Inactivation modifiers of Na+ currents and the gating of rat brain Na+ channels in planar lipid membranes

Rat brain Na⁺ channels whose inactivation process had been removed either by batrachotoxin (BTX) or veratridine (VT) were reconstituted into planar lipid membranes. The voltage dependence of the open probability (P_o) of the channel, of the opening and closing rate constants, and the conductance and relative permeability for Na⁺ and K⁺ were studied in voltage-clamp conditions in the presence of agents known to modify the inactivation of Na⁺ currents. In relation to alkaloids (BTX, VT, and aconitine), it was found that once a Na⁺ channel was modified by BTX or VT, the addition of another alkaloid did not change further the gating and permeation properties of the channel over a period of about 1 h. Once the inactivation process of the channels is removed by BTX, the addition of a proteolytic enzyme (trypsin) or an halogenated compound (chloramine-T, CT) induced profound and specific modifications on the opening and closing events of Na⁺ channels: (1) the voltage dependence of the channel P_o shifted to more hyperpolarized potentials; (2) this voltage shift can be explained by equal hyperpolarizing voltage shifts of the opening and closing rate constants of the channel; (3) although the gating properties of the channel were modified by these compounds, the permeation properties of the channel, as evaluated by the conductance and the selectivity to Na⁺ and K⁺ ions, were unaltered; (4) trypsin and CT were active only in the intracellular side of the channel and were irreversible within the time course of the experiments, suggesting covalent modifications of the channel. Inactivation modifiers also affected the gating of toxin-activated single Na⁺ channels. This alteration is compatible with a simple increase in the intracellular potential as seen by the voltage sensor of the channel.