Anaesthesia by the n-alkanes. A comparative study of nerve impulse blockage and the properties of black lipid bilayer membranes

Screening for bilayer-active and likely cytotoxic molecules reveals bilayer-mediated regulation of cell function

A perennial problem encountered when using small molecules (drugs) to manipulate cell or protein function is to assess whether observed changes in function result from specific interactions with a desired target or from less specific off-target mechanisms. This is important in laboratory research as well as in drug development, where the goal is to identify molecules that are unlikely to be successful therapeutics early in the process, thereby avoiding costly mistakes. We pursued this challenge from the perspective that many bioactive molecules (drugs) are amphiphiles that alter lipid bilayer elastic properties, which may cause indiscriminate changes in membrane protein (and cell) function and, in turn, cytotoxicity. Such drug-induced changes in bilayer properties can be quantified as changes in the monomer↔dimer equilibrium for bilayer-spanning gramicidin channels. Using this approach, we tested whether molecules in the Pathogen Box (a library of 400 drugs and drug-like molecules with confirmed activity against tropical diseases released by Medicines for Malaria Venture to encourage the development of therapies for neglected tropical diseases) are bilayer modifiers. 32% of the molecules in the Pathogen Box were bilayer modifiers, defined as molecules that at 10 µM shifted the monomer↔dimer equilibrium toward the conducting dimers by at least 50%. Correlation analysis of the molecules' reported HepG2 cell cytotoxicity to bilayer-modifying potency, quantified as the shift in the gramicidin monomer↔dimer equilibrium, revealed that molecules producing <25% change in the equilibrium had significantly lower probability of being cytotoxic than molecules producing >50% change. Neither cytotoxicity nor bilayer-modifying potency (quantified as the shift in the gramicidin monomer↔dimer equilibrium) was well predicted by conventional physico-chemical descriptors (hydrophobicity, polar surface area, etc.). We conclude that drug-induced changes in lipid bilayer properties are robust predictors of the likelihood of membrane-mediated off-target effects, including cytotoxicity.

Incorporation and localisation of alkanes in a protomembrane model by neutron diffraction

Protomembranes at the origin of life were likely composed of short-chain lipids, readily available on the early Earth. Membranes formed by such lipids are less stable and more permeable under extreme conditions, so a novel membrane architecture was suggested to validate the accuracy of this assumption. The model membrane includes the presence of a layer of alkanes in the mid-plane of the protomembrane in between the two monolayer leaflets and lying perpendicular to the lipid acyl chains. Here, we investigated such a possibility experimentally for membranes formed by the short-chain phospholipid 1,2-didecanoyl-sn-glycero-3-phophocholine, including or not the alkanes eicosane, squalane or triacontane by means of neutron membrane diffraction and contrast variation. We found strong indications for incorporation of two of the three alkanes in the membrane mid-plane through the determination of neutron scattering length density profiles with hydrogenated vs deuterated alkanes and membrane swelling at various relative humidities indicating a slightly increased bilayer thickness when the alkanes are incorporated into the bilayers. The selectivity of the incorporation points out the role of the length of the n-alkanes with respect to the capacity of the membrane to incorporate them.

Filling the Gap with Long n-Alkanes: Incorporation of C20 and C30 into Phospholipid Membranes

Investigating how hydrophobic molecules mix with phospholipid bilayers and how they affect membrane properties is commonplace in biophysics. Despite this, a molecular-level empirical description of a membrane model as simple as a phospholipid bilayer with long linear hydrophobic chains incorporated is still missing. Here, we present an unprecedented molecular characterization of the incorporation of two long n-alkanes, n-eicosane (C20) and n-triacontane (C30) with 20 and 30 carbons, respectively, in phosphatidylcholine (PC) bilayers using a combination of experimental techniques (²H NMR, ³¹P NMR, ¹H-¹³C dipolar recoupling solid-state NMR, X-ray scattering, and cryogenic electron microscopy) and atomistic molecular dynamics (MD) simulations. At low hydration, deuterated C20 and C30 yield ²H NMR spectra evidencing anisotropic-motion, which demonstrates their miscibility in PC membranes up to a critical alkane-to-acyl-chain volume fraction, ϕ_c. The acquired ²H NMR spectra of C20 and C30 have notably different lineshapes. At low alkane volume fractions below ϕ_c, CHARMM36 MD simulations predict such ²H NMR spectra qualitatively and thus enable an atomistic-level interpretation of the spectra. Above ϕ_c, the ²H NMR lineshapes become characteristic of motions in the intermediate-regime that, together with the MD simulation results, suggest the onset of immiscibility between the alkane molecules and the acyl chains. For all the systems investigated, the phospholipid molecular structure is unperturbed by the presence of the alkanes. However, at conditions of excess hydration and at surprisingly low alkane fractions below ϕ_c, a peak characteristic of isotropic motion is observed in both the ²H spectra of the alkanes and ³¹P spectra of the phospholipids, strongly indicating that the incorporation of the alkanes induces a reduction on the average radius of the lipid vesicles.

