Saturation effects and rectifier properties of sodium channels in human skeletal muscle.

Sodium outward currents were measured in human myoballs with the whole-cell recording method. The electro-chemical gradient of the sodium ions across the cell membrane was modified over a wide range by variations of the clamped membrane potential and of the internal and external sodium concentration. Up to 50 mV positive to the sodium equilibrium potential, ENa, the current-voltage relation is linear. At a potential 80 mV positive to ENa the sodium outward current has a maximum and decreases with a further increase in electrochemical gradient. Investigating the instantaneous current change in experiments in which the membrane potential was changed while the channels were already open we could exclude the possibility that the gates of activation or inactivation are responsible for this effect. Therefore we postulate that the sodium channel has a valve-like mechanism producing a negative slope conductance at highly positive membrane potentials, a current saturation with self-inhibition by the intracellular sodium concentration, and a blockade of the channel on reduction of the extracellular sodium concentration.