scispace - formally typeset
Search or ask a question

Showing papers on "Conductance published in 1980"


Journal ArticleDOI
02 Oct 1980-Nature
TL;DR: The first observations of currents through individual Na+ channels under physiological conditions using an improved version of the extracellular patch-clamp technique on cultured rat muscle cells support earlier inferences about channel gating and show a single-channel conductance of approximately 18 pS.
Abstract: The voltage- and time-dependent conductance of membrane Na+ channels is responsible for the propagation of action potentials in nerve and muscle cells. In voltage-step-clamp experiments on neurone preparations containing 104–107 Na+ channels the membrane conductance shows smooth variations in time, but analysis of fluctuations1,2 and other evidence3 suggest that the underlying single-channel conductance changes are stochastic, rapid transitions between ‘closed’ and ‘Open’ states as seen in other channel types. We report here the first observations of currents through individual Na+ channels under physiological conditions using an improved version of the extracellular patch-clamp technique4–6 on cultured rat muscle cells. Our observations support earlier inferences about channel gating and show a single-channel conductance of approximately 18 pS.

413 citations


Journal ArticleDOI
TL;DR: A phenomenological theory considering the output characteristics of oxidative phosphorylation has been worked out by adopting the formalism of linear nonequilibrium thermodynamics and in isolated rat livers perfused in a metabolic resting state, the condition of conductance matching is fulfilled.
Abstract: A phenomenological theory considering the output characteristics of oxidative phosphorylation has been worked out by adopting the formalism of linear nonequilibrium thermodynamics. The linearity of oxidative phosphorylation in the range of the output forces of practical interest has been experimentally verified. The efficiency of oxidative phosphorylation is zero if either a load with a zero conductance (open-circuited situation) or a load with an infinite conductance (short-circuited situation) is attached to oxidative phosphorylation. In between these extreme conductances there exists a finite load conductance permitting oxidative phosphorylation to operate with optimal efficiency. The necessary and sufficient condition for optimal efficiency was found to be L33/L11=√1−q2 where L11 is the phenomenological conductance of phosphorylation, L33 the phenomenological conductance of the load and q the degree of coupling of oxidative phosphorylation driven by respiration. This condition was called conductance matching. Under the condition of conductance matching, four output functions of oxidative phosphorylation of practical interest were optimized. A maximal net rate of oxidative phosphorylation occurs at a degree of coupling qf= 0.78. A maximal output power of oxidative phosphorylation, i.e. net rate times established phosphate potential, results at qp= 0.91. The maximization of the function net rate times efficiency yielded an economic degree of coupling qecp= 0.95 for maximal ATP flow. Finally, maximization of the function output power times efficiency led to a degree of coupling qecp= 0.97. This last function simultaneously maximizes net rate of ATP production, developed phosphate potential and efficiency and reflects therefore the most economic solution to the output problem under the condition of conductance matching. In isolated rat livers perfused in a metabolic resting state, the condition of conductance matching is fulfilled. In addition, the degree of coupling of oxidative phosphorylation under these conditions corresponds to the economic degree of coupling qecp.

354 citations


Journal ArticleDOI
TL;DR: The open-channel conductance properties of a voltage-gated channel from sarcoplasmic reticulum were studied in planar phospholipid membranes, and the results are consistent with an Eyring- type barriers as it permeates the channel.
Abstract: The open-channel conductance properties of a voltage-gated channel from sarcoplasmic reticulum were studied in planar phospholipid membranes The channel is ideally selective for K+ over Cl- and for K+ over Ca++ In symmetrical 1 M solutions, the single-channel conductance (in pmho) falls in the order: K+ (214) > NH4+ (157) > Rb+ (125) > Na+ (72) > La+ (81) > Cs+ (< 3) In neutral bilayers, the channel conductance saturates with ion activity according to a rectangular hyperbolic relation, with half-saturation activities of 54 mM for K+ and 34 mM for Na+ Under symmetrical salt conditions, the K+:Na+ channel conductance ratio increases with salt activity, but the permeability ratio, measured by single-channel bi-ionic potentials, is constant between 20 mM and 25 M salt; the permeability ratio is equal to the conductance ratio in the limit of low-salt concentration The channel conductance varies < 5% in the voltage range -100 to +70 mV The maximum conductance varies K+ and Na+ is only weakly temperature dependent (delta H++ = 46 and 53 kcal/mol, respectively), but that of Li+ varies strongly with temperature (delta H++ = 13 kcal/mol) The channel's K+ conductance is blocked asymmetrically by Cs+, and this block is competitive with K+ The results are consistent with an Eyring-type barriers as it permeates the channel The data conform to Luger's (1973 Biochem Biophys Acta 311:423-441) predictions for a "pure" single-ion channel

