Showing papers on "Conductance published in 1990"

Voltage dependence of NMDA-activated macroscopic conductances predicted by single-channel kinetics

Abstract: The conductance activated in many mammalian CNS neurons by the glutamate analog NMDA is inhibited at hyperpolarized potentials by extracellular magnesium. Whole-cell recordings from hippocampal neurons in culture were used to determine the voltage dependence of the NMDA conductance in the presence of extracellular magnesium concentrations from 1 microM to 10 mM. The conductance-voltage data are well fitted by a gating function derived from rate constants determined in an earlier study of the kinetic behavior of single channels activated by NMDA. The results are consistent with the assumption that magnesium inhibits current through the NMDA-activated channel by directly blocking the ion pore. In addition, another voltage-dependent blocking or flicker- producing mechanism has to be invoked to account for the behavior of the conductance at both the single-channel and whole-cell level, especially at low concentrations of extracellular magnesium.

Single-electron charging and periodic conductance resonances in GaAs nanostructures.

Abstract: Narrow channels interrupted by two controlled potential barriers and having a tunable electron density were made in GaAs, and their conductance was measured at low temperatures. Reproducible and accurately periodic oscillations of the conductance with changing density are found to correspond to the sequential addition of single electrons to the segment of the channel between the barriers. Detailed examination of the line shape of the conductance versus density provides a new insight into the transport mechanism.

Potential Fluctuations in Heterostructure Devices

Abstract: Ionized donors in a doped heterostructure have random positions, which gives rise to a random potential with long-ranged fluctuations. This has a small effect on fast electrons, allowing high mobilities in two-dimensional electron gases, but becomes much more significant when the density of electrons is low. We have analysed the effect of this random potential using a semi-classical, self-consistent method to calculate the density of electrons. This includes a realistic potential from a patterned gate, and is followed by a quantum-mechanical calculation of the conductance.

Properties of channels reconstituted from the major intrinsic protein of lens fiber membranes.

Abstract: Detergent-solubilized plasma membrane protein of either adult bovine or calf lens and high-performance liquid chromatography-purified major intrinsic protein (MIP) of the lens were reconstituted into unilamellar vesicles and planar lipid bilayers. Freeze-fracture studies showed that the density of intramembrane particles in the vesicles was proportional to the protein/lipid ratio. At high ratios, these particles crystallized into tetragonal arrays as does MIP in lens fibers. Channels induced by either purified MIP or detergent-solubilized protein had essentially identical properties. The conductance of multichannel membranes was maximal near 0 mV and decreased to 0.49 +/- 0.08 of the maximum value at voltages greater than 80 mV. The dependence of the conductance on voltage was well fit by a two-state Boltzmann distribution. Voltage steps greater than 30 mV elicited an ohmic current step followed by a slow (seconds) biexponential decrease. The amplitudes and time constants depended on the magnitude but not the sign of the voltage. Steps from 100 mV to voltages less than 30 mV caused the channels to open exponentially with a millisecond time constant. Analysis of latency to first closure after a voltage step gave nearly the same time constants as multichannel kinetics. Single-channel conductance is proportional to salt concentration from 0.1 to 1.0 M in KCl. In 0.1M KCl, the channel had two preferred conductance states with amplitudes of 380 and 160 pS, as well as three additional substates. Multi- and single-channel data suggest that the channel has two kinetically important open states. The channel is slightly anion selective. The properties of the channel do not vary appreciably from pH 7.4 to 5.8 or from pCa 7 to 2. We propose that a channel with these properties could contribute to maintenance of lens transparency and fluid balance.

Permeation in the dihydropyridine-sensitive calcium channel. Multi-ion occupancy but no anomalous mole-fraction effect between Ba2+ and Ca2+.

