The effects of pressure on excitable cells

The living state: How cellular excitability is controlled by the thermodynamic state of the membrane

The thermodynamic (TD) properties of biological membranes play a central role for living systems. It has been suggested, for instance, that nonlinear pulses such as action potentials (APs) can only exist if the membrane state is in vicinity of a TD transition. Herein, two membrane properties in living systems - excitability and velocity - are analyzed for a broad spectrum of conditions (temperature (T), 3D-pressure (p) and pH-dependence). Based on experimental data from Characean cells and a review of literature we predict parameter ranges in which a transition of the membrane is located (15-35°C below growth temperature; 1-3pH units below pH7; at ∼800atm) and propose the corresponding phase diagrams. The latter explain: (i) changes of AP velocity with T,p and pH.(ii) The existence and origin of two qualitatively different forms of loss of nonlinear excitability ("nerve block", anesthesia). (iii) The type and quantity of parameter changes that trigger APs. Finally, a quantitative comparison between the TD behavior of 2D-lipid model membranes with living systems is attempted. The typical shifts in transition temperature with pH and p of model membranes agree with values obtained from cell physiological measurements. Taken together, these results suggest that it is not specific molecules that control the excitability of living systems but rather the TD properties of the membrane interface. The approach as proposed herein can be extended to other quantities (membrane potential, calcium concentration, etc.) and makes falsifiable predictions, for example, that a transition exists within the specified parameter ranges in excitable cells.

Insights into high-pressure acclimation: comparative transcriptome analysis of sea cucumber Apostichopus japonicus at different hydrostatic pressure exposures

Background

Global climate change is predicted to force the bathymetric migrations of shallow-water marine invertebrates. Hydrostatic pressure is proposed to be one of the major environmental factors limiting the vertical distribution of extant marine invertebrates. However, the high-pressure acclimation mechanisms are not yet fully understood.

Results

In this study, the shallow-water sea cucumber Apostichopus japonicus was incubated at 15 and 25 MPa at 15 °C for 24 h, and subjected to comparative transcriptome analysis. Nine samples were sequenced and assembled into 553,507 unigenes with a N50 length of 1204 bp. Three groups of differentially expressed genes (DEGs) were identified according to their gene expression patterns, including 38 linearly related DEGs whose expression patterns were linearly correlated with hydrostatic pressure, 244 pressure-sensitive DEGs which were up-regulated at both 15 and 25 MPa, and 257 high-pressure-induced DEGs which were up-regulated at 25 MPa but not up-regulated at 15 MPa.

Conclusions

Our results indicated that the genes and biological processes involving high-pressure acclimation are similar to those related to deep-sea adaptation. In addition to representative biological processes involving deep-sea adaptation (such as antioxidation, immune response, genetic information processing, and DNA repair), two biological processes, namely, ubiquitination and endocytosis, which can collaborate with each other and regulate the elimination of misfolded proteins, also responded to high-pressure exposure in our study. The up-regulation of these two processes suggested that high hydrostatic pressure would lead to the increase of misfolded protein synthesis, and this may result in the death of shallow-water sea cucumber under high-pressure exposure.

Comparative transcriptome analysis of Eogammarus possjeticus at different hydrostatic pressure and temperature exposures

Hydrostatic pressure is an important environmental factor affecting the vertical distribution of marine organisms. Laboratory-based studies have shown that many extant shallow-water marine benthic invertebrates can tolerate hydrostatic pressure outside their known natural distributions. However, only a few studies have focused on the molecular mechanisms of pressure acclimatisation. In the present work, we examined the pressure tolerance of the shallow-water amphipod Eogammarus possjeticus at various temperatures (5, 10, 15, and 20 °C) and hydrostatic pressures (0.1–30 MPa) for 16 h. Six of these experimental groups were used for transcriptome analysis. We found that 100% of E. possjeticus survived under 20 MPa at all temperature conditions for 16 h. Sequence assembly resulted in 138, 304 unigenes. Results of differential expression analysis revealed that 94 well-annotated genes were up-regulated under high pressure. All these findings indicated that the pressure tolerance of E. possjeticus was related to temperature. Several biological processes including energy metabolism, antioxidation, immunity, lipid metabolism, membrane-related process, genetic information processing, and DNA repair are probably involved in the acclimatisation in deep-sea environments.

