Effects of chemical modification of amino and sulfhydryl groups on the voltage-clamped frog node of Ranvier

Several reagents that react with sulfhydryl and amino groups were applied to voltage-clamped single nerve fibres of the frog. The fibres were exposed to comparable amounts of the chemical reagents for relatively short times. 3-(p-Hydroxyphenyl)propionic acid N-hydroxysuccinimide ester (HPPS), a substance which preferentially modifies amino groups, irreversibly reduced the size of the sodium and potassium current. The effect of HPPS on the Na current could be removed only partially by hyperpolarizing prepulses. N-ethylmaleimide (NEM), a reagent that preferentially reacts with sulfhydryl groups produced a small decrease of the sodium current which was removed almost completely by hyperpolarizing prepulses. NEM and HPPS shifted the voltage dependence of sodium inactivation,h (E), to more negative values of membrane potential, but had little effect on the time course of sodium activation and inactivation. Pretreatment of a fibre with NEM did not prevent the action of HPPS; however, pretreatment of a fibre with HPPS decreased considerably the shift of theh (E) curve caused by NEM. Our results suggest that modification of membrane bound amino groups affects the size of the ionic currents and the inactivation process. Although reagents that react with sulfhydryl groups were found to affect channel function, no definite evidence for the presence or absence of a functionally important sulfhydryl group on sodium channels has been obtained.Abbreviations NEM N-ethylmaleimide - MOPS morpholinopropanesulfonic acid - DTDP dithiodipyridine - SPDP N-succinimidyl-3-(2-pyridyldithio)propionate - HPPS 3-(p-hydroxyphenyl)propionic acid N-hydroxy-succinimide ester