Fig. 5.

Passive conductance enables a rapid change in transmembrane K⁺ concentrations. A: illustration of experimental paradigm where the K⁺ ions in the electrode were fully replaced by Na⁺ ions while the bath K⁺ remained at physiological [K⁺]_e of 3.5 mM. B: single astrocyte recorded under the condition shown in A in current-clamp mode; a −2-nA holding current (I_holding) pulse was applied at incremental durations from 1 to 5 s. In between these I_holding pulses, the cell was maintained at I_holding = 0 for V_m recovery back to resting levels. The longer the duration of the I_holding pulse, the larger the induced maximal V_m hyperpolarization. Also, the longer the duration of the I_holding pulse, the more negative the reversal potential (V_rev), indicating more accumulation of K⁺ inside astrocytes (see expanded scale in C). D: illustration of experimental paradigm where both the electrode and bath K⁺ ions were substituted by Na⁺ ions. E. under the experimental condition shown in D, V_rev returned to the resting potential level immediately, indicating a high selectivity of passive conductance to K⁺. F: according to the Goldman-Hodgkin-Katz equation, the estimated intracellular K⁺ concentrations ([K⁺]_i), corresponding to the V_rev values, are plotted against the I_holding pulse durations. Values are means ± SE.