40 mV, the time constants for activation (tau m) and inactivation (tau h) of the Na current, and for activation (tau n) of the K current were all reduced by the pentane. 4. The curve of the steady-state inactivation parameter (h infinity) for the Na current against membrane potential was shifted by the pentane in a hyperpolarizing direction (at h infinity = 0.5 this shift was approx. -15 mV in 275 microM-pentane) and the slope at all potentials was reduced. 5. The curve of the steady-state activation parameter (m infinity) for the Na current against membrane potential was also shifted by the pentane in a hyperpolarizing direction (in 153 microM-pentane, 10 mV at m infinity = 0.5). 6. The maximum Na and K conductances gNa and gK were lowered by the pentane, though not usually completely reversibly. 7. The changes in position and slope of the steady-state inactivation curve have been tentatively accounted for in terms of an increase in membrane thickness.

Some effects of aliphatic hydrocarbons on the electrical capacity and ionic currents of the squid giant axon membrane

1. The electrical properties of squid giant axons were examined by means of admittance bridges at frequencies from 0.5 to 300 kHz. A simple equivalent circuit was used to estimate the membrane capacity. 2. The calculated membrane capacities decreased monotonically over the whole frequency range. 3. At 100 kHz and higher frequencies the membrane capacity was independent of potential. 4. At frequencies greater than 20 kHz, exposure of the axons to saturated or 0.9 saturated solutions of n-pentane (275-306 micrometer) reduced the capacity per unit area by 0.1-0.15 micro F cm-2. 5. At 1 kHz the effect of the saturated pentane solutions depended on the membrane potential. In axons having potentials between -60 and zero mV the pentane solutions lowered the capacity, whereas for potentials between -160 and -60 mV they produced little or no change. 6. Saturated solutions of n-hexane, n-heptane and n-octane exhibited qualitatively similar, but quantitatively smaller influences on the membrane capacity, the changes declining as the chain length increased. 7. Under voltage clamp, the peak inward and steady-state outward currents were partially suppressed by the hydrocarbons. Saturated solutions of n-pentane usually reduced the former (reversibly) by 60-80% and the latter by 20-40%. Solutions of n-hexane, n-heptane and n-octane appeared to have successively less effect. Except in deteriorating axons, none of the hydrocarbons produced any consistent changes in the passive membrane resistance, the resting potential or in the reversal potential of the transient inward current. 8. Both the changes in the clamp currents and in the membrane capacity were largely, though not usually completely, reversible. In the hydrocarbon solution the axons deteriorated more rapidly than normal. 9. The responses of axons of Doryteuthis plei to the hydrocarbons were very similar to those of Loligo forbesi with the exception that for the former all observed changes were some five times faster. 10. The time courses of the peak inward and steady-state outward currents on exposure of the axons to n-pentane resembled the time course of the change in membrane capacity at 100 kHz. 11. The simplest interpretation of the high frequency capacity results is suggested to be that, as for lipid bilayers, the membranes become thicker through adsorption of the hydrocarbon.

Burden and dose-related neurochemical effects of intermittent cyclohexane vapour inhalation in rats

Intermittent daily inhalation exposure to 300, 1000 or 2000 ppm of cyclohexane vapour resulted in a dose-dependent solvent concentration in the perirenal fat in rats. The linear relationship changed between the first and second week of exposure as the body solvent burden decreased, despite the continued exposure: this was especially clear in the brain cyclohexane analyses. The salient feature in the brain was the reduction in the activity of azoreductase, while no change could be found in the RNA or glutathione content or in glutathione peroxidase activity. The azoreductase activity was somewhat below the control range after a 2-week withdrawal period, while no solvent could be found and other biochemical variables were within the control ranges.

The organization of n-alkanes in lipid bilayers

The interaction of n-alkanes (C6--C16) with phosphatidylcholine has been studied by the combined use of differential scanning calorimetry, X-ray diffraction and monolayer techniques. It has been found that the thermal properties and ultrastructure of lipid-alkane vesicles are strongly dependent on the length of the n-alkanes. Long alkanes, such as tetradecane and hexadecane, increase the transition temperature of dimyristoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine, while the X-ray data indicate that these long alkanes align parallel to the lipid acyl chains. In contrast, shorter alkanes, such as hexane and octane, decrease and broaden the thermal transition and electron density profiles show that these alkanes increase bilayer width by partitioning between the apposing monolayers of the bilayer. For lipids in the gel and liquid crystalline states, the short alkanes form an alkane region in the geometric center of the bilayer.

The interaction of n-octanol with black lipid bilayer membranes

The electrical capacities of black lipid films formed from monoolein + n-hexadecane and monoolein + squalane (or squalene) solutions have been measured in the presence of various concentrations of n-octanol. In addition, partition coefficients for n-octanol between n-hexadecane and 0.1 M NaCl, dielectric constants for octanol-hexadecane mixtures and the interfacial tension of films and film-forming lipid solutions against the aqueous phases have been determined. It is concluded that in "solvent-free" bilayers the octanol is unlikely to have changed the bilayer thickness by more than about 1 A. The bilayer tension, on the other hand, increases appreciably in the presence of octanol.