Abstract: We investigated the mechanism whereby ions cross dihydropyridine-sensitive (L-type) Ca channels in guinea pig ventricular myocytes. At the single-channel level, we found no evidence of an anomalous mole-fraction effect like that reported previously for whole-cell currents in mixtures of Ba and Ca. With the total concentration of Ba + Ca kept constant at 10 (or 110) mM, neither conductance nor absolute unitary current exhibits a paradoxical decrease when Ba and Ca are mixed, thereby weakening the evidence for a multi-ion permeation scheme. We therefore sought independent evidence to support or reject the multi-ion nature of the L-type Ca channel by measuring conductance at various permeant ion concentrations. Contrary to the predictions of models with only one binding site in the permeation pathway, single-channel conductance does not follow Michaelis-Menten kinetics as Ba activity is increased over three orders of magnitude. Two-fold variation in the Debye length of permeant ion solutions has little effect on conductance, making it unlikely that local surface charge effects could account for these results. Instead, the marked deviation from Michaelis-Menten behavior was best explained by supposing that the permeation pathway contains three or more binding sites that can be occupied simultaneously. The presence of three sites helps explain both a continued rise in conductance as [Ba2+] is increased above 110 mM, and the high single-channel conductance (approximately 7 pS) with 1 mM [Ba2+] as the charge carrier; the latter feature enables the L-type channel to carry surprisingly large currents at physiological divalent cation concentrations. Thus, despite the absence of an anomalous mole-fraction effect between Ba and Ca, we suggest that the L-type Ca channel in heart cells supports ion flux by a single-file, multi-ion permeation mechanism.

Visibility of synaptically induced conductance changes: theory and simulations of anatomically characterized cortical pyramidal cells

Abstract: A recent report has provided evidence that there are no significant increases in the neuronal input conductance during the response of cortical cells in cat visual cortex to non-preferred visual stimuli (Douglas et al., 1988). A criticism of experiments of this kind is that changes in the membrane conductance occurring in the dendritic tree may not be visible from electrodes that impale the soma. Our paper describes theoretical and numerical results concerning the visibility of synaptically induced conductance changes from intracellular electrodes, in both ideal and anatomically well-characterized cortical neurons. Based on earlier work by Rall (1967), we here derive theoretical expressions for the change in input conductance at any location in a passive dendritic tree resulting from activation of a single synapse and obtain bounds for the effects of multiple synapses. We find that the conductance change measured at the cell body is always less than the sum of the synaptic conductance changes and that this observed conductance change does not depend on the synaptic reversal potential. For the case of an infinite dendritic cylinder, the change in input resistance due to a single synaptic input decays exponentially with distance of the synapse from the recording site. Numerical simulations of synaptic inputs that change approximately as fast as the membrane time-constant produce an increase in input conductance that is only slightly less visible than that of a constant input. We also compute the changes in somatic input conductance of 2 morphologically identified pyramidal cells from cat visual cortex during activity of a single inhibitory basket cell with known synaptic input locations. We find that the increase in conductance due to the activity of the inhibitory basket cells is clearly visible from the cell body of the pyramidal cells and that a 70% reduction in the amplitude of excitation is associated with at least a 30% increase in somatic input conductance, which would be visible in intracellular recordings. Taken together with the negative experimental evidence of Douglas et al. (1988), our results cast doubt on a large class of models of direction selectivity that rely on synaptically mediated inhibitory conductance increases to veto or block excitatory conductances increases.

Unbinding of charge-anticharge pairs in two-dimensional arrays of small tunnel junctions

Abstract: We describe the behavior of charges in two-dimensional arrays of normal-metal tunnel junctions with very small capacitance. A Kosterlitz-Thouless-Berezinskii phase transition with unbinding of charge-anticharge pairs occurs at a transition temperature of about Tc = e2/8πCkB, with C the junction capacitance. We calculate the influence of tunneling conductance; Tc is reduced with increasing conductance and no transition occurs for junction conductance above (14 kΩ)–1. In the superconducting state a similar transition occurs at a 4 times higher Tc. We present the first experimental results on the conductive transition of an array in the normal and superconducting states.

Conductance of 1,1-electrolytes in acetonitrile solutions from −40° to 35°C

Abstract: Conductance data for perchlorates of Li+, K+, Me4N+, Et4N+, Pr4N+, Bu4N+, iodides of K+, Me4N+, i-Am3BuN+, and tetraphenylborates of Na+, Bu4N+ and i-Am3BuN+ in acetonitrile solution in the temperature range −40° to 35°C are reported Λ° (limiting molar conductance) and KA (association constant) are evaluated for several temperatures using a conductance equation based on the chemical model of electrolyte solutions including short range forces Limiting molar ion conductances, λ ΰ i , at −35°, −25°, −15°, −5°, 5°, 15° and 25°C are evaluated from temperature dependent limiting transference numbers Enthalpies and entropies of association, obtained from the temperature dependence of the association constants, are also presented

Current-clamp analysis of a time-dependent rectification in rat optic nerve.

