Asymmetrical displacement current and its relation with the activation of sodium current in the membrane of frog myelinated nerve

Abstract

1. Sodium currents (I_Na) and asymmetrical displacement currents (I _D ) were measured in the same nerve fibres fromRana esculenta under similar conditions. 2. For exploring possible kinetic and steady state relations betweenI_Na andI _D the following quantities were compared: (i) the activation of the sodium channels and (ii) the charge displacement ofI _D . 3. The delay of sodium activation increased after hyperpolarization. A corresponding effect on the displacement of charge was not observed. 4. Upon a small depolarization sodium activation rose slower than the displacement of charge, whereas at large depolarizations sodium activation reached a steady level before the charge displacement. 5. Upon repolarization to various levels between −52 and 12 mV relative to the resting potential, the ratio between the time constants of charge displacement and of sodium tail current varied between 3 and 1. 6. In the steady state the sodium activation was one half at about the same potential as the charge displacement but exhibited a clearly steeper voltage dependence. 7. Blockage of sodium channels with tetrodotoxin did not affect the asymmetrical displacement current. Replacing a part of external Na by tris did not alter the sodium activation process. 8. The results indicate that the asymmetrical displacement of charge may reflect states of the gating mechanism in sodium channels but cannot be considered as a correlate of the Hodgkin Huxleym variable.