Figure 7.

External K⁺ competes with Mg²⁺ block of Kir1.1 currents. (A) Analysis of block of K⁺ currents by 3 mM Mg²⁺ at three different external [K⁺] according to Eqs. 2 and 3. Lines represent K_Mg(0) = 4.6 mM and zδ_Mg = 0.19 (11 mM K⁺); K_Mg(0) = 9.9 mM and zδ_Mg = 0.18 (55 mM K⁺); and K_Mg(0) = 20 mM and zδ_Mg = 0.19 (110 mM K⁺). (B) K_Mg(0) plotted versus [K⁺]. The straight line represents a linear regression fit of the data. The intercept on the ordinate implies that K_Mg = 2.5 mM in the absence of K⁺, and the slope provides an estimate of 15 mM for the dissociation constant for K⁺ displacement of Mg²⁺. (C) Ratios of currents at V_m = −100 mV in the presence and absence of 3 mM Mg²⁺ with different ions in the external solution. (D) External Mg²⁺ decreases the apparent affinity for K⁺. g is plotted as a function of [K⁺] for no added divalents (black) and with 3 mM Mg²⁺ in the external solution (red). Lines represent the best fits to Eq. 1, with g_max = 50 ± 1 pS and K_m = 11.0 ± 1.3mM (control), and g_max = 67 pS ± 1 and K_m = 96 ± 4 mM (+3 mM Mg²⁺).