Showing papers on "Conductance published in 1981"
TL;DR: It is proposed that the intramembrane particles that comprise gap junctions in early amphibian embryos are voltage- sensitive channels, arising from a reversible two-state system in which voltage linearly affects the energy difference between states.
Abstract: The conductance of junctions between amphibian blastomeres is strongly voltage dependent. Isolated pairs of blastomeres from embryos of Ambystoma mexicanum, Xenopus laevis, and Rana pipiens were voltage clamped, and junctional current was measured during transjunctional voltage steps. The steady-state junctional conductance decreases as a steep function of transjunctional voltage of either polarity. A voltage-insensitive conductance less than 5% of the maximum remains at large transjunctional voltages. Equal transjunctional voltages of opposite polarities produce equal conductance changes. The conductance is half maximal at a transjunctional voltage of approximately 15 mV. The junctional conductance is insensitive to the potential between the inside and outside of the cells. The changes in steady-state junctional conductance may be accurately modeled for voltages of each polarity as arising from a reversible two-state system in which voltage linearly affects the energy difference between states. The voltage sensitivity can be accounted for by the movement of about six electron charges through the transjunctional voltage. The changes in junctional conductance are not consistent with a current-controlled or ionic accumulation mechanism. We propose that the intramembrane particles that comprise gap junctions in early amphibian embryos are voltage-sensitive channels.
361 citations
TL;DR: It is suggested that four melittin monomers are needed to form a channel and, furthermore, that a minimum of four equivalent electronic charges need to be displaced by the electrical field to explain the voltage dependence of the conductance.
344 citations
TL;DR: In this article, a re-analysis of data published in the literature demonstrates that the minimal conductance of mammals and birds is about 50 % greater when measured during the activity time α of the animal than during the rest time ρ (Figs 1 and 3).
316 citations
TL;DR: It is proposed that the gap junctions between amphibian blastomeres are comprised of voltage-sensitive channels and that the junctional conductance undergoes a transient recovery when the transjunctional voltage polarity is reversed.
Abstract: We have proposed that the gap junctions between amphibian blastomeres are comprised of voltage-sensitive channels. The kinetic properties of the junctional conductance are here studied under voltage clamp. When the transjunctional voltage is stepped to a new voltage of the same polarity, the junctional conductance changes as a single exponential to a steady-state level. The time constant of the conductance change is determined by the existing transjunctional voltage and is independent of the previous voltage. For each voltage polarity, the relations between voltage, time constant, and steady-state conductance are well modeled by a reversible two-state reaction scheme in which the calculated rate constants for the transitions between the states are exponential functions of voltage. The calculated rate constant for the transition to the low-conductance state is approximately twice as voltage dependent as that for the transition to the high-conductance state. When the transjunctional voltage polarity is reversed, the junctional conductance undergoes a transient recovery. The polarity reversal data are well modeled by a reaction scheme in which the junctional channel has two gates, each with opposite voltage sensitivity, and in which an open gate may close only if the gate in series with it is open. A simple explanation for this contingent gating is a mechanism in which each gate senses only the local voltage drop within the channel.
230 citations
TL;DR: It is concluded that the K+ conductance of the nodal membrane is composed of three components corresponding to three different and distinct types of K+ channels.
