The inhibition of sodium currents by quaternary derivatives of lidocaine was studied in single myelinated nerve fibers. Membrane currents were diminished little by external quaternary lidocaine (QX). QX present in the axoplasm (<0.5 mM) inhibited sodium currents by more than 90%. Inhibition occurred as the sum of a constant, tonic phase and a variable, voltage-sensitive phase. The voltage-sensitive inhibition was favored by the application of membrane potential patterns which produce large depolarizations when sodium channels are open. Voltage-sensitive inhibition could be reversed by small depolarizations which opened sodium channels. One explanation of this observation is that QX molecules enter open sodium channels from the axoplasmic side and bind within the channels. The voltage dependence of the inhibition by QX suggests that the drug binds at a site which is about halfway down the electrical gradient from inside to outside of the sodium channel.

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