Characterization of maintained voltage-dependent K(+)-channels induced in Xenopus oocytes by rat brain mRNA

The voltage-dependent K+ currents encoded by rat brain mRNA were studied in Xenopus oocytes after the voltage-dependent Na+ currents and the Ca(2+)-activated Cl- currents were eliminated pharmacologically. This paper describes the maintained K+ currents (IK), defined primarily by resistance to inactivation for 1 s at a holding potential of -40 mV. IK activates at potentials more positive than -60 to -70 mV and consists of both low-threshold and high-threshold components. IK 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 IK; the inactivating component is inhibited by TEA. Sucrose density gradient fractionation partially resolves the RNA encoding the several components of IK; most IK 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.