Nerve Fibres Of Xenopus Laevis Research Articles

Effect of the flavoid phloretin on Ca 2+-activated K + channels in myelinated nerve fibres of Xenopus laevis

The effects of the flavonoid phloretin on K + channels in amphibian myelinated nerve were studied by patch clamping. The open probability of Ca 2+-activated K + channels was greatly increased by external phloretin (10–200 μM) due to a shift of the membrane potential for half-maximal activation, E 50, of −63.9 mV (80 μM phloretin). Open times were prolonged and closed times shortened. Channel activation by phloretin developed slowly ( τ on = 33.4 s) and its washout was even slower ( τ off,1 = 4.7 and τ off,2 = 183.2 s). In contrast, submillimolar phloretin blocked two delayed rectifier K + channels (I and F) whereas the gating of the ATP-sensitive and the flickering K + channel were unaffected. Phloretin may directly affect the voltage sensor of K + channels.

The effect of phloretin on single potassium channels in myelinated nerve.

The effect of phloretin, a dipolar organic compound, on single potassium channel currents of myelinated nerve fibres of Xenopus laevis has been investigated, using inside-out patches prepared by the method of Jonas et al. (1989). The I channel, a potential dependent K channel with intermediate deactivation kinetics, was reversibly blocked by 20 microM phloretin applied on the inside; the block was strongest at negative membrane potentials and less pronounced at positive potentials. Phloretin shifted the curve relating open probability to membrane potential towards more positive potentials and reduced its slope and maximum. This confirms previous findings on the effect of phloretin on the voltage dependence of the fast macroscopic K conductance. Single channel conductance and deactivation kinetics were not altered by phloretin.

Mechanisms of the tetrahydroaminoacridine effect on action potential and ion currents in myelinated axons

9-Amino-1,2,3,4-tetrahydroacridine (THA) in the range of 10–300 μM was shown to prolong the action potential in myelinated nerve fibres of Xenopus laevis. Voltage-clamp experiments showed that THA, besides reducing the Na + and the K + current, modified the Na + current inactivation and the K + current activation. The effects were frequency dependent. Quantitative models were developed and used in computer simulations of the THA effect on the action potential. The computations showed that the observed effects on the ion currents were sufficient to explain the observed prolongation of the action potential. The models further suggest that THA binds to Na + channels in an open state and from the axoplasmic side while it binds to K + channels in a closed state. The findings suggest an explanation to some aspects of the clinical effects of THA on Alzheimer patients.

Sodium and potassium currents recorded during an action potential

A simple method was used to measure directly sodium and potassium currents underlying the action potential in single nerve fibres of Xenopus laevis. A short rectangular stimulus under current-clamp conditions elicited an action potential which was digitally stored and later used as command when voltage-clamping the same fibre. The currents thus obtained nearly reproduced the original rectangular stimulus. Adding first 100 nM TTX and subsequently 100 nM TTX plus 10 mM TEA to the extracellular Ringer solution revealed the sodium and the potassium currents during an action potential. They were converted to permeabilities by use of the constant-field equation and are in good agreement with the curves which had been calculated from conventional voltage-clamp data. Thus experimentally determined currents and permeabilities are shown as they are changing during an action potential.

Development of sodium permeability inactivation in nodal membranes.

