Fig. 7.

Analysis of the block by extracellular Ba²⁺ of wild-type Kir7.1 and mutant Kir7.1M125R inXenopus oocytes. A, Ramp current responses to voltage ramps of 2 sec duration between −150 and +60 mV show the voltage dependence of I_Kir7.1 block by 1 and 5 mm Ba²⁺. B, Current inhibition relative to a maximum block by a saturating concentration of Ba²⁺ is plotted versus the concentration of the blocking cation at a holding potential ofV_h = −80 mV. Curves are least-squares fits of data points to a Hill equation (1/1 + [A/K_i]ⁿ, giving a K_i for Ba²⁺ of 670 μm for Kir7.1 (•) and aK_i for Ba²⁺ of 27 μm for Kir7.1M125R (○). K_iis the concentration of cation producing 50% block; Aand n are variables. Insets, Shown are continuous recordings of wild-type and mutant Kir7.1 currents at −80 mV under Ba²⁺ block. The application of the blocker is indicated by black bars. C, Time- and voltage-dependent block of wild-type and mutant Kir7.1M125R channels by Ba²⁺. Shown are single-exponential fits to the time course of the current block by a saturating concentration of Ba²⁺ (10 mm for Kir7.1 and 500 μm for Kir7.1M125R). Insets, Shown are the complete current responses of Kir7.1 and Kir7.1M125R channels to 500 msec voltage steps between 0 and −120 mV. D, Relationship of time constants of activation (τ_on) and the membrane potential for wild-type Kir7.1 (•) and mutant Kir7.1M125R (○) channels at 96 mm external K⁺.