Kinetics of local anesthetic inhibition of neuronal sodium currents. pH and hydrophobicity dependence (original) (raw)
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Kinetic analysis of phasic inhibition of neuronal sodium currents by lidocaine and bupivacaine
Biophysical Journal, 1990
Phasic ("use-dependent") inhibition of sodium currents by the tertiary amine local anesthetics, lidocaine and bupivacaine, was observed in voltage-clamped node of Ranvier of the toad, Bufo marinus. Local anesthetics were assumed to inhibit sodium channels through occupation of a binding site with 1:1 stoichiometry. A three-parameter empirical model for state-dependent anesthetic binding to the Na channel is presented: this model includes two discrete parameters that represent the time integrals of binding and unbinding reactions during a depolarizing pulse, and one continuous parameter that represents the rate of unbinding of drug between pulses. The change in magnitude of peak sodium current during a train of depolarizing pulses to 0 mV was used as an assay of the extent of anesthetic binding at discrete intervals; estimates of model parameters were made by applying a nonlinear least-squares algorithm to the inhibition of currents obtained at two or more depolarizing pulse rates. Increasing the concentration of drug increased the rate of binding but had little or no effect on unbinding, as expected for a simple bimolecular reaction. The dependence of the model parameters on pulse duration was assessed for both drugs: as the duration of depolarizing pulses was increased, the fraction of channels binding drug during each pulse became significantly larger, whereas the fraction of occupied channels unbinding drug remained relatively constant. The rate of recovery from block between pulses was unaffected by pulse duration or magnitude. The separate contributions of open (0) and inactivated (I) channel binding of drug to the net increase in block per pulse were assessed at 0 mV: for lidocaine, the forward binding rate ko was 1.4 * 105 M-ls-1, k, was 2.4 * 104 M-1s-1; for bupivacaine, ko was 2.5-105 M-ls-1, k, was 4.4 * 104 M-ls-1. These binding rates were similar to those derived from time-dependent block of maintained Na currents in nodes where inactivation was incomplete due to treatment with chloramine-T. The dependence of model parameters on the potential between pulses (holding potential) was examined. All three parameters were found to be nearly independent of holding potential from-70 to-100 mV. These results are discussed with respect to established models of dynamic local anesthetic-Na channel interactions.
British Journal of Pharmacology, 2004
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.
Mechanisms of use-dependent block of sodium channels in excitable membranes by local anesthetics
Biophysical Journal, 1984
Many local anesthetics promote reduction in sodium current during repetitive stimulation of excitable membranes. Use-, frequency-, and voltage-dependent responses describe patterns of peak INa when pulse width, pulse frequency, and pulse amplitude are varied. Such responses can be viewed as reflecting voltage-sensitive shifts in equilibrium between conducting, unblocked channels and nonconducting, blocked channels. The modulated-receptor hypothesis postulates shifts in equilibrium as the result of a variable-affinity receptor and modified inactivation gate kinetics in drug-complexed channels. An alternative view considers drug blocking in the absence of these two features. We propose that drug binds to a constant-affinity channel receptor where receptor access is regulated by the channel gates. Specifically, we view channel binding sites as guarded by the channel gate conformation, so that unlike receptors where ligands have continuous access, blocking agent access is variable during the course of an action potential. During the course of an action potential, the m and h gates change conformation in response to transmembrane potential. Conducting channels with both gates open leave the binding site unguarded and thus accessible to drug, whereas nonconducting channels, with gates in the closed conformation, act to restrict drug access to unbound receptors and possibly to trap drug in drug-complexed channels. We develop analytical expressions characterizing guarded receptors as "apparently" variable-affinity binding sites and predicting shifts in "apparent" channel inactivation in the hyperpolarizing direction. These results were confirmed with computer simulations. Furthermore, these results are in quantitative agreement with recent investigations of lidocaine binding in cardiac sodium channels.
Molecular and kinetic determinants of local anaesthetic action on sodium channels
Toxicology Letters, 1998
(1) Local anaesthetics (LA) rely for their clinical actions on state-dependent inhibition of voltage-dependent sodium channels. (2) Single, batrachoxin-modified sodium channels in planar lipid bilayers allow direct observation of drug-channel interactions. Two modes of inhibition of single-channel current are observed: fast block of the open channels and prolongation of a long-lived closed state, some of whose properties resemble those of the inactivated state of unmodified channels. (3) Analogues of different parts of the LA molecule separately mimic each blocking mode: amines-fast block, and water-soluble aromatics-closed state prolongation. (4) Interaction between a v-conotoxin derivative and diethylammonium indicate an intrapore site of fast, open-state block. (5) Site-directed mutagenesis studies suggest that hydrophobic residues in transmembrane segment 6 of repeat domain 4 of sodium channels are critical for both LA binding and stabilization of the inactivated state.
