Dependence of SR Ca²⁺ content on the concentration of [Mg²⁺]_free and [ATP]_free. SR Ca²⁺ loading was estimated by application of 20 mM caffeine at the end of the experiments (after 6–8 min) and recording of the resulting fluorescence transient. (A) SR Ca²⁺ content at constant total ATP ([Mg²⁺]_free and [ATP]_free varied simultaneously). (B) SR Ca²⁺ content at constant [Mg²⁺]_free (0.2 mM) but variable [ATP]_free. (D) SR Ca²⁺ content at constant [ATP]_free (0.3 mM) but variable [Mg²⁺]_free.