Chemical modification of sodium channel surface charges in frog skeletal muscle by trinitrobenzene sulphonic acid.

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Abstract
The effects of externally applied trinitrobenzene sulfonic acid (TNBS) on Na currents of voltage-clamped frog skeletal muscle fibers were investigated. TNBS is a membrane-impermeant reagent which reacts specifically with amino groups under physiological conditions. TNBS shifts the voltage dependence of steady-state Na current inactivation (h.infin.) by .apprx. 18 mV to more hyperpolarized potentials when measured at pH 9. This effect of TNBS is irreversible, suggesting that the reagent permanently modifies some membrane component(s). Time constants for the development of and recovery from inactivation of Na current are similarly shifted to more hyperpolarized potentials by TNBS treatment. External TNBS did not affect the completeness of Na current inactivation during depolarizing pulses. The activation of Na current is less affected by TNBS than is inactivation. The voltage dependence of peak Na current activation was shifted by .apprx. 4 mV by TNBS treatment at pH 9. The kinetics of Na current activation during depolarization were little affected by TNBS. The TNBS-induced shift in h.infin. and Na current activation are independent of pH when measured at pH between 7.4 and 11. TNBS may increase the negative surface charge of the membrane by reacting with membrane amino groups having a high pK.