title: Ion selectivity and gating of small conductance Ca(2+)-activated K+ channels in cultured rat adrenal chromaffin cells.
abstract: 1. The ion selectivity and gating of apamin-sensitive, small conductance Ca(2+)-activated K+ (SK) channels were studied in cultured rat adrenal chromaffin cells using patch clamp techniques. 2. The amplitude of slow tail currents showed a bell-shaped dependence on depolarization potentials. Slow tail currents were abolished in a Ca(2+)-free external solution or by adding 100 microM Cd2+ to the external solution. Reversal potentials followed the predictions of the Nernst equation for a K+ electrode. 3. Slow tail currents were largely blocked by external application of apamin (dissociation constant, Kd, 4.4 nM), (+)-tubocurarine (Kd, 20 microM), and tetraethylammonium (Kd, 5.4 mM). 4. The relative permeability (PX/PK, where X may be any one of the ions listed) of SK channels was: Tl+ (1.87) > K+ (1.0) > Rb+ (0.81) > Cs+ (0.16) > NH4+ (0.11). Na+, Li+ and methylamine were not measurably permeant (PX/PK < 0.005). Open SK channels seem to have an effective pore diameter of 0.34-0.38 nm. The relative conductance (gX/gK) was: Tl+ (1.29) > K+ (1.0) > Rb+ (0.85) > Cs+ (0.45) approximately NH4+ (0.44). 5. With mixtures of Tl+ and K+, SK channels showed anomalous mole-fraction behaviour. 6. Ca2+ dependence of SK channel gating was studied using inside-out macropatches. The [Ca2+] required for half-maximal activation and the Hill coefficient were 0.69 microM and 1.7, respectively, and independent of membrane potentials. 7. Single-channel conductance was 13-14 pS (160 mM K+).
authors.affiliation: Department of Physiology and Biophysics, University of Washington, School of Medicine, Seattle 98195.
authors.forename: Y B
authors.initials: YB
authors.lastname: Park
mesh_terms: D000313:Adrenal Medulla / Q000166:cytology; D000818:Animals; D001030:Apamin / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D004553:Electric Conductivity; D004594:Electrophysiology; D015640:Ion Channel Gating; D008297:Male; D018408:Patch-Clamp Techniques; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000032:analysis / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D012680:Sensitivity and Specificity; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D014403:Tubocurarine / Q000494:pharmacology
chemical_list: D026902:Potassium Channel Blockers; D015221:Potassium Channels; D013757:Tetraethylammonium Compounds; D001030:Apamin; D019789:Tetraethylammonium; D002118:Calcium; D014403:Tubocurarine