On the slowly rising phase of the sodium gating current in the squid giant axon

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Abstract
High–resolution records of the sodium gating current in the squid giant axon demonstrate the existence of a slowly rising phase that is first apparent at pulse potentials slightly below zero, and becomes increasingly pronounced at more positive potentials. At +80 mV the current reaches its peak with a delay of 30 μs at 10°C. It is suggested 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. The kinetics of the slowly rising phase can only be explained by the incorporation of an appropriate degree of voltage–dependent cooperativity between the S4 voltage–sensors for their two initial transitions.

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