Chemical modification of potassium channel gating in frog myelinated nerve by trinitrobenzene sulphonic acid.

We investigated the actions of externally applied trinitrobenzene sulphonic acid (TNBS) on the K currents of voltage-clamped frog myelinated nerve fibres. TNBS treatment irreversibly slowed the tail currents for K channel closing up to 10-fold. Time constants for the tail currents appear shifted 60-80 mV in the hyperpolarizing direction following TNBS treatment. The time course of K channel opening was unaffected for depolarizing pulses to potentials positive to -20 mV. For smaller pulses to potentials between -80 and -30 mV the activation time course was slower following TNBS treatment. TNBS had little or no effect on the steady-state conductance-voltage relation of K channels determined from tail current amplitude. The instantaneous current-voltage relation for K channels and potency of block by external TEA were unaffected by TNBS. K tail currents showed fast and slow components both before and after TNBS treatment. Reaction with TNBS caused the fast component to decline in amplitude and the slow component to increase both in magnitude and time constant. The rate of reaction increased with increasing pH. Full expression of altered K channel kinetics depends upon the ionic composition of the external solution. Tail currents were up to 10-fold slower when measured in 117.5 mM-K than when measured with 80% of the external K replaced by Na. The differential effects of TNBS on K channel closing and opening were modelled in a three-state kinetic scheme, with an increase in the energy barrier for closing an open channel following TNBS treatment.