Role of sodium and potassium permeabilities in the depolarization of denervated rat muscle fibres.

1. Na+ and K+ flux measurements and membrane potential (Vm) determinations were performed on normal and denervated rat extensor digitorum longus (e.d.l.) muscles. 2. The mean Vm in normal muscle fibres was -74.6 mV. During the first week after denervation Vm fell about 20 mV following an S-shaped time course. 3. In that period the Na+ permeability (PNa) increased and the K+ permeability (PK) decreased, so that by the sixth day post-denervation, the PNa/PK ratio was increased by a factor of 2.7. 4. The decrease in PK preceded the increase in PNa. 5. No major contribution to the fall of Vm by a reduced activity of an electrogenic Na+ pump could be detected. 6. A good agreement was found between the experimental values of the depolarization and those calculated using the constant-field equation assuming Cl- is at equilibrium and no significant change of the intracellular K+ concentration ([K+]i) during the first week after denervation. 7. It is concluded that the depolarization promoted by denervation in e.d.l. rat muscle fibres can be fully explained in terms of changes in PNa and PK.