214 citations


Journal ArticleDOI
TL;DR: The basic features of the motoneurone action potential were reconstructed by simulations based on voltage clamp measurements of the voltage dependent conductance systems and previous measurements of passive membrane properties, and indicated that a major portion of the delayed depolarization following the action potential is attributable to capacitative currents from the dendrites.
Abstract: 1. The soma membrane of cat motoneurones was voltage-clamped in vivo using intracellular current and voltage electrodes whose tips were separated by at least 5 micrometer. 2. Depolarization activates two separate, non-interacting K conductance systems whose rates of activation and decay differ by a factor of about 10. These conductances have a similar reversal potential, in the range of -6 to -21 mV (these and all subsequent voltages are expressed relative to the resting potential). Both conductances show linear 'instantaneous' current-voltage relationships. The steady-state magnitudes of both conductances increase with increasing depolarization. Neither conductance inactivates substantially during prolonged depolarizations. 3. The faster K conductance is similar to that described for squid axons and frog node. Activation begins at about +30 mV and is more than 90% complete within 5 msec of a depolarizing voltage step to +50 mV. Activation kinetics appear to be nonlinear. This fast K conductance contributes to the fast falling phase of the action potential. Following repolarization, this conductance decays with a time constant of 2-4 msec. 4. The slower K conductance activates during depolarizations of 10 mV or greater. The activation and decay of this conductance can be described by first-order exponential functions with time constants ranging from 20 to 50 msec. The slow K conductance underlies the prolonged hyperpolarization that follows motoneurone action potentials. Evidence from other studies suggests that this slow K conductance is regulated by intracellular Ca ions. 5. In addition to the two K conductance systems activated by depolarization, motoneurones exhibit another distinct conductance system that is activated by hyperpolarization. This third system has a reversal potential near the resting potential. Activation of this conductance during a hyperpolarizing voltage step can be fitted by a single exponential function with a time constant of 50-60 msec over the range -20 to -50 mV. This hyperpolarization-activated conductance accounts for some aspects of the anomalous rectification reported in cat motoneurones. 6. When the clamp circuit was turned off and the motoneurones were stimulated to discharge repetitively by depolarizing current steps, the apparent soma threshold voltage increased as the applied current (and discharge frequency) increased. 7. The basic features of the motoneurone action potential were reconstructed by simulations based on voltage clamp measurements of the voltage dependent conductance systems and previous measurements of passive membrane properties. These simulations assumed that the kinetics of the fast Na and K conductance systems in motoneurones can be described by equations of the same form as the Hodgkin-Huxley equations. These action potential reconstructions indicated that a major portion of the delayed depolarization following the action potential is attributable to capacitative currents from the dendrites...

197 citations


Journal ArticleDOI
TL;DR: Comparison of the efficacy of Ba++ block at various internal K+ concentrations suggests that Ba++ is probably a simple competitive inhibitor of K+ interaction with the K+ conductance.
Abstract: Ba++ ion blocks K+ conductance at concentrations in the nanomolar range. This blockage is time and voltage dependent. From the time dependence it is possible to determine the forward and reverse rate constants for what appears to be an essentially first-order process of Ba++ interaction. The voltage dependence of the rate constants and the dissociation constants place the site of interaction near the middle of the membrane field. Comparison of the efficacy of Ba++ block at various internal K+ concentrations suggests that Ba++ is probably a simple competitive inhibitor of K+ interaction with the K+ conductance. The character of Ba++ block in high external K+ solutions suggests that Ba++ ion may be "knocked-off" the site by inward movement of external K+. Examination of the effects of other divalent cations suggests that the channel may have a closed state with a divalent cation inside the channel. The relative blockage at different temperatures implies a strong interaction between Ba++ and the K+ conductance.

192 citations


Journal ArticleDOI
TL;DR: The following equation based on 230 conductance values for 192 species of mammals of body weights ranging from 3.5 to 150,000 g describes the relationship of conductance below thermal neutrality to body weight in mammals.

167 citations


Journal ArticleDOI
TL;DR: The ratio of observed channels to alpha-bungarotoxin sites present showed that a significant fraction of acetylcholine receptor in the membrane was functional and the quantitative aspects of the cation channel, the desensitization, and the ligand binding properties were in close agreement with established values.
Abstract: Planar bilayer membranes containing functional acetylcholine receptor were formed from vesicles of Torpedo marmorata electric organ without extracting the acetylcholine receptor from its native environment. Native vesicles were transformed into monolayers which subsequently were apposed into planar bilayers. In the absence of agonists the membrane conductance was similar to that of lipid bilayers. Addition of carbamoylcholine or succinylcholine caused increased membrane conductance and this could be competitively inhibited by d-tubocurarine and suppressed by alpha-bungarotoxin. The amplitude of the conductance response was proportional to the number of alpha-bungarotoxin binding sites in the bilayers. Asymmetric membranes could be formed with the ligand binding sites on only one membrane surface. Desensitization of acetylcholine receptor was evident from equilibrium and kinetic data of the carbamoylcholine-activated conductance. Carbamoylcholine-induced membrane permeability was about 7 times higher for K+ and Na+ ions than for Cl-. At low levels of conductance, single-channel fluctuations of 20-25 pS in conductance and 1.3-msec lifetime were resolved in physiological saline containing carbamoylcholine. The ratio of observed channels to alpha-bungarotoxin sites present showed that a significant fraction of acetylcholine receptor in the membrane was functional. The quantitative aspects of the cation channel, the desensitization, and the ligand binding properties were in close agreement with established values. This transformation of natural acetylcholine receptor vesicles to planar bilayers conserves the essential properties of the in vivo receptor.

