Sodium channel permeation in squid axons. II: Non‐independence and current‐voltage relations.

Abstract

The concentration and voltage dependence of current through the Na channels of squid giant axons was studied. The permeant cations Na, K and NH4 were used. The Na channel current at a fixed voltage saturates as the internal permeant ion concentration is increased. The half-saturation activities at 50 mV were 623, 268 and 161 mM for Na, NH4 and K, respectively. These Km values for Na and K appear to be voltage-dependent. Instantaneous current-voltage relations were determined with 2 different internal Na concentrations and with Na outside and K inside. A 3-barrier, 2-site ion permeation model previously used in describing Na channel reversal potentials accounts for the Na channel currents as well.