Slow inactivation and reactivation of the K+ channel in squid axons. A tail current analysis.

Potassium current inactivation and reactivation in squid axons were measured from tail current amplitudes after voltage clamp prepulses to the potassium equilibrium potential, EK, in seawater containing elevated levels of potassium ion concentration, Ko. Little or no inactivation resulted with prepulses lasting less than 100 ms. Longer pulses caused the current to inactivate in two phases, one between 0.1 and 1 s, and a second phase between 5 and 100 s. Inactivation was incomplete. The time constant of the tail current after a prepulse to EK was independent of pulse duration (0.1-120 s). Inactivation was independent of Ko (10 less than or equal to Ko less than or equal to 300 mM), and it was independent of membrane potential, V, for -40 less than or equal to V less than or equal to 0 mV. Reactivation was measured with a three-pulse protocol. The reactivation time course was sigmoidal with a delay of approximately 100 ms before significant reactivation occurred. These results were described by a model consisting of three inactivated states arranged in a linear sequence. The rate constants of the model are of the form (A + B exp (CV), or 1/(A + B exp (CV], which are required to describe the non-inactivating conductance component.