Kinetics of sodium current and gating current in the frog node of Ranvier

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Abstract
The experiments were done on voltage-clamped nodes of Ranvier of the frog. The aim was to study the kinetics of sodium currentI Na and gating currentI gat over a large potential range (−92 to −12 mV) and to compare the time constants for the turning-on ofI Na orI gat with those for the turning-off measured at the same potential. Sodium tail currents were recorded at different postpulse potentials. Inactivation was inhibited by a few min treatment with 0.5 mM chloramine-T (Wang 1984). The sodium permeability was activated by a 0.4 ms pulse from holding potential (−92 mV) to about 0 mV. At the peak ofI Na the membrane was repolarized to postpulse potentials between −92 and −12 mV. AtE> −60 mV the tail currents decayed with two time constants, τ1 and τ2, reflecting presumably the turning-off and the inactivation of the sodium permeability. The relation between τ1 and postpulse potential was bellshaped with a maximum at −32 mV. The tail currents could also be fitted by the Hodgkin-Huxley equation with the sodium activation variablem raised to the second or third power. AtE<−50 mV τm off was equal to 2 τ1 or 3 τ1, respectively, whereas atE>−25 mV τm off was equal to τ1. In addition, the time constant of the turning-on of sodium activationm (τm on) was determined, assumingI Na ∼m 2 (with a small initial delay) orI Na ∼m 3 (without an initial delay). At −22 mV and −12 mV the ratio τm off/τm on was close to 1. At −42 mV and −32 mV it was larger than 1 (1.22 and 1.65 for them 2 andm 3 fit, respectively, at −32 mV). A similar pulse program was used to measure the turning-on and turning-off ofI gat in the presence of 300 nM TTX. In the potential range −52 to −22 mV, no significant difference between τoff and τon (measured at the same potential) was found. This is in conflict with the findings of Dubois and Schneider (1982) who reported an inequality τoff < τon. Comparison between τoff of charge movement and τ1 of the tail current yielded τoff/τ1 = 2.8 at −92 mV. This agrees with previous measurements of Neumcke et al. (1976).