135 citations


Journal ArticleDOI
TL;DR: It is concluded that each channel’s activation is determined by the configuration of a single tethered QBr molecule, and the same rate-limiting step governs the opening and closing of channels for both reversible and tethered agonists.
Abstract: After disulphide bonds are reduced with dithiothreitol, trans-3-(alpha-bromomethyl)-3’-[alpha-(trimethylammonium)methyl]azobenzene (trans-QBr) alkylates a sulfhydryl group on receptors. The membrane conductance induced by this “tethered agonist” shares many properties with that induced by reversible agonists. Equilibrium conductance increases as the membrane potential is made more negative; the voltage sensitivity resembles that seen with 50 [mu]M carbachol. Voltage- jump relaxations follow an exponential time-course; the rate constants are about twice as large as those seen with 50 mu M carbachol and have the same voltage and temperature sensitivity. With reversible agonists, the rate of channel opening increases with the frequency of agonist-receptor collisions: with tethered trans-Qbr, this rate depends only on intramolecular events. In comparison to the conductance induced by reversible agonists, the QBr-induced conductance is at least 10-fold less sensitive to competitive blockade by tubocurarine and roughly as sensitive to “open-channel blockade” bu QX-222. Light-flash experiments with tethered QBr resemble those with the reversible photoisomerizable agonist, 3,3’,bis-[alpha-(trimethylammonium)methyl]azobenzene (Bis-Q): the conductance is increased by cis {arrow} trans photoisomerizations and decreased by trans {arrow} cis photoisomerizations. As with Bis-Q, ligh-flash relaxations have the same rate constant as voltage-jump relaxations. Receptors with tethered trans isomer. By comparing the agonist-induced conductance with the cis/tans ratio, we conclude that each channel’s activation is determined by the configuration of a single tethered QBr molecule. The QBr-induced conductance shows slow decreases (time constant, several hundred milliseconds), which can be partially reversed by flashes. The similarities suggest that the same rate-limiting step governs the opening and closing of channels for both reversible and tethered agonists. Therefore, this step is probably not the initial encounter between agonist and receptor molecules.

134 citations


Journal ArticleDOI
TL;DR: High resolution experiments of the alamethicin pore demonstrate the existence of a further pore state at low conductance values, which seems to be impermeable to Ca2+, Cl, Tris-H+ and Hepes-, whereas the higher conductance states are not.

113 citations


Journal ArticleDOI
TL;DR: A voltage-dependent, K+-selective ionic channel from sarcoplasmic reticulum of rabbit skeletal muscle has been studied in a planar phospholipid bilayer membrane to study the mechanism by which the channel undergoes transitions between its conducting and nonconducting states.
Abstract: A voltage-dependent, K+-selective ionic channel from sarcoplasmic reticulum of rabbit skeletal muscle has been studied in a planar phospholipid bilayer membrane. The purpose of this work is to study the mechanism by which the channel undergoes transitions between its conducting and nonconducting states. Thermody- namic studies show that the "open" and "closed" states of the channel exist in a voltage-dependent equilibrium, and that the channel displays only a single open state; the channel conductance is 120 pmho in 0.1 M K +. The channel's gating process follows single exponential kinetics at all voltages tested, and the individual opening and closing rate constants are exponentially dependent on voltage. The individual rate constants may also be determined from a stochastic analysis of channel fluctua- tions among multiple conductance levels. Neither the thermodynamic nor the kinetic parameters of gating depend on the absolute concentration of channels in the bilayer. The results are taken as evidence that the channel gates by an unusually simple two- state conformational mechanism in which the equivalent of 1.1 net charges are moved across the membrane during the formation of the open channel. Of the events leading to the contraction of vertebrate skeletal muscle, those involved in the release of Ca ++ from the sarcoplasmic reticulum (SR) mem- brane are the least understood. It is clear that the permeability of the SR membrane to Ca ++ is greatly increased during the release process (Endo, 1977), but it is not known to what extent other ions participate in any voltage and conductance changes that may occur during Ca ++ release. One obvious question to arise from any consideration of Ca ++ movements is: what other ions move across the SR membrane to maintain electroneutralit y and, hence, to permit the rapid, massive fluxes of Ca ++ into and out of the SR throughout the contraction-rela xation cycle? Indirect approaches have yielded estimates of the overall SR conductance (Vergara et al., 1978), but the ionic basis of this is entirely unknown. In particular, the role of K +, the overwhelmingly abundant ion on both sides of the SR membrane in vivo (Somlyo et al., 1977)) is obscure.

