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

The partition of sulfate, Ca(++), and Mg(++) across the membrane of the sartorius muscle has been studied, and the effect of various concentrations of these ions in the Ringer solution on the cellular level of Na(+), K(+), and Cl(-) has been determined. The level of the three divalent ions in toad plasma and muscle in vivo has been assayed. Muscle was found to contain an almost undetectable amount of inorganic sulfate. Increases in the external level of these ions brought about increases in intracellular content, calculated from the found extracellular space as determined with radioiodinated serum albumin or inulin. Less of the cell water is available to sulfate than to Cl(-), and the Mg(++) space is less than the Na(+) space. An amount of muscle water similar to that found for Li(+) and I(-) appears to be available to these divalent ions. Sulfate efflux from the cell was extremely rapid, and it was not found possible to differentiate kinetically between intra- and extracellular material. These results are consistent with the theory of a three phase system, assuming the muscle to consist of an extracellular phase and two intracellular phases. Mg(++) and Ca(++) are adsorbed onto the ordered phase, and increments in cellular content found on raising the external level are assumed to occur in the free intracellular phase.