The kinetics of recovery and development of potassium channel inactivation in perfused squid (Loligo pealei) giant axons.

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Abstract
K+ currents were studied at a normal (-69 mV) and at a depolarized (-49 mV) membrane potential in voltage-clamped squid giant axons perfused with 350 mM K+ and bathed in K+-free artificial seawater containing tetrodotoxin to block the Na+ channels. Steady-state and instantaneous K+ currents were reduced by > 50% at corresponding voltages at the depolarized membrane potential. Instantaneous chord conductance-voltage curves showed that the depolarized membrane potential caused a uniform reduction of K+ conductance across the voltage range under study. The driving force for K+ ions was comparable at both membrane potentials when a short (2 ms) prepulse was used to open the K+ channels. When a longer (7.5 ms) prepulse was used, the driving force was actually larger at the depolarized membrane potential. The depolarized membrane potential did drive some K+ ions into the periaxonal space. The amount of K+ ions driven into the periaxonal space was estimated by 2 independent methods, with similar results. The resulting increase of K+ ions in the periaxonal space (10 mM) was .apprx. 40 times too small to account for the large reduction in currents in terms of a reduced driving force for K+ ions. The kinetics of recovery and development of inactivation were monitored by repeatedly applying a 7.5 ms test pulse followed by a long conditioning potential. Both recovery and development of inactivation, from the depolarized membrane potential, were described by the sum of 2 exponential terms plus a constant. The time constant-voltage curves for both phases of inactivation peaked at about -54 mV at 10.degree. C. The time constant of the slow phase of inactivation at -54 mV was .apprx. 12.4 s, while the corresponding time constant for the fast phase was .apprx. 2.3 s. The slow relaxation had an apparent plateau of .apprx. 11 s at more depolarized membrane potentials. Recovery from inactivation was rapid at hyperpolarized membrane potentials. The steady-state inactivation curve of the K+ channel was incomplete in the depolarizing region; an apparent platequ was reached with .apprx. 75% of the K+ current inactivated.