Pressure Effects on Artificial and Cellular Membranes

We review the combined effect of temperature and pressure on the structure, dynamics and phase behavior of lipid bilayers, differing in chain length, headgroup structure and composition as revealed by thermodynamic, spectroscopic and scattering experiments. The effect of additives, such as ions, cholesterol, and anaesthetics is discussed as well. Our data include also reports on the effect of pressure on the lateral organization of heterogeneous lipid membranes and lipid extracts from cellular membranes, as well as the influence of peptide and protein incorporation on the pressure-dependent structure and phase behavior of lipid membranes. Moreover, the effects of pressure on membrane protein function are summarized. Finally, we introduce pressure as a kinetic variable for studying the kinetics of various lipid phase transformations.

Explaining bathymetric diversity patterns in marine benthic invertebrates and demersal fishes: physiological contributions to adaptation of life at depth

Bathymetric biodiversity patterns of marine benthic invertebrates and demersal fishes have been identified in the extant fauna of the deep continental margins. Depth zonation is widespread and evident through a transition between shelf and slope fauna from the shelf break to 1000 m, and a transition between slope and abyssal fauna from 2000 to 3000 m; these transitions are characterised by high species turnover. A unimodal pattern of diversity with depth peaks between 1000 and 3000 m, despite the relatively low area represented by these depths. Zonation is thought to result from the colonisation of the deep sea by shallow-water organisms following multiple mass extinction events throughout the Phanerozoic. The effects of low temperature and high pressure act across hierarchical levels of biological organisation and appear sufficient to limit the distributions of such shallow-water species. Hydrostatic pressures of bathyal depths have consistently been identified experimentally as the maximum tolerated by shallow-water and upper bathyal benthic invertebrates at in situ temperatures, and adaptation appears required for passage to deeper water in both benthic invertebrates and demersal fishes. Together, this suggests that a hyperbaric and thermal physiological bottleneck at bathyal depths contributes to bathymetric zonation. The peak of the unimodal diversity-depth pattern typically occurs at these depths even though the area represented by these depths is relatively low. Although it is recognised that, over long evolutionary time scales, shallow-water diversity patterns are driven by speciation, little consideration has been given to the potential implications for species distribution patterns with depth. Molecular and morphological evidence indicates that cool bathyal waters are the primary site of adaptive radiation in the deep sea, and we hypothesise that bathymetric variation in speciation rates could drive the unimodal diversity-depth pattern over time. Thermal effects on metabolic-rate-dependent mutation and on generation times have been proposed to drive differences in speciation rates, which result in modern latitudinal biodiversity patterns over time. Clearly, this thermal mechanism alone cannot explain bathymetric patterns since temperature generally decreases with depth. We hypothesise that demonstrated physiological effects of high hydrostatic pressure and low temperature at bathyal depths, acting on shallow-water taxa invading the deep sea, may invoke a stress-evolution mechanism by increasing mutagenic activity in germ cells, by inactivating canalisation during embryonic or larval development, by releasing hidden variation or mutagenic activity, or by activating or releasing transposable elements in larvae or adults. In this scenario, increased variation at a physiological bottleneck at bathyal depths results in elevated speciation rate. Adaptation that increases tolerance to high hydrostatic pressure and low temperature allows colonisation of abyssal depths and reduces the stress-evolution response, consequently returning speciation of deeper taxa to the background rate. Over time this mechanism could contribute to the unimodal diversity-depth pattern.

Flying-patch patch-clamp study of G22E-MscL mutant under high hydrostatic pressure

High hydrostatic pressure (HHP) present in natural environments impacts on cell membrane biophysical properties and protein quaternary structure. We have investigated the effect of high hydrostatic pressure on G22E-MscL, a spontaneously opening mutant of Escherichia coli MscL, the bacterial mechanosensitive channel of large conductance. Patch-clamp technique combined with a flying-patch device and hydraulic setup allowed the study of the effects of HHP up to 90 MPa (as near the bottom of the Marianas Trench) on the MscL mutant channel reconstituted into liposome membranes, in addition to recording in situ from the mutant channels expressed in E. coli giant spheroplasts. In general, against thermodynamic predictions, hydrostatic pressure in the range of 0.1-90 MPa increased channel open probability by favoring the open state of the channel. Furthermore, hydrostatic pressure affected the channel kinetics, as manifested by the propensity of the channel to gate at subconducting levels with an increase in pressure. We propose that the presence of water molecules around the hydrophobic gate of the G22E MscL channel induce hydration of the hydrophobic lock under HHP causing frequent channel openings and preventing the channel closure in the absence of membrane tension. Furthermore, our study indicates that HHP can be used as a valuable experimental approach toward better understanding of the gating mechanism in complex channels such as MscL.

