The Effects of Several Alcohols on the Properties of the Squid Giant Axon

The effects of several alcohols on the resting potential, action potential, and voltage-clamp currents of the squid giant axon have been measured. All the alcohols employed are similar in that they depress maximum sodium conductance much more than maximum potassium conductance. Octyl alcohol differs from the others (C₂ through C₅) in that it has less tendency to depolarize the axon. Depolarization is always accompanied by a decrease of g_K near the resting potential, such that the ratio g_K/g_leak is decreased. Steady-state inactivation of the sodium ion current is unaffected by alcohols, as is membrane capacity. Resting membrane conductance is usually decreased by alcohols. The findings are discussed in relation to work on monomolecular films.     

The Influence of H+ on the Membrane Potential and Ion Fluxes of Nitella

The resting membrane potential of the Nitella cell is relatively insensitive to [K]_o, but behaves like a hydrogen electrode. K⁺ and Cl^- effluxes from the cell were measured continuously, while the membrane potential was changed either by means of a negative feedback circuit or by external pH changes. The experiments indicate that P_K and P_Cl are independent of pH but are a function of membrane potential. Slope ion conductances, G_K, G_Cl, and G_Na were calculated from efflux measurements, and their sum was found to be negligible compared to membrane conductance. The possibility that a boundary potential change might be responsible for the membrane potential change was considered but was ruled out by the fact that the peak of the action potential remained at a constant level regardless of pH changes in the external solution. The conductance for H⁺ was estimated by measuring the membrane current change during an external pH change while the membrane potential was clamped at K⁺ equilibrium potential. In the range of external pH 5 to 6, H⁺ chord conductance was substantially equal to the membrane conductance. However, the [H]_i measured by various methods was not such as would be predicted from the [H]_o and the membrane potential using the Nernst equation. In artificial pond water containing DNP, the resting membrane potential decreased; this suggested that some energy-consuming mechanism maintains the membrane potential at the resting level. It is probable that there is a H⁺ extrusion mechanism in the Nitella cell, because the potential difference between the resting potential and the H⁺ equilibrium potential is always maintained notwithstanding a continuous H⁺ inward current which should result from the potential difference.     

An analysis of the change in K-permeability on depolarization in terms of affinity and numbers of total channels.

A theoretical relation between permeability and ionic concentrations in a bathing solution has been derived by assuming that only channels unoccupied by a competing non-permeable ion can transport ions specific for that channel. The affinities of the channel to the ion and the competitor are expressed by dissociation constants of the ion-site and competitor-site complexes in the channel.Analyses of the relation of K permeability to [K]_o obtained from myelinated nerve fibres and Nitella cells revealed that the affinity of sites in K channels was independent of membrane potential, whereas K conductivity increased with depolarization. The value of the dissociation constant of the K⁺-site complex, K₁, was estimated as 1244 mm for myelinated nerve, and $\text{K}{}_1\text{exp(ψ}{}_0\text{F}\text{RT}$) for Nitella was 17.5 mm (ψ₀ is the surface potential at the outer surface of membrane). The dependence on voltage of the total number of K channels was estimated from the dependence of K conductance on membrane potential at [K]_o = [K]₁ (obtained from the theoretical magnitude of K current computed by using the dissociation constants described above). It should be noted that when the channels are partially saturated with K⁺, neither the chord conductance nor the “permeability coefficient”, as defined in the Goldman and Hodgkin-Katz formulation, correctly represents the dependence on membrane potential of the total number of channels.     

Condition of optimum conduction of synaptic excitation along the dendritic fiber

Existence of an optimum radius of fibre is shown, when physical parameters of the intracellular medium and the nonexcitable dendritic membrane are determined and the length and load resistances are fixed. It provides the maximum potential for one end of the fibre if synaptic conductance is determined for another one. Conductance of excitation along the dendritic fibre of changing thickness is optimum for little synaptic conductance when the fibre radius increases and for high conductance when the radius decreases. The formula for calculating an optimum dendritic spine neck radius is proposed.     

