Ionic current through batrachotoxin-modified sodium channels of the ranvier node membrane at high positive and negative potentials

Ionic current through batrachotoxin (BTX)-modified sodium channels within a wide range of membrane potentials were measured by the voltage clamp method on the membrane of a myelinated frog nerve fiber. At high positive voltages (above +80 mV) the current decreased with time; with an increase in voltage the steady-state level of the currents fell. The results of measurement of "instant" currents showed that this phenomenon is connected with a decrease in overall conductivity of the modified channels. Scorpion toxin had no significant effect on the kinetics of decline of the currents. This indicates that they are due to processes which differ from ordinary inactivation. In the presence of procaine, at high positive voltages slow (tens of milliseconds) potential-dependent blocking of BTX-modified channels was observed. An increase in negative potentials above ?100 mV caused a decrease in "instant" currents, connected with rapid potential-dependent blocking of BTX-modified sodium channels by calcium ions.