Muscle: a three phase system. The partition of monovalent ions across the cell membrane.

The partition of Li(+), Br(-), and I(-) across the membrane of the sartorius muscle of the toad Bufo marinus has been investigated both at the steady state and with kinetic methods. Li(+) was found to have access to an amount of muscle water similar to that of Na(+). Br(-) and I(-) could be regarded as being interchangeable with cellular Cl(-). None of the foreign ions caused significant losses of cellular K(+). Li(+) efflux from the cell was slower in muscles which were equilibrated for long periods in Li(+) than in short equilibrated muscles. Na(+) efflux from Li(+)-treated muscles was similar in rate to normal controls, but the amount of Na(+) in the slow fraction was increased by Li(+). I(-) efflux was extremely rapid, and it was not possible to differentiate kinetically between intra- and extracellular material. These results have been found to be consistent with the hypothesis of a three phase system for muscle.