Intracellular Ca2+ activates a fast voltage-sensitive K+ current in vertebrate sympathetic neurones

Many neurones, when depolarized, exhibit two components of outward K+ current—the voltage-sensitive delayed rectifier current originally described in squid axons by Hodgkin and Huxley1, and an additional current triggered by the entry of Ca2+ ions2. These two currents have been termed IK and IC, respectively3. Previous experiments have indicated that both forms of K+ current are also present in vertebrate sympathetic neurones4–6. We have now studied the properties of IC in bullfrog sympathetic neurones, uncontaminated with IK, by injecting Ca2+ ions into the cells and measuring the resultant outward currents under voltage-clamp, in the manner previously used for large molluscan neurones7,8. We find three interesting properties of IC in these vertebrate neurones. First, it shows strong voltage sensitivity independent of the voltage sensitivity of the Ca2+ channels (which have been bypassed by the injection technique). Second, IC activates and deactivates very rapidly (τC≤20 ms at 0 mV), with stepped changes in membrane potential. Current fluctuation analysis and patch-clamp records of single-channel currents yielded evidence for appropriate short-lifetime ionic channels with a maximum conductance of ∼100 pS. Finally IC in ganglion cells is highly sensitive to external tetraethylammonium. We deduce that in these neurones IC is a fast current which can contribute a substantial fraction to the repolarizing current during an action potential.