Irreversible inhibition of sodium current and batrachotoxin binding by a photoaffinity-derivatized local anesthetic

Does scorpion bite lead to development of resistance to the effect of local anaesthetics?

A patient posted for vaginal hysterectomy was administered subarachnoid block, which failed, so was repeated in one space above. The block failed again, after waiting for 30 min. Patient gave a history of scorpion bite twice, once at the age of 17 years on her right foot and again about 8 months back. Thereafter, balanced general anaesthesia was given. On eighth post-operative day, after explaining about her possible special condition (?Resistance to local anaesthetic agents), the patient was given left median, ulnar and radial nerve blocks at the wrist and local infiltration near the anatomical snuff box. There was neither sensory nor motor block. The scorpion venom is known to affect the pumping mechanism of sodium channels in the nerve fibres, which are involved in the mechanism of action of local anaesthetic drugs, it may be responsible for the development of 'resistance' to the action of local anaesthetic agents.

Quaternary ammonium block of mutant Na+ channels lacking inactivation: features of a transition-intermediate mechanism

1. The quaternary ammonium (QA) lidocaine derivative QX-314 (2-(triethylamino)-N-(2,6-dimethylphenyl)-acetamide) induces internal pore blockade of single cardiac Na+ channels enzymatically modified (papain) to eliminate fast inactivation. The mechanism involves dual, interacting blocking modes (rapid and discrete) with binding domains deep in the pore from the cytoplasmic mouth, and where the rapid blocked configuration serves as a transition-intermediate for the development of discrete block. The primary goals of this study were to test for this mechanism in a recombinant Na+ channel genetically engineered to selectively lack fast inactivation, and if present, to explore the underlying structural features. 2. Fast inactivation was removed in rat skeletal muscle mu1 Na+ channels (RSkM1) with an IFM-QQQ mutation in the cytoplasmic III-IV interdomain (QQQ). QQQ was expressed in Xenopus oocytes and single-channel activity was studied in cell-free, inside-out membrane patches. Application of QX-314 (QX, 0-4 mM) to the cytoplasmic membrane surface caused two distinct modalities of single-channel blockade: reduction of unitary current and interruptions of current lasting tens of milliseconds. These are consistent with rapid and discrete pore block, respectively. The voltage and concentration dependence of block indicates that the modes interact and have binding sites that share a deep location in the pore, at approximately 65 % of the membrane electric field in from the cytoplasmic mouth. 3. Mutation of phenylalanine (F1579) in domain IV-S6, critical in local anaesthetic block, to alanine in QQQ (QQQ-F1579A) disabled discrete block but notably failed to alter rapid block, single-channel gating and slope conductance. 4. Amplitude distribution analysis was applied to long bursts (> 50 ms) of QQQ-F1579A activity to investigate the kinetics of rapid block. Computed rapid blocking and unblocking rate constants are 42 000 +/- 18 000 m-1 ms-1 and 82 +/- 22 ms-1, respectively (n = 3, -20 mV). 5. The results support a general transition-intermediate mechanism that governs internal QX and local anaesthetic pore block of voltage-gated Na+ channels and provide insight into underlying structural features.

Tolperisone: evaluation of the lidocaine-like activity by molecular modeling

Tolperisone (1), a muscle relaxant with lidocaine-like activity, was compared to lidocaine (2) by molecular modeling methods. Conformational search analysis has been employed to find the global minima of these compounds along with numerous low energy conformations from which specific conformers were extracted that show good superimposition of the structural features important for protein binding. Two additional compounds, mepi- (3) and bupivacaine (4), were included in the analysis to validate the method as these ligands show very close structural and pharmacological relationship to lidocaine (2) and are assumed to bind to an identical site. As a result we find conformers of all four ligands that have exactly the same position and orientation of the potential sites for hydrogen bonding with the rest of the molecule showing close comparison of the three-dimensional geometry. Semiempirical calculations furthermore reveal good agreement of the electrostatic potentials of these conformations indicating similar interactions with a receptor. We conclude that tolperisone (1) and lidocaine (2) despite their chemical divergence can still attach to identical protein binding sites.