Figure 3.

(A) K_IR channel conductance over a range of V_m values with increasing concentrations of K⁺. With 3 mM [K⁺] _o at -40 mV (the approximate resting V_m; dotted arrows), K_IR channel conductance is very low. Raising K⁺ greatly increases K_IR channel activity at a given V_m; channel activity is also increased by membrane hyperpolarization. Data were plotted according to the eq. 1 in the text. (B) The relationship between external K⁺ concentration and K_IR channel conductance based on the K⁺- and voltage-dependence of K_IR2 channels, calculated using eq. 1 (see text), and measured effects of external K⁺ on PA SM V_m [26,37]. Elevation of [K⁺]_o from 3 mM to 8 mM causes an enormous, near-maximal increase in K_IR channel conductance, with further elevations to 15 and 25 mM causing only small subsequent increases in conductance. (C) Subtracted 100-μM Ba²⁺-sensitive currents from a freshly dissociated EC from a rat PA in response to a voltage ramp from -140 to 0 mV indicating the presence of functional K_IR2 channels in these cells. External K⁺ was 6 mM in this experiment.