CHARACTERIZATION OF MAINTAINED VOLTAGE-DEPENDENT K+-CHANNELS INDUCED IN XENOPUS OOCYTES BY RAT-BRAIN MESSENGER-RNA

The voltage-dependent K+ currents encoded by rat brain mRNA were studied in Xenopus oocytes after the voltage-dependent Na+ currents and the Ca2+-activated Cl- currents were eliminated pharmacologically. This paper describes the maintained K+ currents (I(K)), defined primarily by resistance to inactivation for 1 s at a holding potential of -40 mV. I(K) activates at potentials more positive than -60 to -70 mV and consists of both low-threshold and high-threshold components. I(K) is partially blocked by both tetraethyl ammonium (TEA) and 4-aminopyridine (4-AP), which appear to be blocking the same component. Long depolarizing pulses result in incomplete inactivation of I(K); the inactivating component is inhibited by TEA. Sucrose density gradient fractionation partially resolves the RNA encoding the several components of I(K); most I(K) arises from size classes between 3.8 and 9.5 kb. The study gives further evidence for the existence of numerous distinct RNA populations that encode brain K+ channels different from previously reported cloned K+ channels that have been expressed in Xenopus oocytes.