title: Cation permeability of a cloned rat epithelial amiloride-sensitive Na+ channel.
abstract: 1. Conductance of heterotrimeric rat epithelial Na+ channels (alpha, beta, gamma-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 alpha, beta, gamma-rENaC conductance measured in symmetrical mixtures of Na(+)-Li+ (1 M) exhibited an anomalous mole fraction dependence, with a minimum at 4:1 Li+ to Na+ molar ratio. 3. Permeability ratios PK/PNa and PLi/PNa of the channel calculated from the bionic reversal potentials were dependent on ion concentration: PK/PNa was 0.11 +/- 0.01, and PLi/PNa was 1.6 +/- 0.3 at 50 mM; PK/PNa was 0.04 +/- 0.01 and PLi/PNa was 2.5 +/- 0.4 at 3 M, 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 allowing double occupancy, developed on the basis of single ion current-voltage relationships, are in agreement with the observed conductance maximum in single ion experiments, conductance minimum in the mole fraction experiments, non-linearity of the current-voltage curves in bionic experiments, and the concentration dependence of permeability ratios. 5. Computer simulations using the 2S3B model recreate the ion concentration dependencies of single-channel conductance observed for the immunopurified bovine renal amiloride-sensitive Na+ channel, and short-circuit current in frog skin, thus supporting the hypothesis that ENaCs form a core conduction unit of epithelial Na+ channels.
authors.affiliation: Department of Physiology and Biophysics, University of Alabama at Birmingham 35294-0005, USA. Ismailov@PhyBio.bhs.uab.Edu
authors.forename: I I
authors.initials: II
authors.lastname: Ismailov
authors.forename: V G
authors.initials: VG
authors.lastname: Shlyonsky
authors.forename: O
authors.initials: O
authors.lastname: Alvarez
authors.forename: D J
authors.initials: DJ
authors.lastname: Benos
mesh_terms: D000818:Animals; D001665:Binding Sites; D002412:Cations / Q000378:metabolism*; D003198:Computer Simulation; D004553:Electric Conductivity; D053503:Epithelial Sodium Channels; D008051:Lipid Bilayers / Q000378:metabolism; D008094:Lithium / Q000493:pharmacokinetics; D008954:Models, Biological; D010539:Permeability; D010743:Phospholipids / Q000737:chemistry / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D051381:Rats; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000378:metabolism*
chemical_list: D002412:Cations; D053503:Epithelial Sodium Channels; D008051:Lipid Bilayers; D010743:Phospholipids; D015222:Sodium Channels; D008094:Lithium; D012964:Sodium; D011188:Potassium