Transient K + current and periaxonal potassium accumulation in the giant axon of the cockroach Periplaneta americana

Abstract

Potassium currents were studied under voltage-clamp conditions in the cockroach isolated giant axon in the presence of 10 −7M tetrodotoxln. At room temperature, a transient K + current (inactivation time constant ∼ 5ms at + 30mV), having characteristics similar to the I A current, could be activated if the holding potential (HP) was maintained at values negative to −70mV. This current was not present at HP −50 mV or if the test pulse applied from a negative HP was preceded by a ∼ 15–20 ms conditioning predepolarization to −40 mV. Although this transient current could be attributed to the existence of inactivating K + channels, the current decrease during a voltage pulse could be likely explained by the external K + accumulation in the periaxonal space, revealed by the instantaneous K + tail current analysis. Even though the K + accumulation in the isolated cockroach axon is less than in many similar preparations, it can increase the external K + concentration by a factor of 10 under the experimental conditions and shift the equilibrium potential for K + ions by 30–50 mV towards positive values at the end of a 35 ms voltage pulse to +30 mV. Thus, the accumulation phenomena should always be taken into account in the studies of ionic conductances.