Relief of Na+ block of Ca2+-activated K+ channels by external cations.

The flickery block of single Ca2+-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 (Yellen, G., 1984b, J. Gen. Physiol., 84:157-186). 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.