Quinidine blocking of sodium and potassium channels in myelinated nerve fibers

Abstract

Experiments by the voltage clamp method showed that external application of quinidine (5 × 10⁻⁵ M) to the Ranvier node membrane of the frog nerve fiber inhibitis both sodium and potassium currents. Blocking of the sodium current is considerably intensified by repetitive depolarization of the membrane (1–10 Hz); the rate of development of the block increases with an increase in stimulation frequency. After the end of stimulation the sodium current gradually returns to its initial level (with a time constant of the order of 30 sec at 12°C). Unlike repetitive depolarization with short (5 msec) stimuli, a prolonged shift (1 sec) of potential toward depolarization has no significant effect on quinidine blocking of the sodium current. Analysis of the current-voltage characteristic curves showed that quinidine blocks outward sodium current more strongly than inward. Batrachotoxin protects sodium channels against the blocking action of quinidine in a concentration of 10⁻⁵ M. Inhibition of the outward potassium currents by quinidine is distinctly time-dependent in character: Initially the potassium current rises to a maximum, then falls steadily to a new stationary level. The results agree with the view that quinidine, applied externally, penetrates through the membrane in the basic form and blocks open sodium and potassium channels from within in the charged (protonated) form. The similarity in principle between the action of quinidine and local anesthetics on the sodium suggests that these compounds bind with the same receptor, located in the inner mouth of the sodium channel.