Cell water, sodium, and potassium in stimulated red and white mammalian muscles

Potassium and sodium concentration and inulin space were measured in samples of rat soleus, plantaris, and gastrocnemius muscles after indirect stimulation in situ for 4 min, 30 min, and 6 hr. Two samples were taken from the gastrocnemius muscle: one from its outer layers, containing mostly white fibers, and another from the deep layer containing relatively more dark fibers. The cationic changes which follow low-frequency stimulation are largest in white gastrocnemius fibers, and decrease in this order: plantaris muscle, red gastrocnemius fibers, and soleus. When the stimulation frequency was increased, predominantly red components of the triceps surae also showed a marked cationic "debt." The same occurred with an intermittent tetanus alternating with stimulation at 10/sec. Intermittent tetanus alternating with resting periods or continuous tetanus did not cause any significant change in cationic distribution.