ON THE SLOWLY RISING PHASE OF THE SODIUM GATING CURRENT IN THE SQUID GIANT-AXON

High-resolution records of the sodium gating current in the squid gian t axon demonstrate the existence of a slowly rising phase that is firs t apparent at pulse potentials slightly below zero, and becomes increa singly pronounced at more positive potentials. At +80 mV the current r eaches its peak with a delay of 30 mu s at 10 degrees C. It is suggest ed that this current is generated by the first two steps labelled R--> P and P-->A in the S4 units of all four domains of the series-parallel gating system, activating the channel before its opening by the third steps A-->B in domains I, II and III in conjunction with hydration. T he kinetics of the slowly rising phase can only be explained by the in corporation of an appropriate degree of voltage-dependent cooperativit y between the S4 voltage-sensors for their two initial transitions.