Effects of potassium and rubidium on the resting potential of muscle

Some Cation Interactions in Muscle

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.

Rubidium and cesium fluxes in muscle as related to the membrane potential

The reduction of membrane potential in frog sartorius muscle produced by rubidium and cesium ions has been studied over a wide concentration range and compared with depolarization occasioned by potassium ions. The constant field theory of passive flux has been used to predict the potential changes observed. The potential data suggest certain permeability coefficient ratios and these are compared with ratios obtained from flux data using radioactive tracers. The agreement of the flux with the potential data is good if account is taken of the inhibition of potassium flux which occurs in the presence of rubidium and cesium ions. A high temperature dependence has been observed for cesium influx (Q(10) = 2.5) which is correlated with the observation that cesium ions depolarize very little at low temperatures. The observations suggest that cesium ions behave more like sodium ions at low temperatures and more like potassium ions at room temperature with respect to their effect on the muscle cell resting potential. The constant field theory of passive ion flux appears to be in general agreement with the experimental results observed if account is taken of the dependence of permeability coefficients on the concentrations of ions used and of possible interactions between the permeabilities of ions.