Interleaflet coupling of n-alkane incorporated bilayers

The relationship between the membrane bending modulus (κ) and compressibility modulus (K_A) depends on the extent of coupling between the two monolayers (leaflets). Using neutron spin echo (NSE) spectroscopy, we investigate the effects of n-alkanes on the interleaflet coupling of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers. Structural studies with small-angle X-ray and neutron scattering (SAXS and SANS) showed that the bilayer thickness increased with increasing n-alkane length, while NSE suggested that the bilayers became softer. Additional measurements of the membrane thickness fluctuations with NSE suggested that the changes in elastic moduli were due to a decrease in coupling between the leaflets upon addition of the longer n-alkanes. The decreased coupling with elongating n-alkane length was explained based on the n-alkane distribution within the bilayers characterized by SANS measurement of bilayers composed of protiated DPPC and deuterated n-alkanes. A higher fraction of the incorporated long n-alkanes were concentrated at the central plane of the bilayers and decreased the physical interaction between the leaflets. Using NSE and SANS, we successfully correlated changes in the mesoscopic collective dynamics and microscopic membrane structure upon incorporation of n-alkanes.

Application of small-angle neutron diffraction to the localization of general anesthetics in model membranes

We set out to explore the applicability of small-angle neutron diffraction (SAND) to the localization of biomembrane components by studying the general anesthetic n-decane in a model lipid bilayer system composed of dioleoyl-phosphocholine (DOPC). Samples in the form of planar membrane multilayers were hydrated by varied mixtures of deuterated and protonated water, and examined by the means of SAND. Neutron scattering length density (NSLD) profiles of the system were then reconstructed from the experimental data. We exploited the significantly different neutron scattering properties of hydrogen and deuterium atoms via labeling in addition to water contrast variation. Enhancing the signals from particular components of bilayer system led to a set of characteristic membrane profiles and from their comparison we localized n-decane molecules unequivocally in the bilayer's hydrocarbon chain region.

Phase separation of a ternary lipid vesicle including n-alkane: Rugged vesicle and bilayer flakes formed by separation between highly rigid and flexible domains

We investigate the phase separation of a ternary lipid bilayer including n-alkane and construct the ternary phase diagram. When a certain proportion of a long n-alkane is mixed with a binary mixture of lipids, which exhibit the disordered liquid-crystalline phase and the ordered gel phase at room temperature, we observed the characteristic morphology of bilayers with phase separation. The ordered bilayer forms flat and rigid domains, which is connected or rimmed with flexible domains in the disordered phase. The asymmetric emergence of the phase separation region close to the ordered phase side is interpreted based on the almost equal distribution of the n-alkane to the ordered and disordered phase domains.

Study of procaine and tetracaine in the lipid bilayer using molecular dynamics simulation

Despite available experimental results, the molecular mechanism of action of local anesthetics upon the nervous system and contribution of the cell membrane to the process are still controversial. In this work, molecular dynamics simulations were performed to investigate the effect of two clinically used local anesthetics, procaine and tetracaine, on the structure and dynamics of a fully hydrated dimyristoylphosphatidylcholine lipid bilayer. We focused on comparing the main effects of uncharged and charged drugs on various properties of the lipid membrane: mass density distribution, diffusion coefficient, order parameter, radial distribution function, hydrogen bonding, electrostatic potential, headgroup angle, and water dipole orientation. To compare the diffusive nature of anesthetic through the lipid membrane quantitatively, we investigated the hexadecane/water partition coefficient using expanded ensemble simulation. We predicted the permeability coefficient of anesthetics in the following order: uncharged tetracaine > uncharged procaine > charged tetracaine > charged procaine. We also shown that the charged forms of drugs are more potent in hydrogen bonding, disturbing the lipid headgroups, changing the orientation of water dipoles, and increasing the headgroup electrostatic potential more than uncharged drugs, while the uncharged drugs make the lipid diffusion faster and increase the tail order parameter. The results of these simulation studies suggest that the different forms of anesthetics induce different structural modifications in the lipid bilayer, which provides new insights into their molecular mechanism.

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.

Molecular structure influences the stability of membrane penetrating biointerfaces

Nanoscale patterning of hydrophobic bands on otherwise hydrophilic surfaces allows integration of inorganic structures through biological membranes, reminiscent of transmembrane proteins. Here we show that a set of innate molecular properties of the self-assembling hydrophobic band determine the resulting interface stability. Surprisingly, hydrophobicity is found to be a secondary factor with monolayer crystallinity the major determinate of interface strength. These results begin to establish guidelines for seamless bioinorganic integration of nanoscale probes with lipid membranes.

