Using Lidocaine and Benzocaine to Link Sodium Channel Molecular Conformations to State-Dependent Antiarrhythmic Drug Affinity

Abstract

A subset share characteristics that make them effective as antiarrhythmic drugs, ie, they exhibit high affinity, use-dependent block of Na current (I Na ) at high heart rates. Despite extensive study, there remains uncertainty regarding how observed block relates to specific drug-channel conformations. Several vocabularies have emerged to describe block, which in general, have their basis in kinetic models of Na channel gating and assume preferential binding to one or more states that produce no 1 or altered 2 gating. Recent availability of crystal structures in combination with mutagenesis data now allow for linking electrophysiological data, kinetic states, and drug block to specific channel conformations.It is generally accepted that lidocaine and lidocaine-like drugs bind in the inner pore of voltage-gated Na channels. Scanning mutagenesis studies with various Na channel isoforms and multiple lidocaine-like drugs have identified only one amino acid residue, a phenylalanine (Phe) in domain IV, S6 (DIVS6), which, when mutated, alters use-dependent drug affinity by more than ten-fold. When this Phe (1759 in Na V 1.5) is mutated to nonaromatic residues 3-8 or to unnatural amino acids with different electron withdrawing capabilities 9 the mutated channel shows a marked decrease in high-affinity LA block. Homology modeling with K channels predicts that this Phe faces the pore just below the selectivity filter. 10,11 This orientation of Phe is supported by the finding that its cysteine mutant is accessible to MTS reagents applied from inside the pore when the channel is maintained in an open state. 12 Furthermore, it has been shown by us 13 and others 14 that use-dependent block is intimately associated with altered movements of the structurally distant S4 segments in domains III and IV.Block assayed from negative holding potentials at low rates of stimulation is affected very little by channel mutations in contract to their effects on use-dependent block. This lower affinity block is usually called tonic block, although it has also been called rested-state block (or closed-state block)

Molecular basis for class Ib anti-arrhythmic inhibition of cardiac sodium channels

“…Consistent with earlier studies, we found almost no use-dependent inhibition by benzocaine 20,25,33,39 ( Supplementary Fig. S5), suggesting that a protonated amine is necessary for use-dependent inhibition.…”

“…It is generally appreciated that sodium channel inhibitor action involves a number of different mechanisms 12 including pore block 13 , electrostatic interactions between the cationic charge on the drug and sodium ions at the selectivity filter 14,15 , stabilization of either fast or slow non-conducting states of the channel 16,17 , gating charge immobilization [18][19][20] , and targeted uncoupling of channel gating from voltage-sensing 21 . Previous studies have shown that site-directed mutation of two strictly conserved aromatic residues in transmembrane segment 6 (S6) of the fourth sodium channel domain (DIV), Phe1760 and Tyr1767 in Na v 1.5, can virtually abolish channel inhibition by all classes of anti-arrhythmic drugs 12,[22][23][24] .…”

Prokaryotic NavMs channel as a structural and functional model for eukaryotic sodium channel antagonism

“…Residues F1774 and Y1781 of DIV (and residues in DI and DIII) of eukaryotic sodium channels have been identified as important for binding channel-blocker compounds (7)(8)(9)(10)(16)(17)(18) and use-dependence drug-binding (8,17). The residues in DIV correspond to residues T207 and F214 in NavMs (Figs.…”

“…Although there are as yet no crystal structures of eukaryotic sodium channels, crystal structures of several prokaryotic sodium channels in different conformational states have been determined, including ones with closed (2), partially (3) and fully (4) open pores, and two potentially inactivated forms (5,6). Mutations in human sodium channels (hNa v s) have been linked to channelopathies such as epilepsy, cardiac arrhythmia, and chronic pain syndromes; consequently sodium channel blockers have been developed as anticonvulsant, antiarrhythmic, and local anesthetic drugs (7)(8)(9)(10). Several eukaryotic calcium channel blocker drugs have previously been found to bind and block prokaryotic sodium channels (11)(12)(13).…”

Voltage-dependent blockade by bupivacaine of cardiac sodium channels expressed in Xenopus oocytes

“…state [25] . The latter type of inactivation only occurs at very high concentrations and is therefore considered to be a low-affi nity block, so this was not a concern in the current residues within S6, such as L1280 and P1759 [10] .…”