Interstitial space of mouse skeletal muscle.

1. A new preparation of mouse skeletal muscle, prepared from pectoral muscles, is described.2. The sorbitol space of this muscle, both in vivo and in vitro, has been measured with dynamic loading of the muscle in vitro as an experimental variable.3. The Na(+) and K(+) contents of the muscle have been determined and the apparent intracellular concentration for these ions calculated both in vivo and after incubation in vitro.4. Histological studies on the incubated muscle have been made so as to permit comparison of the changes in the chemical measurements with changes in the ultrastructure of the muscle.5. The results of these experiments show that there is an increase in the apparent extracellular space of the muscle following incubation. This increase is constant, and independent of the load, with the important exception that unloaded muscles do not reach an equilibrium during the period of incubation and have a much greater apparent extracellular space.6. Intracellular Na(+) and K(+) concentrations are consistent with the sorbitol being restricted to an extracellular phase in the loaded muscle; but the evidence implies that sorbitol in the unloaded muscle penetrates into a space from which Na(+) is excluded.7. The total water content of the muscle per unit weight is unchanged by incubation, indicating that the apparent change in sorbitol space is in the ratio of intracellular space to extracellular space rather than by addition of water to the extracellular space. The significance of these results is discussed with reference to the use of such preparations for in vitro studies.