Activation of squid axon K+ channels. Ionic and gating current studies.

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Abstract
Data obtained from measurements of ionic and gating currents were used to study the process of K+ channel activation in squid giant axons. A marked improvement in the recording of K+ channel gating currents .**GRAPHIC**. was obtained by total replacement of Cl- in the external solution by NO-3, which eliminated .apprx. 50% of the Na+ channel gating current with no effect on .**GRAPHIC**. The midpoint of the steady state charge-voltage (Qrel - V) relationship was .apprx. 40 mV hyperpolarized to that of the steady state activation (fo -V) curve, which was an indication that the channel had many nonconducting states. Ionic and gating currents had similar time constants for both ON and OFF pulses. This eliminated any Hodgkin-Huxley nx scheme for K+ channel activation. An interrupted pulse paradigm showed that the last step in the activation process was not rate limiting. .**GRAPHIC**. showed a nonartifactual rising phase, which indicated that the 1st step was either the slowest step in the activation sequence or was voltage independent.