Fusion of biomimetic stealth probes into lipid bilayer cores

Many biomaterials are designed to regulate the interactions between artificial and natural surfaces. However, when materials are inserted through the cell membrane itself the interface formed between the interior edge of the membrane and the material surface is not well understood and poorly controlled. Here we demonstrate that by replicating the nanometer-scale hydrophilic-hydrophobic-hydrophilic architecture of transmembrane proteins, artificial "stealth" probes spontaneously insert and anchor within the lipid bilayer core, forming a high-strength interface. These nanometer-scale hydrophobic bands are readily fabricated on metallic probes by functionalizing the exposed sidewall of an ultrathin evaporated Au metal layer rather than by lithography. Penetration and adhesion forces for butanethiol and dodecanethiol functionalized probes were directly measured using atomic force microscopy (AFM) on thick stacks of lipid bilayers to eliminate substrate effects. The penetration dynamics were starkly different for hydrophobic versus hydrophilic probes. Both 5- and 10 nm thick hydrophobically functionalized probes naturally resided within the lipid core, while hydrophilic probes remained in the aqueous region. Surprisingly, the barrier to probe penetration with short butanethiol chains (E(o,5 nm) = 21.8k(b)T, E(o,10 nm) = 15.3k(b)T) was dramatically higher than longer dodecanethiol chains (E(o,5 nm) = 14.0k(b)T, E(o,10 nm) = 10.9k(b)T), indicating that molecular mobility and orientation also play a role in addition to hydrophobicity in determining interface stability. These results highlight a new strategy for designing artificial cell interfaces that can nondestructively penetrate the lipid bilayer.

Computational studies on the interactions of inhalational anesthetics with proteins

Despite the widespread clinical use of anesthetics since the 19th century, a clear understanding of the mechanism of anesthetic action has yet to emerge. On the basis of early experiments by Meyer, Overton, and subsequent researchers, the cell's lipid membrane was generally concluded to be the primary site of action of anesthetics. However, later experiments with lipid-free globular proteins, such as luciferase and apoferritin, shifted the focus of anesthetic action to proteins. Recent experimental studies, such as photoaffinity labeling and mutagenesis on membrane proteins, have suggested specific binding sites for anesthetic molecules, further strengthening the proteocentric view of anesthetic mechanism. With the increased availability of high-resolution crystal structures of ion channels and other integral membrane proteins, as well as the availability of powerful computers, the structure-function relationship of anesthetic-protein interactions can now be investigated in atomic detail. In this Account, we review recent experiments and related computer simulation studies involving interactions of inhalational anesthetics and proteins, with a particular focus on membrane proteins. Globular proteins have long been used as models for understanding the role of protein-anesthetic interactions and are accordingly examined in this Account. Using selected examples of membrane proteins, such as nicotinic acetyl choline receptor (nAChR) and potassium channels, we address the issues of anesthetic binding pockets in proteins, the role of conformation in anesthetic effects, and the modulation of local as well as global dynamics of proteins by inhaled anesthetics. In the case of nicotinic receptors, inhalational anesthetic halothane binds to the hydrophobic cavity close to the M2-M3 loop. This binding modulates the dynamics of the M2-M3 loop, which is implicated in allosterically transmitting the effects to the channel gate, thus altering the function of the protein. In potassium channels, anesthetic molecules preferentially potentiate the open conformation by quenching the motion of the aromatic residues implicated in the gating of the channel. These simulations suggest that low-affinity drugs (such as inhalational anesthetics) modulate the protein function by influencing local as well as global dynamics of proteins. Because of intrinsic experimental limitations, computational approaches represent an important avenue for exploring the mode of action of anesthetics. Molecular dynamics simulations-a computational technique frequently used in the general study of proteins-offer particular insight in the study of the interaction of inhalational anesthetics with membrane proteins.

Interactions of anesthetics with their targets: non-specific, specific or both?

What makes a general anesthetic a general anesthetic? We shall review first what general anesthesia is all about and which drugs are being used as anesthetics. There is neither a unique definition of general anesthesia nor any consensus on how to measure it. Diverse drugs and combinations of drugs generate general anesthetic states of sometimes very different clinical quality. Yet the principal drugs are still considered to belong to the same class of 'general anesthetics'. Effective concentrations of inhalation anesthetics are in the high micromolar range and above, and even for intravenous anesthetics they do not go below the micromolar range. At these concentrations, many molecular and higher level targets are affected by inhalation anesthetics, fewer probably by intravenous anesthetics. The only physicochemical characteristic shared by anesthetics is the correlation of their anesthetic potencies with hydrophobicity. These correlations depend on the group of general anesthetics considered. In this review, anesthetic potencies for many different targets are plotted against octanol/water partition coefficients as measure of hydrophobicity. Qualitatively, similar correlations result, suggesting several but weak interactions with proteins as being characteristic of anesthetic actions. The polar interactions involved are weak, being roughly equal in magnitude to hydrophobic interactions. Generally, intravenous anesthetics are noticeably more potent than inhalation anesthetics. They differ considerably more between each other in their interactions with various targets than inhalation anesthetics do, making it difficult to come to a decision which of these should be used in future studies as representative 'prototypical general anesthetics'.

