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 (Moczydlowsk i, 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 a mines on batrachotoxin(BTX)-activated bovine cardiac sodium channels i ncorporated into planar lipid bilayers. We applied blocking amines fro m the intracellular side and examined the structural determinants of f ast, 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 cha nnel is not required. Block, however, was significantly enhanced by ad dition of an aromatic ring, or by the lengthening of aliphatic side ch ains, suggesting that a hydrophobic domain strengthens binding while t he amine group blocks the pore. For most blockers, depolarizing potent ials enhanced block, with the charged amine group apparently traversin g 45-60% of the transmembrane voltage. By contrast, block by phenylhyd razine was essentially voltage-independent. The relatively rigid plana r structure of phenylhydrazine may prevent the charged amino end from entering the electric field when the aromatic ring is bound. The relat ion between structural features of different blockers and their sensit ivity to voltage suggests that the transmembrane voltage drops complet ely over less than 5 Angstrom, We raise the possibility that the propo sed hydrophobic binding domain overlaps the endogenous receptor for th e inactivation gate. If so, our data place limits on the distance betw een this receptor and the intrapore site at which charged amines bind.