Cation permeation of the amphibian motor end-plate

Measurements of acetylcholine-induced single-channel conductance and null potentials at the amphibian motor end-plate in solutions containing Na, K, Li and Cs ions (Gage & Van Helden, 1979; J. Physiol. (London) (in press) were analyzed in terms of three models. Two of these models, the "neutral" site channel model and the "charged" site channel model were developed to cater for three cations. Both were shown to be able to explain the dependence of single-channel conductance on membrane potential and gave the following sequences of equilibrium constants and mobilities. KLi/KNa/KK/KCs = 7:1.1:1:0.9 and UCs/UK/UNa/ULi = 1.4:1:0.58:0.13 at 8 degrees C. Similar sequences were obtained at 20 degrees C. Although the neutral model fitted the data for relative conductances in Li-, Cs- and Na-solutions slightly better than the charged model, experiments done in normal [NaCl] and [NaCl]/2 solutions could only be fitted by the neutral model. In contrast, the third model, the Constant Field Equation, was unable to fit the conductance data in any of the above situations. The data available suggests that permeation is through "long" neutral channels, lined with high field-strength negative polar groups and including one or possibly more high resistance barriers for anions.