Neuronal sensitivity to hyperoxia, hypercapnia, and inert gases at hyperbaric pressures

As ambient pressure increases, hydrostatic compression of the central nervous system, combined with increasing levels of inspired Po2, Pco2, and N2 partial pressure, has deleterious effects on neuronal function, resulting in O2 toxicity, CO2 toxicity, N2 narcosis, and high-pressure nervous syndrome. The cellular mechanisms responsible for each disorder have been difficult to study by using classic in vitro electrophysiological methods, due to the physical barrier imposed by the sealed pressure chamber and mechanical disturbances during tissue compression. Improved chamber designs and methods have made such experiments feasible in mammalian neurons, especially at ambient pressures <5 atmospheres absolute (ATA). Here we summarize these methods, the physiologically relevant test pressures, potential research applications, and results of previous research, focusing on the significance of electrophysiological studies at <5 ATA. Intracellular recordings and tissue Po2 measurements in slices of rat brain demonstrate how to differentiate the neuronal effects of increased gas pressures from pressure per se. Examples also highlight the use of hyperoxia (<or=3 ATA O2) as a model for studying the cellular mechanisms of oxidative stress in the mammalian central nervous system.

Ultrastructural localization of nitric oxide synthase and endothelin in the renal and mesenteric arteries of the golden hamster: differences during and after arousal from hibernation

This is a study of the electron-immunocytochemical localization of nitric oxide synthase (type III) and endothelin in renal and mesenteric artery endothelial cells of normal (active) and hibernating hamsters, as well as hamsters exposed to the cold but not hibernating, and hamsters aroused for 2h following hibernation. In the renal artery of hibernating hamsters and cold-exposed hamsters, a subpopulation of nitric oxide synthase-positive endothelial cells displayed immunoprecipitate predominantly in the vicinity of the Golgi complex indicating intracellular translocation from the cytoplasm to the Golgi complex. In hibernating animals, the percentages of both nitric oxide synthase-positive and endothelin-positive endothelial cells were notably lower than those observed either in active, cold-exposed or aroused animals. These changes may reflect a reduced endothelial contribution to the maintenance of vascular tone in these vessels during hibernation and an upregulation of expression of nitric oxide synthase and endothelin in the endothelium early on during arousal from hibernation.

Structural and electrophysiological effects of local anesthetics and of low temperature on myelinated nerves: implication of the lipid chains in nerve excitability

X-ray scattering and electrophysiological experiments performed on toad sciatic nerves as a function of the exposure to either low temperature or tetracaine yielded the following results: (i) the main structural effect is to thicken the individual membranes, thus to stiffen the acyl chains and increase the repeat distance of the one-dimensional lattice, phenomena that are typical of lipid-containing systems with disordered chains; (ii) the electrophysiological effect is to decrease the amplitude and velocity of the compound action potential; (iii) the structural and physiological effects of the two agents are practically identical. Since the structural and the electrophysiological parameters have different origins in the nerves (the structure regards the myelin sheath, the electrical signals originate at the nodes of Ranvier) it is inferred that tetracaine and low temperature exert similar effects on the membranes of both the myelin sheath and the nodes of Ranvier. Also, since local anesthetics act by inhibiting the Na+ channels, these observations suggest that the acyl chain conformation modulates the channel function and thus the generation of action potential.

Regulation of potassium conductance in the cellular membrane at early embryogenesis

At the early stages of development of the fresh water fish loach (Misgurnus fossilis) the resting membrane potential (Er) of cleaving cells oscillates periodically with an amplitude of 8-12 mV. Er oscillation correlates with the cell cycle and is accompanied by changes of K+ conductivity. Two types of K(+)-selective ionic channels with conductance of approximately 70 and 25 pS in symmetrical (150 mM KCl) solution were observed in the membrane of cleaving loach embryos. 'High' conductance and 'low' conductance channels were recorded in approximately 90% and 10% of patches investigated (n = 275), respectively? The activity of 'high' conductance channels was regulated by the application of pressure to the membrane, ie these channels were stretch-activated (SA). The activity of SA channels changes dramatically during the cell-cleavage cycle. At the beginning of interphase the probability of SA channels being in the open state (P0) was minimal, while at prometaphase the probability was increased 10-100-fold. Application of ATP to the cytoplasmic inside-out patches induced a reversible elevation of stretch sensitivity of the SA channels in 50% of the patches, while the non-hydrolyzable analogue of ATP was not effective. Combined application of ATP, cAMP and cAMP-dependent protein kinase (PK) induced a reversible elevation in the SA channel activity while inhibitors of PK prevented its activating effects. Phosphatase inhibitors prolonged the activating effect of PK on SA channels. We propose that oscillations of the resting potential during the cell-cleavage cycle arise due to modulation of SA channel sensitivity to stretch through cAMP-dependent phosphorylation.