108 citations


Journal ArticleDOI
TL;DR: In this article, an improved system of conductance probes is used to identify the flow patterns in two phase horizontal, near horizontal and upward flows, and the results show that this system is very well suited to distinguish among flow patterns consistent with visual observations.

Journal ArticleDOI
TL;DR: In this article, the authors present a general model of the frequency dependence of conductance and capacitance in Schottky diodes and obtain independent, consistent values of the depletion width and of the density of states at the Fermi level and below from both conductance at low and high modulation frequencies.
Abstract: We present a general model of the frequency dependence of conductance and capacitance in a‐Si:H Schottky diodes. In order to circumvent several questionable assumptions required in the analysis of capacitance voltage characteristics, the frequency dependence of sputtered a‐Si:H devices is measured with no applied dc voltage. We obtain independent, consistent values of the depletion width and of the density of states at the Fermi level and below from both conductance and capacitance at both low and high modulation frequencies. We show that the linear frequency dependence of conductance cannot be attributed to hopping conductance, but rather to the interaction of gap states with free carriers. Our study shows that the interaction kinetics of the states around the Fermi level with the conduction‐band carriers is so fast that the response of the diode is limited by the band transport of these carriers, which rapidly thermalize and distribute themselves through the continuum of states from the conduction band ...

Journal ArticleDOI
TL;DR: The findings reported here are consistent with the assumption that the different porins form large aqueous channels in the lipid bilayer membranes and that the single condutance unit is a trimer.
Abstract: The three types of porin (matrix-proteins) fromSalmonella typhimurium with molecular weights of 38,000, 39,000 and 40,000 were reconstituted with lipid bilayer membranes either as a trimer or as an oligomer (complex I). The specific conductance of the membranes increased several orders of magnitude after the addition of the porins into the aqueous phase bathing the membranes. A linear relationship between protein concentration in the aqueous phase and membrane conductance was found. In the case of lower protein concentrations (10−12m), the conductance increased in a stepwise fashion with a single conductance increment of 2.3 nS in 1m KCl. For a given salt the conductance increment was found to be largely independent of the particular porin (38 K, 39K or 40 K) and on the state of aggregation, although porin oligomers showed an up to 10 times smaller conductance increase in macroscopic conductance measurements. The conductance pathway has an ohmic current voltage characteristic and a poor selectivity for different alkali ions. Further information on the structure of the pores formed by the different porins fromSalmonella was obtained from the selectivity for various ions. From the permeability of the pore for large ions (Tris+, glucosamine+, Hepes−_ a minimum pore diameter of 0.8 nm is estimated. This value is in agreement with the size of the pore as calculated from the conductance data for 1m KCl (1.4 nm for a pore length of 7.5 nm). The pore diameter may well account for the sugar permeability which has been found in reconstituted vesicles. The findings reported here are consistent with the assumption that the different porins form large aqueous channels in the lipid bilayer membranes and that the single condutance unit is a trimer. In addition, it is suggested that one trimer contains only one pore rather than a bundle of pores.

Journal ArticleDOI
TL;DR: In this article, the Hodgkin-Huxley type K channels with an average single channel conductance (γK) of 2.40±0.15 pS were studied in internally perfused neurone somata from land snails.
Abstract: Delayed potassium channels were studied in internally perfused neurone somata from land snails. Relaxation and fluctuation analysis of this class of ion channels revealed Hodgkin-Huxley type K channels with an average single channel conductance (γ K) of 2.40±0.15 pS. The conductance of open channels is independent of voltage and virtually all K channels seem to be open at maximum K conductance (g K) of the membrane. Voltage dependent time constants of activation ofg K, calculated from K current relaxation and from cut-off frequencies of power spectra, are very similar indicating dominant first-order kinetics. Ion selectivity of K channels was studied by ion substitution in the external medium and exhibited the following sequence: T1+>K+>Rb+>Cs+>NH 4 + >Li+>Na+. The sequence of the alkali cations does not conform to any of the sequences predicted by Eisenman's theory. However, the data are well accommodated by a new theory assuming a single rate-limiting barrier that governs ion movement through the channel.

