Potential-dependent changes in ionic selectivity of batrachotoxin-modified sodium channels of frog nerve fiber

Ionic currents through sodium channels modified by batrachotoxin were measured by the voltage clamp method on a myelinated frog nerve fiber membrane. The reversal potential (E_rev) of steady-state currents was shown to be on the average 5 mV less positive than E_rev corresponding to the initial (peak) values of the currents. The results of control experiments using procaine and tetrodotoxin showed that the change in E_rev observed during a depolarizing pulse is not connected with the presence of unmodified sodium channels or unblocked potassium channels, with nonlinearity of leakage, or with a change in transmembrane gradients of current-carrving cations. In experiments with measurement of "instant" currents it was shown that E_rev becomes less positive as the amplitude and duration of preliminary depolarization increase. The results support the view that sodium-potassium selectivity of batrachotoxin-modified sodium channels depends on potential.