ION CONDUCTION THROUGH C-TYPE INACTIVATED SHAKER CHANNELS

C-type inactivation of Shaker potassium channels involves entry into a state (or states) in which the inactivated channels appear nonconduct ing in physiological solutions. However, when Shaker channels, from wh ich fast N-type inactivation has been removed by NH2-terminal deletion s, are expressed in Xenopus oocytes and evaluated in inside-out patche s, complete removal of K+ ions from the internal solution exposes cond uction of Na+ and Li+ in C-type inactivated conformational states. The present paper uses this observation to investigate the properties of ion conduction through C-type inactivated channel states, and demonstr ates that both activation and deactivation can occur in C-type states, although with slower than normal kinetics. Channels in the C-type sta tes appear ''inactivated'' (i.e., nonconducting) in physiological solu tions due to the summation of two separate effects: first, internal K ions prevent Na+ ions from permeating through the channel; second, C- type inactivation greatly reduces the permeability of K+ relative to t he permeability of Na+, thus altering the ion selectivity of the chann el.