Abstract: 1. Rat optic nerves were studied using intra-axonal and whole-nerve recording techniques in a sucrose-gap chamber. Constant-current pulses were applied across the outer compartments of the chamber to achieve a current clamp. 2. The nerves displayed a prominent time-dependent conductance increase elicited by a hyperpolarizing constant-current pulse, as evidenced by a relaxation or 'sag' in membrane potential towards resting potential. The inward current began at about 80 ms and reached a steady level over the next 100-200 ms. Its magnitude progressively increased with increasing levels of hyperpolarization. 3. The inward current elicited by hyperpolarization was reduced, but not abolished, when Na+ was reduced from the normal bath concentration of 151 mM to 0 mM. In Na(+)-free solutions the bath K+ concentration, [K+]o, was varied between 0 and 5 mM; the inward current was greatest when [K+]o was 5 mM and was abolished when [K+]o was zero. 4. The inward current was not abolished by tetrodotoxin (TTX), tetraethylammonium (TEA) or 4-aminopyridine (4-AP) suggesting that conventional voltage-dependent sodium and potassium channels do not underlie the time-dependent conductance increase. Low concentrations of Cs+ completely blocked the inward current, and Ba2+ induced a partial block. External application of divalent cations (Cd2+ and Mg2+) did not block the inward current. These properties are similar to the inwardly rectifying conductance observed in a central nervous system neurone. 5. Stimulus-response curves obtained during the hyperpolarization pulse, before and during the conductance increase, indicate that excitability is increased during the conductance increase. This along with the intra-axonal recordings demonstrates that the origin of the increased conductance is axonal and not glial. 6. It is concluded that central nervous system myelinated fibres in rat optic nerve display a prominent time-dependent conductance increase in response to hyperpolarization that depends on both Na+ and K+ and is blocked by Cs+. This conductance is similar to an inward rectifier described for a variety of neurone types. The increased axonal excitability observed during the conductance increase suggests that its functional role may be to maintain or stabilize axonal excitability during periods of intense action potential activity.

Kinetics of the conductance evoked by noradrenaline, inositol trisphosphate or Ca2+ in guinea-pig isolated hepatocytes.

Abstract: 1. Guinea-pig hepatocytes respond to noradrenaline (NA, 5-10 microM) with a large membrane conductance increase to K+ and Cl-. The response has a long initial delay (range 2-30 s). Following the delay, the K+ conductance (studied in Cl(-)-free solutions) rises quickly to a peak in 1-2 s and is maintained in the continued presence of NA, though often with superimposed oscillations of conductance. The roles of intracellular Ca2+ and D-myo-inositol 1,4,5-trisphosphate (InsP3) in this complex response have been investigated by rapid photolytic release of intracellular Ca2+ (from Nitr5-Ca2+ buffers) or InsP3 from 'caged' InsP3. 2. A rapid increase of intracellular [Ca2+] produced an immediate membrane conductance increase which rose approximately exponentially to a new steady level, consistent with a direct activation of Ca2(+)-dependent ion channels. 3. Following a pulse of InsP3, conductance rose after a brief delay (range 70-1500 ms) which was shortest at high [InsP3] or if the initial cytosolic [Ca2+] had been raised above normal levels. The maximum conductance produced by InsP3 was similar in each cell to the peak recorded with NA and could be evoked by InsP3 concentrations of 0.5-1 microM. 4. The rates of rise of conductance increased with InsP3 concentration in the range 0.25-12.5 microM (range 10-90%, rise times 90-1000 ms), indicating that InsP3-evoked Ca2(+)-efflux from stores increases with InsP3 concentration in this range. 5. Photochemically released InsP3 and Ca2+ activate at physiological concentrations the same membrane conductances as NA. The results indicate that the long initial delay in NA action occurs prior to or during generation of InsP3. The mechanism of the delay and the subsequent apparently all-or-none conductance increase during NA action are discussed in terms of the high co-operativity in InsP3 and Ca2+ actions and an additional positive feedback step. 6. Evidence was found of a negative interaction between [Ca2+] and InsP3-evoked Ca2+ release. The time course of the recovery of InsP3-evoked Ca2+ release following a rise of cytosolic [Ca2+] suggests that this interaction may be important in regulating oscillatory responses of [Ca2+] during hormonal stimulation of guinea-pig hepatocytes.

