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 (IK), defined primarily by resistance to inactivation for 1 s at a holding potential of -40 mV. IKactivates at potentials more positive than -60 to -70 mV and consists of both low-threshold and high-threshold components. IKis 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 IKarises 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. © 1991.
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