1. The time course of inactivation of the sodium permeability was studied in myelinated nerve fibres of Xenopus laevis using the voltage-clamp technique of Nonner (1969). The potassium currents were blocked by 10 mM-tetraethylammonium chloride (TEA). The remaining currents were corrected for leakage and capacity currents. 2. The decay of the sodium current was double-exponential confirming Chiu's (1977) findings The slower time constant was observed in TEA-free solutions by clamping to VK in high potassium concentrations. 3. The fast time constant was similar to tau h of Frankenhaeuser (1960), whereas the slower time constant was about four times larger and also decreased with increasing depolarization. 4. Arrhenius plots of both time constants can be well approximated by straight lines for temperatures between 0 and 25 degrees C. 5. The entire sodium current induced by single voltage-clamp pulses was fitted by a non-linear least-square routine to the Frankenhaeuser-Huxley equations extended by Chiu's three-state kinetics. With rare exceptions for potentials between 37 and 70 mV the fit was better without a delay in the onset of the inactivation of the sodium current. 6. The time course of inactivation was also studied in two-pulse experiments where a prepulse of varying duration was directly followed by a test pulse. The peak sodium currents were normalized by the associated peak currents without a prepulse. 7. The relative peak sodium current as a function of the prepulse duration had a sigmoid time course. The early deviation from an exponential decay is due to the activation arising during the prepulse and is implicit in the classical equations quoted above. It is therefore not necessarily a sign of a delay in the inactivation process. 8. To eliminate errors due to activation in two-pulse experiments, the decaying part of the test sodium currents was extrapolated back to the onset of the test pulse. These current values at t = 0 plotted as a function of the prepulse duration revealed no delay at the beginning of inactivation. Within the accuracy of our measurements a possible delay of more than 100 microseconds (at 5-10 degrees C) could be excluded.

Effect of the nonionic detergent triton X-100 on sodium permeability of the myelinated nerve fibre of Xenopus laevis.

The effect of the nonionic detergent Triton X-100 (TX-100) on the Na permeability (PNa) properties of the nodal membrane in myelinated nerve fibres of Xenopus laevis was analysed with potential clamp technique. Application of TX-100 caused a rapid initial decrease in PNa that was reversible at wash out as well as a slow irreversible block. Both effects dependend on [TX-100] and duration of exposure. The reversible reduction of PNa at the steady state was 50% at 40--60 micrometer TX-100. The slope of the Hill plot for the reaction was 1.75 indicating a deviation from a first order reaction. The equilibrium dissociation constant (KD) for n = 1.75 was 0.9 X 10(-7) (M1.75). KD calculated from the rate constants for onset and offset of the reversible reaction (KD = k2/k1) was 1.5 X 10(-7) (M1.75). The possibility that the action of TX-100 involves membrane proteins is discussed.

Slow action of Ca on myelinated nerve fibres of Xenopus laevis.

Single myelinated nerve fibres were isolated and the nodal currents were recorded with potential clamp technique. Rapid solution changes were performed by use of a recording chamber so shaped that it had a minimal dead space. A volume ten times larger than the dead space flowed past the node within about 20 ms. The half-time of the change in Na current (INa)amplitude associated with changes between solutions of different [Na]:s was 100-150 ms, whereas the half-time of the change in INa associated with changes between solutions of different [Ca]:s was 200-300 ms. The external [Ca] affects INa through its action on the Na permeability properties of the nodal membrane. The difference in rate of effect between changes in [Na] and [Ca] was also noticeable when simultaneous changes were performed. These findings are discussed with regard to the properties of the nodal gap.

Effects of ionic concentration on sodium permeability properties of myelinated nerve fibres of Xenopus laevis.

1. The nodal currents of single myelinated nerve fibres were recorded under potential clamp conditions, and the effect of [Ca], [Na] and [K] in the external solution on some of the Na permeability properties were analysed. 2. [Ca], [Na] and [K] all affected the position of the steady-state Na inactivation (h) curve on the potential axis. The curve was displaced in positive direction by high ionic concentration. 3. The shift associated to different [Ca] was largest in low [Na] and [K]. Similarly the shift associated to different [Na] and [K] was largest in low [Ca]. 4. The maximum peak sodium permeability (max. peak - PNa) was affected by the [Ca], [Na] and [K]. It was greater in (i) low [Ca], (ii) high ([Na] + [K]) and (iii) high [Na]:[K] ratio. 5. The effect of [Ca] on peak - Na was mainly a consequence of a change in PNa (which is the value of PNa if activation were complete, m = 1, and inactivation fully removed, h = 1).

Effect of aconitine on the sodium permeability of the node of Ranvier.

