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B Neumcke

Publications and source records attributed to B Neumcke.

At least 37 records · Page 2Linked to original sources
PubMed1987

A high-conductance anion channel in adult amphibian skeletal muscle.

Membrane patches were excised from enzymatically dissociated frog toe muscle. High-conductance anion channels could be induced in previously quiet patches by 20-120 s depolarizations beyond +20 mV and then studied in the potential range from -80 to +60 mV for a long time. From reversal potentials the estimated permeability ratios PCl/PNa and PCl/Pglucuronate were near 3.5 and 4, respectively. There were probably 5 or more conductance levels (substates) for a single channel, the most common in symmetrical 110 mM NaCl being 260 and 70 pS at 10 degrees C. Gating was complex, with rapid and slow events and several gating modes, including periods of rapid flickering. Channels closed reversibly at potentials more negative than -50 mV. The channel was blocked by application to the cytoplasmic face of tannic acid, gallic acid, and zinc but not of DIDS or 9-anthracene-carboxylic acid, and it was blocked by extracellular zinc.

Animals↗
PubMed1987

Conductance properties and voltage dependence of an anion channel in amphibian skeletal muscle.

Single anion-selective channels were studied in excised membrane patches of adult frog toe muscle. The conductance gamma and the probability po of the main open state were determined for various ionic compositions of the extra- and intracellular solutions. gamma = 280 pS in symmetrical 110 mM NaCl, pH 7.4 solutions at 15 degrees C. Higher gamma values were found at elevated internal or external NaCl concentrations, in 70 mM external CaCl2 and at lower extracellular pH. The po(E) curve declined steeply with hyperpolarization and was shifted towards more negative potentials at increased internal ionic strength and at higher external pH. Positive shifts were induced by extracellular Ca. The results show that the anion channel saturates at Cl concentrations greater than 110 mM, that the potential profile of an open channel is almost symmetrical and that channel gating is affected by neighboring channels. It is suggested that the anion channel has a voltage sensor (effective gating charge 4.3) and a similar collection of local fixed charges on the extra- and intracellular sides as voltage-gated cation channels.

Animals↗
PubMed1987

A comparison of sodium currents in rat and frog myelinated nerve: normal and modified sodium inactivation.

1. Sodium currents were measured under voltage-clamp conditions in Ranvier nodes of rat and frog nerve fibres at 20 degrees C. Voltage errors due to the resistance in series with the nodal membrane were minimized by reducing sodium currents with tetrodotoxin in the extracellular solutions. 2. The stationary and kinetic properties of sodium activation and inactivation were determined for a wide range of potentials (V) from -40 to 160 mV with respect to the initial holding level (V = 0 mV). 3. The curves m infinity(V) and h infinity(V) of stationary sodium activation and inactivation were not different in rat and frog fibres. 4. The time constants tau m, tau h of sodium activation and inactivation were normally larger in the rat than in the frog. At moderate depolarizations (0 less than or equal to V less than or equal to 80 mV) tau m in the rat was 15-50% larger; the ratio of the rat to the frog tau h values was usually smaller. Thus tau m/tau h = 0.116 for the rat and 0.0965 for the frog at V = 60 mV (potential with maximum peak sodium inward current). 5. Sodium inactivation in rat nerve was slowed and became incomplete by application of intra-axonal iodate or by treatment with external Anemonia toxin II (ATX II), chloramine-T or Ruthenium Red. Peak sodium currents were not increased by these substances. 6. Wash-out of ATX II from frog nerve was rapid and complete but partly irreversible in rat nerve. This suggests different properties or accessibilities of sodium channels in frog and rat nodes.

Action Potentials↗
PubMed1985

Comparison of the effects of Anemonia toxin II on sodium and gating currents in frog myelinated nerve.

Na+ and gating currents were measured in myelinated frog nerve fibres without and in the presence of 7 microM Anemonia toxin II in the extracellular solution. From the experiments, kinetic parameters of Na+ currents and of gating charge displacements during ('on' response) and after ('off' response) depolarizations were determined. The following parallel modifications of Na+ currents and charge displacements by Anemonia toxin II were observed: the toxin reduces the maximum Na+ permeability and the 'on' charge displacement; Na+ activation and 'on' charge displacement become faster; Na+ inactivation and the decline of the 'off' charge displacement with increasing pulse duration (charge immobilization) are prolonged; slow components of 'on' charge displacements are diminished. The observations support the notion that the fast 'on' charge displacement is connected with the process of Na+ activation, while Na+ inactivation is linked to charge immobilization. Our experiments suggest that slow 'on' charge displacements during longer depolarizations are correlated with the process of Na+ inactivation.

Animals↗
PubMed1985

[Sodium channels in electrically excitable membranes].

