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

Abstract

The kinetics of the sodium tail current (tail INa) in myelinated frog nerve fibers in the region of repolarization potentials (Vr) from −40 to −70 mV has two exponential components: fast and slow. The component composition of the tail INa depends on Vr: with an increase in negative values of Vr the contribution of the slow component of the tail (θs) decreases, and at Vr values higher than −80 mV, the tail INa follows virtually one fast exponential curve. The component composition of the tail INa at a fixed level of Vr depends on the initial conditions: with an increase in the duration of the test pulse (Vt) 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 INa. Scorpion toxin causes slowing of the kinetics of the tail INa at all values of Vr, mainly on account of an increase in θs. 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.