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

Abstract

• 1. 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.

• 2. 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.

• 3. 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 τ_{m off} was equal to 2 τ₁ or 3 τ₁, respectively, whereas atE>−25 mV τ_{m off} was equal to τ₁.

• 4. In addition, the time constant of the turning-on of sodium activationm (τ_{m 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 τ_{m off}/τ_{m 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).

• 5. 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 τ_off and τ_on (measured at the same potential) was found. This is in conflict with the findings of Dubois and Schneider (1982) who reported an inequality τ_off < τ_on.

• 6. Comparison between τ_off of charge movement and τ₁ of the tail current yielded τ_off/τ₁ = 2.8 at −92 mV. This agrees with previous measurements of Neumcke et al. (1976).