Squalane is in the midplane of the lipid bilayer: implications for its function as a proton permeability barrier

A recently proposed model for proton leakage across biological membranes [Prog. Lipid Res. 40 (2001) 299] suggested that hydrocarbons specifically in the center of the lipid bilayer inhibit proton leaks. Since cellular membranes maintain a proton electrochemical gradient as a principal energy transducer, proton leakage unproductively consumes cellular energy. Hydrocarbons in the bilayer are widespread in membranes that sustain such gradients. The alkaliphiles are unique in that they contain up to 40 mol% isoprenes in their membranes including 10-11 mol% squalene [J. Bacteriol. 168 (1986) 334]. Squalene is a polyisoprene hydrocarbon without polar groups. Localizing hydrocarbons in lipid bilayers has not been trivial. A myriad of physical methods including fluorescence spectroscopy, electron-spin resonance, nuclear magnetic resonance as well as X-ray and neutron diffraction have been used to explore this question with various degrees of success and often contradictory results. Seeking unambiguous evidence for the localization of squalene in membranes or lipid bilayers, we employed neutron diffraction. We incorporated 10 mol% perdeuterated or protonated squalane, an isosteric analogue of squalene, into stacked bilayers of dioleoyl phosphatidyl choline (DOPC) doped with dioleoyl phosphatidyl glycerol (DOPG) to simulate the negative charges found on natural membranes. The neutron diffraction data clearly show that the squalane lies predominantly in the bilayer center, parallel to the plane of the membrane.

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.

Identification of shallow and deep membrane-penetrating forms of diphtheria toxin T domain that are regulated by protein concentration and bilayer width

The alpha-helix-rich, hydrophobic transmembrane (T) domain of diphtheria toxin is believed to play a central role in membrane insertion by the toxin and in the translocation of its catalytic domain across membranes. In this report, T domain structure was studied using site-directed single-Cys mutants. The residues chosen, 322 (near the amino-terminal end of helix TH8), 333 (within helix TH8), and 356 (within helix TH9) were substituted with Cys and labeled with the fluorescent probe bimane. (Residues 333 and 356 should be located within the bilayer in the transmembrane state, and residue 322 should not penetrate the bilayer.) After insertion of T domain into model membrane vesicles, the location of bimane label relative to the lipid bilayer was characterized by its fluorescence emission and by its quenching with nitroxide-labeled phospholipids. It was found that when the T domain is added to dioleoylphosphatidylcholine-containing vesicles, all three residues reside close to the outer surface. However, at high T domain concentration or in thinner dimyristoleoylphosphatidylcholine-containing vesicles, a large fraction of residues 333 and 356 penetrate deeply into the membrane. In contrast, residue 322 remains exposed to aqueous solution under these conditions. These conclusions were confirmed by a novel antibody binding method. Antibodies that quench the fluorescence of 4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-3-indacene++ + (BODIPY) groups were used to evaluate the exposure of BODIPY-labeled 322, 333, and 356. Maximum exposure of residues 333 and 356 to externally added antibody was only observed under conditions in which bimane fluorescence showed that these residues do not penetrate the bilayer. In contrast, residue 322 remained exposed under all conditions. We propose that the deeply penetrating T domain conformation represents a transmembrane or near-transmembrane state. The regulation of the transmembrane/nontransmembrane equilibrium should be a key to understanding diphtheria toxin membrane insertion and translocation. Our results suggest that toxin-toxin interactions may play an important role in regulating this behavior.

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.

Stimulation by menthol of Cl secretion via a Ca(2+)-dependent mechanism in canine airway epithelium

1. To investigate the effect of menthol on airway epithelial ion transport function, we studied the bioelectrical properties of canine cultured tracheal epithelium by Ussing's short-circuit technique in vitro. 2. Addition of menthol (10(-3) M) to the mucosal but not the submucosal solution increased the short-circuit current (Isc) from 6.2 +/- 0.9 to 14.0 +/- 2.2 microA cm-2 (P < 0.001), and this effect was accompanied by increases in transepithelial potential difference and conductance. The response was dose-dependent, with the maximal increase from the baseline value and the concentration required to produce a half-maximal effect (EC50) being 6.4 +/- 0.9 microA cm-2 (P < 0.001) and 40 microM, respectively. 3. Other cyclic alcohols, including menthone and cyclohexanol, had no effect on the electrical properties. 4. The menthol-induced increase in Isc was not altered by pretreatment of the cells with amiloride, indomethacin, or propranolol but was abolished by diphenylamine-2-carboxylate, furosemide or substitution of Cl with iodide in the medium. 5. Menthol (10(-3) M) increased cytosolic levels of free calcium ([Ca2+]i) from 98 +/- 12 to 340 +/- 49 nM (P < 0.01) in fura-2-loaded tracheal epithelium but did not affect the intracellular adenosine 3',5'-cyclic monophosphate content. 6. These results suggest that menthol stimulates Cl secretion across airway epithelium, probably through a Ca(2+)-dependent mechanism, and might thus influence mucociliary transport in the respiratory tract.

