Phenol derivatives accelerate inactivation kinetics in one inactivation-deficient mutant human skeletal muscle Na(+) channel

Interaction of local anesthetics with a peptide encompassing the IV/S4–S5 linker of the Na+ channel

The peptide pIV/S4-S5 encompasses the cytoplasmic linker between helices S4-S5 in domain IV of the voltage-gated Na+ channel, residues 1644-1664. The interaction of two local anesthetics (LA), lidocaine and benzocaine, with pIV/S4-S5 has been studied by DOSY, heteronuclear NMR 1H-15N-HSQC spectroscopy and computational methods. DOSY indicates that benzocaine, a neutral ester, exhibits stronger interaction with pIV/S4-S5 than lidocaine, a charged amine-amide. Weighted average chemical shifts, Deltadelta(1H-15N), show that benzocaine affects residues L1653, M1655 and S1656 while lidocaine slightly perturbs residues I1646, L1649 and A1659, L1660, near the N- and C-terminus, respectively. Computational methods confirmed the stability of the benzocaine binding and the existence of two binding sites for lidocaine. Even considering that the approach of studying the peptide in the presence of a co-solvent (TFE/H2O, 30%/70% v/v) has an inherently limited implication, our data strongly support the existence of multiple LA binding sites in the IV/S4-S5 linker, as suggested in the literature. In addition, we consider that LA can bind to the S4-S5 linker with diverse binding modes and strength since this linker is part of the receptor for the "inactivation gate particle". Conditions for devising new functional studies, aiming to better understand Na+ channel functionality as well as the various facets of LA pharmacological activity are proposed in this work.

An improved model for the binding of lidocaine and structurally related local anaesthetics to fast-inactivated voltage-operated sodium channels, showing evidence of cooperativity

1. The interaction of lidocaine-like local anaesthetics with voltage-operated sodium channels is traditionally assumed to be characterized by tighter binding of the drugs to depolarized channels. As inactivated and drug-bound channels are both unavailable on depolarization, an indirect approach is required to yield estimates for the dissociation constants from channels in inactivated states. The established model, originally described by Bean et al., describes the difference in affinity between resting and inactivated states in terms of the concentration dependence of the voltage shift in the availability curve. We have tested the hypothesis that this model, which assumes a simple Langmuir relationship, could be improved by introducing a Hill-type exponent, which would take into account potential sources of cooperativity. 2. Steady-state block by lidocaine was studied in heterologously (HEK 293) expressed human skeletal muscle sodium channels and compared with experimental data previously obtained for 2,6-dimethylphenol, 3,5-dimethyl-4-chlorophenol, and 4-chlorophenol. Cells were clamped to membrane potentials from -150 to -5 mV, and a subsequent test pulse was used to assess the number of channels available to open. 3. All compounds shifted the voltage dependence of channel availability in the direction of negative prepulse potentials. Prediction of the concentration dependence of the voltage shift in the availability curve was improved by the modified model, as shown by a marked reduction in the residual sum of squares. 4. For all compounds, the Hill-type exponent was significantly greater than one. These results could be interpreted in the light of the contemporary hypothesis that lidocaine functions as an allosteric gating effector to enhance sodium channel inactivation by strengthening the latch mechanism of inactivation, which is considered to be a particle-binding process allosterically coupled to activation. Alternatively, they could be interpreted by postulating additional binding sites for lidocaine on fast-inactivated sodium channels.