Transient impairment of the axolemma following regional anaesthesia by lidocaine in humans.

The local anaesthetic lidocaine is known to block voltage-gated Na(+) channels (VGSCs), although at high concentration it was also reported to block other ion channel currents as well as to alter lipid membranes. The aim of this study was to investigate whether the clinical regional anaesthetic action of lidocaine could be accounted for solely by the block of VGSCs or whether other mechanisms are also relevant. We tested the recovery of motor axon conduction and multiple measures of excitability by 'threshold-tracking' after ultrasound-guided distal median nerve regional anaesthesia in 13 healthy volunteers. Lidocaine caused rapid complete motor axon conduction block localized at the wrist. Within 3 h, the force of the abductor pollicis brevis muscle and median motor nerve conduction studies returned to normal. In contrast, the excitability of the motor axons at the wrist remained markedly impaired as indicated by a 7-fold shift of the stimulus-response curves to higher currents with partial recovery by 6 h and full recovery by 24 h. The strength-duration properties were abnormal with markedly increased rheobase and reduced strength-duration time constant. The changes in threshold during electrotonus, especially during depolarization, were markedly reduced. The recovery cycle showed increased refractoriness and reduced superexcitability. The excitability changes were only partly similar to those previously observed after poisoning with the VGSC blocker tetrodotoxin. Assuming an unaltered ion-channel gating, modelling indicated that, apart from up to a 4-fold reduction in the number of functioning VGSCs, lidocaine also caused a decrease of passive membrane resistance and an increase of capacitance. Our data suggest that the lidocaine effects, even at clinical 'sub-blocking' concentrations, could reflect, at least in part, a reversible structural impairment of the axolemma.

Coffee drinking enhances the analgesic effect of cigarette smoking.

Nicotine (from cigarette smoke) and caffeine (from coffee) have analgesic effects in humans and experimental animals. We investigated the combined effects of coffee drinking and cigarette smoking on pain experience in a group of moderate nicotine-dependent, coffee drinking, young smokers. Pain threshold and pain tolerance were measured during cold pressor test following the habitual nocturnal deprivation of smoking and coffee drinking. Smoking increased pain threshold and pain tolerance in both men and women. Coffee drinking, at a dose that had no independent effect, doubled the increase in pain threshold induced by smoking. The effect could not be explained by a cumulative raise in blood pressure. Our data suggest that caffeine enhances the analgesic effect of nicotine.