K Permeability of Nitella clavata in the Depolarized State

Membrane current responses to sudden potential changes were recorded in solutions of various K]_o on 52 internodal cells of Nitella clavata. The membrane current after sudden depolarization had a component sensitive to K]_o which increased with time from 0.3 to 2.0 s and remained steady thereafter. This late current became zero at values of E and K]_o which suggests that the current was nearly all carried by K⁺. The potassium conductivity represented by this current increased with depolarization, with a half-maximum value at about -70 mV, and saturation at about -30 to -20 mV. The potassium conductance also increased with increasing K]_o, but less rapidly than predicted for constant potassium permeability. This failure of the conductance to increase with K]_o was relatively the same at all membrane potentials and may be explained by a model with a finite number of channels. No attempt was made to model the dependence of g_K on time after depolarization or on membrane potential. However, the finding that the membrane potential did not affect the way in which the permeability depended on K]_o suggests that the membrane potential change does not affect the affinity of the sites, and that the increase in g_K with time after depolarization is brought about by an increase in the number of channels with such sites.