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 (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+ ~ Rb+ > NH4/+ > Cs+, with no measurable permeability to Li+ or CH3NH3/+. However, Na0 carried detectable inward current for I(DR) but not I(M)-Na0/+ also blocked inward K+ current for I(DR) (but not I(M)), at an apparent electrical distance (δ) ~0.4, with extrapolated dissociation constant (K(D)) ~1 M at 0 mV. Much of the instantaneous rectification of I(DR) in physiologic ionic conditions resulted from block by Na0/+. 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) ~0.2 M at 0 mV for I(DR)), with δ~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 (C(Rb)/C(K), and concentration-dependent apparent K(D)'s and δ's for block by Na0/+ and Cs0/+. 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 Cs0/+, whereas for I(DR) even a three-barrier, two-site model was not fully adequate.