Effect of chlordimeform on nerve-muscle system of the larva of the waxmoth, Galleria mellonella

The action of chlordimeform on the nerve-muscle preparation of the larvae of the waxmoth Galleria mellonella has been studied by means of microelectrodes. Exitatory junction potential evoked by nerve stimulation is reversibly suppressed by 2 × 10^?3 M chlordimeform, and spike-like component is abolished. The resting membrane potential of the muscle fibre and the action potential from the nerve terminal are not affected at 5 × 10^?3 M chlordimeform. The depolarizing membrane response caused by outward current and the effective membrane resistance are not appreciably affected. It appears that chlordimeform exerts its blocking action on the neuromuscular junction rather than the conductance mechanism of muscle fibre membrane.     

Reversal potentials corresponding to mechanical stimulation and leakage current in Myxicola giant axons

The response of a Myxicola infundibulum giant axon to a transverse mechanical stimulus is an increase in membrane conductance. The similarity of the reversal potential for this conductance increase and the reversal potential for leakage current, together with other similarities, suggest similar pathways for these two processes. Depolarization of the reversal potential with increased mechanical stimulus is best explained in terms of a gradual change in the mechanically-stimulated ionic pathways.     

Electrotonic conduction of excitation and inhibition along the dendritic fiber with the uniformly distributed synaptic conductance

The existence of an optimum radius of fibre is shown, when the physical parameters of the intracellular medium and the nonexcitable dendritic membrane are known and the length and load resistance are fixed. This provides the maximum potential for the proximal end of the fibre if synaptic conductance is distributed uniformly along the fibre. A formula for calculating the soma potential of the whole neuron is proposed. The optimal ratio l/lambda is 0.9193 ... if the volume of the fibre and synaptic conductivity are fixed.     

Intracellular sodium activity and sodium transport inNecturus gallbladder epithelium

Summary Ion-sensitive glass microelectrodes, conventional microelectrodes and isotope flux measurements were employed inNecturus gallbladder epithelium to study intracellular sodium activity, [Na] _i , electrical parameters of epithelial cells, and properties of active sodium transport. Mean control values were: [Na] _i : 9.2 to 12.1mm; transepithelial potential difference, _ms : –1.5 mV (lumen negative); basolateral cell membrane potential, _es : –62 mV (cell interior negative); sodium conductance of the luminal cell membrane,g _Na: 12 mho cm^–2; active transcellular sodium flux, 88 to 101 pmol cm^–2 sec^–1 (estimated as instantaneous short-circuit current). Replacement of luminal Na by K led to a decrease of the intracellular sodium activity at a rate commensurate to the rate of active sodium extrusion across the basolateral cell membrane. Mucosal application of amphotericin B resulted in an increase of the luminal membrane conductance, a rise of intracellular sodium activity, and an increase of short-circuit current and unidirectional mucosa to serosa sodium flux. Conclusions: (i) sodium transport across the basolateral membrane can proceed against a steeper chemical potential difference at a higher rate than encountered under control conditions; (ii) the luminal Na-conductance is too low to accommodate sodium influx at the rate of active basolateral sodium extrusion, suggesting involvement of an electrically silent luminal transport mechanism; (iii) sodium entry across the luminal membrane is the rate-limiting step of transcellular sodium transport and active sodium extrusion across the basolateral cell membrane is not saturated under control conditions.     

Excitation trapping and charge separation in Photosystem II in the presence of an electrical field

To investigate the effects of a membrane potential on excitation trapping and charge separation in Photosystem II we have studied the chlorophyll fluorescence yield in osmotically swollen chloroplasts subjected to electrical field pulses. Significant effects were observed only in those membrane regions where a large membrane potential opposing the photochemical charge separation was built up. When the fluorescence yield was low, close to F₀, a much higher yield, up to F_max, was observed during the presence of the membrane potential. This is explained by an inhibition by the electrical field of electron transfer to the quinone acceptor Q, resulting in a decreased trapping of excitations. A field pulse applied when the fluorescence yield was high, Q and the donor side being in the reduced state, had the opposite effect: the fluorescence was quenched nearly to F₀. This field-induced fluorescence quenching is ascribed to reversed electron transfer from Q^? to the intermediate acceptor, pheophytin. Its field strength dependence suggests that the midpoint potential difference between pheophytin and Q is at most about 300 mV. Even then it must be assumed that electron transfer between pheophytin and Q spans 90% of the potential difference across the membrane.     