Abstract: 1. In voltage clamped myelinated fibres, the K+ current was recorded in high-K+ media to allow analysis without complications due to K+ accumulation. 2. After a depolarization, the tail of K+ current following repolarization decreases in two phases: a fast phase lasting about 20 msec and a slow exponential phase lasting several hundred milliseconds. When the duration of the depolarization is increased, the amplitude at time zero of the fast phase increases (activation of the conductance) and then decreases slowly (inactivation of the conductance). Simultaneously, the amplitude of the slow phase, extrapolated to time zero of repolarization, increases slowly and reaches a steady-state level (about 20% of the maximum instantaneous current) after about 600 msec of depolarization. 3. The fast phase of the tail current is blocked by external application of 4-aminopyridine (4-AP) (KD = 10(-5)M). The slow phase is unaltered by 4-AP (10(-7)-10(-2)M). 4. In the presence of 4-AP (10-3M), the remaining slow K+ current, activated by depolarizations, does not inactivate. 5. During depolarizations and repolarizations, the conductance of the slow current (GKs) varies exponentially. The steady-state value of the slow conductance and its time constant of activation vary with voltage. The variation of the slow conductance with time and voltage can be described by a closed-open mode, assuming that each channel is gated by one particle. The activation kinetics of the slow current is unaltered by long lasting (500 msec) prepolarizations. 6. The fast K+ conductance, calculated from the fast tail current, is fully inactivated at the end of a 3 min depolarization to 0 mV. 7. The fast K+ conductance can be decomposed into two components: one component (GKf1) activating between -80 and -30 mV and inactivating very slowly (tau = 45 sec at E = 0 mV); one component (GKf2) activating between -40 mV and +30 mV and inactivating slowly (tau = 2 sec at E = 0 mV). t = 12 degrees C. 8. The maximum slow and fast conductances increase with [K]0. While the maximum fast conductance tends to saturate at high external K+ concentrations, the maximum slow conductance shows no sign of saturation. 9. A comparison between motor and sensory fibres shows that, while the amplitude of maximum slow and fast conductances are identical for both types of fibres, the amplitude of fast-1 conductance is larger and consequently the amplitude of fast-2 is smaller in motor than in sensory fibres. The different spike frequency adaptations observed on both types of fibres are discussed in relation to these different relative fast conductances amplitudes. 10. It is concluded that the K+ conductance of the nodal membrane is composed of three components (GKS, GKf1 and GKf2) corresponding to three different and distinct types of K+ channels.
212 citations
TL;DR: 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.
183 citations
TL;DR: Planar bilayers formed from Escherichia coli outer membrane vesicles exhibit conductance properties similar to those previously observed in bilayers reconstituted from aggregates of matrix protein, the major outer membrane protein, providing the reference data for the reconstitution of membrane function from individual components.
Abstract: Planar bilayers formed from Escherichia coli outer membrane vesicles exhibit conductance properties similar to those previously observed in bilayers reconstituted from aggregates of matrix protein, the major outer membrane protein. Discrete conductance steps are observed, reflecting voltage-dependent transmembrane channels. These exist in clusters which are activated by voltage. After activation, channels close with increasing potentials and reopen reversibly at lower voltage. Depending on the sign of the potential, two distinct closed states of the pores are observed. Cooperative interactions, hysteresis effects, relaxation times, and values of channel conductance depend on cluster size. These properties provide the reference data for the reconstitution of membrane function from individual components. Planar bilayers were formed from vesicles containing either solubilized matrix protein in a homogeneous trimeric state or bacterial glycolipid (lipopolysaccharide), or both. Activation of channel conductance required the presence of glycolipid and the formation of channel clusters, leading to conductance properties of the channels closely resembling those observed in native outer membranes. At very low concentrations of trimers, irreversible association to clusters by lateral diffusion was observed. Nearly quantitative recoveries of channels allowed the assignment of three pores per trimer.
139 citations
TL;DR: The calcium-induced hyperpolarizing conductance, producing an outward current at −50 mV, followed the inward current and reached maximum during the late decline in [Ca2+]i, which showed a particular dependence on voltage.
Abstract: Controlled quantities of 100 mM aqueous CaCl2 solutions were pressure injected into voltage-clamped neurons with a resolution of 10−11 1. Ca2+-selective microelectrodes monitored the time course of changes in [Ca2+]i. At a membrane potential of −50 mV CaCl2 quantities in the range of 1% of the cell volume induced an inward current, associated with a conductance increase and having an equilibrium potential between −20 and +20 mV, which accompanied the rise in [Ca2+]i. An artifactual origin of the inward current by the injection procedure or by calcium screening of membrane sites could be excluded. The calcium-induced hyperpolarizing conductance, producing an outward current at −50 mV, followed the inward current and reached maximum during the late decline in [Ca2+]i. In most cases its development was separated from the inward current by an intermediate relative decrease of the membrane conductance. Neither of the two transient conductance increases showed a particular dependence on voltage. Renewed Ca2+ injection quickly decreased the calcium-induced hyperpolarizing conductance for several seconds. Ca2+ injections below 0.05% of the cell volume mostly produced pure outward currents or hyperpolarizing responses. Partial substitution of extracellular CaCl2 by NiCl2 decreased the hyperpolarizing response but not the initial inward current. The immediate effects of increased [Ca2+]i are activation of a depolarizing conductance and the partial block of the late hyperpolarizing conductance. The latter is probably produced through intermediate steps after increasing [Ca2+
i.
