Shaker B K+ conductance in Na+ solutions lacking K+ ions: a remarkably stable non‐conducting state produced by membrane depolarizations.

1. Shaker B K+ channels, expressed in the insect cell line Sf9, were studied in zero K+, Na+ or N‐methyl‐D‐glucamine (NMG)‐containing solutions. In the absence of K+ ions on both sides of the membrane, the K+ conductance collapsed with the delivery of short depolarizing pulses that activated the channels. The collapse of the conductance was fully prevented when the channels were kept closed at a holding potential of ‐80 mV. 2. The fall in K+ conductance had the notable characteristic of being strikingly stable. At ‐80 mV or more negative holding potentials, the conductance never recovered (cells observed for up to 1 h). 3. The extent of collapse of the K+ conductance depended on the number of depolarizing activating pulses applied in zero K+ solutions. For moderate to low frequencies of pulsing (1 to 0.002 Hz), the extent of the collapse did not depend on the frequency. 4. K+, Rb+, Cs+ and NH4+ added to the external Na+ solution impeded the fall in K+ conductance. 5. TEA added to the external, zero K+, Na(+)‐containing solution also precluded the fall of the conductance. The protection by TEA paralleled its block of the outward K+ currents recorded with standard recording solutions. 6. The fall in K+ conductance was prevented by depolarized holding potentials. 7. The K+ conductance that was thought to be irreversibly lost at ‐80 mV or more negative holding potentials was fully recovered, however, after a prolonged (tens of seconds to minutes) change in the holding potential to depolarized values (above ‐50 mV). Full recovery could be obtained at any time after the former halt of the conductance.