Effects of isolation and high helium pressure on the nucleolus of sympathetic neurons in the rat superior cervical ganglion

In prokaryotes, unicellular eukaryotes and cell-free systems, pressure is known to exert an inhibitory effect on protein synthesis and RNA metabolism, the mechanism(s) of which remain to be investigated in detail. The purpose of the present in vitro study was to compare ultrastructural and quantitative changes of the nucleolus, which is the site of ribosome biogenesis, in sympathetic neurons of rat superior cervical ganglia (SCG) maintained for 2, 3 and 5 h in NCTC 109 medium and subjected to pressure or not. In control SCG (left) the nucleolus greatly increased in volume (+ 33%) 2 h after excision, in comparison with SCG fixed immediately. This overall enlargement was found to reflect a marked increase in all nucleolar components (from 16 to 87%). After 5 h, volumes of nucleolus, fibrillar centers and vacuolar component returned to control values, whereas dense fibrillar and granular components remained affected. Such early and transient changes are regarded as reflecting basic metabolic changes associated with increased nucleolar RNA that should be of primary concern to experiments using SCG transplanted in culture media. Compression under helium up to 180 atmospheric pressure for 1 h of right SCG maintained for 2 h in culture medium, was shown to induce, on the contrary, a marked decrease in nucleolar volume (-39%) and in volumes of all nucleolar components (from -36 to -51%). When they were kept at constant high pressure for 1 and 3 h a progressive recovery of volumes of nucleoli and nucleolar components was observed. Consequently, compression was shown to exert opposite effects to those of isolation of SCG. Present data are interpreted as an inhibitory effect of pressure on ribosome biogenesis. Such observations on a vertebrate neuron might open a new field in the search for cellular mechanisms underlying the effects of pressure on living organisms and especially on the nervous system.

Pressure study on thermal transitions of oleic acid polymorphs by high-pressure differential thermal analysis

Transition temperatures for oleic acid polymorphs (alpha, beta and gamma) and the melt were measured under various pressures up to 200 MPa by means of high-pressure differential thermal analysis. The pressure dependences of the transition temperatures were analyzed by applying the Clapeyron equation, and the volume changes associated with the phase transitions were estimated as follows; delta V gamma-alpha = 5.9 cm3/mol, delta V alpha-melt = 29.2 cm3/mol and delta V beta-melt = 41.1 cm3/mol. The volume changes for the gamma-alpha and alpha-melt transitions were discussed in relation to the crystal structures of the alpha and gamma modifications. The experiments showed that the beta modification, whose structural data are lacking, is the most dense. This correlates with the fact that the beta form is thermodynamically most stable among the three modifications of oleic acid.

Stretch-activated ion channels modulate the resting membrane potential during early embryogenesis

By using the patch-clamp technique, stretch-activated ionic channels were found in the membrane of cleaving freshwater fish embryos at the early stages of embryogenesis (2-256 cells). The application of negative pressure to the pipette increased the frequency of activation and the duration of bursts. This type of channel has a preferential K+ selectivity. When bathed on both membrane surfaces with 140 mM KCl the channel conductance was 71 pS. The kinetic behaviour did not depend markedly on either membrane potential (in the range from -70 to +70 mV) or calcium concentration on the cytoplasmic side of the membrane. On continuous recording, the probability of the channel being open was found to change periodically over a 5- to 20-fold range for different cells. These variations correlated with changes in resting potential and membrane conductance during the cell cycle. These results suggest that the oscillation of resting potential within the cell cycle is associated with the operation of stretch-activated ion channels.

