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  • 1
    Electronic Resource
    Electronic Resource
    Cooperativity of tetrodotoxin action in the frog node of Ranvier (1985)
    Springer
    Pflügers Archiv 405 (1985), S. 237-243 
    Details
    ISSN: 1432-2013
    Keywords: Node of Ranvier ; Na channels ; Tetrodotoxin ; Cooperativity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract 1. The steady state effects and rates of action of tetrodotoxin (TTX) on sodium current were studied in the voltage clamped frog node of Ranvier. 2. Inactivation of the sodium current was separated into fast and slow phases. Both phases were assumed to correspond to two different currents (fast and slow) flowing through fast and slow channels (Benoit et al. 1985). 3. The dose-response curve of the steady state effect of tetrodotoxin on the fast current was sigmoid. An analysis of this effect in double logarithmic coordinates gave a Hill coefficient of 1.74. 4. The rates of tetrodotoxin action on the fast current were determined by the reversible reduction of the peak current recorded at a potential (+20 mV) at which the slow current was relatively small. 5. After an initial delay, the onset of TTX effect followed an exponential function of time whose constant decreased with increasing tetrodotoxin concentrations. Expressed as the time corresponding to a reduction of 2% of the current, the delay (δt2%) increased from about 100 ms with 300 nM-TTX to about 30 s with 1 nM-TTX. 6. When tetrodotoxin was removed, the offset developed quasi-instantaneously and followed an exponential function of time whose constant was independent of the toxin concentration. 7. Both steady state and rates of tetrodotoxin effects could be fitted well if one assumed that the block of one fast channel occurred after binding of two TTX molecules to two cooperative sites.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00582567
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  • 2
    Electronic Resource
    Electronic Resource
    Late sodium current in the node of Ranvier (1975)
    Springer
    Pflügers Archiv 357 (1975), S. 145-148 
    Details
    ISSN: 1432-2013
    Keywords: Sodium Permeability ; Delayed Currents ; Node of Ranvier
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Voltage clamp experiments carried out on nodes of Ranvier of myelinated fibres ofRana esculenta showed that a small fraction of sodium channels fail to inactivate. Thus during long lasting depolarizing pulses there is a small Na-current superimposed on the leakage and potassium currents. This late Na-current appears more marked in sensory fibres than in motor ones.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00584552
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  • 3
    Electronic Resource
    Electronic Resource
    The steady-state potassium conductance of the ranvier node at various external K-concentrations (1977)
    Springer
    Pflügers Archiv 370 (1977), S. 185-194 
    Details
    ISSN: 1432-2013
    Keywords: Node of Ranvier ; Potassium conductance ; Cation selectivity ; Gating mechanism
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary 1. Voltage clamp experiments were carried out on single myelinated fibres of the frog to analyse the changes of the potassium conductance (g K ) resulting from variations of the external K-concentration ([K]0). The use ofg K is justified by the observation that instantaneous K-currents vary linearly withE despite asymmetries between [K]0 and [K]i. 2. At constant membrane potential and for inward going currents,g K increases as a Michaëlian function of [K]0. We propose, therefore, that external K-ions bind specific membrane sites controllingg K . The apparent equilibrium constant (K app ) of the binding reaction decreases with depolarization following a double exponential function. The rate constants of this function depend on the external Ca-concentration. 3. An empirical equation is derived which satisfactorily describes the variation ofg K as a function ofE and [K]0. The fact that either the increase in [K]0 or the membrane depolarization can lead to the opening of the channels is regarded as evidence that the binding of K to specific sites (“K-receptors”) is a necessary step for the formation of conducting channels. The affinity of the receptor-sites for K-ions depends on the electrical field in the membrane. 4. K-free Ringer solutions markedly reduce the steady-state K-current and its rate of activation. These effects are not observed if the lack of potassium is compensated by addition of caesium. It is concluded that Cs can replace K at the sites controlling the formation of the channel. 5. High external concentrations of Cs-ions reduce the inward K-currents when [K]0 is high and tend to increase outward K-currents, especially when [K]0 is low. These results are explained assuming that Cs-ions enter the channel with K-ions moving in single file. They stop the in-moving file when reaching a selectivity structure deeply located in the channel. 6. The fraction of the membrane thickness (δ) at which Cs-ions occupy a blocking position in the K-channel, when K and Cs are present at high concentrations in the external medium, is estimated to be: δ=0.6. From this result, it is deduced that the voltagesensitive K-receptors are located in the external half of the membrane.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00581693