Leak Current Rectification in Myxicola Giant Axons : Constant field and constant conductance components

Early leak current, i.e. for times similar to the time to peak of the transient current was measured in Myxicola giant axons in the presence of tetrodotoxin. The leak current-voltage relation rectifies, showing more current for strong depolarizing pulses than expected from symmetry around the holding potential. A satisfactory practical approximation for most leak corrections is constant resting conductance. The leak current-voltage curve rectifies less than expected from the constant field equation. These curves cannot be reconstructed by summing the constant field currents for sodium and potassium using a P_Na/P_K ratio obtained in the usual way, from zero current constant field fits to resting membrane potential data. Nor can they be reconstructed by summing the constant field current for potassium with that for any other single ion. They can be reconstructed, however, by summing the constant field current for potassium with a constant conductance component. It is concluded that the leak current and the resting membrane potential, therefore, are determined by multiple ionic components, at least three and possibly many. Arguments are presented suggesting that ion permeability ratios obtained in the usual way, by fitting the constant field equation to resting membrane potential data should be viewed with skepticism.     

The expression of the skeletal muscle force-length relationship in vivo: A simulation study

The force-length relationship is one of the most important mechanical characteristics of skeletal muscle in humans and animals. For a physiologically realistic joint range of motion and therefore range of muscle fibre lengths only part of the force-length curve may be used in vivo, i.e. only a section of the force-length curve is expressed. A generalised model of a mono-articular muscle-tendon complex was used to examine the effect of various muscle architecture parameters on the expressed section of the force-length relationship for a 90° joint range of motion. The parameters investigated were: the ratio of tendon resting length to muscle fibre optimum length (L_TR:L_F·OPT) (varied from 0.5 to 11.5), the ratio of muscle fibre optimum length to average moment arm (L_F·OPT:r) (varied from 0.5 to 5), the normalised tendon strain at maximum isometric force (c) (varied from 0 to 0.08), the muscle fibre pennation angle (θ) (varied from 0° to 45°) and the joint angle at which the optimum muscle fibre length occurred (φ). The range of values chosen for each parameter was based on values reported in the literature for five human mono-articular muscles with different functional roles. The ratios L_TR:L_F·OPT and L_F·OPT:r were important in determining the amount of variability in the expressed section of the force-length relationship. The modelled muscle operated over only one limb at intermediate values of these two ratios (L_TR:L_F·OPT=5; L_F·OPT:r=3), whether this was the ascending or descending limb was determined by the precise values of the other parameters. It was concluded that inter-individual variability in the expressed section of the force-length relationship is possible, particularly for muscles with intermediate values of L_TR:L_F·OPT and L_F·OPT:r such as the brachialis and vastus lateralis. Understanding the potential for inter-individual variability in the expressed section is important when using muscle models to simulate movement.     

Potassium Ion Current in the Squid Giant Axon: Dynamic Characteristic

Measurements of the potassium current in the squid axon membrane have been made, after changes of the membrane potential to the sodium potential of Hodgkin and Huxley (HH), from near the resting potential, from depolarizations of various durations and amplitudes, and from hyperpolarizations of up to 150 mv. The potassium currents I given by I = I_∞ {1 - exp [- (t + t₀)/τ]}²⁵, where t₀ is determined by the initial conditions, represent the new data and approximate the HH functions in the regions for which they are adequate. A corresponding modification for the sodium current does not appear necessary. The results support the HH assumptions of the independence of the potassium and sodium currents, the dependence of the potassium current upon a single parameter determined by the membrane potential, and the expression of this parameter by a first order differential equation, and, although the results drastically modify the analytical expressions, they very considerably extend the range of apparent validity of these assumptions. The delay in the potassium current after severe hyperpolarization is used to estimate a potassium ion mobility in the membrane as 10^-5 of its value in aqueous solutions.     

Synaptic Electrogenesis in Eel Electroplaques

Whether evoked by neural or by chemical stimulation, the synaptic membrane of eel electroplaques contributes a depolarizing electrogenesis that is due to an increased conductance for Na and K. The reversal potential (E_S) is the same for the two modes of synaptic activation. It is inside-positive by about 30–60 mv, or about midway between the emf's of the ionic batteries for Na (E_Na) and K(E_K). The total conductance contributed by synaptic activity (G_S) varied over a fivefold range, but the individual ionic branches, G_SSNa, and G_SSK, change nearly equally so that the ratio G_SSNa:G_SSK is near unity. G_SSK increases independently of the presence or absence of Na in the bathing medium, and independently of the presence or absence of the electrically excitable G_K channels. When activated, the synaptic membrane appears to be slightly permeable to Ca and Mg. When the membrane is depolarized into inside positivity the conductance of the synaptic components decreases and approaches zero for large inside-positive values. Thus, the synaptic components become electrically excitable when the potential across the membrane becomes inside-positive, responding as do the nonsynaptic components, with depolarizing inactivation.     

