Summary of Ca²⁺ spark frequency variations and their dependence on [Mg²⁺]_free, [ATP]_free, and [MgATP]. Spark frequencies were determined from the peak (P; 0–30 s; gray triangles and lines) and from the steady-state (SS; 3–6 min; black circles and lines) after solution exchange, as shown by lines in Fig. 2. (A) Spark frequencies at constant total ATP ([Mg²⁺]_free and [ATP]_free varied simultaneously). (B) Spark frequencies at constant [Mg²⁺]_free (0.2 mM) but variable [ATP]_free. (C) Spark frequencies at constant [ATP]_free (0.3 mM) but variable [Mg²⁺]_free. The spark frequencies were plotted versus [Mg²⁺]_free (left), [ATP]_free (middle), and [MgATP] (right) to isolate the dependence on each compound from the others.