Journal ArticleDOI
Raymond M. Fuoss1
TL;DR: Correlations between the values found for R and DeltaG and properties characteristic of salt and solvent are discussed, andThermodynamic derivations are independent of model.
Abstract: Earlier theories of electrolytic conductance are reviewed; all of these, with the exception of the Arrhenius-Ostwald theory, are based on physical models. Their theory failed to describe the conductance of strong electrolytes because it did not include the effects (then unsuspected) of long-range forces on mobility. Thermodynamic derivations are independent of model; applied to the postulated equilibrium A(+) + B(-) right arrow over left arrow A(+)B(-) between free ions and nonconducting paired ions, the thermodynamic pairing constant K(a) equals a(p)/(a+/-)(2), and DeltaG, the difference in free energy between paired ions (activity = a(p)) and free ions (activity = a(+/-)), equals (-RT ln K(a)). Converting to the molarity scale, K(a) = (1000 rho/M)[1 - gamma)/cy(2)(y(+/-))(2)]. Here rho is the density of the solvent of molecular weight M, c is stoichiometric concentration of electrolyte (mol/liter), gamma is the fraction of solute present as unpaired ions, and y(+/-) is their activity coefficient. The corresponding conductance function Lambda = Lambda(c;Lambda(0),R, big up tri, openG)involves three parameters: Lambda(0), the limiting equivalent conductance; R, the sum of the radii of the cospheres of the ions; and DeltaG. Conductance data for cesium bromide and for lithium chloride in water/dioxane mixtures and for the alkali halides in water are analyzed to determine these parameters. Correlations between the values found for R and DeltaG and properties characteristic of salt and solvent are then discussed.

Journal ArticleDOI
TL;DR: In this article, the specific conductances of LiClO4, KSCN, KPF6, and Et4NPF6 in propylene carbonate-dimethoxyethane mixtures are reported for moderate to high concentrations at temperatures between −45°C to 25°C.
Abstract: The specific conductances of LiClO4, KSCN, KPF6, and Et4NPF6 in propylene carbonate-dimethoxyethane mixtures are reported for moderate to high concentrations at temperatures between −45°C to 25°C. The data analysis was established on the base of an empirical conductance equation permitting equally well adapted fits for all electrolyte solutions irrespective of temperature, electrolyte compound and solvent composition. The effects on the conductance resulting from ion-ion, ion-solvent interactions and solvent viscosity are rationalized in terms of the thermodynamic and hydrodynamic parameters characterizing the behaviour of electrolytes in dilute solutions. Some basic aspects are discussed for optimizing the conductance of battery electrolytes, especially those of high energy density batteries.

Journal ArticleDOI
TL;DR: Intracellular recordings from single neurones demonstrated two types of conductance changes in response to applications of L-glutamic acid and a number of structurally related compounds, suggesting at least two distinct conductance mechanisms are linked to excitatory amino acid receptors.
Abstract: Neurones of foetal mouse spinal cord or brain were grown dissociated in tissue culture. Intracellular recordings from single neurones demonstrated two types of conductance changes in response to applications of L-glutamic acid and a number of structurally related compounds. In the presence of 10 mM MgCl2 the responses consisted of an increase in membrane conductance. However, if this divalent cation was kept at ambient concentration a decrease of membrane conductance was also noted (perhaps to potassium ions). Often this decrease in conductance was associated with the appearance of regenerative and spikelike potentials even in the presence of tetrodotoxin. All amino acids tested with the exception of DL-kainic acid produced such a change in conductance. It is concluded that at least two distinct conductance mechanisms are linked to excitatory amino acid receptors.

Journal ArticleDOI
TL;DR: Many of the observations are compatible with the idea that scorpion venoms increase the number of channels which go from the peak Conductance state into the maintained conductance state, but the alternative hypothesis that peak conductance and maintained Conductance reflect two separate populations of Na channels cannot be ruled out.
Abstract: 1. The effect of externally applied scorpion venoms (0.1--0.5 mg/ml., species Leiurus quinquestriatus and Centruroides sculpturatus) on the Na currents of intracellularly perfused squid giant axons has been studied with the voltage-clamp method. 2. The venoms from the two species had the same effect. They reduced the size of the peak conductance but had little effect on its kinetics (time to peak, time constant of inactivation) and on its steady-state inactivation. The venoms increased markedly, however, the maintained conductance and the time constants of its turning-on and turning-off. 3. The voltage dependence of the maintained conductance was determined (a) by fitting a modified Hodgkin--Huxley equation to the Na currents and (b) by measuring the tail currents at the end of depolarizing pulses. The maintained conductance rose with increasing depolarization from a minimum at -20 mV to a maximum at 40 mV. The peak conductance, by contrast, was constant in the positive potential range. 4. The ratio maintained conductance in venom to maintained conductance in control varied between 2 and 7 (depending on the venom concentration and the time of treatment) and was not significantly dependent on membrane potential. 5. Peak current and maintained current reversed sign at the same potential and were both blocked by tetrodotoxin. 6. During a pulse to -2 mV preceded by a pre-pulse to -42 mV the Na conductance showed a rapid initial increase followed by a slower decay and a subsequent slow increase, reflecting the activation and inactivation of the peak conductance and the slow development of the maintained conductance. 7. Many of the observations are compatible with the idea that scorpion venoms increase the number of channels which go from the peak conductance state into the maintained conductance state (open in equilibrium or formed from closed in equilibrium or formed from open transition of the inactivation gate, see Chandler & Meves (1970 a, b)). But the alternative hypothesis that peak conductance and maintained conductance reflect two separate populations of Na channels cannot be ruled out.