93 citations
TL;DR: Calculations suggested that the volume flux in naturally exuding root systems is relatively insensitive to changes in the conductance.
Abstract: The response of solute and volume fluxes of Phaseolus root systems to applied AbA was observed under conditions of applied pressure which were used to enhance the volume flow. The growth regulator elicited three separate responses: a transient release of solutes to the xylem which was responsible for an initial increase in volume flux; a long term increase in the total ion flux; and a long term decrease in the hydraulic conductance of the root systems. The exact response was highly dependent on the magnitude of the pre-AbA volume flux density, the relative contributions of osmotic and pressure-induced flow, and the applied dosage. Calculations suggested that the volume flux in naturally exuding root systems is relatively insensitive to changes in the conductance.
81 citations
TL;DR: In this article, a method for calculating the relationship between the density of localized states in an amorphous semiconductor and the experimentally measured field-effect conductance is presented.
Abstract: A method for calculating the relationship between the density of localized states in an amorphous semiconductor and the experimentally measured field-effect conductance is presented. Commonly used simplifying assumptions of a constant space-charge density, zero-temperature statistics and a parabolic band-bending potential profile are removed and the errors that are introduced by using these approximations calculated. The method is used to show that the existence of a reported peak in the density of states at 0·4 eV below the conduction-band mobility edge of amorphous silicon (the Ex peak) cannot be proved through field-effect-conductance measurements.
64 citations
01 Jan 1981
TL;DR: 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 and it is believed that there are two K+-transport pathways in the Riccia membrane.
TL;DR: In this article, it was shown that the logarithmic corrections to the two-dimensional conductance arise from both interaction and incipient localisation effects, and that a transition between these two type of behaviour can be achieved by a change of electron temperature in the presence of a magnetic field, or just by the application of magnetic field.
Abstract: It is shown that the logarithmic corrections to the two-dimensional conductance arise from both interaction and incipient localisation effects. A transition between these two type of behaviour can be achieved by a change of electron temperature in the presence of a magnetic field, or just by the application of a magnetic field. The magnetic field suppresses the weak localisation and enhances the effects of the interaction. Results on conductance, magnetoconductance and Hall effect are presented and discussed.
TL;DR: The open-channel conductance properties of a voltage-gated Cl- channel derived from Torpedo californica electroplax and incorporated into planar bilayers were studied and it is shown that ions permeating the channel do not sense the surface potential of the bulk membrane.
Abstract: The open-channel conductance properties of a voltage-gated Cl- channel derived from Torpedo californica electroplax and incorporated into planar bilayers were studied by several approaches. In neutral bilayers the channel conductance saturates with Cl- activity according to a rectangular hyperbolic relation with a half-saturation activity of 75 mM and a maximum conductance of 32 pmho. The observation of identical behavior in charged membranes implies that ions permeating the channel do not sense the surface potential of the bulk membrane. The Cl-:Br- permeability ratio, measured under biionic conditions, is independent of salt concentration. SCN- ion reversibly blocks the channel. The voltage dependence of the block implies the existence of two separate blocking sites within the channel: one accessible from the cis side only (the side to which vesicles are added) and the other accessible from the trans side only. The block at each site is competitive with Cl-. The results are consistent with a single-ion Eyring model of the conduction process in which the ion must traverse three kinetic barriers as it permeates the channel and in which the channel can accommodate at most one ion at a time.
TL;DR: Isolated neurones from the snail Helix pomatia were investigated under voltage clamp at 21‐23 degrees C and single channel current was not affected by the membrane potential (since the equilibrium potential was high) nor by inactivation.
