Analysis of the nerve-blocking action of mercurochrome, a fluorescent thiol reagent

Giant axons isolated from the circumesophageal connectives of the lobster were used to determine the effects of the thiol reagent, mercurochrome, on the excitability of the axonal membrane. Intracellular microelectrode recordings demonstrated that mercurochrome depolarized the resting membrane potential only slightly but completely blocked conduction of the propagated action potential. Partial conduction block was markedly potentiated by repeated electrical stimulation. To investigate whether mercurochrome produced a specific shift in membrane ion selectivity, the compound was tested in low Na + and high K + media. Calculation of transference numbers demonstrated that mercurochrome lacked any marked Na or K ion channel specificity. Thiol group protection experiments suggest that some of the effects of mercurochrome may not necessarily involve thiols. Treatment of the axons with a disulphide reducing agent prior to the application abolished the potentiating effects of electrical stimulation, but decreased the sensitivity of the nerve fibres to mercurochrome possibly through the involvement of non-specific disulphide groups.