Lidocaine, which has been in clinical use for over 60 years, is one of the most widely used local anaesthetics and is useful for treating ventricular arrhythmias. The mechanism of action of lidocaine has been intensively investigated, yet there remain important unresolved questions. Although there is evidence that lidocaine can interact with multiple targets, lidocaine's primary clinically relevant target is believed to be voltage-gated sodium channels. Voltage-gated sodium channels play a critical role in the generation and propagation of action potentials in neurons and muscle cells. Lidocaine inhibition of voltage-gated sodium currents involves complex voltage and use dependence that is thought to be crucial for many of the therapeutic effects. An elegant study published in this issue of The Journal of Physiology by Sheets & Hanck (2007) identifies crucial components of the molecular mechanism that underlies the use-dependent block of sodium channels by lidocaine.
Lidocaine binds to voltage-gated sodium channels in a 1: 1 fashion and prevents the flow of sodium ions through the channel pore. Mutagenesis experiments have identified multiple residues lining the pore that are important in the lidocaine block (Nau & Wang, 2004) and molecular models have been developed to account for how lidocaine binds to these residues and impedes the flow of sodium ions (Fozzard et al. 2005; McNulty et al. 2007). Several theories have been proposed to account for the complexities of lidocaine's action, including the modulated receptor hypothesis (Hille, 1977), which postulates that the receptor on the sodium channels can exist in multiple configurations and the affinity and binding rates of lidocaine depend on channel's state, which in turn can depend on voltage. However, the molecular mechanisms accounting for the voltage- and use-dependent block of sodium channels by lidocaine have remained elusive. Sheets & Hanck investigated the proposal that the outward movement of the voltage sensors of sodium channels (the S4 segments) are critical to the establishment of the high affinity receptor. They focus on the S4 segments of domains III an IV as they previously determined that lidocaine locked movement of the gating charge associated with these S4s but not the S4 segments of domains I and II (Sheets & Hanck, 2003). Cysteine mutagenesis of specific charged residues in the S4s coupled with biotinoylaminoethylmethanethiosulphonate (MTSEA-biotin) modification was cleverly used to lock the S4 segments in the depolarized, or outward, configuration to determine if the position of the S4 segments was indeed crucial to the formation of the high affinity lidocaine binding site.
Gating current measurements, which are a tour de force in mammalian expression systems, were used to confirm that the S4 segments could be locked, or stabilized, in the depolarized configuration. Sheets & Hanck (2007) show that stabilizing the S4 segments into the outward configuration locks the sodium channels into the high affinity state, essentially providing proof of the concept behind the modulated receptor hypothesis for lidocaine block. They extended their findings by investigating interactions between the fast inactivation gate, gating currents, S4 segment positions and lidocaine. Removal of fast inactivation by cysteine mutagenesis of the inactivation particle coupled with MSTET treatment greatly reduced the ability of lidocaine to block sodium currents, but did not eliminate lidocaine's ability to reduce gating current (presumably by stabilizing S4 segments in the outward configuration). Locking the S4 segments of domains III and IV in the outward configuration in fast-inactivation deficient channels also significantly enhanced lidocaine's affinity for the sodium channels, although this was still 20-fold lower than the affinity of locked channels with intact inactivation. These data suggest that positioning of the S4 segments and the fast-inactivation particle work in a concerted manner to set up the high affinity binding of lidocaine.
As Sheets and Hanck point out, the modified channels with the S4s locked in position may not replicate a specific natural configuration and therefore their data do not necessarily address some of the controversies regarding which precise state of the channel has the highest affinity for lidocaine. Although their experiments on channels with defective fast inactivation clearly confirm that the fast inactivation gate contributes to high affinity binding, other studies have suggested that slow-inactivation of sodium channels may contribute to increased affinity for lidocaine, and this remains an intriguing possibility. While Sheets & Hanck focus on the cardiac sodium channel, it is highly likely that the outward positioning of the DIII and DIV S4s is necessary to achieve the high affinity configuration for neuronal sodium channel isoforms and therefore this work may have important implications for understanding the local anaesthetic actions of lidocaine too. Other clinically relevant compounds, such as anticonvulsants, also show complex use dependence and bind at the same site, or an overlapping site, as lidocaine. It will be interesting to see if other sodium channel modulators derive their voltage- or use-dependent properties from movement of S4 segments in only domains III and IV or if some might involve the S4s in domains I and II.