title: Ion permeation and block of M-type and delayed rectifier potassium channels. Whole-cell recordings from bullfrog sympathetic neurons.
abstract: Ion permeation and conduction were studied using whole-cell recordings of the M-current (I(M)) and delayed rectifier (IDR), two K+ currents that differ greatly in kinetics and modulation. Currents were recorded from isolated bullfrog sympathetic neurons with 88 mM [K+]i and various external cations. Selectivity for extracellular monovalent cations was assessed from permeability ratios calculated from reversal potentials and from chord conductances for inward current. PRb/PK was near 1.0 for both channels, and GRb/GK was 0.87 +/- 0.01 for IDR but only 0.35 +/- 0.01 for I(M) (15 mM [Rb+]o or [K+]o). The permeability sequences were generally similar for I(M) and IDR: K+ approximately Rb+ > NH4+ > Cs+, with no measurable permeability to Li+ or CH3NH3+. However, Na+ carried detectable inward current for IDR but not I(M). Nao+ also blocked inward K+ current for IDR (but not IM), at an apparent electrical distance (delta) approximately 0.4, with extrapolated dissociation constant (KD) approximately 1 M at 0 mV. Much of the instantaneous rectification of IDR in physiologic ionic conditions resulted from block by Nao+. Extracellular Cs+ carried detectable inward current for both channel types, and blocked I(M) with higher affinity (KD = 97 mM at 0 mV for I(M), KD) approximately 0.2 M at 0 mV for IDR), with delta approximately 0.9 for both. IDR showed several characteristics reflecting a multi-ion pore, including a small anomalous mole fraction effect for PRb/PK, concentration-dependent GRb/GK, and concentration-dependent apparent KD's and delta's for block by Nao+ and Cso+. I(M) showed no clear evidence of multi-ion pore behavior. For I(M), a two-barrier one-site model could describe permeation of K+ and Rb+ and block by Cso+, whereas for IDR even a three-barrier, two-site model was not fully adequate.
authors.affiliation: Department of Neurosciences, Case Western Reserve University, Cleveland, Ohio 44106, USA. bmb5@po.cwru.edu
authors.forename: B M
authors.initials: BM
authors.lastname: Block
authors.forename: S W
authors.initials: SW
authors.lastname: Jones
mesh_terms: D000818:Animals; D004305:Dose-Response Relationship, Drug; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000494:pharmacology*; D015221:Potassium Channels / Q000187:drug effects* / Q000502:physiology; D011892:Rana catesbeiana; D012413:Rubidium / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology; D013564:Sympathetic Nervous System / Q000187:drug effects*
chemical_list: D015221:Potassium Channels; D012964:Sodium; D012413:Rubidium; D011188:Potassium