Decrease of recurrent and feed-forward inhibitions under high pressure of helium in rat hippocampal slices

The effect of high helium pressure on inhibitory synaptic transmission was studied in rat hippocampal slices with extracellular recordings. Both feed-forward and recurrent GABAergic inhibition were tested in the CA1 region with paired-pulse stimulation paradigms. The efficiency of both types of inhibition decreased under high pressure (80 atm). However, the depression of synaptic and antidromic field potentials induced by perfusion of GABA or muscimol were not significantly affected by pressure. High pressure induced hyperexcitability of CA1 pyramidal cells. This effect was reduced by the application of 2-aminophosphonovalerate or GABA. The present results suggest that: (1) high pressure reduces the efficiency of the GABAergic inhibitory transmission but does not affect the sensitivity of GABAA receptors; (2) two different processes (reduction of GABAergic inhibition and facilitation of N-methyl-D-aspartate-mediated excitation) might be a direct consequence of the change in the voltage-sensitive ion channels under high pressure and might be involved in the development of the pressure-induced hyperexcitability of CA1 pyramidal cells.

Effects of high pressure on nervous conduction velocity in man

The velocities of motor and sensory nervous conduction and of neuro-muscular transmission were measured in four subjects during a simulated dive at 4.6 MPa (46 bars). The results show an increase in motor distal latency in the ulnar nerve, especially during decompression, with reversibility of the effect on return to ambient conditions. The hypothesis of an interaction of dissolved gases with the membranes of ischaemic cells is proposed.

Altered cell-averaged microviscosity of murine peritoneal macrophages undergoing activation in vivo or in vitro

The cell-averaged microviscosity of intact murine peritoneal mononuclear phagocytes in various stages of activation was assessed by quantifying fluorescent depolarization of 1,6-diphenyl-1,3,5-hexatriene. Macrophages activated in vivo with Mycobacterium bovis, strain BCG, were significantly more fluid than resident peritoneal macrophages, responsive macrophages elicited with thioglycollate broth, proteose peptone broth, or fetal bovine serum, or primed macrophages elicited with pyran copolymer, MVE-2. Specifically, the cell-averaged microviscosity decreased from a mean of 3.47 +/- .07 eta 25 degrees C (poise) (range of 3.32 to 3.67 p) to 2.62 eta 25 degrees C. Exposure of responsive macrophages in vitro to bacterial endotoxin plus hybridoma supernatants containing macrophage-activating factor or purified recombinant interferon gamma resulted in decreased microviscosity; the largest effect was seen after 24 hr. Macrophages primed in vivo with MVE-2 and treated in vitro with endotoxin also developed decreased microviscosity. Similar changes in microviscosity were observed in a plasma membrane-enriched fraction isolated from macrophages activated in vitro with interferon gamma and endotoxin, thus suggesting that the cell-averaged measurements reflected changes in membrane viscosity. The optimum concentration of MAF-inducing decreased overall microviscosity was identical to that for inducing tumoricidal capacity. Taken together, the data indicate activation of lytic capacity in murine macrophages is closely associated with decreased cell-averaged microviscosity and that this change reflects, at least in part, decreased microviscosity of the plasma membrane of these cells.

The action of oxygen and oxygen at high pressure on inhibitory transmission

The effect of 100% oxygen at ambient pressure, 100% oxygen at 1.7 Atmospheres Absolute (ATA), 100% oxygen at 5.1 ATA, helium at 1.7 ATA and helium at 5.1 ATA on inhibitory synaptic transmission was studied using the lobster walking leg neuromuscular preparation. Exposure to 100% oxygen at ambient pressure, at 1.7 ATA or at 5.1 ATA produced a decrease in inhibitory transmission manifest as a fall in inhibitory synaptic conductance (Ginh). The largest decrease in Ginh was seen in 100% oxygen at ambient pressure, while a progressively smaller decrease was seen in 100% oxygen at 1.7 ATA and 5.1 ATA, respectively. Also associated with 100% oxygen at ambient pressure was the disappearance of inhibitory junction potentials. Pressurization with helium produced a fall in Ginh at 5.1 ATA but no change or a slight increase at 1.7 ATA. The action of either 100% oxygen at ambient and at 1.7 or 5.1 ATA or helium at 1.7 or 5.1 ATA was shown to be on presynaptic parameters since the percent decrease in Ro induced by exogenous application of gamma-aminobutyric acid (GABA), the inhibitory transmitter, was the same in either 100% oxygen at ambient pressure, 100% oxygen or helium at 1.7 ATA and 5.1 ATA. The similarity in action of oxygen to the action of isoniazid, a known glutamic acid decarboxylase (the enzyme that catalyzes the production of GABA) inhibitor in the same preparation suggests that one possible site of oxygen action is on GABA production.

