Activation of Squid Axon K+ Channels

We have used data obtained from measurements of ionic and gating currents to study the process of K* channel activation in squid giant axons . A marked improvement in the recording ofK' channel gating currents (Ig) was obtained by total replacement of Cl- in the external solution by NO- which eliminates ^-50% of the Na' channel gating current with no effect on Ig . The midpoint of the steady state charge-voltage (QJe , - V) relationship is ^-40 mV hyperpolarized to that of the steady state activation (fo - V) curve, which is an indication that the channel has many nonconducting states . Ionic and gating currents have similar time constants for both ON and OFF pulses . This eliminates any Hodgkin-Huxley n" scheme for K+ channel activation . An interrupted pulse paradigm shows that the last step in the activation process is not rate limiting . Ig shows a nonartifactual rising phase, which indicates that the first step is either the slowest step in the activation sequence or is voltage independent . These data are consistent with the following general scheme for K+ channel activation :