Comparison of large-conductance Ca(2+)-activated K+ channels in artificial bilayer and patch-clamp experiments

We compared the gating, ion conduction, and pharmacology of large-conductance Ca(2+)-activated K+ channels (BK channels) from canine colon in artificial lipid bilayers and in excised patches. Both protocols identified 270-pS K(+)-selective channels activated by depolarization and Ca2+ (approximately 130-mV shift of half-activation voltage per 10-fold change in Ca2+) that were inhibited by extracellular tetraethylammonium (TEA) and charybdotoxin. These similarities suggest that the same BK channels are studied in the two techniques. However, we found three quantitative differences between channels in artificial bilayers and patches. 1) Channels in artificial bilayers required fivefold higher free Ca2+ or 80-mV stronger depolarization for activation. 2) The voltage dependence of TEA block was smaller for channels in artificial bilayers. The apparent distance across the membrane field for the TEA binding site was 0.031 for channels in artificial bilayers and 0.23 for channels in patches. 3) ATP (2 mM) decreased open probability (Po) of channels in artificial bilayers, whereas channels in patches were unaffected. Neither GTP nor UTP reduced Po of channels in artificial bilayers. It is possible that these differences may be due to a lack of molecular identity between the channels studied in the two protocols. Alternatively, they may be attributed to alterations in channel properties during reconstitution or to influences of the artificial lipid environment.

Predicting skin permeability

Published permeability coefficient (Kp) data for the transport of a large group of compounds through mammalian epidermis were analyzed by a simple model based upon permeant size [molecular volume (MV) or molecular weight (MW)] and octanol/water partition coefficient (Koct). The analysis presented is a facile means to predict the percutaneous flux of pharmacological and toxic compounds solely on the basis of their physiocochemical properties. Furthermore, the derived parameters of the model have assignable biophysical significance, and they provide insight into the mechanism of molecular transport through the stratum corneum (SC). For the very diverse group of chemicals considered, the results demonstrate that SC intercellular lipid properties alone are sufficient to account for the dependence of Kp upon MV (or MW) and Koct. It is found that the existence of an "aqueous-polar (pore) pathway" across the SC is not necessary to explain the Kp values of small, polar nonelectrolytes. Rather, their small size, and consequently high diffusivity, accounts for their apparently larger-than-expected Kp. Finally, despite the size and breadth of the data set (more than 90 compounds with MW ranging from 18 to greater than 750, and log Koct ranging from -3 to +6), the postulated upper limiting value of Kp for permeants of very high lipophilicity cannot be determined. However, the analysis is able to define the physicochemical characteristics of molecules which should exhibit these maximal Kp values.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of short-chain phospholipids on the acetylcholine-activated ion channel

The effects of a homologous series of short-chain phospholipids, the phosphatidylcholines from dihexanoylglycerophosphocholine (Hxo2GroPCho) to didecanoylglycerophosphocholine, on the nicotinic acetylcholine-activated ion channel in cultured rat muscle cells were investigated. Standard patch-clamp techniques were used to measure single-channel currents in excised patches. All phospholipids investigated markedly reduced the frequency of channel opening in a concentration-dependent manner. Other parameters, such as the mean open time, the duration and frequency of brief closures within an opening, and channel amplitude, were not significantly affected. This effect was independent of the side of the membrane to which the phospholipid was added. Dose/response curves were obtained for Hxo2-, diheptanoyl(Hpo2)- and dinonanoyl(Nno2)GroPCho. The concentration leading to 50% reduction in channel activity decreased upon ascending the homologous series from 16.69 microM Hxo2GroPCho to 4.52 microM and 0.043 microM for Hpo2- and Nno2GroPCho, respectively. The more hydrophobic the molecule the more effective it was, and hence the higher its affinity to the binding site. Calculation of the standard free-energy change of adsorption into the site led to a value of -3.1 kJ/mol, which indicates a very hydrophobic binding site. In conclusion, the phospholipids interact in a non-specific way with the lipid membrane thereby disturbing proper channel function.

Effects of small organic molecules on phospholipid phase transitions

Small organic molecules are known to exhibit a wide spectrum of physiological or pharmacological effects and many of them are thought to be membrane associated. Therefore a great number of studies is devoted to the interaction between these molecules and phospholipid model membranes. Results obtained for molecular species of varying hydrophobic/hydrophilic balances will be described. It will be shown that, in general, these different molecules induce similar effects on phospholipid phase transitions, although they are located differently in the membrane. Detailed studies of these interactions will help to understand these processes on a molecular level.

