The penetration of some cations into muscle.

The influx of Na(+), K(+), Rb(+), and Cs(+) into frog sartorius muscle has been followed. The results show that a maximum rate is found for K(+), while Na(+) and Cs(+) penetrate much more slowly. Similar measurements with Ca(++), Ba(++), and Ra(++) show that Ba(++) penetrates at a rate somewhat greater than that of either Ca(++) or Ra(++). All these divalent cations, however, penetrate at rates much slower than do the alkali cations. The results obtained are discussed with reference to a model that has been developed to explain the different penetration rates for the alkali cations.