Influence of trophic substances in the regulation of resting membrane potential and ionic concentration in skeletal muscle.

We studied the ionic and water content and the resting membrane potential of rat extensor digitorum longus EDL muscles at different times after unilateral nerve crush. Intracellular potassium concentration decreased progressively during the 1st week after nerve crush whereas intracellular sodium concentration increased significantly. At about day 10, when functional reinnervation (presence of end-plate potentials and miniature end-plate potentials) was detected, the above changes tended to return to control values. In addition, there was a significant difference between muscles with long and short nerve stumps. These results suggest a neurogenic dependency of muscle hydroelectrolytic composition. The decrease in resting membrane potential was greatest after 6.5 days of denervation when changes in the internal ionic concentration were maximum; however, these ionic changes contributed little to the decrease. The recovery of the resting membrane potential commenced at least 48 h before the first signs of functional reinnervation (10th day). This finding suggested an important contribution of some neurotrophic material in early stages of the reinnervation when nerve-muscle contacts were already established. Later, the contribution of mechanical activity to the restoration of the RMP became apparent (20th day); fibrillation potentials had disappeared by that time.