Kinetics of local anesthetic inhibition of neuronal sodium currents. pH and hydrophobicity dependence

Abstract

This study assesses the importance of local anesthetic charge and hydrophobicity in determining the rates of binding to and dissociation from neuronal Na channels. Five amide-linked local anesthetics, paired either by similar pKa or hydrophobicity, were chosen for study: lidocaine, two tertiary amine lidocaine homologs, a neutral lidocaine homolog, and bupivacaine. Voltage-clamped nodes of Ranvier from the sciatic nerve of Bufo marinus were exposed to anesthetic externally, and use-dependent ("phasic") block of Na current was observed. Kinetic analysis of binding (blocking) rates was performed using a three parameter, piecewise-exponential binding model. Changes in extracellular pH (pHo) were used to assess the role of drug protonation in determining the rate of onset of, and recovery from, phasic block. For those drugs with pKa's in the range of pHo tested (6.2-10.4), the forward binding rate during a depolarizing pulse increased at higher pH, consistent with an increase in either intracellular or intramembrane concentration of drug. The rate for unbinding during depolarization was independent of pHo. The dissociation rate between pulses also increased at higher pHo. The pHo dependence of the dissociation rate was not consistent with a model in which the cation is trapped relentlessly within a closed channel. Quantitative estimates of dissociation rates show that the cationic form of lidocaine dissociates at a rate of 0.1 s-1 (at 13 degrees C); for neutral lidocaine, the dissociation rate is 7.0 s-1. Furthermore, the apparent pKa of bound local anesthetic was found to be close to the pKa in aqueous solution, but different than the pKa for "free" local anesthetic accessible to the depolarized channel.

Subunit and Frequency-Dependent Inhibition of Acid Sensing Ion Channels by Local Anesthetic Tetracaine

“…pH < 7.0), the penetration of local anesthetics into neuron is dramatically reduced by decrease of the non-ionized form. Accordingly, its effect on voltage-gated sodium channels is diminished in severe acidic conditions [47]. In contrast to its action on sodium channels, our studies showed that tetracaine still has significant effect on ASIC current at pH level as low as 4.5.…”

Evidence for a Multivalent Interaction of Symmetrical, N-Linked, Lidocaine Dimers with Voltage-Gated Na+ Channels

“…In the present study, however, the most hydrophobic molecules [i.e., compounds 5 and 6 (the relative increase in logD value being ϳ 0.5 per methylene unit)] had lower in vitro potencies and exhibited significantly faster dissociation rates from VGSCs than compounds 3 or 4. Differences in local anesthetic pK a values and hence the relative proportions of cationic and uncharged free amine forms, also can influence the extent and/or kinetics of inhibition of VGSCs (Chernoff and Strichartz, 1990). The calculated pK a value (Pallas; CompuDrug International, Inc., Sedona, AZ) for lidocaine is 8.03.…”

State-dependent inhibition of sodium channels by local anesthetics: A 40-year evolution

“…Since the LA receptor of Na + channels is situated within the permeation pathway accessible by permanently charged LA derivatives only from the intracellular surface [6], clinic applications of LAs with a tertiary amine moiety require that LA drugs first penetrate the membrane phase in their neutral (unprotonatcd) form. The neutral form may enter and exit its receptor site via the transmembrane segments of Na + channels from the lipid environment [12, 43]. Such a hydrophobic pathway through the membrane phase is not well defined and the significance of this direct hydrophobic pathway has not been evaluated but it may play an important role for a neutral LA drug, benzocaine.…”