Modulation of potassium channels in human T lymphocytes: effects of temperature.

Abstract: 1. The predominant channels found in lymphocytes with patch-clamp whole-cell recordings are voltage-gated K+ channels. Several lines of evidence suggest that these channels are involved in lymphocyte function. Most lymphocyte functions are temperature sensitive and have not been correlated with electrophysiology at different temperatures. We have examined the effect of temperature on the voltage-dependent K+ channel in normal human T lymphocytes. Both macroscopic current and single-channel events were studied with whole-cell recordings at temperatures from 5 to 42 degrees C. 2. Peak conductance, activation rate, inactivation rate and rate of recovery from inactivation all increased progressively as the temperature increased. The effect of temperature on channel opening processes was greater at low temperatures. In contrast, the inactivation process was most sensitive to temperature changes above room temperature. Arrhenius plots of conductance and kinetic parameters were curvilinear with no obvious break-points. 3. The increase in whole-cell conductance at 37 degrees C was due to both an increase in the single-channel conductance and in the probability that each channel is open at any time. 4. K+ currents were fitted by Hodgkin-Huxley equations with n4j kinetics providing the best description of the currents at all temperatures tested. 5. Steady-state activation- and inactivation-voltage curves shifted in opposite directions with warming, resulting in a greater area of overlap of the curves ('window' current). The increase in resting K+ channel activity predicted by a greater window current was confirmed with single-channel measurements. 6. The present study has shown that the behaviour of K+ channels in human T lymphocytes is temperature dependent.

Conductance oscillations periodic in the density of one-dimensional electron gases

Abstract: Results are reported of a detailed study of the conductance oscillations in one-dimensional (1D) Si inversion layers. A comparison is made with results for GaAs accumulation layers. The conductance oscillates by as much as a factor of 100 and is accurately periodic in the number of electrons per unit length. The period varies randomly from sample to sample, and changes when a single sample is warmed to room temperature and remeasured at low temperature. Multiple periods are often observed, and the amplitude of the individual frequency components can be altered by moving the electron gas from side to side with a transverse electric field. These observations suggest that the period is determined by the distance between charged defects near the 1D channel. Measurements of the temperature dependence indicate that the oscillatory conductance reflects a periodic energy for a thermally activated conductance mechanism as well as a parallel oscillatory tunneling mechanism. The period of the conductance oscillations is found to be surprisingly independent of magnetic field B. However, the random modulation of the amplitude of the oscillations is reduced by a B field normal to the semiconductor surface, as it is by increasing temperature.

A large, multiple-conductance chloride channel in normal human T lymphocytes.

Abstract: Chloride (Cl) channels have been proposed to play roles in lymphocyte functions including volume regulation and cellular cytotoxicity; however, direct studies of such channels in normal human lymphocytes are lacking. In the present study we describe a large conductance Cl channel observed in about 50% of excised, inside-out patches from normal human peripheral T lymphocytes. The channel has multiple conductance states with linear single-channel current-versus-voltage relationships in symmetrical Cl solutions. The most prevalent state is the largest, which has a conductance of about 365 pS. The channel closes in a voltage-dependent manner at both negative and positive potentials, but does not show voltage-dependent inactivation. The probability of opening is maximal between −15 mV and +15 mV and the voltage dependence is well described by two Boltzmann equations with half-maximal probabilities at −22.8 mV and +18.0 mV. The slopes of the voltage dependence suggest two gates in series with 5.7 and 9.6 equivalent charges. The channel was about 30 times more selective for Cl− than for Na+ or K+ under balanced osmolarity but less selective (approx. 11∶1) under a large osmotic gradient. The single-channel conductance increased with Cl concentration with an apparent saturation at about 581 pS and a Michaelis-Menten constant of about 120 mM. The selectivity sequence among anions, determined from changes in reversal potential was: I− > NO3−> Br−, Cl− > F−, isethionate, HCO3−> SO42−> gluconate, propionate > aspartate ≫ Na+, K+ and was apparently the same for subconductance states. The sequence determined from measurable values of single-channel conductance was: I− > NO3−> Br− > Cl− > F− > HCO3−, isethionate. The channel was rapidly and reversibly blocked by 1 mM Zn2+ or 1 mM Ni2+ added to the cytoplasmic face. Possible roles of this maxi-Cl channel in lymphocyte function are discussed.