Properties of the large ion-permeable pores formed from protein F of Pseudomonas aeruginosa in lipid bilayer membranes

The incorporation of porin protein F from the outer membrane of Pseudomonas aeruginosa into artificial lipid bilayers results in an increase of the membrane conductance by many orders of magnitude. The membrane conductance is caused by the formation of large ion-permeable channels with a single-channel conductance in the order of 5 nS for 1 M alkali chlorides. The conductance has an ohmic current vs. voltage relationship. Further information on the structure of the pore formed by protein F was obtained by determining the single-channel conductance for various species differing in charge and size, and from zero-current potential measurements. The channel was found to be permeable for large organic ions (Tris⁺, N(C₂H₅)₄⁺, Hepes^?) and a channel diameter of 2.2 nm could be estimated from the conductance data (pore length of 7.5 nm). At neutral pH the pore is about two times more permeable for cations than for anions, possibly caused by negative charges in the pore. The consistent observation of large water filled pores formed by porin protein F in model membrane systems is discussed in the light of the known low permeability of the Ps. aeruginosa outer membrane towards antibiotics. It is suggested that this results from a relatively low proportion of open functional porin protein F pores in vivo.     

The Sodium-Potassium Exchange Pump: Relation of Metabolism to Electrical Properties of the Cell: I. Theory

The Na-K exchange pump is represented as a net stoichiometrically coupled reaction, r, involving ATP, Na⁺, and K⁺, and is located in the active region of the cell membrane. The reaction rate is J_r = L_rr (-ΔF_r), where ΔF_r is the free energy change of the reaction. ΔF_r includes membrane potential ø₂ in the absence of 1:1 coupling between Na⁺ and K⁺, and the reaction rate is potential dependent under these conditions. At the same time the pump will produce a potential H which is the difference between membrane potential and the diffusion potential as calculated with constant field assumptions. In the absence of 1:1 coupling, the pump is electrogenic. The feedback relation between reaction rate and membrane potential makes the membrane resistance in the presence of the pump less than or equal to the resistance in its absence, at the same membrane potential. H depends on stoichiometry, reaction rate, and passive ionic conductances. Experimental verification of the model will depend on the accuracy of permeability determinations. Dissipation and efficiency of transport can be calculated also.     

A study of the current-voltage relationships of electrogenetic active and passive membrane elements in riccia fluitans

Potassium- and proton-dependent membrane potential, conductance, and current-voltage characteristics (I–V curves) have been measured on rhizoid cells of the liverwort Riccia fluitans. The potential difference (E_m) measured with microelectrodes across plasmalemma and tonoplast is depolarized to the potassium-sensitive diffusion potential (E_D) in the presence of 1 mM NaCN, 1 mM NaN₃, or at temperatures below 6°C. Whereas the temperature change from 25°C to 5°C decreases the membrane conductance (g_m) from 0.71 to 0.43 S ? m^?2, 1 mM NaCN increases g_m by about 25%. The membrane displays potassium-controlled rectification which gradually disappears at temperatures below 5°C. The potassium pathway can be described by an equivalent circuit of a diode and an ohmic resistor in parallel. In the potential interval of E_D ± 100 mV the measured I-V curves roughly fit the theoretical curves obtained from a modified diode equation. ⁸⁶Rb⁺(K⁺)-influx is voltage sensitive: In the presence of 1 mM NaCN, ⁸⁶Rb⁺-influx follows a hyperbolic function corresponding to a low conductance at low [K⁺]_o and high conductance at high [K⁺]_o. On the contrary ⁸⁶Rb⁺-influx is linear with [K⁺]_o when pump activity is normal. It is believed that there are two K⁺-transport pathways in the Riccia membrane, one of which is assigned to the low conductance (0.2 S · m^?2), the other to a temperature-dependent facilitated diffusion system with a higher conductance (7.7 S · m^?2). The electrogenic pump essentially acts as a current source and consumes about 39% of the cellular ATP-turnover. In the presence of 30 μM CCCP the saturation current of 0.1 A · m^?2 is doubled to about 0.2 A · m^?2, and the electromotive force of ?360 mV switches to ?250 mV. It is suggested that this may be due to a change in stoichiometry from one to two transported charges per ATP hydrolyzed.     

