Ion permeation and block of M-type and delayed rectifier potassium channels - Whole-cell recordings from bullfrog sympathetic neurons

Ion permeation and conduction were studied using whole-cell recordings of the M-current (I-M) and delayed rectifier (I-DR), 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. P-Rb/P-K was near 1.0 for both channels, and G(Rb)/G(K) was 0.87 +/- 0.01 for I-DR but only 0.35 +/- 0.01 for I-M (15 mM [Rb+](0) or [K+](0)). The permeability sequences were generally similar for I-M and I-DR: K+ similar to Rb+ > NH4+ > Cs+, with no measurable permeability to Li+ or CH3NH3+. However, Na+ carried detectable inward current for I-DR but not I-M. Na-0(+) also blocked inward K+ current for I-DR (but not I-M), at an apparent electrical distance (delta) similar to 0.4, with extrapolated dissociation constant (K-D) similar to 1 M at 0 mV. Much of the instantaneous rectification of I-DR in physiologic ionic conditions resulted from block by Na-0(+). Extracellular Cs+ carried detectable inward current for both channel types, and blocked I-M with higher affinity (K-D = 97 mM at 0 mV for I-M, K-D similar to 0.2 M at 0 mV for I-DR), with delta similar to 0.9 for both. I-DR showed several characteristics reflecting a multi-ion pore, including a small anomalous mole fraction effect for P-Rb/P-K, concentration-dependent G(Rb)/G(K), and concentration-dependent apparent K-D's and delta's for block by Na-0(+) and Cs-0(+). I-M showed no clear evidence of multiion pore behavior. For I-M, a two-barrier one-site model could describe permeation of K+ and Rb+ and block by Cs-0(+), whereas for I-DR even a three-barrier, two-site model was not fully adequate.