Activity-induced recovery of excitability in K+-depressed rat soleus muscle

Increased extracellular K+ concentration ([K+](o)) can reduce excitability and force in skeletal muscle. Here we examine the effects of muscle activation on compound muscle action potentials (M waves), resting membrane potential, and contractility in isolated rat soleus muscles. In muscles incubated for 60 min at 10 mM K+, tetanic force and M wave area decreased to 23 and 24%, respectively, of the control value. Subsequently, short (1.5 s) tetanic stimulations given at 1-min intervals induced recovery of force and M wave area to 81 and 90% of control levels, respectively, within 15 min (P< 0.001). The recovery of force and M wave was associated with a partial repolarization of the muscle fibers. Experiments with tubocurarine suggest that the force recovery was related to activation of muscle Na+-K+ pumps caused by the release of some compound from sensory nerves in response to muscle activity. In conclusion, activity produces marked recovery of excitability in K+-depressed muscle, and this may protect muscles against fatigue caused by increased [K+](o) during exercise.