Menthol blocks dihydropyridine-insensitive Ca2+ channels and induces neurite outgrowth in human neuroblastoma cells

Voltage-gated Ca2+ channels were identified in LA-N-5 human neuroblastoma cells using the Ca2+ sensitive fluorescent probe, fura-2. Using a variety of "classical" Ca2+ channel blockers, we have demonstrated the presence of both dihydropyridine (DHP)-sensitive and -insensitive channel types that can be activated by depolarization of the cells with either high K+ or gramicidin in the bathing solution. Brief exposure of LA-N-5 cells to menthol blunted the depolarization-induced Ca2+ influx though both DHP-sensitive and DHP-insensitive channels. This effect is concentration dependent (50% maximal blocking effect with 0.25 mM menthol), rapid in onset, and readily reversible. The specificity of the Ca2(+)-channel blocking effect of menthol was demonstrated in parallel studies using compounds with similar structures: menthone blocked Ca2+ channels with about half the potency of menthol, while cyclohexanol was without effect. Addition of either menthol or menthone to LA-N-5 cultures induced neurite outgrowth, cellular clustering, and reduction of cell growth in a dose-dependent fashion that correlated with the ability of these compounds to inhibit the DHP-insensitive Ca2+ influx. Cyclohexanol had no biologic activity. Taken together, the parallel potency for blockade of DHP-insensitive Ca2+ influx with the biologic activity of menthol suggests a role for certain types of Ca2+ channels in triggering growth and morphologic changes in LA-N-5 cells.

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)

Chain-length dependence of n-alkane solubility in phosphatidylcholine bilayers: a 2H-NMR study

Phosphatidylcholine bilayer membranes containing 2H-labelled n-alkanes have been studied by 2H-NMR as a model system for the investigation of molecular theories of general anesthesia. The solubilities of n-alkanes in lipid bilayers have been determined by measurement of the relative intensities of a powder pattern signal arising from orientationally ordered, membrane-soluble alkane and a sharp signal in the 2H-NMR spectrum resulting from isotropically reorienting alkane. The ordering profiles for the ordered n-alkane as determined from the quadrupole splittings for different segments along the chain are similar to those described earlier for n-hexane, n-octane and n-dodecane, suggesting that the restricted motions undergone by the n-alkanes of chain length from 6 to 19 are basically similar. For this homologous series of n-alkanes, it was found that membrane solubility dropped sharply at an alkane chain length which depended on lipid chain length, degree of unsaturation, cholesterol concentration in the bilayer, and temperature. The results show that the incorporation of n-alkanes in lipid bilayers is a complex function of lipid composition. The implications of these results in relationship to the observed 'cut-off' in anesthetic potency in the n-alkane homologous series are discussed.

The effects of homologous series of anaesthetics on a resting potassium conductance of the squid giant axon

The effects of n-alkanes (n-pentane to n-octane), n-alkanols (n-pentanol to n-undecanol) and two carboxylic esters (methyl pentanoate and methyl octanoate) on the conductance of squid giant axons in a high potassium, zero sodium bathing solution have been examined. Sodium and delayed rectifier potassium channels were as far as possible pharmacologically blocked. A substantial fraction of the measured conductance is attributed to a recently-described, voltage-independent, potassium channel. Anaesthetics block this channel but its sensitivity is markedly different from those of other squid axon ion channels.

The interaction of homologous series of alkanols with sodium channels in nerve membrane vesicles

The potency of members of the homologous series of alkanols to block 22Na uptake through sodium channels stimulated by veratridine was studied in membrane vesicles obtained from lobster walking leg nerves. A cut-off was revealed at the level of 1-undecanol. However, secondary isomers of inactive primary homologues, such as 5-dodecanol and 5-tridecanol, were able to block ion flux. From the concentration required for an equipotent effect, it was calculated that the standard free energy for adsorption of primary alkanols is -725 cal/mol CH2. Furthermore, since the concentration required for an equipotent effect for primary isomer was found to be lower than that obtained for secondary isomers, it is concluded that the latter are less potent than the former. The similarity between this set of results and those obtained in intact frog sciatic nerve (J. Requena et al., J. Membrane Biol., 84:229-238, 1985) offers further support to the notion that the procedure employed to isolate the membrane vesicles does preserve the Na channels. However, the mechanism of alcohol inhibition of the Na channel in isolated membrane vesicles would seem to be somewhat different from that preferred in axons. While in vesicles the block needs to be thought in terms of a reduction in the number of conducting Na channel, in axons this is considered to be the less likely mode of action, mainly because under veratridine it is not possible to invoke a shift in the steady-state activation or inactivation.

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.

