Two components of Ca-dependent potassium current in identified neurones ofAplysia californica

Abstract

Outward tail currents measured inAplysia neurones after termination of depolarizing voltage-clamp pulses consist of rapidly decaying voltage-dependent K currents and slow tail currents of much slower time course. The rapidly decaying voltage-dependent tail currents were blocked with aminopyridines, and measurements of the slow tail currents were made following decay of any residual rapid tail currents. The slow tail current exhibited two components of differing sensitivity to externally applied tetraethylammonium (TEA) ions. In some neurones of the abdominal ganglion (L-2, L-4), virtually all of the slow tail current was resistant to blockage by TEA, while in others (L-3, L-6) 80% or more of the slow tail current was blocked by low TEA concentrations (K _D<1 mM), the remaining slow tail current being resistant to TEA. This TEA-resistant slow tail current was identified as a K current because it reversed near the K equilibrium potential (E _K), the reversal potential was shifted by changes in the external K concentration, and it could be blocked by injection of Cs⁺. It was abolished by replacement of external Ca²⁺ by Co²⁺ or Ba²⁺, by addition of Cd²⁺, or by injection of EGTA, and thus determined to be a Ca-dependent current. Intracellular injection of TEA or external application of aminopyridine or apamine had little or no effect on the TEA-resistant slow tail current. Quinidine reduced the TEA-sensitive, but not the TEA-resistant current. Both the TEA-sensitive and the TEA-resistant components of the slow tail current exhibited similar time courses of decay. Thus, neurones ofAplysia appear to contain different proportions of two classes of Ca-dependent K channels that differ in their sensitivity to certain channel blocking agents.