Effects of high-pressure helium on gamma-[3H]aminobutyric acid release from the isolated frog spinal cord

The application of high pressure in vivo causes a hyperexcitability syndrome involving tremors and convulsions. Drugs that potentiate GABA transmission protect animals against this syndrome. It is possible that changes in gamma-aminobutyric acid (GABA) transmission may underlie the hyperexcitability. We have therefore investigated the effects of pressure on the components of GABA transmission in vitro. After incubation with [3H]GABA, hemisected frog spinal cords were superfused inside a pressure chamber and perfusate fractions were collected every 10 min. Helium, at 50 or 100 atm, did not alter the spontaneous release of GABA, but if electrical stimulation had been applied previously, then pressure caused a prolonged increase in GABA release. Helium at 50 atm did not alter the evoked release during electrical stimulation, but at 100 atm this was increased. This increase was smaller in the absence of calcium. No corresponding changes in [14C]urea efflux were seen, suggesting that the effects were not due to nonselective membrane permeability changes. The results are consistent with the known effects of pressure on neuronal activity, such as repetitive firing, but they do not suggest a selective action on the GABA release process.

The effects of pressure and cholesterol on rotational motions of perylene in lipid bilayers

Using steady-state fluorescence polarization measurements, an isothermal pressure-induced phase transition was observed in dimyristoyl-L-alpha-phosphatidylcholine multilamellar vesicles containing perylene. The temperature-to-pressure equivalence, dT/dP, estimated from the phase transition pressure, P1/2, is about 22 K X kbar-1, which is comparable to values determined from diphenylhexatriene polarization (Chong, P.L.-G. and Weber, G. (1983) Biochemistry 22, 5544-5550). In addition, we have employed a new method, introduced in this paper, to calculate the rate of in-plane rotation (Rip) and the rate of out-of-plane rotation (Rop) of perylene in lipid bilayers. The effects of pressure and cholesterol on the rotational rates of perylene in two lipid bilayer systems have been examined. They are 1-palmitoyl-2-oleoyl-L-alpha-phosphatidylcholine (POPC) multilamellar vesicles (MLV) and 50 mol% cholesterol in POPC (MLV). Rop is smaller than Rip due to the fact that the out-of-plane rotation requires a larger volume change than the in-plane rotation. Cholesterol seems not to affect Rop significantly, but pressure causes a decrease in Rop by about a factor of three. In contrast, the effects of pressure and cholesterol on Rip are less straightforward. At 1 atm cholesterol increases Rip by a factor of about two. Similarly, in the absence of cholesterol 1.5 kbar pressure essentially triples Rip. However, if both cholesterol is added and pressure is applied, Rip decreases sharply. The possible interactions between cholesterol and perylene are discussed.

Pressure dependence of sodium gating currents in the squid giant axon

Asymmetric displacement currents, Ig, were measured in squid axons at different hydrostatic pressures, P, up to 60 MPa. Potassium and sodium currents were abolished by intracellular Cs+ and TEA+, by extracellular Tetrodotoxin (TTX), and by Na+ substitution with Tris+. The time course of Ig became progressively slower with increasing pressure, and the amplitude decreased. With appropriate scaling in time and amplitude, Ig records at any given P could be made to superimpose very well with those obtained at atmospheric pressure. The same scaling factors yielded a good superposition of all records obtained for voltage steps to membrane potentials in the range -30 to +42 mV. The ratio between the amplitude and time factors was larger than unity and increased with P, indicating a progressive decrease (up to 35% at 60 MPa) of the total charge displaced, Q, with no significant change in its voltage dependence. The time-scaling factor increased exponentially with P, as expected if all the steps involved in the opening of a sodium channel, and producing a major charge redistribution, have the same activation volume, delta V not equal to g approximately 17 cm3/mol. This value is roughly one-half of that characterizing the pressure dependence of sodium current activation, suggesting that some late, rate-limiting step in the opening of sodium channels has a large activation volume without being accompanied by an easily detected charge movement. Part of the decrease of Q with pressure could be attributed to an increase in sodium inactivation. However, we cannot exclude the possibility that there is a reversible reduction in the number of fast activating sodium channels, similar to the phenomenon that has been reported to occur at low temperatures (Matteson and Armstrong 1982).

