It has been possible to treat potassium, rubidium, and cesium ion entry into frog sartorius muscle by the use of a model which assumes a limited number of sites at the cell surface. The ion concentration in an outer surface layer is regarded as the main factor determining the rate of inward movement. It is supposed that the concentration of ions in the external solution is effective in promoting inward movement only to an extent determined by the fraction of sites occupied. Equations are derived from the model which fit the inward flux versus applied concentration curves experimentally determined for the three ions. The ions were found to compete for the postulated sites in various bi-ionic mixtures, the competition being satisfactorily described by equations derived from the model. The constants assigned to each ion remain invariant and independent of gradients in electrochemical potential. The order of decreasing exchange rate found is K > Rb > Cs. The order of decreasing site affinity found is Rb > K > Cs which is the same order as that observed for the ion selectivity deduced from analytical measurements of cation preference after equilibration in various equimolal mixtures (Lubin and Schneider (21)). The manner in which such a model might affect the application of a theory which assumes electrical driving forces as well is discussed.

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