Measurement of the Thermal Contact Conductance and Thermal Conductivity of Anodized Aluminum Coatings

Abstract: An experimental investigation was conducted to determine the thermal contact conductance and effective thermal conductivity of anodized coatings. One chemically polished Aluminum 6061-T6 test specimen and seven specimens with anodized coatings varying in thickness from 60.9 {mu}m to 163.8 {mu}m were tested while in contact with a single unanodized aluminum surface. Measurements of the overall joint conductance, composed of the thermal contact conductance between the anodized coating and the bare aluminum surface and the bulk conductance of the coating material, indicated that the overall joint conductance decreased with increasing thickness of the anodized coating and increased with increasing interfacial load. Using the experimental data, a dimensionless expression was developed that related the overall joint conductance to the coating thickness, the surface roughness, the interfacial pressure, and the properties of the aluminum substrate. By subtracting the thermal contact conductance from the measured overall joint conductance, estimations of the effective thermal conductivity of the anodized coating as a function of pressure were obtained for each of the seven anodized specimens. At an extrapolated pressure of zero, the effective thermal conductivity was found to be approximately 0.02 W/m-K. In addition to this extrapolated value, a single expression for predicting the effective thermal conductivitymore » as a function of both the interface pressure and the anodized coating thickness was developed and shown to be within {plus minus}5 percent of the experimental data over a pressure range of 0 to 14 MPa.« less

Anomalous H + and D + conductance in H 2 O–D 2 O mixtures

Abstract: A KNOWLEDGE of proton-transfer dynamics and hydrogen-bonding in water and aqueous solutions is necessary for the understanding of many important chemical and biological processes. For example, quantum effects related to proton transfer (or tunnelling) in H+(H2O)n clusters of liquid water (where n = 1,2,· · ·) are known to have a dominant role in the proton conductance mechanism1,2 and are responsible for the high conductances of H+ and OH– in water. A new quantum theoretical approach to this process has been presented3, which is based on the hypothesis that there are quantum correlations4–8 between each H+ and the protons of the surrounding water molecules, leading to the formation of coherent dissipative structures3,8. From further investigations, one of us predicted that an anomalous decrease of H+ conductance in H2O–D2O mixtures would take place9. Having thought of an experiment to test these predictions9 we now report the experimental results and conclude that an anomalous decrease in proton conductance does indeed occur.

Distinction between dipolar and inductive effects in modulating the conductance of gramicidin channels.

Abstract: The ion permeability of transmembrane channels formed by the linear gramicidins is altered by amino acid sequence substitutions. We have previously shown that the polarity of the side chain at position one is important in modulating a channel's conductance and ion selectivity [Russel et al. (1986) Biophys. J. 49, 673-686]. Changes in polarity could alter ion permeability by (through-space) ion-dipole interactions or by (through-bond) inductive electron shifts. We have addressed this question by investigating the permeability characteristics of channels formed by gramicidins where the NH2-terminal amino acid is either phenylalanine or one of a series of substituted phenylalanines: p-hydroxy-, p-methoxy-, o-fluoro-, m-fluoro-, or p-fluorophenylalanine. The electron-donating or -withdrawing nature, as quantified by the Hammett constant, ranges from -0.37 to +0.34 for these side chains. Channels formed by these gramicidins show a more than 2.5-fold variation in their Na+ conductance, but the conductance variations do not rank in the order of the Hammett constants of the side chains. Inductive effects cannot therefore be of primary importance in the modulation of the gramicidin single-channel conductance by these side chains. The results support previous suggestions that electrostatic interactions between side chain dipoles and permeating ions can modify the energy profile for ion movement through the gramicidin channel and thus alter the conductance.

