How many conductance states do potassium channels have?

The steady-state potassium conductance gK( Xo) of the Rana node of Ranvier is a sigmoid function of membrane potential which ranges from 0 to perhaps 0.15 ,umho (Dodge, 1963). This voltage-dependent increase in potassium conductance could be achieved by various physical mechanisms, but two limiting cases may conveniently be distinguished: On the one hand, each individual channel might have only two conductance states (open and closed) so that total potassium conductance would reflect the number of open channels. Alternatively, individual channels might each increase their conductance in much the same way as does the total membrane. Of course, intermediate mechanisms can be envisioned in which a channel has more than two conductance states and the membrane potential determines what fraction of the time a channel spends in each of its various conductance states. Most membrane biophysicists favor some particular mechanism, but no direct experimental support is presently available for any specific view. The only biological system for which information is available, the acetylcholine-induced conductance increase at the frog neuromuscular junction, appears to operate by altering the number of channels which open; individual channels seem to have only two conductance states, open and closed, which differ by 20-30 pmho (Anderson and Stevens, 1973). We have found, using fluctuation analysis, that frog node potassium channels may also have only two conductance states with a single channel conductance of approximately 4 pmho. In the preliminary experiments reported here, single channel conductance yK was estimated by the equation