Hydrostatic pressure modifies the action of octanol and atropine on frog endplate conductance

The effects of octanol, ethanol and atropine were examined on the time course of decay (tau D) of miniature endplate currents (m.e.p.cs) in the frog neuromuscular junction at normal and high pressure. Octanol (25-100 microM) decreased reversibly the tau D of m.e.p.cs in a dose-dependent manner, 100 microM reducing tau D to 0.39 of the control value. Higher concentrations (200-500 microM) additionally depressed the amplitude of m.e.p.cs. Hydrostatic pressure (3.19 and 5.25 MPa) reduced the tau D of octanol (25-100 microM)-shortened m.e.p.cs. Thus 3.19 MPa and 5.25 MPa reduced the tau D in the presence of 100 microM octanol to 0.75 and 0.78 of the octanol treated values. This effect was not completely reversed on decompression. The m.e.p.c. amplitude is reversibly decreased by pressure in the presence of octanol. Hydrostatic pressure (3.19-15.55 MPa) did not modify the effect of ethanol on tau D. At 10.40 and 15.55 MPa the tau D was increased equally in the absence or presence of ethanol. Atropine (60 microM) reduced the tau D and amplitude of m.e.p.cs to 0.33 and 0.63 of the control values. These effects were completely reversible. Hydrostatic pressure (3.19 and 5.25 MPa) reduced the tau D of atropine-shortened m.e.p.cs to 0.82 and 0.77 of the atropine-treated values respectively. This effect was not completely reversed on decompression. Hydrostatic pressure also reversibly depressed the amplitude of atropine-treated m.e.p.cs. The implications of these drug-hydrostatic pressure interactions are discussed.

Interacting effects of temperature, pressure and cholesterol content upon the molecular order of dioleoylphosphatidylcholine vesicles

Dioleoylphosphatidylcholine (DOPC) multilamellar vesicles containing varying amounts of cholesterol (0-50 mol%) were studied by measuring the polarisation of diphenylhexatriene fluorescence at 6, 23.5, and 35.5 degrees C, and at hydrostatic pressures up to 1.5 kbar . Interactions between temperature and pressure were quantified as the temperature-pressure equivalence which was approximately 19-23 K X kbar -1 for all binary mixtures of cholesterol and DOPC. Polarisation was linearly related to cholesterol/DOPC ratio, except at low temperature. In all cases pressure caused an increase in polarisation (i.e., an increase in molecular order) but did not alter the slope of the graph relating polarisation to cholesterol/DOPC ratio. The relative ordering effect of cholesterol and pressure was quantified by calculating the cholesterol-pressure equivalence. An increase in cholesterol/DOPC ratio of approximately 0.35-0.50 increased polarisation by an amount equivalent to an increase in pressure of 1 kbar . Cholesterol-pressure equivalence tended to decrease as temperature decreased and pressure increased; that is, as membrane order increased.

Effects of pressure on the phase transition of bilayers in liposomes. Influence of cholesterol and alpha-tocopherol

The effects of pressure on the gel-to-liquid crystalline phase transition temperature of dimyristoylphosphatidylcholine (DMPC) bilayers containing cholesterol, alpha-tocopherol, and alpha-tocopheryl acetate were studied by fluorescence depolarization. The transition temperature of cholesterol mixtures (greater than 7.5 mol%) was lower than that of 100% DMPC at atmospheric pressure, but it became higher than the latter on increase in pressure. The thermodynamic parameters of the transition (delta V, delta S, delta H) were estimated and the functions of cholesterol and alpha-tocopherols in the bilayers are discussed.

The effect of temperature on the growth and decay times of miniature end-plate currents in the mouse diaphragm

The temperature dependence of both the growth and the decay phases of miniature end-plate currents was investigated in the mouse diaphragm. The relationship between the log of the time constant of decay and reciprocal temperature was linear, that between the growth phase duration and temperature was not linear, the Q10 being higher over the low end of the temperature range. The possible reasons for the non-uniform Q10 of the growth time of the miniature end-plate current are discussed.

