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

Abstract

1. Delayed K+-currents and displacement currents were studied with a voltage-clamp technique. 2. In normal fibres, the conductance of the delayed channel grows e-fold per 3 millivolts 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 fourfold. 3. By contrast, 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. 4. Tetraethylammonium ions (TEA) are known to slow the kinetics of delayed K+-channels fivefold but fail, like tetracaine, to change the kinetics of the displacement currents. 5. Both tetracaine and TEA have thus large effects on the 'gating' of the delayed channel, yet little or none on the displacement currents. This suggests that the displacement currents in skeletal muscle 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.