Abstract: 1. Isolated neurones from the snail Helix pomatia were investigated under voltage clamp at 21-23 degrees C. The cells were internally dialysed and the current through small electrically isolated patches of the membrane was measured. The area of the patches was 30-500 micrometers 2 (1/1000-1/100 of the cell surface). The internal resistance of the membrane patches was 10(9)-10(10) omega. 2. In order to obtain the maximum conductance of the calcium channels an external solution containing 130 mM-Ba2+ or Ca2+ and an internal solution containing Tris glutamate and 5 mM-EGTA were used. Fluctuations due to the activity of calcium channels have been detected and analysed. 3. The power density spectra of barium current fluctuations were calculated for conductance values from 3% to 30% of the maximum conductance in the frequency band 1-1000 Hz. They were fitted to a spectral density function of the Lorentz form. The half-power frequency of the spectra was 227 +/- Hz (S.E.). It did not reveal any distinct voltage dependence. 4. The current flowing through a single calcium channel was calculated from the variance-to-mean relationship. Its value for the transfer of Ba2+ ions is iBa = 0.20 +/- 0.02 pA (S.E.). Single channel current was not affected by the membrane potential (since the equilibrium potential was high) nor by inactivation. The maximum calcium inward current which flows through a single calcium channel is about 0.1 pA and corresponds to a conductance gamma Ca = 0.5 pS (calculated for an equilibrium potential of 200 mV). This estimate gives an upper limit to gamma Ca. 5. The parameters of calcium channels modified by external EGTA have been evaluated. Measurements were performed in an external solution containing 200 mM-Na+. The current carried by a single modified calcium channel is iNa = 1.0 +/- 0.2 pA (S.E.) (gamma Na approximately 8.0 pS).
TL;DR: The results suggest that the change in K+ conductance proceeds in a multi‐step transition or (and) that the K+ Conductance is determined by several types of K+ channels.
Abstract: 1 In voltage clamped myelinated nerve fibres, the K+ conductance has been calculated from current recordings obtained in low and high K+ media, taking into account the changes in EK resulting from accumulation of depletion of K+ ions near to nodal membrane 2 At the end of a depolarization, the instantaneous K+ current reverses at a potential (instantaneous reversal potential) differing from the Nernst potential calculated using the external and internal bulk concentrations (theoretical Nernst potential) During a depolarization, EK, as estimated from the instantaneous reversal potential, changes continuously This change depends on the size, the duration and the direction of the time dependent K+ current The variation of EK is attributed to continuous changes in K+ concentration near the membrane during voltage pulses which turn on the K+ conductance 3 The chord conductance [GK = IK/(E-EK), as calculated using the instantaneous reversal potential values for EK, has been analysed as a function of time and membrane potential As previously reported it increases with the initial K+ concentration in the external medium 4 The time course of the K+ current depends on both the kinetics of the conductance increase and the rate of change in the driving force for K The kinetics of the conductance increase can satisfactorily be described by a single exponential function following a delay after the onset of the depolarizing voltage clamp pulse 5 This delay increases when the holding potential is made more negative It decreases with membrane depolarization and it is independent of the external K+ concentration At a given membrane potential, the turning on of the K+ conductance is found to be faster at high than at low external K+ concentrations 6 At repolarization the turning off of the conductance cannot be described by a single exponential function It is faster at low than at high external K+ concentrations 7 The results suggest that the change in K+ conductance proceeds in a multi-step transition or (and) that the K+ conductance is determined by several types of K+ channels
TL;DR: The lifetimes of channels formed by natural gramicidin and its dimeric analog in monoglyceride lipid bilayers of various compositions were investigated and no simple relationship between channel conductance and surface tension or bilayer thickness was found.
TL;DR: Measurements indicate that the higher steady-state conductance observed in KCl at a given proteolipid concentration in a multi-channel membrane presumably results because more channels incorporate in the presence of KCl than in the absence of NaCl.
Abstract: Proteolipids extracted from bovine kidney plasma membrane induce irreversible changes in the electrical properties of lipid bilayers formed from diphytanoyl phosphatidylcholine. The interaction with the proteolipid produces channels which are cation selective. At low protein concentrations (i.e., <0.6 μg/ml), the single-channel conductance is approximately 10 pS in 100mm KCl and 3 pS in 100mm NaCl. In the presence of protein concentrations above 1 μg/ml, another population of channels appears. These channels have a conductance of about 100 pS in 100mm KCl and 30 pS in 100mm NaCl. Further, these channels are voltage dependent in KCl, closing when the voltage is clamped at values ≧30 mV. The steady-state membrane conductance, measured at low voltages, was found to increase proportional to a high power (2–3) of the proteolipid concentration present in one of the aqueous phases. In 100mm NaCl, the conductance increases at protein concentrations above 5 μg/ml, whereas in 100mm KCl in increases at protein concentrations above 0.6 μg/ml. These measurements indicate that the higher steady-state conductance observed in KCl at a given proteolipid concentration in a multi-channel membrane presumably results because more channels incorporate in the presence of KCl than in the presence of NaCl.
