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

Abstract

1. (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 μM Anemonia Toxin II or by internal application of 20 or 40 mM IO 3 −. 2. (2) At depolarizations of 24 and 32 mV the spectral density of Na + current fluctuations could be described as the sum of two contributions, S h ( f) and S m ( 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, S h1(f) + S h2(f) , of two separate Lorentzian functions. 3. (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 IO 3 −. 4. (4) The kinetics of Na + current inactivation were described as A 1 · exp(-t/ τ h1) + A 2 · exp(-t/τ h2 ) + B . The time constant, τ h1 , of fast Na + inactivation was th in normal and modified nerve fibres. The slow inactivation time constant, τ h2 , increased with increasing depolarizations in modified fibres but decreased under control conditions. In all cases τ h2 showed a similar voltage dependence as the time constant found by fitting the low frequency fluctuations of Na + current with one Lorentzian function, S h ( f). 5. (5) It is concluded that Anemonia Toxin II and IO 3 − 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 of Na + current fluctuations. 7 μM external Anemonia Toxin II modifies more than 17% and 20 or 40 mM internal IO 3 − more than 8% of all Na + channels. The inactivation gates in modified channels experience an electric field different from that in normal fibres.