Formation of ion channels by a negatively charged analog of gramicidin a

Summary O-pyromellitylgramicidin is a derivative of gramicidin in which three carboxyl groups are introduced at the terminal hydroxyl end of the peptide. Experiments with artificial lipid membranes indicate that this negatively charged analog forms ion-permeable channels in a way similar to that of gramicidin. If O-pyromellitylgramicidin is added to only one aqueous solution, the membrane conductance remains small, but increases by several orders of magnitude if the same amount is also added to the other side. In accordance with the dimer model of the channel, the membrane conductance under symmetrical conditions is proportional to the square of the aqueous concentration of O-pyromellitylgramicidin over a wide range. The ratio _PG/ _G of the single-channel conductance of O-pyromellitylgramicidin to that of gramicidin is close to unity at high ionic strength, but increases more than fivefold at smaller ionic strength (0.01m). This observation is explained in terms of an electrostatic effect of the fixed negative charges localized near the mouth of the channel. In a mixture of O-pyromellitylgramicidin and gramicidin, unit conductance steps of intermediate size are observed in addition to the conductance steps corresponding to the pure compounds, indicating the formation of hybrid channels. Hybrid channels with preferred orientation may be formed if small amounts of gramicidin and O-pyromellitylgramicidin are added to opposite sides of the membrane. These hybrid channels show a distinct asymmetry in the current-voltage characteristic.     

Survey and synthesis of intra- and interspecific variation in stomatal sensitivity to vapour pressure deficit

Responses of stomatal conductance (g_s) to increasing vapour pressure deficit (D) generally follow an exponential decrease described equally well by several empirical functions. However, the magnitude of the decrease – the stomatal sensitivity – varies considerably both within and between species. Here we analysed data from a variety of sources employing both porometric and sap flux estimates of g_s to evaluate the hypothesis that stomatal sensitivity is proportional to the magnitude of g_s at low D ( ≤ 1 kPa). To test this relationship we used the function g_s = g_sref–m· lnD where m is the stomatal sensitivity and g_sref = g_s at D = 1 kPa. Regardless of species or methodology, m was highly correlated with g_sref (average r² = 0·75) with a slope of approximately 0·6. We demonstrate that this empirical slope is consistent with the theoretical slope derived from a simple hydraulic model that assumes stomatal regulation of leaf water potential. The theoretical slope is robust to deviations from underlying assumptions and variation in model parameters. The relationships within and among species are close to theoretical predictions, regardless of whether the analysis is based on porometric measurements of g_s in relation to leaf-surface D (D_s), or on sap flux-based stomatal conductance of whole trees (G_Si), or stand-level stomatal conductance (G_S) in relation to D. Thus, individuals, species, and stands with high stomatal conductance at low D show a greater sensitivity to D, as required by the role of stomata in regulating leaf water potential.     

GABA-gated anion channels in intact crayfish opener muscle fibres and stretch-receptor neurons are neither activated nor desensitized by glutamate

Summary The influence of glutamate on the GABA-activated Cl^- conductance was studied in the slowly adapting stretch-receptor neuron and dactylopodite opener muscle fibre of the crayfish (Astacus astacus) using a two-microelectrode and a three-microelectrode voltage clamp, respectively. Glutamate (0.5–1.0 mM) had no effect on the GABA-activated conductance in either preparation. This indicates that the availability of the inhibitory channels for activation of GABA is not influenced by glutamate. The present results are in sharp contrast to those obtained by Franke et al. (J Comp Physiol A 159:591–609, 1986) in experiments on excised membrane patches, which suggested that glutamate is capable of both activating and desensitizing inhibitory postsynaptic channels in the crayfish opener muscle fibre.Abbreviations GABA -aminobutyric acid - G_GABA and G _GABA ^p GABA-gated conductance and peak conductance - HEPES N-2-hydroxyethylpiperazine-N-2-ethanesulphonic acid - I current - SRN stretch-receptor neuron - V_m and V_l membrane voltage in two- and three-microelectrode voltage clamp, respectively