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  • 4
    Electronic Resource
    Electronic Resource
    Potassium accumulation in the perinodal space of frog myelinated axons (1975)
    Springer
    Pflügers Archiv 358 (1975), S. 111-124 
    Details
    ISSN: 1432-2013
    Keywords: Node of Ranvier ; Potassium Permeability ; Ionic Accumulation ; Series Resistance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary 1. Voltage clamp experiments were carried out on frog myelinated fibres to study the origin of the transient inward current occuring when the membrane is repolarized after long lasting depolarizing pulses (tail current denominated “I p” by Frankenhaeuser). 2. The “tail” of inward current measured during repolarization after break of the depolarizing pulse is insensitive to external application of TTX, is abolished by external treatment with TEA or Cs and decreases when the outward K-current during the pulse is diminished. 3. The time course of the “tail” current is exponential. Its direction depends on the duration of the depolarizing pulse and on the membrane potential level at repolarization. 4. It is concluded that the tail of inward current during repolarization is carried by K-ions accumulated in the perinodal space during a depolarizing pulse. The data suggest that the tail reflects the time course of the restoration of the K-concentration to its initial level. The tail current itself contributes to this restoration depending on the Em value at repolarization. 5. It is shown that one of the two phenomenological models proposed by Frankenhaeuser and Hodgkin to account for the external potassium accumulation observed in the squid giant axon may be also applied to the Ranvier node. Assuming that the thickness of the space is 2900 Å and that the K-permeability of the barrier is 0.019 cm/sec, it is possible to account for the observed changes in [K]0 during a long lasting depolarizing pulse. 6. The existence of such a barrier would introduce an electrical resistance in series with the nodal membrane of roughly 150000 Ω.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00583922
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  • 5
    Electronic Resource
    Electronic Resource
    Cesium induced rectifications in frog myelinated fibres (1975)
    Springer
    Pflügers Archiv 355 (1975), S. 361-364 
    Details
    ISSN: 1432-2013
    Keywords: Node of Ranvier ; Cs-Ions ; Potassium Channel
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Voltage clamp experiments done on nodes of Ranvier show that external application of Cesium blocks the inward but not the outward potassium currents. Internal application of Cs ions reduces the outward K-current and the inward K-current is not affected. These results support the hypotheses that K ions cross the K-channel after dehydration at superficial sites where competition may occur with Cs-ions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00579857
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  • 6
    Electronic Resource
    Electronic Resource
    Distribution of nitrogen in common bean (Phaseolus vulgaris L.) genotypes selected for differences in nitrogen fixation ability (1991)
    Springer
    Plant and soil 138 (1991), S. 303-311 
    Details
    ISSN: 1573-5036
    Keywords: breeding ; common bean ; dinitrogen fixation ; 15N-depleted fertilizer ; N partitioning ; Phaseolus vulgaris L. ; selection
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract The improvement of N2 fixation in legumes may lead to increased yields and reduced fertilizer requirement. Levels of N2 fixation were determined for three cultivars and nine progeny lines from two inbred backcross common bean (Phaseolus vulgaris L.) populations that were grown at Hancock, Wissconsin in 1984 and 1985 using 15N-depleted (NH4)2SO4. The high N2-fixing line ‘Puebla 152’ was the donor parent for both inbred backcross populations and the cultivars ‘Porrillo Sintetico’ and ‘Sanilac’ were the recurrent parents for populations 21 and 24, respectively. Total N yield, fixed N2 and % N derived from the atmosphere were determined for whole plants and plant parts at the R3 (50% bloom) and R9 (maturity) growth stages. Significant year-by-line interactions were found for N2 fixation traits among the population 21 lines and parents, but not for population 24 lines and their parents. Measures of N2 fixation at R3 were inadequate to predict N2 fixation at R9. Population 24 lines and parents differed for N2 fixation ability at R9, and fixed N2 was correlated with maturity. The recovery of an inbred backcross progeny line, 24-21, which matured earlier and fixed more N2 than the recurrent parent ‘Sanilac’ indicated that N2 fixation was heritable and that favorable alleles, independent of maturity, were recovered from a late-maturing, high N2-fixing donor parent by utilizing the inbred backcross breeding method. Since most fixed N2 and non-fixed N (〉80%) was found in the seeds at maturity, and most lines did not vary for the distribution of nitrogen throughout the plant, selection for improved remobilization of nitrogen to the seed to increase yield is impractical in this genetic material. The highest N2-fixing lines tended to have high and similar % Ndfa in all plant parts.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00012256
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