Slow sodium inactivation in nerve after exposure to sulhydryl blocking reagents.

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Abstract
Exposure to N-ethylmaleimide (NEM), a reagent that binds covalently to protein sulfhydryl groups, results in a specific reduction in Na conductance in crayfish (Procambarus clarkii] axons. Resting potential, the delayed rise in K conductance, and the selectivity of the Na channel are unaffected. Na currents are only slightly increased by hyperpolarizing prepulses of up to 50 ms duration, but can be restored to .apprx. 70% their value before treatment if this duration is increased to 300-800 ms. The time to peak Na current and the time constant of decay of Na tail currents are unaffected by NEM, suggesting that the Na activation system remains unaltered. Kinetic studies suggest that NEM reacts with a ''slow'' Na inactivation system present in normal axons which may be seen after depolarization produced by lowering the holding potential or increasing the external K+ concentration. NEM also perturbs the fast h [probability variable in Hodgkin-Huxley equation] inactivation system, and in a potential-dependent manner. At small depolarizations .tau.h is decreased, while at strong depolarizations it is increased over control values. Experiments with structural analogs of NEM suggest that sulfhydryl block is involved, but do not rule out an action similar to that of local anesthetics. p-Chloromercuriphenylsulfonic acid (PCMBS), another reagent with high specificity for SH groups, also blocks Na currents, but restoration with prolonged hyperpolarizations is not possible.