Changes in sleep and wakefulness following single and repeated exposures to toluene vapor in rats

Male rats with indwelling electrodes for electroencephalographic (EEG), electromyographic (EMG) and electrooculographic (EOG) recordings were exposed via inhalation to 900 ppm and 2700 ppm toluene vapor continuously for a 8-h period or repeatedly for 3 weeks at a rate of 8 h/day and 5 days/week. Rats exposed to a clean air-stream under the same exposure schedules served as controls. Polygraphic recordings were made on 3 consecutive days after cessation of the single 8-h and repeated 3-week exposures to 900 ppm and 2700 ppm toluene vapor or clean airstream. Amounts of time spent in wakefulness (W), slow-wave sleep (SWS) and paradoxical sleep (PS) were quantified by visual inspection of the polygraphic records. Single exposure to toluene produced a prolonged PS latency and a long-lasting increase in SWS at the expense of depressed W, whereas repeated exposures prolonged both SWS and PS latencies, abolished the initial increase of SWS and increased the light-phase level of W on Days 1 and 2. The prolonged PS latency and the decreased light-phase PS on Day 2 induced by single exposure to toluene still persisted after repeated exposures. There were no statistically significant differences in attenuation of brain and blood toluene levels between single and repeated exposures to toluene vapor of 900 ppm and 2700 ppm.

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.

Evaluation of surface tension and ion occupancy effects on gramicidin A channel lifetime

The surface tension of glycerylmonooleate-hexadecane lipid bilayer membranes and the lifetime of gramicidin A channels were measured at various concentrations of the surrounding solutions. For HCl the surface tension is essentially constant at approximately 5 mN/m up to approximately 1 M, whereas the average lifetime increases approximately 40-fold. At higher concentrations the surface tension decreases markedly. For CsCl the surface tension is constant up to about 1 M then increases with salt level. The average lifetime in this case increases about sixfold. In both cases the lifetime levels off and even decreases at higher salt levels. The increase in lifetime observed with ion activity is therefore qualitatively different from, and not explained by, the established dependence of lifetime on membrane properties (Elliot, J.R., D. Needham, J.P. Dilger, and D.A. Haydon. 1983. Biochim. Biophys. Acta. 735:95-103). We have previously proposed that ion occupancy is a determinant of channel stability, and to test this hypothesis the voltage dependence of channel lifetime was measured in asymmetrical solutions. For the case of a potassium chloride solution on one side of the membrane and a hydrogen chloride solution, on the other, the voltage dependence of the lifetime is asymmetrical. The asymmetry is such that when the electrical field is applied in the direction of the chemical gradient for each of the ions, the channel lifetime approaches, at increasing field strengths, that of a symmetrical solution of the respective ion. The voltage dependence of the surface tension, on the other hand, is negligible for the range of voltages used. These results, and the earlier findings that the order of the lifetimes for the alkali cations generally agree with the order of the permeability selectivity of the gramicidin A channel, support the hypothesis that ion occupancy is a major factor determining the lifetime of gramicidin A channels. The effects of multivalent blockers and osmotic agents were also tested. Ba2", La3+,and Mg2" decrease the lifetime and conductance markedly. Sucrose and urea increase the lifetime and decrease the conductance. The voltage dependence of the lifetime in symmetrical solutions was examined. Contrary to previous reports it was found that the lifetimes for K+, Cs', and H+ are voltage dependent. For 0.5 M HCI the lifetime decreases monotonically by .60% at 150 mV, and for 0.5 M KCI the lifetime increases by -60% at 200 mV. Below 10 mM there is no effect of voltage for H+, K+, and Cs+. These effects of blockers, osmotic agents, and voltage on the lifetime, as well as the lack of effect of voltage at low salt levels, are consistent with the occupancy hypothesis.

Synergistic stimulation of pregnenolone synthesis in rat adrenal mitochondria by n-hexane and cardiolipin

n-Hexane and cardiolipin each stimulate pregnenolone production by isolated rat adrenal mitochondria. Following corticotropin (ACTH) stimulation, mitochondrial cholesterol metabolism exhibits a fast phase lasting 2 min, followed by a 10-fold slower metabolism. ACTH suppression by dexamethazone or cycloheximide (CX) treatment removes this fast phase. n-Hexane, at concentrations approaching 80% of the aqueous solubility limit (approximately 0.08 mM), selectively stimulates the slow phase of metabolism, while cardiolipin (100 microM) stimulates only the fast phase. Other alkanes and ethers are effective. The effect of n-hexane is dependent on mitochondrial integrity, as evidenced by decreased effects in hypoosmotically shocked mitochondria (outer membrane disrupted) and ineffectiveness in sonicated mitochondria (both membranes disrupted). n-Hexane apparently enhances the transfer of outer membrane cholesterol to inner membrane P-450scc. Stimulation by cardiolipin is retained by disrupted mitochondria and may involve enhanced availability of P-450scc to inner membrane cholesterol. When added together, these agents produce more than additive effects on cholesterol metabolism. Preincubation with n-hexane did not increase reactive cholesterol, suggesting that enhanced cholesterol transport occurs only in concert with metabolism of inner membrane cholesterol. Uptake of alkanes into mitochondrial membranes may effect structural changes that facilitate outer to inner membrane cholesterol transfer, but major changes are excluded by the effectiveness of isocitrate as a reductant for P-450scc. In combination, n-hexane and cardiolipin reproduce the effect of the ACTH-sensitive sterol regulatory peptide on mitochondria [R. C. Pedersen and A. C. Brownie (1983) Proc. Natl. Acad. Sci. USA 80, 1882-1886], suggesting that peptide action on adrenal mitochondria may resolve into two analogous components.

