Relief of Na ' Block of Ca " - activated K + Channels by External Cations

The flickery block of single Ca'-activated K+ channels that is produced by internally applied Na' can be relieved by millimolar concentrations of external K+. This effect of K+ on the kinetics of Na' block was studied by the method of amplitude distribution analysis described in the companion paper It appears that K+ relieves block by increasing the exit rate of the blocking ion from the channel, not by competitively slowing its entrance rate. This suggests that a K ion that enters the channel from the outside can expel the blocking Na ion, which entered the channel from the inside. Cs" which cannot carry current through the channel, and Rb+, which carries a reduced current through the channel, are just as effective as K+ in relieving the block by internal Na'. The kinetics of block by internal nonyltriethylammonium (C9) are unaffected by the presence of these ions in the external bathing solution .