The effect of aconitine (10−5–10−6 g/ml) on membrane potentials and membrane currents of myelinated nerve fibres of Xenopus laevis was investigated. The following observations were made: a) Current clamp conditions: Slow depolarization (10–15 mV), decrease of amplitude and maximum rate of rise of action potential, finally inexcitability. With inward current pulses ‘hyperpolarizing responses’ could be elicited at membrane potentials more negative than the resting potential (Er). Neither spontaneous activity nor repetitive responses to electrical stimuli were observed. No effects of aconitine were found in Na-free solutions or in the presence of tetrodotoxin. b) Voltage clamp conditions: Development of steady inward current at normal resting potential due to formation of a non-inactivating sodium permeability; heavily poisoned nodes therefore exhibit an N-shaped steady-state current voltage relation with negative slope at membrane potentials more negative thanEr. These non-inactivating sodium channels open more slowly than normal sodium channels, and can only be closed by hyperpolarizing the membrane by about 50 mV. The majority of sodium channels have almost normalτm; theirm∞ andh∞-V relations are shifted by 10–15 mV towardsEr. It is concluded that these changes of the sodium permeability account for the changes of electrical activity observed after treatment with aconitine.

Effects of ionic concentration on permeability properties of nodal membrane in myelinated nerve fibres of Xenopus laevis. Potential clamp experiments.

AbstractThe specific sodium and potassium permeability mechanisms in the nodal membrane were analysed. The current to potential relations were measured with various external concentrations of NaC1, KC1 and CaC12. PNa and PK were calculated. The measurements showed that the permeability to potential curve was shifted along the potential axis and in the same direction for changes in the uni‐univalent salt concentration as well as for changes in [Ca]. The shift caused by changes in [Ca] was larger at low uni‐univalent salt concentrations and the shift caused by [K] and [Na] increased with decreased [Ca]. It was assumed that the membrane has negative charges fixed to its outside surface. The permeability shifts were approximately predicted by a charge density of‐5.5 μC μ cm‐2. A decrease in [Na], but not in [Ca] caused a decrease in PK. This behaviour was not predicted.

Influence of calcium ions on the ionic currents of nodes of Ranvier treated with scorpion venom.

Voltage clamp experiments were done on myelinated nerve fibres of Xenopus laevis to study the effect of calcium on the ionic currents of the nodal membrane treated with scorpion venom. Increasing the calcium concentration of the medium from 1.8 to 7.2 mM produced the following changes of the sodium and potassium permeabilities,PNa andPK: a) The incomplete sodium inactivation, which is typical for nodes of Ranvier treated with scorpion venom, is abolished. The curves relatingm∞ and τm to membrane potential are shifted by 8–10 mV towards larger depolarizations.PNa and the time constants of the fast and slow components of the sodium inactivation remain practically unchanged. b) The maximum potassium permeability,PK, which is reduced by scorpion venom to 45% of the value observed in the normal node, increases to 68% by addition of calcium ions; τn is unchanged.

Potassium inactivation in single myelinated nerve fibres of Xenopus laevis.

1. Voltage clamp measurements were performed on single myelinated nerve fibres of the frog Xenopus laevis. 2. During long-lasting depolarizations the potassium current decayed in a fast phase with a time constant of about 0.6 sec and a following slow phase with a time constant between 3.6 (V=0) and 20 sec (V=100 mV). 3. The decay of the potassium current was the result of an inactivation of the potassium permeability and not of a shift of the potassium equilibrium potential as shown by experiments in isotonic KCl solution. 4. At a hyperpolarization of −20 mV the potassium inactivation was fully removed. It remained incomplete even at large depolarizations. The steady-state inactivation curve was S-shaped but not symmetrical. 5. The experimental results could be described by extending the Hodgkin-Huxley equations introducing two terms of potassium inactivation.

Conduction velocity in myelinated nerve fibres of Xenopus laevis.

1. The relationship between conduction velocity and nerve diameter in single myelinated nerve fibres from Xenopus laevis was measured and was found to be linear.2. The relationship between conduction velocity and temperature in nerve fibres of various diameters was measured over the range 15-30 degrees C and was found to be linear for each fibre.3. The slope of the conduction velocity-temperature relationship was directly proportional to the diameter of the nerve fibre.4. The form of the conduction velocity-temperature relationship was determined by numerical solution of the Frankenhaeuser-Huxley equations for the case of a propagated action potential. The computation predicted that conduction velocity is a linear function of temperature over the range studied.

