Effect of Mg²⁺ on the regulation of mitochondrial Ca²⁺. (A–C) Large Ca²⁺ additions (Ca²⁺ additions were 15.3 µM each) in the presence of zero (black), 0.1 (blue), or 1 mM (red) MgCl_2. (D–F) Small Ca²⁺ additions (first Ca²⁺ addition 0.2 µM, subsequent 0.011 µM each) with 0 (black), 0.1 (green), 0.5 (blue), and 1 mM (red) MgCl₂.