Changes in [Mg²⁺]_free affect the frequency of spontaneous Ca²⁺ sparks. Representative examples of confocal line-scan images recorded from permeabilized cardiomyocytes. (A) Solution change from 1 mM [Mg²⁺]_free (control) to nominally Mg²⁺ free (with 3 mM [ATP]_free). (B) To 0.5 mM [Mg²⁺] (with 0.54 mM [ATP]_free). (C) To 3 mM [Mg²⁺] (with 0.11 mM [ATP]_free). The images were obtained before the solution change (left; 1 mM [Mg²⁺]), 5–30 s after (middle), and 3–6 min after (right) exchange of solution.