[Potential-dependent changes in the ionic selectivity of batrachotoxin-modified sodium channels of a frog nerve fiber].

Currents through batrachotoxin-modified sodium channels in frog myelinated fibres were measured under voltage-clamp conditions. Reversal potential (Erev) of steady-state currents is shown to be about 5 mV less positive than Erev of initial (peak) currents. Control experiments with procaine and tetrodotoxin in external solutions showed that this shift of Erev during depolarizing pulse cannot be accounted for by the presence of unmodified sodium channels, unblocked potassium channels, nonlinearity of the leakage or any changes in transmembrane gradients of current-carrying cations. "Instantaneous" current measurements showed that Erev becomes less positive as amplitude and duration of preliminary depolarization increase. The results obtained are consistent with assumption that sodium-potassium selectivity of the batrachotoxin-modified channels depends on potential.