A three-state model for inactivation of sodium permeability.

The inactivation of Na+ permeability in single myelinated motor nerve fibres of Rana esculenta was investigated under voltage and current clamp conditions at 20 degrees C in Ringer's solution and under blocked K+ currents. Development of inactivation and its recovery was described by two potential-dependent time constants: The smaller time constant followed the usual bell-shaped function of membrane potential, whereas the larger one was monotone-increasing with more negative potentials. Several three-state models for inactivation were investigated. The experiments could best be approximated by a model with two open and one closed state for inactivation following: open in equilibrium closed in equilibrium open. Rate constants were determined for all transitions shown from the voltage clamp experiments. The action potentials computed by means of the proposed model were in good agreement with those measured, both in Ringer's solution and under blocked K+ current conditions.