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 activat ion on compound muscle action potentials (M waves), resting membrane potent ial, 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) tetani c stimulations given at 1-min intervals induced recovery of force and M wav e area to 81 and 90% of control levels, respectively, within 15 min (P< 0.0 01). The recovery of force and M wave was associated with a partial repolar ization of the muscle fibers. Experiments with tubocurarine suggest that th e 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 act ivity. 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.