External Mg²⁺ and Ca²⁺ block K⁺ currents through Kir1.1 channels. (A) Currents at different voltages in the presence of 11 mM K⁺ plus 99 mM Na⁺ in the pipette solution and 110 mM K⁺ in the bath solution. (B) Currents under the same conditions as in A but with 10 mM Mg²⁺ in the pipette solution. (C) i-V relationships in the absence of divalents and in the presence of 10 mM Mg²⁺ or 10 mM Ca²⁺ in the external solution. (D) Analysis of block by external Mg²⁺ and Ca²⁺ according to Eqs. 2 and 3. The lines represent best fits to the data, with K_iMg = 13 mM and zδ_Mg = 0.18, and K_Ca = 31 mM and zδ_Ca = 0.17.