British Journal of Pharmacology, 2004
Voltage-gated Na þ channels are transmembrane proteins that are essential for the propagation of action potentials in excitable cells. Na v 1.7 and Na v 1.8 dorsal root ganglion Na þ channels exhibit different kinetics and sensitivities to tetrodotoxin (TTX). We investigated the properties of both channels in the presence of lidocaine, a local anesthetic (LA) and class I anti-arrhythmic drug. 2 Na v 1.7 and Na v 1.8 Na þ channels were coexpressed with the b 1-subunit in Xenopus oocytes. Na þ currents were recorded using the two-microelectrode voltage-clamp technique. 3 Dose-response curves for both channels had different EC 50 (dose producing 50% maximum current inhibition) (450 mM for Na v 1.7 and 104 mM for Na v 1.8). Lidocaine enhanced current decrease in a frequency-dependent manner. Steady-state inactivation of both channels was also affected by lidocaine, Na v 1.7 being the most sensitive. Only the steady-state activation of Na v 1.8 was affected while the entry of both channels into slow inactivation was affected by lidocaine, Na v 1.8 being affected to a larger degree. 4 Although the channels share homology at DIV S6, the LA binding site, they differ in their sensitivity to lidocaine. Recent studies suggest that other residues on DI and DII known to influence lidocaine binding may explain the differences in affinities between Na v 1.7 and Na v 1.8 Na þ channels. 5 Understanding the properties of these channels and their pharmacology is of critical importance to developing drugs and finding effective therapies to treat chronic pain.
Journal of General Physiology, 1996
A B S T RA C T We have recently reported that brain sodium channels display periods with high (low-/Q) and low (high-/Q) levels of lidocaine-induced open channel block (Salazar, B.C., D.O. Flash, J.L. Walewski, and E. Recio-Pinto. 1995. Brain Res. 699:305-314). In the present study, we further characterize this phenomenon by studying the effects of the permanently charged lidocaine analogue, QX-314. We found that the detection of high-and low-/Q periods does not require the presence of the uncharged form of lidocaine. The level of block, for either period, at various QX-314 concentrations indicated the presence of a single local anesthetic binding site. Increasing the concentration of QX-314 decreased the lifetime of the high-/Q periods while it increased the lifetime of the low-/Q periods. These results could be best fitted to a model with two open channel conformations that display different local anesthetic Kd values (low and high/Q), and in which the channel area defining the local anesthetic/Q consists of multiple interacting regions. Amplitude distribution analysis showed that changes in the/Q values reflected changes in the kon rates, without changes in the kof f rates. Both lidocaine and QX-314 were found to be incapable of blocking small-channel subconductance states (5-6 pS). Changes in the local anesthetic kon rates for blocking the fully open state and the lack of local anesthetic block of the small subconductance state are consistent with the presence of channel conformational changes involving the intracellular permeation pathway leading to the local anesthetic binding site. Key words: lidocaine 9 QX-314 9 brain voltage-dependent sodium channels 9 open channel block
Local Anesthetic Neurotoxicity Does Not Result from Blockade of Voltage-Gated Sodium Channels
Anesthesia & Analgesia, 1995
To investigate whether local anesthetic neurotoxicity results from sodium channel blockade, we compared the effects of intrathecally administered lidocaine, bupivacaine, and tetrodotoxin (TTX), the latter a highly selective sodium channel blocker, on sensory function and spinal cord morphology in a rat model. First, to determine relative anesthetic potency, 25 rats implanted with intrathecal catheters were subjected to infusions of lidocaine (n = 8), bupivacaine (n = 81, or TTX (n = 9). The three drugs produced parallel dose-effect curves that differed significantly from one another: the EC,, values for lidocaine, bupivacaine, and TTX were 28.2 mM (0.66%), 6.6 mM (0.19%), and 462 nM, respectively. Twenty-five additional rats were then given intrathecal lidocaine (n = 8), bupivacaine (n = 8), or TTX (n = 9) at concentrations 10 times the calculated EC,, for sensory block. Lidocaine and bupivacaine induced persistent sensory impairment, whereas TTX did not. Finally, 28 rats were given either intrathecal bupivacaine (n = 10) or TTX (n = 9) at 10 times the EC,,, or normal saline (n = 9). Significant sensory impairment again occurred after infusion of bupivacaine, but not after infusion of TTX or saline. Neuropathologic evaluation revealed moderate to severe nerve root injury in bupivacaine-treated animals; histologic changes in TTX-and saline-treated animals were minimal, similar, and restricted to the area adjacent to the catheter. These results indicate that local anesthetic neurotoxicity does not result from blockade of the sodium channel, and suggest that development of a safer anesthetic is a realistic goal.
Developmental Brain Research, 2003
Sodium currents were recorded in CA1 hippocampal cells from new-born (P 4 -10 ) and older (P >22 ) rats, using whole-cell voltage clamp techniques. The effects of local anaesthetics (procaine and lidocaine) were studied in both cell populations. Parameters defining steady-state inactivation, removal of inactivation and the affinity of the anaesthetic molecules to the inactivated state were determined at both stages of maturation. Procaine and lidocaine induced a hyperpolarizing shift in steady-state inactivation curves, and slowed the rate of recovery from the inactivated state. Procaine disclosed differences between immature and older cells in what concerns block of the closed (resting) channels, drug affinity and binding to the inactivated state, i.e. the binding rate of procaine was found higher and the affinity lower in younger cells. The characteristics of procaine and lidocaine block on CA1 sodium currents differed in some particular aspects: magnitude of block on resting channels, shift in the voltage dependence and voltage sensitivity of steady-state inactivation, slow recovery from inactivation and usedependent block. D