Properties of sodium channels in electrically excitable membranes can be obtained from sodium currents, gating currents, sodium-current fluctuations and currents through single channels. Gating processes in sodium channels are related to intramembranous charge displacements and are modified by toxins, local anaesthetics and chemical agents. From the results a schematic diagram of a sodium channel can be derived.

Action Potentials↗
PubMed1984

Heterogeneity of external surface charges near sodium channels in the nodal membrane of frog nerve.

The conductance gamma and the number of No of Na channels in the nodal membrane of frog nerve fibres were determined from ensemble average values of the Na current and the variance of Na current fluctuations. Replacement of extracellular Cl- by NO3- shifts the voltage dependencies of all Na gating parameters towards more negative voltages, reduces gamma by a factor of 0.84 and hardly changes the number No of channels not blocked by 8 nM TTX. Adding 0.1 mM LaCl3 to the extracellular solution shifts the voltage dependencies of all Na gating parameters towards more positive voltages, reduces gamma by a factor of 0.75 and hardly changes the number No of channels not blocked by 8 nM TTX. It is concluded that changes of the external surface potential induced by Cl-, NO3- replacement do not alter the local Na+ concentration in the outer mouth of the Na channel and hardly affect the TTX binding to toxin receptors. Surface potential changes by addition of LaCl3 also have no clear effect on TTX binding. The reduction of gamma in 0.1 mM LaCl3 is probably due to a direct interaction of La3+ with Na channels. Our results suggest a heterogeneous distribution of external fixed surface charges in the outer mouth of the Na channel, at the TTX binding site and near the Na channel gates.

Animals↗
PubMed1984

The effect of local anaesthetics on the components of the asymmetry current in the squid giant axon.

The effects of local anaesthetics and holding potential on sodium and asymmetry currents were studied in intracellularly dialysed squid giant axons. The asymmetry currents were fractionated into their inactivating and non-inactivating components, and the charge displacements Qi and Qn of the two components were determined for pulse potentials between -20 and +40 mV. The charged local anaesthetic RAD 366, a quaternary derivative of lidocaine, applied internally at a concentration in the dialysis solution of 1 mM, did not change Qn, but reduced Qi about 3-fold. The neutral local anaesthetic benzocaine, applied externally at a concentration in the bathing solution of 1 mM, had effects very similar to RAD 366. It did not change Qn, but reduced Qi and the sodium current about 2 . 5-fold. Unlike local anaesthetics, steady membrane depolarization had essentially equal effects on Qn, Qi and sodium current. Lowering the holding potential from -98 to -60 mV for several minutes reduced all three variables to about half. Models of sodium channel voltage-gating are discussed which implicate both Qn and Qi, and which account for the selective blockage of Qi by sodium inactivation and local anaesthetics.

Action Potentials↗
PubMed1983

Alteration of the conductance of Na+ channels in the nodal membrane of frog nerve by holding potential and tetrodotoxin.

(1) Na+ currents and Na+-current fluctuations were measured in myelinated frog nerve fibres at 15 degrees C during 7.7 ms depolarizations to V = 40, 60 and 80 mV. (2) The conductance gamma of a single Na+ channel and the number No of channels per node were calculated from ensemble average values of the mean Na+ current and the variance of Na+-current fluctuations. (3) For a hyperpolarizing holding potential of VH = -28 mV the mean values of the channel conductance and number were gamma = 9.8 pS and No = 74000. (4) After changing the holding potential to the resting potential (VH = 0) the conductance gamma increased by a factor of 1.37 whereas the number No decreased by a factor of 0.60. (5) Addition of 8 nM tetrodotoxin at a holding potential of VH = -28 mV increased gamma by a factor of 1.55 and reduced No by a factor of 0.25. (6) The increase of the channel conductance at reduced channel numbers suggests negative cooperativity between Na+ channels in the nodal membrane.

Animals↗
PubMed1982

Sodium currents and sodium-current fluctuations in rat myelinated nerve fibres.