Oxygen gas sensing characteristics at ambient pressure of undoped and lithium-doped ZnO-sputtered thin films

Abstract: Characteristics of sensing oxygen, present in some vol.% concentration in nitrogen at atmospheric pressure, for ZnO thin films, are reported here; these characteristics were obtained by means of simple electrical conductance measurements. The thin films were grown by reactive sputtering, in a 28% O2/Ar atmosphere, starting from a ZnO target; either pure or 1 mol.% Li doped. The lithium content of the films was measured using nuclear reaction analysis. The crystallographic orientation and the surface structure of the films were studied using X-ray diffraction and reflection high energy electron diffraction respectively. The undoped films showed a higher (002) preferential orientation with respect to the doped films. The conductance of lithium-doped films decreased more than undoped film conductance in the presence of oxygen; the conductance of both thin films followed a power law at operating temperatures higher than 673 K, while at a temperature lower than 673 K conductance vs. oxygen partial pressure followed a linear law. The different response laws of conductance towards oxygen are explained as resulting from the migration of oxygen vacancies from the material bulk towards the surface at temperatures higher than 673 K and as oxygen ionosorption on to the surface sites at temperatures lower than 673 K.

Potassium and chloride conductances in rat Leydig cells: effects of gonadotrophins and cyclic adenosine monophosphate.

Abstract: 1. The effects of gonadotrophins (luteinizing hormone and human chorionic gonadotrophin) and cyclic AMP on ionic conductances were investigated using the tight-seal whole-cell recording technique in Leydig cells freshly isolated from nature rat testis by enzymatic treatment. 2. In resting cells, the predominant ionic conductance is a voltage-dependent K+ conductance resembling the delayed rectifier K+ conductance of T-lymphocytes. This conductance is characterized by: (1) a time-dependent inactivation for potentials more positive than +20 mV, (2) a reversal potential near -65 mV, (3) a sensitivity to intracellular Cs+, and (4) a sensitivity to extracellular TEA and 4-aminopyridine. 3. A Cl- conductance is also present resembling the Cl- background conductance in squid axons and heart cells. In resting cells, this conductance contributes only a small component of the total outward current obtained with depolarizing pulses. 4. Gonadotrophins (human chorionic gonadotrophin, porcine luteinizing hormone and ovine luteinizing hormone) have little effect on the K+ conductance. They transiently increase a Cl- conductance after a delay of up to 30 s. This response does not occur if the hormones are applied late in the whole-cell recording. Gonadoliberine (GnRH) does not affect the Cl- or K+ conductance. 5. Internal cyclic AMP (100 microM) mimics all these effects while internal application of a GTP-ATP mixture induces a similar response, which is, however, sustained rather than transient. 6. The Cl- conductance was studied quantitatively with a GTP-ATP internal solution. This conductance is activated by depolarizing voltage steps to test potentials of -40 mV or more. Under these conditions, the instantaneous current observed as soon as the depolarizing pulse is applied displays outward rectification and reverses near ECl. During the pulses, a strong inactivation is observed for potentials greater than +40 mV. This conductance is independent of external and internal calcium. 7. It is concluded that the gonadotrophins act through a cyclic AMP-dependent process to activate a Cl- conductance. This conductance is different to the hyperpolarization-activated Cl- conductance and the calcium-activated Cl-conductance also present in the membrane of resting cells.

Effects of channel opening and disorder on the conductance of narrow wires.

Abstract: The crossover from ballistic to diffusive transport in narrow two-dimensional conductors is studied as a function of Fermi energy and disorder. For an ordered sample, conductance quantization is confirmed. As the disorder is increased, the sharp conductance steps as a function of Fermi energy are rounded and preceded by pronounced dips. The origin of these dips is explained in terms of ``level repulsion'' between Lyapunov exponents of the total transfer matrix. For even larger disorder, the channel-opening signature in a given sample is obscured by universal conductance fluctuations. However, this structure can be restored, even for samples longer than the elastic mean free path, by ensemble averaging over different realizations of the disorder. The conditions for carrying out experimentally such an ensemble averaging are specified.

L-type Ca-channels : similar Q10 of Ca-, Ba- and Na-conductance points to the importance of ion-channel interaction

Abstract: The temperature-dependence of currents through L-type Ca-channels was studied in myocytes isolated from the urinary bladder of the guinea pig. Currents were measured at 22 °C and 35 °C with Ca-, Ba- and Na-ions as charge carrier. The higher temperature increased the open channel conductance for Ca-ions from 8.5 to 16 pS (Q10 =1.63±0.07, mean ± S.D.), for Ba-ions from 24 to 43 pS (Q10=1.55±0.06), and for Na-ions (pH 9) from 74 to 131 pS (Q10 of 1.55±0.09). The differences in the Q10's are not significant, the activation energy approximates a common high value of 34.8±2.5 kJ/mol. A three barrier model with intra-channel binding predicts high Q10's for Ca and Ba but not for Na. To fit the results we postulate that the temperature-dependence reflects multiple ion-channel interactions within a central permeability barrier, e.g. polar groups substituting part of the ionic water shell.