ACCOMMODATION IN MYELINATED NERVE FIBRES OF XENOPUS LAEVIS AS COMPUTED ON THE BASIS OF VOLTAGE CLAMP DATA.

AbstractThe equation system earlier derived from the voltage clamp analysis on myelinated nerve fibres from Xeonpus laevis was programmed for a digital computer. The response of the nerve model to linearly rising currents was computed for a number of quantitative modifications of the nerve model. The agreement between the computations and the earlier experimental results was qualitatively satisfactory. The quantitative agreement was not perfect but the discrepancies were not greater than what is expected from the known errors in the voltage clamp technique. The computations indicated that changes of any of the constants of the nerve model have effects on the rate of accommodation. The greatest effects were obtained by changing the inactivation of the sodium permeability. It was concluded, on the basis of the results obtained in the present and in the earlier investigation, that the slow excitability changes of the nerve fibres are well predicted by the equation system describing the ionic currents. Further, it was concluded that the normal variation in accommodation among myelinated nerve fibres from Xenopus laevis is accounted for mainly by variations in the rate constants ah although this may not be the only variable of importance. A variation in the turning on of the potassium permeability might also be significant.

INACTIVATION OF THE SODIUM-CARRYING MECHANISM IN MYELINATED NERVE FIBRES OF XENOPUS LAEVIS.

INACTIVATION OF THE SODIUM-CARRYING MECHANISM IN MYELINATED NERVE FIBRES OF XENOPUS LAEVIS.

THE SPECIFICITY OF THE INITIAL CURRENT IN MYELINATED NERVE FIBRES OF XENOPUS LAEVIS. VOLTAGE CLAMP EXPERIMENTS.

THE SPECIFICITY OF THE INITIAL CURRENT IN MYELINATED NERVE FIBRES OF XENOPUS LAEVIS. VOLTAGE CLAMP EXPERIMENTS.

THE EFFECT OF TEMPERATURE ON THE SODIUM AND POTASSIUM PERMEABILITY CHANGES IN MYELINATED NERVE FIBRES OF XENOPUS LAEVIS.

The Journal of PhysiologyVolume 169, Issue 2 p. 431-437 ArticleFree Access The effect of temperature on the sodium and potassium permeability changes in myelinated nerve fibres of Xenopus laevis B. Frankenhaeuser, B. FrankenhaeuserSearch for more papers by this authorL. E. Moore, L. E. MooreSearch for more papers by this author B. Frankenhaeuser, B. FrankenhaeuserSearch for more papers by this authorL. E. Moore, L. E. MooreSearch for more papers by this author First published: 01 November 1963 https://doi.org/10.1113/jphysiol.1963.sp007269Citations: 175AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume169, Issue2November 1, 1963Pages 431-437 RelatedInformation

A QUANTITATIVE DESCRIPTION OF POTASSIUM CURRENTS IN MYELINATED NERVE FIBRES OF XENOPUS LAEVIS.

A QUANTITATIVE DESCRIPTION OF POTASSIUM CURRENTS IN MYELINATED NERVE FIBRES OF XENOPUS LAEVIS.

Delayed currents in myelinated nerve fibres of Xenopus laevis investigated with voltage clamp technique.

The Journal of PhysiologyVolume 160, Issue 1 p. 40-45 ArticleFree Access Delayed currents in myelinated nerve fibres of Xenopus laevis investigated with voltage clamp technique B. Frankenhaeuser, B. FrankenhaeuserSearch for more papers by this author B. Frankenhaeuser, B. FrankenhaeuserSearch for more papers by this author First published: 01 January 1962 https://doi.org/10.1113/jphysiol.1962.sp006832Citations: 65AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume160, Issue1January 1, 1962Pages 40-45 RelatedInformation