Journal ArticleDOI
M. Tanielian1, M. Chatani1, H. Fritzsche1, V. Šmíd1, Peter D. Persans1 
TL;DR: In this paper, a-Si:H planar cinductance G is strongly affected by the space charge layers near surface and the substrate, and changes in G are accompanied by corresponding changes in thermal activation energy, which can be reversed by removing the adsorbed or deposited layers by heating in vacuum.
Abstract: Becauseof the low density of gap states in a-Si:H the planar cinductance G is strongly affected by the space charge layers near surface and the substrate. Several surface layer substances, such as H2O and NH3, act as donors and increase or decrease G of highly resistive n-type or p-type a-Si:H films, respectively, by many orders of magnitude. Other substances such as Se act as acceptors and thus produce the opposite effect. • †G • increses with Se thickness and saturates near 1000A of Se. This is a measure of the space charge width in Se. Changes in G are accompanied by corresponding changes in thermal activation energy. The changes in G can be reversed by removing the adsorbed or deposited layers by heating in vacuum. H2O and NH3 adsorption causes a rapid rise of G of n-type a-Si:H followed by a slow decrease over many hours. The magnitude and time constants of these effects depend on the initial oxide layer thickness. The slow decrease of G is essentially absent for adsorption in situ on freshly deposited a-Si:H.

Journal ArticleDOI
TL;DR: In this paper, the response of stomatal conductance in Pinus sylvestris L. to a sequence of progressively changed photon flux densities showed hysteresis when the direction of the sequence was reversed.
Abstract: . The response of stomatal conductance in Pinus sylvestris L. to a sequence of progressively changed photon flux densities showed hysteresis when the direction of the sequence was reversed. Hysteresis was most evident when 1 h was allowed for stabilization at a temperature of 10°C and a leaf-air vapour pressure difference of 0.5 kPa. The hysteresis was largely eliminated by a stabilization time of 2.5 h or a temperature of 20°C. Elimination of self shading also largely eliminated the hysteresis and resulted in light saturation of stomatal conductance at about 600 μE m−2 s−1 whereas with the normal grouping of fasicles light saturation was not achieved at 1750 μE m−2 s−1 even with bilateral illumination. Hysteresis was also eliminated by reduction in the maximum attainable conductance as a result of large leaf-air vapour pressure differences (> 1.8 kPa) but reducing the ambient CO2 concentration to the compensation concentration or below had no effect on hysteresis. In addition to the hysteresis, there was a carry-over effect of the previous treatment. When the direction of the sequence of photon flux densities was changed, stomatal conductance continued to change in the direction appropriate to the previous sequence for at least 1 h. The presence of a transportable chemical intermediate is postulated, the amount or activity of which would take some time to change after a change in photon flux density. The presence of such an intermediate could account for both the sluggishness of the stomata and the carry over effect. As a result of the sluggish behaviour and carryover, in the field stomatal conductance will tend to follow the general trend in photon flux density and will be very insensitive to short term fluctuations.

Journal ArticleDOI
TL;DR: The electrical potential and conductance of the perfused frog lens preparation was measured using a two internal microelectrode technique and direct evidence for the existence of a voltage-dependent conductance is presented in the form of conductance-voltage curves which show rectification similar to squid axon.

Journal ArticleDOI
TL;DR: The results of experiments showed that the relative magnitude of i chi, fast is not greatly changed by substantially increasing the external K concentration in order to reduce the proportionate effect of K accumulation on the K concentration, reinforcing the conclusion that the process of increasing the extracellular K concentration cannot account for all the time‐dependent changes in outward current during depolarization.
Abstract: 1. Voltage-clamp experiments on frog atrial muscle were designed to distinguish effects due to K accumulation in extracellular spaces from those due to activation of K conductance mechanisms in the membrane. 2. The set of instantaneous current-voltage relations obtained at various external K concentrations following depolarization to about -10 mV for several seconds was found to be quite different from that obtained before the depolarization. Hence the process of increasing the extracellular K concentration cannot account for all the time-dependent changes in outward current during depolarization. 3. Although the instantaneous current-voltage relations obtained at different values of external K concentration before prolonged depolarization show the cross-over phenomenon (Noble, 1965), those obtained at the end of the depolarization did not show this feature. It is concluded that the current-voltage relations for the channels conducting the time-dependent K current do not show cross-over. 4. These results were used to construct a model involving both K activation and K accumulation. This model successfully reproduces the appearance of a very slow component in outward current decay tails which, when subtracted by semi-exponential curve-stripping leaves a component with the real time constant of conductance change. The model does not however reproduce the appearance of a fast decaying component without adding a second conductance mechanism, or assuming non-exponential decay of a single conductance mechanism. 5. It is therefore suggested that i chi, fast is not a perturbation of i chi, slow or of iK1 by the process of K accumulation. This conclusion is reinforced by the results of experiments showing that the relative magnitude of i chi, fast is not greatly changed by substantially increasing the external K concentration in order to reduce the proportionate effect of K accumulation on the K concentration.

