Time course of the sodium influx in squid giant axon during a single voltage clamp pulse.

1. Sodium influx measurements were carried out on internally perfused squid giant axons under conditions of membrane potential control.2. The ratio ;measured extra sodium influx/calculated ionic influx from the inward current record' is close to unity and is independent of the duration of the rectangular pulse of membrane potential.3. The variation of the sodium permeability with time during the voltage clamp pulse is obtained by subtracting the sodium influx obtained from the ;tail' current record from the measured extra sodium influx.4. Evidence was obtained indicating that the measured change in sodium permeability during a sudden decrease of the membrane potential from its resting level involves two processes, namely, a transient increase in sodium permeability (activation), and a decrease in sodium permeability (inactivation).5. The extra influx of sodium during a voltage clamp pulse is not decreased by raising the internal sodium concentration from 0 to 100 mM, supporting the validity of the independence principle.