title: Delayed rectifier current of bullfrog sympathetic neurons: ion-ion competition, asymmetrical block and effects of ions on gating.
abstract: 1. The delayed rectifier (DR) K+ channel pore was probed using different permeant and blocking ions applied intra- and extracellularly. Currents were recorded from bullfrog sympathetic neurons using whole-cell patch-clamp techniques. 2. With intra- and extracellular Cs+ (0 K+), there were large, tetraethylammonium (TEA)-sensitive currents. Adding K+ back to the extracellular solution revealed that the current with Cs+i was K+ selective (permeability ratio PCs/PK = 0.17 +/- 0.02, n = 4) and showed a strong anomalous mole fraction effect. 3. There were also large non-inactivating currents with Na+i and Na+o (0 K+). The current with Na+i was K+ selective (Na+o vs. K+o: PNa/PK = 0.022 +/- 0.005, n = 5), and was TEA sensitive with K+o but not with Na+o. 4. Permeant ions affected gating kinetics. DR currents activated faster in K+ than in Cs+, and activated faster with increasing concentrations of either K+ or Cs+. Deactivation was slowed by increased K+ or Cs+ concentration, with no difference between K+ and Cs+. 5. The pore was also characterized using intracellular blocking ions. A wide variety of monovalent cations (TEA, N-methyl-D-glucamine, arginine, choline, CH3NH3+, Li+, Cs+ and Na+) blocked DR channels from the inside in a voltage-dependent manner: KD at 0 mV was 2.9 mM for TEA and 134-487 mM for the others, at apparent electrical distances (delta) of 0.33-0.79. There was no detectable block by 10 mM Mgi2+. Apart from TEA, the organic cations did not block from the outside. 6. The permeability to Na+ in the absence of K+, and the strong anomalous mole fraction effects observed for Cs+o + K+o mixtures, suggest that DR channels select for K+ using ion-ion competition. The block by large intracellular cations shows that the pore is asymmetrical. The loss of high affinity TEAo block with Na+i and Na+o, and the effects of permeant ions on gating, suggest that channel conformation may be affected by ions in the pore.
authors.affiliation: Department of Neurosciences, Case Western Reserve University, Cleveland, OH 44106, USA.
authors.forename: B M
authors.initials: BM
authors.lastname: Block
authors.forename: S W
authors.initials: SW
authors.lastname: Jones
mesh_terms: D000818:Animals; D001120:Arginine / Q000378:metabolism; D002586:Cesium / Q000378:metabolism; D002794:Choline / Q000378:metabolism; D051668:Delayed Rectifier Potassium Channels; D005728:Ganglia, Sympathetic / Q000166:cytology*; D015640:Ion Channel Gating; D008274:Magnesium / Q000378:metabolism; D008536:Meglumine / Q000378:metabolism; D009474:Neurons / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels / Q000378:metabolism*; D024642:Potassium Channels, Voltage-Gated; D011892:Rana catesbeiana; D012964:Sodium / Q000378:metabolism; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000378:metabolism
chemical_list: D051668:Delayed Rectifier Potassium Channels; D015221:Potassium Channels; D024642:Potassium Channels, Voltage-Gated; D013757:Tetraethylammonium Compounds; D002586:Cesium; D019789:Tetraethylammonium; D008536:Meglumine; D001120:Arginine; D012964:Sodium; D008274:Magnesium; D002794:Choline; D011188:Potassium