1. Sodium currents and fluctuations of sodium currents were measured in myelinated fibres of rat sciatic nerve under voltage clamp at 20 degrees C.2. Relaxations of sodium currents during various test potentials were recorded in the presence of 6 nM-TTX in the extracellular solution. The activation of sodium currents at low depolarizations could be described with the m(2) formulation. At increasing potentials higher powers of m up to 4 were required. The mid-point of the P(Na) (E) curve was located near E = -32 mV. Sodium inactivation during various depolarizations developed in two phases.3. The resistance in series with the nodal membrane was calculated from peak sodium currents without and with 6 nM-TTX in the extracellular solution. The resistance varied between different fibres and ranged between 190 and 620 kOmega.4. From peak sodium currents at the same mambrane potential without and in the presence of TTX an apparent equilibrium dissociation constant of 1.6 nM was calculated for TTX binding to sodium channels.5. The conductance gamma and the number N(0) (corrected for series-resistance effects) of sodium channels were evaluated from ensemble average values of the mean sodium current and the variance of sodium-current fluctuations at the beginning of a test pulse. The mean values were gamma = 14.5 pS, N(0) = 21,000 per node.6. The spectral density of stationary sodium-current fluctuations exhibited two relaxation components whose time constants were comparable to those of sodium activation and inactivation. At low depolarizations the variance produced by inactivation fluctuations was larger than predicted by the m(3). h formulation.7. It is concluded that individual sodium channels of rat and frog nerve have similar gating properties. In mammalian nodes the number of sodium channels is lower and the single-channel conductance higher than in amphibian nodes.

Action Potentials↗
PubMed1981

Fluctuation analysis of Na+ channels modified by batrachotoxin in myelinated nerve.

(1) Single myelinated nerve fibers of Rana esculenta were treated with the steroidal alkaloid batrachotoxin, and Na+ currents and Na+-current fluctuations were measured near the resting potential under voltage-clamp conditions. Between test pulses the fibres were held at hyperpolarizing membrane potentials. (2) The spectral density of Na+-current fluctuations was fitted by the sum of a 1/f component and a Lorentzian function. The time constant tau c = 1/(2 pi fc) obtained from the corner frequency fc of the Lorentzian function approximately agreed with the activation time constant tau m of the macroscopic currents. (3) The conductance gamma of a single Na+ channel modified by batrachotoxin was calculated from the integral of the Lorentzian function and the steady-state Na+ current. At the resting potential V = 0 we obtained gamma - 1.6 pS, higher gamma-values of 3.2 and 3.45 pS were found at V = --8 and --16 mV, respectively. (4) The conductance of a modified Na+ channel is significantly lower than the values 6.4 to 8.85 pS reported in the literature for normal Na+ channels. Hence, our experiments are in agreement with the view that batrachotoxin acts in an 'all-or-none' manner on Na+ channels and creates a distinct population of modified channels.

Animals↗
PubMed1981

Block of Na channels in the membrane of myelinated nerve by benzocaine.

The actions of the neutral local anesthetic benzocaine on Na channels were studied in voltage-clamp experiments on single myelinated nerve fibres of the frog by measurements of sodium currents, asymmetry currents, and sodium current fluctuations. 2. 1 mM benzocaine reduced the peak Na currents during various depolarizations V between 20 and 120 nV to 63% of their control values but did not change the time constant of Na activation. 3. 1 mM benzocaine altered asymmetry currents during 1 ms pulses V between 20 and 120 mV in the same was as the early Na currents: It reduced the amplitude to 64% but did not affect the kinetics of the currents. 4. The charge displacement of the asymmetry current during the pulse (Qon) was compared with the charge displacement after the pulse (Qoff). Without benzocaine the relative charge Qoff/Qon Declined to a constant level (0.42 at V = 40mV, 0.25 at V = 100 mV) with increasing pulse durations. In the presence of 1 mM benzocaine the charges Qoff after pulses to V = 40 or 100 mV are almost independent of pulse duration and approximately equal to the control Qoff values after 5.6 ms pulses. Thus, the immobilizations caused by Na inactivation and benzocaine are not additive. 5. Na currents and Na-current fluctuations were recorded during depolarizations V between 24 and 48 mV in the presence of 0.1 mM benzocaine and 7 microM Anemonia toxin II. A lower limit of 8.6 pS was derived for the conductance of a single Na channel. The value agrees with other estimates of the conductance of Na channels which had not been treated by local anesthetics. This suggests an "all-or-none blocking" of Na channels by benzocaine.

Animals↗
PubMed1981

K-current fluctuations in inward-rectifying channels of frog skeletal muscle.

K currents and K-current fluctuations were recorded in inwardly rectifying K channels of frog skeletal muscle under voltage-clamp conditions. External application of 0.2 to 10 mM Cs reduces the inward mean K current but produces a distinct increase of the spectral density of K-current fluctuations. The additional fluctuations arise from the random blocking by Cs ions. From the variance of current fluctuations, the steady-state current and the probability of the open unblocked channel an effective single-channel conductance gamma* was calculated. Gamma* strongly depends on the external Cs concentration (7.8 pS at 0.2 mM Cs, 2.1 pS at 10 mM Cs). This dependence is interpreted in terms of a two-step blocking process: (1) a fast exchange of Cs ions between the external solution and a first binding site inside the channel which leads to the Cs-modulated effective single-channel conductance, and (2) a slow Cs binding to a second site deeper in the channel which produces the observed current fluctuations. With this hypothesis we obtained a real single-channel conductance of gamma approximately equal to 10 pS and a real density of n approximately equal to 4 inwardly rectifying channels per micrometer2 of muscle surface area.

