Ion conductance of the Ca2+-activated maxi-K+ channel from the embryonic rat brain

By using single-channel recording techniques, we measured the conductance (g(K)) of the Ca2+-activated Maxi-K+ channel from the embryonic rat brain, and examined its dependence on K+ ions present in equimolar concentrations on both sides of the membrane patch. With ionic strength maintained constant by substitution of N-methyl-D-glucamine for K+, g(K) has a sigmoidal dependence upon [K+]. This result has been obscured in previous work by variations in ionic strength, which has a marked effect on single-channel conductance, especially in the limit for which this variable approaches zero, The g, versus [K+] relationship is described, theoretically, by a three-barrier, two-binding-site model in which the barrier that an ion must cross to leave the channel is decreased as [K+] is increased.