Time-course of averaged Ca²⁺ spark frequencies after changes in [Mg²⁺]_free and [ATP]_free. Traces are offset slightly for clarity. SpF denotes spark frequency (per s⁻¹ 100 μm⁻¹). [Mg²⁺]_free was initially 1 mM in all experiments (control solution). For composition of the test solution, see symbol legend on the right. (A) At the moment indicated by the bar below the traces, test solutions were applied with different levels of [Mg²⁺]_free and [ATP]_free, but at constant ATP_total (3 mM). (B) [ATP]_free was varied at constant [Mg²⁺]_free (0.2 mM). (C) [Mg²⁺]_free was varied at constant [ATP]_free (0.3 mM). Time intervals for data considered in further analyses are indicated by horizontal lines. P, peak values; SS, steady-state values.