Abstract
The permeation of K+ and Na+ through the pore of a K+ channel from the plasma membrane of rye roots was studied in planar 1-palmitoyl-2-oleoyl phosphatidylethanolamine bilayers. The pore contains at least two ion-binding sites which can be occupied simultaneously. This was indicated by: (i) biphasic relationships with increasing cation concentration of both channel conductance at the zero-current (reversal) potential of the channel (Erev) and unitary-current at a specified voltage and (ii) a decline in Erev in the presence of equimolar Na+ (cis):K+ (trans) as the cation concentration was increased. To determine the spatial characteristics and energy profiles for K+ and Na+ permeation, unitary-current/voltage data for the channel were fitted to a three energy-barrier, two ion-binding site (3B2S) model. The model allowed for simultaneous occupancy of binding sites and ionic repulsion within the pore, as well as surface potential effects. Results suggested that energy peaks and energy wells (ion binding sites) were situated asymmetrically within the electrical distance of the pore, the trans energy-well being closer to the center of the pore than its cis counterpart; that the energy profile for K+ permeation differed significantly from that of Na+ in having a higher cis energy peak and a deeper cis energy well; that cations repelled each other within the pore and that vestibule surface charge was negligible. The model successfully simulated various aspects of K+ and Na+ permeation including: (i) the complexities in current rectification of a wide range of contrasting ionic conditions; (ii) the biphasic relationships with increasing cation concentration of both channel conductance at Erev and unitary-current at a specified voltage; (iii) the decline in Erev in equimolar Na+ (cis):K+ (trans) as cation concentrations were increased and (iv) the complex relationships between mole fraction and Erev at total cation concentrations of 100 and 300 mM.