Journal ArticleDOI
TL;DR: In this paper, the surface potential as a function of applied bias is evaluated using results of high and low frequency capacitance measurements and the two methods are found to yield roughly similar results.
Abstract: Conductance and capacitance measurements have been used to investigate the interface properties of MOS capacitors formed by depositing an insulating layer of SiO 2 on n -type GaAs. The surface potential as a function of applied bias is evaluated using results of high and low frequency capacitance measurements and the two methods are found to yield roughly similar results. The interface state density evaluated from the conductance and capacitance measurements is found to peak near the center of the band gap and also near the conduction band edge. Plots of conductance vs frequency indicate the presence of both fast and slow interface states in these materials.

Journal ArticleDOI
TL;DR: Voltage-current curves for the isolated midgut of the tobacco hornworm were determined by transient and steady voltage clamping, suggesting that a major effect of metabolic inhibition is to decrease the active conductance of the potassium transport pathway.
Abstract: Voltage-current curves for the isolated midgut of the tobacco hornworm were determined by transient and steady voltage clamping over the range of 200 to −200 mV. Over this range the transient method yields a linear relation while the steady method usually yields a curve consisting of two lines of differing slope which intersect at zero voltage. The difference between the results of the methods is due to a slow decline in total conductance which accompanies steady voltage clamping. Holding the midgut at short circuit increases the total conductance of the tissue in a manner consistent with increasing shunt conductance; this effect was seen in both diet-reared and leaf-reared animals. When potassium transport is inhibited by substitution of choline or sodium for potassium in bathing solution the total conductance decreases and the voltage-current curve intersects the normal curve in the hyperpolarizing region. Applying a simple equivalent circuit analysis to the results from partial or total potassium replacement suggests that the electromotive force of the potassium transport system is of the order of 140–190 mV. The conductance decrease during inhibition of potassium transport by transient anoxia is of similar magnitude, suggesting that a major effect of metabolic inhibition is to decrease the active conductance of the potassium transport pathway.

Journal ArticleDOI
TL;DR: The electrical conductance of the frog lens membranes was observed to increase when the lens was depolarized by current, and to decrease when theLens was hyperpolarized, which reduced the extent to which lens conductance was dependent on voltage.
Abstract: 1. The electrical conductance of the frog lens membranes was observed to increase when the lens was depolarized by current, and to decrease when the lens was hyperpolarized. 2. The total lens conductance (GT) could be described by a voltage-dependent component (GH) together with a voltage-insensitive component (GO). 3. Conductance measured at the resting potential increased from 9.7 to 14.4 x 10(-5) S when the lens was depolarized by 9.1 mV in 10(-4) M-ouabain. The increase of conductance could be explained by an increase of GH which resulted from the depolarization alone. 4. Potassium-rich solutions influenced the conductance by increasing GH as the result of depolarization and by decreasing (blocking) GO. Small increases in the external potassium concentration (from 2.5 to 5 mM), which did not depolarize the lens, resulted in a decrease in resting conductance due to the blockade of GO. 5. Conductance-voltage relationships established in 2.5 (control), 5, 12.5 and 25 mM external potassium could each be fitted to a single mathematical model by assuming that GO had been reduced for 5.0 (control) to 4.5, 3 and 1 x 10(-5) S respectively by those increased potassium concentrations. 6. Barium (2 mM) depolarized the lens by 14.3 mV and decreased the resting conductance by 39%. When the lens was depolarized by a step current pulse in the presence of barium, the conductance increased with time after the onset of the current. Hyperpolarization elicited less time dependence of the conductance. 7. Barium reduced the extent to which lens conductance was dependent on voltage.

Journal ArticleDOI
TL;DR: The results suggest that the mechanism of ion permeation during anomalous rectification changes at about 10 degrees C, and the Q10 of the activation time constant is membrane potential‐dependent above 10 degreesC.
Abstract: 1. The effect of temperature upon the anomalous or inward rectification of the K conductance in the immature egg membrane of the starfish, Mediaster aequalis, was studied by using voltage clamp technique. 2. The K conductance decrease with a relatively small Q10 (1.62) down to about 10 degrees C; below 10 degrees C, the Q10 is much greater (5.8 at [K+]o = 25 mM). The smaller Q10 is independent of [K+]o, whereas the larger one depends on [K+]o. 3. The activation of the rectification depends on V-VK, rather than V alone, at all temperatures at constant internal K concentration. 4. The K conductance at a given V-VK is approximately proportional to the square root of [K+]o at a fixed [K]i above 10 degrees C while the conductance depends substantially less on [K+]o below this temperature. 5. The logarithm of the activation time constant of the inward rectification depends linearly on the membrane potential at all temperatures. 6. The slope of the relation is strongly temperature dependent above about 10 degrees C whilst the dependence is much less below 10 degrees C; i.e. the Q10 of the activation time constant is membrane potential-dependent above 10 degrees C. 7. The results suggest that the mechanism of ion permeation during anomalous rectification changes at about 10 degrees C.

