Amine Blockers of the Cytoplasmic Mouth of Sodium Channels: a Small Structural Change Can Abolish Voltage Dependence

Many drugs block sodium channels from the cytoplasmic end (Moczydlowski, E., A. Uehara, X, Guo, and J. Heiny. 1986. Isochannels and blocking modes of voltage-dependent sodium channels. Ann. N.Y. Acad. Sci. 479:269-292.). Lidocaine, applied to either side of the membrane, induces two blocking modes, a rapid, voltage-dependent open-channel block, and a block of the inactivated channel that occurs on a 1000-fold slower timescale. Here we describe the actions of several lidocaine-related amines on batrachotoxin(BTX)-activated bovine cardiac sodium channels incorporated into planar lipid bilayers. We applied blocking amines from the intracellular side and examined the structural determinants of fast, open-channel block. Neither hydroxyl nor carbonyl groups, present in the aryl-amine link of lidocaine, were necessary, indicating that hydrogen bonding between structures in the aryl-amine link and the channel is not required. Block, however, was significantly enhanced by addition of an aromatic ring, or by the lengthening of aliphatic side chains, suggesting that a hydrophobic domain strengthens binding while the amine group blocks the pore. For most blockers, depolarizing potentials enhanced block, with the charged amine group apparently traversing 45-60% of the transmembrane voltage. By contrast, block by phenylhydrazine was essentially voltage-independent. The relatively rigid planar structure of phenylhydrazine may prevent the charged amino end from entering the electric field when the aromatic ring is bound. The relation between structural features of different blockers and their sensitivity to voltage suggests that the transmembrane voltage drops completely over less than 5 A. We raise the possibility that the proposed hydrophobic binding domain overlaps the endogenous receptor for the inactivation gate. If so, our data place limits on the distance between this receptor and the intrapore site at which charged amines bind.