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.