Differential effects of tetracaine on delayed potassium channels and displacement currents in frog skeletal muscle.

Abstract

Delayed K+-currents and displacement currents were studied with a voltage-clamp technique. In normal [Rana temporaria] fibers, the conductance of the delayed channel grows e-fold per 3 mV at sufficiently negative potentials and reaches a limiting value of 2-10 m mho/cm2 (mean 5.8 m-mho/cm2) at positive potentials. Adding tetracaine (2 mM) reduces the limiting conductance, shifts the voltage-dependence of the delayed channel to +25 mV more positive potentials and slows the kinetics 4-fold. The displacement currents are virtually unaltered by 2 mM tetracaine. Their voltage-dependence is shifted by less than 5 mV and their kinetics are unaffected. Tetraethylammonium ions (TEA) are known to slow the kinetics of delayed K+-channels 5-fold but fail, like tetracaine, to change the kinetics of the displacement currents. Both tetracaine and TEA have large effects on the gating of the the delayed channel, yet little or none on the displacement currents. The displacement currents in skeletal muscle probably are for the most part unrelated to the opening and closing of delayed channels. It is estimated that gating the delayed channel in muscle may require no more than 1 or 2% of the observed charge displacement.