Cation permeability of a cloned rat epithelial amiloride-sensitive Na+ channel

Abstract

1. Conductance of heterotrimeric rat epithelial Na+ channels (α,β,γ-rENaCs) for Li+ and Na+ in planar lipid bilayers was a non-linear function of ion concentration, with a maximum of 30.4 ± 2.9 pS and 18.5 ± 1.9 pS at 1 M Li+ and Na+, respectively. 2. The α,β,γ-rENaC conductance measured in symmetrical mixtures of Na+-Li+ (1M) exhibited an anomalous mole fraction dependence, with a minimum at 4:1 Li+ to Na+ molar ratio. 3. Permeability ratios P(K)/P(Na) and P(Li)/P(Na) of the channel calculated from the biionic reversal potentials were dependent on ion concentration: P(K)/P(Na) was 0.ll ± 0.01, and P(Li)/P(Na) was 1.6 ± 0.3 at 50 mM; P(K)/P(Na) was 0.04 ± 0.01 and P(Li)/P(Na) was 2.5 ± 0.4 at 3 M, but differed but differed from the ratios of single-channel conductances in symmetrical Li+ Na+ or K+ solutions. The permeability sequence determined by either method was Li+ > Na+ + > K+ >> Rb+ > Cs+. 4. Predictions of a model featuring two binding sites and three energy barriers (2S3B), and