TRANSIENT K-AXON OF THE COCKROACH PERIPLANETA-AMERICANA( CURRENT AND PERIAXONAL POTASSIUM ACCUMULATION IN THE GIANT)

Potassium currents were studied under voltage-clamp conditions in the cockroach isolated giant axon in the presence of 10(-7) M tetrodotoxin . At room temperature, a transient K+ current (inactivation time const ant approximate to 5 ms at +30 mV), having characteristics similar to the I-A current, could be activated if the holding potential (HP) was maintained at values negative to -70 mV. This current was not present at HP -50 mV or if the test pulse applied from a negative HP was prece ded by a approximate to 15-20 ms conditioning predepolarization to -40 mV. Although this transient current could be attributed to the existe nce of inactivating K+ channels, the current decrease during a voltage pulse could be likely explained by the external K+ accumulation in th e periaxonal space, revealed by the instantaneous K+ tail current anal ysis. Even though the K+ accumulation in the isolated cockroach axon i s 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 po sitive 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 stu dies of ionic conductances.