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.

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)

Effect of menthol on two types of Ca currents in cultured sensory neurons of vertebrates

The effect of menthol on voltage-dependent Ca currents was investigated in cultured dorsal root ganglion cells from chick and rat embryos. Bath application of menthol (0.1-1 mM) had different effects on the various Ca currents present in these neurons. Below -20 mV, the low threshold Ca currents were reduced in amplitude in a dose-dependent manner by menthol with little changes of their activation kinetics. In contrast to this, the time course of inactivation of the high-threshold Ca currents, activated above -20 mV from a holding potential of -80 mV, was drastically accelerated by external menthol. The action of menthol was unchanged with more positive holding potentials (-50 mV). Thus, a proposed third type of Ca current with transient activation and complete deactivation below -50 mV was either not present or not affected by menthol. Menthol exerted its action only when applied from the outside. Its effect was completely reversible within 15-20 min of wash-out. Our findings are consistent with the idea that menthol acts on two types of Ca channels coexisting on the membrane of cultured sensory neurons. Menthol blocks currents through the low voltage-activated Ca channel, and facilitates inactivation gating of the classical high voltage-activated Ca channel.

Petroleum hydrocarbon toxicity in vitro: effect of n-alkanes, benzene and toluene on pulmonary alveolar macrophages and lysosomal enzymes of the lung

The in vitro effects of straight chain alkanes (nC6-nC10), benzene and toluene on pulmonary alveolar macrophages (PAM) of rats and rabbits was studied. The concentrations used ranged from 0.02 to 1.0 mM. All hydrocarbons used in the study were cytotoxic to isolated cultured PAM cells in a dose-dependent manner. The LC50 for these hydrocarbons towards rat PAM cells was estimated to be 1.0 mM for nC8, 2 mM for nC7, 5 mM for nC9 and 10 mM for nC6, nC10, benzene and toluene. Rabbit PAM cells were more sensitive to the hydrocarbons, resulting in and LC50 half that for rat PAM cells. Hydrocarbons also caused extracellular release of the lysosomal enzymes cathepsin D (EC 3.4.23.5) and cathepsin B (EC 3.4.22.1) in a manner corresponding with cell damage. There was more cathepsin D activity released from cells than cathepsin B. In addition, hydrocarbons also caused the release of cathepsin B and D from isolated lysosomes, and there was 10-15% more enzyme activity released in the culture medium of lysosomes exposed to concentrations of 0.5 and 1.0 mM compared to PAM cell cultures of either rats or rabbits. Hydrocarbons also caused loss of cell respiration and stimulated a dose-dependent and a time-dependent increase in lipid peroxidation. The two alkanes nC7 and nC8 caused the greatest increase in lipid peroxidation and the greatest loss of cell respiration. The results indicate that there is a relationship between chain length of alkanes and their cytotoxicity to PAM cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of n-pentane on voltage-clamped squid nerve sodium currents. A reinterpretation using kinetics of ordered systems

The sodium-current voltage-clamp data of Haydon and Kimura obtained on squid nerves treated with n-pentane (J. Physiol. 312 (1981) 57) are fitted with a previously described model (K.A. Rubinson, J. Physiol. 281 (1978) 14P; Biophys. Chem. 15 (1982) 245). The apparently complex action of the perturbant can be interpreted as due to a shift in shielding of the applied potential jumps, a change in channel conductivity, and an increase in the rate constant of channel shutoff. The shift in shielding due to n-pentane is found to be quantitatively the same for variables describing both kinetic and equilibrium quantities, which are independent. The transmembrane sodium potential remains unchanged, however.

Calcium channel current inactivation is selectively modulated by menthol

The effect of menthol on Ca channel current inactivation was studied in identified Helix neurons. External application of menthol accelerated the Ca-dependent rapid phase of inactivation. Menthol restored a fast inactivation phase after the Ca-dependent inactivation had been removed by strongly buffering changes in intracellular free Ca or by using Ba ions as current carriers. The menthol-induced inactivation was unchanged by variations in intracellular free Ca. A sensitizing effect of menthol on Ca-dependent inactivation appeared unlikely. Instead the results indicate a modulating action of menthol on Ca inactivation.

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.