Permeation of Large Tetra-Alkylammonium Cations through Mutant and Wild-Type Voltage-Gated Sodium Channels as Revealed by Relief of Block at High Voltage

Many large organic cations are potent blockers of K⁺ channels and other cation-selective channels belonging to the P-region superfamily. However, the mechanism by which large hydrophobic cations enter and exit the narrow pores of these proteins is obscure. Previous work has shown that a conserved Lys residue in the DEKA locus of voltage-gated Na⁺ channels is an important determinant of Na⁺/K⁺ discrimination, exclusion of Ca²⁺, and molecular sieving of organic cations. In this study, we sought to determine whether the Lys(III) residue of the DEKA locus interacts with internal tetra-alkylammonium cations (TAA⁺) that block Na⁺ channels in a voltage-dependent fashion. We investigated block by a series of TAA⁺ cations of the wild-type rat muscle Na⁺ channel (DEKA) and two different mutants of the DEKA locus, DEAA and DERA, using whole-cell recording. TEA⁺ and larger TAA⁺ cations block both wild-type and DEAA channels. However, DEAA exhibits dramatic relief of block by large TAA⁺ cations as revealed by a positive inflection in the macroscopic I–V curve at voltages greater than +140 mV. Paradoxically, relief of block at high positive voltage is observed for large (e.g., tetrapentylammonium) but not small (e.g., TEA⁺) symmetrical TAA⁺ cations. The DEKA wild-type channel and the DERA mutant exhibit a similar relief-of-block phenomenon superimposed on background current rectification. The results indicate: (a) hydrophobic TAA⁺ cations with a molecular diameter as large as 15 Å can permeate Na⁺ channels from inside to outside when driven by high positive voltage, and (b) the Lys(III) residue of the DEKA locus is an important determinant of inward rectification and internal block in Na⁺ channels. From these observations, we suggest that hydrophobic interfaces between subunits, pseudosubunits, or packed helices of P-region channel proteins may function in facilitating blocker access to the pore, and may thus play an important role in the blocking and permeation behavior of large TAA⁺ cations and potentially other kinds of local anesthetic molecules.