Submaximal potassium-induced contractures were studied in isolated mouse hemidiaphragms (HD) and compared with extensor digitorum longus (EDL) and soleus. 2. In the HD, the contracture was triphasic: an initial small, fast phase followed by a larger slow phase, and a late phase which was seen on prolonged exposure. 3. Costal segments of the HD responded to elevated potassium with a larger fast phase, while dorsal or ventral segments produced a larger slow phase. 4. In calcium-free solution, the fast phase appeared to be unaffected, while the slow phase was potentiated and its time course abbreviated. The late phase was greatly attenuated or abolished. 5. In the soleus, but not the EDL muscle, contracture amplitude in calcium-free solution was potentiated and the time course was shortened. 6. The characteristics of potassium-induced contractures of both the soleus and EDL muscles are to be seen in the response of the hemidiaphragm, but there is a regional variation in their proportional contribution which suggests a heterogeneity in diaphragm composition.