Changes in the Conductance of the Neuronal Nicotinic Acetylcholine Receptor Channel Induced by Magnesium

Abstract: We have investigated the effect of magnesium on the single-channel conductance of neuronal nicotinic acetylcholine receptors (nAChRs) in nerve growth-factor treated rat pheochromocytoma (PC12) cells. The patch-clamp technique was used to record single-channel currents from cell attached and excised, outside-out patches in the presence of various internal and external Mg$^{2+}$ concentrations. Mg$^{2+}$ reduced the single-channel conductance in a concentration-dependent manner with an IC$_{50}$ of 9.2 mM for external Mg$^{2+}$ (inward conductance) and 0.69 mM for internal Mg$^{2+}$ (outward conductance). Both estimated and measured conductances for divalent cation-free CsCl solutions were around 60 pS. We also find that divalent cations are not involved in the inward rectification of whole-cell ACh-induced currents in these cells. Our results imply that the amino acids screened by divalent cations sense electric fields only weakly and are presumably outside the lipid bilayer. They also suggest that the density and the number of charges (or both) differ on either side of the ion pore.

A chloride channel from lobster walking leg nerves. Characterization of single-channel properties in planar bilayers.

Abstract: A novel, small conductance of Cl- channel was characterized by incorporation into planar bilayers from a plasma membrane preparation of lobster walking leg nerves. Under conditions of symmetrical 100 mM NaCl, 10 mM Tris-HCl, pH 7.4, single Cl- channels exhibit rectifying current-voltage (I-V) behavior with a conductance of 19.2 +/- 0.8 pS at positive voltages and 15.1 +/- 1.6 pS in the voltage range of -40 to 0 mV. The channel exhibits a negligible permeability for Na+ compared with Cl- and displays the following sequence of anion permeability relative to Cl- as measured under near bi-ionic conditions: I- (2.7) greater than NO3- (1.8) greater than Br- (1.5) greater than Cl- (1.0) greater than CH3CO2- (0.18) greater than HCO3- (0.10) greater than gluconate (0.06) greater than F- (0.05). The unitary conductance saturates with increasing Cl- concentration in a Michaelis-Menten fashion with a Km of 100 mM and gamma max = 33 pS at positive voltage. The I-V curve is similar in 10 mM Tris or 10 mM HEPES buffer, but substitution of 100 mM NaCl with 100 mM tetraethylammonium chloride on the cis side results in increased rectification with a 40% reduction in current at negative voltages. The gating of the channel is weakly voltage dependent with an open-state probability of 0.23 at -75 mV and 0.64 at +75 mV. Channel gating is sensitive to cis pH with an increased opening probability observed for a pH change of 7.4 to 11 and nearly complete inhibition for a pH change of 7.4 to 6.0. The lobster Cl- channel is reversibly blocked by the anion transport inhibitors, SITS (4-acetamido, 4'-isothiocyanostilbene-2,2'-disulfonic acid) and NPPB (5-nitro-2-(3-phenylpropylamino)benzoic acid). Many of these characteristics are similar to those previously described for small conductance Cl- channels in various vertebrate cells, including epithelia. These functional comparisons suggest that this invertebrate Cl- channel is an evolutionary prototype of a widely distributed class of small conductance anion channels.

Multiple conductance levels of the dihydropyridine-sensitive calcium channel in GH3 cells.

Abstract: Calcium channels in GH3 cells exhibit at least five conductance levels when examined in cell-attached or outside-out patches. These channels resemble the high threshold Ca2+ current in their range of activation and inactivation, and in their sensitivity to dihydropyridines (DHP). Mean open times for the five levels were brief (less than 1 msec) in control solutions but increased in the presence of BAY K 8644. In 100 mM Ba2+ and BAY K 8644, the five predominant slope conductances were 8-9, 12-13, 16-18, 23-24, and 28 pS. The present study is the first report of multiple levels of the DHP-sensitive Ca2+ channel occurring with high frequency in native membranes. The range of conductance levels that we observed encompasses the range of conductances found for two other different types of Ca2+ channels and indicates that unit conductance should be used with caution as a distinguishing characteristic for identification of different channel types.