Journal ArticleDOI
TL;DR: Electron microscopic examination of peanut leaves did not reveal any major structural differences between stomata on the two surfaces that would account for the differences in conductance, and it was indicated that two-thirds of the peanut leaf's total net photosynthesis can be attributed to diffusion of CO(2) through the adaxial leaf surface.
Abstract: Conductance to gaseous transfer is normally considered to be greater from the abaxial than from the adaxial side of a leaf. Measurements of the conductance to water vapor of peanut leaves ( Arachis hypogaea L.) under well watered and stress conditions in a controlled environment, however, indicated a 2-fold higher conductance from the adaxial side of the leaf than from the abaxial. Studies of conductance as light level was varied showed an increase in conductance from either surface with increasing light level, but conductance was always greater from the adaxial surface at any given light level. In contrast, measurements of soybean ( Glycine max [L.] Merr.) and snapbean ( Phaseolus vulgaris L.) leaf conductance showed an approximate 2-fold greater conductance from the abaxial surface than from the adaxial. Approximately the same number of stomata were present on both peanut leaf surfaces and stomatal size was similar. Electron microscopic examination of peanut leaves did not reveal any major structural differences between stomata on the two surfaces that would account for the differences in conductance. Light microscope studies of leaf sections revealed an extensive network of bundle sheaths with achloraplastic bundle sheath extensions; the lower epidermis was lined with a single layer of large achloraplastic parenchyma cells. Measurements of net photosynthesis made on upper and lower leaf surfaces collectively and individually indicated that two-thirds of the peanut leaf9s total net photosynthesis can be attributed to diffusion of CO 2 through the adaxial leaf surface. Possibly the high photosynthetic efficiency of peanut cultivars as compared with certain other C 3 species is associated with the greater conductance of CO 2 through their upper leaf surfaces.

Journal ArticleDOI
TL;DR: The potassium conductance system of sensory and motor fibers from the frog Rana esculenta were studied and compared by means of the voltage clamp and it was found that the potassium accumulation is faster and more pronounced in sensory fibers.

Journal ArticleDOI
TL;DR: Inhibitory post‐synaptic currents were recorded under voltage clamp from neurones of Aplysia buccal ganglia to permit the speculation that lengthening average channel lifetime, and therefore time constant of decay, with decreasing temperature, may have adaptive significance in maintaining synaptic efficacy.
Abstract: 1. Inhibitory post-synaptic currents (i.p.s.c.s) were recorded under voltage clamp from neurones of Aplysia buccal ganglia. 2. The synaptic charge, Q, transferred by each i.p.s.c. was calculated as the time integral of the synaptic current, approximated by numerical integration. For typical i.p.s.c.s recorded at or near resting potential, Q = -100 to -500 pC. The majority of the charge is transferred during the interval between the peak and a time one time constant later. 3. In order to characterize an alternative measurement of synaptic efficacy, the slope of the Q vs. membrane potential curve was calculated and defined as the time integral of conductance, b. Values of b ranged from 2.6 to 51 pC/mV, averaging 14 pC/mV. For i.p.s.c.s recorded in thirty-one cells at room temperature, b was well correlated with Gpeak, the peak synaptic conductance (r = 0.86). 4. In most synapses, the time integral of conductance, which incorporates both amplitude and duration, may be a more revealing measure of synaptic efficacy than peak conductance. 5. Size of synaptic response was determined as a function of temperature, T. While Gpeak decreases with decreasing temperature over the range 9-22 degrees C, b peaks at 12-18 degrees C and decreases at higher and lower values of T. The data permit the speculation that lengthening average channel lifetime, and therefore time constant of decay, with decreasing temperature, may have adaptive significance in maintaining synaptic efficacy.

Journal ArticleDOI
TL;DR: The effect of potential on channel lifetime and conductance was more pronounced in denervated than in normal fibres, and there was little difference in net charge transfer through the two types of channel under similar conditions.
Abstract: 1. The average lifetime and conductance of acetylcholine-activated channels were measured in normal and denervated, voltage-clamped toad sartorius muscle fibres at 10 degrees C. 2. The null potential was -4 +/- 1 mV for subsynaptic channels in normal fibres and -6 +/- 3 mV for extrasynaptic channels in denervated fibres. 3. There was a linear relationship between variance of conductance fluctuations and mean conductance for acetylcholine-induced currents up to 50 nA, in denervated fibres clamped at -50 mV. The ratio gave a channel conductance of 14 pS. 4. At the same membrane potential, the average lifetime of extrasynaptic channels in denervated fibres was approximately double, whereas channel conductance was approximately half, that of subsynaptic channels in normal fibres: there was little difference in net charge transfer through the two types of channel under similar conditions. 5. Single channel conductance increased, whereas average channel lifetime decreased, as the membrane potential became more positive (depolarized). The effect of potential on channel lifetime and conductance was more pronounced in denervated than in normal fibres.