Animals↗
PubMed1980

Modification of sodium inactivation in myelinated nerve by Anemonia toxin II and iodate. Analysis of current fluctuations and current relaxations.

(1) Na+ currents and Na+ current fluctuations were measured in single myelinated nerve fibres of Rana esculenta under voltage-clamp conditions. The process of Na+ inactivation was modified by external treatment with 7 microM Anemonia Toxin II or by internal application of 20 or 40 mM IO3(-). (2) At depolarization of 24 and 32 mV the spectral density of Na+ current fluctuations could be described as the sum of two contributions, Sh(f) and Sm(f), representing the spectrum from fluctuations of the inactivation (h) and activation (m) gates, respectively. At higher depolarizations of 40 and 48 mV the low frequency (h) fluctuations could be better fitted by the sum, Sh1(f)+Sh2(f), of two separate Lorentzian functions. (3) The Na+ current and the variance of Na+ current fluctuations between 150 and 450 ms after depolarization are increased by one order of magnitude after application of Anemonia Toxin II or IO3(-). (4) The kinetics of Na+ current inactivation were described as A1 x exp(-t/tau h1) + A2 x exp(-t/tau h2) + B. The constant, tau h1, of fast Na+ inactivation was the same in normal and modified nerve fibres. The slow inactivation time constant, tau h2, increased with increasing depolarizations in modified fibres but decreased under control conditions. In all cases tau h2 showed a similar voltage dependence as the time constant found by fitting the low frequency fluctuations of Na+ current with one Lorentzian function, Sh(f). (5) It is concluded that Anemonia Toxin II and IO3(-) modify a fraction of Na+ channels in an all-or-none manner. A lower limit of the number of modified Na+ channels is estimated from the Na+ current and the variance Na+ current fluctuations. 7 microM external Anemonia Toxin II modifies more than 17% and 20 or 40 mM internal IO3(-) more than 8% of all Na+ channels. The inactivation gates in modified channels experience an electric field different from that in normal fibres.

Animals↗
PubMed1980

Differences between K channels in motor and sensory nerve fibres of the frog as revealed by fluctuation analysis.

Differences between K channels in the nodal membrane of sensory and motor myelinated nerve fibres of the frog were investigated by fluctuation analysis. Spectral densities, S(f), between 3 Hz and 5 kHz were determined from K-current fluctuations measured between 145 and 460 ms after the onset of depolarizations V between 16 and 80 mV. Fits by the sum of a 1/f component and Lorentzian spectra corresponding to Hodgkin-Huxley n4-kinetics gave significant deviations from the measured spectra. The best fit was obtained by: S(f) = S1/[1+(f/fc)1.5]+S2. The first term can be interpreted as a diffusion spectrum which would originate from gating of K channels governed by an electrodiffusion process. To describe the spectral density at frequencies above 1 kHz it was necessary to add the plateau S2. Time constants taun* = 1/(2pifc) are roughly equal to the conventional Hodgkin-Huxley time constant taun only for pulses V < 40mV. At higher depolarizations taun increases with increasing depolarization in contrast to taun. The variance, var, of conductance fluctuations was determined by integration of the first component of S(f). From var, the probability of the open channel state, and the steady-state K current the single-channel conductance gamma and the number N of K channels per node were calculated; all parameters were corrected for K accumulation during depolarizing pulses. gamma and N were found to be only weakly voltage-dependent. The mean values over all voltages are for motor fibres: gamma=2.7 pS, N = 5.7 x 10(4), and for sensory fibres: gamma = 4.6 pS, N = 5.2 x 10(4). The results suggest two different kinds of K channels in motor and sensory nerve fibres.

Animals↗
PubMed1980

Increased charge displacement in the membrane of myelinated nerve at reduced extracellular pH.

Asymmetry currents were measured in nodes of myelinated nerve fibers from Rana esculenta at extracellular pH values of 5.2, 7.0, and 8.1 by averaging the currents during and after 1-ms depolarizing and hyperpolarizing voltage pulses. The charge displacement in the nodal membrane was obtained by numerical integration of the asymmetry currents. Lowering the pH from 7.0 to 5.2 significantly slows down the kinetics of the fast charge displacement during depolarization but hardly affects the kinetics after repolarization. The pH reduction increases the maximum charge displacement during depolarization by 46%. No differences between asymmetry currents were found between pH 7.0 and 8.1. It is concluded that protonation by extracellular H+ ions may increase the net charge or the transition range of mobile subunits in the nerve membrane.

Animals↗