An electrophysiological analysis of oxygen and pressure on synaptic transmission

The effect of oxygen at high pressure (OHP), helium at 150 PSIG and 100% oxygen at ambient pressure on excitatory synaptic transmission was studied using the lobster walking leg neuromuscular preparation. Both 100% oxygen at 150 PSIG (7135 mm Hg oxygen) and helium at 150 PSIG (7000 mm Hg helium plus 135 mm Hg oxygen) produced a significant decrease in the amplitude of the junction potential (Vejp). The decrease in Vejp induced by OHP, however, was greater than with pressure alone. OHP also produced a significant decrease in short term facilitation. Exposure to 100% oxygen at ambient pressure produced a transient increase in Vejp and a large increase in frequency of miniature junction potentials. In each case the change in Vejp was due to changes in presynaptic release of transmitter since quantal content per fiber (M') was shown to decrease for OHP and helium at 150 PSIG and to transiently rise with 100% oxygen at ambient pressure. In addition, the response to exogenously applied glutamate (the putative neurotransmitter) was not affected by OHP, 150 PSIG helium or 100% oxygen at ambient pressure. This further indicates a presynaptic site of action.

Effects of ketamine and of high pressure on the responses to gamma-aminobutyric acid of the rat superior cervical ganglion in vitro

1 The method of Brown & Marsh (1974) for recording of surface potentials from the rat superior cervical ganglion has been adapted for use in a high pressure chamber in order to study the effects of high pressure of helium and the possible interactions with the effects of general anaesthetics. 2 Helium pressure of 130 atm did not alter the amplitude of the responses recorded from the ganglion in response to gamma-aminobutyric acid (GABA) application (9.7 and 19.4 microM) but the amplitude of responses to a nicotinic agonist were depressed. 3 Ketamine, at concentration between 18 and 180 microM, considerably potentiated the responses of the ganglion to GABA. 4 Helium pressure (130 atm) did not reverse the potentiation of GABA by ketamine. 5 The results are discussed in connection with the ability of ketamine to oppose the behavioural effects of high pressure.

Increase in lipid microviscosity of unilamellar vesicles upon the creation of transmembrane potential

Diffusion potential of potassium ions was found in unilamellar vesicles of phosphatidyl choline. The vesicles, which included potassium sulfate buffered with potassium phosphate were diluted into an analogous salt solution made of sodium sulfate and sodium phosphate. The diffusion potential was created by the addition of the potassium-ionophore, valinomycin. The change in lipid microviscosity, ensuing the formation of membrane potential, was measured by the conventional method of fluorescence depolarization with 1,6-diphenyl-1,3,5-hexatriene as a probe. Lipid microviscosity was found to increase with membrane potential in a nonlinear manner, irrespective of the potential direction. Two tentative interpretations are proposed for this observation. The first assumes that the membrane potential imposes an energy barrier on the lipid flow which can be treated in terms of Boltzmann-distribution. The other interpretation assumes a decrease in lipid-free volume due to the pressure induced by the electrical potential. Since increase in lipid viscosity can reduce lateral and rotational motions, as well as increase exposure of functional membrane proteins, physiological effects induced by transmembrane potential could be associated with such dynamic changes.

The possible mechanisms of the high pressure-induced motor disturbances in the cat

High pressures elicit a high-frequency tremor (8-12 c/sec) in mammals, the mechanisms of which are still unknown. The present study shows that: (1) in spite of many similarities observed between the EMG characteristics of harmaline-induced tremor and pressure-induced tremor, cerebellar lesions which suppress the harmaline-induced tremor, do not modify the characteristics of the pressure-induced tremor; (2) at depth, the caudal part of the spinal cat (section at T9-T10 spinal level) displays irregular spontaneous EMG activities which can be clonic or rhythmic (4-8 c/sec), and a neuromuscular stretch hyperreflexivity. These data suggest that the origin of the pressure-induced tremor is spinal and neuromuscular rather than cerebellar.

Effects of pressure and pressure antagonists on the growth and membrane-bound ATP-ase of Acholeplasma laidlawii B

1. Arrhenius plots of the membrane-bound ATP-ase were constructed at pressures of 300, 600 and 900 atm. Pressure shifts the plots to higher temperatures with an increase in slope. 2. These data are partially consistent with a kinetic model in which a phase transition in the lipids associated with the ATP-ase determines the activity of the enzyme. They are also consistent with a model in which the non-linear Arrhenius plot is caused by the low temperature inactivation of the enzyme, upon which pressure acts directly. 3. Pentanol inhibits the ATP-ase without affecting the Arrhenius break temperature, and therefore does not act by affecting the phase-state of lipids associated with the enzyme. The pentanol-enzyme interaction yielded the following: delta H 46 Kcal mol-1 and delta S 114 cal mol-1 deg-1. 4. Pressure inhibits cell growth in a way which is partially offset by comparable partial pressures of helium and hydrogen. Its action probably involves the ordering of the membrane bilayer which is counteracted by the fluidising effect of the gases.