Molecular properties of local anesthetics as predictors of affinity for nicotinic acetylcholine receptors

Muscle-Type Nicotinic Receptor Modulation by 2,6-Dimethylaniline, a Molecule Resembling the Hydrophobic Moiety of Lidocaine

To identify the molecular determinants responsible for lidocaine blockade of muscle-type nAChRs, we have studied the effects on this receptor of 2,6-dimethylaniline (DMA), which resembles lidocaine’s hydrophobic moiety. Torpedo marmorata nAChRs were microtransplanted to Xenopus oocytes and currents elicited by ACh (I_ACh), either alone or co-applied with DMA, were recorded. DMA reversibly blocked I_ACh and, similarly to lidocaine, exerted a closed-channel blockade, as evidenced by the enhancement of I_ACh blockade when DMA was pre-applied before its co-application with ACh, and hastened I_ACh decay. However, there were marked differences among its mechanisms of nAChR inhibition and those mediated by either the entire lidocaine molecule or diethylamine (DEA), a small amine resembling lidocaine’s hydrophilic moiety. Thereby, the IC₅₀ for DMA, estimated from the dose-inhibition curve, was in the millimolar range, which is one order of magnitude higher than that for either DEA or lidocaine. Besides, nAChR blockade by DMA was voltage-independent in contrast to the increase of I_ACh inhibition at negative potentials caused by the more polar lidocaine or DEA molecules. Accordingly, virtual docking assays of DMA on nAChRs showed that this molecule binds predominantly at intersubunit crevices of the transmembrane-spanning domain, but also at the extracellular domain. Furthermore, DMA interacted with residues inside the channel pore, although only in the open-channel conformation. Interestingly, co-application of ACh with DEA and DMA, at their IC₅₀s, had additive inhibitory effects on I_ACh and the extent of blockade was similar to that predicted by the allotopic model of interaction, suggesting that DEA and DMA bind to nAChRs at different loci. These results indicate that DMA mainly mimics the low potency and non-competitive actions of lidocaine on nAChRs, as opposed to the high potency and voltage-dependent block by lidocaine, which is emulated by the hydrophilic DEA. Furthermore, it is pointed out that the hydrophobic (DMA) and hydrophilic (DEA) moieties of the lidocaine molecule act differently on nAChRs and that their separate actions taken together account for most of the inhibitory effects of the whole lidocaine molecule on nAChRs.

Effects of 2 mg.kg⁻¹ of intravenous lidocaine on the latency of two different doses of rocuronium and on the hemodynamic response to orotracheal intubation

BACKGROUND AND OBJECTIVES

Lidocaine potentiates the effects of neuromuscular blockers and attenuates the hemodynamic response to orotracheal intubation. The objective of the present study was to test the effects of lidocaine on the latency of two different doses of rocuronium and on the hemodynamic response to intubation.

METHODS

Eighty patients were distributed in 4 groups: Groups 1 and 2 received 0.6 mg.kg(-1) of rocuronium; patients in Group 2 also received 2 mg.kg(-1) of lidocaine before intubation. Patients in Groups 3 and 4 received 1.2 mg.kg(-1) of rocuronium; patients in Group 4 received additional 2 mg.kg(-1) of lidocaine. The latency of the neuromuscular blockade was measured by acceleromyography. Hemodynamic evaluation was performed at baseline, immediately before, and 1 minute after orotracheal intubation (OI).

RESULTS

Statistically significant differences were not observed between the latency from 0.6 mg.kg(-1) and 1.2 mg.kg(-1) of rocuronium in patients who received lidocaine before induction and those who did not. The latency in patients who received 0.6 mg.kg(-1) of rocuronium with lidocaine was statistically similar to that of those who received 1.2 mg.kg(-1) rocuronium independently of whether lidocaine was administered or not. Patients who did not receive lidocaine before induction showed the same increases in systolic, diastolic, and mean arterial pressure and heart rate after OI, which was not observed in those patients who received lidocaine.

CONCLUSIONS

Intravenous lidocaine before anesthetic induction was capable of attenuating the hemodynamic response associated to OI maneuvers, but it did not reduce the latency of the neuromuscular blockade produced by two different doses of rocuronium.