TL;DR: The lens conductance-voltage relationships were mapped out in control and calcium-free solutions, and as the two curves cross over at a particular voltage it appears that removing the external calcium also affects the membrane voltage-sensitive potassium channels.
TL;DR: Alamethicin appears to allow voltage-dependent lipid exchange ("flip-flop") between leaflets of a planar bilayer in membranes with one leaflet of phosphatidyl serine and one of phosphorus ethanolamine, and the shape of the nonactin current-voltage curve accurately reports the difference in surface potential between the two sides of the membrane.
TL;DR: A model based on a simplified form of the Gouy-Chapman theory of the electrified double layer to explain the experimental results suggests that there is a negative fixed charge associated with the pore.
TL;DR: In this paper, a 3-terminal ac technique and blocking electrodes were used to measure the conductance and capacitance of the vitreous silver ion conductor Ag 6 I 4 WO 4 (WO 4) with a range of frequencies from 1 to 10 7 Hz and temperatures from 77 to 370 K.
TL;DR: Analysis of the data for membrane potentials from +10 to +150 mV shows that the inhibition of the Na+ currents at high positive potentials cannot be described by a titration curve of a single acid group.
TL;DR: A mutant of Paramecium tetraurelia (teaB) shows a shift in the positive direction of the voltage sensitivity of the Ca conductance and the depolarization inactivation curve by 10 mV with no change in the total conductance under voltage clamp.
Abstract: Under voltage clamp, a mutant ofParamecium tetraurelia (teaB) shows a shift in the positive direction of the voltage sensitivity of the Ca conductance and the depolarization inactivation curve by 10 mV with no change in the total conductance. This effect can be mimicked in the wild type by the addition of external Ca2+ or Mg2+. The mutation also shifts the resting potential and the voltage sensitivities of the delayed rectification (depolarization-sensitive) K conductance and the anomalous rectification (hyperpolarization-sensitive) K conductance in the positive direction to a similar extent. This systematic shift of channel voltage sensitivities is best explained by the reduction of the surface negative charges of the membrane due to the mutation.
TL;DR: The results indicate that the increase in intracellular [Ca 2+ ] is primarily responsible for the light-induced increase of K + conductance during the dip, and the possible source of the Ca 2+ is, at least in part, extracellular due to activation of an inward Ca 2- current.
TL;DR: In this paper, the proper relationship between the physiological variable, thermal conductance, and heating or cooling time is not recognized, owing to conflicting definitions of conductance which either fail to relate to mean heat flow through the body surface or fail to separate body conductance from that of the environment.
TL;DR: A simplified two-state model is examined where the relaxation time of the voltage sensor is assumed to be considerably faster than the fastest changes of ionic conductance and significant changes in the occupancy of voltage sensor states in response to an applied oscillating electric field are predicted.
Abstract: An alternating component of potential across the membrane of an excitable cell may change the membrane conductance by interacting with the voltage-sensing charged groups of the protein macromolecules that form voltage-sensitive ion channels. Because the probability than a voltage sensor is in a given state is a highly nonlinear function of the applied electric field, the average occupancy of a particular state will change in an oscillating electric field of sufficient magnitude. This "rectification" at the level of the voltage sensors could results in conformational changes (gating) that would modify channel conductance. A simplified two-state model is examined where the relaxation time of the voltage sensor is assumed to be considerably faster than the fastest changes of ionic conductance. Significant changes in the occupancy of voltage sensor states in response to an applied oscillating electric field are predicted by the model.
TL;DR: In this paper, the authors used the existing concepts for the universal low temperature properties of glasses to calculate the contribution of supposed localized centers in the surface of solids to the heat transfer into adjacient helium.
Abstract: The existing concepts for the universal low temperature properties of glasses are used to calculate the contribution of supposed localized centers in the surface of solids to the heat transfer into adjacient helium. With a similar coupling constant and density of states as in bulk glass, the mechanism can account for the strength and the experimental features of the “anomalous Kapitza conductance” which is universal for all but the most perfect surfaces.
TL;DR: This paper presents a mathematical model for analyzing systematic errors associated with the membrane conductance of multicellular muscle preparations as determined in a sucrose gap apparatus, and its dependence on several experimental parameters is discussed.