Kinetics of sodium tail current during repolarization of axon membrane normally and in the presence of scorpion toxin

The kinetics of the sodium tail current (tail I_Na) in myelinated frog nerve fibers in the region of repolarization potentials (V_r) from -40 to -70 mV has two exponential components: fast and slow. The component composition of the tail I_Na depends on V_r: with an increase in negative values of V_r the contribution of the slow component of the tail (θ_s) decreases, and at V_r values higher than -80 mV, the tail I_Na follows virtually one fast exponential curve. The component composition of the tail I_Na at a fixed level of V_r depends on the initial conditions: with an increase in the duration of the test pulse (V_t) the amplitude of the fast component of the tail falls much faster than the amplitude of the slow component. In that case the kinetics of the fall in amplitude of the fast component corresponds to the kinetics of inactivation of I_Na. Scorpion toxin causes slowing of the kinetics of the tail I_Na at all values of V_r, mainly on account of an increase in θ_s. For qualitative interpretation of the results a kinetic scheme assuming the presence of two open states of the sodium channel of the axon membrane is suggested. The hypothesis is put forward that scorpion toxin interacts with the same site of the gating mechanism of the channel as DDT and trinitrophenol.